Effect of high-fat diet on gene expression of GLUT4 and insulin receptor in soleus muscle

24-h glucose level and variability in response to carbohydrate overfeeding assessed using continuous glucose monitoring system are associated with daily carbohydrate intake

BACKGROUND & AIMS

The quantity gap between daily and loaded carbohydrates may affects blood glucose response to carbohydrate intake; however, no study has investigated the difference in 24-h span. This study aimed to determine differences in the 24-h glucose levels and variability in response to single-day carbohydrate overfeeding based on daily carbohydrate intake in healthy Japanese men.

METHODS

Twenty male college students completed a 3-day dietary record and were divided into two groups based on whether their daily carbohydrate intake exceeded the median intake (H-CHO) or not (L-CHO). Thereafter, the participants consumed a high-carbohydrate diet (carbohydrate 8.1 g/kg/d) for 1 day. The 24-h glucose levels and glucose variability (CONGA1) were measured using a continuous glucose monitoring system.

RESULTS

The mean daily carbohydrate intakes in the L-CHO and H-CHO groups were 3.9 ± 0.5 and 5.8 ± 0.6 g/kg/d, respectively (p < 0.001). The peak 24-h glucose level was not differ between the L-CHO group and the H-CHO group (8.0 ± 0.8 vs. 8.0 ± 1.0; p = 0.886). The mean 24-h glucose level was higher in the L-CHO group than in the H-CHO group (6.0 ± 0.3 vs. 5.6 ± 0.3 mmol/L; p = 0.010). The CONGA1 was higher in the L-CHO group than in the H-CHO group (5.40 ± 0.41 vs. 4.95 ± 0.25; p = 0.008).

CONCLUSIONS

Mean glucose level and glucose variability in response to carbohydrate overfeeding were high in the individuals with small daily carbohydrate intake. These findings suggest that the large quantity gap between daily and loaded carbohydrates cause worse glucose control during carbohydrate overfeeding.

Association between PLA2 gene polymorphisms and treatment response to antipsychotic medications: A study of antipsychotic-naïve first-episode psychosis patients and nonadherent chronic psychosis patients

Here we investigated whether antipsychotic treatment was influenced by three polymorphisms: rs10798059 (BanI) in the phospholipase A2 (PLA2)G4A gene, rs4375 in PLA2G6, and rs1549637 in PLA2G4C. A total of 186 antipsychotic-naïve first-episode psychosis patients or nonadherent chronic psychosis individuals (99 males and 87 females) were genotyped by polymerase chain reaction analysis/restriction fragment length polymorphism. At baseline, and after 8 weeks of treatment with various antipsychotic medications, we assessed patients' Positive and Negative Syndrome Scale (PANSS) scores, PANSS factors, and metabolic syndrome-related parameters (fasting plasma lipid and glucose levels, and body mass index). We found that PLA2G4A polymorphism influenced changes in PANSS psychopathology, and PLA2G6 polymorphism influenced changes in PANSS psychopathology and metabolic parameters. PLA2G4C polymorphism did not show any impact on PANSS psychopathology or metabolic parameters. The polymorphisms' effect sizes were estimated as moderate to strong, with contributions ranging from around 6.2-15.7%. Furthermore, the polymorphisms' effects manifested in a gender-specific manner.

Remodeling of the soleus muscle of ovariectomized old female rats submitted to resistance training and different diet intake

Sarcopenia is a common condition that is associated mainly with hormonal factors, nutritional status, physical activity, leading to a lower quality of life. Thus, this study aimed to evaluate the effects of diets with vegetable or animal proteins (AP) associated with resistance training on the structure of the soleus muscle in aged Wistar rats. The histochemical technique was used for the typing of muscle fibers, the cross-sectional area of myocytes, and volume densities of myocytes and interstitium. Picrosirius stain was used to quantify the collagen density. Diet intake, mainly animal protein, associated with resistance training leaded to muscle remodeling, and increased deposit of collagen fibers. We observed hypertrophy in animal groups that consumed animal protein diet, even the sedentary group, although more evident in those trained.

