Oxidized low density lipoprotein inhibits inducible nitric oxide synthase,GTP cyclohydrolase I and transforming growth factor beta gene expression in rat macrophages

Several studies have already demonstrated that oxidized- LDL decreases nitr ic oxide (NO) generation by cytokine-stimulated macrophages. However, the m echanisms of such an inhibition have not been yet elucidated. NO generation by inducible nitric oxide synthase (iNOS) is dependent on the presence of cofactors for NO generation, tetrathydrobiopterin (BH4) among them. The NO generation by these cells is also regulated by some endogenous inhibitors, like TGF-beta. Therefore, the aim of our recent study was to investigate th e influence of ox-LDL on the expression of iNOS and GTP cyclohydrolase I(GT P-CH I), the key enzyme involved in the BH, synthesis as well as the ox-LDL effect on TGF-beta expression in rat macrophages stimulated with IFN gamma (250 U/ml) and LPS (500 ng/ml). Macrophages, activated in this way, expres s iNOS, GTP-CH I, and TGF-beta mRNA. This expression was inhibited when the macrophages were preincubated for 24 hours with ox-LDL (100 mu g/ml). Quan titative PCR revealed about 10-fold inhibition of iNOS gene expression by o x-LDL. As a consequence of down-regulation of iNOS and GTP-CH I genes, almo st 3-fold diminished generation of NO2- by rat macrophages was observed. An inhibition of the TGF beta mRNA expression was also found. Our studies indicate that decreased NO generation by ox-LDL treated macroph ages may be the result of the diminished expression of both iNOS and GTP-CH I genes. This effect may be mediated by the activity of certain endogenous inhibitors of gene expression, however, our studies exclude the TGF-beta a s a candidate for this activity.