HIRc Cells Research Articles

A simple and sensitive method for glutamine:fructose-6-phosphate amidotransferase assay

For measuring glutamine:fructose-6-phosphate amidotransferase (GFAT) activity in cultured cells, an enzyme method—GDH method—was set up with high-efficiency, high-sensitivity and simple operation by determining the formed glutamate. During the process of making samples, reduced glutathione (GSH, 5 mM) and glucose-6-phophate Na 2 (5 mM) were added to the buffer for scraping the cells. The range of protein content in the samples was 80–150 μg. In the GFAT activity assay, the end product reduced acetylpyridine adenine dinucleotide (APADH) was determined at 370 nm directly. The suitable concentrations of the reactants fructose-6-phosphate (F-6-P), glutamine, acetylpyridine adenine dinucleotide (APAD) and glutamate dehydrogenase (GDH) were 0.8, 6 and 0.3 mM and 6 U, respectively. However, the excess of APAD may interfere with the APADH measurement. The reaction time course was 90 min. The GFAT activity in 3T3-L1, L6, HepG2 and HIRc cells were 1.84–8.51 nmol glutamate/mg protein·min.

Association of the polymorphisms in the 5′-untranslated region of PTEN gene with type 2 diabetes in a Japanese population

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is known to act as a lipid phosphatase hydrolyzing phosphatidylinositol (PI)(3,4,5)P 3 to PI(4,5)P 2. Since the PI3-kinase product, PI(3,4,5)P 3, is an important second messenger leading to the metabolic action of insulin, PTEN functions as a potent negative regulator of insulin signaling and its gene is one of the possible candidates involved in susceptibility to the development of type 2 (non-insulin-dependent) diabetes. In the present study, we investigated the polymorphisms of the PTEN gene in Japanese patients with type 2 diabetes and non-diabetic control subjects. We identified three mutations of the gene in the type 2 diabetes patients. Among these mutations, the frequency of the substitution of C with G at position −9 (−9C→G) (SNP1), located in the untranslated region of exon 1, was significantly higher in type 2 diabetic patients than in control subjects. In addition, transfection of the PTEN gene with SNP1 resulted in a significantly higher expression level of PTEN protein compared with that of the wild-type PTEN gene in Cos1 and Rat1 cells. Furthermore, insulin-induced phosphorylation of Akt in HIRc cells was decreased more greatly by transfection of SNP1 PTEN gene than that of wild-type PTEN gene. These findings suggest that the change of C to G at position −9 of the PTEN gene is associated with the insulin resistance of type 2 diabetes due possibly to a potentiated hydrolysis of the PI3-kinase product.

Role of the Src Homology 2 (SH2) Domain and C-Terminus Tyrosine Phosphorylation Sites of SH2-Containing Inositol Phosphatase (SHIP) in the Regulation of Insulin-Induced Mitogenesis

To examine the role of SHIP in insulin-induced mitogenic signaling, we used a truncated SHIP lacking the SH2 domain (ΔSH2-SHIP) and a Y917/1020F-SHIP (2F-SHIP) in which two tyrosines contributing to Shc binding were mutated to phenylalanine. Wild-type (WT)-, ΔSH2-, and 2F-SHIP were transiently transfected into Rat1 fibroblasts overexpressing insulin receptors (HIRc). Insulin-stimulated tyrosine phosphorylation of WT-SHIP and ΔSH2-SHIP, whereas tyrosine phosphorylation of 2F-SHIP was not detectable, indicating that 917/1020-Tyr are key phosphorylation sites on SHIP. Although SHIP can bind via its 917/1020-Tyr residues and SH2 domain to Shc PTB domain and 317-Tyr residue, respectively, insulin-induced SHIP association with Shc was more greatly decreased in 2F-SHIP cells than that inΔ SH2-SHIP cells. Insulin stimulation of Shc association with Grb2, which is important for p21ras-MAP kinase activation, was decreased by overexpression of WT- and 2F-SHIP. Importantly, insulin-induced Shc·Grb2 association was not detectably reduced in ΔSH2-SHIP cells. In accordance with the extent of Shc association with Grb2, insulin-induced MAP kinase activation was relatively decreased in both WT-SHIP and 2F-SHIP cells, but not in ΔSH2-SHIP cells. To examine the functional role of SHIP in insulin’s biological action, insulin-induced mitogenesis was compared among these transfected cells. Insulin stimulation of thymidine incorporation and bromodeoxyuridine incorporation was decreased in WT-SHIP cells compared with that of control HIRc cells. Expression of 2F-SHIP also significantly reduced insulin-induced mitogenesis, whereas it was only slightly affected by overexpression of ΔSH2-SHIP. Furthermore, the reduction of insulin-induced mitogenesis in WT-SHIP cells was partly compensated by coexpression of Shc. These results indicate that SHIP plays a negative regulatory role in insulin-induced mitogenesis and that the SH2 domain of SHIP is important for its negative regulatory function.

