Abstract C083: HDAC6 inhibition increases proteasome activity and modulates the myeloma immunopeptidome to promote cytotoxic T-cell activity
Authors: Rana Priyanka S. , Ignatz-Hoover James J. , Driscoll James J. 
Published: December 2023

Abstract
The proteasome functions as an essential component of immune surveillance by generating peptides from intracellular antigens that are then presented to cytotoxic T-cells. The vast majority of defined antitumor T-cell responses involves the proteasomal processing of intracellular proteins and their presentation in the context of major histocompatibility complex (MHC) class I molecules to antigenic peptide-specific CD8+ T-cells. Such antigenic peptides generated by the proteasome are generally 8-10 amino acids in length and mirror the linear sequence of the parental protein. However, immune evasion is a hallmark of cancer cells that disrupt the antigen processing and presentation machinery. Tumors develop strategies to evade host immunity and resist current immunotherapies, while others display a low tumor mutation burden limiting the presence of tumor antigenicity. Here, we postulated that pharmacologic enhancement of proteasome catalytic activity would increase MHC class I antigen presentation, modulate the myeloma immunopeptidome, and promote CD8+ T-cells anti-myeloma activity. A cell-based, high-throughput screen revealed that the histone deacetylase 6 (HDAC6) inhibitors tubastatin-A, ACY-738 and ACY-1215 increased proteasome activity in multiple myeloma (MM) cells. HDAC6 inhibitors increased pan-MHC class-I expression in MM cells as well as a panel of cancer cell lines. Treatment of MM patient tumor cells with HDAC6 inhibitors increased the cytotoxic effect of autologous patient-derived T-cells. Mechanism-based studies demonstrated that pharmacologic blockade or genetic ablation of the HDAC6 ubiquitin-binding (BUZ) domain disrupted HDAC6 association with HR23B, a substrate-shuttle factor that delivers ubiquitinylated cargo to proteasomes. HDAC6 inhibitors or HDAC6-BUZ domain deletion reduced the association of HR23B with HDAC6 and promoted binding of HR23B to proteasomes and a consequent increase in proteasome activity. The results validate the HDAC6-BUZ domain as an attractive drug discovery target and demonstrate that proteasome activators enhance antitumor immunity. Proteasome activators represent a paradigm-shifting approach to overcome mechanisms of immune escape in cancer cells. FDA-approved drugs that increase proteasome activity and boost antigen presentation and can be repositioned as cancer immunotherapeutics to overcome the existing bottlenecks in drug development. Taken together, our results highlight the breadth and magnitude of the proteasome in governing fundamental cellular processes and the immense potential of therapeutics that exploit proteasomes to treat human disease.
Citation Format: Priyanka S. Rana, James J. Ignatz-Hoover, James J. Driscoll. HDAC6 inhibition increases proteasome activity and modulates the myeloma immunopeptidome to promote cytotoxic T-cell activity [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr C083.