Abstract 4508: Defining PIM kinase protein regulation and PROTAC for pre-clinical testing

Abstract The PIM kinase family are serine/threonine kinases known to be overexpressed in castration-resistant prostate cancer (CRPC). PIM1, an isoform of PIM, is known to regulate cell survival, migration, and proliferation. Uniquely, PIM kinases are constitutively active due to the lack of a regulatory domain. Therefore, PIM protein levels are reflected in PIM activity and are regulated by protein degradation and synthesis. PIM inhibitors have been developed, targeting the PIM kinase activity and inhibiting phosphorylation of downstream targets, but have had limited efficacy. Our lab and others demonstrate that PIM inhibitor treatment increases total PIM protein levels, suggesting that PIM kinases control protein stability via autophosphorylation. To elucidate the mechanism of autophosphorylation dependent regulation of PIM1, we used phospho-proteomics to identify potential autophosphorylation sites on PIM1. We have identified overexpression of deubiquitinase USP28 inhibiting degradation of PIM1, leading to the investigation of the role of F-box protein Fbw7 in regulating PIM1. Dominant-negative expression of Fbw7 results in increased PIM1 total protein. Additionally, coimmunoprecipitation of Fbw7 knockdown cells resulted in decreased PM1 ubiquitination compared to control. Therefore, we have identified Fbw7 as a putative E3 ligase responsible for PIM1 degradation. To address the limited efficacy of PIM inhibitors, we have developed the first PIM-PROTAC (PIMTAC) with our collaborators. Proteolysis targeting chimera (PROTAC) is a promising approach to eliminating the pro-tumorigenic effects of PIM. The PIMTAC selectively targets PIM kinases for degradation and we have demonstrated in vitro efficacy. PIMTAC successfully maintains degradation of all isoforms of PIM for over 72 hours. In vitro assessment of anti-tumor efficacy demonstrated increased apoptotic death with PIMTAC alone compared to PIM inhibitor or docetaxel. Furthermore, dual treatment of PIMTAC and docetaxel had more apoptotic death compared to PIM inhibitor with docetaxel. We aim to develop the PIMTAC for pre-clinical testing. Citation Format: Hope Liou, John Brognard, Rolf Swenson, Pedro Torres-Ayuso, Noel Warfel. Defining PIM kinase protein regulation and PROTAC for pre-clinical testing abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 4508.