Retinoids induced astrocytic differentiation with down regulation of telomerase activity and enhanced sensitivity to taxol for apoptosis in human glioblastoma T98G and U87MG cells

We hypothesized that induction of differentiation with retinoid could increase sensitivity to microtubule-binding drug taxol (TXL) for apoptosis in human glioblastoma T98G and U87MG cells. Treatment of cells with 1 μM all-trans retinoic acid (ATRA) or 1 μM 13-cis retinoic acid (13-CRA) for 7 days induced astrocytic differentiation, overexpression of glial fibrillary acidic protein (GFAP), and also down regulated telomerase expression and activity, thereby increased sensitivity to TXL for apoptosis. Treatment of glioblastoma cells with TXL triggered production of reactive oxygen species (ROS), induced phosphorylation of p38 mitogen-activated protein kinase (MAPK), and activated the redox-sensitive c-Jun NH₂-terminal kinase 1 (JNK1) pathway. Moreover, TXL activated Raf-1 kinase for phosphorylation and inactivaion of anti-apoptotic Bcl-2 protein. The events of apoptosis included increase in expression of Bax, down regulation of Bcl-2 and baculoviral inhibitor-of-apoptosis protein (IAP) repeat containing (BIRC) proteins, mitochondrial release of cytochrome c and Smac into the cytosol, increase in intracellular free [Ca²⁺], and activation of calpain, caspase-9, and caspase-3. Increased activity of caspase-3 cleaved inhibitor of caspase-activated DNase (ICAD) to release and translocate CAD to the nucleus for DNA fragmentation. Involvement of stress signaling kinases and proteolytic activities of calpain and caspase-3 in apoptosis was confirmed by pretreating cells with specific inhibitors. Taken together, our results suggested that retinoid (ATRA or 13-CRA) induced astrocytic differentiation with down regulation of telomerase activity to increase sensitivity to TXL to enhance apoptosis in glioblastoma cells. Thus, combination of retinoid and TXL could be an effective therapeutic strategy for controlling the growth of glioblastoma.