several multifunctional proteins are involved in both transcriptional and translational control ( wilkinson and shyu , 2001 ) . here
we focus on a family of such multifunctional proteins , the y - box protein family , in terms of its significance in cell proliferation and cancer . as different aspects of the y - box proteins
have already been reviewed ( matsumoto and wolffe , 1998 ; swamynathan et al , 1998 ; evdokimova and ovchinnikov , 1999 ; kohno et al , 2003 ) , we briefly appraise the structure and functions of the y - box proteins with the emphasis on recent findings .
we then summarize the role of a y - box protein yb-1 in cancer and its use in the clinical setting .
y - box proteins ( or y - box binding proteins ) are so named because they were originally identified as dna binding proteins that are capable of associating with the y - box ( inverted ccaat - box ) sequence of the major histocompatibility complex class ii gene .
y - box proteins have thus far been known to regulate positively or negatively a number of genes , such as multidrug resistance 1 , cyclin a , cyclin b1 , matrix metalloproteinase 2 and collagen alpha2(i ) ( higashi et al , 2003a ; jurchott et al , 2003 ; kohno et al , 2003 ) .
however , members of y - box protein family are also found in the cytoplasm and associated with mrnas as major components of messenger ribonucleoprotein particles .
y - box proteins regulate translation in a dose - dependent manner ; low concentrations of y - box proteins activate translation and high concentrations repress it ( evdokimova and ovchinnikov , 1999 ) .
collectively , y - box proteins have been implicated in the regulation of mrna metabolism in multiple steps in both the nucleus and the cytoplasm , including transcription , splicing , mrna stability and translation .
y - box proteins consist of three domains : the n - terminal domain , the cold shock domain ( csd ) and the c - terminal tail domain ( figure 1 ) .
the csd is a highly conserved nucleic acid binding domain that confers rna- and single - stranded and double - stranded dna binding activities to the y - box proteins .
both the short n - terminal and the c - terminal tail domains are less conserved among the y - box proteins .
the charged c - terminal tail domain of vertebrate y - box proteins , consisting of alternating clusters of acidic / aromatic and basic amino acids , is likely to account for its rna - binding activity and ability for associating with various proteins .
y - box proteins have been shown to interact with a number of cellular and viral proteins that are involved in various cellular processes ( figure 2 ) .
+ + and   indicate clusters of basic and acidic / aromatic amino acids .
numbers in parentheses indicate the references as follows : ( 1 ) shnyreva et al , 2000 , ( 2 ) kohno et al , 2003 ; swamynathan et al , 1998 and references therein , ( 3 ) higashi et al , 2003b , ( 4 ) safak et al , 1999 , ( 5 ) zou et al , 1997 , ( 6 ) kojic et al , 2004 , ( 7 ) chansky et al , 2001 , ( 8) moraes et al , 2003 , ( 9 ) funke et al , 1996 , ( 10 ) raffetseder et al , 2003 , ( 11 ) wilhelm et al , 2000 , ( 12 ) matsumoto et al , 2005 , ( 13 ) balda et al , 2003 , ( 14 ) moorthamer et al , 1999 , ( 15 ) matsumoto and wolffe , 1998 ; evdokimova and ovchinnikov , 1999 and references therein , ( 16 ) balda and matter , 2000 , ( 17 ) frankel et al , 2005 . in a variety of cell types , y - box proteins are predominantly found in the cytoplasm . however , given that y - box proteins regulate transcription , they are expected to localize to the nucleus .
y - box proteins are translocated into the nucleus by a number of conditions and mechanisms , including uv irradiation , hyperthermia , interferon - gamma treatment , adenovirus infection , interaction with p53 and a splicing factor srp30c and high levels of ectopic yb-1expression ( higashi et al , 2003a ; kohno 2003 ; raffetseder et al , 2003 and references therein ; zhang et al , 2003 ) .
both the csd and the tail domains are implicated in nuclear localization of yb-1 ; the tail domain seems to contain a non - canonical nuclear localization signal and the isolated csd also contributes to nuclear retention ( bader and vogt , 2005 ) .
y - box proteins are capable of nucleocytoplasmic shuttling , which allows them to contribute to the coupling control of transcription and translation .