Palmitate- and C6 ceramide-induced Tnnt3 pre-mRNA alternative splicing occurs in a PP2A dependent manner

Background

In a previous study, we showed that consumption of diets enriched in saturated fatty acids causes changes in alternative splicing of pre-mRNAs encoding a number of proteins in rat skeletal muscle, including the one encoding skeletal muscle Troponin T (Tnnt3). However, whether saturated fatty acids act directly on muscle cells to modulate alternative pre-mRNA splicing was not assessed. Moreover, the signaling pathway through which saturated fatty acids act to promote changes in alternative splicing is unknown. Therefore, the objective of the present study was to characterize the signaling pathway through which saturated fatty acids act to modulate Tnnt3 alternative splicing.

Methods

The effects of treatment of L6 myotubes with saturated (palmitate), mono- (oleate), or polyunsaturated (linoleate) fatty acids on alternative splicing of pre-mRNA was assessed using Tnnt3 as a marker gene.

Results

Palmitate treatment caused a two-fold change (p < 0.05) in L6 myotube Tnnt3 alternative splicing whereas treatment with either oleate or linoleate had minimal effects compared to control myotubes. Treatment with a downstream metabolite of palmitate, ceramide, had effects similar to palmitate on Tnnt3 alternative splicing and inhibition of de novo ceramide biosynthesis blocked the palmitate-induced alternative splicing changes. The effects of palmitate and ceramide on Tnnt3 alternative splicing were accompanied by a 40–50% reduction in phosphorylation of Akt on S473. However, inhibition of de novo ceramide biosynthesis did not prevent palmitate-induced Akt dephosphorylation, suggesting that palmitate may act in an Akt-independent manner to modulate Tnnt3 alternative splicing. Instead, pre-treatment with okadaic acid at concentrations that selectively inhibit protein phosphatase 2A (PP2A) blocked both palmitate- and ceramide-induced changes in Tnnt3 alternative splicing, suggesting that palmitate and ceramide act through PP2A to modulate Tnnt3 alternative splicing.

Conclusions

Overall, the data show that fatty acid saturation level and ceramides are important factors modulating alternative pre-mRNA splicing through activation of PP2A.

Electronic supplementary material

The online version of this article (10.1186/s12986-018-0326-3) contains supplementary material, which is available to authorized users.

High Fat Diet Exposure during Fetal Life Enhances Plasma and Hepatic Omega-6 Fatty Acid Profiles in Fetal Wistar Rats

Pregnant rats were fed a high fat diet (HFD) for the first (HF1), second (HF2), third (HF3) or all three weeks (HFG) of gestation. Maintenance on a HFD during specific periods of gestation was hypothesized to alter fetal glycemia, insulinemia, induce insulin resistance; and alter fetal plasma and hepatic fatty acid (FA) profiles. At day 20 of gestation, fetal plasma and hepatic FA profiles were determined by gas chromatography; body weight, fasting glycemia, insulinemia and the Homeostasis Model Assessment (HOMA-insulin resistance) were also determined. HF3 fetuses were heaviest concomitant with elevated glycemia and insulin resistance (p < 0.05). HFG fetuses had elevated plasma linoleic (18:2 n-6) and arachidonic (20:4 n-6) acid proportions (p < 0.05). In the liver, HF3 fetuses displayed elevated linoleic, eicosatrienoic (20:3 n-6) and arachidonic acid proportions (p < 0.05). HFG fetuses had reduced hepatic docosatrienoic acid (22:5 n-3) proportions (p < 0.05). High fat maintenance during the final week of fetal life enhances hepatic omega-6 FA profiles in fetuses concomitant with hyperglycemia and insulin resistance thereby presenting a metabolically compromised phenotype.

