Abstract PR002: Loss of protein phosphatase PP2A activity drives AR addiction in prostate cancer

Abstract Reversible protein phosphorylation plays a fundamental role in numerous biological processes and is regulated by a dynamic interplay between protein kinases and phosphatases. Disruption of this balance due to aberrant activation of kinases and inactivation of phosphatases is a hallmark of cancer. The highly conserved protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase with a complex heterotrimeric composition involved in the regulation of numerous critical aspects of cellular function. PP2A is one of the most abundant enzymes, accounting for up to 1% of the total cellular protein in some tissues and represents the majority of serine/threonine phosphatase activities in many tissues. PP2A is comprised of a core dimer consisting of a scaffolding ‘‘A’’ subunit (PP2A-A), and a catalytic ‘‘C’’ subunit (PP2A-C), which form a heterotrimeric complex with one of at least 20 substrate-directing regulatory “B” subunits. PP2A heterotrimerization is regulated through Leucine309 methylation of catalytic C subunit, which influence the binding affinity of the B subunits to the AC dimer and thus the substrate specificity. Loss of the tumor suppressive activity of PP2A is associated with prostate cancer; however, the molecular basis for this loss is not well understood. Here, we report that the leucine carboxy methyltransferase (LCMT1), which methylates the L309 of the catalytic C subunit of the PP2A heterodimer, acts as a suppressor of androgen receptor (AR) addicted prostate cancer (PCa). In a series of tissue microarrays (TMAs) representing 17,747 localized PCa, low methyl-PP2A-C levels were negatively associated with critical parameters of disease progression including high Gleason score, and lymph node metastasis. Biologically, LCMT1 silencing leads to increased AR phosphorylation/activity and enhanced CRPC growth in vivo. Conversely, LCMT1 dependent methyl sensitive PP2A heterotrimers target AR and its critical coactivator MED1 for dephosphorylation through binding of the PP2A-B56α regulatory subunit to two conserved small linear motifs (SLiMs) on MED1, resulting in the eviction of the AR-MED1 complex from the chromatin and loss of target gene expression. Mechanistically, LCMT1 is regulated by PI3K-AKT-mTOR-S6K1-mediated phosphorylation-induced degradation requiring the β-TRCP E3 ubiquitin ligase. Targeted degradation of LCMT1 through this mechanism is associated with acquired resistance to second-generation anti-androgens such as enzalutamide, apalutamide and darolutamide resulting in restored AR-addiction in CRPC cells. Finally, feedforward stabilization of LCMT1 by a small molecule activator of phosphatase (SMAP) results in attenuation of AR signaling and tumor regression in enzalutamide refractory PCa xenografts. Collectively, these results demonstrate that decreased methyl-PP2A-C is a novel prognostic marker for PCa progression and that the loss of LCMT1 is a major determinant of AR-addicted advanced PCa, and support the therapeutic potential of selective small molecules modulators of PP2A for metastatic prostate cancer treatment. Citation Format: Irfan A. Asangani. Loss of protein phosphatase PP2A activity drives AR addiction in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr PR002.