y - box proteins become associated with nascent transcripts cotranscriptionally and are presumed to accompany mrna into the cytoplasm ( soop et al , 2003 ) .
interestingly , a mouse y - box protein msy2 preferentially associates with mrnas that are transcribed from genes containing y - box sequences in their promoter regions and stores those mrnas in male germ cells ( yang et al , 2005a ) .
experiments with overexpression or down - regulation of the y - box proteins in cultured cells or animals have shown that the amount of y - box proteins must be precisely controlled ( see below ) .
therefore , it is important to understand how the synthesis of y - box proteins is regulated .
recent data have shown that the synthesis of a y - box protein , yb-1 ( y - box binding protein-1 ) , is regulated both at transcriptional and post - transcriptional levels .
yb-1 mrna accumulates when cells are treated with cisplatin or uv irradiation ( ohga et al , 1996 ) .
transcription of the yb-1 gene is stimulated by p73 through an enhanced recruitment of the c - myc - max complexes to e - box sequences in the yb-1 promoter ( uramoto et al , 2002 ) .
yb-1 protein represses translation of yb-1 mrna by binding to specific elements in the 5- and 3-untranslated regions ( fukuda et al , 2004 ; skabkina et al , 2005 ) .
this self - regulation may contribute towards maintaining the concentration of yb-1 protein optimal in a cell . in human and mouse
, there are three y - box proteins , two of which are expressed in both somatic cells and germ cells ( table 1 ) . the most extensively studied y - box protein , yb-1
human contrin and mouse msy2 are germ cell - specific members of the y - box protein family .
analyses of the effects of targeting y - box genes in chicken cells and mice have been widely carried out in the last three years .
chicken b - cell lymphoma dt40 cells are widely used to study functional consequences of disrupting specific genes because of the high frequency of homologous recombination .
the yb-1 ( or yb-1b ) gene in dt40 cells has been disrupted by two independent groups of investigators ; one group reported that yb-1 cells show slow - growth phenotype and increased dna content ( swamynathan et al , 2002 ) .
the other group of researchers found that heterozygous disruption resulted in no growth defects but homozygous gene disruptants exhibited a slow and cold - sensitive growth phenotype ( matsumoto et al , 2005 ) .
one research group tried to disrupt yb-1 gene in mice but encountered difficulties in disrupting both alleles of the yb-1 gene ( shibahara et al , 2004 ) .
however , as was the case in chicken cells , another group recently reported homozygous yb-1gene disruption , showing the importance of yb-1 in late stages of embryonic development ( lu et al , 2005 ) .
they observed developmental defects of yb-1embryos after embryonic day 13.5 including craniofacial lesions , hemorrhage and respiratory failure , with yb-1 mef cells showing premature senescence and hypersensitivity to different cellular stresses .
the reason for the presence or absence of the haplo - insufficient phenotypes is currently unknown . in mice lacking msy2 ,
both male and female homozygotes are sterile , a consequence of disturbed spermatogenesis due to reduction of postmeiotic germ - cell mrnas in male and oocyte loss in female ( yang et al , 2005b ) .
overall , studies designed to reduce or deplete a y - box protein in cells or whole organisms underscore the significance of y - box proteins in appropriate cell growth , stress responses and development . y - box proteins in human and mouse the role of yb-1 in cancer progression
has attracted attention in recent years.yb-1 has been found to be upregulated during prostate cancer tumor progression ( gimenez - bonafe et al , 2004 ) . increased yb-1 expression has been correlated with dna topoisomerase ii and proliferating cell nuclear antigen expression in human lung cancer ( gu et al , 2001 ) and colorectal cancer ( shibao et al , 1999 ) and linked to markers of cellular proliferation in osteosarcoma ( oda et al , 1998 ) .
yb-1 has been identified as a cell cycle stage - specific transcription factor ( jurchott et al , 2003 ) .
nuclear accumulation of yb-1 in hela cells was demonstrated to transcriptionally activate cyclin a and b1 genes , which are crucial for cell cycle progression .
increase in cyclin a has been reported to be associated with poor clinical outcome in breast cancer ( michalides et al , 2002 ) .
in addition , yb-1 is believed to promote metastasis by enhancing the transcription of gelatinase a , a matrix metalloproteinase that facilitates cell migration ( cheng et al , 2002 ) . recently , berquin et al .
