Histone deacetylase inhibitors sensitize glioblastoma models to temozolomide and reprogram immunosuppressive myeloid cells

Histone deacetylase inhibitors (HDACis) are promising anti-cancer agents but remain underexplored in glioblastoma (GBM). This study evaluated the effects of three HDACis—CAY10603, vorinostat (SAHA), and valproic acid (VPA)—on human GBM cell lines (U87, MGG8) with immortalized human astrocytes (IHAs) as healthy controls. HDACis were tested alone or in combination with temozolomide (TMZ), the standard chemotherapy for GBM, in both 2D (monolayer) and 3D (neurosphere) cultures. Additionally, co-culture of GBM cells with macrophages (M0, biochemically differentiated from THP-1 human monocytes) was used to examine the impact of HDACis on cancer-immune interactions. Results demonstrated that all three HDACis significantly reduced cell viability and synergistically enhanced the effect of TMZ. CAY10603 and SAHA induced early apoptosis and upregulated caspase 3 (CASP3) expression, whereas VPA primarily induced late apoptosis and necrosis in GBM cultures. VPA induced both G0/G1 and G2/M cell cycle arrest, while SAHA and CAY10603 only induced G2/M arrest. mRNA expression analysis following HDACi treatment in U87 neurospheres revealed that HDACis inhibited expression of markers for epithelial-to-mesenchymal transition (EMT), proliferation, and stemness pathways. In U87-M0 co-cultures, we observed significant upregulation of stemness markers and the pro-inflammatory cytokine TNF-α following CAY10603 and VPA treatments. In contrast, TMZ monotherapy upregulated the expression of the immunosuppressive cytokine TGF-β. These findings suggest that HDAC inhibition—including the novel small molecule CAY10603—sensitizes GBM to temozolomide and confers potent anti-tumor effects that combat GBM (e.g., reducing proliferation, EMT, stemness). Our in vitro findings —e.g., with 3D neurospheres that better mimic physiological tumor growth than 2D monolayers—warrant future in vivo testing of HDACis alone or in combination with chemotherapy.