Downregulation of Grb2 contributes to the insulin-sensitizing effect of calorie restriction

Mechanosensing in Metabolism

Electrical mechanosensing is a process mediated by specialized ion channels, gated directly or indirectly by mechanical forces, which allows cells to detect and subsequently respond to mechanical stimuli. The activation of mechanosensitive (MS) ion channels, intrinsically gated by mechanical forces, or mechanoresponsive (MR) ion channels, indirectly gated by mechanical forces, results in electrical signaling across lipid bilayers, such as the plasma membrane. While the functions of mechanically gated channels within a sensory context (e.g., proprioception and touch) are well described, there is emerging data demonstrating functions beyond touch and proprioception, including mechanoregulation of intracellular signaling and cellular/systemic metabolism. Both MR and MS ion channel signaling have been shown to contribute to the regulation of metabolic dysfunction, including obesity, insulin resistance, impaired insulin secretion, and inflammation. This review summarizes our current understanding of the contributions of several MS/MR ion channels in cell types implicated in metabolic dysfunction, namely, adipocytes, pancreatic β-cells, hepatocytes, and skeletal muscle cells, and discusses MS/MR ion channels as possible therapeutic targets. © 2024 American Physiological Society. Compr Physiol 14:5269-5290, 2024.

Effect of Selenium Treatment on Central Insulin Sensitivity: A Proteomic Analysis in β-Amyloid Precursor Protein/Presenilin-1 Transgenic Mice

Prior studies have demonstrated a close association between brain insulin resistance and Alzheimer’s disease (AD), while selenium supplementation was shown to improve insulin homeostasis in AD patients and to exert neuroprotective effects in a mouse model of AD. However, the mechanisms underlying the neuroprotective actions of selenium remain incompletely understood. In this study, we performed a label-free liquid chromatography-tandem mass spectrometry (LC–MS/MS) quantitative proteomics approach to analyze differentially expressed proteins (DEPs) in the hippocampus and cerebral cortex of Aβ precursor protein (APP)/presenilin-1 (PS1) mice following 2 months of treatment with sodium selenate. A total of 319 DEPs (205 upregulated and 114 downregulated proteins) were detected after selenium treatment. Functional enrichment analysis revealed that the DEPs were mainly enriched in processes affecting axon development, neuron differentiation, tau protein binding, and insulin/insulin-like growth factor type 1 (IGF1)-related pathways. These results demonstrate that a number of insulin/IGF1 signaling pathway-associated proteins are differentially expressed in ways that are consistent with reduced central insulin resistance, suggesting that selenium has therapeutic value in the treatment of neurodegenerative and metabolic diseases such as AD and non-alcoholic fatty liver disease (NAFLD).

SWELL1 regulates skeletal muscle cell size, intracellular signaling, adiposity and glucose metabolism

Maintenance of skeletal muscle is beneficial in obesity and Type 2 diabetes. Mechanical stimulation can regulate skeletal muscle differentiation, growth and metabolism; however, the molecular mechanosensor remains unknown. Here, we show that SWELL1 (Lrrc8a) functionally encodes a swell-activated anion channel that regulates PI3K-AKT, ERK1/2, mTOR signaling, muscle differentiation, myoblast fusion, cellular oxygen consumption, and glycolysis in skeletal muscle cells. LRRC8A over-expression in Lrrc8a KO myotubes boosts PI3K-AKT-mTOR signaling to supra-normal levels and fully rescues myotube formation. Skeletal muscle-targeted Lrrc8a KO mice have smaller myofibers, generate less force ex vivo, and exhibit reduced exercise endurance, associated with increased adiposity under basal conditions, and glucose intolerance and insulin resistance when raised on a high-fat diet, compared to wild-type (WT) mice. These results reveal that the LRRC8 complex regulates insulin-PI3K-AKT-mTOR signaling in skeletal muscle to influence skeletal muscle differentiation in vitro and skeletal myofiber size, muscle function, adiposity and systemic metabolism in vivo.

