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

Pharmacological Investigation of Hypoalbuminemia on the Prolonged and Potentiated Action of Midazolam in Rats

Midazolam (MDZ) is clinically used for its sedative and anticonvulsant properties. However, its prolonged or potentiated effects are sometimes concerning. The main binding protein of MDZ is albumin, and reduced serum albumin levels could lead to MDZ accumulation, thereby potentiating or prolonging its effects. Previous investigations have not thoroughly examined these phenomena from a behavioral pharmacology standpoint. Consequently, this study aimed to evaluate both the prolonged and potentiated effects of MDZ, as well as the effects of serum albumin levels on the action of MDZ in low-albumin rats. Male Wistar rats were classified into control (20% protein diet), low-protein (5% protein), and non-protein groups (0% protein diet) and were fed the protein-controlled diets for 30 d to obtain low-albumin rats. The locomotor activity and muscle relaxant effects of MDZ were evaluated using the rotarod, grip strength, and open-field tests conducted 10, 60, and 120 min after MDZ administration. Serum albumin levels decreased significantly in the low-protein and non-protein diet groups compared with those in the control group. Compared with the control rats, low-albumin rats demonstrated a significantly shorter time to fall, decreased muscle strength, and a significant decrease in the distance traveled after MDZ administration in the rotarod, grip strength, and open-field tests, respectively. Decreased serum albumin levels potentiated and prolonged the effects of MDZ. Hence, serum albumin level is a critical parameter associated with MDZ administration, which should be monitored, and any side effects related to decreased albumin levels should be investigated.

The role of increased FGF21 in VLDL-TAG secretion and thermogenic gene expression in mice under protein malnutrition

Protein malnutrition promotes hepatic lipid accumulation in growing animals. In these animals, fibroblast growth factor 21 (FGF21) rapidly increases in the liver and circulation and plays a protective role in hepatic lipid accumulation. To investigate the mechanism by which FGF21 protects against liver lipid accumulation under protein malnutrition, we determined whether upregulated FGF21 promotes the thermogenesis or secretion of very-low-density lipoprotein (VLDL)-triacylglycerol (TAG). The results showed that protein malnutrition decreased VLDL-TAG secretion, but the upregulation of FGF21 did not oppose this effect. In addition, protein malnutrition increased expression of the thermogenic gene uncoupling protein 1 in inguinal white adipose and brown adipose tissue in an FGF21-dependent manner. However, surgically removing inguinal white adipose tissue did not affect liver triglyceride levels in protein-malnourished mice. These data suggest that FGF21 stimulates thermogenesis under protein malnutrition, but this is not the causative factor underlying the protective role of FGF21 against liver lipid accumulation.

Very low-density lipoprotein receptor increases in a liver-specific manner due to protein deficiency but does not affect fatty liver in mice

Very low-density lipoprotein receptor (VLDLR) is a member of the LDL receptor family that is involved in the uptake of VLDL into cells. Increased hepatic VLDLR under endoplasmic reticulum (ER) stress has been shown to cause fatty liver. In this study, the effect of dietary protein restriction on hepatic VLDLR and the role of VLDLR in fatty liver were investigated using Vldlr knockout (KO) mice. Growing wild-type (WT) and KO mice were fed a control diet containing 20% ​​protein or a low protein diet containing 3% protein for 11 days. In WT mice, the amount of hepatic Vldlr mRNA and VLDLR protein increased by approximately 8- and 7-fold, respectively, due to protein restriction. Vldlr mRNA and protein levels increased in both type 1 and type 2 VLDLR. However, neither Vldlr mRNA nor protein levels were significantly increased in heart, muscle, and adipose tissue, demonstrating that VLDLR increase due to protein restriction occurred in a liver-specific manner. Increased liver triglyceride levels during protein restriction occurred in KO mice to the same extent as in WT mice, indicating that increased VLDLR during protein restriction was not the main cause of fatty liver, which was different from the case of ER stress.

Endogenous testosterone reduces hepatic lipid accumulation in protein-restricted male rats

OBJECTIVES

Protein deficiency is known to cause ectopic fat accumulation in the liver. The aim of this study was to analyse the mechanism of suppression of hepatic fat accumulation by testosterone and to clarify the mechanism behind the gender difference in fatty liver formation due to protein deficiency.

