Towards more effective Glioblastoma Multiforme therapies: HDAC6 and HDAC8 combined targeting

Glioblastoma Multiforme (GBM) is the most common and lethal tumor of the central nervous system in adults, with recurrence and poor prognosis. To date, therapeutic intervention consists of surgical resection followed by radio and chemotherapy. Temozolomide (TMZ) is the standard treatment for GBM, but most patients do not respond or develop resistance. Among the aberrant pathways involved in GBM, several histone deacetylases (HDACs), such as HDAC6 and HDAC8, have been reported to be upregulated. Indeed, we previously demonstrated that the specific inhibition of HDAC6 reduces GBM progression. In this work, we explored whether the combined inhibition of HDAC6 and HDAC8 could be more effective than single inhibition, due to the different specificity of the two HDACs. To accomplish this, we performed in vivo analyses using the GBM zebrafish model zic:RAS, in which tumor development is induced by the specific oncogene expression in neural cells. In parallel, we employed the human GBM cell lines U87MG and T98G, which differ in their genetic profile and sensitivity to TMZ treatment, as in vitro models. Our results revealed that the combined inhibition of HDAC6 and HDAC8 induced a stronger reduction of the vitality of the GBM cell lines, as well as of tumor growth in the zic:RAS zebrafish larvae, compared to the single treatments. Additionally, the HDAC6/HDAC8 combined inhibition enhanced TMZ sensitivity both in vivo and in vitro. From a functional point of view, our in vitro findings suggest that only HDAC6 inhibition impairs the acidic compartments, leading to the disruption of lysosomal trafficking. In zebrafish, blocking HDAC8, but not HDAC6, results in an apoptotic response. To better characterize the cellular and molecular mechanisms involved, we will perform omics analyses of the acetylation status and the transcriptional profile of both our models when HDAC6 and HDAC8 are inhibited. This integrated approach provides a powerful platform to evaluate the effect of the selective and combined HDAC6/HDAC8 inhibition, and to dissect the biological mechanisms that could be relevant for GBM onset and resistance. Our results support the potential of the dual inhibition as a promising therapeutic approach for GBM and other HDAC6/HDAC8 overexpressing cancers.