Rina Hemi

A Molecular Basis for Insulin Resistance ELEVATED SERINE/THREONINE PHOSPHORYLATION OF IRS-1 AND IRS-2 INHIBITS THEIR BINDING TO THE JUXTAMEMBRANE REGION OF THE INSULIN RECEPTOR AND IMPAIRS THEIR ABILITY TO UNDERGO INSULIN-INDUCED TYROSINE PHOSPHORYLATION

Tumor necrosis factor α (TNFα) or chronic hyperinsulinemia that induce insulin resistance trigger increased Ser/Thr phosphorylation of the insulin receptor (IR) and of its major insulin receptor substrates, IRS-1 and IRS-2. To unravel the molecular basis for this uncoupling in insulin signaling, we undertook to study the interaction of Ser/Thr-phosphorylated IRS-1 and IRS-2 with the insulin receptor. We could demonstrate that, similar to IRS-1, IRS-2 also interacts with the juxtamembrane (JM) domain (amino acids 943–984) but not with the carboxyl-terminal region (amino acids 1245–1331) of IR expressed in bacteria as His6fusion peptides. Moreover, incubation of rat hepatoma Fao cells with TNFα, bacterial sphingomyelinase, or other Ser(P)/Thr(P)-elevating agents reduced insulin-induced Tyr phosphorylation of IRS-1 and IRS-2, markedly elevated their Ser(P)/Thr(P) levels, and significantly reduced their ability to interact with the JM region of IR. Withdrawal of TNFα for periods as short as 30 min reversed its inhibitory effects on IR-IRS interactions. Similar inhibitory effects were obtained when Fao cells were subjected to prolonged (20–60 min) pretreatment with insulin. Incubation of the cell extracts with alkaline phosphatase reversed the inhibitory effects of insulin. These findings suggest that insulin resistance is associated with enhanced Ser/Thr phosphorylation of IRS-1 and IRS-2, which impairs their interaction with the JM region of IR. Such impaired interactions abolish the ability of IRS-1 and IRS-2 to undergo insulin-induced Tyr phosphorylation and further propagate the insulin receptor signal. Moreover, the reversibility of the TNFα effects and the ability to mimic its action by exogenously added sphingomyelinase argue against the involvement of a proteolytic cascade in mediating the acute inhibitory effects of TNFα on insulin action.

Tumor Necrosis Factor α-induced Phosphorylation of Insulin Receptor Substrate-1 (IRS-1) POSSIBLE MECHANISM FOR SUPPRESSION OF INSULIN-STIMULATED TYROSINE PHOSPHORYLATION OF IRS-1

Tumor necrosis factor-α (TNF) has been suggested to be the mediator of insulin resistance in infection, tumor cachexia, and obesity. We have previously shown that TNF diminishes insulin-induced tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1). The current work examines potential mechanisms that mediate this event. TNF effect on IRS-1 in Fao hepatoma cells was not associated with a significant reduction in insulin receptor tyrosine kinase activity as measured in vitro but impaired the association of IRS-1 with phosphatidylinositol 3-kinase, localizing TNF impact to IRS-1. TNF did not increase protein-tyrosine phosphatase activity and protein-tyrosine phosphatase inhibition by vanadate did not change TNF effect on IRS-1 tyrosine phosphorylation, suggesting that protein-tyrosine phosphatases are not involved in this TNF effect. In contrast, TNF increased IRS-1 phosphorylation on serine residues, leading to a decrease in its electrophoretic mobility. TNF effect on IRS-1 tyrosine phosphorylation was not abolished by inhibiting protein kinase C using staurosporine, while inactivation of Ser/Thr phosphatases by calyculin A and okadaic acid mimicked it. Our data suggest that TNF induces serine phosphorylation of IRS-1 through inhibition of serine phosphatases or activation of serine kinases other than protein kinase C. This increased serine phosphorylation interferes with insulin-induced tyrosine phosphorylation of IRS-1 and impairs insulin action.

