MKP-1 switches arginine metabolism from nitric oxide synthase to arginase following endotoxin challenge

L-Arginine (L-arg) is metabolized to nitric oxide (NO) by inducible NO synthase (iNOS) or to urea and L-ornithine (L-orn) by arginase. NO is involved in the inflammatory response, whereas arginase is the first step in polyamine and proline synthesis necessary for tissue repair and wound healing. Mitogen-activated protein kinases (MAPK) mediate LPS-induced iNOS expression, and MAPK phosphatase-1 (MKP-1) plays a crucial role in limiting MAPK signaling in macrophages. We hypothesized that MKP-1, by attenuating iNOS expression, acts as a switch changing L-arg metabolism from NO production to L-orn production after endotoxin administration. To test this hypothesis, we performed studies in RAW264.7 macrophages stably transfected with an MKP-1 expression vector in thioglyollate-elicited peritoneal macrophages harvested from wild-type and Mkp-1(-1-) mice, as well as in vivo in wild-type and Mkp-1(-1-) mice. We found that overexpression of MKP-1 resulted in lower iNOS expression and NO production but greater urea production in response to LPS. Although deficiency of MKP-1 resulted in greater iNOS expression and NO production and lower urea production in response to LPS, neither the overexpression nor the deficiency of MKP-1 had any substantial effect on the expression of the arginases.