The effect of thioredoxin on the expression of proopiomelanocortin-derived peptides, the melanocortin 1 receptor and cell survival of normal human keratinocytes.

An important constituent of the cellular antioxidant buffering system that controls the redox state of proteins is thioredoxin (TRX), a 13 kDa protein that catalyzes thiol-disulfide exchange reactions, regulates activation of transcription factors, and possesses several other biologic functions similar to cytokines. We have previously reported that TRX released from UVB-irradiated keratinocytes stimulates melanogenesis by upregulating MSH receptor expression and its binding activity in melanocytes. The purpose of this study was to examine the effects of TRX on keratinocytes as an autocrine factor. TRX suppressed the UVB-induced production and secretion of alpha-melanocyte stimulating hormone (alpha-MSH) and of adrenocorticotropic hormone (ACTH), and also suppressed proopiomelanocortin (POMC) mRNA expression by normal human keratinocytes; however, TRX upregulated melanocortin 1 receptor (MC1-R) expression synergistically with UVB in normal human keratinocytes. These results suggest that exogenous TRX regulates expression of those genes in different manners. Furthermore, addition of an antibody against TRX induced cell death in keratinocytes, probably due to enhanced signaling of MSH, as it has been shown that MSH suppresses heat shock protein (hsp) 70 expression in differentiated keratinocytes, which express high levels of MC1-R and decreases their survival rate during oxidative stress. Taken together, the results suggest that keratinocyte-derived TRX regulates the expression of stress inducible neuropeptides and their receptor, and is critically involved in the survival of keratinocytes.