Role of the Src homology 2 (SH2) domain and C-terminus tyrosine phosphorylation sites of SH2-containing inositol phosphatase (SHIP) in the regulation of insulin-induced mitogenesis.

To examine the role of SHIP in insulin-induced mitogenic signaling, we used a truncated SHIP lacking the SH2 domain (deltaSH2-SHIP) and a Y917/1020F-SHIP (2F-SHIP) in which two tyrosines contributing to Shc binding were mutated to phenylalanine. Wild-type (WT)-, deltaSH2-, and 2F-SHIP were transiently transfected into Rat1 fibroblasts overexpressing insulin receptors (HIRc). Insulin-stimulated tyrosine phosphorylation of WT-SHIP and deltaSH2-SHIP, whereas tyrosine phosphorylation of 2F-SHIP was not detectable, indicating that 917/1020-Tyr are key phosphorylation sites on SHIP. Although SHIP can bind via its 917/1020-Tyr residues and SH2 domain to Shc PTB domain and 317-Tyr residue, respectively, insulin-induced SHIP association with Shc was more greatly decreased in 2F-SHIP cells than that in deltaSH2-SHIP cells. Insulin stimulation of Shc association with Grb2, which is important for p21ras-MAP kinase activation, was decreased by overexpression of WT- and 2F-SHIP. Importantly, insulin-induced Shc x Grb2 association was not detectably reduced in deltaSH2-SHIP cells. In accordance with the extent of Shc association with Grb2, insulin-induced MAP kinase activation was relatively decreased in both WT-SHIP and 2F-SHIP cells, but not in deltaSH2-SHIP cells. To examine the functional role of SHIP in insulin's biological action, insulin-induced mitogenesis was compared among these transfected cells. Insulin stimulation of thymidine incorporation and bromodeoxyuridine incorporation was decreased in WT-SHIP cells compared with that of control HIRc cells. Expression of 2F-SHIP also significantly reduced insulin-induced mitogenesis, whereas it was only slightly affected by overexpression of deltaSH2-SHIP. Furthermore, the reduction of insulin-induced mitogenesis in WT-SHIP cells was partly compensated by coexpression of Shc. These results indicate that SHIP plays a negative regulatory role in insulin-induced mitogenesis and that the SH2 domain of SHIP is important for its negative regulatory function.

