Insulin signaling in chicken liver and muscle

This review addresses the control exerted by insulin through its receptor on the general metabolism and gene expression in chicken liver and muscle. Compared with mammals, chickens have similar concentrations of circulating insulin, but still maintain high plasma glucose levels. This may be a consequence of the low sensitivity of the chicken to exogenous insulin. In order to determine whether this low sensitivity is the result of differences in insulin receptor signaling between mammals and birds, insulin receptors have been characterized in several chicken tissues and two insulin receptor substrates (IRS-1 and Shc) have been described in liver and muscle. Compared with mammals current knowledge of insulin signaling in birds is incomplete. This is particularly evident when considering the number of isoforms of the components involved in the insulin cascade (IRSs, AKT, ERK and others) many of which may have not been characterized in the chicken. Despite these shortfalls in available data, it appears that insulin signaling in chicken liver is similar to that in mammals, but is unlike that in mammals in muscle. In leg muscle, chickens differ from mammals in the early steps of the insulin signaling cascade (IR, IRS-1 and PI3K) where PI3K activity is about 30-fold greater in the chicken than in the rat. This "constitutive" hyperactivity of PI3K in chicken muscle may over-stimulate a feedback inhibitory pathway described in mammals thereby desensitizing chicken muscle to insulin.     

Modulation of early steps in insulin action in the liver and muscle of epinephrine treated rats

Epinephrine is known to produce insulin resistance, but the exact molecular mechanism involved is unknown. In the present study we have examined the levels and phosphorylation state of the insulin receptor and of insulin receptor substrate 1 (IRS-1), as well as the association between IRS-1 and phosphatidylinositol 3-kinase (PI 3-kinase) in the liver and muscle of rats treated with epinephrine. The results demonstrate a decrease in insulin-stimulated receptor and IRS-1 phosphorylation levels which was accompanied by a reduction in the association of IRS-1 with PI 3-kinasein vivo in liver and muscle of epinephrine treated rats. These data suggest that molecular post-receptor defects may explain some aspects of the insulin resistance induced by catecholamines.     

Statin modulates insulin signaling and insulin resistance in liver and muscle of rats fed a high-fat diet

Recent studies have shown that statins might have relevant effects on insulin resistance in animal models and in humans. However, the molecular mechanisms that account for this improvement in insulin sensitivity are not well established. The aim of the present study was to investigate the effect of a statin on insulin sensitivity and insulin signaling in liver and muscle of rats fed on a high-fat diet (HFD) for 4 weeks, treated or not with lovastatin during the last week. Our data show that treatment with lovastatin results in a marked improvement in insulin sensitivity characterized by an increase in glucose disappearance rate during the insulin tolerance test. This increase in insulin sensitivity was associated with an increase in insulin-induced insulin receptor (IR) tyrosine phosphorylation and, in parallel, a decrease in IR serine phosphorylation and association with PTP1B. Our data also show that lovastatin treatment was associated with an increase in insulin-stimulated insulin receptor substrate (IRS) 1/phosphatidylinositol 3-kinase/Akt pathway in the liver and muscle of HFD-fed rats in parallel with a decrease in the inflammatory pathway (c-jun N-terminal kinase and I kappa beta kinase (IKKbeta)/inhibitor of kappaB/nuclear factor kappaB) related to insulin resistance. In summary, statin treatment improves insulin sensitivity in HFD-fed rats by reversing the decrease in the insulin-stimulated IRS-1/phosphatidylinositol 3-kinase/Akt pathway in liver and muscle. The effect of statins on insulin action is further supported by our findings that HFD rats treated with statin show a reduction in IRS-1 serine phosphorylation, I kappa kinase (IKK)/inhibitor of kappaB/nuclear factor kappaB pathway, and c-jun N-terminal kinase activity, associated with an improvement in insulin action. Overall, these results provide important new insight into the mechanism of statin action in insulin sensitivity.     

