Free fatty acids induce lipid accumulation, autophagy and apoptosis in human sebocytes

Background: A disruption of sebocyte differentiation and lipogenesis has fatal consequences and can cause a wide spectrum of skin diseases, from acne vulgaris to sebaceous carcinoma, however, the relevant molecular mechanisms have not been fully clarified. Objectives: The induction of autophagy and apoptosis in human sebocytes in response to biologically relevant fatty acids was investigated. Methods: Free fatty acids (arachidonic acid, linoleic acid, palmitic acid and palmitoleic acid) and the pan-caspase inhibitor QVD-Oph were added in the supernatant of cultured human SZ95 sebocytes. Individual relevant proteins were analysed by Western blotting. Apoptosis and cell viability were determined, and typical autophagy structures were detected through electron microscopy. To obtain cell growth curves, cell confluence was continuously monitored by real-time cell analysis. Results: Fatty acids induced the development of intracellular lipid droplets with subsequent apoptosis, whereas arachidonic acid caused the most rapid effect. Cleavage products of caspase-3 were only detected in arachidonic acid-induced apoptosis. The high basal apoptotic rate of cultured SZ95 sebocytes was strongly suppressed by QVD-Oph. Fatty acid-induced apoptosis was also markedly inhibited by QVD-Oph, whereas intracellular lipid droplets further accumulated. While cell viability after incubation with linoleic acid, palmitic acid or palmitoleic acid and QVD-Oph was comparable with non-treated controls, arachidonic acid significantly reduced cell viability and cell density despite the concomitant pan-caspase inhibitor treatment. Using electron microscopy, typical autophagy structures were detected, such as autophagosomes and autolysosomes, at the basal level, which became more pronounced after treatment with fatty acids. Conclusions: Our findings contribute to a better understanding of the inflammation-associated mechanisms of lipogenesis and cell death induction in human sebocytes and may help to unveil the effects of fatty acid-rich human nutrition.