Androgen receptor splice variants drive castration-resistant prostate cancer metastasis by activating distinct transcriptional programs.

One critical mechanism through which prostate cancer (PCa) adapts to treatments targeting androgen receptor (AR) signaling is the emergence of ligand-binding domain-truncated and constitutively active AR splice variants, particularly AR-V7. While AR-V7 has been intensively studied, its ability to activate distinct biological functions compared with the full-length AR (AR-FL), and its role in regulating the metastatic progression of castration-resistant PCa (CRPC), remain unclear. Our study found that, under castrated conditions, AR-V7 strongly induced osteoblastic bone lesions, a response not observed with AR-FL overexpression. Through combined ChIP-seq, ATAC-seq, and RNA-seq analyses, we demonstrated that AR-V7 uniquely accesses the androgen-responsive elements in compact chromatin regions, activating a distinct transcription program. This program was highly enriched for genes involved in epithelial-mesenchymal transition and metastasis. Notably, we discovered that SOX9, a critical metastasis driver gene, was a direct target and downstream effector of AR-V7. Its protein expression was dramatically upregulated in AR-V7-induced bone lesions. Moreover, we found that Ser81 phosphorylation enhanced AR-V7's pro-metastasis function by selectively altering its specific transcription program. Blocking this phosphorylation with CDK9 inhibitors impaired the AR-V7-mediated metastasis program. Overall, our study has provided molecular insights into the role of AR splice variants in driving the metastatic progression of CRPC.

Mithramycin induces promoter reprogramming and differentiation of rhabdoid tumor.

Rhabdoid tumor (RT) is a pediatric cancer characterized by the inactivation of SMARCB1, a subunit of the SWI/SNF chromatin remodeling complex. Although this deletion is the known oncogenic driver, there are limited effective therapeutic options for these patients. Here we use unbiased screening of cell line panels to identify a heightened sensitivity of rhabdoid tumor to mithramycin and the second-generation analogue EC8042. The sensitivity of MMA and EC8042 was superior to traditional DNA damaging agents and linked to the causative mutation of the tumor, SMARCB1 deletion. Mithramycin blocks SMARCB1-deficient SWI/SNF activity and displaces the complex from chromatin to cause an increase in H3K27me3. This triggers chromatin remodeling and enrichment of H3K27ac at chromHMM-defined promoters to restore cellular differentiation. These effects occurred at concentrations not associated with DNA damage and were not due to global chromatin remodeling or widespread gene expression changes. Importantly, a single 3-day infusion of EC8042 caused dramatic regressions of RT xenografts, recapitulated the increase in H3K27me3, and cellular differentiation described in vitro to completely cure three out of eight mice.

Bone loss from Wnt inhibition mitigated by concurrent alendronate therapy.

Dysregulated Wnt signaling is associated with the pathogenesis of cancers, fibrosis, and vascular diseases. Inhibition of Wnt signaling has shown efficacy in various pre-clinical models of these disorders. One of the key challenges in developing targeted anti-cancer drugs is to balance efficacy with on-target toxicity. Given the crucial role Wnts play in the differentiation of osteoblasts and osteoclasts, acute inhibition of Wnt signaling is likely to affect bone homeostasis. In this study, we evaluated the skeletal effect of small molecule inhibitor of an o-acyl transferase porcupine (PORCN) that prevents Wnt signaling by blocking the secretion of all Wnts. Micro-computed tomography and histomorphometric evaluation revealed that the bones of mice treated with two structurally distinct PORCN inhibitors LGK974 and ETC-1922159 (ETC-159) had loss-of-bone volume and density within 4 weeks of exposure. This decreased bone mass was associated with a significant increase in adipocytes within the bone marrow. Notably, simultaneous administration of a clinically approved anti-resorptive, alendronate, a member of the bisphosphonate family, mitigated loss-of-bone mass seen upon ETC-159 treatment by regulating activity of osteoclasts and blocking accumulation of bone marrow adipocytes. Our results support the addition of bone protective agents when treating patients with PORCN inhibitors. Mitigation of bone toxicity can extend the therapeutic utility of Wnt pathway inhibitors.

LRP1 Suppresses Bone Resorption in Mice by Inhibiting the RANKL-Stimulated NF-κB and p38 Pathways During Osteoclastogenesis.

Single-nucleotide polymorphisms in the LRP1 gene coding sequence are associated with low bone mass, and cell culture studies suggest that LRP1 plays a role in osteoblast proliferation and osteoblast-mediated osteoclastogenesis. However, the in vivo function of LRP1 in bone homeostasis has not been explored. In this work, we studied the osteoclast-specific role of LRP1 in bone homeostasis using a Ctsk-Cre;Lrp1f/f mouse model on the C57BL/6J background. These mice had a dramatically decreased trabecular bone mass with markedly more osteoclasts, while the osteoblast activity was unaffected or slightly increased. The cortical bone parameters were largely unaltered. Upon RANKL treatment, Lrp1-deficient bone marrow monocytes more efficiently differentiated into osteoclasts and showed elevated p65 NFκB and p38 signaling. Consistently, Lrp1-overexpressing Raw264.7 cells were desensitized to RANKL-induced p38 and p65 activation and osteoclastogenesis. Moreover, RANKL treatment led to a sharp decrease of LRP1 protein and RNA in BMMs. Overall, our data suggest that osteoclast-expressed LRP1 is a crucial regulator of bone mass. It inhibits the NFκB and p38 pathways and lessens the efficiency of RANKL-induced osteoclastogenesis. © 2018 American Society for Bone and Mineral Research.