Compound cellular stress maximizes apoptosis independently of p53 in glioblastoma

We examined the apoptotic response of two glioblastoma cells, p53 wild type U87 and p53 mutated T98G, to doxorubicin, bortezomib, and vorinostat, which respectively target DNA, 26S proteasome and histone deacetylase, to clarify p53's function in apoptosis. We demonstrated that doxorubicin induced apoptosis in U87 cells but not in T98G cells. The level of p53 was definitively correlated to the extent of DNA damage and apoptosis initiation. Dominant-negative p53 reduced p21 expression, but did not affect doxorubicin-induced apoptosis, so the transcriptional activity of p53 seemed not to participate in doxorubicin-induced apoptosis. However, p53 concentrated into the nucleus during heavy apoptosis. Bortezomib could induce apoptosis in U87 with high sensitivity and T98G cells with low sensitivity. In contrast, vorinostat promoted apoptosis in both U87 and T98G cells and reduced the basal level of p53 in U87 cells, indicating that p53 played no role in the vorinostat-induced apoptosis. To clearly define the role of p53 in bortezomib- and doxorubicin-induced apoptosis, we combined doxorubicin with bortezomib to treat U87 cells to assess this combination's effect on apoptosis and p53 status. Interestingly, the combination of doxorubicin with bortezomib engendered compound stress, resulting in a synergistic outcome for apoptosis in U87 cells. However, the amounts of p53 in the total count and in the nucleus were much lower with the combination than with doxorubicin alone, suggesting that p53 played no role in either the compound stress, doxorubicin-only or bortezomib-induced apoptosis.