Characterization of the chicken muscle insulin receptor

Insulin receptors are present in chicken skeletal muscle. Crude membrane preparations demonstrated specific 125I-insulin binding. The nonspecific binding was high (36-55% of total binding) and slightly lower affinity receptors were found than are typically observed for crude membrane insulin binding in other chicken tissues. Affinity crosslinking of 125I-insulin to crude membranes revealed insulin receptor alpha-subunits of Mr 128K, intermediate between those of liver (134K) and brain (124K). When solubilized and partially purified on wheat germ agglutinin (WGA) affinity columns, chicken muscle insulin receptors exhibited typical high affinity binding, with approximately 10(-10) M unlabeled insulin producing 50% inhibition of the specific 125I-insulin binding. WGA purified chicken muscle insulin receptors also exhibited insulin-stimulated autophosphorylation of the beta-subunit, which appeared as phosphorylated bands of 92- and 81K. Both bands were immunoprecipitated by anti-receptor antiserum (B10). WGA purified membranes also demonstrated dose-dependent insulin-stimulated phosphorylation of the exogenous substrate poly(Glu,Tyr)4:1. However, unlike chicken liver, chicken muscle insulin receptor number and tyrosine kinase activity were unaltered by 48 hr of fasting or 48 hr of fasting and 24 hr of refeeding. Thus, despite the presence of insulin receptors in chicken muscle showing normal coupling to receptor tyrosine kinase activity, nutritional alterations modulate these parameters in a tissue-specific manner in chickens.     

Impaired insulin action despite upregulation of proximal insulin signaling: novel insights into skeletal muscle insulin resistance in liver cirrhosis

BACKGROUND/AIMS: Disturbance in glucose metabolism is a common feature in liver diseases and this is associated with skeletal muscle insulin resistance. However, the underlying molecular mechanisms are unclear. To characterize skeletal muscle insulin resistance associated with liver disease, we examined muscles from animals after an acute, 5 weeks perturbation of the common bile duct. Clinical findings, elevated plasma levels of liver enzymes and histological examinations confirmed cirrhosis. METHODS/RESULTS:: Cirrhotic animals were insulin resistant and this was associated with reduced glucose transport into muscles. Interestingly, activity in the proximal part of the insulin signaling cascade was not decreased, as evinced by increased activity of key enzymes in the signal to glucose transport. Expression of the glucose transporter, GLUT4, was normal. So together these results indicate that signaling downstream of PKB/Akt and/or the translocation of GLUT4 is impaired in skeletal muscle from cirrhotic animals. CONCLUSIONS: In conclusion, in an animal model of liver cirrhosis whole body insulin resistance is associated with insulin resistance in skeletal muscles. Unlike other common forms of insulin resistance, muscles from cirrhotic animals have increased activity in the proximal insulin signaling cascade. This emphasizes the fact that skeletal muscle insulin resistance associated with liver cirrhosis is a unique entity.     

Loss of sensitivity to insulin at early events of the insulin signaling pathway in the liver of growth hormone-transgenic mice.

Growth hormone (GH) excess is associated with secondary hyperinsulinemia, but the molecular mechanism and consequences of this alteration are poorly understood. To address this problem we have examined the levels and phosphorylation state of the insulin receptor (IR) and the insulin receptor substrate-1 (IRS-1), the association between IRS-1 and the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) as well as the PI 3-kinase activity in the livers of GH-transgenic mice. As expected, IR levels were reduced in the liver of GH-transgenic mice (55% of normal values) as determined by immunoblotting with an anti-IR beta-subunit antibody. IR and IRS-1 phosphorylation as determined by immunoblotting with antiphosphotyrosine antibody were increased in basal conditions by 315% and 560% respectively. After a bolus administration of insulin in vivo, IR phosphorylation increased by 40% while IRS-1 phosphorylation did not change. Insulin administration to control (normal) mice produced 670% and 300% increases in the IR and IRS-1 phosphorylation respectively. In the GH-transgenic animals, basal association of PI 3-kinase with IRS-1 as well as PI 3-kinase activity in liver was increased by 200% and 280% respectively, and did not increase further after administration of insulin in vivo, indicating a complete insensitivity to insulin at these levels. In conclusion, GH excess and the resulting secondary hyperinsulinemia were associated with alterations at the early steps of insulin action in liver. IR concentration was reduced, while IR and IRS-1 phosphorylation, IRS-1/PI 3-kinase association, and PI 3-kinase activity appeared to be maximally activated under basal conditions, thus making this tissue insensitive to further stimulation by exogenous insulin in vivo.     