The Functional Role of CrkII in Actin Cytoskeleton Organization and Mitogenesis

Crk is a member of a family of adapter proteins predominantly composed of Src homology 2 and 3 domains, whose role in signaling pathways is presently unclear. Using an in situ electroporation system which permits the introduction of glutathione S-transferase (GST) fusion proteins into cells, we found that c-CrkII bound to p130(cas), but not to paxillin in serum-starved rat-1 fibroblasts overexpressing the human insulin receptor (HIRc cells) in vivo. 17 nM insulin stimulation dissociated the binding of c-CrkII to p130(cas), whereas 13 nM insulin-like growth factor-I, 16 nM epidermal growth factor (EGF), and 10% serum each showed little or no effect. We found that stress fiber formation is consistent with a change in the p130(cas).c-CrkII interactions before and after growth factor stimulation. Microinjection of either GST-Crk-SH2 or -Crk-(N)SH3 domains, or anti-Crk antibody each inhibited stress fiber formation before and after insulin-like growth factor-I, EGF, and serum stimulation. Insulin stimulation by itself caused stress fiber breakdown and there was no additive effect of microinjection. Microinjection of anti-p130(cas) antibody also blocked stress fiber formation in quiescent cells. Microinjection of the Crk-inhibitory reagents also inhibited DNA synthesis after insulin-like growth factor-I, EGF, and serum stimulation, but not after insulin. These data suggest that the complex containing p130(cas).c-CrkII may play a crucial role in actin cytoskeleton organization and in anchorage-dependent DNA synthesis.

Relative Involvement of Shc Tyrosine 239/240 and Tyrosine 317 on Insulin Induced Mitogenic Signaling in Rat1 Fibroblasts Expressing Insulin Receptors

Shc is phosphorylated on Tyr-239/240 and/or Tyr-317, which serves as a docking site for Grb2. To clarify the relative involvement of Shc Tyr-239/240 and Tyr-317 in insulin-induced mitogenesis, we generated expression vectors for Y317F (1F)-Shc, Y239/240F (2F)-Shc, and Y239/240/317F (3F)-Shc, and stably transfected them into Rat1 fibroblasts expressing insulin receptors (HIRc). Insulin-induced Shc phosphorylation and subsequent association with Grb2 was enhanced in wild-type (WT)-Shc cell. In contrast, insulin-stimulated Shc phosphorylation and Shc·Grb2 association were significantly decreased in 1F-Shc and 3F-Shc cells, while these were only slightly affected and almost comparable in 2F cells compared with those in parental HIRc cells. The kinetics of MAP kinase activation closely paralleled the kinetics of Shc phosphorylation and Shc·Grb2 association. Thus, insulin stimulation of MAP kinase activation occurred more rapidly in WT-Shc cells, and the activation was delayed in 1F-Shc and 3F-Shc cells, while it was comparable in 2F-Shc cells compared with that in HIRc cells. Furthermore, WT-Shc cells displayed enhanced sensitivity to insulin stimulation of thymidine incorporation. Importantly, the sensitivity was significantly decreased in 1F-Shc and 3F-Shc cells, while it was almost comparable in 2F-Shc cells compared with that in HIRc cells. These results indicate that Shc Tyr-317 is more predominant insulin-induced phosphorylation site than Tyr-239/240 for coupling with Grb2 leading to MAP kinase activation and mitogenesis in Rat1 fibroblasts.

Involvement of the Ras/extracellular signal-regulated kinase signalling pathway in the regulation of ERCC-1 mRNA levels by insulin.

Expression of DNA repair enzymes, which includes ERCC-1, might be under the control of hormonal and growth factor stimulation. In the present study it was observed that insulin increased ERCC-1 mRNA levels both in Chinese hamster ovary cells overexpressing human insulin receptors (HIRc cells) and in fully differentiated 3T3-L1 adipocytes. To investigate the mechanisms underlying the increase in ERCC-1 gene expression in HIRc cells, we used a variety of pharmacological tools known to inhibit distinct signalling pathways. None of these inhibitors affected the amount of ERCC-1 mRNA in unstimulated cells. The pretreatment of cells with two chemically unrelated phosphatidylinositol 3'-kinase inhibitors, wortmannin and LY294002, failed to block the doubling of ERCC-1 mRNA content by insulin. Similarly, inhibition of pp70 S6 kinase by rapamycin had no apparent effects on this insulin response. In contrast, altering the p21(ras)-dependent pathway with either manumycin, an inhibitor of Ras farnesylation, or PD98059, an inhibitor of the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (ERK) kinase, suppressed the induction of ERCC-1 mRNA by insulin (P<0.001). Furthermore inhibition of RNA and protein synthesis negatively regulated the expression of this insulin-regulated gene (P<0.005). These results suggest that insulin enhances ERCC-1 mRNA levels by the activation of the Ras-ERK-dependent pathway without the involvement of the phosphatidylinositol 3'-kinase/pp70 S6 kinase.

