mTOR inhibition by rapamycin increases ceramide synthesis by promoting transforming growth factor‐β1/Smad signaling in the skin

Although mammalian target of rapamycin (mTOR) mediates a wide variety of biological functions, little information is available on the effect of mTOR on the functions of skin cells. In this study, we investigated effects of mTOR inhibition by rapamycin on ceramide synthesis in the skin of rats and human keratinocytes and its regulatory mechanisms. The phosphorylation of p70 S6 kinase, which indicates mTOR activation, was induced in the skin of rats fed a high‐fat diet, but this abnormality was reversed by supplementation with rapamycin. Ceramide levels and the mRNA levels of serine palmitoyltransferase (SPT) and transforming growth factor (TGF)‐β1 were suppressed in the skin of rats fed high‐fat diets, but this abnormality was reversed by supplementation with rapamycin. TGF‐β1‐induced SPT mRNA expression was blocked by SB525334, an inhibitor of TGF‐β1‐induced Smad2/3 nuclear localization, in human keratinocytes. Rapamycin‐induced SPT mRNA expression was blocked by an anti‐TGF‐β1 antibody or SB525334 in human keratinocytes. These results show that mTOR inhibition by rapamycin increases ceramide synthesis by promoting TGF‐β1/Smad signaling in the skin.