Fatty acids and insulin resistance in muscle and liver

Free fatty acids (FFAs) circulate round the body and represent important nutrients and the key oxidative fuel for the heart and resting skeletal muscle. In addition, FFAs are thought to be potent signalling molecules. Growing evidence indicates that FFAs may be involved in type 2 diabetes mellitus and obesity by mediating insulin resistance. In 1963, it was postulated that accumulated glucose-6-phosphate as a result of increased FFA oxidation leads to decreased glucose uptake. An alternative hypothesis is that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity, which appears to be a consequence of decreased insulin receptor substrate-1-associated phosphatidyl inositol 3 kinase activity. Moreover, FFAs can arise locally, and increased intramyocellular and hepatocellular lipids have been shown to be associated with insulin resistance. This paper reviews the main aspects of FFA metabolism in the development of insulin resistance in skeletal muscle and liver, as well as the role of ectopic lipid deposits as a local source of FFAs. Finally, the role of thiazolidinediones as modulators of FFA-induced insulin resistance will be discussed.     

Tissue-specific impairment of insulin signaling in vasculature and skeletal muscle of fructose-fed rats.

The relation between insulin resistance/hyperinsulinemia and cardiovascular diseases has attracted much attention. Insulin affects not only glucose metabolism, but also protein synthesis and cell growth. Insulin stimulates both the phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kinase (MAPK) pathways, but the relationship between cardiovascular disease and selective insulin signal pathways is unclear. We investigated the tissue specificity and intracellular signal transduction selectivity of insulin resistance in the vasculature and skeletal muscle of fructose-fed rats (FFR). Sprague-Dawley rats were fed either normal rat chow (control rats) or fructose-rich chow. Normal saline with or without 1,000 (microg/kg) insulin was injected, and then the thoracic aorta or soleus muscle was removed under anesthetization. Insulin-induced tyrosine phosphorylation of insulin receptor beta subunit (IRbeta) and insulin receptor substrate-1 (IRS-1) and tyrosine/threonine phosphorylation of p44/42 MAPK (ERK-1/2) were evaluated. There were no significant differences in the degree of phosphorylation of IRbeta or ERK-1/2 in the thoracic aorta or in the soleus muscle between FFR and controls. However, tyrosine phosphorylation of IRS-1 in the soleus muscle of FFR was significantly reduced to 80% (p<0.001) of that in controls. The results suggest that PI3-K pathway in skeletal muscle is selectively impaired in FFR, and this impairment may induce hyperinsulinemia, which in turn may stimulate the MAPK pathway and lead to atherosclerosis. Thus PI3-K pathway may be one of the factors underlying the onset of cardiovascular disease in patients with insulin resistance.     

Dissociation between fat-induced in vivo insulin resistance and proximal insulin signaling in skeletal muscle in men at risk for type 2 diabetes

The effect of short- (2 h) and long-term (24 h) low-grade Intralipid infusion on whole-body insulin action, cellular glucose metabolism, and proximal components of the insulin signal transduction cascade was studied in seven obese male glucose intolerant first degree relatives of type 2 diabetic patients [impaired glucose tolerance (IGT) relatives] and eight matched control subjects. Indirect calorimetry and excision of vastus lateralis skeletal muscle biopsies were performed before and during hyperinsulinemic euglycemic clamps combined with 3[(3)H]glucose. Clamps were performed after 0, 2, or 24 h Intralipid infusion (0.4 ml.kg(-1).min(-1)). Insulin-stimulated glucose disposal decreased approximately 25% after short- and long-term fat infusion in both IGT relatives and controls. Glucose oxidation decreased and lipid oxidation increased after both short- and long-term fat infusion in both groups. Insulin-stimulated glucose oxidation was higher after long-term as compared with short-term fat infusion in control subjects. Short- or long-term infusion did not affect the absolute values of basal or insulin-stimulated insulin receptor substrate-1 tyrosine phosphorylation, tyrosine-associated phosphoinositide 3-kinase (PI 3-kinase) activity, insulin receptor substrate-1-associated PI 3-kinase activity, or Akt serine phosphorylation in IGT relatives or matched controls. In fact, a paradoxical increase in both basal and insulin-stimulated PI 3-kinase activity was noted in the total study population after both short- and long-term fat infusion. Short- and long-term low-grade Intralipid infusion-induced (or enhanced) whole-body insulin resistance and impaired glucose metabolism in IGT relatives and matched control subjects. The fat-induced metabolic changes were not explained by impairment of the proximal insulin signaling transduction in skeletal muscle.     