Docosahexaenoic Acid Supplementation in Pregnancy Modulates Placental Cellular Signaling and Nutrient Transport Capacity in Obese Women

Context

Maternal obesity in pregnancy has profound impacts on maternal metabolism and promotes placental nutrient transport, which may contribute to fetal overgrowth in these pregnancies. The fatty acid docosahexaenoic acid (DHA) has bioactive properties that may improve outcomes in obese pregnant women by modulating placental function.

Objective

To determine the effects of DHA supplementation in obese pregnant women on maternal metabolism and placental function.

Design

Pregnant women were supplemented with DHA or placebo. Maternal fasting blood was collected at 26 and 36 weeks' gestation, and placentas were collected at term.

Setting

Academic health care institution.

Subjects

Thirty-eight pregnant women with pregravid body mass index ≥30 kg/m2.

Intervention

DHA (800 mg, algal oil) or placebo (corn/soy oil) daily from 26 weeks to term.

Main Outcomes

DHA content of maternal erythrocyte and placental membranes, maternal fasting blood glucose, cytokines, metabolic hormones, and circulating lipids were determined. Insulin, mTOR, and inflammatory signaling were assessed in placental homogenates, and nutrient transport capacity was determined in isolated syncytiotrophoblast plasma membranes.

Results

DHA supplementation increased erythrocyte (P < 0.0001) and placental membrane DHA levels (P < 0.0001) but did not influence maternal inflammatory status, insulin sensitivity, or lipids. DHA supplementation decreased placental inflammation, amino acid transporter expression, and activity (P < 0.01) and increased placental protein expression of fatty acid transporting protein 4 (P < 0.05).

Conclusions

Maternal DHA supplementation in pregnancy decreases placental inflammation and differentially modulates placental nutrient transport capacity and may mitigate adverse effects of maternal obesity on placental function.

Role of growth factor receptor-bound 2 in CCl4-induced hepatic fibrosis

BACKGROUND

Growth Factor Receptor-bound 2 (GRB2) plays a crucial role in regulation of cellular function including proliferation and differentiation, and we previously identified GRB2 as promoting HSCs (HSCs) proliferation. However, the underlying mechanisms that are involving in the regulation of GRB2 in hepatic fibrogenesis remain unknown.

METHODS

In the present study, we tested the function of GRB2 in hepatic fibrosis. Hepatic fibrosis was induced by subcutaneous CCl₄ administration at a dose of 3mL/kg in rats. The rat HSC cell line HSC-T6 were cultured for proliferation investigation by CCK-8 and BrdU incorporation method. The levels of GRB2, HMGB1, PI3K/AKT, COL1A1 and α-SMA were analyzed by western blot or real-time PCR.

RESULTS

showed that the expression of GRB2 and HMGB1 was obviously increased in liver tissues of hepatic fibrosis rats accompanied by up-regulation of COL1A1 and α-SMA. In cultured HSCs, application of exogenous HMGB1 induced cell proliferation and cell proliferation rate concomitantly with up-regulation of GRB2 expression and PI3K/AKT phosphorylation. The effects of HMGB1-induced proliferation of HSCs and up-regulation of COL1A1 and α-SMA were abolished by GRB2 siRNA. HMGB1-induced proliferation of HSCs and up-regulation of COL1A1 and α-SMA was reversed in the presence of LY294002, an inhibitor of PI3K inhibitor.

CONCLUSIONS

These findings suggest that GRB2 plays an important role in CCl₄-induced hepatic fibrosis by regulating HSCs' function, and up-regulation of GRB2 induced by HMGB1 is mediated via the PI3K/AKT pathway.

SWELL1 is a regulator of adipocyte size, insulin signalling and glucose homeostasis

Adipocytes undergo considerable volumetric expansion in the setting of obesity. It has been proposed that such marked increases in adipocyte size may be sensed via adipocyte-autonomous mechanisms to mediate size-dependent intracellular signalling. Here, we show that SWELL1 (LRRC8a), a member of the Leucine-Rich Repeat Containing protein family, is an essential component of a volume-sensitive ion channel (VRAC) in adipocytes. We find that SWELL1-mediated VRAC is augmented in hypertrophic murine and human adipocytes in the setting of obesity. SWELL1 regulates adipocyte insulin-PI3K-AKT2-GLUT4 signalling, glucose uptake and lipid content via SWELL1 C-terminal leucine-rich repeat domain interactions with GRB2/Cav1. Silencing GRB2 in SWELL1 KO adipocytes rescues insulin-pAKT2 signalling. In vivo, shRNA-mediated SWELL1 knockdown and adipose-targeted SWELL1 knockout reduce adiposity and adipocyte size in obese mice while impairing systemic glycaemia and insulin sensitivity. These studies identify SWELL1 as a cell-autonomous sensor of adipocyte size that regulates adipocyte growth, insulin sensitivity and glucose tolerance.

