Thyrotropin‐releasing hormone and oestrogen differentially regulate prolactin and prolactin receptor expression in female human skin and hair follicles in vitro

Background Human skin and scalp hair follicles are both a nonclassical target and an extrapituitary source of prolactin (PRL), which is a potent hair growth modulator. However, how the expression of PRL and PRL receptor (PRLR) is regulated in human skin is unknown. Objectives To investigate whether two key stimulators of pituitary PRL secretion, thyrotropin‐releasing hormone (TRH) and oestrogen, also regulate cutaneous PRL and PRLR expression. Methods Female scalp skin and/or microdissected hair follicles were treated for 6 days in serum‐free organ culture with oestrogen (100 nmol L^?1), TRH (1–10 ng mL^?1, 3–30 nm) or vehicle control. Quantitative immunohistomorphometry of skin and hair follicle sections was complemented with quantitative polymerase chain reaction for PRL and PRLR in cultured hair follicles and/or female human outer root sheath (ORS) keratinocytes. Results Oestrogen treatment significantly upregulated PRL and PRLR immunoreactivity in selected skin and hair follicle compartments, at the gene and protein level (P < 0·05). TRH significantly increased PRL immunoreactivity and transcription in hair follicles (P < 0·05); however, while it also increased PRLR transcription in hair follicles, it downregulated PRLR immunoreactivity in the hair follicle ORS (P < 0·05). Conclusions Our pilot study shows that two key endocrine controls of pituitary PRL secretion, oestrogen and TRH, also regulate PRL and PRLR expression in human skin. This provides novel insights into the regulation of extrapituitary PRL and PRLR expression, and invites exploration of oestrogen and TRH as novel therapeutic agents in the management of skin and hair diseases characterized by aberrant PRLR‐mediated signalling.