Second Messenger Of Insulin Action Research Articles

Pioglitazone Therapy Increases Insulin-Stimulated Release of d-Chiro-Inositol-Containing Inositolphosphoglycan Mediator in Women with Polycystic Ovary Syndrome.

Insulin resistance in women with polycystic ovary syndrome (PCOS) may be mediated, in part, by a deficiency in the insulin-stimulated release of a d-chiro-inositol-inositolphosphoglycan (DCI-IPG) mediator of insulin action. Supporting this idea, several studies have reported improved insulin sensitivity in both lean and obese women with PCOS after oral administration of DCI. Pioglitazone improves insulin sensitivity in women with PCOS, but it is unknown whether this may be contributed by enhanced insulin-stimulated release of the DCI-IPG second messenger. The study aimed to determine if pioglitazone increases release of biologically active DCI-IPG per unit insulin released during an oral glucose tolerance test (OGTT). A randomized, double-blind placebo-controlled trial was conducted in 32 women with PCOS at a tertiary referral center in Venezuela. The intervention comprised administration of pioglitazone 45 mg daily or matched placebo for 6 months. Outcome measures included area under curves (AUC) of DCI-IPG (AUCDCI-IPG), insulin (AUCinsulin), and the ratio of AUCDCI-IPG to AUCinsulin during a 2-hr OGTT. After treatment with pioglitazone, AUCinsulin during the OGTT decreased and whole body insulin sensitivity, as determined by the Matsuda index, increased significantly only in the pioglitazone group. The ratio of AUCDCI-IPG/AUCinsulin increased in the pioglitazone group by 1.85-fold (P < 0.0001) with no significant change in the placebo group. Change in Matsuda index correlated with change in DCI-IPG released per unit of insulin during OGTT (r = 0.47, P < 0.01). In women with PCOS, pioglitazone increased insulin-stimulated release of the DCI-IPG second messenger of insulin action, which may contribute to its insulin-sensitizing effect in these women.

Can a Dietary Supplement Prevent Gestational Diabetes Mellitus?

Gestational diabetes mellitus (GDM), glucose intolerance with onset or first recognition during pregnancy, has been increasing (1) and will increase further with widespread adoption of new diagnostic criteria recommended by the American Diabetes Association (ADA) (2). GDM, even at the milder end of the diagnostic spectrum, is associated with fetal macrosomia, neonatal adiposity, preeclampsia, and cesarean section (3), which can be reduced by diagnosis and treatment (4,5). Such treatment is not without cost (6), and an effective, relatively simple, inexpensive approach to prevention could result in significant savings to the health care system, not to mention decreasing morbidity. In this issue of Diabetes Care , D’Anna et al. (7) describe a randomized controlled trial (RCT) of such a potential prevention strategy. Insulin resistance is characteristic of human pregnancy and may have evolved to ensure the fetus a continued supply of nutrients even in times of famine. Most gravidas increase insulin release and maintain euglycemia, while those with GDM are unable to do so adequately. While metformin, an insulin-sensitizing drug, initially appeared to prevent GDM in nonrandomized cohort studies, a double-blind RCT did not demonstrate efficacy (8). Inositol, present in many foods, is a component of inositolphosphoglycans, a second messenger for insulin action (9) (Fig. 1), and two of its nine isoforms, myo -inositol and chiro -inositol, have been used as insulin-sensitizing agents to treat insulin-resistant states such as polycystic ovary syndrome (PCOS) in doses ranging from 200 mg/day to 4 g/day (10–12). The authors of the current study have also demonstrated in an RCT a beneficial effect of myo -inositol in treating the metabolic syndrome in postmenopausal women (13). These same authors (14) reported a lower incidence of GDM (17 vs. 54%) among …

Inositol Phosphoglycan P-Type in Preeclampsia

A state of insulin resistance has been demonstrated in active preeclampsia, and women with clinical evidence of insulin resistance are at higher risk to develop this syndrome during pregnancy. Recently, inositol phosphoglycan P-type, a putative second messenger of insulin action, has been implicated in the pathophysiology of preeclampsia and is increased in the placenta, amniotic fluid, and maternal urine of preeclamptic women compared with normal pregnant women. We report here a case-control study to assess the potential of urinary levels of inositol phosphoglycan P-type as a screening test for preeclampsia. Twenty-seven preeclamptic women and 47 healthy pregnant women were recruited. A polyclonal antibody-based ELISA was developed to detect levels of inositol phosphoglycan P-type in urine. Its content in urinary specimens was found to be 30-fold higher in preeclamptic subjects than control subjects (329.1+/-21.8 versus 9.2+/-1.5; P<0.001), with a higher level in all of the preeclamptic cases. For 6 women who developed preeclampsia, >1 gestational date sample of urine was available, and retrospective analysis showed a significant time-related increase of the urinary level of inositol phosphoglycan P-type <or=7 weeks before clinical diagnosis of preeclampsia. Urinary level of inositol phosphoglycan P-type increased after diagnosis indicating a possible pathophysiological threshold level and steeply decreased after delivery.

