P53 ACCUMULATION PROMOTES DEPHOSPHORYLATION AND PROTEOLYTIC CLEAVAGE OF RETINOBLASTOMA PROTEIN IN HUMAN-MALIGNANT GLIOMA-CELLS

The turner suppressor gene products, p53 and RE, modulate cellular res ponses to genotoxic stress including cancer chemotherapy. Expression o f the murine temperature-sensitive mutant p53(val135) in wild-type (32 .5 degrees C), but not mutant (38.5 degrees C), conformation results i n complete growth arrest, but not apoptosis, in three human malignant glioma cell lines, LN-18, LN-229 and LN-308. Here we report that force d expression of p53(val135) promotes RB hypophosphorylation and proteo lytic cleavage, generating a p68 pRB fragment. RE cleavage occurs both with mutant and wild-type p53 conformation, Tn contrast to myeloid ce lls, cleavage of RB is not associated with apoptosis in malignant glio ma cells. Further, RE cleavage does not sensitize these cells to the c ytotoxic effects of cytarabine or etoposide (VP-16). Established proap optotic stimuli fur glioma cells such as exposure to CD95 ligand or do xorubicin fail to induce cleavage of RE. Physiologically, RE is inacti vated via CDK4-dependent phosphorylation, and CDK4 activity is control led by the CDK inhibitor, CDKN2/MTS1/p16. The apparent lack of biologi cal effects of RE cleavage in The glioma cells suggested consitutive i nactivation of the Re pathway. Consistent with this hypothesis, we det ected a homozygous loss of the CDKN2 alleles in LN-18 and LN-229 cells and amplification of CDK4 in LN-308 cells, Further studies need to de fine whether p53-mediated dephosphorylation and cleavage of,PB mediate s apoptosis in cancer cells with an intact regulatory circuit involvin g RE and the CDKN2/p16 and CDK4 proteins.