regulation of protein translation is a critical step of the gene expression process , which allows cellular adaptation during stress conditions by rapidly reprograming the proteome output without the requirement for changes in rna synthesis . in conditions , such as heat shock , hypoxia ,
endoplasmic reticulum ( er ) stress , and apoptosis , an immediate change in protein levels is required , stressing the importance of translational regulation , responsible for rapid adaptation to physiological conditions .
however , there is an emerging body of evidence that shows a limited correlation between the transcriptome and the corresponding proteome , suggesting that when it comes to translation , not all transcripts are treated equally .
epidermal growth factor ( egf ) treatment of serum - starved hela cells resulted in only 4.8% differentially expressed genes ( degs ) , where a deg represents a significant change in both the transcriptome and translatome in the same direction ( homodirectionally ) . in opposition ,
the 95.2% of uncoupled degs represent a significant change in either the transcriptome or translatome or an inverse relationship between the transcriptome and translatome . using parallel genome - scale measurements of mrna and corresponding protein levels and half - lives ,
mrnas were found to explain 40% of the variability in protein levels , with translation efficiency being the best predictor of protein levels in mouse fibroblasts .
accordingly , translation control is considered to play a central role in eukaryotic gene expression . as new evidence
is being uncovered , scientists have now started to appreciate the critical role of mrna translation in tumor progression . in a wide range of cancer types , inappropriate translation of oncogenes , tumor suppressors , and eukaryotic translation initiation factors is a critical process in cancer cell proliferation [ 46 ] . even during times of stress , when global levels of protein synthesis are reduced ,
these transcripts encode prosurvival proteins that are translated by alternative ( noncanonical ) mechanisms [ 1 , 7 , 8 ] .
studies continue to provide new knowledge with regard to the developmental causes and possible novel treatments of various types of cancers [ 47 , 9 ] .
significantly , much of this information can be tied back to the paradigm of translational regulation and its critical contribution to our understanding of cancer etiology . in this review
, we will first discuss the mechanism of canonical translation initiation , followed by noncanonical mechanisms that utilize rna sequence features including upstream open reading frames ( uorfs ) and internal ribosome entry site- ( ires- ) mediated translation mechanisms , the role of eukaryotic initiation factors in noncanonical translation , and their significance in cancer progression .
in addition to uorfs- and ires - mediated translation regulation , other noncanonical translation mechanisms exist , such as gamma interferon - activated inhibitor of translation ( gait ) complex , 5 terminal oligopyrimidine ( 5 top ) elements , and au - rich elements ( ares ) [ 10 , 11 ] .
these mechanisms are out of the scope of this review article . 
 canonical translation initiation .
eukaryotic cap - dependent translation initiation includes the recognition and recruitment of mrna onto the small ribosomal ( 40s ) subunit , followed by ribosomal scanning in a 53 direction .
subsequently , the 60s large ribosomal subunit is recruited , forming the 80s initiation complex . at this stage ,
an initiator methionyl - trna ( met - trnai ) is in the ribosomal peptidyl ( p ) site at the mrna start codon [ 1 , 13 ] .
canonical initiation is a complex process utilizing more than 25 proteins , including a minimum of twelve eukaryotic initiation factors ( eifs ) .
the rate of initiation varies between different mrnas and is influenced by accessibility to the methylated guanosine cap structure ( mg cap ) at the 5 terminus of the mrna , the length and secondary structure of the 5 untranslated region ( utr ) , the sequence and secondary structure surrounding the start codon , and the poly(a ) tail [ 15 , 16 ] .
initiation begins with the assembly of eif4f complex comprising eif4e , eif4 g , and eif4a onto the 5mg cap ( figure 1 ) .
eif4e binds to the mg cap , which then interacts with the multidomain scaffold protein eif4 g and the atp - dependent rna helicase eif4a .
the ternary complex ( eif2-gtp - met - trnai ) associated with a 40s ribosomal subunit is then recruited to the 5 end of the mrna via a critical link to eif4 g ( captured at the cap via eif4e ) mediated by eif3 , forming the 43s preinitiation complex .
eif1 and eif1a assist in stimulating recruitment of the ternary complex , as well as acting synergistically to promote continuous ribosomal scanning for aug start codons .
this 43s preinitiation complex then scans the 5 utr of the mrna , with the help of eif4a , until an initiation codon in optimal context is recognized .
eif5 and eif5b then mediate subsequent hydrolysis of gtp to release the bound initiation factors from the 48s complex , leaving the start codon in the ribosomal p - site with the met - trnai and allowing 60s ribosomal subunit to bind [ 1 , 19 ] .
the now competent 80s initiation complex then proceeds to translation elongation ( figure 1 ) . 
 noncanonical translation initiation . during stress conditions , such as hypoxia , nutrient deprivation , or endoplasmic reticulum ( er ) stress , alternative mechanisms that are mediated by cis - acting sequences in specific mrna subsets , such as uorf and ires ,
drive the translation of stress response mrnas [ 1 , 68 , 18 , 20 ] .
studies have revealed that a decent portion of human transcripts is known to contain uorfs ( upstream of the initiation codon of the coding region ) that function as translation or mrna stability regulators .
recent ribosome profiling data reveals that uorfs can exist in an out - of - frame relative to the main coding sequence .
however , an overlap can also occur between uorfs and the coding sequence , in which alternative translation of an upstream in - frame start codon of a gene can possibly produce an extended protein product .
the mechanism of uorf - mediated translation functions primarily during eif2 phosphorylation conditions and enhances the expression of proteins involved in cell - cycle regulation and apoptosis .
typical examples of uorf - mediated translation regulation include general control nonderepressible 4 ( gcn4 ) , the yeast transcriptional activator , and activating transcription factor 4 ( atf4 ) in mammals .
normally , translation initiation occurs from the start codons located in the 5 utr of mrna which leads to the translation of small uorfs .
additionally , reinitiation of terminating ribosomes will typically not occur on the downstream cistron ; thus , the translation of the main coding sequence is inhibited in these conditions .
however , during stress conditions eif2 phosphorylation attenuates translation of uorf sequences and allows the translation of main coding sequence .
eif2 phosphorylation and reduced availability of the eif2-gtp - trnai ternary complex favor translation reinitiation at the gcn4p coding region ( in yeast ) , subsequently resulting in activation of numerous genes [ 25 , 26 ] .
gcn4p activates these target genes by binding to them and functioning as a transcription factor .
the complexity of rna structure in the 5 utr also plays a crucial role in uorf - mediated translation .
for example , translation of -site app - cleaving enzyme 1 ( bace1 ) , which is implicated in alzheimer 's disease ( ad ) progression , is regulated through uorfs
. however , high gc content and complexity of the rna secondary structure are also crucial decisive factors for uorf - mediated translation of bace1 .
additionally , recent genome - level studies indicate that rna secondary structure is negatively correlated with uorf translational efficiency as upstream ( relative to uorfs ) structures restrict or even arrest ribosomal preinitiation complex ( pic ) , whereas structures downstream of uorfs enhance translation initiation of coding sequences .
