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 responses to genotoxic stress including cancer chemotherapy. Expression of the murine temperature-sensitive mutant p53(val135) in wild-type (32.5 degrees C), but not mutant (38.5 degrees C), conformation results in complete growth arrest, but not apoptosis, in three human malignant glioma cell lines, LN-18, LN-229 and LN-308. Here we report that forced expression of p53(val135) promotes RB hypophosphorylation and proteolytic cleavage, generating a p68 pRB fragment. RE cleavage occurs both with mutant and wild-type p53 conformation, Tn contrast to myeloid cells, cleavage of RB is not associated with apoptosis in malignant glioma cells. Further, RE cleavage does not sensitize these cells to the cytotoxic effects of cytarabine or etoposide (VP-16). Established proapoptotic stimuli fur glioma cells such as exposure to CD95 ligand or doxorubicin fail to induce cleavage of RE. Physiologically, RE is inactivated via CDK4-dependent phosphorylation, and CDK4 activity is controlled by the CDK inhibitor, CDKN2/MTS1/p16. The apparent lack of biological effects of RE cleavage in The glioma cells suggested consitutive inactivation of the Re pathway. Consistent with this hypothesis, we detected 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 define whether p53-mediated dephosphorylation and cleavage of,PB mediates apoptosis in cancer cells with an intact regulatory circuit involving RE and the CDKN2/p16 and CDK4 proteins.