HDAC6 and HDAC8 inhibition enhances the efficacy of Temozolomide by rewiring sphingolipid metabolism and glioma cell fate

Glioblastoma multiforme (GBM) is the most aggressive brain tumor in adults, characterized by rapid progression, therapeutic resistance, and poor survival. Standard treatments, including surgery, radiotherapy, and chemotherapy with Temozolomide (TMZ), have limited effectiveness, underscoring the need for improved therapeutic strategies. HDAC6 and HDAC8, two histone deacetylases frequently overexpressed in GBM, have emerged as potential therapeutic targets. In addition, deregulation of sphingolipid metabolism and signaling, particularly reduced levels of the pro-apoptotic lipid ceramide, has been implicated in GBM growth and drug resistance. In this study, we firstly investigated the effects of HDAC6 and HDAC8 selective inhibitors, Tubastatin A and PCI34051, respectively, on the viability of two human GBM cell lines, U87MG and T98G. In all lines, both inhibitors reduced cell viability individually, with a stronger additive effect when combined. Co-treatment with TMZ further enhanced this reduction, showing a synergistic effect and increased drug sensitivity. We also examined the possibility that these inhibitors affect sphingolipid metabolism and acidic compartments, as we previously demonstrated in U87MG cells treated with HDAC6 inhibitor. Our results demonstrated that, also in T98G cells, HDAC6 inhibition influences both processes, increasing ceramide levels and lysosomal stress. On the contrary, HDAC8 inhibition did not affect acidic compartments, but did impact on ceramide metabolism in both cell lines. These results reveal that dual inhibition of HDAC6 and HDAC8 enhances TMZ sensitivity and alters sphingolipid balance in GBM, supporting its potential as a promising therapeutic avenue for GBM and other HDAC6/8 overexpressing cancers.