1,2-Diacylglycerol and ceramide levels in rat skeletal muscle and liver in vivo. Studies with insulin, exercise, muscle denervation, and vasopressin

Insulin increases a biochemically distinct pool of diacylglycerol in the rat soleus muscle

Insulin stimulates the incorporation of glucose-carbon into diacylglycerol (DAG) in rat skeletal muscle, and its ability to do so is enhanced severalfold after the muscle is denervated (S. J. Heydrick, N. B. Ruderman, T. J. Kurowski, H. A. Adams, and K. S. Chen. Diabetes 40: 1707-1711, 1991). The present studies were carried out to assess the nature of this newly synthesized DAG and to identify factors other than insulin that determine its rate of appearance in the incubated rat soleus muscle. In control muscles, incubated at a medium glucose concentration of 6-7.5 mM, insulin (10 mU/ml) increased DAG content (mass) by 20-25% and increased the incorporation of a 14C label from extracellular [14C]glucose into DAG by 200-300%. The labeling of DAG reached a plateau within 20 min, at which time the labeled DAG comprised a very small percentage of total muscle DAG. Molecular species analysis revealed that DAG species having fatty acids of 18:2/20:4 and 18:2/18:2 each constituted approximately 2% of total DAG content but contained 20 and 15%, respectively, of the glucose-derived label in DAG. In contrast, 16:0/18:1 accounted for > 80% of total DAG content but only 18% of the total label incorporated into DAG. Insulin did not alter this pattern. Denervation also did not alter the molecular species profiles of the labeled DAGs or DAG analyzed by mass. An increased incorporation of glucose-carbon into DAG was observed in muscles incubated with 30 mM glucose in place of the usual 7.5-mM concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Diacylglycerol/protein kinase C signalling: a mechanism for insulin resistance?

It is proposed that an intracellular cycle exists to limit or terminate the insulin signal. The cycle involves increased synthesis of sn-1,2-diacylglycerol (DAG) in response to insulin. The DAG activates protein kinase C (PKC) which phosphorylates glycogen synthase either directly or through other protein kinases to render it inactive. Protein kinase C may also inhibit the insulin receptor by phosphorylation of receptor serine residues. Insulin resistance could then arise as a consequence of a persistent increase in DAG levels. Such an increase could occur in three different ways. Chronic hyperinsulinaemia could increase DAG levels by de-novo synthesis from phosphatidic acid, by hydrolysis of phosphatidylcholine, or by hydrolysis of glycosyl-phosphatidylinositol; DAG is also formed by hydrolysis of phosphatidylinositol 4,5-biphosphate (PIP2). This reaction, known as the 'PI response,' may be the connection between hypertension and insulin resistance. A third mechanism for an increase in DAG involves neural abnormalities. Thus, muscle denervation in the rat is characterized both by a profound insulin resistance and a large increase in DAG. It is possible that a similar increase occurs in humans and may explain the association between denervation, inactivity, and insulin resistance.

Effect of sphingoid bases on basal and insulin-stimulated 2-deoxyglucose transport in skeletal muscle

Incubation of rat soleus muscles with 50 microM sphingosine or 50 microM sphinganine augmented basal 2-deoxy-D-glucose (2DG) transport 32%, but reduced the response to 0.1 and 1.0 mU insulin/ml by 17 and 27%, respectively. When the muscles were incubated with 50 microM phytosphingosine, a 63-93% increase in basal 2DG transport was observed. However, this treatment had no effect on insulin-stimulated 2DG transport. The phytosphingosine-induced increase in basal 2-DG transport was inhibited 93 and 98% with 35 and 70 microM cytochalasin B, respectively, suggesting that it is mediated by glucose transporters. Cellular accumulation of L-glucose, which is not mediated by glucose transporters, was not affected by phytosphingosine. It is concluded that (a) both sphingosine and sphinganine increase basal 2DG transport in muscle but diminish insulin-stimulated transport, and (b) phytosphingosine stimulates basal 2DG transport in muscle by a mechanism involving glucose transporters.

Inhibitors such as staurosporine, H-7 or polymyxin B cannot be used in skeletal muscle to prove the role of protein kinase C on insulin action

The precise role of protein kinase C in insulin action in skeletal muscle is not well defined. Based on the fact that inhibitors of protein kinase C block some insulin effects, it has been concluded that some of the biological actions of insulin are mediated via protein kinase C. In this study, we present evidence that inhibitors of protein kinase C such as staurosporine, H-7 or polymyxin B cannot be used to ascertain the role of protein kinase C in skeletal muscle. This is based on the following experimental evidences: a) staurosporine, H-7 and polymyxin B markedly block in muscle the effect of insulin on System A transport activity; however, this effect of insulin is not mimicked in muscle by TPA-induced stimulation of protein kinase C, b) H-7 and polymyxin B block insulin action on System A transport activity in an additive manner to the inhibitory effect of phorbol esters, c) staurosporine, H-7 and polymyxin B block the effect of insulin on lactate production, a process that is activated by insulin and TPA in an additive fashion, and d) staurosporine completely blocks the tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle.

Effects of insulin and phospholipase C in control and denervated rat skeletal muscle

Phospholipase C (PLC), an enzyme that increases endogenous 1,2-diacylglycerol (DAG), caused dose-dependent stimulation of 2-deoxy-D-glucose (2-DG) uptake in rat soleus muscles; the maximal effect was less than that of insulin. In denervated muscles the effect of insulin on 2-DG uptake was markedly reduced, whereas the response to PLC was identical to that of control muscles. Both PLC and insulin stimulated glucose incorporation into glycogen in control but not in denervated solei. Amino acid transport was unaffected by PLC; however, the enzyme completely inhibited the stimulation of amino acid transport by insulin. PLC did not activate the insulin receptor tyrosine kinase but decreased activation of the receptor by insulin in vivo. Basal muscle DAG content increased after denervation. Incubation with PLC markedly increased DAG in control and in denervated muscle. Insulin increased total DAG mass less than PLC in control muscles and did not affect DAG in denervated muscles. In media without added Ca2+, PLC stimulation of DAG production was impaired, and 2-DG uptake was unresponsive to PLC. The data are consistent with, but do not prove, that a subpopulation of DAGs may participate in insulin-mediated stimulation of glucose transport. They also suggest that the denervation-induced insulin resistance of glucose transport may reflect impaired generation of certain DAGs involved in the signaling cascade.