Anti-androgens induce Rab11a-exosome secretion in prostate cancer by suppressing amino acid-sensitive PAT4-mTORC1 signalling

Abstract

Advanced prostate cancer is typically treated with anti-androgens to reduce cancer growth, but patients almost inevitably develop treatment resistance and castration-resistant disease. Recently, extracellular vesicles known as exosomes, which are secreted from the endosomal compartments in which they are formed, have been implicated in drug resistance mechanisms. Here we investigate whether growth regulation by the amino acid-dependent kinase complex, mechanistic Target of Rapamycin Complex 1 (mTORC1), and associated extracellular vesicle secretion might be involved in the adaptive responses to anti-androgens. We show that expression and intracellular localisation of the glutamine-sensing PAT4 (SLC36A4) amino acid transporter is increased in malignant versus benign prostatic tissue, mirroring earlier in vivo fly studies suggesting that these transporters are more effective at promoting growth from internal versus cell surface membranes. Furthermore, androgens induce PAT4 expression in prostate cancer cell lines and PAT4 is required for a proportion of androgen-stimulated mTORC1 activation and growth. Consistent with previous studies in other cancer cell lines, we find that glutamine depletion, PAT4 knockdown and mTORC1 inhibition all independently increase the production of a specific exosome subtype, Rab11a-exosomes, which has recently been implicated in pro-tumorigenic signalling responses to mTORC1 inhibition. Furthermore, we show that these exosomes are also induced by anti-androgens. We hypothesise that the uptake of Rab11a-exosomes by cells with higher PAT4 levels could provide a growth-promoting boost, enabling them to out-compete others with lower PAT4 expression, resulting in tumours that are more resistant to nutrient-deprivation and anti-androgen treatment.