The Ca2+-Binding Capacity of Epidermal Furin Is Disrupted by H2O2-Mediated Oxidation in Vitiligo

Abstract

The Ca2+-dependent precursor convertase furin is abundantly expressed in epidermal keratinocytes and melanocytes. In this context, it is noteworthy that proopiomelanocortin (POMC) cleavage is also processed by furin, leading to ACTH, β-lipotropin, and β-endorphin. All prohormone convertases including furin are regulated by Ca2+. Because numerous epidermal peptides and enzymes are affected by H2O2-mediated oxidation, including the POMC-derived peptides α-MSH and β-endorphin as shown in the epidermis of patients with vitiligo, we here asked the question of whether furin could also be a possible target for this oxidation mechanism by using immunofluorescence, RT-PCR, Western blotting, Ca2+-binding studies, and computer modeling. Our results demonstrate significantly decreased in situ immunoreactivity of furin in the epidermis of patients with progressive vitiligo (n = 10), suggesting H2O2-mediated oxidation. This was confirmed by 45Ca2+-binding studies with human recombinant furin identifying the loss of one Ca2+-binding site from the enzyme after oxidation with H2O2. Computer simulation supported alteration of one of the two Ca2+-binding sites on furin. Taken together, our results implicate that the Ca2+-dependent proteolytic activity of this convertase is targeted by H2O2, which in turn could contribute to the reduced epidermal expression of the POMC-derived peptides α-MSH and β-endorphin as documented earlier in patients with vitiligo.