Phosphorylation of Ser357 of rat insulin receptor substrate-1 mediates adverse effects of protein kinase C-delta on insulin action in skeletal muscle cells

The importance of muscle glycogen phosphorylase in glial cells function

The three isoforms of glycogen phosphorylase - PYGM, PYGB, and PYGL - are expressed in glial cells. Unlike PYGB and PYGL, PYGM is the only isoform regulated by Rac1. This specific regulation may confer a differential functional role compared with the other glycogen phosphorylases-PYGB and PYGL. The involvement of muscle glycogen phosphorylase in glial cells and its association with post-translational modifications (PTMs) of proteins through O-glycosylation is indeed a fascinating and emerging area of research. The dual role it plays in metabolic processes and the regulation of PTMs within the brain presents intriguing implications for various neurological conditions. Disruptions in the O-GlcNAcylation cycle and neurodegenerative diseases like Alzheimer's disease (AD) is particularly noteworthy. The alterations in O-GlcNAcylation levels of specific proteins, such as APP, c-Fos, and tau protein, highlight the intricate relationship between PTMs and AD. Understanding these processes and the regulatory function of muscle glycogen phosphorylase sheds light on its impact on protein function, signaling pathways, cellular homeostasis, neurological health, and potential interventions for brain-related conditions.

The serine phosphorylations in the IRS-1 PIR domain abrogate IRS-1 and IR interaction

Serine phosphorylations on insulin receptor substrate 1 (IRS-1) by diverse kinases aoccur widely during obesity-, stress-, and inflammation-induced conditions in models of insulin resistance and type 2 diabetes. In this study, we define a region within the human IRS-1, which is directly C-terminal to the PTB domain encompassing numerous serine phosphorylation sites including Ser307 (mouse Ser302) and Ser312 (mouse 307) creating a phosphorylation insulin resistance (PIR) domain. We demonstrate that the IRS-1 PTB-PIR with its unphosphorylated serine residues interacts with the insulin receptor (IR) but loses the IR-binding when they are phosphorylated. Surface plasmon resonance studies further confirm that the PTB-PIR binds stronger to IR than just the PTB domain, and that phosphorylations at Ser307, Ser312, Ser315, and Ser323 within the PIR domain result in abrogating the binding. Insulin-responsive cells containing the mutant IRS-1 with all these four serines changed into glutamates to mimic phosphorylations show decreased levels of phosphorylations in IR, IRS-1, and AKT compared to the wild-type IRS-1. Hydrogen-deuterium exchange mass spectrometry experiments indicating the PIR domain interacting with the N-terminal lobe and the hinge regions of the IR kinase domain further suggest the possibility that the IRS-1 PIR domain protects the IR from the PTP1B-mediated dephosphorylation.

Phosphorylation Codes in IRS-1 and IRS-2 Are Associated with the Activation/Inhibition of Insulin Canonical Signaling Pathways

Insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) are signaling adaptor proteins that participate in canonical pathways, where insulin cascade activation occurs, as well as in non-canonical pathways, in which phosphorylation of substrates is carried out by a diverse array of receptors including integrins, cytokines, steroid hormones, and others. IRS proteins are subject to a spectrum of post-translational modifications essential for their activation, encompassing phosphorylation events in distinct tyrosine, serine, and threonine residues. Tyrosine residue phosphorylation is intricately linked to the activation of the insulin receptor cascade and its interaction with SH2 domains within a spectrum of proteins, including PI3K. Conversely, serine residue phosphorylation assumes a different function, serving to attenuate the effects of insulin. In this review, we have identified over 50 serine residues within IRS-1 that have been reported to undergo phosphorylation orchestrated by a spectrum of kinases, thereby engendering the activation or inhibition of different signaling pathways. Furthermore, we delineate the phosphorylation of over 10 distinct tyrosine residues at IRS-1 or IRS-2 in response to insulin, a process essential for signal transduction and the subsequent activation of PI3K.

Remimazolam induced cognitive dysfunction in mice via glutamate excitotoxicity

Objective

Several lines of evidence demonstrated the role of anesthetic drugs in cognitive functions. Some anesthetic agents have been confirmed to be associated with long-term spatial memory and learning in aged animal models.

Methods

C57BL/6 mice were divided into four different groups based on different concentrations of remimazolam treatments. Behavioral phenotype was observed by open field, rota rod, Morris water maze, and elevated plus maze test. Western blot was performed to see the expression pattern of different proteins. Confocal microscopy images were taken for neuronal and glial cells to see the effect of remimazolam on CNS cells.

Results

We showed that remimazolam, a new anesthetic drug, impaired cognitive behavior. Repetitive doses of remimazolam have been found to induce neuronal loss with a significant change in morphology. Here, we showed that a higher concentration of remimazolam had a significant effect on CNS cell activation. We showed that remimazolam caused memory dysfunction by inducing neuronal apoptosis via glutamate excitotoxicity. It also exhibited amyloid β plaque in the brain via abnormal phosphorylation of tau protein. Remimazolam-mediated regulation of glial cells in mouse cortex was observed and robust activation of astrocytes and microglial cells was found. Finally, we assessed the behavioral phenotype of mice and found that treatment with remimazolam induced significant behavioral changes and memory dysfunction.