( 2005 ) has also shown that yb-1 may induce epidermal growth factor ( egf ) independence in mammary epithelial cells via activation of the egf receptor pathway , thereby contributing to breast tumor aggressiveness . in
yet another recent paper , bergmann and colleagues ( 2005 ) , using a transgenic mouse model , showed that overexpression of yb-1 may cause breast cancer through the induction of genetic instability .
on the other hand , yb-1 may have anti - oncogenic activity as it is reported to be capable of blocking oncogenic cell transformation ( bader and vogt , 2005 ) .
the phosphoinositide 3-kinase ( pi 3-kinase ) pathway is known to show gain of function in human cancers ( bader and vogt 2004 ) .
the catalytic subunits of pi 3-kinase , p110 ( of which p3k is a homolog ) and akt are oncoproteins and yb-1 is specifically known to inhibit p3k and akt - induced transformation involving protein synthesis ( bader et al . , 2003 ) .
yb-1 may interefere with the synthesis of growth - related proteins including growth factors , receptors , kinases , transcriptional regulators and cell cycle proteins associated with p3k and akt pathways ( zimmer et al , 2000 ; bader and vogt 2004 ) .
a seminal paper describing yb-1 expression in cancer tissues was first reported by royer 's group in breast cancer ( bargou et al , 1997 ) .
the pathological significance of yb-1 in a variety of cancers is shown in table 2 .
overexpression of yb-1 and pathological significance in human cancers substantial yb-1 expression was demonstrated in multidrug - resistant breast , gastric and pancreatic cell lines ( holm et al , 2004 ) .
altered drug sensitivity to cisplatin , a very potent and widely used anti - cancer agent and mitomycin c has been observed following treatment of cells with antisense yb-1 ( ohga et al , 1996 ; torigoe et al , 2005 ) .
expression of yb- 1 protein has been reported to reflect the chemosensitivity of ovarian serous adenocarcinoma ( kamura et al , 1999 ) and breast cancer ( janz et al , 2002 ; huang et al , 2005 ) .
increased nuclear localization of yb-1 has been observed in acquired cisplatin - resistant ovarian cancer ( yahata et al , 2002 ) .
yb-1 expression has also been shown to be associated with p - glycoprotein ( pgp ) expression in breast cancer cells resulting in drug resistance ( bargou et al , 1997 ; saji et al , 2003 ; huang et al , 2005 ) .
pgp , encoded by the mdr1 gene , is a member of the atp - binding cassette transporter superfamily of proteins involved in the protection of cells from xenobiotics and drugs ( kuwano et al , 2003 ) .
pgp has become an important molecular target for limiting chemoresistance as it plays a major role in the development of multidrug - resistant tumor type and is known to mediate resistance to a wide range of anticancer agents ( kuwano et al , 1999 ) .
bay and co - workers have recently demonstrated a direct interaction between yb-1 and pgp using the computer - based resonance recognition model ( huang et al , 2005 ) .
the same investigators observed the occurrence of raised recurrence rates in breast tumor patients with high yb-1 expression who underwent a chemotherapy regime which contained anthracycline ( a pgp substrate ) . besides breast cancer , yb-1 has been correlated with pgp in ovarian cancer ( huang et al , 2004 ) , prostate cancer ( gimenez - bonafe et al , 2004 ) and osteosarcoma ( oda et al , 1998 ) .
yb-1 has been shown to bind p53 ( okamoto et al , 2000 ) and interaction with p53 could be necessary for the self - defense of cells exposed to dna - damaging agents ( kuwano et al , 2003 ) . as mentioned earlier , p73 , a close relative of the p53 family , has also been observed to stimulate transcription of the yb-1 promoter by enhancing the recruitment of the cmyc - max complex to its target gene ( uramoto et al , 2002 ) .
c - myc , an oncogene with a dual function in cell proliferation and apoptosis can confer resistance to cisplatin .
p73 is known to induce apoptosis ( irwin et al , 2000 ) . and p73 overexpressing clones have been observed to be cisplatin resistant ( gong et al , 1999 ) .