Magnesium and glucose homeostasis

Magnesium is an important ion in all living cells being a cofactor of many enzymes, especially those utilising high energy phosphate bounds. The relationship between insulin and magnesium has been recently studied. In particular it has been shown that magnesium plays the role of a second messenger for insulin action; on the other hand, insulin itself has been demonstrated to be an important regulatory factor of intracellular magnesium accumulation. Conditions associated with insulin resistance, such as hypertension or aging, are also associated with low intracellular magnesium contents. In diabetes mellitus, it is suggested that low intracellular magnesium levels result from both increased urinary losses and insulin resistance. The extent to which such a low intracellular magnesium content contributes to the development of macro- and microangiopathy remains to be established. A reduced intracellular magnesium content might contribute to the impaired insulin response and action which occurs in Type 2 (non-insulin-dependent) diabetes mellitus. Chronic magnesium supplementation can contribute to an improvement in both islet Beta-cell response and insulin action in non-insulin-dependent diabetic subjects.

Insulin resistance in uremia: In vitro model in the rat liver using human serum to study mechanisms

We have previously demonstrated in a rat model of chronic uremia that the liver is resistant to insulin. To further investigate the mechanism(s) of insulin resistance in uremia, primary cultures of normal rat hepatocytes were incubated with varying dilutions ( 1 10 to 1 10,000 ) of sera from undialyzed end stage uremic and normal humans for 20 hours. We then studied insulin action, binding, and postbinding events. Dilutions of uremic sera as low as 1 1,000 rendered the hepatocytes resistant to maximal concentrations of insulin with regard to [ 14C]acetate incorporation into lipids. The dose response curve for insulin-stimulated [ 14C]aminoisobutyric acid uptake demonstrated a shift to the right in hepatocytes incubated with uremic sera when compared with those incubated with normal sera. The 125I-insulin binding sites and affinity, 125I-insulin internalization and degradation, insulin receptor structure, autophosphorylation of the insulin receptor, and its tyrosine-specific kinase activity were normal in the hepatocytes rendered resistant to insulin by uremic sera. However, these cells failed to generate the chemical mediator or second messenger of insulin action, as assessed by its ability to stimulate pyruvate dehydrogenase (PDH) in liver mitochondria from normal rats. We concluded that uremic sera renders normal rat hepatocytes resistant to insulin. Insulin resistance is a postinsulin receptor kinase defect possibly due to lack of the generation of the chemical mediator of insulin action. This in vitro cell model may be useful to further define the mechanism(s) and the serum factor(s) responsible for insulin resistance in uremia in the absence of complicating hormonal and substrate changes that occur in vivo.

Partial purification from human mononuclear cells and placental plasma membranes of an insulin mediator which stimulates pyruvate dehydrogenase and suppresses glucose-6-phosphatase

A substance capable of stimulating pyruvate dehydrogenase (PDH) and suppressing glucose-6-phosphatase (G-6-Pase) in a cell-free system was prepared from insulin-treated human placental plasma membranes and peripheral blood mononuclear cells by formic acid extraction. This material was partially purified by molecular-exclusion chromatography, ion-exchange chromatography, and hydroxylapatite chromatography. This was found to stimulate pyruvate dehydrogenase and inhibit glucose-6-phosphatase in a dose-dependent manner. The amount or ability of this substance to stimulate pyruvate dehydrogenase was increased in the proportion to the concentration of insulin. The stimulation of pyruvate dehydrogenase by the factor was eliminated when sodium fluoride was presented in the assay of the activation. This result implied that the activation of pyruvate dehydrogenase was mediated by the stimulation of the phosphatase of pyruvate dehydrogenase complex. Each material isolated from insulin-treated human placental plasma membranes and mononuclear cells shared a number of important characteristics of putative second messengers of insulin action as follows: (i) heat and acid stability; (ii) a similar molecular weight; (iii) increased activity of pyruvate dehydrogenase in a insulin-dependent manner; and (iv) stimulated pyruvate dehydrogenase by the sodium fluoride-sensitive mechanism. This human putative second messenger of insulin action was eluted from the anion-exchange resin AG1-X8 at an ionic strength of 3–4 m, as well as from the hydroxylapatite column at a phosphate concentration of 2–3 m.

