Insulin Rapidly Increases Diacylglycerol by Activating De Novo Phosphatidic Acid Synthesis

Abstract

The mechanisms whereby insulin increases diacylglycerol in BC3H-1 myocytes were examined. When [3H]arachidonate labeling of phospholipids was used as an indicator of phospholipase C activation, transient increases in [3H]diacylglycerol were observed between 0.5 and 10 minutes after the onset of insulin treatment. With [3H]glycerol labeling as an indicator of de novo phospholipid synthesis, [3H]diacylglycerol was increased maximally at 1 minute and remained elevated for 20 minutes. [3H]Glycerol-labeled diacylglycerol was largely derived directly from phosphatidic acid. Insulin increased de novo phosphatidic acid synthesis within 5 to 10 seconds; within 1 minute, this synthesis was 60 times greater than that of controls. Thus, the initial increase in diacylglycerol is due to both increased hydrolysis of phospholipids and a burst of de novo phosphatidic acid synthesis. After 5 to 10 minutes, de novo phosphatidic acid synthesis continues as a major source of diacylglycerol. Both phospholipid effects of insulin seem important for generating diacylglycerol and other phospholipid-derived intracellular signaling substances.

Fructose Stimulates Na/H Exchange Activity and Sensitizes the Proximal Tubule to Angiotensin II

“…22,33 Furthermore, although proximal tubules do not metabolize glucose (they are gluconeogenic), they can metabolize fructose and form diacylglycerol, one of the activators of PKC. 29,30,34,35 In agreement to this possibility, our results show that the effects of fructose can be blunted by PKC inhibition. Finally, fructose might modify the surface expression of NHE3.…”

“…28 Third, fructose metabolism might generate secondary metabolites, such as diacylglycerol, that further activate PKC. [29][30][31][32] It has been shown that the stimulatory effects of angiotensin II on proximal tubule transport are mediated by PKC activation. 22,33 Furthermore, although proximal tubules do not metabolize glucose (they are gluconeogenic), they can metabolize fructose and form diacylglycerol, one of the activators of PKC.…”

Diacylglycerol directly stimulates the insulin receptor tyrosine kinase

“…The activated receptor kinase phosphorylates its primary substrate, IRS-1, on YMXM motifs, thus converting IRS-1 into a docking protein for SH2 domain-containing proteins such as phosphatidylinositol 3-kinase or Grb2 [11]. Insulin stimulation has been reported to cause diacylglycerol levels to increase in several cell types [12,13]. The possibility that this increase is mediated by phospholipase Cy is suggested by the finding that insulin stimulates the activity of this enzyme in plasma membrane preparations [14].…”

Glucose‐induced synthesis of diacylglycerol de novo is associated with translocation (activation) of protein kinase C in rat adipocytes

“…Lipids were extracted and DAG was purified by thinlayer chromatography [6,8,11], and counted for radioactivity. DAG content of lipid extracts was measured by the DAG kinase method (Lipidex) of Preiss et al [20].…”

“…The DAG which derives from phospholipase Cmediated hydrolysis of phosphatidylinositol 4',5'-bisphosphate (PIP2) [3] causes PKC to translocate from cytosol to the membrane, and this serves as evidence of PKC activation [4]. Some agonists, such as insulin, do not stimulate PIP2 hydrolysis significantly, but increase DAG through enhanced synthesis of phosphatidic acid (PA) de novo [5][6][7][8][9][10][11], and hydrolysis of a phosphatidyl- inositol (PI)-glycan [12] and phosphatidylcholine [11,13,14]. Insulin stimulates PKC translocation [15], but it is uncertain whether DAG derived from the de novo pathway contributes to this activation.…”