HDAC6 and HDAC8 combined targeting to counteract Glioblastoma Multiforme progression and treatment resistance

Glioblastoma Multiforme (GBM) is the most common and lethal primary brain tumor in adults. Standard care, comprising surgical resection followed by radio and chemotherapy with Temozolomide (TMZ), shows limited efficacy, highlighting the need for new therapeutic strategies. Among the aberrant pathways involved in GBM, histone deacetylases (HDACs) are frequently upregulated. In particular, HDAC6 and HDAC8 have emerged as potential therapeutic targets, paving the way for their combined inhibition as an alternative treatment approach. In this work, we investigated the effects of HDAC6 and HDAC8 combined targeting by pharmacological inhibition with the selective compounds Tubastatin A and PCI34051. We employed two human GBM cell lines, U87-MG and T98G, and the GBM zebrafish model zic:RAS. Our results revealed that the combined inhibition of HDAC6 and HDAC8 significantly reduced GBM cell viability, as well as tumor growth in vivo. Co-treatment with TMZ further enhanced the reduction in cell viability and was also effective in vivo. Mechanistically, HDAC6 inhibition induced lysosomal stress in both cell lines, ultimately resulting in autophagic disruption and apoptosis in T98G cells. Instead, HDAC8 inhibition did not affect these processes, but both inhibitors impacted ceramide metabolism by increasing the acid sphingomyelinase activity. Since these data suggest that HDAC6 and HDAC8 inhibition might alter partially divergent pathways, we are performing omics analyses to better elucidate the mechanisms regulated by HDAC8 inhibition. The therapeutic relevance of the combined targeting is further supported by the negative impact of HDAC6/HDAC8 inhibition on the side population, a cellular subset related to cancer stemness and known to mediate treatment resistance and disease relapse. Therefore, taken together, our findings support the potential of the combined inhibition as a promising therapeutic approach for GBM and other HDAC6/HDAC8 overexpressing cancers.