Insulin stimulates phosphorylation of a 120-kDa glycoprotein substrate (pp120) for the receptor-associated protein kinase in intact H-35 hepatoma cells

Hepatic Insulin Clearance: Mechanism and Physiology

Upon its secretion from pancreatic β-cells, insulin reaches the liver through the portal circulation to exert its action and eventually undergo clearance in the hepatocytes. In addition to insulin secretion, hepatic insulin clearance regulates the homeostatic level of insulin that is required to reach peripheral insulin target tissues to elicit proper insulin action. Receptor-mediated insulin uptake followed by its degradation constitutes the basic mechanism of insulin clearance. Upon its phosphorylation by the insulin receptor tyrosine kinase, carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) takes part in the insulin-insulin receptor complex to increase the rate of its endocytosis and targeting to the degradation pathways. This review summarizes how this process is regulated and how it is associated with insulin-degrading enzyme in the liver. It also discusses the physiological implications of impaired hepatic insulin clearance: Whereas reduced insulin clearance cooperates with increased insulin secretion to compensate for insulin resistance, it can also cause hepatic insulin resistance. Because chronic hyperinsulinemia stimulates hepatic de novo lipogenesis, impaired insulin clearance also causes hepatic steatosis. Thus impaired insulin clearance can underlie the link between hepatic insulin resistance and hepatic steatosis. Delineating these regulatory pathways should lead to building more effective therapeutic strategies against metabolic syndrome.

CEACAM1 in Liver Injury, Metabolic and Immune Regulation

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is a transmembrane glycoprotein that is expressed on epithelial, endothelial and immune cells. CEACAM1 is a differentiation antigen involved in the maintenance of epithelial polarity that is induced during hepatocyte differentiation and liver regeneration. CEACAM1 regulates insulin sensitivity by promoting hepatic insulin clearance, and controls liver tolerance and mucosal immunity. Obese insulin-resistant humans with non-alcoholic fatty liver disease manifest loss of hepatic CEACAM1. In mice, deletion or functional inactivation of CEACAM1 impairs insulin clearance and compromises metabolic homeostasis which initiates the development of obesity and hepatic steatosis and fibrosis with other features of non-alcoholic steatohepatitis, and adipogenesis in white adipose depot. This is followed by inflammation and endothelial and cardiovascular dysfunctions. In obstructive and inflammatory liver diseases, soluble CEACAM1 is shed into human bile where it can serve as an indicator of liver disease. On immune cells, CEACAM1 acts as an immune checkpoint regulator, and deletion of Ceacam1 gene in mice causes exacerbation of inflammation and hyperactivation of myeloid cells and lymphocytes. Hence, hepatic CEACAM1 resides at the central hub of immune and metabolic homeostasis in both humans and mice. This review focuses on the regulatory role of CEACAM1 in liver and biliary tract architecture in health and disease, and on its metabolic role and function as an immune checkpoint regulator of hepatic inflammation.

Recombinational branch migration by the RadA/Sms paralog of RecA in Escherichia coli

RadA (also known as 'Sms') is a highly conserved protein, found in almost all eubacteria and plants, with sequence similarity to the RecA strand exchange protein and a role in homologous recombination. We investigate here the biochemical properties of the E. coli RadA protein and several mutant forms. RadA is a DNA-dependent ATPase, a DNA-binding protein and can stimulate the branch migration phase of RecA-mediated strand transfer reactions. RadA cannot mediate synaptic pairing between homologous DNA molecules but can drive branch migration to extend the region of heteroduplex DNA, even without RecA. Unlike other branch migration factors RecG and RuvAB, RadA stimulates branch migration within the context of the RecA filament, in the direction of RecA-mediated strand exchange. We propose that RadA-mediated branch migration aids recombination by allowing the 3’ invading strand to be incorporated into heteroduplex DNA and to be extended by DNA polymerases.

DOI:http://dx.doi.org/10.7554/eLife.10807.001

Damage to the DNA of a cell can cause serious harm, and so cells have several ways in which they can repair DNA. Most of these processes rely on the fact that each of the two strands that make up a DNA molecule can be used as a template to build the other strand. However, this is not possible if both strands of the DNA break in the same place. This form of damage can be repaired in a process called homologous recombination, which uses an identical copy of the broken DNA molecule to repair the broken strands. As a result, this process can only occur during cell division shortly after a cell has duplicated its DNA.

One important step of homologous recombination is called strand exchange. This involves one of the broken strands swapping places with part of the equivalent strand in the intact DNA molecule. To do so, the strands of the intact DNA molecule separate in the region that will be used for the repair, and the broken strand can then use the other non-broken DNA strand as a template to replace any missing sections of DNA. The region of the intact DNA molecule where the strands need to separate often grows during this process: this is known as branch migration. In bacteria, a protein called RecA plays a fundamental role in controlling strand exchange, but there are other, similar proteins whose roles in homologous recombination are less well known.

Cooper and Lovett have now purified one of these proteins, called RadA, from the Escherichia coli species of bacteriato study how it affects homologous recombination. This revealed that RadA can bind to single-stranded DNA and stimulate branch migration to increase the rate of homologous recombination. Further investigation revealed that RadA allows branch migration to occur even when RecA is missing, but that RadA is unable to begin strand exchange if RecA is not present. The process of branch migration stabilizes the DNA molecules during homologous recombination and may also allow the repaired DNA strand to engage the machinery that copies DNA.

Cooper and Lovett also used genetic techniques to alter the structure of specific regions of RadA and found out which parts of the protein affect the ability of RadA to stimulate branch migration. Future challenges are to find out what effect RadA has on the structure of RecA and how RadA promotes branch migration.