Genome-wide DNA methylation changes in skeletal muscle between young and middle-aged pigs

BACKGROUND

Age-related physiological, biochemical and functional changes in mammalian skeletal muscle have been shown to begin at the mid-point of the lifespan. However, the underlying changes in DNA methylation that occur during this turning point of the muscle aging process have not been clarified. To explore age-related genomic methylation changes in skeletal muscle, we employed young (0.5 years old) and middle-aged (7 years old) pigs as models to survey genome-wide DNA methylation in the longissimus dorsi muscle using a methylated DNA immunoprecipitation sequencing approach.

RESULTS

We observed a tendency toward a global loss of DNA methylation in the gene-body region of the skeletal muscle of the middle-aged pigs compared with the young group. We determined the genome-wide gene expression pattern in the longissimus dorsi muscle using microarray analysis and performed a correlation analysis using DMR (differentially methylated region)-mRNA pairs, and we found a significant negative correlation between the changes in methylation levels within gene bodies and gene expression. Furthermore, we identified numerous genes that show age-related methylation changes that are potentially involved in the aging process. The methylation status of these genes was confirmed using bisulfite sequencing PCR. The genes that exhibited a hypomethylated gene body in middle-aged pigs were over-represented in various proteolysis and protein catabolic processes, suggesting an important role for these genes in age-related muscle atrophy. In addition, genes associated with tumorigenesis exhibited aged-related differences in methylation and expression levels, suggesting an increased risk of disease associated with increased age.

CONCLUSIONS

This study provides a comprehensive analysis of genome-wide DNA methylation patterns in aging pig skeletal muscle. Our findings will serve as a valuable resource in aging studies, promoting the pig as a model organism for human aging research and accelerating the development of comparative animal models in aging research.

Early gene expression changes in skeletal muscle from SOD1(G93A) amyotrophic lateral sclerosis animal model

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by loss of motor neurons. Familial ALS is strongly associated to dominant mutations in the gene for Cu/Zn superoxide dismutase (SOD1). Recent evidences point to skeletal muscle as a primary target in the ALS mouse model. Wnt/PI3 K signaling pathways and epithelial-mesenchymal transition (EMT) have important roles in maintenance and repair of skeletal muscle. Wnt/PI3 K pathways and EMT gene expression profile were investigated in gastrocnemius muscle from SOD1(G93A) mouse model and age-paired wild-type control in the presymptomatic ages of 40 and 80 days aiming the early neuromuscular abnormalities that precede motor neuron death in ALS. A customized cDNA microarray platform containing 326 genes of Wnt/PI3 K and EMT was used and results revealed eight up-regulated (Loxl2, Pik4ca, Fzd9, Cul1, Ctnnd1, Snf1lk, Prkx, Dner) and nine down-regulated (Pik3c2a, Ripk4, Id2, C1qdc1, Eif2ak2, Rac3, Cds1, Inppl1, Tbl1x) genes at 40 days, and also one up-regulated (Pik3ca) and five down-regulated (Cd44, Eef2 k, Fzd2, Crebbp, Piki3r1) genes at 80 days. Also, protein-protein interaction networks grown from the differentially expressed genes of 40 and 80 days old mice have identified Grb2 and Src genes in both presymptomatic ages, thus playing a potential central role in the disease mechanisms. mRNA and protein levels for Grb2 and Src were found to be increased in 80 days old ALS mice. Gene expression changes in the skeletal muscle of transgenic ALS mice at presymptomatic periods of disease gave further evidence of early neuromuscular abnormalities that precede motor neuron death. The results were discussed in terms of initial triggering for neuronal degeneration and muscle adaptation to keep function before the onset of symptoms.

