Models of insulin action on metabolic and growth response genes

Egr1 plays a major role in the transcriptional response of white adipocytes to insulin and environmental cues

It is believed that insulin regulates metabolic functions of white adipose tissue primarily at the post-translational level via the PI3K-Akt-mediated pathway. Still, changes in transcription also play an important role in the response of white adipocytes to insulin and environmental signals. One transcription factor that is dramatically and rapidly induced in adipocytes by insulin and nutrients is called Early Growth Response 1, or Egr1. Among other functions, it directly binds to promoters of leptin and ATGL stimulating the former and inhibiting the latter. Furthermore, expression of Egr1 in adipocytes demonstrates cell autonomous circadian pattern suggesting that Egr1 not only mediates the effect of insulin and nutrients on lipolysis and leptin production but also, coordinates insulin action with endogenous circadian rhythms of adipose tissue.

IER3 Promotes Expansion of Adipose Progenitor Cells in Response to Changes in Distinct Microenvironmental Effectors

Adipose tissue expansion is well-orchestrated to fulfill the energy demand. It results from adipocyte hypertrophy and hyperplasia due to adipose progenitor cell (APC) expansion and differentiation. Chronic low grade inflammation and hypoxia take place in obese adipose tissue microenvironment. Both of these events were shown to impact the APC pool by promoting increased self-renewal along with a decrease in the APC differentiation potential. However, no common target has been identified so far. Here we show that the immediate early response 3 gene (IER3) is preferentially expressed in APCs and is essential for APC proliferation and self-renewal. Experiments based on RNA interference revealed that impairing IER3 expression altered cell proliferation through ERK1/2 phosphorylation and clonogenicity. IER3 expression was induced by Activin A, which plays a crucial role in adipocyte differentiation as well as by a decrease in oxygen tension through HIF1-induced transcriptional activation. Interestingly, high levels of IER3 were detected in native APCs (CD34+/CD31- cells) isolated from obese patients and conditioned media from obese adipose tissue-macrophages stimulated its expression. Overall, these results indicate that IER3 is a key player in expanding the pool of APC while highlighting the role of distinct effectors found in an obese microenvironment in this process.

4E-BPs Control Fat Storage by Regulating the Expression of Egr1 and ATGL

Early growth response transcription factor Egr1 controls multiple aspects of cell physiology and metabolism. In particular, Egr1 suppresses lipolysis and promotes fat accumulation in adipocytes by inhibiting the expression of adipose triglyceride lipase. According to current dogma, regulation of the Egr1 expression takes place primarily at the level of transcription. Correspondingly, treatment of cultured adipocytes with insulin stimulates expression of Egr1 mRNA and protein. Unexpectedly, the MEK inhibitor PD98059 completely blocks insulin-stimulated increase in the Egr1 mRNA but has only a moderate effect on the Egr1 protein. At the same time, mTORC1 inhibitors rapamycin and PP242 suppress expression of the Egr1 protein and have an opposite effect on the Egr1 mRNA. Mouse embryonic fibroblasts with genetic ablations of TSC2 or 4E-BP1/2 express less Egr1 mRNA but more Egr1 protein than wild type controls. (35)S-labeling has confirmed that translation of the Egr1 mRNA is much more effective in 4E-BP1/2-null cells than in control. A selective agonist of the CB1 receptors, ACEA, up-regulates Egr1 mRNA, but does not activate mTORC1 and does not increase Egr1 protein in adipocytes. These data suggest that although insulin activates both the Erk and the mTORC1 signaling pathways in adipocytes, regulation of the Egr1 expression takes place predominantly via the mTORC1/4E-BP-mediated axis. In confirmation of this model, we show that 4E-BP1/2-null MEFs express less ATGL and accumulate more fat than control cells, while knock down of Egr1 in 4E-BP1/2-null MEFs increases ATGL expression and decreases fat storage.

