El receptor soluble de insulina y el síndrome metabólico.

The metabolic syndrome describes a group of signs that increase the likelihood for developing type 2 diabetes mellitus, cardiovascular diseases and some types of cancer. The action of insulin depends on its binding to membrane receptors on its target cells. We wonder if blood insulin could travel bound to proteins and if, in the presence of hyperinsulinemia, a soluble insulin receptor might be generated. We used young adult Wistar rats (which have no predisposition to obesity or diabetes), whose drinking water was added 20 % of sugar and that were fed a standard diet ad libitum for two and six months. They were compared with control rats under the same conditions, but that had running water for consumption. At two months, the rats developed central obesity, moderate hypertension, high triglyceride levels, hyperinsulinemia, glucose intolerance and insulin resistance, i.e. metabolic syndrome. Electrophoresis of the rats' plasma proteins was performed, followed by Western Blot (WB) for insulin and for the outer portion of the insulin receptor. The bands corresponding to insulin and to the receptor external part were at the same molecular weight level, 25-fold higher than that of free insulin. We demonstrated that insulin, both in control animals and in those with hyperinsulinemia, travels bound to the receptor outer portion (ectodomain), which we called soluble insulin receptor, and that is released al higher amounts in response to plasma insulin increase; in rats with metabolic syndrome and hyperinsulinemia, plasma levels are much higher than in controls. Soluble insulin receptor increase in blood might be an early sign of metabolic syndrome.

El síndrome metabólico es un conjunto de signos que aumentan la probabilidad de desarrollar diabetes mellitus tipo 2, enfermedades cardiovasculares y algunos tipos de cáncer. La acción de la insulina depende de su unión a los receptores en la membrana de sus células diana. Para responder a la pregunta de si la insulina en la sangre podría viajar unida a proteínas y si en presencia de hiperinsulinemia podría generarse un receptor soluble de insulina, utilizamos ratas wistar (no tienen predisposición a la obesidad ni a la diabetes), adultas jóvenes, a cuya agua de consumo se adicionó 20 % de azúcar y a las que se les administró dieta estándar ad libitum, durante dos y seis meses; fueron comparadas con ratas control que tuvieron las mismas condiciones, pero con agua corriente para consumo. A los dos meses, las ratas desarrollaron obesidad central, hipertensión moderada, triglicéridos altos, hiperinsulinemia, intolerancia a la glucosa y resistencia a la insulina, es decir, síndrome metabólico. Se realizó electroforesis de las proteínas del plasma de las ratas, seguida de Western Blot para insulina y para la porción externa del receptor de insulina. Las bandas correspondientes a la insulina y la parte externa del receptor estaban al mismo nivel de peso molecular, 25 veces mayor que el de la insulina libre. Demostramos que la insulina, tanto en animales testigo como en aquellos con hiperinsulinemia, viaja unida a la porción externa del receptor (ectodominio), al cual denominamos receptor soluble de insulina, que se libera en mayor cantidad en respuesta al incremento en la insulina plasmática; en las ratas con síndrome metabólico e hiperinsulinemia, los niveles en plasma son mucho mayores que en los controles. El incremento del receptor soluble de insulina en sangre podría ser un dato temprano de síndrome metabólico.

Alzheimer's disease and insulin resistance: translating basic science into clinical applications.

Alzheimer's disease (AD) and diabetes are currently considered among the top threats to human health worldwide. Intriguingly, a connection between these diseases has been established during the past decade, since insulin resistance, a hallmark of type 2 diabetes, also develops in Alzheimer brains. In this article, the molecular and cellular mechanisms underlying defective brain insulin signaling in AD are discussed, with emphasis on evidence that Alzheimer's and diabetes share common inflammatory signaling pathways. I put forward here a hypothesis on how a cross-talk between peripheral tissues and the brain might influence the development of AD, and highlight important unanswered questions in the field. Furthermore, I discuss a rational basis for the use of antidiabetic agents as novel and potentially effective therapeutics in AD.

Amyloid-ß protein modulates insulin signaling in presynaptic terminals.