Conclusions

This study provides insight into the mechanism of anesthetic drug-induced memory deficits and may help improve the therapeutic effects of anesthesia agents in clinical applications.

McArdle Disease: New Insights into Its Underlying Molecular Mechanisms

McArdle disease, also known as glycogen storage disease type V (GSDV), is characterized by exercise intolerance, the second wind phenomenon, and high serum creatine kinase activity. Here, we recapitulate PYGM mutations in the population responsible for this disease. Traditionally, McArdle disease has been considered a metabolic myopathy caused by the lack of expression of the muscle isoform of the glycogen phosphorylase (PYGM). However, recent findings challenge this view, since it has been shown that PYGM is present in other tissues than the skeletal muscle. We review the latest studies about the molecular mechanism involved in glycogen phosphorylase activity regulation. Further, we summarize the expression and functional significance of PYGM in other tissues than skeletal muscle both in health and McArdle disease. Furthermore, we examine the different animal models that have served as the knowledge base for better understanding of McArdle disease. Finally, we give an overview of the latest state-of-the-art clinical trials currently being carried out and present an updated view of the current therapies.

Angiotensin II type 2 receptor inhibits expression and function of insulin receptor in rat renal proximal tubule cells

Both renin-angiotensin systems and insulin participate in kidney-involved blood pressure regulation. Activation of angiotensin II type 2 receptor (AT₂R) decreases sodium reabsorption in renal proximal tubule (RPT) cells, whereas insulin produces the opposite effect. We presume that AT₂R has an inhibitory effect on insulin receptor expression in RPT cells, which may affect renal sodium transport and therefore be of physiological or pathological significance. Our present study found that activation of AT₂R inhibited insulin receptor expression in a concentration and time-dependent manner in RPT cells from Wistar-Kyoto (WKY) rats. In the presence of a protein kinase C (PKC) inhibitor (PKC inhibitor peptide 19-31, 10^-6 mol/L) or a phosphatidylinositol 3 kinase inhibitor (wortmannin, 10^-6 mol/L), the inhibitory effect of AT₂R on insulin receptor was blocked, indicating that both PKC and phosphatidylinositol 3 kinase were involved in the signaling pathway. There was a linkage between AT₂R and insulin receptor which was determined by both laser confocal microscopy and coimmunoprecipitation. However, the effect of AT₂R activation on insulin receptor expression was different in RPT cells from spontaneously hypertensive rats (SHRs). Being contrary to the effect in WKY RPT cells, AT₂R stimulation increased insulin receptor in SHR RPT cells. Insulin (10^-7 mol/L, 15 minutes) enhanced Na⁺-K⁺-ATPase activity in both WKY and SHR RPT cells. Pretreatment with CGP42112 decreased the stimulatory effect of insulin on Na⁺-K⁺-ATPase activity in WKY RPT cells, whereas pretreatment with CGP42112 increased it in SHR RPT cells. It is suggested that activation of AT₂R inhibits insulin receptor expression and function in RPT cells. The lost inhibitory effect of AT₂R on insulin receptor expression may contribute to the pathophysiology of hypertension.

Genome-wide SNP data unveils the globalization of domesticated pigs

BACKGROUND

Pigs were domesticated independently in Eastern and Western Eurasia early during the agricultural revolution, and have since been transported and traded across the globe. Here, we present a worldwide survey on 60K genome-wide single nucleotide polymorphism (SNP) data for 2093 pigs, including 1839 domestic pigs representing 122 local and commercial breeds, 215 wild boars, and 39 out-group suids, from Asia, Europe, America, Oceania and Africa. The aim of this study was to infer global patterns in pig domestication and diversity related to demography, migration, and selection.

RESULTS

A deep phylogeographic division reflects the dichotomy between early domestication centers. In the core Eastern and Western domestication regions, Chinese pigs show differentiation between breeds due to geographic isolation, whereas this is less pronounced in European pigs. The inferred European origin of pigs in the Americas, Africa, and Australia reflects European expansion during the sixteenth to nineteenth centuries. Human-mediated introgression, which is due, in particular, to importing Chinese pigs into the UK during the eighteenth and nineteenth centuries, played an important role in the formation of modern pig breeds. Inbreeding levels vary markedly between populations, from almost no runs of homozygosity (ROH) in a number of Asian wild boar populations, to up to 20% of the genome covered by ROH in a number of Southern European breeds. Commercial populations show moderate ROH statistics. For domesticated pigs and wild boars in Asia and Europe, we identified highly differentiated loci that include candidate genes related to muscle and body development, central nervous system, reproduction, and energy balance, which are putatively under artificial selection.

