ER{alpha}-mediated nuclear sequestration of RSK2 is required for ER+ breast cancer tumorigenesis

Although ribosomal protein S6 kinase A3 (RSK) activation status positively correlates with patient responses to anti-estrogen hormonal therapies, the mechanistic basis for these observations is unknown. Using multiple in vitro and in vivo models of ER+ breast cancer, we report that ERα sequesters active RSK2 into the nucleus to promote neoplastic transformation and facilitate metastatic tumor growth. RSK2 physically interacted with ERα through its N-terminus to activate a pro-neoplastic transcriptional network critical to the ER+ lineage in the mammary gland, thereby providing a gene signature that effectively stratified patient tumors according to ERα status. ER+ tumor growth was strongly dependent on nuclear RSK2, and transgenic mice engineered to stably express nuclear RSK2 in the mammary gland developed high grade ductal carcinoma in situ. Mammary cells isolated from the transgenic model and introduced systemically successfully disseminated and established metastatic lesions. Anti-estrogens disrupted the interaction between RSK2 and ERα, driving RSK2 into the cytoplasm and impairing tumor formation. These findings establish RSK2 as an obligate participant of ERα-mediated transcriptional programs, tumorigenesis, and divergent patient responses to anti-estrogen therapies.

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