Synchronization in G0/G1 enhances the mitogenic response of cells overexpressing the human insulin receptor A isoform to insulin

Evaluating mitogenic signaling specifically through the human insulin receptor (IR) is relevant for the preclinical safety assessment of developmental insulin analogs. It is known that overexpression of IR sensitizes cells to the mitogenic effects of insulin, but it is essentially unknown how mitogenic responses can be optimized to allow practical use of such recombinant cell lines for preclinical safety testing. We constitutively overexpressed the short isoform of the human insulin receptor (hIR-A, exon 11-negative) in L6 rat skeletal myoblasts. Because the mitogenic effect of growth factors such as insulin is expected to act in G0/G1, promoting S-phase entry, we developed a combined topoinhibition + serum deprivation strategy to explore the effect of G0/G1 synchronization as an independent parameter in the context of serum deprivation, the latter being routinely used to reduce background in mitogenicity assays. G0/G1 synchronization significantly improved the mitogenic responses of L6-hIR cells to insulin, measured by ³H-thymidine incorporation. Comparison with the parental L6 cells using phospho-mitogen-activated protein kinase, phospho-AKT, as well as ³H-thymidine incorporation end points supported that the majority of the mitogenic effect of insulin in L6-hIR cells was mediated by the overexpressed hIR-A. Using the optimized L6-hIR assay, we found that the X-10 insulin analog was more mitogenic than native human insulin, supporting that X-10 exhibits increased mitogenic signaling through the hIR-A. In summary, this study provides the first demonstration that serum deprivation may not be sufficient, and G0/G1 synchronization may be required to obtain optimal responsiveness of hIR-overexpressing cell lines for preclinical safety testing.

Synergistic premalignant effects of chronic ethanol exposure and insulin receptor substrate-1 overexpression in liver

AIM

Insulin receptor substrate, type 1 (IRS-1) transmits growth and survival signals, and is overexpressed in more than 90% of hepatocellular carcinomas (HCCs). However, experimental overexpression of IRS-1 in the liver was found not to be sufficient to cause HCC. Since chronic alcohol abuse is a risk factor for HCC, we evaluated potential interactions between IRS-1 overexpression and chronic ethanol exposure by assessing premalignant alterations in gene expression.

METHODS

Wild-type (wt) or IRS-1 transgenic (Tg) mice, constitutively overexpressing the human (h) transgene in the liver, were pair-fed isocaloric liquid diets containing 0% or 24% ethanol for 8 weeks. The livers were used for histopathologic study and gene expression analysis, focusing on insulin, insulin-like growth factor (IGF) and wingless (WNT)-Frizzled (FZD) pathways, given their known roles in HCC.

RESULTS

In wt mice, chronic ethanol exposure caused hepatocellular microsteatosis with focal chronic inflammation, reduced expression of proliferating cell nuclear antigen (PCNA) and increased expression of IGF-I and IGF-I receptor. In hIRS-1 Tg mice, chronic ethanol exposure caused hepatic micro- and macrosteatosis, focal chronic inflammation, apoptosis and disordered lobular architecture. These effects of ethanol in hIRS-1 Tg mice were associated with significantly increased expression of IGF-II, insulin, IRS-4, aspartyl-asparaginyl beta hydroxylase (AAH), WNT-1 and FZD 7, as occurs in HCC.

CONCLUSION

In otherwise normal liver, chronic ethanol exposure mainly causes liver injury and inflammation with impaired DNA synthesis. In contrast, in the context of hIRS-1 overexpression, chronic ethanol exposure may serve as a cofactor in the pathogenesis of HCC by promoting expression of growth factors, receptors and signaling molecules known to be associated with hepatocellular transformation.

Glucose induces MafA expression in pancreatic beta cell lines via the hexosamine biosynthetic pathway