Phosphorylation of Insulin Receptor Substrate-1 (IRS-1) by Protein Kinase B Positively Regulates IRS-1 Function

Incubation of cells with insulin leads to a transient rise in Tyr phosphorylation of insulin receptor substrate (IRS) proteins, accompanied by elevation in their Ser(P)/Thr(P) content and their dissociation from the insulin receptor (IR). Wortmannin, a phosphatidylinositol 3-kinase inhibitor, selectively prevented the increase in Ser(P)/Thr(P) content of IRS-1, its dissociation from IR, and the decrease in its Tyr(P) content following 60 min of insulin treatment. Four conserved phosphorylation sites within the phosphotyrosine binding/SAIN domains of IRS-1 and IRS-2 served asin vitro substrates for protein kinase B (PKB), a Ser/Thr kinase downstream of phosphatidylinositol 3-kinase. Furthermore, PKB and IRS-1 formed stable complexes in vivo, and overexpression of PKB enhanced Ser phosphorylation of IRS-1. Overexpression of PKB did not affect the acute Tyr phosphorylation of IRS-1; however, it significantly attenuated its rate of Tyr dephosphorylation following 60 min of treatment with insulin. Accordingly, overexpression of IRS-14A, lacking the four potential PKB phosphorylation sites, markedly enhanced the rate of Tyr dephosphorylation of IRS-1, while inclusion of vanadate reversed this effect. These results implicate a wortmannin-sensitive Ser/Thr kinase, different from PKB, as the kinase that phosphorylates IRS-1 and acts as the feedback control regulator that turns off insulin signals by inducting the dissociation of IRS proteins from IR. In contrast, insulin-stimulated PKB-mediated phosphorylation of Ser residues within the phosphotyrosine binding/SAIN domain of IRS-1 protects IRS-1 from the rapid action of protein-tyrosine phosphatases and enables it to maintain its Tyr-phosphorylated active conformation. These findings implicate PKB as a positive regulator of IRS-1 functions.

INSULINLIKE GROWTH FACTOR-1 INHIBITS CELL DEATH INDUCED BY CYCLOHEXIMIDE IN MCF-7 CELLS: A MODEL SYSTEM FOR ANALYZING CONTROL OF CELL DEATH

SummaryProlonged exposure of cells to the potent protein synthesis inhibitor cycloheximide terminates in cell death. In the present study we investigated the effect of insulinlike growth factor-1, insulin, and epidermal growth factor on cell death induced by cycloheximide in the confluent MCF-7 cells, and correlated this effect to the inhibition rate of protein synthesis. Cell death was evaluated by measuring either dead cells by the trypan blue dye exclusion test or by the release of lactic dehydrogenase into the culture medium. After 48 h incubation, cycloheximide (10 to 50µg/ml) was shown to induce cell death in a concentration-dependent manner. Insulinlike growth factor-1, at physiologic concentrations (0.2 to 5 ng/ml), reduced this cell death. Insulin at supraphysiologic concentrations (1 to 10µg/ml) mimicked the effect of insulinlike growth factor-1, whereas epidermal growth factor (10 to 50 ng/ml) had no effect. More than 90% of protein synthesis measured by [3H]leucine incorporation was inhibited by 10 to 50µg/ml cyclohexmide. Insulinlike growth factor-1 and insulin at the concentrations that reduced cell death to control level, had no effect on the protein synthesis inhibition rate induced by cycloheximide. These results indicate that inhibition of cell death by insulinlike growth factor-1 does not depend on protein synthesis and may be mediated via a posttranslational modification effect.

Total and high molecular weight adiponectin are elevated in patients with Laron syndrome despite marked obesity.

OBJECTIVE Patients with Laron syndrome (LS; primary GH insensitivity) caused by molecular defects of the GH receptor gene, are characterized by dwarfism, profound obesity, and hyperlipidemia. The aim of the current study was to evaluate adiponectin levels in LS, as obesity is known to be associated with low adiponectin. DESIGN AND METHODS We studied nine untreated LS adult patients (5 males, 4 females) and six girls with LS receiving once-daily treatment by IGF1. Total and high molecular weight (HMW) adiponectin levels, adiponectin multimers distribution, and metabolic indices were analyzed in serum samples obtained during several years of follow-up. RESULTS Adiponectin levels in the severely obese adult LS patients (percent body fat; females 61.0+/-2.5%, males 40.6+/-8.1%) were two- to three-fold higher than those reported for subjects of corresponding age, gender and degree of adiposity. Total adiponectin was significantly higher in females compared with males (21.4+/-3.5 vs 10.2+/-4.6 microg/ml, P<0.001). The elevated adiponectin in LS subjects was associated with an increased abundance of the HMW isoform, and positively correlated with body fat percentage (r=0.65, P=0.017) and leptin (r=0.65, P=0.012). There was no correlation between adiponectin levels (total and HMW) and the degree of insulin resistance in LS subjects or their blood lipids levels. Adiponectin was also high in young girls with LS (22.9+/-7.4 microg/ml) and did not change during long-term IGF1 replacement therapy. CONCLUSION Adiponectin hypersecretion in LS, despite profound obesity, suggests that GH activity may negatively impact adiponectin secretion from adipocytes.

p38 mitogen-activated protein kinase-dependent transactivation of ErbB receptor family: a novel common mechanism for stress-induced IRS-1 serine phosphorylation and insulin resistance.