ireses are rna sequence elements that were initially discovered in the 5 leader sequences of poliovirus and encephalomyocarditis virus genomic rna that lack the 5 cap structure but nonetheless are efficiently translated in the host cell [ 18 , 29 ] .
the viral ires elements comprise secondary and tertiary structures that play a role in direct interactions with the translation initiation machinery .
mutations in viral ireses such as hepatitis c virus ( hcv ) , classical swine fever virus ( csfv ) , and cricket paralysis virus ( crpv ) affect the secondary and tertiary rna structures and render these ireses inactive .
these viral ireses are classified based on structural and sequence similarities , as well as their requirement for eifs and other protein factors for translation initiation [ 19 , 20 ] .
picornavirus ires elements are the examples of types i and ii ireses which require eif4 g , eif4a , and eif3 to assemble 48s initiation complex .
type iii viral ireses require eif4 g , with hcv ires being an exception ; hcv ires interacts with eif3 for recruitment of the 40s ribosomal subunit in close proximity to the start codon , circumventing the requirement for the 5 cap structure [ 1 , 19 ] .
although ires - mediated translation operates independently of many canonical initiation factors , it requires rna - binding proteins known as ires trans - acting factors ( itafs ) .
many cellular mrnas are known to comprise ires elements , but they do not share structural or sequence similarities , unlike their viral counterparts [ 20 , 31 ] .
however , similar to viral ireses , cellular ireses participate in multiple interactions with the canonical initiation factors and itafs to recruit the ribosome [ 1 , 20 ] .
in fact , despite sequence and structural dissimilarities , cellular ireses are reported to share critical itafs .
ires elements have been identified in mrnas encoding stress response proteins ( pro- and antiapoptotic ) , such as x - linked inhibitor of apoptosis ( xiap ) , cellular inhibitor of apoptosis 1 ( ciap1 ) , b cell lymphoma extralarge ( bcl - xl ) , bcl-2 , bag-1 , apoptotic protease - activating factor 1 ( apaf-1 ) , p53 , l - myc , n - myc , c - myc , and death - associated protein 5 ( dap5 ) [ 1 , 8 , 3234 ] .
translation initiation switches from cap - dependent to ires - dependent mode during stress conditions such as hypoxia , vascular lesions , serum deprivation , -irradiation , apoptosis , growth arrest , and angiogenesis .
this shift is attributed to eif2 phosphorylation , eif4e - bp dephosphorylation , and eif4 g cleavage , any of which can inhibit canonical translation initiation [ 33 , 36 ] . although the cellular ires elements are activated under stress conditions , these ireses differ in their requirement for eifs . for example , the l - myc ires requires the eif4f complex and interaction of both poly(a ) tail binding protein ( pabp ) and eif3 with eif4 g for translation . on the other hand , partial silencing assays ( using the knockdown plasmid , psilencer31 ( si31 ) , and hippuristanol ( eif4a inhibitor ) treatment ) have demonstrated that c- and n - myc ireses can function only in presence of the c - terminal domain of eif4gi , eif4a , and eif3 ; this ires does not require full - length eif4gi or pabp .
er stress caused by the accumulation of unfolded proteins triggers the unfolded protein response ( upr ) which in turn modulates both transcription and translation of key regulators ( e.g. , atf4 ) of the cellular stress response .
a recent study has shown that an alternatively spliced variant of the previously discussed human atf4 transcript ( variant v1 ) is translated by an ires - mediated mechanism .
this variant also has a unique 5 leader sequence and is found to be less abundant than other variants in numerous human tissues .
interestingly , these variants were shown to be translated by different mechanisms by using luciferase reporters and modifying 5 leader sequences with stem - loop insertions .
chan et al . found high gc content in the long and highly structured 5 leader region of v1 in comparison to v2 , suggesting a possibility of ires - mediated translation .
additional tests involving inhibition of critical canonical eifs ( e.g. , eif4g1 ) and eif2 phosphorylation indeed confirmed the ires - mediated translation of v1 during upr . during amino acid starvation , viral infection , or endoplasmic reticulum stress , several kinases are activated that induce eif2 phosphorylation ( figure 2 ) .
this phosphorylation , in turn , decreases eif2-gtp - trnai ternary complex activity , resulting in suppression of cap - dependent translation .
however , several viral and cellular mrnas are insensitive to this mode of translation inhibition .
this suggests that mrnas can employ alternative factors or mechanisms to recruit the eif2-gtp - trnai ternary complex .
examples of cellular ireses unaffected by eif2 phosphorylation are xiap , c - myc , cationic amino acid transporter-1 ( cat-1 ) , and n - myc .
the exact mechanism by which the consequences of eif2 phosphorylation are avoided by the hcv ires is not known .
however , some ireses ( csfv and hcv ) employ eif5b , an orthologue of prokaryotic if2 , during conditions of increased eif2 phosphorylation .
the eif5b - eif3 mediated mechanism involves eif3 stimulating trnai binding to the 40s subunit ( in the ires/40s complex ) in an eif5b - dependent manner , which allows for the formation of the 48s initiation complex and , subsequently , the translation - competent 80s ribosome .
like if2 , eif5b binds and delivers initiator trna during translation initiation on these ireses [ 8 , 30 ] .
we have recently found that the xiap ires uses a similar mechanism during eif2 phosphorylation conditions .
this finding suggests that eif5b - dependent activation of ires - mediated xiap mrna translation would act as a critical prosurvival switch in cells under stress .
moreover , a recent publication from our lab suggests that the xiap ires does not require eif4 g , eif4e , and eif4a for initiation complex formation .
the inhibition of eif4a activity by hippuristanol or pateamine a treatment did not affect the ability of xiap ires to form initiation complexes , suggesting eif4a is not required for ires - mediated translation of xiap
. additionally , eif3 and pabp bind synergistically with in vitro - transcribed , uncapped , and poly(a)-tailed xiap ires rna and recruit ribosomes near the start codon   ( figure 3 ) .
although cellular ireses do not share structural similarities in studies conducted thus far , the secondary structure is indeed important as we have found it to be required for efficient recruitment of eif3 and the ribosome .
a recent study suggests that eif3 subunit d plays a key role in alternative mechanisms of translation initiation by a noncanonical mechanism of cap recognition .
specifically , the study classifies this method as a novel , eif4e - independent mechanism of translation initiation .
found that the eif3d subunit alone provided rnase protection to the mrna of the early response transcription factor c - jun by directly interacting with a mature , methylated 5 cap structure .
the specificity of eif3d binding to only a subset of capped mrnas is suggested to occur through an  rna gate 
domain that regulates this novel cap - binding function of eif3d . in the larger context of eif3 specialized translation pathways ,
eif3d seems to play a critical role as a cap - binding protein that helps cells regulate protein synthesis even during times when the eif3f complex ( required for canonical translation ) is inactivated or inhibited .
mitochondria play an important role in the intrinsic pathway of apoptosis , which is regulated by the bcl-2 family of proteins .