CONCLUSIONS

Key events related to domestication, dispersal, and mixing of pigs from different regions are reflected in the 60K SNP data, including the globalization that has recently become full circle since Chinese pig breeders in the past decades started selecting Western breeds to improve local Chinese pigs. Furthermore, signatures of ongoing and past selection, acting at different times and on different genetic backgrounds, enhance our insight in the mechanism of domestication and selection. The global diversity statistics presented here highlight concerns for maintaining agrodiversity, but also provide a necessary framework for directing genetic conservation.

A new glycotoxins inhibitor attenuates insulin resistance in liver and fat cells

Glycotoxins/Advanced glycation end products (AGEs) have implications in development of diabetes and related diseases. In the present study we deciphered the mechanisms of action of URM-II-81, a new derivative of isatin, in alleviation of insulin resistance in human hepatocytes and murine adipocytes. URM-II-81 reduced AGEs formation and receptor for advanced glycation end products (RAGE) expression in both cell types. We also observed suppression of methylglyoxal (MGO) mediated ROS production and deactivation of PKC-α. URM-II-81 restored proximal insulin signaling by modulating IRS-1 phosphorylation. URM-II-81 also alleviated MGO mediated diminished distal insulin signaling by increasing protein kinase B (PKB) and glycogen synthase kinase 3-beta (GSK-3-beta) phosphorylation. Glycogen synthesis was also increased in hepatocytes after treatment with URM-II-81. In adipocytes URM-II-81 prevented MGO induced reduced glucose uptake. We conclude that URM-II-81 can be a possible treatment target to address glycotoxins induced insulin resistance.

A bis-Schiff base of isatin improves methylglyoxal mediated insulin resistance in skeletal muscle cells

Methylglyoxal (MGO) is a highly reactive advanced glycation end products (AGEs) precursor and its abnormal accumulation causes damage to various tissues and organs. In our previous study, we synthesized a novel MGO inhibitor, MK-I-81, a bis-Schiff base derivative of isatin. In this study we demonstrate the mechanism of action of MK-I-81, on insulin resistance in skeletal muscle cells. MK-I-81 reduced AGEs formation and restored proximal insulin signaling by modulating IRS-1 phosphorylation. MK-I-81 also alleviated MGO mediated diminished distal insulin signaling by increasing protein kinase B and glycogen synthase kinase 3-beta phosphorylation. We also observed that MK-I-81 prevented reduced glucose uptake and glycogen synthesis induced by MGO in muscle cells. We found that the mechanism of action by which MK-I-81 reduced insulin resistance was suppression of production of MGO mediated ROS production in C2C12 cells. We evaluated deactivation of PKC-α and receptor for advanced glycation end products (RAGE) after treatment of cells with MK-I-81. MK-I-81 also reduced MGO mediated IRS-1, PKC-α and RAGE interaction in muscle cells. MK-I-81 also promoted nuclear factor erythroid 2-related factor-2 phosphorylation, heme oxygenase-1 and glyoxalase expression levels. We conclude that MK-I-81 can be a potential therapeutic target to address AGEs mediated insulin resistance. A novel Advanced Glycation End products (AGEs) inhibitor, MK-I-81 (a bis Schiff base of isatin), restored AGEs mediated down regulation of insulin signaling via modulating key molecules of proximal and distal insulin signaling. MK-I-81 also increased glucose uptake and glycogen synthesis in muscle cells. Novel bis-Schiff base of isatin showed significant antioxidant activity and also reduced receptor for AGEs (RAGE) expression and PKC-alpha activation therefore; MK-I-81 reduces AGEs induced insulin resistance.

Insulin and metabolic stress stimulate multisite serine/threonine phosphorylation of insulin receptor substrate 1 and inhibit tyrosine phosphorylation

IRS1 and IRS2 are key substrates of the insulin receptor tyrosine kinase. Mass spectrometry reveals more than 50 phosphorylated IRS1 serine and threonine residues (Ser(P)/Thr(P) residues) in IRS1 from insulin-stimulated cells or human tissues. We investigated a subset of IRS1 Ser(P)/Thr(P) residues using a newly developed panel of 25 phospho-specific monoclonal antibodies (αpS/TmAb(Irs1)). CHO cells overexpressing the human insulin receptor and rat IRS1 were stimulated with insulin in the absence or presence of inhibitors of the PI3K → Akt → mechanistic target of rapamycin (mTOR) → S6 kinase or MEK pathways. Nearly all IRS1 Ser(P)/Thr(P) residues were stimulated by insulin and significantly suppressed by PI3K inhibition; fewer were suppressed by Akt or mTOR inhibition, and none were suppressed by MEK inhibition. Insulin-stimulated Irs1 tyrosine phosphorylation (Tyr(P)(Irs1)) was enhanced by inhibition of the PI3K → Akt → mTOR pathway and correlated with decreased Ser(P)-302(Irs1), Ser(P)-307(Irs1), Ser(P)-318(Irs1), Ser(P)-325(Irs1), and Ser(P)-346(Irs1). Metabolic stress modeled by anisomycin, thapsigargin, or tunicamycin increased many of the same Ser(P)/Thr(P) residues as insulin, some of which (Ser(P)-302(Irs1), Ser(P)-307(Irs1), and four others) correlated significantly with impaired insulin-stimulated Tyr(P)(Irs1). Thus, IRS1 Ser(P)/Thr(P) is an integrated response to insulin stimulation and metabolic stress, which associates with reduced Tyr(P)(Irs1) in CHO(IR)/IRS1 cells.