Muscle-specific knock-out of NUAK family SNF1-like kinase 1 (NUAK1) prevents high fat diet-induced glucose intolerance

NUAK1 is a member of the AMP-activated protein kinase-related kinase family. Recent studies have shown that NUAK1 is involved in cellular senescence and motility in epithelial cells and fibroblasts. However, the physiological roles of NUAK1 are poorly understood because of embryonic lethality in NUAK1 null mice. The purpose of this study was to elucidate the roles of NUAK1 in adult tissues. We determined the tissue distribution of NUAK1 and generated muscle-specific NUAK1 knock-out (MNUAK1KO) mice. For phenotypic analysis, whole body glucose homeostasis and muscle glucose metabolism were examined. Quantitative phosphoproteome analysis of soleus muscle was performed to understand the molecular mechanisms underlying the knock-out phenotype. Nuak1 mRNA was preferentially expressed in highly oxidative tissues such as brain, heart, and soleus muscle. On a high fat diet, MNUAK1KO mice had a lower fasting blood glucose level, greater glucose tolerance, higher insulin sensitivity, and higher concentration of muscle glycogen than control mice. Phosphoproteome analysis revealed that phosphorylation of IRS1 Ser-1097 was markedly decreased in NUAK1-deficient muscle. Consistent with this, insulin signaling was enhanced in the soleus muscle of MNUAK1KO mice, as evidenced by increased phosphorylation of IRS1 Tyr-608, AKT Thr-308, and TBC1D4 Thr-649. These observations suggest that a physiological role of NUAK1 is to suppress glucose uptake through negative regulation of insulin signaling in oxidative muscle.

Bioinformatics for the NuGO proof of principle study: analysis of gene expression in muscle of ApoE3*Leiden mice on a high-fat diet using PathVisio

Insulin resistance is a characteristic of type-2 diabetes and its development is associated with an increased fat consumption. Muscle is one of the tissues that becomes insulin resistant after high fat (HF) feeding. The aim of the present study is to identify processes involved in the development of HF-induced insulin resistance in muscle of ApOE3*Leiden mice by using microarrays. These mice are known to become insulin resistant on a HF diet. Differential gene expression was measured in muscle using the Affymetrix mouse plus 2.0 array. To get more insight in the processes, affected pathway analysis was performed with a new tool, PathVisio. PathVisio is a pathway editor customized with plug-ins (1) to visualize microarray data on pathways and (2) to perform statistical analysis to select pathways of interest. The present study demonstrated that with pathway analysis, using PathVisio, a large variety of processes can be investigated. The significantly regulated genes in muscle of ApOE3*Leiden mice after 12 weeks of HF feeding were involved in several biological pathways including fatty acid beta oxidation, fatty acid biosynthesis, insulin signaling, oxidative stress and inflammation.

Mechanisms regulating GLUT4 glucose transporter expression and glucose transport in skeletal muscle

Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and the major glucose transporter expressed in muscle in adulthood is GLUT4. GLUT4 expression is exquisitely regulated in muscle and this seems important in the regulation of insulin-stimulated glucose uptake by this tissues. Thus, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes. Recent information indicates that glut4 gene transcription is regulated by a number of factors in skeletal muscle that include MEF2, MyoD myogenic proteins, thyroid hormone receptors, Kruppel-like factor KLF15, NF1, Olf-1/Early B cell factor and GEF/HDBP1. In addition, studies in vivo indicate that under normal conditions the activity of the muscle-specific GLUT4 enhancer is low in adult skeletal muscle compared with the maximal potential activity that it can attain at high levels of the MRF transcription factors, MEF2, and TRalpha1. This finding indicates that glut4 transcription may be greatly up-regulated via activation of this enhancer through an increase in the levels of expression or activity of these transcription factors. Understanding the molecular basis of the expression of glut4 will be useful for the appropriate therapeutic design of treatments for insulin-resistant states. The nature of the intracellular signals that mediate the stimulation of glucose transport in response to insulin or exercise is also reviewed.

Changes in FAT/CD36, UCP2, UCP3 and GLUT4 gene expression during lipid infusion in rat skeletal and heart muscle

OBJECTIVE

It has been reported that an increased availability of free fatty acids (NEFA) not only interferes with glucose utilization in insulin-dependent tissues, but may also result in an uncoupling effect of heart metabolism. We aimed therefore to investigate the effect of an increased availability of NEFA on gene expression of proteins involved in transmembrane fatty acid (FAT/CD36) and glucose (GLUT4) transport and of the uncoupling proteins UCP2 and 3 at the heart and skeletal muscle level.