Functional Importance of Shc Tyrosine 317 on Insulin Signaling in Rat1 Fibroblasts Expressing Insulin Receptors

Shc is phosphorylated on Tyr-317, which serves as a docking site for Grb2. To investigate the specific role of Shc phosphorylation and Shc.Grb2 coupling on insulin signaling, we generated expression vectors for wild-type (WT-Shc) and a mutant Shc with a Tyr-317 --> Phe substitution (317Y/F-Shc) and stably transfected them into Rat1 fibroblasts expressing insulin receptors (HIRc). From different clonal cell lines, cells expressing 10 times greater amounts of WT-Shc or 317Y/F-Shc compared with endogenous Shc were chosen for analysis of insulin signaling. Insulin-induced Shc phosphorylation and subsequent association with Grb2 was enhanced in WT-Shc cells. Because of competition between Shc and IRS-1 for interaction with the insulin receptor, insulin-stimulated tyrosine phosphorylation of IRS-1 was decreased in WT-Shc cells compared with that in HIRc cells. Likewise, reduction of endogenous Shc expression by antisense Shc mRNA resulted in increased insulin stimulation of IRS-1 phosphorylation. Although 317Y/F-Shc was also able to interact with insulin receptor, decreased amounts of Shc phosphorylation and Shc association with Grb2 were observed in 317Y/F-Shc cells, indicating that 317Y/F-Shc functions as a dominant-negative mutant. The kinetics of mitogen-activated protein (MAP) kinase activation closely paralleled the kinetics of Shc phosphorylation. Thus, insulin stimulation of MAP kinase activation occurred more rapidly and was prolonged in WT-Shc cells, while the activation was delayed and transient in 317Y/F-Shc cells compared with that in HIRc cells. Importantly, WT-Shc cells displayed enhanced sensitivity to insulin stimulation of thymidine and bromodeoxyuridine incorporation, whereas the sensitivity was decreased in 317Y/F-Shc cells. These results indicate that Shc Tyr-317 phosphorylation plays an important role, via coupling with Grb2 and competition with IRS-1, in signal transduction to MAP kinase by insulin, ultimately leading to mitogenesis in Rat1 fibroblasts.

SHPTP2 Serves Adapter Protein Linking between Janus Kinase 2 and Insulin Receptor Substrates

To investigate the role of Janus kinase family (JAK1 and JAK2) in insulin signaling, we assessed their insulin-induced associations with other molecules in the cells overexpressing insulin receptors (HIRc and CHO-IR). After insulin stimulation, pp185 proteins (insulin receptor substrate, IRS) were co-immunoprecipitated with both kinases by αJAK1 and αJAK2 antibodies. However, JAK2 constitutively associated with pp95 protein (IRβ). Moreover, JAK2 also constitutively bound to a protein tyrosine phosphatase containing Src 2 regions (SHPTP2), but JAK1 did not. In HIRc cells expressing PTPase-negative mutant SHPTP2, no association of JAK2 with either pp185 or pp95 was detected. Thus, SHPTP2 might serve as an adapter protein linking between JAK2 and IRS. These results suggest that JAK1 and JAK2 behave differently and they may constitute a new regulatory component in insulin signaling.