Serine phosphorylation sites on IRS2 activated by angiotensin II and protein kinase C to induce selective insulin resistance in endothelial cells

Protein kinase C (PKC) activation, induced by hyperglycemia and angiotensin II (AngII), inhibited insulin-induced phosphorylation of Akt/endothelial nitric oxide (eNOS) by decreasing tyrosine phosphorylation of IRS2 (p-Tyr-IRS2) in endothelial cells. PKC activation by phorbol ester (phorbol myristate acetate [PMA]) reduced insulin-induced p-Tyr-IRS2 by 46% ± 13% and, similarly, phosphorylation of Akt/eNOS. Site-specific mutational analysis showed that PMA increased serine phosphorylation at three sites on IRS2 (positions 303, 343, and 675), which affected insulin-induced tyrosine phosphorylation of IRS2 at positions 653, 671, and 911 (p-Tyr-IRS2) and p-Akt/eNOS. Specific PKCβ2 activation decreased p-Tyr-IRS2 and increased the phosphorylation of two serines (Ser303 and Ser675) on IRS2 that were confirmed in cells overexpressing single point mutants of IRS2 (S303A or S675A) containing a PKCβ2-dominant negative or selective PKCβ inhibitor. AngII induced phosphorylation only on Ser303 of IRS2 and inhibited insulin-induced p-Tyr911 of IRS2 and p-Akt/eNOS, which were blocked by an antagonist of AngII receptor I, losartan, or overexpression of single mutant S303A of IRS2. Increases in p-Ser303 and p-Ser675 and decreases in p-Tyr911 of IRS2 were observed in vessels of insulin-resistant Zucker fatty rats versus lean rats. Thus, AngII or PKCβ activation can phosphorylate Ser303 and Ser675 in IRS2 to inhibit insulin-induced p-Tyr911 and its anti-atherogenic actions (p-Akt/eNOS) in endothelial cells.

Phosphorylation of IRS1 at serine 307 in response to insulin in human adipocytes is not likely to be catalyzed by p70 ribosomal S6 kinase

The insulin receptor substrate-1 (IRS1) is phosphorylated on serine 307 (human sequence, corresponding to murine serine 302) in response to insulin as part of a feedback loop that controls IRS1 phosphorylation on tyrosine residues by the insulin receptor. This in turn directly affects downstream signaling and is in human adipocytes implicated in the pathogenesis of insulin resistance and type 2 diabetes. The phosphorylation is inhibited by rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR) in complex with raptor (mTORC1). The mTORC1-downstream p70 ribosomal protein S6 kinase (S6K1), which is activated by insulin, can phosphorylate IRS1 at serine 307 in vitro and is considered the physiological protein kinase. Because the IRS1 serine 307-kinase catalyzes a critical step in the control of insulin signaling and constitutes a potential target for treatment of insulin resistance, it is important to know whether S6K1 is the physiological serine 307-kinase or not. We report that, by several criteria, S6K1 does not phosphorylate IRS1 at serine 307 in response to insulin in intact human primary adipocytes: (i) The time-courses for phosphorylation of S6K1 and its phosphorylation of S6 are not compatible with the phosphorylation of IRS1 at serine 307; (ii) A dominant-negative construct of S6K1 inhibits the phosphorylation of S6, without effect on the phosphorylation of IRS1 at serine 307; (iii) The specific inhibitor of S6K1 PF-4708671 inhibits the phosphorylation of S6, without effect on phosphorylation of IRS1 at serine 307. mTOR-immunoprecipitates from insulin-stimulated adipocytes contains an unidentified protein kinase specific for phosphorylation of IRS1 at serine 307, but it is not mTOR or S6K1.

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.

The biology of protein kinase C

This review gives a basic introduction to the biology of protein kinase C, one of the first calcium-dependent kinases to be discovered. We review the structure and function of protein kinase C, along with some of the substrates of individual isoforms. We then review strategies for inhibiting PKC in experimental systems and finally discuss the therapeutic potential of targeting PKC. Each aspect is covered in summary, with links to detailed resources where appropriate.

