Involvement of anti-inflammatory heme oxygenase-1 in the inhibitory effect of curcumin on the expression of pro-inflammatory inducible nitric oxide synthase in RAW264.7 macrophages.

Curcumin, at high concentrations (>2 microM), inhibits the production of nitric oxide (NO) and the expression of inducible NO synthase (iNOS) through inactivation of nuclear factor (NF)-kappaB and, at low concentrations, induces the expression of heme oxygenase (HO)-1 in macrophages. Here, we demonstrated that curcumin at low concentrations (0.5-2 microM) can also inhibit NO production and iNOS expression in lipopolysaccharide (LPS)-activated RAW264.7 macrophages only when the cells were pretreated for at least 6h with curcumin. Curcumin induced dose- and time-dependent HO-1 expression, and this was coincident with the inhibitory effects of low concentrations of curcumin on NO production and iNOS expression. Blockage of HO-1 activity or knockdown of HO-1 expression abolished the inhibitory effects of curcumin. Over-expression of HO-1 or exogenous addition of carbon monoxide, a byproduct derived from heme degradation, mimicked the inhibitory action of low concentrations of curcumin. Moreover, LPS-induced NF-kappaB was diminished in macrophages subjected to prolonged treatment with low concentrations of curcumin. Treatment with HO inhibitor abolished the inhibitory effect of curcumin on LPS-induced NF-kappaB activation. Collectively, we provide evidence to support the important role of HO-1 in inhibition of NO production and iNOS expression by curcumin even at low concentrations.