STUDY DESIGN

Euglycemic hyperinsulinemic clamp was performed after 24 h Intralipid(R) plus heparin or saline infusion in lean Zucker rats. Skeletal and heart muscle glucose utilization was calculated by 2-deoxy-[1-(3)H]-D-glucose technique. Quantification of FAT/CD36, GLUT4, UCP2 and UCP3 mRNAs was obtained by Northern blot analysis or RT-PCR.

RESULTS

In Intralipid(R) plus heparin infused animals a significant decrease in insulin-mediated glucose uptake was observed both in the heart (22.62+/-2.04 vs 10.37+/-2.33 ng/mg/min; P<0.01) and in soleus muscle (13.46+/-1.53 vs 6.84+/-2.58 ng/mg/min; P<0.05). FAT/CD36 mRNA was significantly increased in skeletal muscle tissue (+117.4+/-16.3%, P<0.05), while no differences were found at the heart level in respect to saline infused rats. A clear decrease of GLUT4 mRNA was observed in both tissues. The 24 h infusion of fat emulsion resulted in a clear enhancement of UCP2 and UCP3 mRNA levels in the heart (99.5+/-15.3 and 80+/-4%) and in the skeletal muscle (291.5+/-24.7 and 146.9+/-12.7%).

CONCLUSIONS

As a result of the increased availability of NEFA, FAT/CD36 gene expression increases in skeletal muscle, but not at the heart level. The augmented lipid fuel supply is responsible for the depression of insulin-mediated glucose transport and for the increase of UCP2 and 3 gene expression in both skeletal and heart muscle.

Interaction of exercise and diet on GLUT-4 protein and gene expression in Type I and Type II rat skeletal muscle

We determined the interaction of exercise and diet on glucose transporter (GLUT-4) protein and mRNA expression in type I (soleus) and type II [extensor digitorum longus (EDL)] skeletal muscle. Forty-eight Sprague Dawley rats were randomly assigned to one of two dietary conditions: high-fat (FAT, n=24) or high-carbohydrate (CHO, n=24). Animals in each dietary condition were allocated to one of two groups: control (NT, n=8) or a group that performed 8 weeks of treadmill running (4 sessions week-1 of 1000 m @ 28 m min-1, RUN, n=16). Eight trained rats were killed after their final exercise bout for determination of GLUT-4 protein and mRNA expression: the remainder were killed 48 h after their last session for measurement of muscle glycogen and triacylglycerol concentration. GLUT-4 protein expression in NT rats was similar in both muscles after 8 weeks of either diet. However, there was a main effect of training such that GLUT-4 protein was increased in the soleus of rats fed with either diet (P < 0.05) and in the EDL in animals fed with CHO (P < 0.05). There was a significant diet-training interaction on GLUT-4 mRNA, such that expression was increased in both the soleus (100% upward arrowP < 0.05) and EDL (142% upward arrowP < 0.01) in CHO-fed animals. Trained rats fed with FAT decreased mRNA expression in the EDL ( downward arrow 45%, P < 0.05) but not the soleus ( downward arrow 14%, NS). We conclude that exercise training in CHO-fed rats increased both GLUT-4 protein and mRNA expression in type I and type II skeletal muscle. Despite lower GLUT-4 mRNA in muscles from fat-fed animals, exercise-induced increases in GLUT-4 protein were largely preserved, suggesting that control of GLUT-4 protein and gene expression are modified independently by exercise and diet.

