Human inhibitor of growth 1 inhibits hepatoma cell growth and influences p53 stability in a variant-dependent manner.
Inhibitor of growth 1 (ING1) is a type II tumor suppressor that affects cell function by altering chromatin structure and regulating transcription. Recently, three ING1 splice variants have been cloned, but their roles in apoptosis and p53 regulation in human hepatocellular carcinoma (HCC) have not been fully elucidated. The present study found that ING1, in a variant-dependent manner, inhibited hepatoma cell proliferation and colony formation, induced apoptosis and cell cycle arrest at G(0)/G(1) phase, and postponed tumor formation in nude mice. Expression of p33(ING1b) and p24(ING1c) variants, but not p47(ING1a), was markedly reduced in HCC samples. Reverse transcription polymerase chain reaction and western blotting analysis revealed that ectopic overexpression of p33(ING1b) or p24(ING1c) variant increased the expression of p53 downstream genes such as p21(waf1) and bax, and repressed bcl-2 expression (P < 0.01), whereas p47(ING1a) inactivated p21(waf1) promoter (P < 0.01). Furthermore, we found that p33(ING1b) and p24(ING1c) repressed Mdm2 expression (P < 0.01) and competed with Mdm2 for binding to p53. Interestingly, p33(ING1b)and p24(ING1c) did not directly bind to Mdm2 protein but strongly increased p14(arf) expression (P < 0.01) and interacted with p14(arf) protein to stimulate p53. Moreover, we found that ectopic overexpression of p33(ING1b) or p24(ING1c) significantly induced p53 protein acetylation at Lys-373/Lys-382 residue, but did not alter the phosphorylation status of p53.


CONCLUSIONS
ING1 variants p33(ING1b) and p24(ING1c) may modulate p53 activity and subsequently inhibit hepatoma cell growth by at least two possible mechanisms: interacting with Mdm2 and p14(arf) to stabilize and activate p53, or increasing p53 acetylation.
Zhu
Zhi
Z
Department of Pathology, Changhai Hospital, Second Military Medical University, Shanghai, China.
Luo
Zhigang
Z
Li
Yongmei
Y
Ni
Canrong
C
Li
Honghua
H
Zhu
Minghua
M
D000818:Animals; D017209:Apoptosis; D006528:Carcinoma, Hepatocellular / Q000235:genetics / Q000473:pathology*; D002455:Cell Division / Q000235:genetics; D045744:Cell Line, Tumor; D002470:Cell Survival; D002874:Chromosome Mapping; D002882:Chromosomes, Human, Pair 13; D017930:Genes, Reporter; D014644:Genetic Variation; D006801:Humans; D000076023:Inhibitor of Growth Protein 1; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics*; D008113:Liver Neoplasms / Q000235:genetics / Q000473:pathology*; D051379:Mice; D008819:Mice, Nude; D009687:Nuclear Proteins / Q000235:genetics*; D051736:Proto-Oncogene Proteins c-mdm2 / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000235:genetics*
C501503:ING1 protein, human; D000076023:Inhibitor of Growth Protein 1; D047908:Intracellular Signaling Peptides and Proteins; D009687:Nuclear Proteins; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2