Autophagy is a controlled mechanism of intracellular self-digestion with functions in metabolic adaptation to stress, in development, in proteostasis and in maintaining cellular homeostasis in ageing. Deletion of autophagy in epidermal keratinocytes does not prevent the formation of a functional epidermis and the permeability barrier but causes increased susceptibility to damage stress and metabolic alterations and accelerated ageing phenotypes. We here investigated how epidermal autophagy deficiency using Keratin 14 driven Atg7 deletion would affect the lipid composition of the epidermis of young and old mice. Using mass spectrometric lipidomics we found a reduction of age-related accumulation of storage lipids in the epidermis of autophagy-deficient mice, and specific changes in chain length and saturation of fatty acids in several lipid classes. Transcriptomics and immunostaining suggest that these changes are accompanied by changes in expression and localisation of lipid and fatty acid transporter proteins, most notably fatty acid binding protein 5 (FABP5) in autophagy knockouts. Thus, maintaining autophagic activity at an advanced age may be necessary to maintain epidermal lipid homeostasis in mammals.

Background/Objectives: Sebocytes, the primary cell type in sebaceous glands (SGs), produce a lipid mixture called sebum that is released onto the skin surface and is required for skin homeostasis. The lipid receptor Peroxisome Proliferator-Activated Receptor gamma (PPARγ) regulates sebocyte proliferation and lipid synthesis and is involved in acne development. As inhibition of PPARγ has been shown to reduce insulin-induced lipogenesis and Akt/mTOR signalling in SZ95 sebocytes, we here investigated the effects of PPARγ deletion on lipid homeostasis and autophagic stress responses and how the secretomes affect dermal fibroblasts. Methods: SZ95 sebocytes wildtype (WT) and PPARγ knockout (KO) were shifted to low serum and EGF-deficient conditions permissive for autophagy. Untargeted and targeted HPLC-MS/MS analyses were used to analyze native and oxidized lipids, respectively. Protein levels of LC3I/II and p62 were assessed using immunoblots and immunofluorescence microscopy to investigate the autophagic flux. Dermal fibroblasts were exposed to conditioned media. Results: In low serum culture media, KO SZ95 sebocytes displayed significantly altered levels of 23 lipid classes. We observed a significant increase in ether-linked fatty acids as components of complex lipids and detected elevated levels of phospholipid hydroperoxides and aldehydolipids in the KO sebocytes. KO SZ95 sebocytes failed to show the typical responses to lipoxidative stress, such as elevated p62 crosslinking or inclusion body formation, and had reduced LC3II/I ratios as compared to WT cells. PPARγ KO conditioned media promoted a trend towards an inflammatory fibroblast phenotype. Conclusions: These findings suggest that PPARγ in sebocytes may alter the lipidome, elevate redox stress, and affect the autophagic machinery, which could cause accumulation of oxidized lipids and other potentially harmful compounds in sebocytes.