Functional Importance of Amino-terminal Domain of Shc for Interaction with Insulin and Epidermal Growth Factor Receptors in Phosphorylation-independent Manner

Shc has two distinct domains, amino-terminal and SH2 domain, which can interact with activated growth factor receptors. Shc interacts with insulin receptor via Shc-amino-terminal (N) domain, whereas Shc associates with epidermal growth factor (EGF) receptor through both Shc-N and -SH2 domains. In accordance with the different functional roles between insulin and EGF receptors, EGF stimulated tyrosine phosphorylation of Shc faster than insulin. To clarify the functional importance of three distinct Shc domains on insulin and EGF signaling, we microinjected glutathione S-transferase (GST) fusion proteins containing the amino terminus plus collagen homology domain (NCH), collagen homology domain (CH), and Src homology 2 domain (SH2) into Rat1 fibroblasts expressing insulin receptors (HIRc). Bromodeoxyuridine (BrdUrd) incorporation into newly synthesized DNA was subsequently studied to assess the importance of the three distinct domains of Shc. Microinjection of the NCH-GST fusion protein inhibited BrdUrd incorporation induced by both EGF and insulin, whereas microinjection of the SH2-GST fusion protein inhibited EGF, but not insulin stimulation of DNA synthesis. Neither EGF- nor insulin-induced BrdUrd incorporation was inhibited by the CH-GST fusion protein. Following EGF or insulin stimulation, Shc is phosphorylated on single Tyr-317 residue serving as a docking site for Grb2. Microinjection of Shc-N+CH GST fusion protein with Tyr-317 --> Phe replacement (Y317F) also inhibited insulin stimulation of DNA synthesis. Next, we stably overexpressed wild-type Shc or Y317F mutant Shc into HIRc cells. Insulin-induced tyrosine phosphorylation of IRS-1 was compared among the transfected cell lines, since IRS-1 and Shc could competitively interact with insulin receptor. Insulin-stimulated tyrosine phosphorylation of IRS-1 was decreased in both WT-Shc and Y317F-Shc cells compared with that in HIRc cells. Furthermore, overexpression of the Shc-SH2 domain or Shc-N+CH domain with Y317F mutation interfered with EGF-stimulated endogenous Shc phosphorylation. These results suggest that the amino terminus domain of Shc is functionally important in insulin- and EGF-induced cell cycle progression and that the phosphorylation of Shc Tyr-317 residue is independent of Shc interaction with these receptors.

Phosphonate inhibitors of protein-tyrosine and serine/threonine phosphatases.

In all, 15 aryl-containing phosphonates have been synthesized and tested for their effect on protein-tyrosine phosphatase (PTPase) activity. Two compounds, (naphth-2-yl) difluoromethylphosphonic acid (12) and (napthy-1-yl) difluoromethylphosphonic acid (13) have been found to inhibit dephosphorylation of [32P]insulin receptors by PTP-1B, a protein tyrosine phosphatase (PTPase), with IC50 values of 40-50 microM. Compound 12 competitively inhibited insulin-receptor dephosphorylation by PTP-1B. Compound 12 also inhibited PTP-1B-catalysed dephosphorylation of a synthetic tyrosine phosphorylated substrate poly(Glu80-Tyr20) at the same potency, indicating that 12 acted via interaction with the PTPase. Additionally, 12 inhibited insulin-receptor PTPase(s) and epridermal-growth-factor-receptor PTPase(s) present in solubilized membranes from CHO (Chinese-hamster ovary)/HIRc and A431 cells respectively. IC50 values of 40-50 microM were obtained in all cases with compound 12. Of note is the fact that these compounds did not have any effect on insulin-receptor autophosphorylation. Nine out of the 15 compounds potently inhibited serine/threonine phosphatase PP-2A activity without any effect on serine/threonine phosphatase PP-1 when tested at a concentration as high as 675 microM. The most potent compounds acting toward PP-2A had IC50 values of 45-50 microM. These PP-2A inhibitors could be useful tools for studying serine/threonine-phosphatase-mediated signal transduction. Two compounds, 12 and 13, inhibited both tyrosine phosphatase PTP-1B and serine/threonine phosphatase PP-2A with similar potency; IC50 values being 40-50 microM in both cases. Details of the synthesis of compounds 10, 11 and 13 are given in Supplementary Publication SUP 50177 (6 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1995) 305, 9.