MafA is a basic leucine zipper transcription factor that regulates gene expression in both the neuroretina and pancreas. Within the pancreas, MafA is exclusively expressed in the beta cells and is involved in insulin gene transcription, insulin secretion, and beta cell survival. The expression of the mafA gene within beta cells is known to increase in response to high glucose levels by an unknown mechanism. In this study, we demonstrate that pyruvate, which is produced by glycolysis from glucose, is not sufficient to induce mafA gene expression compared with high glucose. This suggests that the signal for MafA induction is independent of ATP levels and that a metabolic event occurring upstream of pyruvate production leads to the induction of MafA. Furthermore, insulin secretion mediated by high glucose is not important for MafA expression. However, the addition of glucosamine to beta cell lines stimulates MafA expression in the absence of high glucose, and inhibition of the hexosamine biosynthetic pathway in the presence of high glucose abolishes MafA induction. Moreover, we demonstrate that the expression of UDP-N-acetylglucosaminyl transferase, the enzyme mediating O-linked glycosylation of cytosolic and nuclear proteins, is essential for glucose-dependent MafA expression. Consistent with this observation, inhibition of N-acetylglucosaminidase, the enzyme involved in the removal of the O-GlcNAc modification from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate stimulates MafA expression under low glucose conditions. The presented data suggest that MafA expression mediated by high glucose requires flux through the hexosamine biosynthetic pathway and the O-linked glycosylation of an unknown protein(s) by UDP-N-acetylglucosaminyl transferase.

Insulin augments GnRH-stimulated LHbeta gene expression by Egr-1

Previous studies have shown that insulin augments GnRH-stimulated LH synthesis and release from primary gonadotrophs. In this study, regulation of LHbeta gene expression by GnRH and insulin was examined in LbetaT2 cells. Endogenous LHbeta mRNA is stimulated 2.4-fold by insulin alone, 2.6-fold by GnRH alone, and 4.7-fold by insulin together with GnRH. This effect of insulin, like GnRH, mapped to sequences -140 to +1 in the mouse LHbeta gene. Insulin together with GnRH stimulates activity of an LHbeta-reporter gene 7.1-fold; whereas, GnRH alone or insulin alone stimulates the reporter activity 2.8- and 3.1-fold, respectively. Blocking the binding of Egr-1 to sequences -51 to -42 in the LHbeta gene inhibits effects of insulin and GnRH. Insulin together with GnRH increases Egr-1 mRNA levels and total Egr-1 binding to LHbeta DNA. These findings indicate that insulin may impact regulation of the reproductive axis at the level of the pituitary.

Evaluation of housekeeping genes in placental comparative expression studies

Preeclampsia and diabetes are complications of pregnancy that contribute to maternal and perinatal mortality worldwide. Results emerging from molecular studies of placentae may elucidate etiologically important genomic alterations. Appropriate application of real time reverse transcription (RT) PCR in comparative gene expression studies requires endogenous housekeeping genes to normalize between sample variations. Ideal housekeeping genes must have stable tissue expression, but few have been specifically studied in the placenta. We sought to identify candidate control genes by analyzing seven functionally distinct housekeeping genes (B2M, GAPDH, HMBS, HPRT, SDHA, TBP, YWHAZ) for their expression stability and level in the placenta. mRNA isolated from 20 placentae was analyzed for gene expression using RT-PCR. Expression stability (M) was assessed using normalization strategies previously used for other tissues. TBP and SDHA were the most stable, with an average expression stability of M = 0.43, followed by YWHAZ (M = 0.44) > HPRT (M = 0.53) > HMBS (M = 0.57) > GAPDH (M = 0.61) > B2M (M = 0.69). The genes tested ranged in abundance, with an approximately 300-fold increase from the lowest (HMBS) to the highest (B2M). By using TBP, SDHA and YWHAZ, with greater expression stability than those housekeeping genes commonly used in placenta studies, gene expression profile comparisons will have more sensitivity and specificity.

The effect of neurotensin on insulin-induced proliferation of human fibroblasts

Neurotensin has been shown to influence growth in a number of cancerous and non-cancerous cells and to enhance the proliferative effects of growth factors without itself inducing proliferation. Here we show that neurotensin potentiates the proliferative effects of insulin on IMR90 human fibroblasts in a concentration and neurotensin receptor type 1-dependent manner. This potentiating effect of neurotensin was blocked by inhibitors of phospholipase C and protein kinase C, was accompanied by an increase in the level of soluble inositol phosphates and did not involve an autocrine factor. These results show that neurotensin can enhance insulin-dependent proliferation of human fibroblasts and suggest a possible role for neurotensin in tissue growth and repair.

