Abstract 5539: PRDM16 is an intrinsic regulator of dormancy in bone disseminated prostate cancer cells
Authors: Nasr Mostafa M. , Li Tao , Bishop Ryan T. , Frieling Jeremy S. , Lynch Conor C. 
Published: March 2024

Abstract
Metastatic prostate cancer (PCa) frequently occurs in the bone and can remain inactive for extended periods after primary tumor treatment. Understanding the intricate molecular mechanisms that govern this phenomenon could open the door to novel therapies, potentially reducing mortality rates. To address this, we developed a novel protocol to induce cellular dormancy in PCa cell lines (RM1, 22Rv1, LAPC4) in vitro. We noted that dormant cells exhibited distinctive clustering and could be reactivated when exposed to serum even after 21 days of stasis. The dormancy program was validated by multiple assays including cell cycle analysis, membrane dye retention, and Edu+ incorporation. RNASeq analysis showed significant transcriptional changes during dormancy induction, with 8 genes commonly upregulated across multiple human and mouse PCa cell lines, of which the transcription factor, PRDM16. We focused on PRDM16 since, 1) it is a known regulator of hematopoietic stem cell quiescence, 2) our emerging data points to bone morphogenetic protein-7 (BMP7), a well-known exogenous mediator of dormancy as an inducer of PRDM16 expression and 3) clinically, PRDM16 is suppressed in primary PCa and active metastases. First, we validated the upregulation of PRDM16 during dormancy in vitro. Silencing PRDM16 led to impaired dormancy phenotype and cell death, while its overexpression significantly mitigated proliferation rates and promoted survival. In vivo, we employed an intra-iliac immunocompetent model (RM1 & C57BL6J) and demonstrated that overexpressing PRDM16 in active RM1 cells resulted in significant reduction of metastasis formation and Ki67 expression concurrent with extended survival when injected in vivo compared to control cells. Prostate cancer cells with PRDM16 over-expression were detected in bone sections as solitary or small clusters (&lt;20 cells) demonstrating an inverse relationship between PRDM16 expression and metastatic outbreak. In defining the targets via which PRDM16 controls dormancy entry, we focused on RAR related orphan receptor C (RORC) based on bioinformatic analysis and publicly available PRDM16 ChIP-Seq. Our data demonstrate that pharmacological inhibition of RORC using GSK805 promoted the apoptosis of dormant PCa cells specifically when compared to their actively growing counterparts. In conclusion, our studies have revealed the critical role of PRDM16 in promoting PCa cell dormancy, in part via upregulating RORC. Translationally, clinically approved RORC inhibitors for non-cancerous conditions (psoriasis) have been developed. We expect, based on our results, that these inhibitors could be repurposed for the eradication of dormant PCa cells in patients with advanced disease.
Citation Format: Mostafa M. Nasr, Tao Li, Ryan T. Bishop, Jeremy S. Frieling, Conor C. Lynch. PRDM16 is an intrinsic regulator of dormancy in bone disseminated prostate cancer cells [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(6_Suppl):Abstract nr 5539.