bridging integrator-1 breast cancer 1 and 2 cyclin - dependent kinase cyclin - dependent kinase inhibitor human homolog of double minute 2 poly(adp - ribosyl)ation poly(adpribose ) polymerase 1 retinoblastoma 1 protein
the discovery of gleevec ( sti-571 ) , a tyrosine kinase inhibitor , was a milestone achievement in clinical oncology and this inhibitor has demonstrated remarkable efficacy in philadelphia chromosome - positive ( ph ) chronic myeloid leukemia . since then , mechanism - based approaches have been used to specifically target various kinases and/or downstream oncogenic pathways that are critically involved in cell cycle progression and tumorigenesis . however , in addition to this approach , a more recent and novel use of small - molecule inhibitors has emerged as a promising endeavor in the field of cancer chemotherapy . here , we briefly review the mechanistic basis of restoration of a tumor suppressor and its potential complications for cancer therapy .
the tumor suppressor function mediated by the retinoblastoma 1 protein ( rb1 ) is principally attributed to its interaction with the e2f transcription factor 1 ( e2f1 ) .
the rb1/e2f1 complex represses a number of e2f1-dependent transcriptional target genes that are required for the transition from g1 to s phase in the cell cycle .
because rb1 is inactivated by phosphorylation mediated by the g1 cyclin - dependent kinases 4 and 6 ( cdk4 and cdk6 ) , restoring rb1 function by inactivating cdk4/6 is theoretically an obvious approach .
although structural similarities among a number of cdk family members hampered the development of a cdk4/6-specific inhibitor for many years , some agents , including palbociclib ( pd-0332991 ) , have recently demonstrated promising results in phase i / ii clinical trials for human malignancies , including breast cancer . above and beyond rb1 , another tumor suppressor that is critical for numerous growth inhibitory pathways
the abundance of wild - type p53 protein is massively reduced as a result of ubiquitin - dependent and human homolog of double minute 2 ( hdm2)-mediated degradation of p53 .
therefore , dissociation of p53 from the p53/hdm2 complex is a reasonable strategy for rescuing p53 function .
based on the crystallographic structure of the p53/hdm2 peptide complex , small p53 peptides that mimic the region of p53 sufficient for hdm2 binding and small - molecule hdm2 antagonists have been shown to disrupt the p53/hdm2 interaction in vitro and in vivo .
some of these , including mi-219 , nutlin-3 , and rg7112 , have been found to be effective preclinically and have consequently moved into phase i / ii clinical trials   although proteasome inhibitors such as bortezomib ( ps-341 ) may not be as specific for stabilizing p53 as these hdm2 inhibitors , other growth - inhibitory gene products , including the cyclin - dependent kinase inhibitor 1b ( cdkn1b or p27 , kip1 ) protein , can also be degraded in an ubiquitin - dependent manner .
therefore , it may be advantageous to re - establish a broad spectrum of growth - inhibitory functions by blocking the proteasome pathway .
although the approach of re - establishing tumor suppressor function in tumors as a therapeutic option is mechanistically intriguing , there are potential dilemmas associated with the systemic restoration of tumor suppressor function .
tumor suppressor genes are frequently mutated or deleted in cancer patients , and given that some of the mutant genes acquire oncogenic potential , this approach may simply reboot a mutant ( i.e. , oncogenic ) tumor suppressor .
even if a tumor suppressor gene is intact , its function should not depend on other cancer - susceptible proteins .
for example , the cyclin - dependent kinase inhibitor 2a ( cdkn2a ) gene is not frequently deleted in cancer cells , but is inactivated by dna methylation . however , epigenetic reactivation of the cdkn2a gene may not be an effective approach if rb1 and/or p53 are deficient , because the tumor suppressor functions of the products of the 2 alternative reading frames of cdkn2a  p16 and p14 proteins  largely depend on rb1 and p53 , respectively .
therefore , for the tumor suppression approach to be fully effective , it will be important to identify a non - mutated ( or non - deleted ) tumor suppressor whose function does not rely on other tumor suppressors that might be already mutated or deleted .
bridging integrator-1 ( bin1 ) was originally identified as a c - myc oncoprotein - interacting tumor suppressor . the bin1 gene itself is rarely mutated or deleted , but is frequently silenced in human cancer cells
moreover , bin1 acts as a tumor suppressor in vitro and in vivo in the absence of rb1 and p53 .