Impaired insulin-receptor autophosphorylation is an early defect in fat-fed, insulin-resistant rats

High-fat feeding results in impaired insulin signaling in skeletal muscle, but the role of the insulin receptor (IR) remains controversial. In the present study, female Fischer 344 rats were fed diets either low in fat [low fat, complex carbohydrate (LFCC)] or high in fat and sucrose (HFS). Insulin-stimulated skeletal muscle glucose transport, measured in purified sarcolemmal vesicles, was lower in rats consuming the HFS diet for 2 and 8 wk compared with LFCC controls (72.9 +/- 3.5, 67.6 +/- 3.5, and 86.1 +/- 3.5 pmol x mg(-1) x 15 s(-1), respectively; P < 0.05). Muscle IR content was unchanged in 2-wk HFS animals but was 50% lower in the 8-wk HFS group (P < 0.001). However, compared with LFCC, insulin-stimulated IR autophosphorylation was 26% lower in 2-wk HFS and 40% lower in 8-wk HFS animals (P < 0.005). Total muscle content of the proposed IR inhibitors cytokine tumor necrosis factor-alpha and membrane glycoprotein PC-1 was not significantly changed in HFS animals at either 2 or 8 wk. These results demonstrate that high-fat feeding induces insulin resistance in muscle concomitant with a diminished IR signaling capacity, although the mechanism remains unknown.

Glucose transporters: structure, function and consequences of deficiency

There are two mechanisms for glucose transport across cell membranes. In the intestine and renal proximal tubule, glucose is transported against a concentration gradient by a secondary active transport mechanism in which glucose is cotransported with sodium ions. In all other cells, glucose transport is mediated by one or more of the members of the closely related GLUT family of glucose transporters. The pattern of expression of the GLUT transporters in different tissues is related to the different roles of glucose metabolism in different tissues. Primary defects in glucose transport all appear to be extremely rare and not all possible deficiencies have been identified. Deficiency of the secondary active sodium/glucose transporters result in glucose/galactose malabsorption or congenital renal glycosuria. GLUT1 deficiency produces a seizure disorder with low glucose concentration in cerebrospinal fluid and GLUT2 deficiency is the basis of the Fanconi-Bickel syndrome, which resembles type I glycogen storage disease.

Metabolic adaptations to a high-fat diet in endurance cyclists

We examined the time course of metabolic adaptations to 15 days of a high-fat diet (HFD). Sixteen endurance-trained cyclists were assigned randomly to a control (CON) group, who consumed their habitual diet (30% +/- 8% mJ fat), or a HFD group, who consumed a high-fat isocaloric diet (69% +/- 1% mJ fat). At 5-day intervals, the subjects underwent an oral glucose tolerance test (OGTT); on the next day, they performed a 2.5-hour constant-load ride at 70% peak oxygen consumption (VO2peak), followed by a simulated 40-km cycling time-trial while ingesting a 10% 14C-glucose + 3.44% medium-chain triglyceride (MCT) emulsion at a rate of 600 mL/h. In the OGTT, plasma glucose concentrations at 30 minutes increased significantly after 5 days of the HFD and remained elevated at days 10 and 15 versus the levels measured prior to the HFD (P < .05). The activity of carnitine acyltransferase (CAT) in biopsies of the vastus lateralis muscle also increased from 0.45 to 0.54 micromol/g/min over days 0 to 10 of the HFD (P < .01) without any change in citrate synthase (CS) or 3-hydroxyacyl-coenzyme A dehydrogenase (3-HAD) activities. Changes in glucose tolerance and CAT activity were associated with a shift from carbohydrate (CHO) to fat oxidation during exercise (P < .001), which occurred within 5 to 10 days of the HFD. During the constant-load ride, the calculated oxidation of muscle glycogen was reduced from 1.5 to 1.0 g/min (P < .001) after 15 days of the HFD. Ingestion of a HFD for as little as 5 to 10 days significantly altered substrate utilization during submaximal exercise but did not attenuate the 40-km time-trial performance.