hence , c - myc and p73 may form a complex necessary in yb-1 mediated drug resistance ( uramoto et al , 2002 ) .
y - box proteins consist of three domains : the n - terminal domain , the cold shock domain ( csd ) and the c - terminal tail domain ( figure 1 ) .
the csd is a highly conserved nucleic acid binding domain that confers rna- and single - stranded and double - stranded dna binding activities to the y - box proteins .
both the short n - terminal and the c - terminal tail domains are less conserved among the y - box proteins .
the charged c - terminal tail domain of vertebrate y - box proteins , consisting of alternating clusters of acidic / aromatic and basic amino acids , is likely to account for its rna - binding activity and ability for associating with various proteins .
y - box proteins have been shown to interact with a number of cellular and viral proteins that are involved in various cellular processes ( figure 2 ) .
+ + and   indicate clusters of basic and acidic / aromatic amino acids .
numbers in parentheses indicate the references as follows : ( 1 ) shnyreva et al , 2000 , ( 2 ) kohno et al , 2003 ; swamynathan et al , 1998 and references therein , ( 3 ) higashi et al , 2003b , ( 4 ) safak et al , 1999 , ( 5 ) zou et al , 1997 , ( 6 ) kojic et al , 2004 , ( 7 ) chansky et al , 2001 , ( 8) moraes et al , 2003 , ( 9 ) funke et al , 1996 , ( 10 ) raffetseder et al , 2003 , ( 11 ) wilhelm et al , 2000 , ( 12 ) matsumoto et al , 2005 , ( 13 ) balda et al , 2003 , ( 14 ) moorthamer et al , 1999 , ( 15 ) matsumoto and wolffe , 1998 ; evdokimova and ovchinnikov , 1999 and references therein , ( 16 ) balda and matter , 2000 , ( 17 ) frankel et al , 2005 . in a variety of cell types , y - box proteins are predominantly found in the cytoplasm . however , given that y - box proteins regulate transcription , they are expected to localize to the nucleus .
y - box proteins are translocated into the nucleus by a number of conditions and mechanisms , including uv irradiation , hyperthermia , interferon - gamma treatment , adenovirus infection , interaction with p53 and a splicing factor srp30c and high levels of ectopic yb-1expression ( higashi et al , 2003a ; kohno 2003 ; raffetseder et al , 2003 and references therein ; zhang et al , 2003 ) . both the csd and
the tail domains are implicated in nuclear localization of yb-1 ; the tail domain seems to contain a non - canonical nuclear localization signal and the isolated csd also contributes to nuclear retention ( bader and vogt , 2005 ) .
y - box proteins are capable of nucleocytoplasmic shuttling , which allows them to contribute to the coupling control of transcription and translation .
y - box proteins become associated with nascent transcripts cotranscriptionally and are presumed to accompany mrna into the cytoplasm ( soop et al , 2003 ) .
interestingly , a mouse y - box protein msy2 preferentially associates with mrnas that are transcribed from genes containing y - box sequences in their promoter regions and stores those mrnas in male germ cells ( yang et al , 2005a ) .
experiments with overexpression or down - regulation of the y - box proteins in cultured cells or animals have shown that the amount of y - box proteins must be precisely controlled ( see below ) .
therefore , it is important to understand how the synthesis of y - box proteins is regulated .
recent data have shown that the synthesis of a y - box protein , yb-1 ( y - box binding protein-1 ) , is regulated both at transcriptional and post - transcriptional levels .
yb-1 mrna accumulates when cells are treated with cisplatin or uv irradiation ( ohga et al , 1996 ) .
transcription of the yb-1 gene is stimulated by p73 through an enhanced recruitment of the c - myc - max complexes to e - box sequences in the yb-1 promoter ( uramoto et al , 2002 ) .
yb-1 protein represses translation of yb-1 mrna by binding to specific elements in the 5- and 3-untranslated regions ( fukuda et al , 2004 ; skabkina et al , 2005 ) .
this self - regulation may contribute towards maintaining the concentration of yb-1 protein optimal in a cell .
in human and mouse , there are three y - box proteins , two of which are expressed in both somatic cells and germ cells ( table 1 ) .
the most extensively studied y - box protein , yb-1 , is ubiquitously expressed in various tissues .