OBJECTIVE Stress stimuli such as tumor necrosis factor (TNF) have been shown to induce insulin receptor substrate (IRS)-1 serine phosphorylation and insulin resistance by transactivation of ErbB receptors. We aimed at elucidating the potential role of p38 mitogen-activated protein kinase (p38MAPK) in mediating stress-induced ErbB receptors activation. RESEARCH DESIGN AND METHODS p38MAPK effect on ErbBs transactivation and insulin signaling was assessed in Fao or HepG2 cells, exposed to stress stimuli, and on metabolic parameters in ob/ob and C57/BL6 mice. RESULTS High-fat diet–fed mice and ob/ob mice exhibited elevated hepatic p38MAPK activation associated with glucose intolerance and hyperinsulinemia. Liver expression of dominant-negative (DN)-p38MAPKα in ob/ob mice reduced fasting insulin levels and improved glucose tolerance, whereas C57/BL6 mice overexpressing wild-type p38MAPKα exhibited enhanced IRS-1 serine phosphorylation and reduced insulin-stimulated IRS-1 tyrosine phosphorylation. Fao or HepG2 cells exposed to TNF, anisomycin, or sphingomyelinase demonstrated rapid transactivation of ErbB receptors leading to PI3-kinase/Akt activation and IRS-1 serine phosphorylation. p38MAPK inhibition either by SB203580, by small interfering RNA, or by DN-p38MAPKα decreased ErbB receptors transactivation and IRS-1 serine phosphorylation and partially restored insulin-stimulated IRS-1 tyrosine phosphorylation. When cells were incubated with specific ErbB receptors antagonists or in cells lacking ErbB receptors, anisomycin- and TNF-induced IRS-1 serine phosphorylation was attenuated, despite intact p38MAPK activation. The stress-induced p38MAPK activation leading to ErbB receptors transactivation was associated with intracellular reactive oxygen species generation and was attenuated by treatment with antioxidants. CONCLUSIONS Hepatic p38MAPK is activated following various stress stimuli. This event is upstream to ErbB receptors transactivation and plays an important role in stress-induced IRS-1 serine phosphorylation and insulin resistance.

Adiponectin and Leptin Concentrations in Dichorionic Twins with Discordant and Concordant Growth

CONTEXT Discordant twin gestation, in which one fetus is growth restricted, is a unique model that can elucidate the mechanism(s) by which the intrauterine environment affects fetal growth. OBJECTIVE The objective of the study was to determine the cord blood adiponectin and leptin concentrations and evaluate their association with birth weight in dichorionic twins, with and without growth discordance. DESIGN, SETTING, PARTICIPANTS, AND MAIN OUTCOME MEASURE: In this cross-sectional study, arterial cord blood adiponectin and leptin concentrations were determined in two groups of newborns: 1) discordant twins, in which one of the twins is growth restricted (small for gestation age and abnormal umbilical arteries Doppler) and the other is appropriate for gestation age (AGA) (n = 14 pairs); and 2) concordant twins, in which both twins are AGA (n = 15 pairs). RESULTS Results were: 1) within the discordant twins group, the median adiponectin concentration was significantly lower in the growth-restricted newborns than in their cotwins (P = 0.004); 2) within the concordant twin group, there was no significant difference in the median cord blood adiponectin concentration between the two AGA twins; 3) the median leptin concentration did not differ between the twins pairs in both study groups; 4) a positive correlation between cord blood adiponectin concentrations and both birth weight (r = 0.7, P < 0.001) and gestational age (r = 0.6, P < 0.02) was found only in the small-for-gestational-age newborns; 5) linear regression model revealed that birth weight is independently associated with circulating adiponectin concentration. CONCLUSIONS Low circulating adiponectin concentrations, previously reported in adults, children, and infants who were born small for gestational age, characterize fetuses with growth restriction and are independently associated with birth weight.