DOI:http://dx.doi.org/10.7554/eLife.10807.002

Serum and glucocorticoid-regulated kinase Sgk1 inhibits insulin-dependent activation of phosphomannomutase 2 in transfected COS-7 cells

Serum- and glucocorticoid-regulated kinase (Sgk1) is considered to be an essential convergence point for peptide and steroid regulation of ENaC-mediated sodium transport. We tried to identify molecular partners of Sgk1 by yeast two-hybrid screening. Yeast two-hybrid screening showed a specific interaction between Sgk1 and phosphomannomutase (PMM)2, the latter of which is an enzyme involved in the regulation of glycoprotein biosynthesis. The interaction was confirmed in intact cells by coimmunoprecipitation and colocalization detected using confocal microscopy. We were then able to demonstrate that Sgk1 phosphorylated PMM2 in an in vitro assay. In addition, we found that the enzymatic activity of PMM2 is upregulated by insulin treatment and that Sgk1 completely inhibits PMM2 activity both in the absence and in the presence of insulin stimulation. These data provide evidence suggesting that Sgk1 may modulate insulin action on the cotranslational glycosylation of glycoproteins.

Activation of serum- and glucocorticoid-induced protein kinase (Sgk) by cyclic AMP and insulin

Sgk (serum- and glucocorticoid-induced protein kinase) is a serine/threonine-specific protein kinase that is transcriptionally regulated by serum, glucorticoids, and mineralocorticoids. Sgk regulates the amiloride-sensitive sodium channel in kidney principal cells. Insulin and insulin-like growth factor-1 stimulate activity of Sgk by a mechanism mediated by phosphoinositide-dependent kinases (PDK)-1 and -2. In this study, we demonstrate that incubation of transfected cells with 8-(4-chlorophenylthio)-cAMP (8CPT-cAMP; 0.2 mm) led to a 2-fold activation of recombinant Sgk expressed in COS7 cells. Furthermore, the combination of insulin plus 8CPT-cAMP elicited a larger response than either agent alone. The effect of insulin was inhibited by wortmannin (100 nm), but not by the cyclic AMP-dependent protein kinase (PKA) inhibitor, H89 (10 microm). As expected, the effect of 8CPT-cAMP was completely blocked by H89. Surprisingly, the effect of 8CPT-cAMP was also inhibited by wortmannin, suggesting that phosphorylation of Sgk by PDK-1 and/or -2 is required for activation by 8CPT-cAMP. Mutational analysis led to similar conclusions. The Thr(369) --> Ala mutant, lacking the PKA phosphorylation site, was activated by insulin but not 8CPT-cAMP. In contrast, the Ser(422) --> Ala mutant, lacking a PDK-2 phosphorylation site, was inactive and resistant to activation by either insulin or 8CPT-cAMP. In summary, Sgk is subject to complex regulatory mechanisms. In addition to regulation at the level of gene expression, the enzymatic activity of Sgk is regulated by multiple protein kinases, including PKA, PDK-1, and PDK-2. Cross-talk among these signaling pathways may play an important role in the pathogenesis of the hypertension associated with hyperinsulinemia, obesity, and insulin resistance.

Comparison of the intracellular trafficking of two alternatively spliced isoforms of pp120, a substrate of the insulin receptor tyrosine kinase

pp120, a substrate of the insulin receptor tyrosine kinase, is a plasma membrane glycoprotein in the hepatocyte. It is expressed as two spliced isoforms differing by the presence (full length) or absence (truncated) of most of the intracellular domain including all phosphorylation sites. Because the two isoforms differ by their ability to regulate receptor-mediated insulin endocytosis and degradation, we aimed to investigate the cellular basis for this functional difference by comparing their intracellular trafficking. During its intracellular assembly, pp120 is transported from the trans-Golgi network to the sinusoidal domain of the plasma membrane before its final transcytosis to the bile canalicular domain. Because both isoforms are expressed in hepatocytes, we examined their intracellular trafficking in NIH 3T3 fibroblasts individually transfected with each isoform. Pulse-chase experiments demonstrated that most of the newly synthesized full-length isoform reached complete maturation at about 60 min of chase. By contrast, only about 40% of the newly synthesized truncated isoform underwent complete maturation, even at more prolonged chase. Moreover, a significant portion of the truncated isoform appeared to be targeted to lysosomes. Abolishing basal phosphorylation on Ser(503) by cAMP-dependent serine kinase by mutating this residue to alanine was correlated with incomplete maturation of full length pp120 in NIH 3T3 cells and hepatocytes. This finding suggests that the intracellular domain of pp120 contains information that regulates its vectorial sorting from the trans-Golgi network to the plasma membrane.

Insulin receptor substrate-4 enhances insulin-like growth factor-I-induced cell proliferation

The insulin receptor substrates (IRSs)-1-4 play important roles in signal transduction emanating from the insulin and insulin-like growth factor (IGF)-I receptors. IRS-4 is the most recently characterized member, which has been found primarily in human cells and tissues. It interacts with SH2-containing proteins such as phosphatidylinositol 3'-kinase (PI3K), Grb2, Crk-II, and CrkL. In this study, we transfected IRS-4 in mouse NIH-3T3 cells that overexpress IGF-I receptors. Clones expressing IRS-4 showed enhanced cellular proliferation when cells were cultured in 1% fetal bovine serum without added IGF-I. Addition of IGF-I enhanced cellular proliferation in cells overexpressing the IGF-I receptor alone but had an even greater proliferative effect in cells overexpressing both the IGF-I receptors and IRS-4. When etoposide and methylmethane sulfonate (MMS), both DNA damaging agents, were added to the cells, they uniformly induced cell cycle arrest. Fluorescence-activated cell sorter analysis demonstrated that the arrest of the cell cycle occurred at the G(1) checkpoint, and furthermore no significant degree of apoptosis was demonstrated with the use of either agent. In cells, overexpressing IGF-I receptors alone, IGF-I addition enhanced cellular proliferation, even in the presence of etoposide and MMS. In cells overexpressing IGF-I receptors and IRS-4, the effect of IGF-I in overcoming the cell cycle arrest was even more pronounced. These results suggest that IRS-4 is implicated in the IGF-I receptor mitogenic signaling pathway.