Diet-induced obesity in mice reduces placental efficiency and inhibits placental mTOR signaling

As in humans, obesity during pregnancy in mice results in elevated maternal insulin levels and metabolic programming of offspring. mTOR signaling regulates amino acid transport and may function as a placental nutrient sensor. Because obesity is a condition with increased nutrient availability, we hypothesized that diet‐induced obesity activates placental mTOR signaling. To test this hypothesis, female C57BL/6J mice were fed an obesogenic diet or standard chow prior to and throughout pregnancy. Fetuses and placentas were collected at gestational day 18. Using Western blot analysis, placental mTOR activity was determined along with energy, inflammatory, and insulin signaling pathways (upstream modulators of mTOR). At gestational day 18, fetal and placental weights did not differ, however, in obese dams, the fetal/placental weight ratio was lower (P <0.01). In placentas from obese dams, mTOR signaling was inhibited, as determined by decreased Rheb and S6K1 expression, and lower rpS6 phosphorylation (P <0.05). In contrast, energy, inflammatory, and insulin signaling pathways were unaffected. Contrary to our hypothesis, diet‐induced obesity in pregnant mice was associated with inhibition of placental mTOR signaling. However, this finding is consistent with the lower fetal/placental weight ratio, indicating reduced placental efficiency.

In this report, we explore placental signaling in an obese mouse model known to result in adverse metabolic programming of the offspring without altering fetal growth. This model parallels with human pregnancies, where maternal obesity also is associated with adverse metabolic programming. In this report, we show that maternal obesity in the mouse leads to reduced placental efficiency potentially caused by lowered signaling activity of the mTOR pathway (a known positive regulator of placental nutrient transport) and that the placentas of obese, hyperinsulinemic dams are insulin resistant.

Suppression of Grb2 expression improved hepatic steatosis, oxidative stress, and apoptosis induced by palmitic acid in vitro partly through insulin signaling alteration

In this study, we aimed to study the role of growth factor receptor-bound protein 2 (Grb2) in palmitic acid-induced steatosis and other "fatty liver" symptoms in vitro. HepG2 cells, with or without stably suppressed Grb2 expression, were incubated with palmitic acid for 24 h to induce typical clinical "fatty liver" features, including steatosis, impaired glucose metabolism, oxidative stress, and apoptosis. MTT and Oil Red O assays were applied to test cell viability and fat deposition, respectively. Glucose uptake assay was used to evaluate the glucose utilization of cells. Quantitative polymerase chain reaction and Western blot were used to measure expressional changes of key markers of insulin signaling, lipid/glucose metabolism, oxidative stress, and apoptosis. After 24-h palmitic acid induction, increased fat accumulation, reduced glucose uptake, impaired insulin signaling, enhanced oxidative stress, and increased apoptosis were observed in HepG2 cells. Suppression of Grb2 in HepG2 significantly reduced fat accumulation, improved glucose metabolism, ameliorated oxidative stress, and restored the activity of insulin receptor substrate-1/Akt and MEK/ERK pathways. In addition, Grb2 deficiency attenuated hepatic apoptosis shown by reduced activation of caspase-3 and fluorescent staining. Modulation of Bcl-2 and Bak1 also contributed to reduced apoptosis. In conclusion, suppression of Grb2 expression in HepG2 cells improved hepatic steatosis, glucose metabolism, oxidative stress, and apoptosis induced by palmitic acid incubation partly though modulating the insulin signaling pathway.

Regulation of insulin sensitivity by serine/threonine phosphorylation of insulin receptor substrate proteins IRS1 and IRS2

