PU.1 is a mediator of the reactive stromal response associated with castration-resistant prostate cancer.

Emerging evidence demonstrates the pivotal role played by the tumor microenvironment, particularly cancer-associated fibroblasts, during the development of castration-resistant prostate cancer. In this study, the molecular composition of the tumor microenvironment of androgen-sensitive and castration-resistant metastatic prostate cancer is investigated by utilizing patient-derived xenograft models. Transcriptomic and histological analysis identify the presence of a pro-fibrotic stroma in the castration-resistant (LAPC9) versus the castration-sensitive (PNPCa, BM18) models, characterized by high levels of collagen and tenascin C deposition and upregulation of inflammatory markers. Intra-tumoral collagen- and tenascin-positive stromal areas specifically correlate with higher tumor invasiveness. Master regulator analysis identifies the transcription factor PU.1 as a mediator of the LAPC9 pro-fibrotic phenotype, whose transcriptional activity can be specifically inhibited by the small molecule DB1976. To test the effect of pharmacologic PU.1 inhibition a novel organoid-fibroblast 3D co-culture system, able to mimic features of the in vivo tumor microenvironment, is established. Inhibition of stromal PU.1 activity reverts the pro-fibrotic phenotype and, in turn, reduces tumor organoid growth. Besides identifying a novel molecular player of the pro-fibrotic stromal phenotype in prostate cancer, this study highlights the applicability of 3D tumor organoid-fibroblast co-cultures as in vitro tools to test the effect of stromal-targeting compounds.