human contrin and mouse msy2 are germ cell - specific members of the y - box protein family .
analyses of the effects of targeting y - box genes in chicken cells and mice have been widely carried out in the last three years .
chicken b - cell lymphoma dt40 cells are widely used to study functional consequences of disrupting specific genes because of the high frequency of homologous recombination .
the yb-1 ( or yb-1b ) gene in dt40 cells has been disrupted by two independent groups of investigators ; one group reported that yb-1 cells show slow - growth phenotype and increased dna content ( swamynathan et al , 2002 ) .
the other group of researchers found that heterozygous disruption resulted in no growth defects but homozygous gene disruptants exhibited a slow and cold - sensitive growth phenotype ( matsumoto et al , 2005 ) .
one research group tried to disrupt yb-1 gene in mice but encountered difficulties in disrupting both alleles of the yb-1 gene ( shibahara et al , 2004 ) .
however , as was the case in chicken cells , another group recently reported homozygous yb-1gene disruption , showing the importance of yb-1 in late stages of embryonic development ( lu et al , 2005 ) .
they observed developmental defects of yb-1embryos after embryonic day 13.5 including craniofacial lesions , hemorrhage and respiratory failure , with yb-1 mef cells showing premature senescence and hypersensitivity to different cellular stresses .
the reason for the presence or absence of the haplo - insufficient phenotypes is currently unknown . in mice lacking msy2 , both male and female homozygotes are sterile , a consequence of disturbed spermatogenesis due to reduction of postmeiotic germ - cell mrnas in male and oocyte loss in female ( yang et al , 2005b ) .
overall , studies designed to reduce or deplete a y - box protein in cells or whole organisms underscore the significance of y - box proteins in appropriate cell growth , stress responses and development .
the role of yb-1 in cancer progression has attracted attention in recent years.yb-1 has been found to be upregulated during prostate cancer tumor progression ( gimenez - bonafe et al , 2004 ) . increased yb-1 expression has been correlated with dna topoisomerase ii and proliferating cell nuclear antigen expression in human lung cancer ( gu et al , 2001 ) and colorectal cancer ( shibao et al , 1999 ) and linked to markers of cellular proliferation in osteosarcoma ( oda et al , 1998 ) .
yb-1 has been identified as a cell cycle stage - specific transcription factor ( jurchott et al , 2003 ) .
nuclear accumulation of yb-1 in hela cells was demonstrated to transcriptionally activate cyclin a and b1 genes , which are crucial for cell cycle progression .
increase in cyclin a has been reported to be associated with poor clinical outcome in breast cancer ( michalides et al , 2002 ) .
in addition , yb-1 is believed to promote metastasis by enhancing the transcription of gelatinase a , a matrix metalloproteinase that facilitates cell migration ( cheng et al , 2002 ) .
( 2005 ) has also shown that yb-1 may induce epidermal growth factor ( egf ) independence in mammary epithelial cells via activation of the egf receptor pathway , thereby contributing to breast tumor aggressiveness . in
yet another recent paper , bergmann and colleagues ( 2005 ) , using a transgenic mouse model , showed that overexpression of yb-1 may cause breast cancer through the induction of genetic instability . on the other hand
, yb-1 may have anti - oncogenic activity as it is reported to be capable of blocking oncogenic cell transformation ( bader and vogt , 2005 ) .
the phosphoinositide 3-kinase ( pi 3-kinase ) pathway is known to show gain of function in human cancers ( bader and vogt 2004 ) .
the catalytic subunits of pi 3-kinase , p110 ( of which p3k is a homolog ) and akt are oncoproteins and yb-1 is specifically known to inhibit p3k and akt - induced transformation involving protein synthesis ( bader et al .
yb-1 may interefere with the synthesis of growth - related proteins including growth factors , receptors , kinases , transcriptional regulators and cell cycle proteins associated with p3k and akt pathways ( zimmer et al , 2000 ; bader and vogt 2004 ) . a seminal paper describing yb-1 expression in cancer tissues
was first reported by royer 's group in breast cancer ( bargou et al , 1997 ) .
the pathological significance of yb-1 in a variety of cancers is shown in table 2 .
