Coordinated induction of inducible nitric oxide synthase and GTP-cyclohydrolase I is dependent on inflammatory cytokines and interferon-γ in HaCaT keratinocytes:: Implications for the model of cutaneous wound repair

Recently we demonstrated a strong expression of inducible nitric oxide synthase (iNOS) and GTP-cyclohydrolase I (GTP-CH I) in the basal keratinocytes of the epidermis adjacent to the wound and of the hyperproliferative epithelium during wound healing. To identify possible mediators of iNOS and GTP-CH I expression during this process, we analyzed the regulation of iNOS and GTP-CH I expression in cultured human keratinocytes. We found a large and long lasting coinduction of iNOS and GTP-CH I expression upon simultaneous treatment of quiescent cells with inflammatory cytokines interleukin-1 beta, tumor necrosis factor-alpha, and interferon-gamma, but not with serum growth factors, The stimulatory effect of interleukin-1 beta, tumor necrosis factor-alpha, and interferon-gamma is strongly synergistic on iNOS and GTP-CH I expression, because these factors alone stimulated GTP-CH I expression, although to a much lesser extent. Furthermore, iNOS mRNA levels are not influenced at all by stimulation with IL-1 beta and revealed only a weak induction after treatment with tumor necrosis factor-alpha and interferon-gamma Induction of iNOS and GTP-CH I gene expression upon cytokine and interferon-gamma exposure is independent of de novo protein synthesis. Because these cytokines are present at the wound site, they might be responsible for iNOS and GTP-CH I induction during cutaneous repair. Serum, which is released upon hemorrhage, is likely to play no stimulatory role in iNOS and GTP-CH I induction during wound healing.