C-Reactive protein augments inducible nitric oxide synthase expression in cytokine-stimulated cardiac myocytes

Nitric oxide (NO) production by inducible NO synthase (iNOS) can exert negative inotropic and cytotoxic effects on cardiac myocytes and may play an important role in the pathogenesis of cardiac dysfunction and remodeling. An elevated serum level of C-reactive protein (CRP) is an important predictive factor for cardiac disorders including acute myocardial infarction and dilated cardiomyopathy. The basic mechanisms responsible for this association are not clear; CRP may merely be a marker of inflammation with no specific role in the pathogenesis of cardiac disease or may directly modulate the disease process. We investigated the effects of CRP on iNOS expression and subsequent NO synthesis in rat cardiac myocytes, and the mechanism by which CRP exerts its effects.

NO production by culture neonatal rat cardiac myocytes was determined by measurement of nitrite contents in the culture medium. The expression of iNOS mRNA and protein were measured by reverse transcription-polymerase chain reaction and Western blotting, respectively. The levels of nuclear factor (NF)-kappaB proteins were analyzed by a gel retardation assay.

Incubation of cardiac myocytes with interleukin-1beta (IL-1beta; 10 ng/ml) caused a significant increase in nitrite production. CRP significantly increased the IL-1beta-induced nitrite production in a dose-dependent manner (10-100 microg/ml). Incubation with IL-1beta induced the expression of iNOS mRNA and protein in cardiac myocytes, and CRP enhanced their expression. Addition of IL-1beta activated NF-kappaB in cardiac myocytes, while CRP did not affect IL-1beta-induced NF-kappaB activation.

These results indicated that CRP directly enhances NO synthesis in IL-1beta-stimulated cardiac myocytes through an NF-kappaB-independent mechanism.