for example , bax and bac proteins activate mitochondria - dependent apoptosis , whereas bcl - xl and bcl-2 proteins inhibit apoptosis . during apoptosis ,
the phosphorylation levels of eif4e and 4e - bp1 are influenced by sgp-2 which , in turn , affects the bcl - xl ires - dependent translation by regulating the stability of eif4f complex .
overexpression of some protooncogenic proteins ( bcl - xl and c - myc ) , as implicated in numerous cancer types , could be a result of elevated levels of eif4f .
dap5 ( p97 ) , eif4g1 , and eif4g2 comprise the eif4 g family and function in the formation of the translation initiation complex .
dap5 domains share homology with the central and c - terminal region of eif4 g that interacts with eif4a , eif3 , mnk1 , and eif2 but not eif4e .
the interaction of dap5 with eif2 and eif4ai facilitates the ires - mediated translation of various cellular mrnas , including those encoding pro- and antiapoptotic proteins , such as c - myc , bcl2 , apaf1 , xiap c - iap1 , and dap5 itself [ 42 , 43 ] .
moreover , the cleavage of dap5 by caspase generates a smaller fragment , p86 , which also facilitates the ires - mediated translation of various cellular mrnas .
the cleavage of eif4gi during apoptosis yields three major cleavage products : n - fag ( n - terminal fragment of apoptotic cleavage of eif4 g ) , m - fag ( middle - fag ) , and c - fag ( c - terminal fag ) ; the cleavage causes disassembly of the eif4f complex as the fragments no longer retain the ability to bind eif4e , eif4a , and eif3 .
surprisingly , eif4 g m - fag alone supports the ires - mediated translation of mrnas , including p97/dap5 , xiap , and c - iap1 . during angiogenesis ,
ires expression of the potent angiogenic factor , vascular endothelial growth factor ( vegf ) , is regulated by its interaction with eif4e and eif4 g .
the direct interaction of eif4g1 with vegf ires enhances its translation during breast cancer progression .
knockdown of eif4gi using rna interference in a chick chorioallantoic membrane system resulted in decreased vegf protein levels and a reduction in angiogenesis .
this reduction was specific to ires - mediated translation , as silencing of eif4gi had no drastic effect on global rates of protein synthesis in normoxic conditions .
the depletion of eif4gi attenuated global mrna translation rates , suggesting a bigger requirement of eif4gi during hypoxic conditions .
the c - terminus of eif4gi stimulates cap - independent translation initiation at the 5 utr of c - myc and vegf . under hypoxic conditions , vegf , fgf-2 , bcl-2 , and hypoxia - inducible factor 1 ( hif1 )
are all overexpressed due to upregulation of ires - mediated translation [ 46 , 47 ] .
this selective translation is mediated by the overexpression of eif4e - bps and eif4 g and is particularly advantageous for cancer cells as vegf , fgf-2 , hif1 , and bcl-2 are all significant factors in promoting tumor growth and survival [ 45 , 48 ] .
the itafs are protein factors that interact specifically with the ires based on the sequence and structure of the mrna and modulate ires - mediated translation .
itafs can act as a molecular chaperone or modify the structure of rna to facilitate direct recruitment of the eukaryotic initiation factors and the ribosome to form 48s initiation complex .
some of the well - characterized itafs are human antigen r ( hur ) , la autoantigen , programed cell death 4 ( pdcd4 ) , polypyrimidine tract binding ( ptb ) protein , heterogeneous nuclear ribonucleoprotein a1 ( hnrnpa1 ) , hnrnac1/c2 upstream of nras ( unr ) , nuclear factor 45 ( nf45 ) , insulin - like growth factor 2-binding protein 1 ( igf2bp1 ) , y - box protein 1 ( yb1 ) , and poly(c ) binding protein ( pcbp ) [ 20 , 49 , 50 ] .
the levels , activity , and localization of these itafs are regulated by various signaling pathways , which in turn regulate the ires - mediated translation .
there are two classes of itafs : one that facilitates while the other represses ires - dependent translation .
for example , nf45 promotes while hnrnpa1 inhibits the ires - mediated translation of xiap [ 51 , 52 ] .
hur directly interacts with the xiap and bcl - xl ires and modulates their translation .
hur interacts with the xiap ires through the rna recognition motifs , rrm1 and rrm2 , to stimulate protein synthesis . on the contrary ,
interaction with the bcl - xl ires decreases translation and enhances the membrane integrity of mitochondrial promoting cell survival [ 53 , 54 ] .
la autoantigen as part of rnp complex enhances xiap ires - mediated translation as demonstrated by in vitro and in vivo assays .
pdcd4 had been considered the general translation inhibitor which sequesters eif4a and inhibits its helicase activity .
however , it is shown that the direct interaction of pdcd4 with the xiap ires is required to inhibit the ires - mediated translation of xiap .
pdcd4 in the absence of activated s6k2 directly binds to the xiap and bcl - xl ires and blocks 48s preinitiation complex formation .
the activation of s6k2 by the fibroblast growth factor 2 ( fgf2 ) results in phosphorylation of pdcd4 and subsequent removal by the proteasomal pathway , which in turn upregulates ires - mediated translation of xiap and bcl - xl .
ptb protein forms ribonucleoprotein complexes with psf , hnrnpl , and other proteins to regulate the gene expression , stability , and localization of mrna during apoptosis .
when trail activates apoptosis , ptb protein forms complex with psf , ybx1 , nono / p54nrb , hnrnpa2/b1 , hnrnpc1/c2 , and ddx3x and regulate the ires activity of mrnas involved in apoptosis .
the interactions could occur in the nucleus prior to splicing and in the cytoplasm aiding recruitment of the ribosome .
cytoplasmic translocation of ptb protein leads to increase in traf1 , p53 , and p47 mrna expression
. specific mutation of cytosine to thymidine observed in domain 2 of c - myc ires derived from multiple myeloma cell lines demonstrated enhanced interactions with ptb protein and y - box binding protein 1 .
this increase in protein binding is correlated with an elevated ires activity of c - myc mrna in multiple myeloma . in human melanoma ,
single nucleotide polymorphism at the ptb protein binding site present on p53 5 utr decreased ires activity , emphasizing the importance of ptb protein as an itaf [ 5862 ] .
overexpression of hnrnpc activates xiap ires activity with no effect on cap - dependent translation .
also , hnrnpc1/c2 interaction with the p53 ires is critical for mrna expression and thus affects transcription of proapoptotic mrna [ 64 , 65 ] .
moreover , hnrnpc interacts with the heptameric uridine sequence in the c - myc ires and enhances c - myc expression only during g2/m phase of cell cycle .
hnrnpa1 , which is part of hnrnp family of proteins , regulates expression of bcl - xl and xiap mrna .
phosphorylation at the rrm1 domain of hnrnpa1 by s6k2 selectively promotes association of bcl - xl or xiap with hnrnpa1 and exports the rna - protein complex into the cytoplasm .
the suppression is relieved by sumoylation of the rrm2 domain of hnrnpa1 , resulting in the decreased affinity of hnrnpa1 to protein and translocation into the nucleus .