Interaction of insulin receptor substrate 3 with insulin receptor, insulin receptor-related receptor, insulin-like growth factor-1 receptor, and downstream signaling proteins

Insulin receptor substrates (IRS) mediate biological actions of insulin, growth factors, and cytokines. All four mammalian IRS proteins contain pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains at their N termini. However, the molecules diverge in their C-terminal sequences. IRS3 is considerably shorter than IRS1, IRS2, and IRS4, and is predicted to interact with a distinct group of downstream signaling molecules. In the present study, we investigated interactions of IRS3 with various signaling molecules. The PTB domain of mIRS3 is necessary and sufficient for binding to the juxtamembrane NPXpY motif of the insulin receptor in the yeast two-hybrid system. This interaction is stronger if the PH domain or the C-terminal phosphorylation domain is retained in the construct. As determined in a modified yeast two-hybrid system, mIRS3 bound strongly to the p85 subunit of phosphatidylinositol 3-kinase. Although high affinity interaction required the presence of at least two of the four YXXM motifs in mIRS3, there was not a requirement for specific YXXM motifs. mIRS3 also bound to SHP2, Grb2, Nck, and Shc, but less strongly than to p85. Studies in COS-7 cells demonstrated that deletion of either the PH or the PTB domain abolished insulin-stimulated phosphorylation of mIRS3. Insulin stimulation promoted the association of mIRS3 with p85, SHP2, Nck, and Shc. Despite weak association between mIRS3 and Grb2, this interaction was not increased by insulin, and may not be mediated by the SH2 domain of Grb2. Thus, in contrast to other IRS proteins, mIRS3 appears to have greater specificity for activation of the phosphatidylinositol 3-kinase pathway rather than the Grb2/Ras pathway.

pp120, a substrate of the insulin receptor tyrosine kinase, is associated with phosphatase activity

pp120, a plasma membrane glycoprotein, is phosphorylated on Tyr488 by the insulin receptor tyrosine kinase. This requires basal phosphorylation on Ser503 by cAMP-dependent serine kinase. Phe513, the other tyrosine residue in the intracellular domain, does not undergo insulin-stimulated phosphorylation. Phe488 mutation abolished basal and insulin-stimulated phosphorylation, whereas, Phe513 plus Phe488 mutation markedly decreased the effect of insulin on pp120 phosphorylation without altering basal phosphorylation in intact cells. To investigate whether basal phosphorylation of pp120 is regulated by a phosphatase activity that requires Tyr513, in vitro phosphorylation assays using partially purified glycoproteins from stably transfected NIH 3T3 cells were performed in the absence of phosphatase inhibitors. Wild-type pp120 was promptly dephosphorylated, whereas, Y513F pp120 was not. Decreasing pp120 expression by antisense cDNA transfection proportionally decreased phosphatase activity in H4-II-E hepatoma cells measured by the p-nitrophenyl phosphate assay. This suggests that pp120 is associated with phosphatase activity that requires an intact Tyr513 residue.

Effect of pp120 on receptor-mediated insulin endocytosis is regulated by the juxtamembrane domain of the insulin receptor

pp120, a substrate of the insulin receptor tyrosine kinase, does not undergo ligand-stimulated phosphorylation by the insulin-like growth factor-1 (IGF-1) receptor. However, replacement of the C-terminal domain of the IGF-1 receptor beta-subunit with the corresponding segment of the insulin receptor restored pp120 phosphorylation by the chimeric receptor. Since pp120 stimulates receptor-mediated insulin endocytosis when it is phosphorylated, we examined whether pp120 regulates IGF-1 receptor endocytosis in transfected NIH 3T3 cells. pp120 failed to alter IGF-1 receptor endocytosis via either wild-type or chimeric IGF-1 receptors. Thus, the effect of pp120 on hormone endocytosis is specific to insulin, and the C-terminal domain of the beta-subunit of the insulin receptor does not regulate the effect of pp120 on insulin endocytosis. Mutation of Tyr960 in the juxtamembrane domain of the insulin receptor abolished the effect of pp120 to stimulate receptor endocytosis, without affecting pp120 phosphorylation by the insulin receptor. These findings suggest that pp120 interacts with two separate domains of the insulin receptor as follows: a C-terminal domain required for pp120 phosphorylation and a juxtamembrane domain required for internalization. We propose that the interaction of pp120 with the juxtamembrane domain is indirect and requires one or more substrates that bind to Tyr960 in the insulin receptor.

