NITRIC-OXIDE SYNTHASE INHIBITION BY HEME OXYGENASE DECREASES MACROPHAGE NITRIC-OXIDE-DEPENDENT CYTOTOXICITY - A NEGATIVE FEEDBACK MECHANISMFOR THE REGULATION OF NITRIC-OXIDE PRODUCTION

Nitric oxide (NO) production in macrophages by inducible nitric oxide synthase (NOS2) has multiple tissue damaging effects and is involved i n the pathogenesis of inflammation and graft rejection. Haem oxygenase (HmOx) is the enzyme which degrades haem. Its inducible isoform, HmOx 1, was recently shown to increase cellular resistance against oxidativ e stress and to decrease inflammation and graft rejection. Since haem is an essential cofactor for NOS2 activity, we investigated the effect s of HmOx1-induction upon NO secretion in macrophages. We induced HmOx 1 in BALB/c bone-marrow-derived macrophages by short-term exposure to haemin (20 mu mol/l, 30 min); then we incubated them for 24 h to allow maximal expression of HmOx1 activity. Next, we activated the macropha ges with lipopolysaccharide (LPS) and measured their NO production and their NO-dependent cytotoxicity against P815 cells. We found that HmO x induction 24 h before LPS activation in mouse macrophages suppresses their production of NO, while HmOx inhibition (with zinc protoporphyr in) increases NO secretion. NOS2 inhibition is reflected by the decrea se of macrophage NO-dependent cytotoxicity against the P815 targets. W e therefore propose that HmOx1 is a physiological inhibitor of NOS2 in activated macrophages because it decreases haem availability for NOS2 synthesis. NOS2 inhibition may explain the antinflammatory effects of HmOx induction which could also be used therapeutically in situations when NO hyperproduction leads to cytotoxic effects such as inflammati on or transplant rejection.