unr acts as either a positive or a negative regulator of apoptosis depending on the cell type . deleting unr in
the embryonic stem cells resulted in decreased expression of p53 , gadd45 g , and caspase-3 , impairing apoptosis signaling pathway in response to gamma irradiation , whereas when unr expression was partially silenced , induction of apoptosis was observed .
unr interaction with the apaf1 ires opens the stem - loop structure of ires enabling ptb protein binding .
ptb proten and unr act synergistically as chaperones to create a single - stranded region for the recruitment of 48s complex .
besides the mentioned positive regulator , itafs such as hnrnpc1 and nucleolin can negatively regulate p53 expression . in conclusion ,
the levels and localization of itafs in the cell are critical for regulation of gene expression . during stress conditions
, proteins are modified mostly by phosphorylation reactions that trigger nuclear to cytoplasmic localization and modulate ires - mediated translation .
eif2 is a heterotrimeric protein , composed of  ,  , and  subunits .
eif2 is a required element of the ternary complex that delivers met - trnai to the 40s ribosomal p - site in translation initiation .
eif2 exists in a gdp- or gtp - bound configuration , which has a critical role in translational control during stress . during translation initiation ,
eif2 bound gtp hydrolysis is induced by eif5 , releasing eif2-gdp in the inactive form .
eif2b catalyzes the exchange of gdp to gtp which is necessary for reformation of the ternary complex . under stress conditions such as an excess of unfolded proteins in the endoplasmic reticulum , or amino acid starvation ,  subunit of eif2
is phosphorylated at serine 51 by one of four members of the eif2 kinase family [ 72 , 73 ] .
inhibition of active eif2-gtp regeneration results in decreased ternary complex thus inhibiting overall translation ( figure 2 ) .
furthermore , eif2 phosphorylation has a role in suppression of tumorigenesis , demonstrated by the ability of protein kinase rna - activated ( pkr ) to promote malignant transformation of nih 3t3 cells [ 70 , 74 ] .
the transformation mechanism inhibits eif2 phosphorylation by decreasing the activity of upstream target pkr , potentially through the formation of inactive pkr heterodimers [ 70 , 74 ] . decreased levels of phosphorylated eif2 were found in osteosarcoma tumors , while increased pkr levels and associated phosphorylated eif2 levels were correlated with tumor cell differentiation [ 74 , 75 ] .
expression levels of phosphorylated eif2 serve as a marker for determining the prognosis of non - small lung cancer ( nsclc ) patients .
er stress is closely associated with solid tumor progression , having implications in cancer proliferation and apoptosis .
downstream mediators and targets of er stress include activating transcription factor 6 , inositol - requiring enzyme 1 ( ire1 ) , and protein kinase rna - like er kinase ( perk ) which is an upstream activator of eif2 phosphorylation [ 77 , 78 ] .
er stress is induced in chronic myeloid leukemia ( cml ) , which activates perk and eif2 phosphorylation .
phosphorylation of eif2 supports cml progression with a prosurvival role , shown in the inhibition of perk , which prevents eif2 phosphorylation .
this , in turn , allows sensitization of cml cells to imatinib and decreases their proliferative abilities .
insulin - like growth factor binding protein-5 ( igfbp-5 ) and protein family member igfbp-3 upregulate expression of growth arrest and dna damage - inducible protein 34 ( gadd34 ) , which assembles an eif2 dephosphorylation complex , enabling regeneration of active eif2 , critical to ternary complex formation .
gadd34 's dephosphorylation activity is required to recommence protein synthesis after periods of global translation inhibition .
however , translation of uorf - containing , prooncogenic protein atf4 is upregulated by eif2 phosphorylation , promoting osteogenesis and osteoblast differentiation . in ovarian cancer cells , autophagy and activation of the perk / eif2 pathway attenuate and
in contrast , protein obg - like atpase 1 ( ola1 ) inhibits protein synthesis and promotes integrated stress response without utilizing eif2 phosphorylation .
ola1 is a gtpase that binds to eif2 , preventing ternary complex formation . in vivo , ola1-knockdown inhibits the mainly prosurvival integrated stress response ( isr ) pathway in cancer cells , which is responsible for restoring cellular homeostasis in response to physiological changes as well as intrinsic stresses such as er stress [ 82 , 83 ] .
inhibition of the isr pathway results in attenuated ccaat - enhancer - binding protein homologous protein ( chop ) levels and promotion of tumor growth and metastasis through cell proliferation .
chop expression is triggered by unfolded protein accumulation in the endoplasmic reticulum ( er ) .
chop induces apoptosis during prolonged stress or stress response malfunction , through the formation of heterodimers with other c / ebp family members .
besides phosphorylation of eif2 , increased levels of eif2 expression are detected in tumor samples in bronchioloalveolar carcinomas of the lung , hodgkin 's lymphoma , gastrointestinal carcinomas , malignant melanoma , and melanocytic neoplasms [ 70 , 85 ] .
eif2 levels were highly expressed , along with eif4e in the germinal centers of reactive follicles when examined in several types of non - hodgkin 's lymphoma .
comparably , eif2 and eif4e were present in the nuclei and cytoplasm of brain tumor cells , with a higher concentration of eif2 in the nuclei of gastrointestinal cancer tumor cells [ 87 , 88 ] .
expressions of eif2 may relate to abnormal protein synthesis , furthering its role in tumorigenesis . as the main effector of both global translation and translation of specific subsets of mrnas , eif2 continually shows potential for cancer therapeutics and treatments .
eukaryotic initiation factor 3 ( eif3 ) is a 13-subunit complex of 800 kilodaltons , required for translation initiation through interactions with the 40s ribosomal subunit , mrna , and other eifs necessary for the formation of competent translation initiation complexes [ 19 , 114 ] .
eif3 , along with eif1a and plausibly eif5 , associates with the ternary complex and the 40s ribosomal subunit to form the 43s preinitiation complex .
eif3 enhances the stability of 43s preinitiation complex through eif3-eif4 g interaction [ 1 , 19 , 89 ] .
primarily , eif3j regulates eif3 interaction with the mrna - binding cleft on the 40s subunit by inhibiting mrna entry and confirming met - trnai is present in the p - site [ 115117 ] .
interestingly , beyond the protein synthesis related functions of the eif3 complex , dysregulation of eif3 subunits has been implicated in several types of cancers [ 70 , 90 ] .
variations in the levels and activity of eif3 subunits are a result of upstream signaling molecules such as protein kinases , involving phosphorylation of eif3 subunits .
for example , the mammalian target of rapamycin complex 1 ( mtorc1 ) , a key protein kinase involved in the regulation of protein synthesis , facilitates interaction between paip1 and eif3 .
the eif3 g subunit directly interacts with paip1 in an rna - independent manner , which enhances paip1-mediated translation stimulation in vivo [ 111 , 118 ] .
stimulation of mtorc1 ( e.g. , by amino acids ) phosphorylates s6k , which ultimately stimulates interaction between eif3 and paip1 .
