AMP-activated kinase reciprocally regulates triacylglycerol synthesis and fatty acid oxidation in liver and muscle: evidence that sn-glycerol-3-phosphate acyltransferase is a novel target

Abstract

AMP-activated kinase (AMPK) is activated in response to metabolic stresses that deplete cellular ATP, and in both liver and skeletal muscle, activated AMPK stimulates fatty acid oxidation. To determine whether AMPK might reciprocally regulate glycerolipid synthesis, we studied liver and skeletal-muscle lipid metabolism in the presence of 5-amino-4-imidazolecarboxamide (AICA) riboside, a cell-permeable compound whose phosphorylated metabolite activates AMPK. Adding AICA riboside to cultured rat hepatocytes for 3 h decreased [14C]oleate and [3H]glycerol incorporation into triacylglycerol (TAG) by 50% and 38% respectively, and decreased oleate labelling of diacylglycerol by 60%. In isolated mouse soleus, a highly oxidative muscle, incubation with AICA riboside for 90 min decreased [14C]oleate incorporation into TAG by 37% and increased 14CO2 production by 48%. When insulin was present, [14C]oleate oxidation was 49% lower and [14C]oleate incorporation into TAG was 62% higher than under basal conditions. AICA riboside blocked insulin's antioxidative and lipogenic effects, increasing fatty acid oxidation by 78% and decreasing labelled TAG 43%. Similar results on fatty acid oxidation and acylglycerol synthesis were observed in C2C12 myoblasts, and in differentiated C2C12 myotubes, AICA riboside also inhibited the hydrolysis of intracellular TAG. These data suggest that AICA riboside might inhibit sn-glycerol-3-phosphate acyltransferase (GPAT), which catalyses the committed step in the pathway of glycerolipid biosynthesis. Incubating rat hepatocytes with AICA riboside for both 15 and 30 min decreased mitochondrial GPAT activity 22-34% without affecting microsomal GPAT, diacylglycerol acyltransferase or acyl-CoA synthetase activities. Finally, purified recombinant AMPKalpha1 and AMPKalpha2 inhibited hepatic mitochondrial GPAT in a time-and ATP-dependent manner. These data show that AMPK reciprocally regulates acyl-CoA channelling towards beta-oxidation and away from glycerolipid biosynthesis, and provide strong evidence that AMPK phosphorylates and inhibits mitochondrial GPAT.

The malonyl-CoA–long-chain acyl-CoA axis in the maintenance of mammalian cell function

“…By contrast, insulin increases the sensitivity of mitochondrial CPTo to malonyl-CoA inhibition [109] and decreases transcription of the CPTo gene in H411E cells [109]. Moreover, the recent observation that AMPK may phosphorylate and inhibit mitochondrial GPAT [110] suggests that insulin may be able to co-ordinate the regulation of CPTo and mitochondrial GPAT activities acutely through its additional ability to activate AMPK phosphatase activity [13]. As mentioned above, AMPK, which is itself phosphorylated and inhibited by a AMPK kinase (AMPKK), phosphorylates and inhibits ACC.…”

“…These interactions are depicted in Scheme 4, from which it is evident that the long-chain acyl-CoA-malonyl-CoA axis is central to their co-ordination. It is intriguing that AMPK appears to inhibit more readily the mitochondrial isoform of GPAT [110]. If preferential utilization of acyl-CoA esters occurs at their site of synthesis (see above), this would suggest that AMPK activation, by inhibiting mitochondrial GPAT, would more specifically tend to favour the activation of mitochondrial (rather that ER or peroxisomal) CPTo by decreasing the competition of mitochondrial GPAT for long-chain acyl-CoA at the mitochondrial-outer-membrane surface.…”

“…Therefore AMPK inhibition of mitochondrial GPAT may be a mechanism through which specificity is imparted to the effects of AMPK on the various CPTo activities located in different intracellular membranes (Scheme 4). The smaller effect (if any) of AMPK on ER GPAT [110] may reflect the requirement for continued phospholipid synthesis at the cytosolic surface of the ER even under conditions characterized by high rates of fatty acid oxidation. Reciprocal regulation of mitochondrial GPAT and CPTo by AMPK would target the effect of changes in kinase activity at the mitochondrial synthesis of lysophosphatidate and acylcarnitine, while sparing lysophosphatidate synthesis in the ER.…”

AMP-activated protein kinase and human cancer: cancer metabolism revisited

“…2,3 AMPK and its attendant upstream kinase LKB1, acutely regulate cellular metabolism and chronically regulate gene expression to promote catabolism while curbing anabolic pathways. [4][5][6][7] LKB1 phosphorylates and activates AMPK in reponse to increases in the AMP/ATP ratio, nutrient starvation, hypoxia, changes in cellular pH and redox status, and increases in the creatine/phosphocreatine ratio. [8][9][10] The role of AMPK and LKB1 in cancer is best understood in the context of cancer cell metabolism and the genetic association of LKB1 and inherited cancer syndromes.…”

Leptinomimetic effects of the AMP kinase activator AICAR in leptin-resistant rats: prevention of diabetes and ectopic lipid deposition

“…GPAT catalyses the first committed step in glycerolipid synthesis [14]. The enzyme AMP-activated protein kinase (AMPK) phosphorylates and inhibits ACC, it acutely diminishes the activity of GPAT and it activates malonyl CoA decarboxylase (MCD), a major enzyme that regulates malonyl CoA degradation [16,17,18]. In addition it diminishes amounts of ACC and GPAT by decreasing the expression of SREBP-1c.…”