Activation of retinoic acid receptor reduces metastatic prostate cancer bone lesions through blocking endothelial-to-osteoblast transition

Metastatic prostate cancer (PCa) in bone induces bone-forming lesions that contribute to progression and therapy resistance. Currently strategies targeting PCa-induced bone formation are lacking. We previously showed that PCa-induced bone originates from endothelial cells (EC) that have undergone endothelial-to-osteoblast (EC-to-OSB) transition in response to tumor-secreted BMP4. Here, we show that activation of retinoic acid receptor (RAR) inhibits EC-to-OSB transition and reduces PCa-induced bone formation. We found that palovarotene, a RAR{gamma} agonist being tested for heterotopic ossification in fibrodysplasia ossificans progressiva, inhibited EC-to-OSB transition and osteoblast mineralization in vitro, and decreased tumor-induced bone formation and tumor growth in several osteogenic PCa models. RAR/{beta}/{gamma} isoform knockdown in 2H11 ECs blocked EC-to-OSB transition and osteoblast mineralization. Pan-RAR agonist ATRA inhibited MycCaP-BMP4-induced bone formation and tumor growth under castration. Furthermore, palovarotene or ATRA reduced plasma Tenascin C, a factor secreted by EC-OSB cells, which may be used to monitor treatment response. Mechanistically, BMP4-activated pSmad1 forms a complex with RAR in the nucleus of 2H11 cells. RAR activation by palovarotene or ATRA causes pSmad1 degradation by recruiting E3-ubiquitin ligase Smurf1 into the nuclear pSmad1/RAR{gamma} complex. Our findings suggest that palovarotene can be repurposed to target PCa-induced bone formation to improve clinical outcomes for bone metastasis.