AMP-activated protein kinase and coordination of hepatic fatty acid metabolism of starved/carbohydrate-refed rats.

Acute increases in the concentration of malonyl-CoA play a pivotal role in mediating the decrease in fatty acid oxidation that occurs in many tissues during refeeding after a fast. In this study, we assess whether such increases in malonyl-CoA in liver could be mediated by malonyl-CoA decarboxylase (MCD), as well as acetyl-CoA carboxylase (ACC). In addition, we examine how changes in the activity of ACC, MCD, and other enzymes that govern fatty acid and glycerolipid synthesis relate temporally to alterations in the activities of the fuel-sensing enzyme AMP-activated protein kinase (AMPK). Rats starved for 48 h and refed a carbohydrate chow diet for 1, 3, 12, and 24 h were studied. Refeeding caused a 40% decrease in the activity of the alpha1-isoform of AMPK within 1 h, with additional decreases in AMPKalpha1 activity and a decrease in AMPKalpha2 occurring between 1 and 24 h. At 1 h, the decrease in AMPK activity was associated with an eightfold increase in the activity of the alpha1-isoform of ACC and a 30% decrease in the activity of MCD, two enzymes thought to be regulated by AMPK. Also, the concentration of malonyl-CoA was increased by 50%. Between 1 and 3 h of refeeding, additional increases in the activity of ACC and decreases in MCD were observed, as was a further twofold increase in malonyl-CoA. Increases in the activity (60%) and abundance (12-fold) of fatty acid synthase occurred predominantly between 3 and 24 h and increases in the activity of mitochondrial sn-glycerol-3-phosphate acyltransferase (GPAT) and acyl-CoA:diaclyglycerol acyltransferase (DGAT) at 12 and 24 h. The results strongly suggest that early changes in the activity of MCD, as well as ACC, contribute to the increase in hepatic malonyl-CoA in the starved-refed rat. They also suggest that the changes in these enzymes, and later occurring increases in enzymes regulating fatty acid and glycerolipid synthesis, could be coordinated by AMPK.

Coordinate regulation of malonyl-CoA decarboxylase, sn-glycerol-3-phosphate acyltransferase, and acetyl-CoA carboxylase by AMP-activated protein kinase in rat tissues in response to exercise.

UNLABELLED: Changes in the concentration of malonyl-CoA in many tissues have been related to alterations in the activity of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in its formation. In contrast, little is known about the physiological role of malonyl-CoA decarboxylase (MCD), an enzyme responsible for malonyl-CoA catabolism. In this study, we examined the effects of voluntary exercise on MCD activity in rat liver, skeletal muscle, and adipose tissue. In addition, the activity of sn-glycerol-3-phosphate acyltransferase (GPAT), which like MCD and ACC can be regulated by AMP-activated protein kinase (AMPK), was assayed. Thirty min after the completion of a treadmill run, MCD activity was increased approximately 2-fold, malonyl-CoA levels were reduced, and ACC and GPAT activities were diminished by 50% in muscle and liver. These events appeared to be mediated via activation of AMPK since: 1) AMPK activity was concurrently increased by exercise in both tissues; 2) similar findings were observed after the injection of 5-amino 4 imidazole carboxamide, an AMPK activator; 3) changes in the activity of GPAT and ACC paralleled that of MCD; and 4) the increase in MCD activity in muscle was reversed in vitro by incubating immunoprecipitated enzyme from the exercised muscle with protein phosphatase 2A, and it was reproduced by incubating immunopurified MCD from resting muscle with purified AMPK. An unexpected finding was that exercise caused similar changes in the activities of ACC, MCD, GPAT, and AMPK and the concentration of malonyl-CoA in adipose tissue. IN CONCLUSION: MCD, GPAT, and ACC are coordinately regulated by AMPK in liver and adipose tissue in response to exercise, and except for GPAT, also in muscle. The results suggest that AMPK activation plays a major role in regulating lipid metabolism in many cells following exercise. They also suggest that in each of them, it acts to increase fatty acid oxidation and decrease its esterification.