RESISTANCE TO P53-MEDIATED GROWTH ARREST AND APOPTOSIS IN HEP 3B HEPATOMA-CELLS

Mutations in the tumor suppressor p53 are a common event in hepatocell ular carcinoma (HCC), Because HCCs typically occur in livers with chro nic injury and impaired function, we have explored the role of mild-ty pe p53 in regulating the growth and differentiation of Hep 3B hepatoma cells, a p53-negative line derived from a liver cancer, Stable Hep 3B cell lines were generated in which inducible p53 was introduced using either a temperature-sensitive mutant (p53val135) or a tamoxifen-regu lated p53-estrogen receptor chimera (p53mER(tm)-pBabepuro), In both ce ll lines, induction of transcriptionally active p53 was confirmed by a ssessing several p53 targets: Mdm2 protein, p21(waf1) mRNA and protein , and the cyclin G promoter, Despite marked induction of p21(waf1), ce lls with active p53 failed to undergo growth arrest, which is probably due to the presence of a non-functional retinoblastoma protein (pRb) in these cells, Apoptosis also was not observed, even after prolonged (48 h) serum starvation or exposure to cisplatinum. Lack of apoptosis was correlated with unchanged bax mRNA levels following p53 induction, Additionally, albumin mRNA levels remained unchanged, and there was n o change in basal transactivation of a reporter containing the promote r of the haptoglobin gene, encoding an acute phase protein, This sugge sts that growth arrest may be required to promote liver-specific gene expression, Overall, our data demonstrate that introduction of transcr iptionally active p53 does not alter the malignant, dedifferentiated p henotype of Hep 3B hepatoma cells, Hence, not all cancer cells are equ ally responsive to the re-activation of wild-type 53, The ability of a cancer cell to undergo p53-mediated phenotypic alterations may depend on the retention of functional downstream effector pathways.