Mechanism of activation of guanine nucleotide exchange factor by insulin

Insulin increases activity of the guanine nucleotide exchange factor (GEF) in Rat-1 fibroblasts transfected with human insulin receptors (HIRc cells), thereby promoting formation of the active form of p21Ras (p21Ras•GTP). In order to identify the upstream molecules mediating this aspect of insulin action, we selectively removed some of these molecules by immunoprecipitation and examined GEF activity in the post-immunoprecipitation lysated of the insulin-treated HIRc cells. The removal of Shc or Grb-2 depleted GEF activity from the cell lysates, whereas immuno-precipitation of the insulin receptors, IRS-1, PLCγ and GAP, were without effect. In summary, the current data demonstrate that a majority of cellular Ras GEF activity after insulin stimulation is associated with Shc and involves interactions among Shc, Grb-2 and Sos.

Insulin inhibits nuclear phosphatase activity: requirement for the C-terminal domain of the insulin receptor.

Insulin's interaction with its receptor initiates a multitude of cellular effects on metabolism, growth, and differentiation. We recently described an insulin-mediated inhibition of nuclear protein phosphatase 2A (PP-2A), which is associated with an increase in phosphorylation of the transcription factor cAMP response element-binding protein. To clarify the role of nuclear PP-2A inhibition in the insulin signaling cascade, we examined the regulation of this phosphatase activity by insulin in Rat-1 fibroblasts overexpressing normal (HIRc) or mutant human insulin receptors (delta CT cells, deletion of a 43-amino acid C-terminal domain). The delta CT cells represent an excellent model of impaired metabolic and intact mitogenic action of insulin. Insulin inhibited nuclear PP-2A activity and enhanced cAMP response element-binding protein phosphorylation in HIRc cells, but not in delta CT cells. The delta CT cells exhibited normal ras activation and blunted mitogen-activating protein kinase phosphorylation and activation in response to insulin (16-fold in HIRc cells vs. 3-fold in delta CT cells), indicating that the mitogen-activating protein kinase pathway is important for the regulation of nuclear PP-2A activity by insulin. We conclude that insulin inhibits nuclear PP-2A activity, and that the carboxy-terminal domain of the insulin receptor is important for this effect.

Rat fibroblast cells overexpressing kinase-inactive human insulin receptors are insulin responsive: influence of growth conditions.

The effects of insulin to stimulate metabolic and mitogenic responses were examined in Rat 1 fibroblast cells that overexpressed either normal (HIRc) or kinase-deficient human insulin receptors. When studied at the optimal growth stage for each cell line, insulin-stimulated responses measured in cells containing kinase-defective receptors with a Lys1018-Ala1018 substitution in the ATP-binding site of the kinase domain (A/K1018). Maximal insulin responsiveness for all these effects, measured as fold-increase over basal, was comparable in parental and HIRc cells (1.8- to 2.4-fold increases). Relative insulin responsiveness for all effects was greatest in A/K 1018 cells. One clone (AK-I) expressing a similar number of kinase-inactive receptors as in the HIRc cells displayed maximal responsiveness of 3.6- to 5.5-fold increases. A second A/K cell line containing 1/10 the number of kinase-inactive receptors displayed responsiveness intermediate between AK-I and parental or HIRc cells (1.5- to 4.8-fold increases). Both clones of kinase-deficient A/K1018 cells displayed impaired insulin sensitivity compared with HIRc cells. These findings suggest that expression of insulin receptor kinase activity is a determinant of insulin sensitivity but not necessarily of the final biological responsiveness of cells to insulin.