this paip1-eif3 interaction is also proposed to stabilize the conformation of circularized mrna by stimulating the eif4g - pabp interaction [ 111 , 118 ] .
moreover , mtor inhibition by rapamycin and pp242 ( inhibitor of mtorc1 and mtorc2 ) significantly decreased s6k1 phosphorylation and subsequently diminished this paip1-eif3 interaction [ 111 , 118 ] .
evidence for the direct association of mtor and s6k1 with eif3 points to probable effects on translation .
specifically , studies have shown that the eif3 complex , as found on the translation preinitiation complex ( eif3-pic ) , functions as a scaffolding structure which is associated with mtor by mitogen / hormone stimulation , whereas s6k1 dissociates from this complex upon similar stimulation .
immunoprecipitation experiments in the immortalized human embryonic kidney cells ( hek293e / t ) have shown that s6k1 associates with eif3b , eif3c , eif3e , and eif3f and mtor coimmunoprecipitates with eif3c [ 94 , 111 ] .
t cells are derived from the original hek293e cell lineage and have been modified to allow for transient transfection of vectors containing the sv40 origin of replication .
interestingly , in nutrient - deprived conditions , the s6k1-eif3 association is observed , whereas the addition of amino acids diminishes this direct interaction .
notably , with insulin treatment , s6k1 dissociates from the eif3-pic , whereas mtor is associated with this complex .
this insulin stimulation results in an increase in cap - dependent translation , suggesting the mtor - eif3-pic association and the subsequent series of phosphorylation events are critical for efficient protein synthesis .
in addition to the critical function of the eif3 complex in translation initiation , many eif3 subunits have been shown to be involved in a diverse set of cellular processes including apoptosis , oncogenesis , and cellular growth and proliferation [ 89 , 100 , 109 , 114 , 120122 ] .
upregulation of eif3a , eif3b , eif3c , eif3d , eif3e , eif3h , and eif3i along with reduced levels of eif3e and eif3f has been observed in several cancers ( table 1 ) .
most notably , increased levels of the largest eif3 subunit , eif3a , have been found in breast , cervix , esophagus , lung , and stomach cancers [ 89 , 90 ] .
the mechanism by which upregulation of eif3a promotes the malignant phenotype in lung cancer ( h1299 ) and breast cancer ( mcf7 ) cells involves enhancing dna synthesis for maintaining cell proliferation . during the s phase of the cell - cycle , eif3a upregulates the translational expression of ribonucleotide reductase m2 , which in turn maintains high levels of dna synthesis [ 89 , 91 ] .
when eif3a is downregulated by using antisense eif3a cdna , ribonucleotide reductase m2 levels ( and , subsequently , dna synthesis ) are significantly reduced . furthermore ,
independently downregulating ribonucleotide reductase m2 expression levels has shown to reduce the extent of malignancy in human cancer cells .
these findings delineate the mechanism of how high expression levels of eif3a maintain cell proliferation in cancer cells . additionally , eif3a is also known to enhance phagocytosis during apoptosis by facilitating the association between apoptotic cells and macrophages .
overexpression of eif3b has been observed in breast , bladder , and prostate cancers ; however , the specific mechanism through which upregulated eif3b promotes the cancer state is still unclear .
increased eif3c transcript levels have been found in human testicular seminomas   as well as increased eif3c gene expression in colon cancer cells .
interestingly , eif3c also directly binds to the neurofibromatosis 2 ( nf2 ) tumor suppressor protein , schwannomin , in sts26 t schwannoma cells .
schwannomin is thought to employ its tumor suppressive functions by binding eif3c and inhibiting eif3c - mediated cell proliferation , possibly due to the role of eif3c during protein translation initiation .
additionally , in meningiomas , which have significantly reduced levels of schwannomin , eif3c is upregulated , suggesting a role of eif3c in tumor growth and proliferation . furthermore , overexpression of eif3c or eif3h resulted in the enhanced translation of cell proliferation mrnas encoding growth - regulating cyclin d1 , c - myc , fibroblast growth factor 2 , and ornithine decarboxylase ( odc ) .
this enhancement of translation rates for these proteins and subsequent production of malignant phenotypes may not be a direct consequence of the overexpression of a single eif3 subunit since enhanced levels of other eif3 subunits ( a , b , c , f , h , and j ) were also noted .
thus , the overexpression of eif3a , eif3b , or eif3c subunits stimulates the expression of other eif3 subunits that further supports the translational components necessary for faster cancer cell growth . like other eif3 subunits
, eif3d is also involved in protein synthesis and has been shown to be upregulated in gastric cancer and mesothelioma [ 70 , 104 ] .
studies using lentivirus - mediated rna interference to knockdown eif3d in the colon ( hct116 ) and non - small cell lung cancer cells ( nsclc  a549 and 95d ) showed significantly reduced cell proliferation ( induced apoptosis ) and inhibited colony formation due to induced cell - cycle arrest in the g2/m phases .
notably , in hct116 cells , eif3d knockdown resulted in phosphorylation of ampk , bad , pras40 , sapk ( stress - activated protein kinase)/jnk , gsk3 , and parp [ poly(adp - ribose ) polymerase ] cleavage .
phosphorylation of bad , a proapoptotic protein , induces apoptosis while phosphorylation of gsk3 promotes reovirus - induced apoptosis .
furthermore , parp cleavage is typically used as a signal for apoptosis induction . eif3d knockdown in nsclc cells resulted in decreased phosphorylation and thus inhibition of akt , hsp27 , and sapk / jnk ( involved in cellular growth and cancer progression pathways ) .
this data supports the crucial role of eif3d in cell proliferation and cancer growth [ 104 , 105 ] . in breast cancer cells ,
reduction of eif3e expression by rnai induces emt ( epithelial - to - mesenchymal transition ) , suggesting a role of eif3e in breast cancer metastasis .
this study and others [ 108 , 124 ] suggest that eif3e normally functions as a tumor suppressor since the reduction of its expression results in enhanced mrna stability and expression of the transcription factors and emt regulators , snail1 and zeb2 .
this suggests that the loss of eif3e directly results in cancer progression and metastasis as emt is induced in breast cancer cells .
conversely , another report has shown that eif3e functions as an oncogene . in this study ,
the knockdown of eif3e using rnai in u2os and mda - mb-231 resulted in a reduction in protein levels of bcl - xl ( antiapoptotic protein ) and urokinase - type plasminogen activator ( plau ) but an increase in mad2l1 ( mitotic checkpoint component ) .
overabundant bcl - xl protein levels are associated with chemoresistance in cancers while plau functions in promoting metastasis in tumors .
interestingly , bcl - xl mrna associates directly with the eif3 complex in an eif3e - dependent manner as determined by rna ip . following eif3e knockdown , the specific changes in protein levels of the mentioned eif3e targets , without any changes in global protein synthesis , suggest that eif3e specifically regulates translation of mrnas involved in tumorigenesis .
furthermore , eif3e gene silencing using sirna in glioblastoma results in cell - cycle arrest in the g1 phase , decreases cell proliferation , and induces both caspase - dependent and caspase - independent apoptosis .