METHODS

Hepatic fat accumulation due to protein deficiency was evaluated in male and female rats before and after sexual maturation. Then, the effects of testosterone on liver lipid, muscle protein metabolism and energy expenditure in adipose tissue were investigated in castrated or testosterone-injected male rats fed control or protein-restricted diet.

RESULTS

Hepatic triglyceride accumulation diminished with sex maturation in male but not in female protein-restricted rats. Protein restriction resulted in a significant increase in hepatic triglyceride content in castrated rats but not in sham-operated rats demonstrating that endogenous testosterone reduces hepatic lipid accumulation in male rats. Protein restriction reduced plasma IGF-I and muscle protein synthesis measured using the SUnSET method. Castration increased the plasma corticosterone level and muscle autophagic activity. Muscle weight was reduced and energy expenditure in adipose tissue was increased only when both factors were combined.

CONCLUSIONS

Muscle protein synthesis downregulation owing to protein restriction and activation of autophagy following castration reduced muscle mass thereby releasing surplus energy and promoting steatosis in protein-restricted castrated rats despite increased energy expenditure in adipose tissue. We hypothesize that endogenous testosterone reduces hepatic lipid accumulation in protein-deficient male rats and provide novel findings on the gender-specific differences in hepatic steatosis.

Effect of dietary vitamin E on oxidative stress-related gene-mediated differences in anxiety-like behavior in inbred strains of mice

It has been reported that the degree of anxiety-like behavior differs between inbred strains of mice, and that this phenomenon was linked to the expression levels of the oxidative stress-related genes glyoxalase 1 (Glo1) and glutathione reductase 1 (Gsr) in the brain. Therefore, we investigated whether antioxidative activity in the brain affects the Glo1 and Gsr mRNA expressions and strain-dependent anxiety-like behavior using mice fed different amounts of vitamin E. First, we measured brain Glo1 and Gsr mRNA levels and evaluated the anxiety-like behaviors presented by C57BL/6J (B6) and DBA/2C (D2) mice. We demonstrated that D2 mice presented both significantly elevated Glo1 and Gsr mRNA levels as well as more prominent anxiety-like behavior in elevated plus-maze and open field tests. Next, we fed mice from these two strains either a control, vitamin E-free, or vitamin E-supplemented diet for four weeks. Plasma, liver, and brain α-tocopherol concentrations changed in a dose-dependent manner. However, neither brain Glo1 and Gsr mRNA levels nor anxiety-like behavior were affected by dietary vitamin E intake. These results demonstrated that while strain-dependent anxiety-like behavior in mice was related to oxidative stress-related gene expression, the regulatory mechanisms for these genes and anxiety-like behaviors were independent of antioxidative activity in the brain. Strain-dependent differences of the anxiety in mice are probably related to the anxiolytic effects of methylglyoxal, a substrate for Glo1 and Gsr.

Dietary protein restriction increases hepatic leptin receptor mRNA and plasma soluble leptin receptor in male rodents

Leptin is an adipokine that regulates adipose tissue mass through membrane-anchored leptin receptor (Ob-R). Extracellular domain of Ob-R in plasma is called soluble leptin receptor (sOb-R), and is the main leptin-binding protein. Based on a previous DNA microarray analysis that showed induction of hepatic Ob-R mRNA in low-protein diet-fed mice, this study aimed to clarify the effect of dietary protein restriction on hepatic Ob-R mRNA and plasma sOb-R levels. First, the effect of protein restriction on hepatic Ob-R mRNA level was examined together with fasting and food restriction using male rats as common experimental model for nutritional research. Hepatic Ob-R mRNA level was increased by feeding low-protein diet for 7 d, although not significantly influenced by 12-h fasting and sixty percent restriction in food consumption. Then, effect of protein restriction on liver Ob-R and plasma sOb-R was investigated using male mice because specific sOb-R ELISA was more available for mice. Hepatic Ob-R mRNA level was also increased in protein restricted-mice although it did not increase in hypothalamus. Hepatic Ob-R protein was decreased, whereas plasma sOb-R was increased by protein restriction. Because the concentration of sOb-R increased without changing plasma leptin concentration, free leptin in plasma was significantly reduced. The direct effect of amino acid deprivation on Ob-R mRNA level was not observed in rat hepatoma cells H4IIE cultured in amino acid deprived medium. In conclusion, dietary protein restriction increased hepatic Ob-R mRNA, resulting in increased plasma sOb-R concentration, which in turn, reduces plasma free leptin level and may modulate leptin activity.

