Dietary protein deprivation decreases the serine phosphorylation of insulin receptor substrate-1 in rat skeletal muscle

Gestational low dietary protein induces intrauterine inflammation and alters the programming of adiposity & insulin sensitivity in the adult offspring

Malnutrition associated with low dietary protein can induce gestational inflammation and sets a long-lasting metabolic impact on the offspring even after replenishment. The work investigated whether a low-protein diet (LPD) during pregnancy and lactation induces intrauterine inflammation and predisposes offspring to adiposity and insulin resistance in their adult life. Female Golden Syrian hamsters were fed LPD (10.0% energy from protein) or a control diet (CD, 20.0 % energy from protein) from preconception until lactation. All pups were switched to CD after lactation and continued until the end. Maternal LPD increased intrauterine inflammation by enhancing neutrophil infiltration, amniotic hsCRP, oxidative stress, and mRNA expression of NFκβ, IL8, COX2, and TGFβ in the chorioamniotic membrane (P<.05). The prepregnancy body weight, placental, and fetal weights, serum AST and ALT were decreased, while blood platelets, lymphocytes, insulin, and HDL were significantly increased in LPD-fed dams (P<.05). A postnatal switch to an adequate protein could not prevent hyperlipidemia in the 6-months LPD/CD offspring. The lipid profile and liver functions were restored over 10 months of protein feeding but failed to normalize fasting glucose and body fat accumulation compared to CD/CD. LPD/CD showed elevated GLUT4 expression & activated pIRS1 in the skeletal muscle and increased expression of IL6, IL1β, and p65-NFκB proteins in the liver (P<.05). In conclusion, present data suggest that maternal protein restriction may induce intrauterine inflammation and affect liver inflammation in the adult offspring by an influx of fats from adipose that may alter lipid metabolism and reduce insulin sensitivity in skeletal muscle.

Disruption of insulin receptor substrate-2 impairs growth but not insulin function in rats

Insulin receptor substrate (IRS)-2, along with IRS-1, is a key signaling molecule that mediates the action of insulin and insulin-like growth factor (IGF)-I. The activated insulin and IGF-I receptors phosphorylate IRSs on tyrosine residues, leading to the activation of downstream signaling pathways and the induction of various physiological functions of insulin and IGF-I. Studies using IRS-2 knockout (KO) mice showed that the deletion of IRS-2 causes type 2 diabetes due to peripheral insulin resistance and impaired β-cell function. However, little is known about the roles of IRS-2 in other animal models. Here, we created IRS-2 KO rats to elucidate the physiological functions of IRS-2 in rats. The body weights of IRS-2 KO rats at birth were lower compared with those of their WT littermates. The postnatal growth of both male and female IRS-2 KO rats was also suppressed. Compared with male WT rats, the glucose and insulin tolerance of male IRS-2 KO rats were slightly enhanced, whereas a similar difference was not observed between female WT and IRS-2 KO rats. Besides the modestly increased insulin sensitivity, male IRS-2 KO rats displayed the enhanced insulin-induced activation of the mTOR complex 1 pathway in the liver compared with WT rats. Taken together, these results indicate that in rats, IRS-2 plays important roles in the regulation of growth but is not essential for the glucose-lowering effects of insulin.

Branched-chain amino acid supplementation restores reduced insulinotropic activity of a low-protein diet through the vagus nerve in rats

Background

Previously, we reported that a low-protein diet significantly reduced insulin secretion in response to feeding within 1 h in rats, suggesting that the insulinotropic effect of dietary protein plays an important role in maintaining normal insulin release. The current study aimed to elucidate whether deficiency of certain amino acids could diminish the insulinotropic activity and to investigate whether reduced insulin secretion in response to a low-protein diet is restored by supplementation with certain amino acids.

Methods

First, we fed male Wistar rats (5–6 rats per group) with diets deficient in every single amino acid or three branched-chain amino acids (BCAAs); within 1–2 h after the onset of feeding, we measured the plasma insulin levels by using an enzyme-linked immunosorbent assay (ELISA). As insulin secretion was reduced in BCAA-deficient groups, we fed low-protein diets supplemented with BCAAs to assess whether the reduced insulin secretion was restored. In addition, we treated the pancreatic beta cell line MIN6 with BCAAs to investigate the direct insulinotropic activity on beta cells. Lastly, we investigated the effect of the three BCAAs on sham-operated or vagotomized rats to assess involvement of the vagus nerve in restoration of the insulinotropic activity.

