Involvement of cutaneous SR-B1 in skin lipid homeostasis

Background: The main functions of the skin are to protect against environmental insults and prevent water loss, which are performed by the complex lipid- and protein matrix present in the outermost layers of the epithelium. The lipidome of these outer layers is mainly composed of ceramides, fatty acids, and cholesterol, which regulates keratinocyte differentiation and skin barrier function. SR-B1 is a multifunctional scavenger receptor that is best known for facilitating uptake of cholesterol from HDL particles in the liver, but it is also expressed in the skin. Objective: To determine the role of SR-B1 in keratinocyte differentiation. Methods: We investigated the relationship between SR-B1 and keratinocyte differentiation using a physiologically relevant model, organotypic skin equivalents (SEs), wherein SR-B1 was knocked down via siRNA transfection. To assess effects of SR-B1 knockdown on keratinocyte differentiation, we performed hematoxylin/eosin staining, RT-PCR, western blotting, and immunohistochemistry. We also examined the effect of SR-B1 knockdown on lipid production by performing Oil Red O staining and thin layer chromatography. Results: SR-B1 knockdown resulted in decreased lipid levels in SEs, specifically ceramides, and in decreased transcript levels of LDLR, PPAR-α and PPAR-γ which are factors involved in regulating ceramide synthesis. In addition, filaggrin levels increased in SR-B1 KD tissues, but neither keratin 14 nor keratin 10 were affected. Conclusion: We conclude that one of the main functions of SR-B1 in the skin is to regulate ceramide levels and thereby maintain the barrier function of the skin, resulting in the protection of cutaneous tissues from outdoor insults.