Rbf/E2F1 control growth and endoreplication via steroid-independent Ecdysone Receptor signalling in Drosophila prostate-like secondary cells

Dysregulation of cell cycle components results in the development and progression of several cancer types. Unusually, loss of the tumour suppressor gene, Retinoblastoma (Rb), and consequent activation of transcription factor E2F1 have been linked to late-stage tumour progression in prostate cancer, rather than early-stage events. This change is associated with an androgen-independent form of cancer, castration-resistant prostate cancer (CRPC), which frequently still requires androgen receptor (AR) signalling. We have previously shown that binucleate secondary cells (SCs) of the Drosophila melanogaster male accessory gland (AG) share several functional and signalling similarities with human prostate epithelial cells. Upon mating, SC growth regulation switches from a steroid-dependent to a steroid-independent form of Ecdysone Receptor (EcR) control that induces genome endoreplication. Here, we demonstrate that the Drosophila Rb homologue, Rbf, and E2F1, as well as cell cycle regulators, Cyclin D (CycD) and Cyclin E (CycE), are key mediators of SC growth and endoreplication both in virgin and mated males. Importantly, we show that the CycD/Rbf/E2F1 axis requires the EcR, but not ecdysone, to trigger CycE-dependent endoreplication and associated growth in SCs after mating, mirroring changes in CRPC. We also demonstrate that excess Rbf activity reversibly suppresses binucleation in adult SCs. Overall, our work reveals mechanistic parallels between the physiological switch to hormone-independent EcR signalling in SCs, and the pathological switch seen in CRPC, and suggests that the latter may represent the dysregulation of a currently unidentified physiological process, which permits AR signalling when androgen levels are low.