Super-induction of HO-1 in macrophages stimulated with lipopolysaccharide by prior depletion of glutathione decreases iNOS expression and NO production.

In the LPS-stimulated macrophages undergoing oxidative burst, intracellular storage of glutathione (GSH) is depleted, expression of iNOS is enhanced, and NO is overproduced. In response to the depletion of GSH, expression of HO-1 is induced and HO activity is elevated. Thus, in macrophages treated with LPS, productions of NO and CO, catalyzed, respectively, by accumulated iNOS and HO-1, are increased in sequence [Biochem. Pharmacol. 68 (2004) 1709]. In support of this, HO-1 is induced in macrophages treated only with buthionine sulfoximine (BSO), an inhibitor of GSH biosynthesis depleting the GSH level. Alternatively, when the macrophages were exposed to spermine NONOate, an exogenous NO-donor, HO-1, was induced also. When the GSH-depleted or BSO-pretreated macrophages were exposed to NO, delivered either exogenously from spermine NONOate or endogenously from LPS-derived elevation of iNOS, super-induction of HO-1 was observed. Moreover, both the BSO and LPS treatments increased the accumulation of HO-1 inducing redox-sensitive transcription factor Nrf2 in the nuclear protein fraction. Thus, when the depletion of GSH is combined with NO delivery, expression of HO-1 is enhanced to a greater extent than that enhanced either by GSH depletion or by NO delivery. In these macrophages with super-induced HO-1 and elevated HO activity, LPS-derived increase in iNOS expression was down-regulated and NO production was suppressed. This indicated that induction of HO-1 caused by the NO overproduced from up-regulated iNOS, in turn, produces a causative inhibition on iNOS expression and NO production. Thus, it appears that there is a reciprocal cross-talk between inductions of HO-1 and iNOS in macrophages stimulated with LPS leading to their survival.