Guanosine triphosphatase-activating protein-associated protein, but not src-associated protein p68 in mitosis, is a part of insulin signaling complexes

The insulin receptor, following insulin stimulation of cells, triggers formation of various signaling complexes. In rat HTC hepatoma cells overexpressing normal human insulin receptors (HTC-IR), p85 regulatory subunit of phosphatidylinositol-3-kinase (PI3K) forms signaling complexes containing the insulin receptor, insulin receptor substrate 1 (IRS-1), guanosine triphosphatase-activating protein (GAP) and 60-70 kDa phosphotyrosine proteins (p60-70). In the present study, we demonstrate that p60-70 interacts directly with the p85 subunit via src homology 2 domain of the latter. Employing antibodies specific to two p85 isoforms, p85alpha and p85beta, we demonstrate that HTC-IR cells express both p85 isoforms, and these isoforms induce the formation of similar signaling complexes in response to insulin. p60-70, present in both alpha-p85alpha and alpha-p85beta immunoprecipitates, is a GAP-associated protein, but is distinct from the p68 src-associated protein in mitosis (Sam68) by several criteria. These data suggest that 1) GAP-associated protein, but not Sam68, is a part of insulin signaling complexes; and 2) p85alpha and p85beta form similar, but distinct, insulin receptor signaling complexes.

Differential effect of pp120 on insulin endocytosis by two variant insulin receptor isoforms

The insulin receptor is expressed as two variably spliced isoforms that differ by the absence (isoform A) or presence (isoform B) of a 12-amino acid sequence encoded by exon 11 at the carboxy terminus of the alpha-subunit. Coexpression of the A isoform and pp120, a substrate of the insulin receptor tyrosine kinase, in NIH 3T3 fibroblasts increased receptor A-mediated insulin endocytosis and degradation by two- to threefold compared with cells expressing receptors alone. Because B is the predominant isoform in the liver and binds insulin with lower affinity than A, we have examined the effect of pp120 on receptor B-mediated endocytosis. In contrast to isoform A, the effect of pp120 on isoform B-mediated insulin internalization and degradation in stably transfected NIH 3T3 cells was minimal.

Annexin II is a novel player in insulin signal transduction. Possible association between annexin II phosphorylation and insulin receptor internalization

Annexin II is a Ca2+-, phospholipid-, and actin- binding protein that was implicated in the regulation of vesicular traffic and endosome fusion. It is a known substrate for protein kinases including the platelet-derived growth factor receptor, src protein-tyrosine kinase, and protein kinase C. In the present study we investigated the possible involvement of annexin II in insulin signal transduction. Phosphorylation of annexin II in response to insulin treatment of intact Chinese hamster ovary (CHO)-T cells was detected by 5 min and reached maximal levels after a 2-3-h incubation with the hormone. However, unlike other receptor substrates, annexin II failed to undergo insulin-induced Tyr phosphorylation under conditions where receptor internalization was inhibited. This was evident in CHO cells, overexpressing the insulin receptor, in which internalization was inhibited either by tyrosine kinase inhibitors or by lowering the temperature to 4 degrees C, and in CHO cells overexpressing various insulin receptor mutants in which normal internalization was impaired. Hence, Tyr phosphorylation of annexin II could be part of the internalization and sorting mechanism of the insulin receptor.

Insulin induces tyrosine phosphorylation of JAK2 in insulin-sensitive tissues of the intact rat

The Janus kinase family of protein tyrosine kinases constitutes a novel type of signal transduction pathway activated in response to a wide variety of polypeptide ligands and has four known members: JAK1, JAK2, JAK3, and Tyk2. In this study, we examined the ability of insulin to stimulate JAK2 tyrosine phosphorylation in insulin-sensitive tissues of the intact rat using immunoprecipitation and immunoblotting. The results demonstrate that after an infusion of insulin, JAK2 is rapidly tyrosine phosphorylated (and the kinase is activated) in the liver, adipose tissue, skeletal muscle, heart, and isolated adipocytes. The presence of phosphorylated JAK2 was detectable after an infusion of insulin that increased serum insulin to physiological postprandial levels (40-70 microunits/ml). Co-immunoprecipitation with anti-insulin receptor antibody, anti-JAK2 antibody, and anti-IRS-1 antibody showed that JAK2 interacts with the insulin receptor and IRS-1 to form stable complexes following stimulation by insulin. In two animal models of insulin resistance the regulation of JAK2 tyrosine phosphorylation after insulin infusion paralleled the phosphorylation of the insulin receptor and of IRS-1. In conclusion, our data indicate that after physiological stimulation by insulin in the intact animal, JAK2 associates with the insulin receptor and is tyrosine phosphorylated in insulin-sensitive tissues in a time- and dose-dependent fashion.

Cloning and characterization of a functional promoter of the rat pp120 gene, encoding a substrate of the insulin receptor tyrosine kinase

Cloning of the 5 -flanking region of the rat pp120 gene has indicated that it is a housekeeping gene: it lacks a functional TATA box and contains several Sp1 binding sites and multiple transcription initiation sites at nucleotides -101, -71, -41, and -27 spread over a GC-rich area. A fragment between nucleotides -21 and -1609 exhibited promoter activity when ligated in a sense orientation into a promoterless luciferase reporter plasmid and transiently transfected into rat H4-II-E hepatoma cells. 5' progressive deletion and block substitution analyses revealed that the three proximal Sp1 boxes (boxes 3, 5, and 6) are required for basal transcription of the pp120 gene. Promoter activity was stimulated 2-3-fold in response to insulin, dexamethasone, insulin plus dexamethasone, and cAMP. Although unaltered by phorbol esters alone, promoter activity was stimulated 4-5-fold in response to phorbol esters plus cAMP. Several motifs resembling response elements for insulin (in the rat phosphoenolpyruvate carboxykinase gene), glucocorticoids, cAMP, and phorbol esters as well as a number of putative binding sites for activating proteins-1 (Jun/Fos) and -2, and liver-specific factors were detected. The role of these sites in tissue-specific expression of pp120 remains to be investigated.

