AUTOCRINE REGULATION OF INDUCIBLE NITRIC-OXIDE SYNTHASE IN MACROPHAGES BY ATRIAL-NATRIURETIC-PEPTIDE

Atrial natriuretic peptide (ANP), a cardiovascular hormone, has been s hown to inhibit synthesis of nitric oxide in lipopolysaccharide (LPS)- activated mouse bone marrow-derived macrophages via activation of its guanylate cyclase-coupled receptor. The goal of the present study was to elucidate the potential sites of inducible nitric-oxide synthase (i NOS) regulation affected by ANP and revealed the following. 1) ANP and dibutyryl-cGMP did not inhibit catalytic iNOS activity measured by th e conversion rate of L-[H-3]arginine to L-[H-3]citrulline in homogenat es of LPS-treated cells. 2) Pretreatment of cells with ANP dose-depend ently reduced the LPS-induced L-[H-3]citrulline production that has be en shown to be due to reduced iNOS protein levels detected by Western blot. 3) ANP does not alter the ratio of catalytically active iNOS dim er versus inactive iNOS monomer considered to be a major post-translat ional regulatory mechanism for the enzyme. 4) Macrophages exposed to A NP display decreased LPS-induced iNOS mRNA levels. 5) Importantly, two basic mechanisms seem to be responsible for this observation, i.e. AN P specifically induced acceleration of iNOS mRNA decay and ANP reduced binding activity of NF-kappa B, the transcription factor predominantl y responsible for LPS-induced iNOS expression in murine macrophages. M oreover, 6) ANP acts via an autocrine mechanism since recently ANP was shown to be secreted by LPS-activated macrophages, and we demonstrate d here that LPS-induced NO synthesis was increased after blocking the binding of endogenous ANP by a receptor antagonist. These observations suggest ANP as a new autocrine macrophage factor regulating NO synthe sis both transcriptionally and post-transcriptionally. ANP may help to balance NO production of activated macrophages and thus may allow suc cessful immune response without adverse effects on host cells.