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

Author summarySex hormones, like testosterone, are steroids required for development and maintenance of reproductive organs, like the prostate, and for growth of prostate cancer in its early stages. However, when cancer patients are given treatments that block production of these steroids, some resistant cells survive and grow, inevitably producing an untreatable and lethal form of the disease. Frequently, these resistant cells still require the steroid receptor, but unlike in normal cells, this receptor works in the absence of hormone. Loss of the human tumour suppressor gene, Retinoblastoma, is often critical in driving this process. Here, we study a prostate-like cell in the fruit fly, in which we have discovered that a steroid receptor can normally drive growth in the absence of hormone after flies have mated. We find that this switch to steroid-independence is controlled by the fly equivalent of Retinoblastoma. We also identify ways to suppress this switch by targeting other genes involved in prostate cancer. Our work therefore suggests the switch to hormone-independent prostate cancer growth in cancer patients may reflect the mis-regulation of a normal physiological process found in flies and humans, and provides clues for the development of treatments that might prevent emergence of human therapy-resistant tumours. In prostate cancer, loss of the tumour suppressor gene, Retinoblastoma (Rb), and consequent activation of transcription factor E2F1 typically occurs at a late-stage of tumour progression. It appears to regulate a switch to an androgen-independent form of cancer, castration-resistant prostate cancer (CRPC), which frequently still requires androgen receptor (AR) signalling. We have previously shown that upon mating, binucleate secondary cells (SCs) of the Drosophila melanogaster male accessory gland (AG), which share some similarities with prostate epithelial cells, switch their growth regulation from a steroid-dependent to a steroid-independent form of Ecdysone Receptor (EcR) control. This physiological change induces genome endoreplication and allows SCs to rapidly replenish their secretory compartments, even when ecdysone levels are low because the male has not previously been exposed to females. Here, we test whether the Drosophila Rb homologue, Rbf, and E2F1 regulate this switch. Surprisingly, we find that excess Rbf activity reversibly suppresses binucleation in adult SCs. We also demonstrate that Rbf, E2F1 and the cell cycle regulators, Cyclin D (CycD) and Cyclin E (CycE), are key regulators of mating-dependent SC endoreplication, as well as SC growth in both 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 endoreplication-associated growth in SCs, mirroring changes seen in CRPC. Furthermore, Bone Morphogenetic Protein (BMP) signalling, mediated by the BMP ligand Decapentaplegic (Dpp), intersects with CycD/Rbf/E2F1 signalling to drive endoreplication in these fly cells. Overall, our work reveals a signalling switch, which permits rapid growth of SCs and increased secretion after mating, independently of previous exposure to females. The changes observed share mechanistic parallels with the pathological switch to hormone-independent AR signalling seen in CRPC, suggesting that the latter may reflect the dysregulation of a currently unidentified physiological process.

Automated summary

Sex hormones, like testosterone, play a crucial role in the development and growth of reproductive organs and prostate cancer. However, treatments that block the production of these hormones can result in the emergence of hormone-independent and lethal forms of the disease. In this study, researchers discovered that a fruit fly's steroid receptor can drive growth in the absence of hormone after mating, and this switch to steroid-independence is regulated by a gene equivalent to Retinoblastoma. Furthermore, they identified potential targets for suppressing this switch and preventing therapy-resistant tumors in humans.