furthermore , a recent study suggests the role of eif3d - eif3e module within the eif3 complex that regulates the translation of specific mrnas involved in maintaining metabolic pathways that are likely disrupted in cancer cells .
critical components of the mitochondrial electron transport chain were downregulated in both yeast and mammalian cells ( nontumorigenic : mcf-10a and nontumorigenic : mcf7 ) that were eif3e - depleted using sirna , whereas glucose metabolism and amino acid biosynthesis processes were upregulated .
their findings suggest that depletion of eif3e triggers a metabolic switch that increases dependence on glycolysis , as respiratory deficiencies alongside increased sensitivity to oxidative stress are also observed when eif3d is depleted in addition to eif3e knockdown .
essentially , this data suggests that the novel function of eif3d - eif3e in maintaining mitochondrial respiration components and serving to adjust metabolic pathways may help us better understand how the cancer - promoting properties of the eif3 complex emerge .
unlike eif3e , eif3f is consistently shown to function as a tumor suppressor in pancreatic cancer .
endogenous levels of both eif3f mrna and protein levels are reduced in pancreatic cancer cells , whereas eif3f - overexpressing nih3t3 cells have shown reduced cell proliferation and induced apoptosis .
likewise , eif3f knockdown in normal human pancreatic epithelial cells has shown an increase in cell proliferation and increased resistance to apoptosis . by utilizing a bicistronic luciferase report system ,
it was shown that eif3f normally inhibits both cap - dependent and cap - independent ( i.e. , ires ) mechanisms of translation initiation .
furthermore , one mechanism of translation inhibition likely involves eif3f - mediated rrna degradation by a direct eif3f - hnrnp k ( rna - binding protein ) interaction . specifically , when eif3f is present , it binds to hnrnp k , preventing it from binding to rrna , which subsequently is degraded , and the translation is reduced . in cancer cells , the loss of eif3f results in an increased binding of hnrnp k to rrna , reducing rrna degradation , and possibly favoring oncogenesis through increased translation .
these findings suggest that eif3f functions as a negative regulator of cell growth due to the naturally reduced levels of eif3f contributing to cancer development and overexpression resulting in apoptosis . despite the vast quantity of correlative evidences between eif3 subunit expression levels and observed cancer phenotypes , it has been difficult to establish a clear mechanism of how cancer progression is directly affected by eif3 expression .
understanding the interactions of the eif3 subunits with antiapoptotic proteins , cellular growth and proliferation proteins , and oncogenic proteins may provide a better understanding of how eif3 subunits contribute to supporting or inhibiting the oncogenic state .
a functional mechanism will most likely involve aspects of translation initiation , preinitiation complex formation , and other processes , such as the recruitment of the preinitiation complex to the cellular mrna and ribosome scanning , all of which are mediated to some degree by a subset of eif3 subunits .
additionally , studies depicting eif3 as both an activator ( for the protooncogene c - jun ) and repressor ( for the negative regulator of cell proliferation btg1 ) of cap - dependent translation , mediated by binding to specific rna structural elements , illustrate the diversity of functions carried out by the eif3 complex through interactions with proteins and nucleic acids .
future studies will need to focus on determining the signaling pathways that are involved in regulation of eif3 and the consequences of eif3-directed therapeutics for human cancers .
this information will assist in the development of novel therapeutics that target eif3 subunits in order to treat cancers and possibly other human diseases . eif4e
the interaction with eif4 g is inhibited by binding of eif4e - binding protein ( 4e - bp ) to eif4e .
eif4 g and 4e - bp are known to compete for a common binding site on eif4e .
the eif4e and 4e - bp interaction is highly regulated via a phosphorylation reaction and acts as a primary target for hindering translation initiation during stress conditions .
activation of the mtor pathway phosphorylates 4e - bp and enhances cap - binding efficiency of eif4e .
in contrast , inhibition of the mtor pathway by amino acid starvation dephosphorylates 4e - bp , resulting in increased association between 4e - bp and eif4e , thus repressing cap - dependent translation .
overexpression of 4e - bp leads to a decrease in the mrna and protein levels of cyclin d1 and an increase in p27 , a cell - cycle regulatory protein that promotes cell - cycle arrest in breast cancer cell lines .
it localizes to the nuclear bodies by interacting with the eif4e - transporter protein ( 4e - t ) . in the nuclear bodies ,
it remains associated with promyelocytic leukemia proteins that share a common binding site with 4e - t and 4e - bp .
thus , an increase in 4e - bp may affect the mrna transport function of eif4e from the nucleus to the cytoplasm , resulting in differential expression of cyclin d1 .
also , dysregulation of 4e - bp phosphorylation is correlated with poor prognosis in lung cancer , breast cancer , melanoma , cervical carcinoma , and astrocytoma [ 131 , 132 ] .
eif4e is phosphorylated at ser209 by p38 mitogen - activated protein kinase ( mapk ) .
p38 mapk phosphorylates mnk , a serine / threonine kinase , and enables mnk interaction with eif4 g and phosphorylation of eif4e .
induction with transforming growth factor  ( tgf ) increases eif4e phosphorylation alongside mesenchymal markers such as n - cadherin , fibronectin , and vimentin , as a result of the noncanonical signaling pathway . as a downstream target of p38 ,
this eventually leads to the upregulation of snail and matrix metalloproteinase 3 , promoting cell invasion and metastasis .
phosphorylation of eif4e is not an absolute requirement for translation but is observed to increase the rate of translation initiation .
genome - wide studies of translating mrna have indicated that eif4e phosphorylation is necessary for synthesizing proteins essential for tumorigenesis , and the levels of eif4e are critical for antiapoptotic protein expression [ 135137 ] . in nasopharyngeal carcinoma ,
the latent membrane protein 1 enhances transcription of many oncogenes such as vegf , c - myc , and matrix metalloproteinases ( mmp ) .
eif4e promoter activity is enhanced by c - myc and , as a feedback mechanism , eif4e increases translation of these oncogenes .
further , overexpression of eif4e leads to increased expression of a subset of proteins , influencing angiogenesis and tumor progression ( vegf and fibroblast growth factor-2 ( fgf-2 ) ) , growth stimulation ( platelet - derived growth factor ) , prosurvival ( bcl-2 and bcl - xl ) , cell - cycle progression ( c - myc , cyclin d1 , and ornithine decarboxylase ) , epithelial - to - mesenchymal transition ( snail and mmp ) , and invasion ( integrin 1 ) [ 134 , 137 , 139142 ] . besides the role of eif4e as a cap - binding protein ,
it is reported to stimulate the helicase activity of eif4a and aid translation of mrnas comprising long , structured 5 utrs . a subset of transcripts harbors a 12-nucleotide motif ( cgg)4 called the g - quadruplex in their 5 utrs . some examples of mrnas including such structures are vegf , bcl-2 , and nras .
these mrnas are sensitive to silvestrol , a drug that inhibits the helicase activity of eif4a , thus indicating the role of eif4a , discrete from mtor - dependent translation regulation [ 144146 ] .