20-HETE induces hyperglycemia through the cAMP/PKA-PhK-GP pathway

We previously generated cytochrome P450 4F2 (CYP4F2) transgenic mice and showed high 20-hydroxyeicosatetraenoic acid (20-HETE) production, which resulted in an elevation of blood pressure. However, it was unclear whether 20-HETE affected glucose metabolism. We measured fasting plasma glucose, insulin, hepatic CYP4F2 expression, and 20-HETE production by hepatic microsomes, and hepatic 20-HETE levels in transgenic mice. We also assessed glycogen phosphorylase (GP) activity and the cAMP/protein kinase A (PKA)-phosphorylase kinase (PhK)-GP pathway, as well as expressions of insulin receptor substrate 1 and glucose transporters in vivo and in vitro. The transgenic mice had overexpressed hepatic CYP4F2, high hepatic 20-HETE and fasting plasma glucose levels but normal insulin level. The GP activity was increased and the cAMP/PKA-PhK-GP pathway was activated in the transgenic mice compared with wild-type mice. Moreover, these alterations were eliminated with the addition of N-hydroxy-N'-(4-butyl-2 methylphenyl) formamidine, which is a selective 20-HETE inhibitor. The results were further validated in Bel7402 cells. In addition, the transgenic mice had functional insulin signaling, and 20-HETE had no effect on insulin signaling in Bel7402 cells, excluding that the observed hyperglycemia in CYP4F2 transgenic mice resulted from insulin dysfunction, because the target tissues were sensitive to insulin. Our study suggested that 20-HETE can induce hyperglycemia, at least in part, through the cAMP/PKA-PhK-GP pathway but not through the insulin-signaling pathway.

Inhibition of GSK3 by lithium, from single molecules to signaling networks

For more than 60 years, the mood stabilizer lithium has been used alone or in combination for the treatment of bipolar disorder, schizophrenia, depression, and other mental illnesses. Despite this long history, the molecular mechanisms trough which lithium regulates behavior are still poorly understood. Among several targets, lithium has been shown to directly inhibit glycogen synthase kinase 3 alpha and beta (GSK3α and GSK3β). However in vivo, lithium also inhibits GSK3 by regulating other mechanisms like the formation of a signaling complex comprised of beta-arrestin 2 (βArr2) and Akt. Here, we provide an overview of in vivo evidence supporting a role for inhibition of GSK3 in some behavioral effects of lithium. We also explore how regulation of GSK3 by lithium within a signaling network involving several molecular targets and cell surface receptors [e.g., G protein coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs)] may provide cues to its relative pharmacological selectivity and its effects on disease mechanisms. A better understanding of these intricate actions of lithium at a systems level may allow the rational development of better mood stabilizer drugs with enhanced selectivity, efficacy, and lesser side effects.

Inhibition of insulin signaling in endothelial cells by protein kinase C-induced phosphorylation of p85 subunit of phosphatidylinositol 3-kinase (PI3K)

The regulation of endothelial function by insulin is consistently abnormal in insulin-resistant states and diabetes. Protein kinase C (PKC) activation has been reported to inhibit insulin signaling selectively in endothelial cells via the insulin receptor substrate/PI3K/Akt pathway to reduce the activation of endothelial nitric-oxide synthase (eNOS). In this study, it was observed that PKC activation differentially inhibited insulin receptor substrate 1/2 (IRS1/2) signaling of insulin's activation of PI3K/eNOS by decreasing only tyrosine phosphorylation of IRS2. In addition, PKC activation, by general activator and specifically by angiotensin II, increased the phosphorylation of p85/PI3K, which decreases its association with IRS1 and activation. Thr-86 of p85/PI3K was identified to be phosphorylated by PKC activation and confirmed to affect IRS1-mediated activation of Akt/eNOS by insulin and VEGF using a deletion mutant of the Thr-86 region of p85/PI3K. Thus, PKC and angiotensin-induced phosphorylation of Thr-86 of p85/PI3K may partially inhibit the activation of PI3K/eNOS by multiple cytokines and contribute to endothelial dysfunction in metabolic disorders.

Mammalian TOR signaling to the AGC kinases

The mechanistic (or mammalian) target of rapamycin (mTOR), an evolutionarily conserved protein kinase, orchestrates cellular responses to growth, metabolic and stress signals. mTOR processes various extracellular and intracellular inputs as part of two mTOR protein complexes, mTORC1 or mTORC2. The mTORCs have numerous cellular targets but members of a family of protein kinases, the protein kinase (PK)A/PKG/PKC (AGC) family are the best characterized direct mTOR substrates. The AGC kinases control multiple cellular functions and deregulation of many members of this family underlies numerous pathological conditions. mTOR phosphorylates conserved motifs in these kinases to allosterically augment their activity, influence substrate specificity, and promote protein maturation and stability. Activation of AGC kinases in turn triggers the phosphorylation of diverse, often overlapping, targets that ultimately control cellular response to a wide spectrum of stimuli. This review will highlight recent findings on how mTOR regulates AGC kinases and how mTOR activity is feedback regulated by these kinases. We will discuss how this regulation can modulate downstream targets in the mTOR pathway that could account for the varied cellular functions of mTOR.

