Hemopexin decreases hemin accumulation and catabolism by neural cells

Abstract

Hemopexin is a serum, CSF, and neuronal protein that is protective after experimental stroke. Its efficacy in the latter has been linked to increased expression and activity of heme oxygenase (HO)-1, suggesting that it facilitates heme degradation and subsequent release of cytoprotective biliverdin and carbon monoxide. In this study, the effect of hemopexin on the rate of hemin breakdown by CNS cells was investigated in established in vitro models. Equimolar hemopexin decreased hemin breakdown, as assessed by gas chromatography, by 60–75% in primary cultures of murine neurons and glia. Extracellular hemopexin reduced cell accumulation of 55Fe-hemin by over 90%, while increasing hemin export or extraction from membranes by four-fold. This was associated with significant reduction in HO-1 expression and neuroprotection. In a cell-free system, hemin breakdown by recombinant HO-1 was reduced over 80% by hemopexin; in contrast, albumin and two other heme-binding proteins had no effect. Although hemopexin was detected on immunoblots of cortical lysates from adult mice, hemopexin knockout per se did not alter HO activity in cortical cells treated with hemin. These results demonstrate that hemopexin decreases the accumulation and catabolism of exogenous hemin by neural cells. Its beneficial effect in stroke models is unlikely to be mediated by increased production of cytoprotective heme breakdown products.

Hemopexin induces neuroprotection in the rat subjected to focal cerebral ischemia

“…This study found that HPX-null mice were more vulnerable to oxidative stress during cerebral ischemia injury than their wild-type counterparts, an observation which was consistent with a recent study in cultured neuronal cells, which showed that HPX decreased heme accumulation and catabolism [24]. Together, these evidences indicate that HPX might offer neuroprotective effects following ischemia-reperfusion injury.…”

“…Taken together, these data provide novel insights that support the clinical utility of HPX in attenuating ischemia-reperfusion injury in CNS. Accumulated evidences [10,17,18,24,29] have proven the protective effect of HPX on different organs and cells. Our current findings are consistent with these studies and provide further evidence for the cytoprotective function of HPX.…”

Protection against Heme Toxicity: Hemopexin Rules, OK?

“…Given the rapid accumulation of heme by cells, the reduction in cellular heme and HO1 induction after addition of HPX is more consistent with the extracellular antioxidant role of HPX to sequester heme, thus limiting unregulated uptake via channels or diffusion, rather than a lack of transport. In the studies of Chen-Roetling et al, 81 HPX removed heme from the membrane fraction but not cytosol, which is consistent with surface heme aggregates. HPX is known to remove heme from membranes.…”

“…14 In spite of the data for intact rats with doubly labeled heme-HPX complexes, 43,65 and publications that demonstrate that heme-HPX induces HO1, 8,[77][78][79] the heme transport function of HPX has been disputed. 80,81 In both cases, a similar experimental design was used: HPX was added to the serum-free medium of cells being cultured with high concentrations of heme (50-100 µM). Given the rapid accumulation of heme by cells, the reduction in cellular heme and HO1 induction after addition of HPX is more consistent with the extracellular antioxidant role of HPX to sequester heme, thus limiting unregulated uptake via channels or diffusion, rather than a lack of transport.…”

Targeting Heme Oxygenase after Intracerebral Hemorrhage

“…After injection, hemin binds with extraordinary affinity to hemopexin [43] , or to albumin when hemopexin binding sites are saturated [44] . In vitro, both complexes are capable of inducing HO-1 at a somewhat reduced rate than hemin per se and with markedly less toxicity [45, 46] . Systemically administered hemin has little or no access to the CNS parenchyma, but induces HO-1 in perivascular cells [47] .…”