NO signaling through cGMP in renal tissue fibrosis and beyond: key pathway and novel therapeutic target.

Nitric oxide (NO) produced by endothelial NO synthase (NOS) in low concentrations is a unique messenger molecule with key homeostatic functions concerning the prevention of pathological vascular and tissue changes such as increases in blood pressure, platelet degranulation, mononuclear cell infiltration, cell proliferation and extracellular matrix protein accumulation. This is in contrast to high levels of NO derived from inducible NOS which act as detrimental effector molecules and free radicals in immune response. Deficiency in NO's protective signaling actions is a major characteristic in numerous experimental and human disease situations. The main function of the NO signaling pathway is activation of the soluble guanylate cyclase (sGC) enzyme with subsequent generation of cyclic guanosine monophosphate (cGMP) as a second messenger and downstream mediator. In the past, attempts to overcome deficiency in endothelial NO effects were focused primarily on increasing the supply with the NO precursor L-arginine or on the use of directly NO-releasing compounds. The clinical impact of these strategies, however, was rather limited. Recent state-of-the-art studies have revealed that NO signaling is highly regulated at the transcriptional level and that deficiency in NO signaling correlates closely with pathological changes. In parallel efforts, novel pharmacological compounds which specifically enhance NO/cGMP signaling have been developed and have demonstrated remarkable efficacy in experimental disease settings. In this review, we summarize the current state of knowledge on the impairment of NO/cGMP signaling and about its pharmacological stimulation. In the first part, experimental renal fibrosis, i.e. the tandem rat model of acute anti-thy1 glomerulonephritis and progressive anti-thy1 renal fibrosis will serve as a paradigm for introducing this new and exciting field. In the second part, we will address the most recent findings on NO signaling in non-renal diseases. Together, these results point out that deficiency in NO/cGMP is a common key pathway as well as a novel therapeutic target in a number of diseases.