A novel view on the mechanisms of action of insulin and other insulin superfamily peptides: involvement of adenylyl cyclase signaling system

A new signaling mechanism common to mammalian insulin, insulin-like growth factor I, relaxin and mollusc insulin-like peptide, and involving receptor-tyrosine kinase==>G(i) protein (betagamma)==>phosphatidylinositol-3-kinase==>protein kinase Czeta==>adenylyl cyclase==>protein kinase A was discovered in the muscles and some other tissues of vertebrates and invertebrates. The authors' data were used to reconsider the problem of participation of the adenylyl cyclase-cAMP system in the regulatory effects of insulin superfamily peptides. A hypothesis has been put forward according to which the adenylyl cyclase signaling mechanism producing cAMP has a triple co-ordinating role in the regulatory action of insulin superfamily peptides on the main cell processes, inducing the mitogenic and antiapoptotic effects and inhibitory influence on some metabolic effects of the peptides. It is suggested that cAMP is a key regulator responsible for choosing the transduction pathway by concerted launching of one (proliferative) program and switching off (suppression) of two others, which lead to cell death and to the predomination of anabolic processes in a cell. The original data obtained give grounds to conclude that the adenylyl cyclase signaling system is a mechanism of signal transduction not only of hormones with serpentine receptors, but also of those with receptors of the tyrosine kinase type (insulin superfamily peptides and some growth factors).

Estradiol and a selective estrogen receptor modulator affect steroid hormone receptor messenger RNA levels and turnover in explant cultures of sheep endometrium

Estrogens upregulate estrogen receptor (ER) and progesterone receptor (PR) gene expression in endometrium immediately before ovulation to prepare it for nurturing embryos. Most in vitro model systems have lost the ability to upregulate expression of the ER gene in response to estradiol (E2) or the ability to express the ER gene at all. Here, we used explant cultures from control and E2-treated ewes and assessed expression of four genes (ER, PR, glyceraldehyde 3-phosphate dehydrogenase [GAPDH], and cyclophilin [CYC] genes) that are upregulated by E2 in vivo on Northern blots. In cultures from control and E2-treated ewes, ER and PR messenger ribonucleic acid (mRNA) levels dropped significantly during 24 h of culture in the absence of E2. Glyceraldehyde 3-phosphate dehydrogenase mRNA levels increased 300% in explants from control ewes to match the higher levels in the endometrium of the E2-treated ewe (in vivo and in explant culture). The only effect of E2 in the explant cultures was to prevent the decrease in PR mRNA. The new selective ER modulator, EM-800 (EM), decreased ER and PR mRNA levels in explants from control ewes but upregulated GAPDH and CYC mRNA levels. The EM treatment in vitro mimicked that of E2 by increasing the half-life of ER mRNA in endometrial explants. These data illustrate distinct, gene-specific effects of the explant culture process, E2, and EM on the expression of endometrial genes.

Early growth response factor-1 mediates insulin-inducible vascular endothelial cell proliferation and regrowth after injury

Hyperinsulinemia in diabetes mellitus is a significant risk factor in the development of atherosclerosis and early restenosis after balloon angioplasty. These manifestations could be mediated by the ability of insulin to potentiate the cellular proliferative and reparative response of vascular cell types to local stimuli. Here we demonstrate that insulin stimulates DNA synthesis in aortic endothelial cells. Reverse transcription-polymerase chain reaction and Northern blotting revealed that insulin induces the expression and transcriptional activity of the immediate early gene and zinc finger transcription protein, early growth response factor-1 (Egr-1). Western immunoblot analysis revealed that insulin-inducible Egr-1 expression was inhibited using phosphorothioate-specific antisense oligonucleotides targeting Egr-1 mRNA. These agents blocked endothelial cell DNA synthesis stimulated by insulin in a dose-dependent manner and inhibited the capacity of insulin to potentiate the reparative response of endothelial cells to mechanical injury in vitro. These oligonucleotides also attenuated wound repair in smooth muscle cells. DNA synthesis induced by insulin was suppressed by inhibitors of two upstream activators of Egr-1, extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-phosphate (PI 3-K), whereas p38 kinase inhibitors had no effect. These present findings demonstrate that insulin-inducible DNA synthesis and repair after injury are processes critically dependent upon the activation of Egr-1. Additionally, they implicate this transcription factor as a potential target for the inhibition of restenosis in diabetics.