Rat fibroblast cells overexpressing kinase-inactive human insulin receptors are insulin responsive: influence of growth conditions

The effects of insulin to stimulate metabolic and mitogenic responses were examined in Rat 1 fibroblast cells that overexpressed either normal (HIRc) or kinase-deficient human insulin receptors. When studied at the optimal growth stage for each cell line, insulin-stimulated responses measured in cells containing kinase-defective receptors with a Lys1018-Ala1018 substitution in the ATP-binding site of the kinase domain (A/K1018). Maximal insulin responsiveness for all these effects, measured as fold-increase over basal, was comparable in parental and HIRc cells (1.8- to 2.4-fold increases). Relative insulin responsiveness for all effects was greatest in A/K 1018 cells. One clone (AK-I) expressing a similar number of kinase-inactive receptors as in the HIRc cells displayed maximal responsiveness of 3.6- to 5.5-fold increases. A second A/K cell line containing 1/10 the number of kinase-inactive receptors displayed responsiveness intermediate between AK-I and parental or HIRc cells (1.5- to 4.8-fold increases). Both clones of kinase-deficient A/K1018 cells displayed impaired insulin sensitivity compared with HIRc cells. These findings suggest that expression of insulin receptor kinase activity is a determinant of insulin sensitivity but not necessarily of the final biological responsiveness of cells to insulin.

Differential Requirement for p21ras Activation in the Metabolic Signaling by Insulin

To evaluate the role of the "Ras pathway" in mediating metabolic signaling by insulin, we employed lovastatin to exhibit isoprenilation of Ras proteins in Rat-1 fibroblasts transfected with human insulin receptors (HIRc cells) and in differentiated 3T3-L1 adipocytes. Lovastatin blocked an ability of insulin to activate p21ras and mitogen-activated protein kinase. Lovastatin also significantly (p < 0.01) reduced insulin effects on thymidine incorporation and glucose incorporation into glycogen. Nevertheless, an effect of insulin on glucose uptake remained unaffected. It appears that in contrast to its mitogenic action and to its effect on glycogenesis, an effect of insulin on glucose uptake does not require p21ras activation.

Signal transduction pathways leading to insulin-induced early gene induction.

We examined the signal transduction pathway leading to insulin stimulation of two immediate early genes, c-fos, and the early growth response gene, Egr-1. In Rat 1 fibroblasts overexpressing normal human insulin receptors (HIRc-B), insulin and IGF-I rapidly and transiently induced the expression of both c-fos and Egr-1 mRNA with maximum accumulation at 30 min, declining to basal levels at 120 min. Insulin (100 ng/mL) increased c-fos and Egr-1 mRNA expression 10-fold (EC50 = 20 ng/mL), whereas IGF-I (100 ng/mL) and serum (20%) led to a 3- and 11.5-fold increase, respectively. Insulin-stimulated c-fos protein expression was maximal at 1 h postinduction and undetectable at 4 h. The effects of insulin and IGF-I on both c-fos mRNA and protein expression were absent in Rat 1 fibroblasts expressing tyrosine kinase-defective human insulin receptors (A/K1018). In cells expressing insulin receptors in which the two C-terminal tyrosines are mutated to phenylalanine (Y/F2 cells), the insulin stimulated increase in Egr-1 and c-fos mRNA was comparable to that of HIRc cells, whereas, in cells expressing C-terminal truncated receptors (delta CT cells), the insulin induced increase in Egr-1 mRNA was normal, but the c-fos mRNA response was severely blunted. As expected, the insulin effect to increase ras GTP formation and MAP kinase activity was negligible in A/K1018 cells but normal, or supernormal, in Y/F2 cells. Importantly, stimulation of ras GTP was increased in delta CT cells, whereas stimulation of MAP kinase activity was almost absent.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin Stimulates the Tyrosine Dephosphorylation of pp125 Focal Adhesion Kinase

