The TRPM5 Antagonist Triphenylphosphine Oxide Increases Sebaceous Lipogenesis and Modulates Immune Phenotype of Human Sebocytes in a TRPM5-Independent Manner

Transient receptor potential melastatin 5 (TRPM5) ion channel is expressed in human hair follicles, where its spontaneous activity contributes to the maintenance of the growing, anagen phase of the hair cycle. Because adjacent sebaceous glands also exhibited TRPM5 immunopositivity, topically applied TRPM5 modulators administered to influence hair growth may also affect sebaceous glands. Hence, we aimed to assess expression of TRPM5 as well as effects of TRPM5 modulators [activators: 2,5-dimethylpyrazine, 2-heptanone; antagonist: triphenylphosphine oxide (TPPO)] on human SZ95 sebocytes, i.e., on the best available in vitro model to study human sebaceous glands. First, using complementary methods [RNA-Seq, RT-qPCR, western blot, siRNA-mediated gene silencing and fluorescent Na(+)- (SBFI AM) and Ca(2+)-measurements (Fura-2 AM)], we found that TRPM5 is not expressed in human sebocytes in a functionally active form. Importantly, while non-cytotoxic (MTT-assay) concentrations of the activators were ineffective, TPPO promoted sebaceous lipogenesis (Nile Red labelling). This effect was TRPM5-independent and was found to be mediated in an Akt- and epidermal growth factor receptor (EGFR)-dependent manner, most likely via the Akt-induced up-regulation of diacylglycerol O-acyltransferase (DGAT)-2. Moreover, TPPO up-regulated interleukin (IL)-6 in an EGFR- and p38α MAPK-dependent manner (RT-qPCR), whereas it decreased the release of IL-8 (ELISA), and down-regulated additional pro-inflammatory cytokines [chemokine (C-X-C motif) ligand [CXCL]-1, CXCL2, CXCL6, colony-stimulating factor 2, IL-32; RNA-Seq]. Collectively, specific TRPM5 modulators are unlikely to exert direct sebaceous gland-related side effects, while safe TPPO analogues may induce beneficial moderate lipogenic and anti-inflammatory effects in dry skin dermatoses.