Anti-Müllerian hormone is highly expressed and secreted from cumulus granulosa cells of stimulated preovulatory immature and atretic oocytes

This study investigated anti-Müllerian hormone (AMH) expression and secretion from cumulus granulosa cells (GC) and steroidogenesis in follicular fluids (FF) with relation to oocyte maturational stages and fertilization capacity in large preovulatory follicles. This prospective study included 53 ovulatory women undergoing intracytoplasmic sperm injection. FF and cumulus GC from 140 large preovulatory follicles were individually obtained during oocyte retrieval. Main outcome measures were oocyte maturation, fertilization and embryo quality. FF were assayed for AMH, progesterone, 17β-oestradiol and testosterone. Cumulus GC were assayed for AMH mRNA expression. AMH mRNA expression and secretion in cumulus GC in preovulatory follicles containing germinal-vesicle (GV) and metaphase-I (MI) oocytes were significantly higher than follicles containing MII oocytes (P<0.01 and P<0.0001, respectively). In addition, FF AMH concentrations from atretic oocytes were significantly higher than from MII oocytes. No correlation was found between AMH expression and secretion to fertilization or embryo quality. FF of MI and GV oocytes had higher concentrations of testosterone and lower progesterone/oestradiol ratios than MII oocytes, and FF of atretic oocytes contained higher testosterone concentrations than FF of MII oocytes. AMH is highly expressed in and secreted from cumulus GC of preovulatory follicles containing premature and atretic oocytes. Anti-Müllerian hormone (AMH) is produced in the female exclusively by granulosa cells. AMH has recently been shown to be one of the most important markers of ovarian reserve and it is highly associated with ovarian follicular development. This study investigates AMH expression and secretion from cumulus granulosa cells (GC) and steroidogenesis in the follicular fluids (FF) with relation to oocyte maturational stages, and fertilization capacity in large preovulatory follicles. We conducted a prospective study with 53 ovulatory women undergoing intracytoplasmic sperm injection. FF and cumulus GC from 140 large preovulatory follicles were individually obtained during oocyte retrieval. The main outcome measures were oocyte maturation, fertilization and embryo quality. FF were assayed for AMH, progesterone, 17β-oestradiol and testosterone. Cumulus GC were assayed for AMH mRNA expression. AMH mRNA expression in cumulus GC and AMH concentrations in FF of preovulatory follicles containing premature oocytes (germinal vesicle (GV) and metaphase I (MI)) were significantly higher than preovulatory follicles containing mature oocytes (MII oocytes). In addition, FF AMH concentrations of atretic oocytes were significantly higher than FF AMH of MII oocytes. No correlation was found between AMH expression and secretion for fertilization or embryo quality. FF of preovulatory MI and GV oocytes had higher levels of testosterone and lower progesterone/oestradiol ratios than MII oocytes, and FF of atretic oocytes contained higher testosterone levels than FF of MII oocytes. This study shows that AMH is highly expressed in and secreted from cumulus GC of preovulatory follicles containing premature and atretic oocytes.

Serum and insulin inhibit cell death induced by cycloheximide in the human breast cancer cell line MCF-7

SummaryContinuous exposure of cells to cycloheximide (CHM) terminates in cell death. This may result from CHM’s inhibition of protein synthesis. In the present study we investigated the effect of serum and insulin on cell death induced by CHM in the human breast cancer cell line MCF-7, and correlated this effect to the inhibition of protein synthesis. Cell death was evaluated by measuring either dead cells by the trypan blue dye exclusion test or by the release of lactic dehydrogenase into the culture medium. CHM (0.1 to 50 µg/ml) was shown to induce cell death in a time- and concentration-dependent manner. Including either fetal bovine serum or insulin in the culture medium inhibited this cell death in a concentration-dependent manner. Protein synthesis as measured by [3H]leucine incorporation was inhibited by the increasing concentration of CHM, However, fetal bovine serum and insulin did not alter the protein synthesis inhibition rate induced by CHM. These results indicate that inhibition of protein synthesis is not enough for cell death to proceed. Insulin or factors present in serum may stabilize some crucial cell proteins (key enzymes, cytoskeletal or membrane components) which are vital for cell life.

Adiponectin levels in adolescent girls with polycystic ovary syndrome (PCOS)

Objective  To determine serum adiponectin concentrations in adolescent girls with and without polycystic ovary syndrome (PCOS) and to assess possible correlations of adiponectin levels with insulin and androgen levels.