Acetyl-CoA carboxylase regulation of fatty acid oxidation in the heart.

The role of acetyl-coenzyme A carboxylase (ACC) in regulating fatty acid oxidation was investigated in isolated fatty acid perfused working rat hearts. Overall fatty acid oxidation rates were determined by addition of 1.2 mM 3Hpalmitate to the perfusate of hearts in which the endogenous triglyceride pool was prelabeled with 14Cpalmitate. Rates of both exogenous and endogenous fatty acid oxidation were measured by simultaneous measurement of 3H2O and 14CO2 production, respectively. A second series of hearts were perfused under similar conditions except that U-14Cglucose was present in the perfusate for measurement of glucose oxidation rates. Addition of dichloroacetate (DCA, 1 mM) to the perfusate resulted in a dramatic stimulation of glucose oxidation (a 411% increase), with a parallel decrease in fatty acid oxidation (from 305 +/- 51 to 206 +/- 40 nmol/g dry weight.min.unit work). DCA treatment increased the contribution of glucose oxidation to ATP production from 7.1 to 30.6%, while decreasing the contribution of overall fatty acid oxidation from 92.9 to 69.4%. Tissue levels of malonyl-CoA in hearts treated with DCA were higher compared to controls (14.0 +/- 0.6 and 10.0 +/- 0.7 nmol/g dry weight, respectively) and were negatively correlated (r = -0.85) with overall fatty acid oxidation rates. Acetyl-CoA levels were also significantly higher in DCA-treated hearts, and a positive correlation (r = 0.88) was seen between myocardial acetyl-CoA and malonyl-CoA levels. This suggests that DCA treatment increased the supply of acetyl-CoA for ACC. Western blots revealed the presence of both the 280-kDa (ACC-280) and the 265-kDa (ACC-265) isoforms of ACC in cardiac tissue, with a predominance of ACC-280. The activity of ACC extracted from hearts was similar in both groups when assayed under optimal conditions of acetyl-CoA and citrate. However, using affinity purified ACC, it was demonstrated that heart ACC (predominantly ACC-280) had a higher Km for acetyl-CoA than ACC isolated from white adipose tissue (predominantly ACC-265). We conclude that ACC is an important regulator of fatty acid oxidation in the heart and that acetyl-CoA supply is a key determinant of heart ACC-280 activity. As acetyl-CoA levels increase, ACC-280 is activated resulting in an increase in malonyl-CoA inhibition of fatty acid oxidation.