Effects of chronic liver diseases on mitochondrial DNA transcription and replication in human liver

To evaluate the effects of chronic liver diseases on mitochondrial DNA (mtDNA) transcription and replication, nuclear respiratory factor-1 (NRF-1) mRNA, mitochondrial transcription factor A (mtTFA) mRNA, a RNA component of ribonuclease (RNase) for mitochondrial RNA processing (MRP), mitochondrial cytochrome b mRNA, and mtDNA were measured in normal, chronically viral-hepatitic and cirrhotic human livers. The mRNA levels of the regulatory factors for mitochondrial gene (NRF-1 and mtTFA) and cytochrome b were significantly increased by chronic hepatitis (160, 280, and 175%, respectively) compared with those in normal livers, but were not different between cirrhotic and normal livers. On the other hand, concentrations of mtDNA and RNA component of RNase MRP were not different among normal, chronically hepatitic, and cirrhotic livers. These results suggest that either persistent hepatitis viral infection or repeated cell necrosis and regeneration in chronically hepatitic liver may be associated with increase in mtDNA transcription.

Suboptimal energy balance selectively up-regulates muscle GLUT gene expression but reduces insulin-dependent glucose uptake during postnatal development

The major facilitative glucose transporters in muscle, GLUT1 (insulin-independent) and GLUT4 (insulin-dependent), are essential for normal growth and metabolism, but factors controlling their expression during postnatal development are poorly understood. We have therefore determined the role of energy status in regulating muscle GLUT gene expression and function in young, growing pigs on a high (H) or low (L) food intake (H =2L) at 35 degrees C or 26 degrees C. RNase protection assays revealed selective up-regulation of GLUT1 and GLUT4 by mild undernutrition 20-24 h after feeding: mRNA levels were elevated in longissimus dorsi (P<0.001) and rhomboideus (P<0.05), but not in diaphragm or cardiac muscles. Assessment of 2-deoxy-glucose uptake in a small isolated muscle, flexor carpi radialis, showed that the 26L group, which had suboptimal energy balance and the greatest GLUT4 expression, had the highest insulin-independent glucose uptake but the lowest insulin-dependent increment: 20% compared with 70% in the other groups. These novel findings are directly relevant to an understanding of mechanisms underlying the development of insulin resistance and demonstrate 1) muscle-specific up-regulation of GLUT gene expression by postnatal undernutrition that is not related simply to myofiber type, but to whole-body function; and 2) that the degree of GLUT up-regulation and the subcellular distribution and function of GLUT proteins are dependent on energy status.

Increased diaphragm expression of GLUT4 in control and streptozotocin-diabetic rats by fish oil-supplemented diets

Dietary fat intake influences plasma glucose concentration through modifying glucose uptake and utilization by adipose and skeletal muscle tissues. In this paper, we studied the effects of a low-fat diet on diaphragm GLUT4 expression and fatty acid composition in control and streptozotocin-induced diabetic rats. Control as well as diabetic rats were divided into three different dietary groups each. Either 5% olive oil, 5% sunflower oil, or 5% fish oil was the only fat supplied by the diet. Feeding these low-fat diets for 5 wk induced major changes in fatty acid composition, both in control and in diabetic rats. Arachidonic acid was higher in diabetic olive and sunflower oil-fed rats with respect to fish oil-fed, opposite to docosahexaenoic acid which was higher in diabetic fish oil-fed rats with respect to the other two groups. Animals receiving a fish oil diet had the lowest plasma glucose concentration. GLUT4 expression in diaphragm, as indicated by GLUT4 protein and mRNA, is modulated both by diabetes and by diet fatty acid composition. Diabetes induced a decrease in expression in all dietary groups. Plasma glucose levels correlated well with the increased amount of GLUT4 protein and mRNA found in fish oil-fed groups. Results are discussed in terms of the influence that arachidonic and n-3 polyunsaturated fatty acids may exert on the transcriptional and translational control of the GLUT4 gene.