we recently demonstrated that bin1 , whose gene promoter is activated by e2f1 , directly interacts with e2f1 and represses its transcription , implying that a negative - feedback loop regulates bin1 gene expression .
interestingly , we found that e2f1 is poly(adp - ribosyl)ated by poly(adp - ribose ) polymerase 1 ( parp1 ) and that parp1 inhibition unlocks the e2f1bin1 negative - feedback loop to vigorously activate the bin1 gene , which induces g2/m arrest in the cell cycle and/or apoptosis .
synthetic lethality, parp inhibitors have been actively used for clinical trials in breast cancer 1 and 2 ( brca1/2)-deficient breast and ovarian cancers . however , it was unclear why parp inhibitors alone also show therapeutic efficacy , even in cancer cells expressing wild - type brca1/2 . based on our recent data ,
the restoration of bin1 by parp inhibitors may offer a mechanistic rationale for expanding the clinical usage of parp inhibitors over a wider range of tumor types , regardless of the status of rb1 , tp53 , and brca1/2  genes ( fig .
1 ) . 
 figure 1.small-molecule inhibitors used in cancer therapy restore the functions of various tumor suppressors in malignant cells .
the tumor suppressor functions of retinoblastoma 1 protein ( rb1 ) , tumor protein p53 ( tp53 , known as p53 ) , and bridging integrator-1 ( bin1 ) can be pharmacologically restored by small - molecule inhibitors , such as palbociclib ( pd-0332991 ) , mi-219 , and olaparib , respectively . in the case of p53 restoration , a human homolog of double minute 2 ( hdm2 ) , named hdm4 ( also known as hdmx ) , might interfere with the effectiveness of hdm2 antagonists , probably because of overlapping functions between hdmx and hdm2 .
p , phosphorylation ; par , poly(adp - ribosyl)ation ; dp1 ( or tfdp1 ) , transcription factor dp1 ; e2f1 , e2f transcription factor 1 ; cdk4/6 , cyclin - dependent kinase 4 and 6 ; g1/s : transition between g1 ( growth1/gap1 ) phase to s ( dna synthesis ) phase in the cell cycle ; g2/m , transition between g2 ( growth2/gap2 ) phase to m ( mitosis ) phase in the cell cycle . 
 
small - molecule inhibitors used in cancer therapy restore the functions of various tumor suppressors in malignant cells .
the tumor suppressor functions of retinoblastoma 1 protein ( rb1 ) , tumor protein p53 ( tp53 , known as p53 ) , and bridging integrator-1 ( bin1 ) can be pharmacologically restored by small - molecule inhibitors , such as palbociclib ( pd-0332991 ) , mi-219 , and olaparib , respectively . in the case of p53 restoration , a human homolog of double minute 2 ( hdm2 ) , named hdm4 ( also known as hdmx ) ,
might interfere with the effectiveness of hdm2 antagonists , probably because of overlapping functions between hdmx and hdm2 .
p , phosphorylation ; par , poly(adp - ribosyl)ation ; dp1 ( or tfdp1 ) , transcription factor dp1 ; e2f1 , e2f transcription factor 1 ; cdk4/6 , cyclin - dependent kinase 4 and 6 ; g1/s : transition between g1 ( growth1/gap1 ) phase to s ( dna synthesis ) phase in the cell cycle ; g2/m , transition between g2 ( growth2/gap2 ) phase to m ( mitosis ) phase in the cell cycle .
chemotherapy and radiotherapy are conventional treatments for eradicating tumors , but cancer often develops therapeutic resistance over time . given that many tumor suppressors are proapoptotic in response to dna damaging agents , it would be clinically pertinent to increase the chemo- and radiosensitivities of cancer by combining standard treatments with agents that can restore the activity of silenced tumor suppressors , provided they are not mutated or deleted , in human malignancies .
this work was partially supported by a grant from the us national institutes of health ( nih ) ( r01ca140379 ) ( to d.s . ) .
 over the last decades , accumulating data have advanced our understanding of the mechanism of action of tumor suppressor proteins and therapeutic strategies to restore tumor suppressor pathways have emerged as a promising approach for cancer therapy . based on our recent findings on bridging integrator-1 ( bin1 ) , 
 we outline potential advantages and disadvantages of chemical activation of tumor suppressors . 