Fasting boosts breast cancer therapy via glucocorticoid activation.

The vast majority of all breast cancers are driven by oestrogen receptor alpha (ERα) activation, and endocrine therapy represents the mainstay treatment for these patients. However, resistance is common and tumours progress despite years of systemic endocrine suppression. Periodic fasting enhances the efficacy of standard endocrine drugs and delay acquired resistance to them, although the underlying mechanisms remain unclear. Here, we show that fasting, in combination with endocrine therapy, induces extensive epigenetic reprogramming in hormone receptor-positive (HR+) breast cancer xenografts, with large-scale activation of glucocorticoid receptor (GR) and progesterone receptor (PR) signalling, and impairment of activator protein-1 (AP-1) family activity. GR-driven gene programs were selectively activated after fasting, and GR knockout perturbed the beneficial effects of fasting in combination with tamoxifen, in vivo. Exogenous administration of GR-ligands fully recapitulated the observed fasting-enhanced anti-oestrogen action, resulting in tumour regression. Finally, elevated progesterone and cortisol levels were detected in the blood of breast cancer patients after fasting, thus providing clinical validation of our animal findings. Our results indicate GR activation to play a pivotal role in fasting’s ability to enhance endocrine drug activity against hormone-receptor positive breast cancer and suggest that corticosteroid administration should be evaluated as an adjuvant to endocrine therapy in this condition.