Overexpression of 1-Acyl-Glycerol-3-Phosphate Acyltransferase-αEnhances Lipid Storage in Cellular Models of Adipose Tissue and Skeletal Muscle

Abstract

Plasma nonesterified fatty acids (NEFA) at elevated concentrations antagonize insulin action and thus may play a critical role in the development of insulin resistance in type 2 diabetes. Plasma NEFA and glucose concentrations are regulated, in part, by their uptake into peripheral tissues. Cellular energy uptake can be increased by enhancing either energy transport or metabolism. The effects of overexpression of 1-acylglycerol-3-phosphate acyltransferase (AGAT)-␣, which catalyzes the second step in triglyceride formation from glycerol-3-phosphate, was studied in 3T3-L1 adipocytes and C2C12 myotubes. In myotubes, overexpression of AGAT-␣ did not affect total [ T ype 2 diabetes is characterized, in part, by excess energy as glucose and nonesterified fatty acids (NEFA) within the plasma compartment. Moreover, an elevated plasma NEFA concentration has been increasingly recognized as a systemic mediator of insulin resistance in type 2 diabetes (1-5). Excess plasma NEFA can inhibit insulin-stimulated glucose utilization in muscle (1-4) and promote hepatic production of glucose (4-6) and VLDL triglyceride (TG) (7,8). Acute systemic administration of NEFA inhibits glucose disposal in muscle in a dose-dependent fashion (4) and increases hepatic glucose output (5,6). Reduction of plasma NEFA concentration improves glucose utilization (9-11), enhances the suppression of hepatic glucose production by insulin (12), and reduces hyperinsulinemia in patients with type 2 diabetes (13). Thus, plasma NEFA elevation may be mechanistically linked to the cluster of metabolic abnormalities seen in type 2 diabetes, including hyperglycemia, hyperinsulinemia, and dyslipidemia.Skeletal muscle and adipose tissue are major sites of energy utilization. Most of the glucose and NEFA taken up by resting muscle is converted to glycogen (14) and TG (15). Because of its mass, skeletal muscle is a significant storage site for excess plasma NEFA in patients with type 2 diabetes (16) and in animal models of the disease (17,18). However, the accumulation of intramuscular TG may increase lipid oxidation and decrease glucose uptake and insulin sensitivity (18,19). In contrast, increasing NEFA and glucose uptake in adipose tissue could reduce systemic NEFA availability and would eventually improve insulin sensitivity in liver and muscle.Energy uptake can be increased by enhancing either of the proposed rate-limiting steps of energy utilization, i.e., energy transport across the plasma membrane or energy metabolism within the cell. Indeed, an increase in glucose transport (20)(21)(22) or metabolism (23,24) promotes glucose utilization in cultured cells and transgenic animals. Moreover, glucose metabolism appears to be rate limiting under conditions of enhanced glucose transport (22,(25)(26)(27)(28)(29). Less is known about the regulation of NEFA uptake. Overexpression of long-chain fatty acid transport protein (FATP) increases NEFA uptake in 3T3-L1 fibroblasts (30). On the other hand, an increase in cytoplasmic fatty acyl-CoA synthase (FACS) activity wit...

Insulin and Angiotensin II Induce the Translocation of Scavenger Receptor Class B, Type I from Intracellular Sites to the Plasma Membrane of Adipocytes

“…Thus, the intracellularly stored GLUT4 glucose transporter and the fatty acid transport proteins are rapidly redistributed to the plasma membrane in presence of insulin (31)(32)(33). Our observation goes along with these findings.…”

In Muscle-Specific Lipoprotein Lipase−Overexpressing Mice, Muscle Triglyceride Content Is Increased Without Inhibition of Insulin-Stimulated Whole-Body and Muscle-Specific Glucose Uptake

“…Because cardiac muscle does not contains glycerol kinase, glycerol must be provided via glucose uptake and subsequent conversion into glycerol-3-phosphate. In vitro evidence in isolated myotubes has shown that increased TG synthesis by overexpressing AGAT increases glucose conversion to cellular lipids, with a concomitant decrease in glycogen formation (40). Our data seem to point in the same direction, although increased conversion in lipids and decreased glycogen formation (Fig.…”

Rat Long Chain Acyl-CoA Synthetase 5 Increases Fatty Acid Uptake and Partitioning to Cellular Triacylglycerol in McArdle-RH7777 Cells

“…Pownall and Hamilton (28) propose that increased intracellular conversion of fatty acids to acyl-CoAs and their subsequent metabolism may "pull" fatty acids into the cell by altering the fatty acid gradient across the plasma membrane. In support of this hypothesis, 1-acyl-glycerol-3-phosphate acyltransferase overexpression increases oleic acid uptake in both adipocytes and myotubes (29). Although plasma membrane proteins may catalyze part of the transport process, metabolism of the incoming fatty acids may be more critical in regulation.…”