Abstract
Uric acid produced by guanine deaminase (GDA) is involved in photoaging and hyperpigmentation. Reactive oxygen species (ROS) generated by uric acid plays a role in photoaging. However, the mechanism by which uric acid stimulates melanogenesis in GDA-overexpressing keratinocytes is unclear. Keratinocyte-derived paracrine factors have been identified as important mechanisms of ultraviolet-induced melanogenesis. Therefore, the role of paracrine melanogenic growth factors in GDA-induced hypermelanosis mediated by uric acid was examined. The relationships between ROS and these growth factors were examined. Primary cultured normal keratinocytes overexpressed with wild type or mutant GDA and those treated with xanthine or uric acid in the presence or absence of allopurinol, H2O2, or N-acetylcysteine (NAC) were used in this study. Intracellular and extracellular bFGF and SCF levels were increased in keratinocytes by wild type, but not by loss-of-function mutants of GDA overexpression. Culture supernatants from GDA-overexpressing keratinocytes stimulated melanogenesis, which was restored by anti-bFGF and anti-SCF antibodies. Allopurinol treatment reduced the expression levels of bFGF and SCF in both GDA-overexpressing and normal keratinocytes exposed to exogenous xanthine; the exogenous uric acid increased their expression levels. H2O2-stimulated tyrosinase expression and melanogenesis were restored by NAC pretreatment. However, H2O2 or NAC did not upregulate or downregulate bFGF or SCF, respectively. Overall, uric acid could be involved in melanogenesis induced by GDA overexpression in keratinocytes via bFGF and SCF upregulation not via ROS generation.