Abstract 2900: BRAFi-induced epigenetic switch drives the reprogramming of resistant cancer-associated fibroblasts
Authors: Xiao Yao , Zhou Linli , Zhang Yuhang 
Published: March 2024

Abstract
Cancer-associated fibroblasts (CAFs) are a potential target for optimizing therapeutic strategies against melanoma. However, a major unknown still exists in CAFs, which is how CAFs maintain and even reinforce their functions in drug therapy and influence melanoma cell drug sensitivity. Yes-associated protein 1 (YAP1) is the master regulator that shuttles between the cytoplasm and nucleus in CAFs. Apparently, proteins associated with nuclear YAP1 in CAFs when treated with BRAF inhibitor (BRAFi) can potentially influence tumor drug response. Here we unveil that CAFs require the YAP1 function to proliferate, migrate, remodel the cytoskeletal machinery and matrix, and promote cancer cell invasion. Ablating YAP1 in CAFs increases the response of BRAF-mutant melanoma cells to BRAFi and MEK inhibitor (MEKi) in vivo and in vitro. Using an approach termed RIME (Rapid Immunoprecipitation Mass Spectrometry), we purified nuclear YAP1 and identified protein arginine methyl transferase 1 (PRMT1) as a major YAP1 binding partner in BRAFi-treated CAFs. Binding PRMT1 to YAP1 was confirmed by Co-immunoprecipitation and proximity ligation assay, and YAP1 deficiency in CAFs diminishes the nuclear accumulation of PRMT1, suggesting BRAFi-induced nuclear translocation of PRMT1 in CAFs requires the formation of YAP1 and PRMT1 complex. Silencing PRMT1 expression in CAFs not only deprives them of their ability to proliferate and to induce BRAF mutant melanoma cell resistance to BRAFi and MEKi. PRMT1 is a type I R-methyltransferase that mediates arginine methylation in human cells, which was known to be recruited to chromatin and exert its enzymatic activity to methylate
Arg3 of histone H4 (H4R3). Using Histone H4 Modification Multiple Assay, we observed a global increase in Histone H4 methylation, especially H4R3 methylation, in CAFs in response to BRAFi stimulation, suggesting increased chromatin unfolding and accessibility and transcription activation. When YAP1 or PRMT1 expression was silenced, H4R3 methylation was notably suppressed, indicating BRAFi-induced H4R3 methylation was indeed driven by the YAP1 and PRMT1 signaling axis. In conclusion, our data disclose a novel BRAFi-induced YAP1-PRMT1-mediated epigenetic mechanism that reprograms CAFs through H4R3 methylation-directed transcriptional activation to reinforce their drug-resistant functions and modulate melanoma cell drug sensitivity. Moreover, the discovery of PRMT1 as a key player in the intricate molecular mechanisms that govern epigenetic regulation in CAFs in response to BRAFi treatment suggest PRMT1 as a potential CAF target in cancer therapy.
Citation Format: Yao Xiao, Linli Zhou, Yuhang Zhang. BRAFi-induced epigenetic switch drives the reprogramming of resistant cancer-associated fibroblasts [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 2900.