Thioridazine (THD) is a common phenothiazine antipsychotic drug reported to suppress growth in several types of cancer cells. We previously showed that THD acts as an antiglioblastoma and anticancer stem-like cell agent. However, the signaling pathway underlying autophagy and apoptosis induction remains unclear. THD treatment significantly induced autophagy with upregulated AMPK activity and engendered cell death with increased sub-G1 in glioblastoma multiform (GBM) cell lines. Notably, through whole gene expression screening with THD treatment, frizzled (Fzd) proteins, a family of G-protein-coupled receptors, were found, suggesting the participation of Wnt/β-catenin signaling. After THD treatment, Fzd-1 and GSK3β-S9 phosphorylation (inactivated form) was reduced to promote β-catenin degradation, which attenuated P62 inhibition. The autophagy marker LC3-II markedly increased when P62 was released from β-catenin inhibition. Additionally, the P62-dependent caspase-8 activation that induced P53-independent apoptosis was confirmed by inhibiting T-cell factor/β-catenin and autophagy flux. Moreover, treatment with THD combined with temozolomide (TMZ) engendered increased LC3-II expression and caspase-3 activity, indicating promising drug synergism. In conclusion, THD induces autophagy in GBM cells by not only upregulating AMPK activity, but also enhancing P62-mediated autophagy and apoptosis through Wnt/β-catenin signaling. Therefore, THD is a potential alternative therapeutic agent for drug repositioning in GBM.

In prostate cancer, p53 maximizes apoptosis in response to severe DNA damage, not DNA replication stress. Here, we examined the apoptotic response of two glioblastoma cells, p53-wild type U87 and a p53-mutated T98G cell, for the same stresses. We ascertained that p53 intensified apoptosis in response to severe DNA damage, not DNA replication stress in glioblastoma. We further asked if p53-mediated apoptosis can be induced by cellular stress other than severe DNA damage. We analyzed two compounds, bortezomib and vorinostat, respective inhibitors of 26S proteasome and histone deacetylase, to evaluate their capacity to activate p53-mediated apoptosis. The cellular stress incited by bortezomib, not vorinostat, activated p53-mediated apoptosis. Next, we asked if the cellular stress generated by combining the two compounds had a synergistic effect on apoptosis. Our results demonstrated that doxorubicin with bortezomib or CFS-1686, or bortezomib with vorinostat have a significant synergistic effect on apoptosis only in p53-wild type cell. Under high stress, p53 translocates from cytosol into the nucleus to cause apoptosis possibly. Together, p53 maximizes apoptosis for cellular stress caused by severe DNA damage, disruption of protein turnover, and for the stress induced by drug combination including doxorubicin with bortezomib or CFS-1686, and bortezomib with vorinostat.