Three Novel Mutations I65S, R66S, and G86R Divulge Significant Conformational Variations in the PTB Domain of the IRS1 Gene

Insulin receptor substrate 1 (IRS1) is one of the major substrates for the IR, and their interaction mediates several downstream insulin signaling pathways. In this study, we have identified three novel mutations in the IRS1 gene of type 2 diabetic (T2D) patients, which reflected in the amino acid changes as I65S, R66S, and G86R in the phosphotyrosine binding domain of the IRS1 protein. The impact of these mutations on the structure and function of the IRS1 protein was evaluated through molecular modeling studies, and distinct conformational fluctuations were recorded. The variable binding affinities and positional displacement of these mutant models were observed in the ligand-binding cleft of IR. The mutant IRS1 models triggered conformational changes in the L1 domain of IR upon their binding. Such structural variations in IRS1 and IR structures due to mutations resulted in variable molecular interactions that could lead to altered insulin transduction, followed by insulin resistance and T2D.

A low-protein diet eliminates the circadian rhythm of serum insulin and hepatic lipid metabolism in mice

Insulin is a key molecule that synchronizes peripheral clocks, such as that in the liver. Although we previously reported that mice fed a low-protein diet showed altered expression of lipid-related genes in the liver and induction of hepatic steatosis, it is unknown whether a low-protein diet impairs insulin secretion and modifies the hepatic circadian rhythm. Therefore, we investigated the effects of the intake of a low-protein diet on the circadian rhythm of insulin secretion and hepatic lipid metabolism in mice. Under 12-h light/12-h dark cycle, mice fed a low-protein diet for 7 days displayed enhanced food intake at the end of the light phase, although central and peripheral PER2 expression rhythm was maintained. Serum insulin levels in mice fed a low-protein diet remained low during the day, and the insulin secretion in OGTT was also markedly lower than in normal mice. In mice fed low-protein diet, hepatic TG accumulation was observed during the nighttime, with relatively high levels of ACC1 mRNA and total ACC proteins. Although there were no differences in the activity rhythm of hepatic mTOR between mice fed a normal or low-protein diet, hepatic IRS-2 expression in mice fed a low-protein diet remained low during the day, with no increase at the beginning of the light period. These results suggested that the low-protein diet eliminated the circadian rhythm of serum insulin and hepatic lipid metabolism in mice, providing insights into our understanding of the mechanisms of hepatic disorders of lipid metabolism.

IGF1 receptor signaling pathways

Insulin-like growth factors (IGFs) bind specifically to the IGF1 receptor on the cell surface of targeted tissues. Ligand binding to the α subunit of the receptor leads to a conformational change in the β subunit, resulting in the activation of receptor tyrosine kinase activity. Activated receptor phosphorylates several substrates, including insulin receptor substrates (IRSs) and Src homology collagen (SHC). Phosphotyrosine residues in these substrates are recognized by certain Src homology 2 (SH2) domain-containing signaling molecules. These include, for example, an 85 kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase (PI 3-kinase), growth factor receptor-bound 2 (GRB2) and SH2-containing protein tyrosine phosphatase 2 (SHP2/Syp). These bindings lead to the activation of downstream signaling pathways, PI 3-kinase pathway and Ras-mitogen-activated protein kinase (MAP kinase) pathway. Activation of these signaling pathways is known to be required for the induction of various bioactivities of IGFs, including cell proliferation, cell differentiation and cell survival. In this review, the well-established IGF1 receptor signaling pathways required for the induction of various bioactivities of IGFs are introduced. In addition, we will discuss how IGF signals are modulated by the other extracellular stimuli or by themselves based on our studies.