ribosome footprinting and polysome profiling experiments have detected more than 250 genes , including myc , mdm2 , cdk6 , and afr6 that are affected by silvestrol treatment making it a promising drug to treat cancer [ 146 , 147 ] . in the case of breast cancer
mucin 1 forms a complex with egfr and activates the pi3  k - akt - mtor pathway .
akt and p70 s6k along with activation of mek - erk signaling promote ubiquitination and degradation of pdcd4 .
this favors translation , as the ma-3 protein binding domain of pdcd4 is known to interact with eif4a n - terminal domain and inhibit its helicase activity .
activation of the helicase activity stimulates the autoinductive pathway and increases levels of oncogenic proteins [ 56 , 148 , 150 ] .
further , pdcd4 inhibits eif4a - eif4 g interaction by interacting with eif4a as well as eif4 g .
colocalization of eif4a and pdcd4 in the cytoplasm inhibits eif4a helicase activity [ 151 , 152 ] .
eif4a also interacts with the paip1 domain , containing sequence similarity to eif4 g , and promotes circularization of the mrna , affecting protein expression involved in apoptosis .
as mentioned earlier , eif4 g interacts with eif4e , eif4a , and eif3 to form the preinitiation complex .
eif4 g consists of two binding sites for eif4a and a single binding site for eif4e , eif3 , pabp , mnk1 , and rna , which interact independently of each other .
consequently , it increases the cap - binding activity of eif4f and joining of the 60s ribosomal subunit to the 40s subunit [ 155157 ] .
interaction of eif4 g with eif4e enhances eif4e - mediated cap recognition and the cap - binding activity of eif4f complex .
therefore , hoxa9 is displaced from eif4e in the cytoplasm to initiate translation . during translation ,
eif4 g and eif3 interactions bring the ternary complex onto the mrna and stabilize the 43s preinitiation complex .
these interactions are mediated by the 3c , 3d , and 3e subunits of eif3 complex .
activation of mtor by insulin , amino acids , or growth factors influences the direct interaction between mtor and eif3 and increases eif4g - eif3 interaction .
eif5 is a 49  kda protein in mammals and 46  kda in saccharomyces cerevisiae [ 158 , 159 ] . eif5 interacts with the 40s initiation complex to mediate hydrolysis of eif2-bound gtp .
this step is critical to initiation complex formation , as the release of multiple eifs including eif3 , eif4e , and eif2-gdp is necessary for the recruitment of 60s ribosomal subunit . in higher eukaryotes including humans ,
eif5 has a 9-residue c - terminal tail that can bind to the eif5b - ctd subdomain .
this is opposed to the binding of the eif1a c - terminus to eif5b - ctd , which is a relatively weak interaction in both humans and yeast .
the interaction between eif1a and eif5b coordinates recruitment and release of one another in s. cerevisiae .
however , in humans , eif1a / eif5b interaction facilitates subunit joining , and recruitment is coordinated separately through eif5/eif5b interaction .
eif5 is a downstream target of casein kinase 2 ( ck2 ) , which phosphorylates eif5 at the major sites : ser389 and ser390 .
ck2 has a significant role in cell proliferation , and both ck2 and eif5 have critical roles in cell - cycle control and progression .
the ck2 enzymatic activity also increases and phosphorylates eif5 , 3 hours after serum stimulation of cell - cycle arrested ( g0 ) human embryonic kidney hek-293 cells and normal human fetal lung fibroblasts tig-7 cells , suggesting a role for ck2 and eif5 in promoting cell proliferation . in tig-7 cells ,
eif5 was associated with ck2 . when ck2 levels were highest and when eif5 mutants were unable to be phosphorylated by ck2 , there was a decrease in growth rate , mature translation initiation complex formation , and expression of cell - cycle - regulated proteins .
nuclear ck2 ( catalytic subunit ) localization is a sign of poor prognosis in prostate cancer and gastric carcinoma [ 163165 ] .
ck2s phosphorylation targets include deleted in breast cancer 1 ( dbc1 ) , eif5 , and endothelin - converting enzyme-1c ( ece-1c ) , which promote cancer cell invasion and progression [ 163165 ] .
depletion of eif5 in s. cerevisiae resulted in the inhibition of cell growth and a decrease in the rate of in vivo protein synthesis . in yeast
, eif5 is able to mimic the effect of eif2 phosphorylation , acting as a translational inhibitor and promoting translation of prooncogenic protein gcn4 ( yeast equivalent of atf4 ) [ 166 , 167 ] .
when overexpressed in s. cerevisiae , eif5 increases the levels of an eif2/eif5 complex , which prevents eif2b interaction and subsequently prevents ternary complex formation .
human 5mp1 protein was found to interact with human eif2s  subunit , similarly to eif5 , eif2b , and kra .
furthermore , in vitro , eif2 demonstrated mutually exclusive interactions with eif5 and 5mp1 , suggesting 5mp1 as a competitive inhibitor of eif5 .
5mp promotes expression of gadd34 ( a downstream target of atf4 ) in tribolium castaneum .
further , 5mp binds eif2 to inhibit general translation and when overexpressed , it promotes atf4 expression in fibrosarcoma [ 166 , 169 ] .
atf4 is expressed in hypoxic- and nutrient - deprived tumor regions , with functions in development , promoting metabolic homeostasis , and cancer cell proliferation .
eif5 demonstrates critical roles in cell - cycle regulation and cell proliferation with specific oncogenic protein interactions .
eif5a is a 17  kda protein that is activated by posttranslational hypusination and functions to mediate cell proliferation , apoptosis , and inflammatory response [ 171 , 172 ] .
hypusination is unique to eif5a and is a posttranslational enzymatic modification that involves two sequential enzymes and the substrate spermidine .
deoxyhypusine synthase ( dhs ) catalyzes nad - dependent cleavage and transfer of an aminobutyl moiety of spermidine to the -amino group of a conserved lysine of eif5a .
the resulting intermediate residue , deoxyhypusine , is hydroxylated by deoxyhypusine hydroxylase ( dohh ) , yielding a hypusine residue , and an active eif5a .
new treatments for chronic myeloid leukemia utilize hypusination inhibitors to deactivate eif5a , creating a target in response to imatinib resistance towards the bcr - abl tyrosine kinase .
inhibition of eif5a results in an antiproliferative effect on bcr - abl positive- and negative - leukemia cell lines in vitro .
eif5a is overexpressed in murine pancreatic intraepithelial neoplasia ( panin ) and in human pancreatic ductal adenocarcinoma ( pdac ) .
pharmacological inhibitors n(1)-guanyl-1,7,-diamineoheptane ( gc7 ) and ciclopirox olamine ( cpx ) are able to inhibit dhs and dohh , respectively , which further inhibits eif5a hypusination and results in eif5a genetic knockdown .