Long-term denervation impairs insulin receptor substrate-1-mediated insulin signaling in skeletal muscle

Long-term denervation is associated with insulin resistance. To investigate the molecular bases of insulin resistance, the downstream signaling molecules of insulin receptor including insulin receptor substrate-1 (IRS-1) and phosphatidylinositol 3-kinase (PI 3-K) were examined in skeletal muscle of rats after 7 days of denervation. Long-term denervation attenuated insulin-stimulated activation of the initial steps of the intracellular signaling pathway. Insulin-stimulated tyrosine phosphorylation of insulin receptor was reduced to 36% (P < .005), as was the phosphorylation of IRS-1 to 34% (P < .0001) of control. While insulin receptor protein level was unchanged, the protein expression of IRS-1 was significantly decreased in denervated muscles. Insulin-stimulated percent tyrosine phosphorylation of IRS-1, normalized to the IRS-1 protein expression, was also reduced to 55% (P < .01) of control in denervated muscle. Denervation caused a decline in the insulin-induced binding of p85 regulatory subunit of PI 3-K to IRS-1 to 61% (P < .001) and IRS-1-associated PI 3-K activity to 57% (P < .01). These results provide evidence that long-term denervation results in insulin resistance because of derangements at multiple points, including tyrosine phosphorylation of insulin receptor and its downstream signaling molecule, IRS-1, protein expression of IRS-1, and activation of PI 3-K.     

Acute ethanol exposure inhibits insulin signaling in the liver

Chronic ethanol consumption may produce hepatic injury and impair the ability of the liver to regenerate principally through its action on insulin signaling. These effects are mediated by insulin receptor substrate-1 (IRS-1) via the mitogen-activated protein kinase/extracellular signal regulated kinase (MAPK/Erk) pathway and by survival signals through phosphatidylinositol-3 kinase (PI3K) and protein kinase B (Akt). Because a protein phosphatase, phosphatase tensin homolog deleted on chromosome 10 (PTEN), has been reported to block insulin signaling through PI3K, we explored acute ethanol effects on signaling in the context of PTEN function. We measured upstream components of the insulin signal transduction pathway and Akt phosphorylation as an indicator of signaling through PI3K, including the generation of survival signals via glycogen synthase kinase 3beta (GSK3beta) and Bcl-2-associated death promoter (BAD). In addition, the physical association between PTEN and PI3K regulatory (p85alpha) and catalytic (p110alpha) subunits was evaluated both in vitro and in vivo. In Huh-7 cells, there was no effect of acute ethanol exposure on tyrosyl phosphorylation of the insulin receptor, IRS-1, and the association of IRS-1 with PI3K. However, Akt phosphorylation was impaired. The association of PTEN with the PI3K p85alpha subunit was substantially increased and led to the inhibition of downstream insulin-mediated survival signals through Akt, GSK3beta, and BAD; the ethanol effect was reversed by PTEN knockdown with small interfering RNA. These results were confirmed in the liver. Conclusion: Short-term ethanol exposure rapidly attenuates insulin signaling. The major cellular mechanism involves the increased association of PTEN with the PI3K p85alpha subunit, which results in reduced phospho-Akt formation and impaired downstream survival signaling. These findings may have relevance to acute toxic effects of ethanol on the liver.     

Placental restriction reduces insulin sensitivity and expression of insulin signaling and glucose transporter genes in skeletal muscle, but not liver, in young sheep