Receptor-mediated internalization of insulin. Potential role of pp120/HA4, a substrate of the insulin receptor kinase

pp120/HA4 is a hepatocyte membrane glycoprotein phosphorylated by the insulin receptor tyrosine kinase. In this study, we have investigated the role of pp120/HA4 in insulin action. Transfection of antisense pp120/HA4 cDNA in H35 hepatoma cells resulted in inhibition of pp120/HA4 expression and was associated with a 2-3-fold decrease in the rate of insulin internalization. Furthermore, insulin internalization in NIH 3T3 fibroblasts co-transfected with insulin receptors and pp120/HA4 was increased 2-fold compared with cells expressing insulin receptors alone. In contrast, no effect on internalization was observed in cells overexpressing a naturally occurring splice variant of pp120/HA4 that lacks the phosphorylation sites in the intracellular domain. Insulin internalization was also unaffected in cells expressing three site-directed mutants of pp120/HA4 in which the sites of phosphorylation by the insulin receptor kinase had been removed (Y488F, Y488F/Y513F, and S503A). Our data suggest that pp120/HA4 is part of a complex of proteins required for receptor-mediated internalization of insulin. It is possible that this function is regulated by insulin-induced phosphorylation of the intracellular domain of pp120/HA4.

Insulin-like growth factor induces phosphorylation of immunoreactive insulin receptor substrate and its association with phosphatidylinositol-3 kinase in human thymocytes

Insulin receptor substrate 1 (IRS-1) is the principle cellular substrate for insulin and insulin-like growth factor I (IGF-I) receptor signaling. After phosphorylation of tyrosine residues within the YMXM or YXXM motifs, IRS-1 associates with phosphatidylinositol-3 kinase (PI3K). This signaling pathway and the presence of an IRS-1-like molecule have been demonstrated in IRS-1-transfected and in nontransfected hematopoietic cell lines, respectively. IGF-I has been implicated in lymphocyte development and function, and recently, we showed that functional type-I IGF receptors are present on human thymocytes and peripheral T cells. In this study, we addressed IGF-I signal transduction in nontransformed, freshly isolated, human thymocytes, as well as in blood T cells. Using Western blot analysis, we found that IGF-I induced phosphorylation of a 160-180-kD protein (pp170) in human thymocytes and that phosphorylated pp170 becomes associated with PI3K and is recognized by anti-IRS-1. In blood T cells, this immunoreactive IRS-1 (irIRS-1) is less abundantly expressed than in thymocytes, as assessed with immunoblotting. As a consequence, phosphorylated pp170 was not or hardly detectable after stimulation with IGF-I, and irIRS-1 was not detected in PI3K immunoprecipitates from lysates of IGF-I-stimulated T cells. However, IGF-I induced the tyrosine phosphorylation of other cellular proteins, indicating that differential expression of irIRS-1 contributes to a distinct signaling pathway in T cells.

Insulin-stimulated phosphorylation of recombinant pp120/HA4, an endogenous substrate of the insulin receptor tyrosine kinase

Insulin binding to the alpha-subunit of its receptor stimulates the receptor tyrosine kinase to phosphorylate the beta-subunit and several endogenous protein substrates, including pp120/HA4, a liver-specific plasma membrane glycoprotein of M(r) 20,000. Analysis of the deduced amino acid sequence of rat liver pp120/HA4 revealed two potential sites for tyrosine phosphorylation in the cytoplasmic domain (Tyr488 and Tyr513), as well as a potential cAMP-dependent protein kinase phosphorylation site (Ser503). To determine which of these sites is phosphorylated in response to insulin, each of these amino acid residues was altered by site-directed mutagenesis. Mutant cDNAs were then expressed by stable transfection in NIH 3T3 cells. Two mutations (Phe488 and Ala503) impaired insulin-induced phosphorylation of pp120/HA4, suggesting that pp120/HA4 undergoes multisite phosphorylation. It seems likely that Tyr488 is phosphorylated by the insulin receptor kinase, and phosphorylation of Ser513 may contribute to the regulation of tyrosine phosphorylation. Since pp120/HA4 is believed to be associated with a Ca2+/Mg(2+)-dependent ecto-ATPase activity, we determined the effects of insulin-induced phosphorylation on this enzymatic activity. In NIH 3T3 cells co-expressing the insulin receptor and pp120/HA4, insulin caused a 2-fold increase in ecto-ATPase activity. Moreover, elimination of the phosphorylation sites of pp120/HA4 impaired the ability of insulin to stimulate the ecto-ATPase activity. These data suggest that tyrosine phosphorylation of pp120/HA4 may regulate Ca2+/Mg(2+)-dependent ecto-ATPase activity.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

The cytoplasmic domain of C-CAM is required for C-CAM-mediated adhesion function: studies of a C-CAM transcript containing an unspliced intron

