Urea-induced inducible nitric oxide synthase inhibition and macrophage proliferation

Background. Atherosclerosis is a major cause of morbidity and mortality in chronic renal failure and is associated with the proliferation of macrophag es within atherosclerotic lesions. Methods. Because the progression of atherosclerosis as a consequence of dec reased nitric oxide (NO) synthesis has been described, we investigated the correlation between the inhibition of inducible NO synthase (iNOS) by urea, macrophage proliferation as assayed by cell counting, tritiated-thymidine incorporation and measurement of cell protein, and macrophage apoptosis. Results. Urea induces a dose-dependent inhibition of inducible NO synthesis in lipopolysaccharide-stimulated mouse macrophages (RAW 264.7) with concom itant macrophage proliferation. Macrophage proliferation as determined by c ell counting became statistically significant at 60 nmol/L urea correspondi ng to a blood urea nitrogen level of 180 mg/100 mL, concentrations seen in uremic patients. iNOS protein expression showed a dose-dependent reduction, as revealed by immunoblotting when cells were incubated with increasing am ounts of urea. The decrease of cytosolic DNA fragments in stimulated macrop hages incubated with urea shows that the proliferative actions of urea are associated with a decrease of diminished NO-mediated apoptosis. Conclusions. These data demonstrate that inhibition of iNOS-dependent NO pr oduction caused by urea enhances macrophage proliferation as a consequence of diminished NO-mediated apoptosis. This fact may be important for the dev elopment of atherosclerotic lesions during chronic renal failure and is in accordance with recently published studies showing that under conditions wi th decreased constitutive NOS activity, iNOS might substitute the synthesis of NO. iNOS expression in vascular smooth muscle cells and macrophages is supposed to prevent restenosis following angioplasty or heart transplant va sculopathy. This is supported by the fact that specific inhibition of endog enous iNOS activity with L-N-6-(1-iminoethly)-lysine accelerates the progre ssion of vasculopathy in transplantation atherosclerosis.