Targeted Degradation of Histone Deacetylase 8 Using Proteolysis Targeting Chimeras Technology: A Promising Approach for Glioblastoma Treatment.

INTRODUCTION: Histone deacetylase 8 (HDAC8) plays a role in glioblastoma progression, making it a promising therapeutic target. While HDAC8 inhibitors (HDAC8is) suppress glioblastoma growth and prolong survival in animal models, they do not eliminate HDAC8. In contrast, HDAC8-targeting proteolysis-targeting chimera (PROTAC), a selective HDAC8 degrader, induces proteasomal degradation of HDAC8 and thus eliminates all of its functions. PURPOSE: In this study, we investigated the antitumor activity and underlying mechanisms of a previously reported HDAC8 PROTAC in glioblastoma cells. METHODS: Cytotoxicity in glioblastoma-derived U-87 MG, A172 and T98G cells and primary human astrocytes (PHA) was assessed via 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assays. Live-cell imaging was performed using an Incucyte(®) Live-Cell Analysis System. Cell proliferation, cell cycle distribution, and apoptosis were analyzed using flow cytometry. HDAC8 and key regulators of cell cycle and apoptosis were quantified via Western blotting. RESULTS: HDAC8 PROTAC effectively degraded HDAC8 and exhibited cytotoxic and antiproliferative effects in human glioblastoma cells, while demonstrating minimal toxicity in PHA. It induced S-phase arrest and reduced Cdk1, Cdk2, Cdk4, Cdk6, and cyclin B1 expression. It elevated caspase-3/7 activation, downregulated Bcl-2, induced apoptosis, and upregulated key endoplasmic reticulum (ER) stress response proteins, including BiP, XBP1s, CHOP, and p-JNK in U-87 MG glioblastoma cells. The HDAC8 PROTAC demonstrated stronger antitumor activity than HDAC8i and pan-HDACi vorinostat. Moreover, the HDAC8 PROTAC showed selective toxicity toward glioblastoma cells compared to primary human astrocytes. CONCLUSION: HDAC8 PROTAC selectively suppressed glioblastoma cell growth and viability by arresting the cell cycle and inducing ER stress-mediated apoptosis via the IRE1α/XBP1s-JNK-CHOP pathway. Hence, HDAC8 PROTAC is a potential therapeutic agent for glioblastoma treatment.