The majority of breast cancers are driven by oestrogen receptor-α (ERα) activation, and endocrine therapy represents the mainstay treatment for these patients1. However, resistance is common and tumours often progress after years of endocrine suppression2. Periodic fasting enhances the efficacy of standard endocrine therapy and delays acquired drug resistance, although the underlying mechanisms remain unclear3. Here we show that fasting induces extensive epigenetic reprogramming in ERα-positive breast cancer xenografts when combined with endocrine therapy, with large-scale activation of glucocorticoid receptor (GR) and progesterone receptor signalling and concomitant reduction in the activity of activator protein-1 (AP-1) family members. GR-driven gene programmes are selectively activated in in vivo models of ERα-positive breast cancer during fasting, and GR knockout hinders the anti-tumour effects of fasting combined with tamoxifen. Exogenous administration of GR ligands recapitulates fasting-enhanced anti-oestrogen action, thus promoting tumour regression. Patients undergoing a cyclic fasting-mimicking diet exhibited increased blood progesterone and cortisol concentrations. Additionally, tumours collected after the fasting-mimicking diet showed an inverse correlation of GR activation with proliferation markers, providing clinical confirmation of our observations in animal models. Our results indicate that GR activation has a pivotal role in the ability of fasting to enhance endocrine therapy activity in breast cancer and suggest that corticosteroid administration should be evaluated as an adjuvant to endocrine therapy in this setting.