Results

Feeding a low-protein diet reduced essential amino acid concentrations in the plasma during an absorptive state, suggesting that reduced plasma amino acid levels can be an initial signal of protein deficiency. In normal rats, insulin secretion was reduced when leucine, valine, or three BCAAs were deficient. Insulin secretion was restored to normal levels by supplementation of the low-protein diet with three BCAAs, but not by supplementation with any single BCAA. In MIN6 cells, each BCAA alone stimulated insulin secretion but the three BCAAs did not show a synergistic stimulatory effect. The three BCAAs showed a synergistic stimulatory effect in sham-operated rats but failed to stimulate insulin secretion in vagotomized rats.

Conclusions

Leucine and valine play a role in maintaining normal insulin release by directly stimulating beta cells, and supplementation with the three BCAAs is sufficient to compensate for the reduced insulinotropic activity of the low-protein diet, through the vagus nerve.

Acetylcholinesterase (AChE) inhibition aggravates fasting-induced triglyceride accumulation in the mouse liver

Although fasting induces hepatic triglyceride (TG) accumulation in both rodents and humans, little is known about the underlying mechanism. Because parasympathetic nervous system activity tends to attenuate the secretion of very-low-density-lipoprotein-triglyceride (VLDL-TG) and increase TG stores in the liver, and serum cholinesterase activity is elevated in fatty liver disease, the inhibition of the parasympathetic neurotransmitter acetylcholinesterase (AChE) may have some influence on hepatic lipid metabolism. To assess the influence of AChE inhibition on lipid metabolism, the effect of physostigmine, an AChE inhibitor, on fasting-induced increase in liver TG was investigated in mice. In comparison with ad libitum-fed mice, 30 h fasting increased liver TG accumulation accompanied by a downregulation of sterol regulatory element-binding protein 1 (SREBP-1) and liver-fatty acid binding-protein (L-FABP). Physostigmine promoted the 30 h fasting-induced increase in liver TG levels in a dose-dependent manner, accompanied by a significant fall in plasma insulin levels, without a fall in plasma TG. Furthermore, physostigmine significantly attenuated the fasting-induced decrease of both mRNA and protein levels of SREBP-1 and L-FABP, and increased IRS-2 protein levels in the liver. The muscarinic receptor antagonist atropine blocked these effects of physostigmine on liver TG, serum insulin, and hepatic protein levels of SREBP-1 and L-FABP. These results demonstrate that AChE inhibition facilitated fasting-induced TG accumulation with up regulation of the hepatic L-FABP and SREBP-1 in mice, at least in part via the activation of muscarinic acetylcholine receptors. Our studies highlight the crucial role of parasympathetic regulation in fasting-induced TG accumulation, and may be an important source of information on the mechanism of hepatic disorders of lipid metabolism.

Kettelhut I, Navegantes LC, Chaves VE, Bertolini GL, Kawashita NH. High glucose uptake in growing rats adapted to a low-protein, high-carbohydrate diet determines low fasting glycemia even with high hepatic gluconeogenesis

The our objective was to investigate the adaptations induced by a low-protein, high-carbohydrate (LPHC) diet in growing rats, which by comparison with the rats fed a control (C) diet at displayed lower fasting glycemia and similar fasting insulinemia, despite impairment in insulin signaling in adipose tissues. In the insulin tolerance test the LPHC rats showed higher rates of glucose disappearance (30%) and higher tolerance to overload of glucose than C rats. The glucose uptake by the soleus muscle, evaluated in vivo by administration of 2-deoxy-[14C]glucose, increased by 81%. The phosphoenolpyruvate carboxykinase content and the incorporation of [1-14C]pyruvate into glucose was also higher in the slices of liver from the LPHC rats than in those from C rats. The LPHC rats showed increases in l-lactate as well as in other gluconeogenic precursors in the blood. These rats also had a higher hepatic production of glucose, evaluated by in situ perfusion. The data obtained indicate that the main substrates for gluconeogenesis in the LPHC rats are l-lactate and glycerol. Thus, we concluded that the fasting glycemia in the LPHC animals was maintained mainly by increases in the hepatic gluconeogenesis from glycerol and l-lactate, compensating, at least in part, for the higher glucose uptake by the tissues.

Disruption of the Availability of Amino Acids Induces a Rapid Reduction of Serine Phosphorylation of Insulin Receptor Substrate-1in Vivoandin Vitro

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signal transduction. It has been shown that the amino acids modulate insulin signaling at the level of IRS-1. Here we show that an amino acid unbalanced diet causes a reduction in serine phosphorylation as well as an elevation in insulin-induced tyrosine phosphorylation of IRS-1 in rat muscle. In fibroblasts and myotube cells, the effect of amino acid deprivation on IRS-1 phosphorylation was evident only when cells were pretreated with reagents causing hyperphosphorylation of serines of IRS-1. But, the target kinases of these reagents were not inactivated by amino acid deprivation, suggesting that amino acid deprivation activates serine/threonine phosphatase(s) of IRS-1. The phosphatases regulated by mammalian target of rapamycin do not appear to participate in the dephosphorylation either. These results suggest that amino acid deprivation dephosphorylates IRS-1 through unidentified serine/threonine phosphatases and thereby potentiates insulin signaling.

