Homeostasis of the sebaceous gland and mechanisms of acne pathogenesis

Sebaceous glands (SGs) are appendages of mammalian skin that produce a mixture of lipids known as sebum. Acne vulgaris is an exceptionally common skin condition, characterized by elevated sebum production, altered sebum composition, and the formation of infundibular cysts, called comedones. Comedo-associated SGs are atrophic, suggesting that comedo formation involves abnormal differentiation of progenitor cells that generate the SG and infundibulum: the 'comedo switch'. Understanding the biological processes that govern SG homeostasis promises to highlight potential aetiological mechanisms underlying acne and other SG-associated skin disorders.In this review, we discuss the clinical data, genetic mouse models and in vitro research that have highlighted major hormones, paracrine factors, transcription factors and signalling pathways that control SG homeostasis. These include, but are not limited to androgens, progestogens and oestrogens; retinoids; receptor tyrosine kinases such as ErbB family receptors, fibroblast growth factor receptor 2 and insulin/insulin-like growth factor 1 receptors; peroxisome proliferator-activated receptor γ; aryl hydrocarbon receptor; and the Wnt signalling pathway. Where possible, the cellular and molecular mechanisms by which these regulatory factors control SG biology are indicated, along with considerations as to how they might contribute to acne pathogenesis.Future research should seek to establish the relative importance, and causative relationships, of altered sebum production, sebum composition, inflammation and abnormal differentiation of sebaceous progenitors to the process of comedo formation in acne. Such an understanding will allow for therapeutic targeting of regulatory factors that control SG homeostasis, with the aim of treating acne.