The insulin receptor substrate proteins IRS1 and IRS2 are key targets of the insulin receptor tyrosine kinase and are required for hormonal control of metabolism. Tissues from insulin-resistant and diabetic humans exhibit defects in IRS-dependent signalling, implicating their dysregulation in the initiation and progression of metabolic disease. However, IRS1 and IRS2 are regulated through a complex mechanism involving phosphorylation of >50 serine/threonine residues (S/T) within their long, unstructured tail regions. In cultured cells, insulin-stimulated kinases (including atypical PKC, AKT, SIK2, mTOR, S6K1, ERK1/2 and ROCK1) mediate feedback (autologous) S/T phosphorylation of IRS, with both positive and negative effects on insulin sensitivity. Additionally, insulin-independent (heterologous) kinases can phosphorylate IRS1/2 under basal conditions (AMPK, GSK3) or in response to sympathetic activation and lipid/inflammatory mediators, which are present at elevated levels in metabolic disease (GRK2, novel and conventional PKCs, JNK, IKKβ, mPLK). An emerging view is that the positive/negative regulation of IRS by autologous pathways is subverted/co-opted in disease by increased basal and other temporally inappropriate S/T phosphorylation. Compensatory hyperinsulinaemia may contribute strongly to this dysregulation. Here, we examine the links between altered patterns of IRS S/T phosphorylation and the emergence of insulin resistance and diabetes.

Mechanisms for increased insulin-stimulated Akt phosphorylation and glucose uptake in fast- and slow-twitch skeletal muscles of calorie-restricted rats

Calorie restriction [CR; ~65% of ad libitum (AL) intake] improves insulin-stimulated glucose uptake (GU) and Akt phosphorylation in skeletal muscle. We aimed to elucidate the effects of CR on 1) processes that regulate Akt phosphorylation [insulin receptor (IR) tyrosine phosphorylation, IR substrate 1-phosphatidylinositol 3-kinase (IRS-PI3K) activity, and Akt binding to regulatory proteins (heat shock protein 90, Appl1, protein phosphatase 2A)]; 2) Akt substrate of 160-kDa (AS160) phosphorylation on key phosphorylation sites; and 3) atypical PKC (aPKC) activity. Isolated epitrochlearis (fast-twitch) and soleus (slow-twitch) muscles from AL or CR (6 mo duration) 9-mo-old male F344BN rats were incubated with 0, 1.2, or 30 nM insulin and 2-deoxy-[(3)H]glucose. Some CR effects were independent of insulin dose or muscle type: CR caused activation of Akt (Thr(308) and Ser(473)) and GU in both muscles at both insulin doses without CR effects on IRS1-PI3K, Akt-PP2A, or Akt-Appl1. Several muscle- and insulin dose-specific CR effects were revealed. Akt-HSP90 binding was increased in the epitrochlearis; AS160 phosphorylation (Ser(588) and Thr(642)) was greater for CR epitrochlearis at 1.2 nM insulin; and IR phosphorylation and aPKC activity were greater for CR in both muscles with 30 nM insulin. On the basis of these data, our working hypothesis for improved insulin-stimulated GU with CR is as follows: 1) elevated Akt phosphorylation is fundamental, regardless of muscle or insulin dose; 2) altered Akt binding to regulatory proteins (HSP90 and unidentified Akt partners) is involved in the effects of CR on Akt phosphorylation; 3) Akt effects on GU depend on muscle- and insulin dose-specific elevation in phosphorylation of Akt substrates, including, but not limited to, AS160; and 4) greater IR phosphorylation and aPKC activity may contribute at higher insulin doses.

The disruption of L-carnitine metabolism by aluminum toxicity and oxidative stress promotes dyslipidemia in human astrocytic and hepatic cells

L-Carnitine is a critical metabolite indispensable for the metabolism of lipids as it facilitates fatty acid transport into the mitochondrion where β-oxidation occurs. Human astrocytes (CCF-STTG1 cells) and hepatocytes (HepG2 cells) exposed to aluminum (Al) and hydrogen peroxide (H₂O₂), were characterized with lower levels of L-carnitine, diminished β-oxidation, and increased lipid accumulation compared to the controls. γ-Butyrobetainealdehyde dehydrogenase (BADH) and butyrobetaine dioxygenase (BBDOX), two key enzymes mediating the biogenesis of L-carnitine, were sharply reduced during Al and H₂O₂ challenge. Exposure of the Al and H₂O₂-treated cells to α-ketoglutarate (KG), led to the recovery of L-carnitine production with the concomitant reduction in ROS levels. It appears that the channeling of KG to combat oxidative stress results in decreased L-carnitine synthesis, an event that contributes to the dyslipidemia observed during Al and H₂O₂ insults in these mammalian cells. Hence, KG may help alleviate pathological conditions induced by oxidative stress.