Insulin injection restored increased insulin receptor substrate (IRS)-2 protein during short-term protein restriction but did not affect reduced insulin-like growth factor (IGF)-I mRNA or increased triglyceride accumulation in the liver of rats

Dietary protein restriction reduces insulin-like growth factor (IGF)-I synthesis and impairs growth. Moreover, insulin secretion is impaired and hepatic insulin signaling is activated presumably through upregulation of insulin receptor substrate (IRS)-2, which can stimulate lipogenesis thereby resulting in steatosis. In order to determine whether impaired insulin secretion is the primary cause of these changes, we injected insulin into protein-restricted rats and compensated for the reduction in insulin secretion for 1 and 7 d. Insulin infusion did not overcome the reduction in liver IGF-I mRNA nor the hepatic triglyceride accumulation. In contrast, it clearly suppressed the upregulation of hepatic IRS-2 on day 1, but not on day 7. Furthermore, insulin elimination increased IRS-2 in H4IIE-C3 cells. In summary, we found that reduced insulin secretion during protein restriction directly increased hepatic IRS-2 as a rapid response on day 1, while additional mechanisms contributed to the upregulation of IRS-2 on day 7.

Tissue-specific effects of protein malnutrition on insulin signaling pathway and lipid accumulation in growing rats

Our previous studies have revealed that protein malnutrition enhances insulin signaling in rat liver and muscle in response to a bolus insulin injection. However, it has not been established whether protein malnutrition up-regulates insulin signaling under physiological conditions, such as feeding. Here, we studied the effects of protein malnutrition on insulin signaling after feeding in rat liver, muscle and white adipose tissue (WAT). Six-week-old rats were fed a 15% casein diet (15C) or a calorie-matched 5% casein diet (5C) for 8 h/day during 14 days. On the 15th day, blood and tissues were collected at various time points after feeding. Feeding-induced insulin secretion was reduced in 5C-fed rats compared to 15C-fed rats. The 5C-feeding suppressed immediate activation of insulin receptor after feeding in the liver, muscle, and WAT. However, 5C-feeding constantly increased tyrosine phosphorylation of insulin receptor substrate (IRS)-2 and threonine phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in the liver during the examined periods, corresponding to the changes of their amounts. In skeletal muscle, 5C-feeding did not appreciably alter insulin signaling. In WAT, 5C-feeding decreased tyrosine phosphorylation of IRS-1 compared to 15C-feeding. Furthermore, hepatic triglyceride content was increased and feeding-induced acetyl-CoA carboxylase 1 gene expression was enhanced in 5C-fed rats. The 5C-feeding decreased insulin-dependent glucose uptake in adipocytes. These results suggest that enhanced insulin signaling through increased IRS-2 and 4E-BP1 levels in the liver and repressed insulin signaling through decreased IRS-1 levels in WAT contribute to the preferential hepatic lipid accumulation under protein malnutrition.

Insulin release, peripheral insulin resistance and muscle function in protein malnutrition: a role of tricarboxylic acid cycle anaplerosis

Pancreatic β-cells and skeletal muscle act in a synergic way in the control of systemic glucose homeostasis. Several pyruvate-dependent and -independent shuttles enhance tricarboxylic acid cycle intermediate (TACI) anaplerosis and increase β-cell ATP:ADP ratio, triggering insulin exocytotic mechanisms. In addition, mitochondrial TACI cataplerosis gives rise to the so-called metabolic coupling factors, which are also related to insulin release. Peripheral insulin resistance seems to be related to skeletal muscle fatty acid (FA) accumulation and oxidation imbalance. In this sense, exercise has been shown to enhance skeletal muscle TACI anaplerosis, increasing FA oxidation and by this manner restores insulin sensitivity. Protein malnutrition reduces β-cell insulin synthesis, release and peripheral sensitivity. Despite little available data concerning mitochondrial metabolism under protein malnutrition, evidence points towards reduced β-cell and skeletal muscle mitochondrial capacity. The observed decrease in insulin synthesis and release may reflect reduced anaplerotic and cataplerotic capacity. Furthermore, insulin release is tightly coupled to ATP:ADP rise which in turn is related to TACI anaplerosis. The effect of protein malnutrition upon peripheral insulin resistance is time-dependent and directly related to FA oxidation capacity. In contrast to β-cells, TACI anaplerosis and cataplerosis pathways in skeletal muscle seem to control FA oxidation and regulate insulin resistance.