Poor growth before birth is associated with impaired insulin sensitivity later in life, increasing the risk of type 2 diabetes. The tissue sites at which insulin resistance first develops after intrauterine growth restriction (IUGR), and its molecular basis, are unclear. We have therefore characterized the effects of placental restriction (PR), a major cause of IUGR, on whole-body insulin sensitivity and expression of molecular determinants of insulin signaling and glucose uptake in skeletal muscle and liver of young lambs. Whole-body insulin sensitivity was measured at 30 d by hyperinsulinaemic euglycaemic clamp and expression of insulin signaling genes (receptors, pathways, and targets) at 43 d in muscle and liver of control (n = 15) and PR (n = 13) lambs. PR reduced size at birth and increased postnatal growth, fasting plasma glucose (+15%, P = 0.004), and insulin (+115%, P = 0.009). PR reduced whole-body insulin sensitivity (-43%, P < 0.001) and skeletal muscle expression of INSR (-36%), IRS1 (-28%), AKT2 (-44%), GLUT4 (-88%), GSK3α (-35%), and GYS1 (-31%) overall (each P < 0.05) and decreased AMPKγ3 expression in females (P = 0.030). PR did not alter hepatic expression of insulin signaling and related genes but increased GLUT2 expression (P = 0.047) in males. Whole-body insulin sensitivity correlated positively with skeletal muscle expression of IRS1, AKT2, HK, AMPKγ2, and AMPKγ3 in PR lambs only (each P < 0.05) but not with hepatic gene expression in control or PR lambs. Onset of insulin resistance after PR and IUGR is accompanied by, and can be accounted for by, reduced expression of insulin signaling and metabolic genes in skeletal muscle but not liver.     

Glimepiride as insulin sensitizer: increased liver and muscle responses to insulin

Aim:  Glimepiride, a low-potency insulin secretagogue, is as efficient on glycaemic control as other sulphonylureas, suggesting an additional insulin-sensitizer role. The aim of the present study was to confirm the insulin-sensitizer role of glimepiride and to show extra-pancreatic effects of the drug.
Methods:  Three-month-old monosodium glutamate (MSG)–induced obese insulin-resistant rats were treated (OG) or not treated (O) with glimepiride for 4 weeks and compared with age-matched non-obese rats (C). Insulin sensitivity in whole body, glucose transporter 4 (GLUT4) protein content, glucose uptake and glycogen synthesis in oxidative skeletal muscle and phospho-glycogen synthase kinase (p-GSK3) and glycogen content in liver were analysed.
Results:  Insulin sensitivity, analysed by the insulin tolerance test, was 30% lower in O than in C rats (p < 0.05), and OG rats recovered this parameter (p < 0.05). In oxidative muscle, glimepiride increased the GLUT4 protein content (50%, p < 0.001) and recovered the obesity-induced reduction (∼20%) of the in vitro insulin-stimulated glucose uptake and incorporation into glycogen. In liver, glimepiride increased p-GSK3 (p < 0.01) and glycogen (p < 0.05) contents.
Conclusion:  The increased GLUT4 protein expression and glucose utilization in oxidative muscle and the increased insulin sensitivity and glycogen storage in liver evidence the insulin-sensitizer effect of glimepiride, which must be important to enable the glimepiride drug to promote an efficient glycaemic control.     

Components of signaling pathways for insulin and insulin-like growth factor-I in muscle myoblasts and myotubes.

To survey and compare the signaling pathways from the insulin and insulin-like growth factor-I (IGF-I) receptors in undifferentiated and differentiated muscle cells, we examined the phosphotyrosine (Ptyr)-containing polypeptides elicited in L6 and Sol8 myoblasts and myotubes by the combination of insulin and IGF-I. These polypeptides were detected by immunoblotting with antibodies against Ptyr. In the L6 myoblasts and myotubes and the Sol8 myoblasts, Ptyr polypeptides of approximately 240, 175, 115, 100, 41, and 37 kilodaltons (kDa) appeared in response to insulin-IGF-I. With the Sol8 myotubes, the 240-, 175-, and 37-kDa Ptyr polypeptides were detected in basal cells, and only the Ptyr content of the 175-kDa one increased in response to insulin-IGF-I. The polypeptides of 175, 41, and 37 kDa were tentatively identified as the insulin receptor substrate 1 (IRS1) and extracellular signal-regulated kinases 1 and 2 (ERK1 and -2), respectively, by immunoblotting with antibodies specific for these proteins, and the 115- and 100-kDa polypeptides are probably the beta-subunits of the insulin and IGF-I receptors. The amounts of IRS1, ERK1, and ERK2 were roughly the same in the L6 and Sol8 myoblasts and myotubes. Thus, differentiation of the myoblasts to myotubes was not accompanied by the detectable appearance of new insulin-IGF-I-elicited Ptyr polypeptides or marked changes in the amounts of known participants in their signaling pathways.     