genetic knockdown of eif5a inhibited pdac cell growth in vitro and orthotopic tumor formation in vivo , potentially through pseudopodium - enriched atypical kinase 1 ( peak1 ) , which is essential to pdac tumor growth , metastasis , and gemcitabine resistance . in melan - a ( a murine melanocyte cell line ) and tm5 ( a murine melanoma cell line derived from melan - a ) , gc7 was used to inhibit eif5a .
more pronounced dna fragmentation was observed in tm5 cells and decreased viability was observed in both cell lines .
additionally , treatment with gc7 was tested on melanoma growth in c57bl/6 mice and found to inhibit further tumor growth , although it did not induce volume reduction of established tumors .
eif5a 's isoform eif5a2 is upregulated in various cancer types including hepatocellular carcinoma , ovarian carcinoma , and colorectal carcinoma ( crc ) [ 176178 ] .
eif5a2 overexpression in cancer cells is correlated to prognosis factors of tumor metastasis and venous infiltration [ 176 , 177 ] . in ovarian carcinoma ,
overexpression of eif5a2 was detected in 7% cystadenomas , 30% borderline tumors , and 53% invasive carcinomas , as opposed to normal expression in normal ovaries . in crc ,
eif5a2 upregulates metastasis - associated protein 1 ( mta1 ) by increasing the enrichment of regulator gene c - myc on mta1s promoter .
this both increases cell motility and invasion in vitro and results in tumor metastasis , inducing epithelial - mesenchymal transition and further promoting crc aggressiveness . in hepatocellular carcinoma cells in vivo , eif5a2 suppression attenuates tumorigenic properties [ 176 , 177 ] .
eif5as unique activation by hypusination creates a desirable mechanism of regulation and knockdown within various cancer lines .
eif5b is a 175  kda protein that promotes 60s ribosome subunit joining and pre-40s subunit proofreading and can indirectly support the trna - meti association with the ribosome in translation initiation [ 179181 ] . in serum - starved thp1 cells , elevated
eif5b levels resulted in increased eif5b complexes with trna - meti , as well as increased phosphorylation of eif2 .
these eif5b complexes are formed during times of attenuated global translation , instead of promoting translation of specific stress - related mrnas .
eif5b is antagonistic of g0 and g0-like states , with eif5b overexpression promoting maturation and cell death .
eif5b depletion is associated with increased phosphor - cdc2 , which is a marker for immaturity .
this suggests a critical role for eif5b in the regulation of cell - cycle transitions .
eif5b interacts with dead - box rna helicase vasa ( vas ) , with reduction of vas - eif5b interaction in drosophila causing female sterility , reduced gurken ( grk ) protein levels , nearly complete loss of germ cell formation , and reduction of somatic posterior patterning .
eif5b interaction with vas is necessary for early drosophila development through the progression of oogenesis and pole plasm assembly , suggesting eif5b as a potential method to regulate vas and subsequently grk . in poliovirus
( pv ) and coxsackie b virus ( cbv ) infection of cultured cells , eif5b is proteolytically cleaved , suggesting that eif5b cleavage is involved in the translation inhibition within enterovirus - infected cells . infection of enteroviruses pv and cbv begins with inhibition of host cell translation , followed by ires - directed enterovirus protein synthesis , with eif5b cleavage beginning at 3 hours after infection , during the attenuation of host cell protein synthesis [ 183 , 184 ] .
eif5b , eif5 , and eif5a all contain links into hallmark stages and mechanisms of cancer proliferation , and manipulation of these proteins offers a potential regulation point of gene expression regulation .
mechanisms utilizing elements of protein translation , specifically in the rate - limiting step of initiation , offer potential methods to diagnose and treat cancer .
translational regulation is capable of efficiently altering specific protein levels in physiological stress conditions that are typical in cancer .
translation is mediated by eukaryotic initiation factors ( eifs ) , which have varying roles in regulating the rate of initiation as described in this review .
the critical role of eif2 phosphorylation has been classified in the integrated stress response , with the requirement of eif4f complex for cap - binding and efficient translation through eif4e and eif4 g .
other mechanisms include eif3 subunit interactions with s6k1 and mtor and eif5a 's necessary activation through posttranslational hypusination , important to the mediation of cell proliferation , apoptosis , and inflammatory response .
varying levels of eifs in various cancer lines and stages , along with the mechanistic background , enforce the use of eifs to regulate gene expression in cancer . during cellular stress induced by cancer , noncanonical translation utilizing uorf elements or ires elements drives translation of specific stress response and adaptation proteins .
proteins such as atf4 and gcn4 have critical roles in the integrated stress response and help determine whether cell proliferation ensues .
the eifs required for ires - dependent translation are specific to the mrna in question , with a subset of cellular ireses not requiring eif4 g and eif4a , while l - myc requires the full eif4f complex and pabp .
ires - dependent translation , which specifically favors about 10 percent of cellular mrnas under cellular stress , offers potential gene regulation targets , including various eifs and itafs with critical implications for cancer treatment .
the mechanisms underlying both canonical and noncanonical translation and the proteins responsible for the processes , are critical in treating the dysregulated gene expression in cancer .
however , these studies are often limited in terms of what one can extract from them , since the observed phenotypical changes in nearly all levels of complexity , starting from the cellular level to the whole organism level , tend to be complicated by several other protein - protein and possibly protein - nucleic acid interactions .
future studies will need to continue building on the present data by taking mechanistic approaches in elucidating the signaling pathways , transcriptomic , translatomic , and proteomic profiles of patient samples and model systems to determine the appropriate methodology to target the function of specific regulators ( e.g. , proteins like initiation factors ) to produce effective novel therapeutics that have intrinsically high specificity .
currently , there are several drugs and antisense oligonucleotides being tested against the initiation factors to increase mortality of cancer cells .
initiation factors , being a common element across various types of translation , hold great potential to be tested for rna - based therapeutics as well as chemical compound - based therapeutics to treat cancer .
 protein synthesis can be segmented into distinct phases comprising mrna translation initiation , elongation , and termination . 
 translation initiation is a highly regulated and rate - limiting step of protein synthesis that requires more than 12 eukaryotic initiation factors ( eifs ) . 
 extensive evidence shows that the transcriptome and corresponding proteome do not invariably correlate with each other in a variety of contexts . 
 in particular , translation of mrnas specific to angiogenesis , tumor development , and apoptosis is altered during physiological and pathophysiological stress conditions . in cancer cells , the expression and functions of eifs 
 are hampered , resulting in the inhibition of global translation and enhancement of translation of subsets of mrnas by alternative mechanisms . 
 a precise understanding of mechanisms involving eukaryotic initiation factors leading to differential protein expression can help us to design better strategies to diagnose and treat cancer . 
 the high spatial and temporal resolution of translation control can have an immediate effect on the microenvironment of the cell in comparison with changes in transcription . 
 the dysregulation of mrna translation mechanisms is increasingly being exploited as a target to treat cancer . in this review , we will focus on this context by describing both canonical and noncanonical roles of eifs , which alter mrna translation . 
1. Introduction
2. Role of Eukaryotic Initiation Factors in Noncanonical Translation
3. IRES
4. Role of Eukaryotic Initiation Factors in Canonical Translation
5. Conclusions