p53 enhances BAK and CD95 expression in human malignant glioma cells but does not enhance CD95L-induced apoptosis

The temperature-sensitive murine p53val(135) mutant was introduced into 3 human malignant glioma cell lines to examine the effects of the p53 status on BCL-2 family protein expression, CD95 expression, and sensitivity to CD95 ligand (CD95L)-induced apoptosis. p53val(135) behaves as a dominant negative mutant at 38.5 degrees C but assumes p53 wild-type properties. In order to dissect (i) specific effects of wild-type versus mutant p53, and (ii) transdominant-negative versus gain-of-function effects of mutant p53, we included glioma cell lines with functional wild-type (LN-229), mutant (LN-18) or deleted (LN-308) p53 genes. Wild-type, but not mutant, p53val(135) promoted G2/M arrest and accumulation of BAK protein in all cell lines. The levels of other BCL-2 family members including BAX, BCL-2, BCL-X or MCL-1 were not consistently modulated by mutant or wildtype p53val135. Wild-type, but not mutant, p53val135 enhanced CD95 expression in all cell lines. CD95L-evoked caspase 3 activity was unaffected by wild-type p53 in all cell lines. Unexpectedly, mutant p53val135 differentially modulated caspase 3 activity in a gain-of-function fash ion in that caspase 3 activity induced by CD95L was enhanced in LN-229 and LN-308 cells but reduced in LN-18 cells. Yet, mutant p53val135 enhanced the sensitivity to CD95L in LN-18 cells, had no effect in LN-229 cells, and decreased the sensitivity of LN-308 cells. Corresponding to the unaltered CD95L-evoked caspase 3 activity, wild-type p53val(135) had no major effect on CD95L-induced apoptosis, except for a moderate sensitization of LN-229 cells but only when protein synthesis was inhibited. Thus, wild-type p53 induces BAK and CD95 expression in human glioma cells without enhancing their susceptibility to CD95-mediated apoptosis, and mutant p53 modulates CD95L-evoked apoptotic signalling in a gain-of-function fashion up-stream and downstream of caspase 3 activation.