Role of FAT/CD36 in novel PKC isoform activation in heart of spontaneously hypertensive rats

Disruption to the sensitive balance of long-chain fatty acids and glucose in the heart could cause cardiovascular diseases. Searching for a possible role of novel protein kinase C (nPKC) in heart with disrupted energy balance, we compared the insulin-resistant spontaneously hypertensive rats (SHR), which carry a nonfunctional variant of the fatty acid transporter FAT/CD36, with the less insulin-resistant congenic strain SHR-4 that is genetically identical except for a segment on chromosome 4 including a wild-type gene for a functional FAT/CD36. We analyzed expression of the nPKC-δ and -ε isoforms plus triacylglycerols (TAG) content in the myocardium of both FAT/CD36 strains and after a high sucrose diet (HSD). Two weeks before killing, males of both strains were randomly divided into two groups and fed either a standard laboratory chow or an HSD. PKC was determined by Western blotting in particulate and cytosolic fractions from left ventricles. The SHR-4 rats exhibited lower serum levels of insulin and free fatty acids than did SHR rats and higher amounts of PKC-ε in the heart particulate fraction. HSD caused accumulation of heart TAG in SHR but not in SHR-4. HSD increased PKC-δ and decreased PKC-ε expression in particulate fraction from left ventricles of SHR-4 while having no effects in SHR. These results demonstrate that reduced insulin resistance in SHR-4 rats with wild-type FAT/CD36 is associated with the insulin signaling pathway involving nPKCs.

Insulin-induced serine phosphorylation of IRS-2 via ERK1/2 and mTOR: studies on the function of Ser675 and Ser907

The identity of specific serine phosphorylation residues of insulin receptor substrate (IRS)-2 and their impact on insulin signal transduction are largely unknown. Ser(675) and Ser(907) of mouse IRS-2 are adjacent to PI 3-kinase or Grb2 binding domains, respectively. Using monoclonal phosphosite-specific antibodies, we demonstrated the phosphorylation of both serines after stimulation of Fao hepatoma cells with insulin, anisomycin, or phorbol esters. Phosphorylation of both sites was a late and prolonged event during insulin treatment and was also detected in liver tissue of insulin-treated as well as refed mice. Inhibition and siRNA-mediated knockdown of ERK1/2 indicated that the insulin-induced phosphorylation of Ser(907) was ERK dependent. Phosphorylation of Ser(907) did not prevent the insulin-induced association of IRS-2 with Grb2, but phosphorylation of the adjacent Tyr(911) was proved to be crucial in HEK 293 cells expressing IRS-2 Ala mutants. The insulin-induced phosphorylation of Ser(675) was prevented by inhibition and siRNA-mediated knockdown of mTOR but not of p70(S6K1). Mutation of Ser(675) to Ala did not affect downstream insulin signaling but increased the half-life of the protein, suggesting an involvement of phospho-Ser(675) in an accelerated degradation of IRS-2. Moreover, the insulin-induced degradation of IRS-2 was blocked by inhibition of mTOR. We conclude that the two novel insulin-dependent serine phosphorylation sites of IRS-2 were not involved in the regulation of the adjacent PI 3-kinase and Grb2 binding domains but might be implicated in the ERK- and mTOR-mediated negative feedback control.

{beta}-Hydroxybutyrate inhibits insulin-mediated glucose transport in mouse oxidative muscle

Blood ketone body levels increase during starvation and untreated diabetes. Here we tested the hypothesis that ketone bodies directly inhibit insulin action in skeletal muscle. We investigated the effect of d,l-beta-hydroxybutyrate (BOH; the major ketone body in vivo) on insulin-mediated glucose uptake (2-deoxyglucose) in isolated mouse soleus (oxidative) and extensor digitorum longus (EDL; glycolytic) muscle. BOH inhibited insulin-mediated glucose uptake in soleus (but not in EDL) muscle in a time- and concentration-dependent manner. Following 19.5 h of exposure to 5 mM BOH, insulin-mediated (20 mU/ml) glucose uptake was inhibited by approximately 90% (substantial inhibition was also observed in 3-O-methylglucose transport). The inhibitory effect of BOH was reproduced with d- but not l-BOH. BOH did not significantly affect hypoxia- or AICAR-mediated (activates AMP-dependent protein kinase) glucose uptake. The BOH effect did not require the presence/utilization of glucose since it was also seen when glucose in the medium was substituted with pyruvate. To determine whether the BOH effect was mediated by oxidative stress, an exogenous antioxidant (1 mM tempol) was used; however, tempol did not reverse the BOH effect on insulin action. BOH did not alter the levels of total tissue GLUT4 protein or insulin-mediated tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 but blocked insulin-mediated phosphorylation of protein kinase B by approximately 50%. These data demonstrate that BOH inhibits insulin-mediated glucose transport in oxidative muscle by inhibiting insulin signaling. Thus ketone bodies may be potent diabetogenic agents in vivo.

Overexpression of kinase-negative protein kinase Cdelta in pancreatic beta-cells protects mice from diet-induced glucose intolerance and beta-cell dysfunction

OBJECTIVE

In vitro models suggest that free fatty acid-induced apoptotic beta-cell death is mediated through protein kinase C (PKC)delta. To examine the role of PKCdelta signaling in vivo, transgenic mice overexpressing a kinase-negative PKCdelta (PKCdeltaKN) selectively in beta-cells were generated and analyzed for glucose homeostasis and beta-cell survival.