Importance of Serum Amino Acid Profile for Induction of Hepatic Steatosis under Protein Malnutrition

We previously reported that a low-protein diet caused animals to develop fatty liver containing a high level of triglycerides (TG), similar to the human nutritional disorder “kwashiorkor”. To investigate the underlying mechanisms, we cultured hepatocytes in amino acid-sufficient or deficient medium. Surprisingly, the intracellular TG level was increased by amino acid deficiency without addition of any lipids or hormones, accompanied by enhanced lipid synthesis, indicating that hepatocytes themselves monitored the extracellular amino acid concentrations to induce lipid accumulation in a cell-autonomous manner. We then confirmed that a low-amino acid diet also resulted in the development of fatty liver, and supplementation of the low-amino acid diet with glutamic acid to compensate the loss of nitrogen source did not completely suppress the hepatic TG accumulation. Only a dietary arginine or threonine deficiency was sufficient to induce hepatic TG accumulation. However, supplementation of a low-amino acid diet with arginine or threonine failed to reverse it. In silico analysis succeeded in predicting liver TG level from the serum amino acid profile. Based on these results, we conclude that dietary amino acid composition dynamically affects the serum amino acid profile, which is sensed by hepatocytes and lipid synthesis was activated cell-autonomously, leading to hepatic steatosis.

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.

Leucine restores murine hepatic triglyceride accumulation induced by a low-protein diet by suppressing autophagy and excessive endoplasmic reticulum stress

Although it is known that a low-protein diet induces hepatic triglyceride (TG) accumulation in both rodents and humans, little is known about the underlying mechanism. In the present study, we modeled hepatic TG accumulation by inducing dietary protein deficiency in mice and aimed to determine whether certain amino acids could prevent low-protein diet-induced TG accumulation in the mouse liver. Mice fed a diet consisting of 3 % casein (3C diet) for 7 days showed hepatic TG accumulation with up-regulation of TG synthesis for the Acc gene and down-regulation of TG-rich lipoprotein secretion from hepatocytes for Mttp genes. Supplementing the 3 % casein diet with essential amino acids, branched-chain amino acids, or the single amino acid leucine rescued hepatic TG accumulation. In the livers of mice fed the 3 % casein diet, we observed a decrease in the levels of the autophagy substrate p62, an increase in the expression levels of the autophagy marker LC3-II, and an increase in the splicing of the endoplasmic reticulum (ER) stress-dependent Xbp1 gene. Leucine supplementation to the 3 % casein diet did not affect genes related to lipid metabolism, but inhibited the decrease in p62, the increase in LC3-II, and the increase in Xbp1 splicing levels in the liver. Our results suggest that ER stress responses and activated autophagy play critical roles in low-protein diet-induced hepatic TG accumulation in mice, and that leucine suppresses these two major protein degradation systems. This study contributes to understanding the mechanisms of hepatic disorders of lipid metabolism.

Rapid increase in fibroblast growth factor 21 in protein malnutrition and its impact on growth and lipid metabolism

Protein malnutrition promotes hepatic steatosis, decreases insulin-like growth factor (IGF)-I production and retards growth. To identify new molecules involved in such changes, we conducted DNA microarray analysis on liver samples from rats fed an isoenergetic low-protein diet for 8 h. We identified the fibroblast growth factor 21 gene (Fgf21) as one of the most strongly up-regulated genes under conditions of acute protein malnutrition (P<0·05, false-discovery rate<0·001). In addition, amino acid deprivation increased Fgf21 mRNA levels in rat liver-derived RL-34 cells (P<0·01). These results suggested that amino acid limitation directly increases Fgf21 expression. FGF21 is a polypeptide hormone that regulates glucose and lipid metabolism. FGF21 also promotes a growth hormone-resistance state and suppresses IGF-I in transgenic mice. Therefore, to determine further whether Fgf21 up-regulation causes hepatic steatosis and growth retardation after IGF-I decrease in protein malnutrition, we fed an isoenergetic low-protein diet to Fgf21-knockout (KO) mice. Fgf21-KO did not rescue growth retardation and reduced plasma IGF-I concentration in these mice. Fgf21-KO mice showed greater epididymal white adipose tissue weight and increased hepatic TAG and cholesterol levels under protein malnutrition conditions (P<0·05). Overall, the results showed that protein deprivation directly increased Fgf21 expression. However, growth retardation and decreased IGF-I were not mediated by increased FGF21 expression in protein malnutrition. Furthermore, FGF21 up-regulation rather appears to have a protective effect against obesity and hepatic steatosis in protein-malnourished animals.

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.