The phosphorylation state of pp125 focal adhesion kinase in response to insulin was examined in parental and transfected Rat-1 fibroblasts expressing both wild-type (HIRc cells) and mutant human insulin receptor cDNAs lacking the C-terminal twin tyrosine phosphorylation sites (YF2 cells) or a deletion mutant lacking the distal 43 amino acids of the beta-subunit (delta CT cells). In HIRc cells insulin stimulated the tyrosine dephosphorylation of pp125fak, whereas IGF-I did not. In contrast, the tyrosine phosphorylation state of pp125fak was unchanged in the parental Rat-1 fibroblasts and the YF2 or delta CT mutant cell lines in response to insulin. Analysis of the supernatants revealed that pp125fak was only one component of the major M(r), 120-130-kDa phosphotyrosine band seen in HIRc cells. We conclude that: 1) in contrast to other growth factors, insulin stimulates the dephosphorylation of pp125fak; 2) the presence of the insulin receptor C-terminal tyrosines 1328 and 1334 is required for the insulin-stimulated tyrosine dephosphorylation of pp125fak, suggesting a possible SH2 domain-dependent interaction; 3) insulin may modulate integrin-mediated signaling through pp125fak by altering the phosphorylation state of pp125fak.

Insulin activates p21Ras and guanine nucleotide releasing factor in cells expressing wild type and mutant insulin receptors.

Insulin stimulates the formation of p21RasGTP in Rat-1 fibroblasts overexpressing wild type (HIRc) or mutant (delta CT and Y/F2) insulin receptors. Maximal insulin effect was observed at 7 min in Y/F2 cells, at 10 min in delta CT cells, and at 15 min in HIRc cells. Mutant insulin receptors which display enhanced mitogenic signaling properties stimulated p21Ras.GTP formation to a greater extent than wild type receptors. The amount of p21Ras was not affected by insulin (Western blotting). Tyrosine kinase inhibitor, Lavendustin A (10 nM), completely prevented insulin-induced activation of p21Ras in all cell lines. Insulin did not lead to GAP phosphorylation, or a change in cellular GAP activity, but did result in marked stimulation of Ras guanine nucleotide releasing factor (GRF) activity (by 48% in HIRc, 71% in delta CT, and 120% in Y/F2 cells). These results indicated that p21Ras.GTP is an important signaling molecule in insulin's mitogenic pathway, but may not participate in metabolic signaling and that insulin's stimulatory effects on p21Ras.GTP formation are mediated through Ras GRF.

Intracellular signaling by a mutant human insulin receptor lacking the carboxyl-terminal tyrosine autophosphorylation sites.

We have recently characterized a mutant insulin receptor (Y/F2) in which the two tyrosines in the carboxyl terminus (Tyr1316, Tyr1322) were mutated to phenylalanine. Compared with wild type receptors, the Y/F2 receptor exhibited markedly enhanced sensitivity to insulin-stimulated DNA synthesis with normal insulin-stimulated glucose uptake (Takata, Y., Webster, N. J. G., and Olefsky, J. M. (1991) J. Biol. Chem. 266, 9135-9139). In this paper, we present further evidence for the divergence of the metabolic and mitogenic signaling pathways utilized by the insulin receptor. The mutant receptor showed normal sensitivity and responsiveness for insulin-stimulated glucose incorporation into glycogen. The insulin sensitivity for phosphorylation of two substrates (pp180 and pp220) was the same in both Y/F2 cells and HIRc cells. Phosphotyrosine content, however, was greater in Y/F2 cells than in HIRc cells, especially in the basal state. Insulin stimulated S6 kinase activity 2-6-fold, with an ED50 of -10 nM in Rat 1 cells and 0.5 nM in HIRc cells. The sensitivity to insulin was enhanced in Y/F2 cells with an ED50 of 0.1 nM. These effects were insulin-specific, since insulin-like growth factor (IGF)-I-stimulated mitogenesis was normal. In summary: 1) Y/F2 receptors exhibit normal metabolic and enhanced mitogenic signaling; 2) the enhanced mitogenic signaling is specific for the insulin receptor in the Y/F2 cells, since IGF-I-stimulated mitogenesis is normal; 3) Y/F2 cells display increased endogenous substrate phosphorylation and augmented insulin-stimulated S6 kinase activity placing these responses among insulin's mitogenic effects; and 4) these results are consistent with the concept that the COOH-terminal tyrosine residues of the insulin receptor are normally inhibitory to mitogenic signaling.