Purpose: The standard treatment for organ-confined prostate cancer (PC) is surgery or radiation, and locally advanced PC is typically treated with radiotherapy alone or in combination with androgen deprivation therapy. Here, we investigated whether Stat5a/b participates in regulation of double strand DNA break repair in PC, and whether Stat5 inhibition may provide a novel strategy to sensitize PC to radiation therapy. Experimental Design: Stat5a/b regulation of DNA repair in PC was evaluated by comet and clonogenic survival assays, followed by assays specific to Homologous Recombination (HR) DNA repair and Non-Homologous End-Joining (NHEJ) DNA repair. For HR DNA repair, Stat5a/b regulation of Rad51 and the mechanisms underlying the regulation were investigated in PC cells, xenograft tumors and patient-derived PCs ex vivo in 3D explant cultures. Stat5a/b induction of Rad51 and HR DNA repair and responsiveness to radiation were evaluated in vivo in mice bearing PC xenograft tumors. Results: Stat5a/b is critical for Rad51 expression in PC via Jak2-dependent mechanisms by inducing Rad51 mRNA levels. Consistent with this, genetic knockdown of Stat5a/b suppressed HR DNA repair while not affecting NHEJ DNA repair. Pharmacological Stat5a/b inhibition potently sensitized PC cell lines and PC tumors to radiation, while not inducing radiation sensitivity in the neighboring tissues. Conclusions: This work introduces a novel concept of a pivotal role of Jak2-Stat5a/b signaling for Rad51 expression and HR DNA repair in PC. Inhibition of Jak2-Stat5a/b signaling sensitizes PC to radiation and, therefore, may provide an adjuvant therapy for radiation to reduce radiation-induced damage to the neighboring tissues.
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