Macrophage-derived heme-oxygenase-1: expression, regulation, and possible functions in skin repair.

BACKGROUND: Expression and enzymatic activity of heme oxygenase (HO) has been implicated in the development, as well as in the resolution, of inflammatory conditions. Because inflammation is central to tissue repair, we investigated the presence and potential functions of HO in an excisional model of normal and diabetes-impaired wound repair in mice. MATERIALS AND METHODS: Expression of HO-1 during cutaneous healing was analyzed by RNase protection assay, Western blot, and immunohistochemical techniques in a murine model of excisional repair. Furthermore, we determined HO-1-dependent release of proinflammatory cytokines from RAW 264.7 macrophages by enzyme-linked immunosorbent assay (ELISA). RESULTS: Upon injury, we observed a rapid and strong increase in HO-1 mRNA and protein levels at the wound site. By contrast to normal repair, late stages of diabetes-impaired repair were associated with elevated HO-1 expression. Besides a few keratinocytes of the hyperproliferative epithelium, immunohistochemistry revealed infiltrating macrophages as the predominant and major source of HO-1 at the wound site. In vitro studies demonstrated the potency of exogenous and also endogenous nitric oxide (NO) to strongly induce HO-1 expression in RAW 264.7 macrophages. However, L-NIL-mediated enzymatic inhibition of inducible NO-synthase (iNOS) at the wound site in vivo was not paralleled by decreased HO-1 levels. In vitro inhibition of HO-1 enzymatic activity by tin protoporphyrin IX (SnPPIX) in RAW 264.7 macrophages markedly attenuated tumor necrosis factor-alpha (TNF-alpha), but strongly increased interleukin-1beta (IL-1beta) release in RAW 264.7 macrophages in vitro. CONCLUSIONS: The observed injury-mediated increase in HO-1 mRNA and protein at the wound site was due to infiltrating HO-1 expressing monocytic cells. Macrophage-derived HO-1 expression was not under regulatory control by NO in skin repair. We provide evidence that HO-1 might exert a regulatory role in macrophage-derived cytokine release.

Dexamethasone inhibits TNF-alpha-induced apoptosis and IAP protein downregulation in MCF-7 cells.

Exposure of human mammary carcinoma cell line MCF-7 to TNF-alpha leads to apoptotic cell death within 24 h. In search for apoptosis-preventing signals, we identified glucocorticoids as potent death-preventing compounds. Ten nM dexamethasone provided a significant protective effect whereas 100 nM dexamethasone roughly blocked 80 - 90% of TNF-alpha-induced apoptosis. Surprisingly, dexamethasone exerted a protective effect even when supplied several hours after TNF-alpha. This points to a powerful inhibition of even advanced apoptotic processes by dexamethasone. To further pinpoint the anti-apoptotic glucocorticoid action, we investigated the expression levels of several members of the inhibitors of apoptosis (IAPs) family of proteins in response to TNF-alpha and dexamethasone. IAP proteins directly block caspase protease activities including caspase-3, caspase-7, and caspase-9. Exposure of MCF-7 cells to TNF caused an extensive downregulation of cIAP1, cIAP2, and XIAP protein levels. The decline of the IAP protein levels temporally paralleled the appearance of apoptotic DNA fragments which started 12 - 14 h following TNF-alpha addition and maximal effects were seen within 24 h. Coincubation of cells with TNF-alpha and dexamethasone potently blocked cIAP1, cIAP2, and XIAP downregulation. TNF-alpha-mediated IAP protein downregulation was not affected by proteasome inhibitors like lactacystin, ALLN or ALLM, whereas it was blocked by the broad-spectrum caspase inhibitor Z-VAD-fmk which also prevented TNF-alpha-induced apoptotic cell death. These data suggest that inhibition of IAP downregulation mediated by a caspase proteolytic activity constitutes the anti-apoptotic action of glucocorticoids in MCF-7 carcinoma cells.

Regulation of interleukin-18 (IL-18) expression in keratinocytes (HaCaT): implications for early wound healing.

Keratinocytes display a high basal level expression of IL-18. Tumor necrosis factor-alpha (TNF-alpha) mediated a large decrease in IL-18 mRNA levels in the human keratinocyte cell line HaCaT, which was accompanied by a subsequent accumulation of IL-18 protein in the cell culture supernatants, which was shown to be biologically active. By contrast, epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha), respectively, strongly decreased IL-18 mRNA expression in HaCaT keratinocytes in the absence of IL-18 protein release from the cells. Notably, a pre-treatment of the cells with EGF, or TGF-alpha clearly attenuated TNF-alpha-induced IL-18 protein, release and bioactivity. For the in vivo situation of cutaneous wound repair, we observed an increase in IL-18 protein, 10 hours post-wounding, that closely correlated to infiltration of neutrophils which are known as producers of TNF-alpha. Our data suggest that bioactive IL-18 might be tightly counter-regulated by platelet- and neutrophil-derived factors at the onset of repair.