Increased susceptibility to metabolic alterations in young adult females exposed to early malnutrition

Early malnutrition during gestation and lactation modifies growth and metabolism permanently. Follow up studies using a nutritional rehabilitation protocol have reported that early malnourished rats exhibit hyperglycemia and/or hyperinsulinemia, suggesting that the effects of early malnutrition are permanent and produce a “programming” effect on metabolism. Deleterious effects have mainly been observed when early-malnutrition is followed by a high-carbohydrate or a high-fat diet.

The aim of this study was to evaluate whether following a balanced diet subsequent to malnutrition can deter the expression of metabolic disease and lead rats to exhibit metabolic responses, similar to those of well-nourished controls.

Young rats, born from dams malnourished during gestation and lactation with a low protein diet, were provided with a regular balanced chow diet upon weaning. At 90 days of age, the effects of rehabilitation were determined under three different feeding conditions: ad libitum, fasting or fasting-reefed satiated.

Early-malnourished rats showed an increased rate of body weight gain. Males under ad libitum conditions showed an elevated concentration of hepatic glycogen and low values of insulin. In the fasting-reefed satiated condition, only early-malnourished females showed an alteration in glucose response and glucagon level, compared with their well-nourished controls.

Data indicate that a balanced diet along life after early malnutrition can mask the expression of metabolic disorders and that a metabolic challenges due to a prolonged fasting and reefed state unmask metabolic deficiencies in early-malnourished females.

Perturbed skeletal muscle insulin signaling in the adult female intrauterine growth-restricted rat

To determine the molecular mechanism(s) linking fetal adaptations in intrauterine growth restriction (IUGR) to adult maladaptations of type 2 diabetes mellitus, we investigated the effect of prenatal seminutrient restriction, modified by early postnatal ad libitum access to nutrients (CM/SP) or seminutrient restriction (SM/SP), vs. early postnatal seminutrient restriction alone (SM/CP) or control nutrition (CM/CP) on the skeletal muscle postreceptor insulin-signaling pathway in the adult offspring. The altered in utero hormonal/metabolic milieu was associated with no change in basal total IRS-1, p85, and p110beta subunits of PI 3-kinase, PKCtheta, and PKCzeta concentrations but an increase in basal IRS-2 (P < 0.05) only in the CM/SP group and an increase in basal phospho (p)-PDK-1 (P < 0.05), p-Akt (P < 0.05), and p-PKCzeta (P < 0.05) concentrations in the CM/SP and SM/SP groups. Insulin-stimulated increases in p-PDK-1 (P < 0.05) and p-Akt (P < 0.0007), with no increase in p-PKCzeta, were seen in both CM/SP and SM/SP groups. SHP2 (P < 0.03) and PTP1B (P < 0.03) increased only in SM/SP with no change in PTEN in CM/SP and SM/SP groups. Aberrations in kinase and phosphatase moieties in the adult IUGR offspring were initiated in utero but further sculpted by the early postnatal nutritional state. Although the CM/SP group demonstrated enhanced kinase activation, the SM/SP group revealed an added increase in phosphatase concentrations with the net result of heightened basal insulin sensitivity in both groups. The inability to further respond to exogenous insulin was due to the key molecular distal roadblock consisting of resistance to phosphorylate and activate PKCzeta necessary for GLUT4 translocation. This protective adaptation may become maladaptive and serve as a forerunner for gestational and type 2 diabetes mellitus.

Effect of food restriction on the insulin signalling pathway in rat skeletal muscle and adipose tissue

We tried to elucidate the effects of a brief and severe model of food restriction on insulin sensitivity in female rats, focusing on key proteins involved in the insulin signalling pathway in skeletal muscle and adipose tissue after 5, 10 and 15 days of food restriction. Using euglycemic clamp, we detected that food-restricted rats are significantly less sensitive to insulin action than control rats. However, the time of restriction promotes a progressive increase on insulin sensitivity. The analysis of the insulin signalling pathway showed a tissue-specific regulation of several proteins involved in insulin signalling. In skeletal muscle, insulin receptor substrate 1 and Glut4 are up-regulated at the end of the food restriction period, just the opposite of what we found in adipose tissue. In conclusion, a 50% reduction of food intake modulates insulin sensitivity through a tissue-specific regulation of the insulin signalling pathway in the main target tissues for this hormone.