Heme Oxygenase-2 Is a Critical Determinant for Execution of an Acute Inflammatory and Reparative Response

Abstract

Heme oxygenase (HO) represents an intrinsic antiinflammatory system based on its ability to regulate leukocyte function and inhibit expression of proinflammatory cytokines. This anti-inflammatory function is linked to the inducible isoform HO-1; the role of the constitutive isoform HO-2 is unknown. The current study was undertaken to investigate the role of HO-2 in the regulation of the acute inflammatory and reparative response by using HO-2-null mice and well-established animal models of epithelial injury and antigen-induced peritonitis. Here we show that in vivo deletion of HO-2 disables execution of the acute inflammatory and reparative response after epithelial injury and leads to an exaggerated inflammatory response in antigen-induced peritonitis. HO-2 deletion was associated with impaired HO-1 induction, indicating that HO-2 is critical for HO-1 expression and that the subsequent failure to up-regulate the HO system may contribute to unresolved inflammation and the development of chronic inflammatory conditions. Indeed, supplementation with the HO bioactive product, biliverdin, rescued the acute inflammatory and reparative response in HO-2-null mice. Thus, HO-2 sets in place a basal tone of anti-inflammatory signals that may be a prerequisite for the ordered execution of an inflammatory and reparative response.

Exacerbated corneal inflammation and neovascularization in the HO-2 null mice is ameliorated by biliverdin

“…Levels of HO-1 mRNA increased by 10-and 40-fold at days 2 and 4 after suture placement, respectively, and decreased by day 7 to a level that was still 8-fold higher than the control level in unsutured cornea ( Figure 6A and B). On the other hand, in the HO-2 null corneas the basal (unsutured) levels of HO-1 mRNA were higher than those of wild type corneas ( Figure 6A), which is in agreement with the observed positive HO-1 immunostaining in HO-2 null corneas ( Figure 1B) and with previous reportings of a compensatory increase in HO-1 expression in the HO-2 null mice (Ding et al, 2006;Seta et al, 2006). Interestingly, as clearly seen in Figure 6B, suture insertion did not significantly induce a further increase in HO-1 mRNA levels.…”

“…Since upregulation of HO-1 in response to suture placement was impaired in HO-2 null mice, it is reasonable to assume that the HO-2 null mice display lower levels of HO activity as was previously demonstrated (Seta et al, 2006). Hence, replenishing the corneas with HO-derived products may rescue the exaggerated inflammatory and neovascular phenotype seen in the HO-2 null corneas.…”

“…We have previously documented that infiltrating macrophages are also a source for the increased HO-1 expression in the injured cornea (Seta et al, 2006). To this end, induction of HO-1 expression in macrophages has been implicated as an important step for the resolution phase of inflammation (Willis et al, 1996).…”

“…In a recent study, we showed that HO-2 displays a prominent constitutive expression in the cornea and that deletion of HO-2 gene markedly impairs the inflammatory and reparative response of the cornea to epithelial injury (Seta et al, 2006). Hence, HO-2 deficiency led to unresolved corneal inflammation and chronic inflammatory complications including ulceration, perforation and neovascularization.…”

“…Measurement of MPO activity was used to quantify polymorphonuclear cells (PMNs) in dissected corneas as previously described (Seta et al, 2006). In brief, tissues were homogenized in potassium phosphate buffer (pH 6.0) containing 0.5% hexadecyltrimethylammonium bromide, followed by three cycles of sonication and freeze-thaw.…”