Benfluorex improves muscle insulin responsiveness in middle-aged rats previously subjected to long-term high-fat feeding

It has been reported that benfluorex ameliorates the insulin resistance induced by high-fat feeding when its administration is initiated at the same time as the change in diet. Here we have examined whether benfluorex reverses insulin resistance when this is established in middle-aged rats chronically maintained on a high-fat diet. Untreated 12-month-old rats that had been subjected to a high-fat diet for the last 6 months showed markedly lower insulin-induced stimulation of 2-deoxyglucose uptake by strips of soleus muscle and a reduced expression of GLUT4 glucose carriers in skeletal muscle. However, animals subjected to the same protocol but treated with benfluorex during the last month of high-fat feeding showed marked improvement in insulin-stimulated glucose transport by soleus muscle. Benfluorex treatment caused a substantial increase in the content of GLUT4 protein in white muscle; however, GLUT4 levels in red muscle remained low. Our results indicate: (i) that benfluorex treatment in middle-aged rats reverses the insulin resistance induced by high-fat feeding in soleus muscle; (ii) benfluorex is active even when it is administered once the insulin-resistant state is already established; (iii) reversion of muscle insulin resistance by benfluorex can occur independently of modifications in GLUT4 protein expression.

Searching for ways to upregulate GLUT4 glucose transporter expression in muscle

1. Skeletal muscle is a major glucose-utilizing tissue in the absorptive state and alterations in muscle insulin-stimulated glucose uptake lead to derangements in whole body glucose disposal. 2. Furthermore, muscle GLUT4 overexpression in transgenic animals ameliorates insulin resistance associated with obesity or diabetes, which suggests that increasing GLUT4 in muscle by pharmacological intervention may be an effective therapy in insulin-resistant states. 3. This highlights the importance of understanding the pathways that upregulate GLUT4 glucose transporter expression in muscle. 4. We review studies describing the regulation of GLUT4 and the information currently available on the mechanisms that control GLUT4 expression in muscle.

Insulin receptor mRNA splicing and altered metabolic control in aged and mildly insulin-deficient rats

Using reverse transcription-competitive polymerase chain reaction, we measured the abundance of the mRNAs encoding the two spliced isoforms of insulin receptor in aged and mildly insulin-deficient rats. Twelve-month-old rats were characterized by peripheral insulin resistance and decreased glucose tolerance. Mild insulin deficiency, obtained by neonatal streptozotocin treatment, was associated with glucose intolerance due to reduced glucose-stimulated insulin response. Both models were associated with a decrease in the relative abundance of the mRNA with exon 11 in liver, heart, adipose tissue, and tibialis muscle, whereas a slight increase was seen in the extensor digitorum longus and no change in the soleus muscle. In the three muscles, the expression of the form without exon 11 largely predominated (>90%). In heart and adipose tissue, the two isoforms were expressed at a similar level in control rats. In both tissues, the form without exon 11 increased in streptozotocin-treated rats, whereas the absolute level of the form with exon 11 decreased in old rats. Although a decreased level of the variant with exon 11 correlated with insulin resistance of whole body glucose uptake, our results indicated that changes in the expression of the insulin receptor variants were secondary events and thus not the cause of the insulin resistance in old and mildly insulin-deficient rats.

Gene expression of insulin signal-transduction pathway intermediates is lower in rats fed a beef tallow diet than in rats fed a safflower oil diet

To elucidate the effects of dietary fatty acid composition on the insulin signaling pathway, we measured the gene expression of the earliest steps in the insulin action pathway in skeletal muscle of rats fed a safflower oil diet or a beef tallow diet. Rats were meal-fed an isoenergetic diet based on either safflower oil or beef tallow for 8 weeks. Both diets provided 45%, 35%, and 20% of energy as fat, carbohydrate, and protein, respectively. Insulin resistance, assessed from the diurnal rhythm of plasma glucose and insulin and the oral glucose tolerance test (OGTT), developed in rats fed a beef tallow diet. Body fat content was greater in rats fed a beef tallow diet versus a safflower oil diet. The level of insulin receptor mRNA, relative expression of the insulin receptor mRNA isoforms, and receptor protein were not affected by the composition of dietary fatty acids. The abundance of insulin receptor substrate-1 (IRS-1) and phosphatidylinositol (PI) 3-kinase mRNA and protein was significantly lower in rats fed a beef tallow diet versus a safflower oil diet. We conclude that long-term feeding of a high-fat diet with saturated fatty acids induces decrease in IRS-1 and PI 3-kinase mRNA and protein levels, causing insulin resistance in skeletal muscle.