Synaptic loss is a major neuropathological correlate of memory decline as a result of Alzheimer's disease (AD). This phenomenon appears to be aggravated by soluble amyloid-ß (Aß) oligomers causing presynaptic terminals to be particularly vulnerable to damage. Furthermore, insulin is known to participate in synaptic plasticity through the activation of the insulin receptor (IR) and the PI3K signaling pathway, while low concentrations of soluble Aß and Aß oligomers aberrantly modulate IR function in cultured neurons. To further examine how Aß and insulin interact in the pathology of AD, the present work analyzes the effect of insulin and Aß in the activation of the IR/PI3K pathway in synaptosomes. We found that insulin increased mitochondrial activity and IR/Akt phosphorylation in synaptosomes taken from both hippocampus and cortex. Also, pretreatment with Aß antagonized insulin's effect on hippocampal synaptosomes, but not vice versa. These results show that Aß can reduce responsiveness to insulin. Combined with evidence that insulin desensitization can increase the risk of developing AD, our results suggest that the initial mechanism that impairs synaptic maintenance in AD might start with Aß changes in insulin sensitivity.

Insulin signaling cascade in the hearts of long-lived growth hormone receptor knockout mice: effects of calorie restriction.

Calorie restriction (CR) improves insulin sensitivity and increases life span in normal but not in long-lived growth hormone-resistant knockout (GHRKO) mice. In this study, we examined interactive effects of GH resistance and long-term CR on cardiac insulin action. GHRKO mice exhibited marked increases in the insulin-induced phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), Akt, and ERK1/2 along with elevated insulin-stimulated IRS-1-associated regulatory subunit of phosphatidylinositol 3-kinase in the heart. These changes were associated with elevated protein levels of IR, IRS-1, and Akt and with a down-regulation of cardiac glucose transporter 4 (GLUT4). In normal mice, CR induced an important increase in the phosphorylation of cardiac Akt without elevation of Akt protein, reaching activation levels similar to those seen in GHRKO mice. This change may be cardioprotective and thus contribute to increased longevity in response to CR. Interestingly, the insulin signaling cascade in the heart of GHRKO mice was unaffected by CR.

[Brain regulation of food intake and expenditure energy: molecular action of insulin, leptin and physical exercise]. / Regulación central de la ingestión alimentaria y el gasto energético: acciones moleculares de la insulina, la leptina y el ejercicio físico.

INTRODUCTION: Overweight and obesity present significant public health concerns because of the link with numerous chronic health conditions. During the last ten years, since the discovery of leptin, great advances were obtained in the characterization oh the hypothalamic mechanisms involved in the control of food intake and thermogenesis. DEVELOPMENT: This review will present some the most recent findings in this field. It will be focused on the actions of leptin and insulin in the hypothalamus and will explore the hypothesis that hypothalamic resistance to the action of these hormones may play a key role in the development of obesity. CONCLUSIONS: The physical activity is an important component on long-term weight control. The exercise markedly increased phosphorylation activity of several proteins involved in leptin and insulin signal transduction in the hypothalamus. Recently our laboratory showed that physical activity increase in sensitivity to leptin- and insulin-induced anorexia after enhances interleukin-6 production. These findings provide support for the hypothesis that the appetite-suppressive actions of exercise may be mediated by the hypothalamus.

Defective insulin and acetylcholine induction of endothelial cell-nitric oxide synthase through insulin receptor substrate/Akt signaling pathway in aorta of obese rats.

The actions of acetylcholine (ACh) on endothelium mainly are mediated through muscarinic receptors, which are members of the G protein-coupled receptor family. In the present study, we show that ACh induces rapid tyrosine phosphorylation and activation of Janus kinase 2 (JAK2) in rat aorta. Upon JAK2 activation, tyrosine phosphorylation of insulin receptor substrate (IRS)-1 is detected. In addition, ACh induces JAK2/IRS-1 and IRS-1/phosphatidylinositol (PI) 3-kinase associations, downstream activation of Akt/protein kinase B, endothelial cell-nitric oxide synthase (eNOS), and extracellular signal-regulated kinase (ERK)-1/2. The pharmacological blockade of JAK2 or PI 3-kinase reduced ACh-stimulated eNOS phosphorylation, NOS activity, and aorta relaxation. These data indicate a new signal transduction pathway for IRS-1/PI 3-kinase/Akt/eNOS activation and ERK1/2 by means of JAK2 tyrosine phosphorylation stimulated by ACh in vessels. Moreover, we demonstrate that in aorta of obese rats (high-fat diet), there is an impairment in the insulin- and ACh-stimulated IRS-1/PI 3-kinase pathway, leading to reduced activation with lower protein levels of eNOS associated with a hyperactivated ERK/mitogen-activated protein kinase pathway. These results suggest that in aorta of obese rats, there not only is insulin resistance but also ACh resistance, probably mediated by a common signaling pathway that controls the activity and the protein levels of eNOS.

