In humans and animal models, increased intramuscular
lipid (IML) stores have been implicated in
insulin resistance.
Malonyl-CoA plays a critical role in cellular lipid metabolism both by serving as a precursor in the synthesis of
lipids and by inhibiting
lipid oxidation. In muscle,
Malonyl-CoA acts primarily as a negative allosteric regulator of
carnitine palmitoyl transferase-1 (CPT1) activity, thereby blocking the transport of long chain fatty acyl CoAs into the mitochondria for oxidation. In muscle, increased
malonyl-CoA, decreased muscle CPT1 activity, and increased IML have all been reported in
obesity. In order to determine whether
malonyl-CoA synthesis might be under transcriptional as well as biochemical regulation, we measured
mRNA content of several key genes that contribute to the cellular metabolism of
malonyl-CoA in muscle biopsies from lean to morbidly obese subjects. Employing quantitative real-time PCR, we determined that expression of mitochondrial
acetyl-CoA carboxylase 2 (ACC2) was increased by 50% with
obesity (P < 0.05). In both lean and obese subjects, expression of mitochondrial ACC2 was 20-fold greater than that of cytoplasmic ACC1, consistent with their hypothesized roles in synthesizing
malonyl-CoA from
acetyl-CoA for CPT1 regulation and lipogenesis, respectively. In addition, in both lean and obese subjects, expression of
malonyl-CoA decarboxylase was approximately 40-fold greater than
fatty acid synthase, consistent with degradation, rather than lipogenesis, being the primary fate of
malonyl-CoA in human muscle. No other genes showed signs of increased
mRNA content with
obesity, suggesting that there may be selective transcriptional regulation of
malonyl-CoA metabolism in human
obesity.