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.

Impact of hyperinsulinemia on myosin heavy chain gene regulation

The purpose of this study was to determine whether hyperinsulinemia alters myosin heavy chain (MHC) gene expression in human skeletal muscle. A biopsy from the vastus lateralis was obtained in young, lean [age 24.6 +/- 1.0 (SE) yr, body fat 11.9 +/- 1.9%, body mass index 26.1 +/- 1.1 kg/m2; n = 10] men before and after 3 h of hyperinsulinemia (hyperinsulinemic-euglycemic clamp). Muscle was analyzed for mRNA of type I, IIa, and IIx MHC isoforms. Hyperinsulinemia (mean of 1,065.7 +/- 9.8 pmol/l during minutes 20 to 180) did not change (P > 0.05) the mRNA concentration of either the type I MHC or type IIA MHC isoforms. In contrast, type IIX MHC mRNA increased (P < 0.05) with hyperinsulinemia compared with the fasted condition. These data indicate that hyperinsulinemia rapidly increases type IIx MHC mRNA in human skeletal muscle.

A sequence element in the GLUT4 gene that mediates repression by insulin

Prolonged treatment of 3T3-L1 adipocytes decreases expression of GLUT4, the insulin-responsive glucose transporter. Expression of promoter-reporter gene constructs that contained 2900 or 785 base pairs of 5'-flanking region of the murine GLUT4 gene was down-regulated by insulin (p < 0.0005), whereas expression of constructs that contained 641, 469, or 78 base pairs of 5'-flanking region was not. Nuclear extract from 3T3-L1 adipocytes protected the region from -707 to -681 in the GLUT4 5'-flanking region from DNase I digestion. Using an oligonucleotide probe that corresponded to this footprinted region, two major protein-DNA complexes were identified by a gel mobility shift assay. Southwestern analysis identified four protein bands with molecular masses from 38 to 46 kDa that bound to the insulin-responsive region probe. A reporter gene construct in which bases -706 to -676 were deleted was not repressed by insulin treatment, confirming that this sequence is necessary for the repression of the GLUT4 promoter by insulin in 3T3-L1 adipocytes. This sequence does not show homology to previously described insulin response elements and thus represents a distinct mechanism of gene regulation by insulin.

Insulin as a growth factor

Various clinical syndromes illustrate the essential role of insulin in modulating somatic growth both in utero and after birth. The effect of insulin on growth is a consequence of direct effects transduced via its homologous receptor and post-receptor signaling pathways and indirect effects on other modulators of growth, such as the growth hormone-IGF axis. Recent insights into the post-receptor mechanisms of insulin signaling provide a scientific framework for the distinction between the traditional role of insulin as a major modulator of metabolism and its role as a promoter of growth.

Effects of okadaic acid on expression of phosphoenolpyruvate carboxykinase in cultured rat hepatocytes

In normal rat hepatocytes in primary culture the level of mRNA encoding the key gluconeogenic enzyme phospho enol pyruvate carboxykinase (PEPCK) is increased by the cyclic AMP analogue, chlorophenylthio cyclic AMP (cpt cAMP), and this response is reversed by insulin. The protein-phosphatase inhibitor okadaic acid diminished the stimulatory effects of cpt cAMP on PEPCK mRNA. Protein kinase A remained fully active in the presence of okadaic acid, therefore, the insulin-mimetic actions of okadaic acid were localised to a site subsequent to initial protein kinase A activation. Insulin produced a decrease in PEPCK mRNA expression which was similar to that of okadaic acid both in extent and mechanism (i.e., lack of change in protein kinase A activation). The effects of okadaic acid on PEPCK mRNA amount were not additive with those of insulin and the effects of insulin were not abolished by okadaic acid. These data suggest that okadaic acid and insulin may interact with the cAMP regulation of the PEPCK gene expression at a common site. The mechanisms by which this may be attained are discussed in relation to what is known about the control of specific protein kinases and protein phosphatases by insulin and okadaic acid and of the importance of protein phosphorylation state to regulation of gene-transcriptional processes.