Cell-CAM105 (also named C-CAM) is a cell surface glycoprotein involved in intercellular adhesion of rat hepatocytes. It has four extracellular immunoglobulin (Ig) domains, a transmembrane domain and a cytoplasmic domain and therefore is a member of the Ig supergene family. We have characterized multiple cDNAs of the C-CAM genes in rat intestine. Sequence analyses showed that rat intestine contained not only the previously reported L-form and S-form C-CAMs (renamed C-CAM1 and C-CAM2 respectively) but also a new isoform, C-CAM3. The C-CAM3 transcript codes for a polypeptide with a truncated C-terminus that lacks 65 amino acids from the previously reported C-CAM1 cytoplasmic domain. Unlike C-CAM1, C-CAM3 did not mediate cell adhesion when expressed in insect cells using the baculoviral expression system. Thus the extra 65 amino acids in the cytoplasmic domain of C-CAM1 are important for adhesion phenotype when expressed in insect cells. Although C-CAM1 and C-CAM2 are encoded by different genes, sequence analysis suggests that C-CAM3 is probably derived from alternative splicing of the C-CAM1 gene. To examine this possibility, we have determined the exon organization of the C-CAM1 gene. C-CAM3 differed from C-CAM1 by the presence of a single unspliced intron which contained a stop codon immediately after the regular splice junction. As a result, translation of C-CAM3 terminates at the point where C-CAM1 and C-CAM3 sequences diverge. To investigate the expression of C-CAM1, C-CAM2 and C-CAM3 in different tissues, we used an RNAase-protection assay to simultaneously assess the levels of expression of these transcripts. Using total RNA prepared from various tissues, we showed that expression of C-CAM3 was tissue-specific, and the C-CAM3 transcript accounted for about 25% of the transcripts derived from the C-CAM1 gene. However, further analysis revealed that C-CAM3 transcript was not present in cytosolic RNA, rather it was enriched in nuclear RNA prepared from hepatocytes. Although C-CAM3 cDNA contains the polyadenylation signal and is polyadenylated, these results indicate that C-CAM3 is probably an incomplete spliced product of C-CAM1 gene.

Insulin receptor substrate 1 mediates insulin and insulin-like growth factor I-stimulated maturation of Xenopus oocytes

Insulin and insulin-like growth factor I (IGF-I) initiate cellular functions by activating their homologous tyrosine kinase receptors. In most mammalian cell types, this results in rapid tyrosine phosphorylation of a high-molecular-weight substrate termed insulin receptor substrate 1 (IRS-1). Previous studies suggest that IRS-1 may act as a "docking" protein that noncovalently associates with certain signal-transducing molecules containing src homology 2 domains; however, direct evidence for the role of IRS-1 in the final biological actions of these hormones is still lacking. We have developed a reconstitution system to study the role of IRS-1 in insulin and IGF-I signaling, taking advantage of the fact that Xenopus oocytes possess endogenous IGF-I receptors but have little or no IRS-1, as determined by immunoblotting with anti-IRS-1 and antiphosphotyrosine antibodies. After microinjection of IRS-1 protein produced in a baculovirus expression system, tyrosyl phosphorylation of injected IRS-1 is stimulated by both insulin and IGF-I in a concentration-dependent manner, with IGF-I more potent than insulin. Furthermore, after IRS-1 injection, both hormones induce a maturation response that correlates well with the amount of injected IRS-1. By contrast, overexpression of human insulin receptors in the Xenopus oocytes does not enhance either IRS-1 phosphorylation or oocyte maturation response upon insulin stimulation. These results demonstrate that IRS-1 serves a critical role in linking IGF-I and insulin to their final cellular responses.

The insulinomimetic agents H2O2 and vanadate stimulate tyrosine phosphorylation of potential target proteins for the insulin receptor kinase in intact cells

H2O2 and vanadate are known insulinomimetic agents. Together they induce insulin's bioeffects with a potency which exceeds that seen with insulin, vanadate or H2O2 alone. We have previously shown that a combination of H2O2 and vanadate, when added to intact cells, rapidly stimulates protein tyrosine phosphorylation, owing to the inhibitory effects of these agents on intracellular protein tyrosine phosphatases (PTPases). Employing Western blotting with anti-phosphotyrosine antibodies, we have now identified in Chinese-hamster ovary (CHO) cells transfected with a wild-type insulin-receptor gene (CHO.T cells) several proteins (e.g. pp180, 125, 100, 60 and 52) whose phosphotyrosine content is rapidly increased upon treatment of the cells with a combination of insulin and 3 mM-H2O2. Tyrosine phosphorylation of these and additional proteins was further potentiated when 100 microM-sodium orthovanadate was added together with H2O2. The effects of insulin, insulin/H2O2, and H2O2/vanadate on tyrosine phosphorylation were markedly decreased in CHO cells transfected with an insulin-receptor gene where the twin tyrosines 1162 and 1163 were replaced with phenylalanine (CHO.YF-3 cells). Similarly, most of these proteins failed to undergo enhanced tyrosine phosphorylation in parental CHO cells incubated in the presence of insulin or the insulinomimetic agents. Our findings suggest that inhibition of PTPase activity by H2O2/vanadate augments the autophosphorylation of tyrosines 1162 and 1163 of the insulin receptor kinase, leading to its activation in an insulin-independent manner. As a result, tyrosine phosphorylation of potential targets for this enzyme takes place. Failure of H2O2/vanadate to induce phosphorylation of these proteins in receptor mutants lacking these twin tyrosine residues supports this hypothesis.

Carcinoembryonic antigen gene family: molecular biology and clinical perspectives

The carcinoembryonic antigen (CEA) gene family belongs to the immunoglobulin super-gene family and can be divided into two main subgroups based on sequence comparisons. In humans it is clustered on the long arm of chromosome 19 and consists of approximately 20 genes. The CEA subgroup genes code for CEA and its classical crossreacting antigens, which are mainly membrane-bound, whereas the other subgroup genes encode the pregnancy-specific glycoproteins (PSG), which are secreted. Splice variants of individual genes and differential post-translational modifications of the resulting proteins, e.g., by glycosylation, indicate a high complexity in the number of putative CEA-related molecules. So far, only a limited number of CEA-related antigens in humans have been unequivocally assigned to a specific gene. Rodent CEA-related genes reveal a high sequence divergence and, in part, a completely different domain organization than the human CEA gene family, making it difficult to determine individual gene counterparts. However, rodent CEA-related genes can be assigned to human subgroups based on similarity of expression patterns, which is characteristic for the subgroups. Various functions have been determined for members of the CEA subgroup in vitro, including cell adhesion, bacterial binding, an accessory role for collagen binding or ecto-ATPases activity. Based on all that is known so far on its biology, the clinical outlook for the CEA family has been reassessed.