[Clinical effects of the alterations that emerge in the signaling mechanisms of the insulin receptor]. / Repercusión de las alteraciones en los mecanismos de señalización del receptor de insulina.

Phosphorilation of subunit beta from insulin receptor induced mainly by insulin, it begins a series of intracellular complex signaling in cascade. Through this way establish multiple effects, which permits to the cell initiate its biological activity. This activity include the glucose metabolism, the regulation of ions and amino acids transport, lipids metabolism, glycogen synthesis, genetic transcription, mRNA expression, synthesis and degradation of proteins, as well as synthesis of DNA. Therefore, a modification in anyone of the proteins involved in the insulin signaling, can take place a dysfunction in the glucose metabolism. The impaired glucose can be due because there are many proteins, ions and enzymes that participate in the downstream pathways of the insulin signaling, it has become difficult to find a single phatophysiologic level as cause of diabetes. In spite of the advances in the study of this disease, it has been reached the conclusion that the glucose control is not enough to impede the complications that characterize to type 2 diabetes, since the organic worsening does not stop, which indicates that insulin signaling dysfunction is directly involved in all cellular process, and a better understanding in the communication ways of this hormone will take to new forms of treatment to impaired insulin response.

Bases genéticas da resistência à insulina, da síndrome metabólica e do diabetes melito tpo 2 / The genetic basis of insulin resistance, metaboilc syndrome and type 2 diabetes mellitus

Acredita-se atualmente que o diabetes tipo 2 ocorra em indivíduos geneticamente predispostos e expostos a uma série de influências ambientais, que precipitam o início da doença. O padrão de herança do diabetes melito tipo 2 é complexo e provavelmente poligênico. Apesar dos esforços, apenas alguns genes têm sido consistentemente relacionados a maior suscetibilidade à doença

Cold-induced PGC-1alpha expression modulates muscle glucose uptake through an insulin receptor/Akt-independent, AMPK-dependent pathway.

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) participates in control of expression of genes involved in adaptive thermogenesis, muscle fiber type differentiation, and fuel homeostasis. The objective of the present study was to evaluate the participation of cold-induced PGC-1alpha expression in muscle fiber type-specific activity of proteins that belong to the insulin-signaling pathway. Rats were exposed to 4 degrees C for 4 days and acutely treated with insulin in the presence or absence of an antisense oligonucleotide to PGC-1alpha. Cold exposure promoted a significant increase of PGC-1alpha and uncoupling protein-3 protein expression in type I and type II fibers of gastrocnemius muscle. In addition, cold exposure led to higher glucose uptake during a hyperinsulinemic clamp, which was accompanied by higher expression and membrane localization of GLUT4 in both muscle fiber types. Cold exposure promoted significantly lower insulin-induced tyrosine phosphorylation of the insulin receptor (IR) and Ser473 phosphorylation of acute transforming retrovirus thymoma (Akt) and an insulin-independent increase of Thr172 phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK). Inhibition of PGC-1alpha expression in cold-exposed rats by antisense oligonucleotide treatment diminished glucose clearance rates during a hyperinsulinemic clamp and reduced expression and membrane localization of GLUT4. Reduction of PGC-1alpha expression resulted in no modification of insulin-induced tyrosine phosphorylation of the IR and Ser473 phosphorylation of Akt. Finally, reduction of PGC-1alpha resulted in lower Thr172 phosphorylation of AMPK. Thus cold-induced hyperexpression of PGC-1alpha participates in control of skeletal muscle glucose uptake through a mechanism that controls GLUT4 expression and subcellular localization independent of the IR and Akt activities but dependent on AMPK.

Modulation of IR/PTP1B interaction and downstream signaling in insulin sensitive tissues of MSG-rats.

PTP1B has been shown to be a negative regulator of the insulin signal transduction in insulin resistant states. Herein we investigated IR/PTP1B interaction and downstream signaling in insulin sensitive tissues of 10 and 28-week-old MSG-insulin resistant rats which represent different stages of insulin resistance. Our results demonstrated that the increase in PTP1B expression and/or association with IR in MSG animals may contribute to the impaired insulin signaling mainly in liver and muscle. Although, adipose tissue of 10-week-old MSG rats showed higher PTP1B expression and IR/PTP1B interaction, they were not sufficient to impair all insulin signaling since IRS-2 phosphorylation and association with PI3-kinase and Akt serine phosphorylation were increased, which may contribute for the increased adiposity of these animals. In 28-week-old-MSG rats there was an increase in IR/PTP1B interaction and reduced insulin signaling in liver, muscle and adipocytes, and a more pronounced insulin resistance.

