7323 Prostatic Acid Phosphatase Is Negatively Regulated By Androgens And Persists In Castration-Resistant Prostate Cancer

Abstract Disclosure: A. Kirschenbaum: None. P. Cheung: None. S. Yao: None. P. Cheung: None. Background: Prostate cancer (PCa) is the most common cancer and second leading cause of cancer death in American men. Most patients with metastatic PCa respond initially to androgen deprivation therapy (ADT) but the majority will progress to lethal, metastatic, castrate-resistant PCa (mCRPC). The identification of markers of mCRPC that are not regulated or negatively regulated by androgens can serve as markers and therapeutic targets in mCRPC. We hypothesized that prostatic acid phosphatase (PAP), a protein phosphatase and 5’ectonucleotidase that is expressed in both the cytoplasm and transmembrane (TM-PAP) of PCa cells, is negatively regulated by androgen signaling and can serve as a therapeutic target. Methods: PAP protein expression was assessed by immunohistochemistry (IHC) in PCa cell line spheroids (VCaP, 22Rv1, LNCaP, C42B) +/- androgen addition as well as in LNCaP s.c. xenografts +/- castration. Protein expression of PAP was assessed with western blotting in PCa cell lines after treatment with androgens (DHT) and anti-androgen (enzalutamide). Results: In spheroids derived from PCa cell lines, there is some IHC expression of PAP with the strongest staining in VCaP and 22Rv1 spheroids. DHT decreased PAP IHC expression in all spheroids. LNCaP and VCaP s.c. xenografts demonstrated that castration decreased tumor growth rates but that in castrate as well as control tumors PAP expression persisted. All forms of PAP including TM-PAP are expressed in VCaP cell line and were negatively regulated by androgen addition. Conclusions: The identification of markers of mCRPC that are not regulated or are negatively regulated by androgen could help identify recalcitrant tumor cells and mount targeted delivery of therapeutic agents. In several androgen sensitive PCa cell lines, DHT addition decreased PAP expression in spheroids. In vivo, PAP expression persists after castration in spite of decreases in tumor growth rates. Finally, DHT decreases and enzalutamide restores PAP expression in vitro. These data demonstrate that PAP is a more progenitor cell marker in PCa that persists after castration and may be targeted in castration-resistant disease. Presentation: 6/3/2024