Basic polycations activate the insulin receptor kinase and a tightly associated serine kinase

The effects of cationic polyamino acids on phosphorylation of the insulin and insulin-like growth factor 1 receptor kinases were studied and the following observations were made. (a) Polylysine stimulated both tyrosine and serine phosphorylation of the insulin receptor and of additional proteins present in lectin-purified membrane preparations from rat liver. (b) Polylysine synergized with insulin to enhance phosphorylation of the insulin receptor and of additional proteins (pp40 and pp110). (c) Polylysine effects were more pronounced upon increasing the polylysine chain length. (d) The effect of polylysine was biphasic with an optimum at 100 micrograms/ml. (e) Polylysine was found ineffective in stimulating the phosphorylation of immobilized insulin receptors. Taken together, these findings support the notion that the action of polylysine involves conformational changes and presumably aggregation of soluble receptors. The same effects of polylysine were obtained with highly purified insulin receptor preparations. Under these conditions polylysine enhanced both serine and tyrosine phosphorylation of the insulin receptor, suggesting that polylysine stimulates the activity of the insulin receptor kinase, and of a serine kinase that is tightly associated with the insulin receptor.

Epidermal growth factor stimulated phosphorylation of a 120-kilodalton endogenous substrate protein in rat hepatocytes

Endogenous substrates of the EGF receptor have been described in transformed cells; however, little is known about substrates in normal tissue. To characterize epidermal growth factor (EGF) receptor phosphorylation and search for endogenous substrates in normal rat hepatocytes, cells were labeled with [32P]orthophosphate, and phosphotyrosine-containing proteins were sought by using a high-affinity, specific anti-phosphotyrosine antibody. Exposure of 32P-labeled freshly isolated hepatocytes to 1 microgram/mL EGF caused phosphorylation of several proteins of Mr 185K, 160K, and 120K. The 185- and 160-kDa proteins (pp185 and pp160) were identified as the intact and proteolyzed forms of the EGF receptor by virtue of their immunoprecipitation with anti-EGF receptor antibody. This antibody failed to recognize the 120-kDa phosphoprotein (pp120). The phosphopeptide map derived from pp120 was by trypsinization and HPLC separation different from that of pp185, further indicating that pp120 is distinct from the EGF receptor. This pp120 was also immunologically distinct from the pp120 substrate of the insulin receptor kinase and from ATP-citrate lyase. Phosphoamino acid analysis revealed pp120 to be phosphorylated on both tyrosine and serine residues. Autophosphorylation of EGF receptor and phosphorylation of pp120 were almost maximal within 1 min of EGF stimulation. The dose-response curves for phosphorylation of the EGF receptor and pp120 were identical (ED50 = 30 ng/mL) and were superimposable with the fractional occupancy of the EGF receptor. In A431 cells, a transformed cell line whose growth is inhibited by EGF, EGF produced a decrease in pp120 phosphorylation. These data suggest that pp120 is an endogenous substrate for the EGF receptor in hepatocytes whose phosphorylation may be closely related to EGF stimulation of cell growth.

Hepatic zonation of insulin-stimulated tyrosine phosphorylation

Zonal distribution of insulin stimulation of hepatic protein tyrosine phosphorylation, detected by immunoblotting with an anti-phosphotyrosine antibody, has been studied in the in situ perfused rat liver by dual-digitonin-pulse perfusion. Insulin promotes the rapid and sustained tyrosine phosphorylation of two proteins (pp150 and pp69) that are present only in the perivenous hepatocytes, while three others (pp46, pp48 and pp96) are stimulated identically in the periportal and perivenous cells. The ability of insulin to rapidly activate acetyl-CoA carboxylase is indistinguishable between the hepatic zones. Hepatic zonation of insulin-stimulated tyrosine phosphorylation could underly differential hepatic insulin responses and might provide clues to the identification of tyrosine phosphorylated proteins linked to insulin regulation of intracellular events.

An insulin-sensitive cytosolic protein kinase accounts for the regulation of ATP citrate-lyase phosphorylation

Purified rat liver ATP citrate-lyase is phosphorylated on serine residues by an insulin-stimulated cytosolic kinase activity partially purified from rat adipocytes [Yu, Khalaf & Czech (1987) J. Biol. Chem. 262, 16677-16685]. The Km for lyase phosphorylation by this hormone-sensitive kinase activity is approx. 3 microM. Two-dimensional tryptic-peptide mapping of the 32P-labelled lyase reveals that the kinase-catalysed phosphorylation occurs primarily on a specific peptide. In intact 32P-labelled adipocytes, insulin enhances the serine phosphorylation of ATP citrate-lyase by 2-3-fold. Tryptic digestion of the 32P-labelled lyase immunopurified from insulin-treated adipocytes also yields one major phosphopeptide. 32P-labelled lyase tryptic peptides derived from labelling experiments in vitro and in vivo exhibit identical electrophoretic and chromatographic migration profiles. Furthermore, radio-sequencing of the phosphopeptide from lyase 32P-labelled in vitro indicates that serine-3 from the N-terminus is phosphorylated by the insulin-stimulated cytosolic kinase, in agreement with previous studies on the position of the phosphoserine residue in ATP citrate-lyase isolated from insulin-treated cells. Taken together, the similarity in site-specific phosphorylation of ATP citrate-lyase from insulin-treated adipocytes to that catalysed by the hormone-activated cytosolic kinase in vitro strongly suggests that this kinase mediates insulin action on lyase phosphorylation in intact cells.

