Prolactin and endocrine therapy resistance in breast cancer: The next potential hope for breast cancer treatment.

Breast cancer, a hormone-dependent tumour, generally includes four molecular subtypes (luminal A, luminal B, HER2 enriched and triple-negative) based on oestrogen receptor, progesterone receptor and human epidermal growth factor receptor-2. Multiple hormones in the body regulate the development of breast cancer. Endocrine therapy is one of the primary treatments for hormone-receptor-positive breast cancer, but endocrine resistance is the primary clinical cause of treatment failure. Prolactin (PRL) is a protein hormone secreted by the pituitary gland, mainly promoting mammary gland growth, stimulating and maintaining lactation. Previous studies suggest that high PRL levels can increase the risk of invasive breast cancer in women. The expression levels of PRL and PRLR in breast cancer cells and breast cancer tissues are elevated in most ER+ and ER- tumours. PRL activates downstream signalling pathways and affects endocrine therapy resistance by combining with prolactin receptor (PRLR). In this review, we illustrated and summarized the correlations between endocrine therapy resistance in breast cancer and PRL, as well as the pathophysiological mechanisms and clinical practices. The study on PRL and its receptor would help explore reversing endocrine therapy-resistance for breast cancer.