Neuronal Nitric Oxide Synthase (nNOS) Expression Is Reduced in Skeletal Muscle from db/db Mice and Glucose and Fatty Acids Show Differential Effects on nNOS Regulation and Myogenic Differentiation.

BACKGROUND

Neuronal nitric oxide synthase (nNOS, encoded by NOS1) is a key regulator of skeletal muscle mass. However, the mechanisms underlying nNOS dysregulation in diabetic skeletal muscle remain unclear.

OBJECTIVES

We examined the relative contributions of hyperglycemia and hyperlipidemia, the hallmarks of type 2 diabetes (T2D), to the regulation of nNOS in a mouse model and C2C12 myotubes.

METHODS

Glucose tolerance, blood parameters, muscle and fat mass, Nos1 DNA methylation levels, and mRNA/protein expression were evaluated in male db/db and C57BL/6J wild type control mice (n = 4/group, 12 wk old). C2C12 myotubes were treated with glucose (5, 25, or 60 mM) or fatty acids (0.05-0.75 mM), including palmitic acid (PA) and oleic acid (OA). Fusion index and mRNA/protein expression were evaluated. All data were analyzed using Student's t-test or analysis of variance.

RESULTS

db/db mice exhibited impaired glucose tolerance (3-fold higher, P < 0.05), hyperlipidemia (triglyceride levels, 2-fold higher, P < 0.01), and reduced muscle mass in plantaris muscle (46% lower, P < 0.01). nNOS protein expression decreased in plantaris muscle (29% lower, P < 0.01) and was positively correlated with muscle mass (r = 0.87, P < 0.01). Nos1μ expression was reduced in diabetic plantaris muscle (64% lower, P < 0.01), whereas Nos1 DNA methylation levels were unchanged. Among the nutritional factors tested, 0.75 mM PA, but not high glucose, reduced nNOS expression (41% lower, P < 0.05) and impaired myotube fusion in C2C12 myotubes (61% lower, P < 0.01). However, OA exhibited no detrimental effects. Importantly, high glucose did not downregulate nNOS expression. The combination treatment demonstrated that PA, but not glucose, was the primary determinant of nNOS downregulation (70% lower, P < 0.01).

CONCLUSIONS

Impaired nNOS signaling may link lipid overload, rather than hyperglycemia, to muscle atrophy in T2D and represents a potential therapeutic target for preserving skeletal muscle mass.