Concanavalin A-induced receptor aggregation stimulates the tyrosine kinase activity of the insulin receptor in intact cells

Concanavalin A (ConA) stimulated the phosphorylation of the beta-subunit of the insulin receptor and an Mr-185,000 protein on serine and tyrosine residues in intact H-35 rat hepatoma cells. This Mr-185,000 protein whose phosphorylation was stimulated by ConA was identical to pp185, a protein reported previously to be a putative endogenous substrate for the insulin receptor tyrosine kinase in rat hepatoma cells. In Chinese hamster ovary (CHO) cells transfected with cDNA of the human insulin receptor, tyrosine-phosphorylation of pp185 was strongly enhanced by ConA compared with the controls, suggesting that the induction of tyrosine-phosphorylation of pp185 was due to stimulation of the insulin receptor kinase by ConA. Moreover, monovalent ConA only slightly induced the tyrosine-phosphorylation of pp185, which was enhanced by the addition of anti-ConA IgG, suggesting that ConA stimulated the insulin receptor kinase mainly by the receptor cross-linking or aggregation in intact cells. These data suggest that the insulin-mimetic action of ConA is related to the autophosphorylation and activation of the insulin receptor tyrosine kinase, as well as the subsequent phosphorylation of pp185 in intact cells.

Further evidence for a two-step model of glucose-transport regulation. Inositol phosphate-oligosaccharides regulate glucose-carrier activity

The insulin effect on glucose uptake is not sufficiently explained by a simple glucose-carrier translocation model. Recent studies rather suggest a two-step model of carrier translocation and carrier activation. We used several pharmacological tools to characterize the proposed model further. We found that inositol phosphate (IP)-oligosaccharides isolated from the drug Actovegin, as well as the alkaloid vinblastine, show a partial insulin-like effect on glucose-transport activity of fat-cells (3-O-methylglucose uptake, expressed as % of equilibrium value per 4 s: basal 5.8%, insulin 59%, IP-oligosaccharides 30%, vinblastine 29%) without inducing carrier translocation. On the other hand, two newly developed anti-diabetic compounds (alpha-activated carbonic acids, BM 130795 and BM 13907) induced carrier translocation to the same extent as insulin and phorbol esters [cytochalasin-B-binding sites in plasma membranes: basal 5 pmol/mg of protein, insulin 13 pmol/mg of protein, TPA (12-O-tetradecanoylphorbol 13-acetate) 11.8 pmol/mg of protein, BM 130795 10.8 pmol/mg of protein], but produce also only 40-50% of the insulin effect on glucose-transport activity (basal 5.8%, insulin 59%, TPA 23%, BM 130795 35%). Almost the full insulin effect was mimicked by a combination of phorbol esters and IP-oligosaccharides (basal 7%, insulin 50%, IP-oligosaccharides 30%, TPA 23%, IP-oligosaccharides + TPA 45%). None of these substances stimulated insulin-receptor kinase in vitro or in vivo, suggesting a post-kinase site of action. The data confirm the following aspects of the proposed model: (1) carrier translocation and carrier activation are two independently regulated processes; (2) the full insulin effect is mimicked only by a simultaneous stimulation of carrier translocation and intrinsic carrier activity, suggesting that insulin acts through a synergism of both mechanisms; (3) IP-oligosaccharides might be involved in the transmission of a stimulatory signal on carrier activity.

Monoclonal antibodies to the insulin receptor mimic metabolic effects of insulin but do not stimulate receptor autophosphorylation in transfected NIH 3T3 fibroblasts

The metabolic actions of insulin and anti-insulin receptor monoclonal antibodies were compared with their effects on insulin receptor phosphorylation in mouse NIH 3T3 fibroblasts transfected with human insulin receptor cDNA. In serum-starved NIH 3T3 HIR3.5 cells, uptake of 2-deoxy-[3H]glucose was stimulated up to 2-fold after 30 min with insulin, with a half-maximal effect at 0.1 nM insulin. Incorporation of [3H]thymidine was stimulated approximately 12-fold after a 16-hr preincubation with insulin, with a half-maximal effect at 2 nM insulin. Phosphorylation of insulin receptor beta-subunit in cells prelabeled with [32P]phosphate was increased 10- to 20-fold within 5 min of adding insulin, with a half-maximal effect at approximately 3 nM insulin. Monoclonal antibodies reacting with four different epitopes on the insulin receptor mimicked the effect of insulin on 2-deoxyglucose uptake. These antibodies also stimulated thymidine incorporation, although the maximum stimulation was only approximately 30% that of insulin. Two antibodies (25-49 and 83-14) showed a similar concentration dependence to insulin in their metabolic effects and in the inhibition of 125I-labeled insulin binding to cells. The other two antibodies (83-7 and 18-44) were somewhat less potent and did not inhibit insulin binding. None of the antibodies significantly increased insulin receptor phosphorylation at concentrations up to 100 nM, which at least in the case of 25-49 and 83-14 was sufficient for full receptor occupancy. It is concluded that the insulin-like metabolic effects of antibodies involve a mechanism of receptor activation that is independent of autophosphorylation and hence that receptor autophosphorylation is not an essential step in triggering at least some events in the insulin signaling pathway.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.