Nitric oxide: a new player in the modulation of energy metabolism.

Nitric oxide (NO) is a key messenger molecule in several cell types. NO formation is catalyzed by a family of NO synthases (NOS) that use L-arginine as a substrate. Rat adipose tissue expresses the inducible, macrophage-type, nitric oxide (NO) synthase isoform (iNOS). Systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) markedly increases the expression and activity of iNOS in both white and brown adipose tissues, as well as in skeletal muscle. iNOS induction can be reproduced in vitro by treatment of cultured white or brown adipocytes or L6 myocytes with LPS and inflammatory cytokines (TNFalpha, IFNgamma). The physiological role of NO in adipose tissues and skeletal muscle is still obscure. Recent evidence suggests that NO may be implicated in the regulation of energy metabolism. Using both pharmacological and genetic models of iNOS invalidation, we have recently begun to uncover a role for NO in the modulation of glucose transport and lipoprotein hydrolysis. These studies support the emerging concept that NO may fulfill the dual role of modulating energy metabolism in both physiological and pathological conditions as well as contributing to local immune defense during inflammatory processes.