RESEARCH DESIGN AND METHODS

Mice were fed a standard or high-fat diet (HFD). Blood glucose and insulin levels were determined after glucose loads. Islet size, cleaved caspase-3, and PKCdelta expression were estimated by immunohistochemistry. In isolated islet cells apoptosis was assessed with TUNEL/TO-PRO3 DNA staining and the mitochondrial potential by rhodamine-123 staining. Changes in phosphorylation and subcellular distribution of forkhead box class O1 (FOXO1) were analyzed by Western blotting and immunohistochemistry.

RESULTS

PKCdeltaKN mice were protected from HFD-induced glucose intolerance. This was accompanied by increased insulin levels in vivo, by an increased islet size, and by a reduced staining of beta-cells for cleaved caspase-3 compared with wild-type littermates. In accordance, long-term treatment with palmitate increased apoptotic cell death of isolated islet cells from wild-type but not from PKCdeltaKN mice. PKCdeltaKN overexpression protected islet cells from palmitate-induced mitochondrial dysfunction and inhibited nuclear accumulation of FOXO1 in mouse islet and INS-1E cells. The inhibition of nuclear accumulation of FOXO1 by PKCdeltaKN was accompanied by an increased phosphorylation of FOXO1 at Ser256 and a significant reduction of FOXO1 protein.

CONCLUSIONS

Overexpression of PKCdeltaKN in beta-cells protects from HFD-induced beta-cell failure in vivo by a mechanism that involves inhibition of fatty acid-mediated apoptosis, inhibition of mitochondrial dysfunction, and inhibition of FOXO1 activation.

Diverse roles for protein kinase C delta and protein kinase C epsilon in the generation of high-fat-diet-induced glucose intolerance in mice: regulation of lipogenesis by protein kinase C delta

AIMS/HYPOTHESIS

This study aimed to determine whether protein kinase C (PKC) delta plays a role in the glucose intolerance caused by a high-fat diet, and whether it could compensate for loss of PKCepsilon in the generation of insulin resistance in skeletal muscle.

METHODS

Prkcd (-/-), Prkce (-/-) and wild-type mice were fed high-fat diets and subjected to glucose tolerance tests. Blood glucose levels and insulin responses were determined during the tests. Insulin signalling in liver and muscle was assessed after acute in vivo insulin stimulation by immunoblotting with phospho-specific antibodies. Activation of PKC isoforms in muscle from Prkce (-/-) mice was assessed by determining intracellular distribution. Tissues and plasma were assayed for triacylglycerol accumulation, and hepatic production of lipogenic enzymes was determined by immunoblotting.

RESULTS

Both Prkcd (-/-) and Prkce (-/-) mice were protected against high-fat-diet-induced glucose intolerance. In Prkce (-/-) mice this was mediated through enhanced insulin availability, while in Prkcd (-/-) mice the reversal occurred in the absence of elevated insulin. Neither the high-fat diet nor Prkcd deletion affected maximal insulin signalling. The activation of PKCdelta in muscle from fat-fed mice was enhanced by Prkce deletion. PKCdelta-deficient mice exhibited reduced liver triacylglycerol accumulation and diminished production of lipogenic enzymes.

CONCLUSIONS/INTERPRETATION

Deletion of genes encoding isoforms of PKC can improve glucose intolerance, either by enhancing insulin availability in the case of Prkce, or by reducing lipid accumulation in the case of Prkcd. The absence of PKCepsilon in muscle may be compensated by increased activation of PKCdelta in fat-fed mice, suggesting that an additional role for PKCepsilon in this tissue is masked.

11beta-hydroxysteroid dehydrogenase type 1 regulates glucocorticoid-induced insulin resistance in skeletal muscle

OBJECTIVE

Glucocorticoid excess is characterized by increased adiposity, skeletal myopathy, and insulin resistance, but the precise molecular mechanisms are unknown. Within skeletal muscle, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) converts cortisone (11-dehydrocorticosterone in rodents) to active cortisol (corticosterone in rodents). We aimed to determine the mechanisms underpinning glucocorticoid-induced insulin resistance in skeletal muscle and indentify how 11beta-HSD1 inhibitors improve insulin sensitivity.

RESEARCH DESIGN AND METHODS

Rodent and human cell cultures, whole-tissue explants, and animal models were used to determine the impact of glucocorticoids and selective 11beta-HSD1 inhibition upon insulin signaling and action.

RESULTS

Dexamethasone decreased insulin-stimulated glucose uptake, decreased IRS1 mRNA and protein expression, and increased inactivating pSer(307) insulin receptor substrate (IRS)-1. 11beta-HSD1 activity and expression were observed in human and rodent myotubes and muscle explants. Activity was predominantly oxo-reductase, generating active glucocorticoid. A1 (selective 11beta-HSD1 inhibitor) abolished enzyme activity and blocked the increase in pSer(307) IRS1 and reduction in total IRS1 protein after treatment with 11DHC but not corticosterone. In C57Bl6/J mice, the selective 11beta-HSD1 inhibitor, A2, decreased fasting blood glucose levels and improved insulin sensitivity. In KK mice treated with A2, skeletal muscle pSer(307) IRS1 decreased and pThr(308) Akt/PKB increased. In addition, A2 decreased both lipogenic and lipolytic gene expression.