substantial yb-1 expression was demonstrated in multidrug - resistant breast , gastric and pancreatic cell lines ( holm et al , 2004 ) . altered drug sensitivity to cisplatin , a very potent and widely used anti - cancer agent and mitomycin c has been observed following treatment of cells with antisense yb-1 ( ohga et al , 1996 ; torigoe et al , 2005 ) .
expression of yb- 1 protein has been reported to reflect the chemosensitivity of ovarian serous adenocarcinoma ( kamura et al , 1999 ) and breast cancer ( janz et al , 2002 ; huang et al , 2005 ) .
increased nuclear localization of yb-1 has been observed in acquired cisplatin - resistant ovarian cancer ( yahata et al , 2002 ) .
yb-1 expression has also been shown to be associated with p - glycoprotein ( pgp ) expression in breast cancer cells resulting in drug resistance ( bargou et al , 1997 ; saji et al , 2003 ; huang et al , 2005 ) .
pgp , encoded by the mdr1 gene , is a member of the atp - binding cassette transporter superfamily of proteins involved in the protection of cells from xenobiotics and drugs ( kuwano et al , 2003 ) .
pgp has become an important molecular target for limiting chemoresistance as it plays a major role in the development of multidrug - resistant tumor type and is known to mediate resistance to a wide range of anticancer agents ( kuwano et al , 1999 ) .
bay and co - workers have recently demonstrated a direct interaction between yb-1 and pgp using the computer - based resonance recognition model ( huang et al , 2005 ) .
the same investigators observed the occurrence of raised recurrence rates in breast tumor patients with high yb-1 expression who underwent a chemotherapy regime which contained anthracycline ( a pgp substrate ) . besides breast cancer , yb-1 has been correlated with pgp in ovarian cancer ( huang et al , 2004 ) , prostate cancer ( gimenez - bonafe et al , 2004 ) and osteosarcoma ( oda et al , 1998 ) .
yb-1 has been shown to bind p53 ( okamoto et al , 2000 ) and interaction with p53 could be necessary for the self - defense of cells exposed to dna - damaging agents ( kuwano et al , 2003 ) . as mentioned earlier , p73 ,
a close relative of the p53 family , has also been observed to stimulate transcription of the yb-1 promoter by enhancing the recruitment of the cmyc - max complex to its target gene ( uramoto et al , 2002 ) .
c - myc , an oncogene with a dual function in cell proliferation and apoptosis can confer resistance to cisplatin . p73 is known to induce apoptosis ( irwin et al , 2000 ) . and p73 overexpressing clones have been observed to be cisplatin resistant ( gong et al , 1999 ) .
hence , c - myc and p73 may form a complex necessary in yb-1 mediated drug resistance ( uramoto et al , 2002 ) .
expression of the yb-1 protein has a prognostic significance in determining disease progression in human cancers .
perhaps more importantly , yb-1 has the potential to be a biological marker which predicts chemotherapy resistance and aid in the selection of appropriate adjuvant chemotherapy .
there has been cumulative evidence in the literature to suggest that yb-1 is involved in pleiotropic resistance to different classes of dna - targeting drugs ( levenson et al , 2000 ) . as clinical drug resistance hampers effective chemotherapy ,
a recent focus in cancer therapeutic strategy is to develop molecular cancer therapeutics ( kuwano et al , 2003 ; holm et al , 2004 ) . in this
regard , yb-1 holds promise as target molecule for the development of novel approaches in overcoming multidrug resistance in cancer chemotherapy ( janz et al , 2002 ) .
 y - box proteins belong to the cold shock domain family of proteins that are known to be involved in both transcriptional and translational control . here , we give a brief overview of the structure , regulation and physiological functions of the y - box proteins . 
 this is followed by examining the role of y - box protein 1 ( yb-1 ) , the most extensively studied of the y - box protein in tumorigenesis , and its clinicopathological significance . 
 yb-1 has the potential to be a prognostic marker and predictor of chemoresistance in human cancers . 
INTRODUCTION
Structure and cellular localization of Y-box proteins
Regulation of the synthesis of Y-box proteins
Physiological functions of Y-box proteins
Role of YB-1 in tumorigenesis
YB-1 and chemoresistance in human tumors
CONCLUSIONS