IKKβ/NF-κB信号通路与骨骼肌胰岛素抵抗

骨骼肌是胰岛素抵抗发生的主要部位.IκB激酶β(IKKβ)/核因子-κB(NF-κB)信号通路通过干扰正常胰岛素信号转导和诱导机体低水平慢性炎性反应,在骨骼肌胰岛素抵抗中发挥重要作用.通过作用于IKKB/NF-κB信号通路达到治疗2型糖尿病的目的,可成为一个新的研究方向.     

Delays in insulin signaling towards glucose disposal in human skeletal muscle

We explored whether the delay that occurs between a rise in plasma insulin and the increase of glucose disposal occurs before, at, or downstream of steps that are believed to be part of the insulin signaling cascade. Skeletal muscle biopsies were obtained from 16 nondiabetic subjects before, and 20 and 180 min after plasma insulin levels had been augmented in euglycemic hyperinsulinemic glucose clamps. Although plasma insulin had reached 98% of its final concentration within 10 min, insulin receptor kinase (IRK) activity, p85 associated with insulin receptor substrate-1 (IRS-1), IRS-1-associated phosphatidylinositol 3-kinase (PI3K) activity, and Thr(308)-protein kinase B (PKB) phosphorylation in the muscle biopsies at 20 min had reached only 60, 48, 34 and 47% respectively of those at 180 min. This suggests a delay before the level of IRK and little or no delay between IRK and PKB activation. The observation that glycogen synthase activity and glucose disposal at 20 min had both only reached 25% of the respective values at 180 min suggests an additional delay downstream of the investigated signaling steps.     

Reversal of insulin resistance postpartum is linked to enhanced skeletal muscle insulin signaling

The restoration of maternal insulin sensitivity postpartum represents an important physiological and metabolic adaptation in a woman's reproductive lifespan. The present study was conducted to examine the potential cellular mechanisms underlying the changes in insulin sensitivity from late pregnancy to postpartum in human skeletal muscle. Nine nonobese women (age, 32 +/- 2 yr; body mass index, 21.2 +/- 0.8 kg/m(2)) with normal glucose tolerance were studied during late pregnancy (30-36 wk) and again approximately 1 yr postpartum using a euglycemic-hyperinsulinemic clamp (5 mm glucose, 40 mU/m(2).min insulin) to determine insulin sensitivity. Biopsies of the vastus lateralis muscle were obtained in the basal state before each clamp. Insulin sensitivity improved by 74% from late pregnancy to 1 yr postpartum (5.5 +/- 0.6 vs. 9.6 +/- 0.9 mg/kg fat-free mass.min; P < 0.005). Skeletal muscle insulin receptor (IR) protein expression increased by 42% postpartum, as measured by ELISA (4.0 +/- 0.6 vs. 5.7 +/- 0.6 ng/g protein; P < 0.05) and by Western blotting of the IR beta-subunit (28.7 +/- 4.7 vs. 42.0 +/- 4.8 arbitrary units; P < 0.003). However, in vitro studies showed that when adjusted for IR concentration, maximal insulin-stimulated (100 nm) IR tyrosine phosphorylation (0.75 +/- 0.06 vs. 0.92 +/- 0.08 U) and IR tyrosine kinase activity (183.8 +/- 27.0 vs. 204.3 +/- 23.7 fmol ATP/ng IR) were unchanged. There was a 69% increase in IR substrate-1 (IRS-1) protein expression (P = 0.05) in muscle postpartum. In addition, the p85alpha regulatory subunit of phosphatidylinositol 3-kinase was markedly reduced by 55% (P < 0.02) postpartum. The change in insulin sensitivity from late pregnancy to postpartum correlated highly with the corresponding change in IRS-1 protein (r = 0.84; P < 0.007). Downstream signaling proteins, including total Akt and p70s6 kinase, and the glucose transporter protein GLUT-4, were similar at both time points. These data suggest that reduced IR tyrosine kinase activity is not a major factor in the IR of pregnancy in lean women with normal glucose tolerance. Rather, the reversal of insulin resistance 1 yr postpartum is accompanied by increased skeletal muscle IRS-1 along with a down-regulation of the p85alpha subunit of phosphatidylinositol 3-kinase. These changes may allow for greater p85/p110 binding to IRS-1 and play a significant physiological role in the underlying metabolic adaptation to normal human pregnancy and restoration of insulin sensitivity postpartum.