CONCLUSIONS

Prereceptor facilitation of glucocorticoid action via 11beta-HSD1 increases pSer(307) IRS1 and may be crucial in mediating insulin resistance in skeletal muscle. Selective 11beta-HSD1 inhibition decreases pSer(307) IRS1, increases pThr(308) Akt/PKB, and decreases lipogenic and lipolytic gene expression that may represent an important mechanism underpinning their insulin-sensitizing action.

Angiotensin II and the development of insulin resistance: implications for diabetes

Angiotensin II (Ang II), the major effector hormone of the renin-angiotensin system (RAS), has an important role in the regulation of vascular and renal homeostasis. Clinical and pharmacological studies have recently shown that Ang II is a critical promoter of insulin resistance and diabetes mellitus type 2. Ang II exerts its actions on insulin-sensitive tissues such as liver, muscle and adipose tissue where it has effects on the insulin receptor (IR), insulin receptor substrate (IRS) proteins and the downstream effectors PI3K, Akt and GLUT4. The molecular mechanisms involved have not been completely identified, but the role of serine/threonine phosphorylation of the IR and IRS-1 proteins in desensitization of insulin action has been well established. The purpose of this review is to highlight recent advances in the understanding of Ang II actions which lead to the development of insulin resistance and its implications for diabetes.

Insulin resistance in polycystic ovary syndrome is associated with defective regulation of ERK1/2 by insulin in skeletal muscle in vivo

Insulin resistance is a recognized feature of PCOS (polycystic ovary syndrome). However, the molecular reason(s) underlying this reduced cellular insulin sensitivity is not clear. The present study compares the major insulin signalling pathways in skeletal muscle isolated from PCOS and controls. We measured whole-body insulin sensitivity and insulin signalling in skeletal muscle biopsies taken before and after acute exposure to hyperinsulinaemia in nine women diagnosed with PCOS and seven controls. We examined the expression, basal activity and response to in vivo insulin stimulation of three signalling molecules within these human muscle samples, namely IRS-1 (insulin receptor substrate-1), PKB (protein kinase B) and ERK (extracellular-signal-regulated kinase) 1/2. There was no significant difference in the expression, basal activity or activation of IRS-1 or PKB between PCOS and control subjects. However, there was a severe attenuation of insulin stimulation of the ERK pathway in muscle from all but two of the women with PCOS (the two most obese), and an accompanying trend towards higher basal phosphorylation of ERK1/2 in PCOS. These results are striking in that the metabolic actions of insulin are widely believed to require the IRS-1/PKB pathway rather than ERK, and the former has been reported as defective in some previous PCOS studies. Most importantly, the molecular defect identified was independent of adiposity. The altered response of ERK to insulin in PCOS was the most obvious signalling defect associated with insulin resistance in muscle from these patients.

Interplay and effects of temporal changes in the phosphorylation state of serine-302, -307, and -318 of insulin receptor substrate-1 on insulin action in skeletal muscle cells

Transduction of the insulin signal is mediated by multisite Tyr and Ser/Thr phosphorylation of the insulin receptor substrates (IRSs). Previous studies on the function of single-site phosphorylation, particularly phosphorylation of Ser-302, -307, and -318 of IRS-1, showed attenuating as well as enhancing effects on insulin action. In this study we investigated a possible cross talk of these opposedly acting serine residues in insulin-stimulated skeletal muscle cells by monitoring phosphorylation kinetics, and applying loss of function, gain of function, and combination mutants of IRS-1. The phosphorylation at Ser-302 was rapid and transient, followed first by Ser-318 phosphorylation and later by phosphorylation of Ser-307, which remained elevated for 120 min. Mutation of Ser-302 to alanine clearly reduced the subsequent protein kinase C-zeta-mediated Ser-318 phosphorylation. The Ser-307 phosphorylation was independent of Ser-302 and/or Ser-318 phosphorylation status. The functional consequences of these phosphorylation patterns were studied by the expression of IRS-1 mutants. The E302A307E318 mutant simulating the early phosphorylation pattern resulted in a significant increase in Akt and glycogen synthase kinase 3 phosphorylation. Furthermore, glucose uptake was enhanced. Because the down-regulation of the insulin signal was not affected, this phosphorylation pattern seems to be involved in the enhancement but not in the termination of the insulin signal. This enhancing effect was completely absent when Ser-302 was unphosphorylated and Ser-307 was phosphorylated as simulated by the A302E307E318 mutant. Phospho-Ser-318, sequentially phosphorylated at least by protein kinase C-zeta and a mammalian target of rapamycin/raptor-dependent kinase, was part of the positive as well as of the subsequent negative phosphorylation pattern. Thus we conclude that insulin stimulation temporally generates different phosphorylation statuses of the same residues that exert different functions in insulin signaling.