A putative second messenger of insulin action regulates hepatic microsomal glucose-6-phosphatase

Physiological concentrations of insulin suppressed rat liver microsomal glucose-6-phosphatase activity in vitro. To attest a hypothesis that a putative second messenger of insulin action (insulin mediator) mediated this process, we isolated the low molecular factor from insulintreated plasma membranes of rat liver, which was acid- and heat-stable substance of a peptide nature. The insulin mediator which was proved to activate the mitochondria pyruvate dehydrogenase suppressed microsomal glucose-6-phosphatase. The insulin mediator was linked to suppression of the gluconeogenic enzyme through the control of non-specific phosphohydroxylase.

Hepatic insulin effects independent of changes in Ca ++ metabolism

The possibility of Ca ++ acting as second messenger for insulin in rat liver was investigated using the net stimulation of 14C-glucose incorporation into glycogen by isolated hepatocytes as an index of insulin action. An insulin effect could be partially sustained in the virtual absence of Ca ++ and Mg ++ and a maximal insulin effect could be observed in the presence of either Ca ++ or Mg ++, suggesting that extracellular Ca ++ is not required for insulin action. Inhibiting the activity of calmodulin, an intracellular mediator of Ca ++ action, with trifluoperazine had little effect on insulin action. The efflux of 45Ca from prelabeled hepatocytes was not altered by the presence of insulin arguing against insulin-induced changes in Ca ++ fluxes. Collectively, these results do not support the role of Ca ++ as second messenger for insulin action in liver.

Partial purification from hepatoma cells of an intracellular substance which mediates the effects of insulin on pyruvate dehydrogenase and low Km cyclic AMP phosphodiesterase

A substance capable of stimulating the activities of pyruvate dehydrogenase and low K m cyclic AMP phosphodiesterase was prepared from H4-II-EC3′ hepatoma cells by acid extraction and partially purified by molecular exclusion chromatography. The material thus prepared by gel chromatography was found to stimulate the activities of these enzymes in a concentration-dependent manner. The amount or activity of the pyruvate dehydrogenase stimulating factor was increased in cells which had been treated with physiological concentrations of insulin (0.2 mU/ml). Increasing the concentration of insulin increased the amount or activity of the factor generated. High concentrations of insulin did not cause a reversal of the effects of insulin. The stimulation of pyruvate dehydrogenase activity by the factor was eliminated when sodium fluoride (75 m m) was present in the enzyme assay, implying that activation was mediated by the pyruvate dehydrogenase phosphatase. The enzyme-stimulating factor isolated from hepatoma cells shares a number of important characteristics with the putative second messenger of insulin prepared from other cell types: (1) it is heat and acid stable, (2) it has a similar apparent molecular weight, (3) it is generated in an insulin-dependent manner, (4) it stimulates the activity of pyruvate dehydrogenase by a fluoride-sensitive mechanism, and (5) it elutes from the anion-exchange resin AG 1-X8 at an ionic strength of 0.4 m. These findings suggest that the stimulator of pyruvate dehydrogenase and of low K m cyclic AMP phosphodiesterase isolated from hepatoma cells has chemical properties identical with those of the putative second messenger of insulin action isolated from a number of other insulin-sensitive tissues.

Membrane bound and cellular cationic changes associated with insulin stimulation of cultured cells

Insulin was employed as a stimulant in our continuing investigations of the molecular mechanisms involved in the coordinate control of cellular metabolism and growth. Incubation of chicken embryo fibroblasts for 16 hours in media containing 0-0.1 U insulin/ml resulted in a 17-fold increase in the rate of 3H-thymidine incorporation into DNA. Concomitantly, there were graded increases in intracellular K+ (14%) AND Mg2+ (22%) and no significant change in Ca2+. These changes in cation content occurred within 10 to 30 minutes and preceded the changes in 3H-thymidine incorporation. Insulin produced a consistent graded decrease in externally bound Mg2+ and Ca2+ and a concomitant increase in bound Na+ and K+ with no significant change in the rates of K+ and Mg2+ efflux. The results are consistent with the concept of Mg2+ as a second messenger for insulin action, as well as with the more general hypothesis that Mg2+ is the centtral agent in the coordinate control of metabolism and growth in animal cells.

MODE OF INSULIN ACTION

A unifying hypothesis is proposed for the mechanism of insulin action in adipose tissue. Insulin both induces displacement of Ca ++ from a membrane-bound pool and inhibits efflux of the ion, thereby facilitating a rise in intracellular free Ca ++ concentration. The former effect could enhance the transport of substrates and ions into the cell, while the latter modulates the activity of some intracellular enzymes to stimulate glyco-genesis, lipogenesis, and decrease lipolysis and glycogenolysis. The calcium ion might act as the missing second messenger for insulin action.