Antifibrotic role of inducible nitric oxide synthase

Long-term treatment in rats with l-NAME, an isoform-non-specific inhibitor of nitric oxide synthase (NOS), leads to fibrosis of the heart and kidney, suggesting that nitric oxide (NO) may play a role in preventing tissue fibrosis. In this process, a likely target of NO is the quenching of reactive oxygen species (ROS) through peroxynitrite formation, and one possible source for this NO is inducible NOS (iNOS). Using Peyronie's disease (PD) tissue from both human specimens and from a rat model of PD as the source of fibrotic tissue, we investigated if NO derived from iNOS could act as such an antifibrogenic defense mechanism by determining whether: (a) tunical ROS and iNOS are increased in PD; and (b) the long-term inhibition of iNOS activity decreases the NO/ROS balance in the tunica albuginea thereby promoting collagen deposition. It was determined that in the human PD plaque, iNOS mRNA and protein, ROS, collagen, and the peroxynitrite marker, nitrotyrosine, were all increased in comparison to the normal tunica. In the rat model of PD, the fibrotic plaque also showed significant increases in iNOS mRNA and protein, nitrotyrosine, ROS as measured by heme oxygenase-1, and collagen when compared with the normal control tunica. When a selective inhibitor of iNOS, L-NIL, was given to rats with the PD-like plaque, this resulted in a decrease in nitrotyrosine levels but intensified ROS levels and collagen deposition. These data demonstrate that: (a) iNOS induction occurs in both the human and rat PD fibrotic plaque; and (b) that the NO derived from iNOS appears to counteract ROS formation and collagen deposition. Because the inhibition of iNOS activity leads to a decrease in the NO/ROS ratio, thereby favoring the development of fibrosis, it is proposed that iNOS induction in this tissue may be a protective mechanism against fibrosis and abnormal wound healing.