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

Background: Expression and enzymatic activity of heme oxygenase (HO) has be en implicated in the development, as well as in the resolution, of inflamma tory conditions. Because inflammation is central to tissue repair, we inves tigated the presence and potential functions of HO in an excisional model o f normal and diabetes-impaired wound repair in mice. Materials and Methods: Expression of HO-1 during cutaneous healing was anal yzed by RNase protection assay, Western blot, and immunohistochemical techn iques in a murine model of excisional repair. Furthermore, we determined HO -1-dependent release of proinflammatory cytokines from RAW 264.7 macrophage s 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 st ages of diabetes-impaired repair were associated with elevated HO-1 express ion. Besides a few keratinocytes of the hyperpro-liferative epithelium, imm unohistochemistry revealed infiltrating macrophages as the predominant and major source of HO-1 at the wound site. in vitro studies demonstrated the p otency of exogenous and also endogenous nitric oxide (NO) to strongly induc e HO-1 expression in RAW 264.7 macrophages. However, L-NIL-mediated enzymat ic 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 enzym atic activity by tin protoporphyrin IX (SnPPIX) in RAW 264.7 macrophages ma rkedly attenuated tumor necrosis factor-alpha (TNF-alpha), but strongly inc reased interleukin-1 beta (IL-1 beta) release in RAW 264.7 macrophages in v itro. 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.