[Chronic complications of polycystic ovary syndrome. Review]. / Complicaciones crónicas del síndrome de ovarios poliquísticos. Revisión.

In addition to neuroendocrine abnormalities, women with polycystic ovary syndrome have insulin resistance and beta-cell dysfunction associated with a high frequency of metabolic syndrome components, such as glucose intolerance, type 2 diabetes mellitus (DM-2), dyslipidemia and a higher risk for endothelial dysfunction, haemostatic abnormalities, hypertension and cardiovascular disease. Obesity, a common finding in this disorder, plays an important role in the development of metabolic and cardiovascular disorders. Early identification of patients and prompt initiation of insulin sensitizing therapy by pharmacological agents or changes in life style such diet and exercise might improve the metabolic and endocrine abnormalities and reduce the risk of DM-2 and cardiovascular disease in these patients.

Growth hormone-induced alterations in the insulin-signaling system.

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the beta-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the beta-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. Interestingly, the modulation of insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those states.

Secreçäo da insulina: efeito autócrito da insulina e modulaçäo por ácidos graxos / Insulin secretion: insulin autocrinous effect and modulation by fatty acids

A insulina exerce um papel central na regulação da homeostase da glicose e atua de maneira coordenada em eventos celulares que regulam os efeitos metabólicos e de crescimento. A sub-unidade 13 do receptor de insulina possui atividade tirosina quinase intrínseca. A autofosforilação do receptor, induzida pela insulina, resulta na fosforilação de substratos protéicos intracelulares, como o substrato-l do receptor de insulina (IRS-1). O IRS-1 fosforilado associa-se a domínios SH2 e SH3 da enzima PI 3-quinase, transmitindo, desta maneira, o sinal insulínico. A insulina parece exercer feedback positivo na sua secreção, pela interação com seu receptor em células B pancreáticas. Alterações nos mecanismos moleculares da via de sinalização insulínica sugerem uma associação entre resistência à insulina e diminuição da secreção deste hormônio, semelhante ao observado em diabetes mellitus tipo 2. Uma das anormalidades associadas à resistência à insulina é a hiperlipidemia. O aumento do pool de ácidos graxos livres circulantes pode modular a atividade de enzimas e de proteínas que participam na exocitose da insulina. Essa revisão descreve também os possíveis mecanismos de modulação da secreção de insulina pelos ácidos graxos em ilhotas pancreáticas.

[Intracellular signals involved in glucose control]. / Señales intracelulares que intervienen en el control de la glucosa.

Many proteins are involved in glucose control. The first step for glucose uptake is insulin receptor-binding. Stimulation of the insulin receptor results in rapid autophosphorylation and conformational changes in the beta chain and the subsequent phosphorylation of the insulin receptor substrate. This results in the docking of several SH2 domain proteins, including PI 3-kinase and other adapters. The final event is glucose transporter (GLUT) translocation to the cell surface. GLUT is in the cytosol but after insulin stimulation, several proteins are activated either in the GLUT vesicles or in the inner membrane. The role of the cytoskeleton is not well known, but it apparently participates in membrane fusion and vesicle mobilization. After glucose uptake, several hexokines metabolize the glucose to generate energy, convert the glucose in glycogen and store it. Type 2 diabetes is characterized by high glucose levels and insulin resistance. The insulin receptor is diminished on the cell surface membrane, tyrosine phosphorylation is decreased, serine and threonine phosphorylation is augmented. Apparently, the main problem with GLUT protein is in its translocation to the cell surface. At present, we know the role of many proteins involved in glucose control. However, we do not understand the significance of insulin resistance at the molecular level with type 2 diabetes.

Modulation of epidermal growth factor receptor phosphorylation by endogenously expressed gangliosides.

The effect of changing the ganglioside composition of Chinese hamster ovary K1 cells on the function of the epidermal growth factor receptor (EGFr) was examined by studying the signalling pathway generated after the binding of epidermal growth factor (EGF) both in cells depleted of glycosphingolipids by inhibiting glucosylceramide synthase activity and in cell lines expressing different gangliosides as the result of stable transfection of appropriate ganglioside glycosyltransferases. After stimulation with EGF, cells depleted of glycolipids showed EGFr phosphorylation and extracellular signal-regulated protein kinase 2 (ERK2) activity as parental cells expressing GM3 [ganglioside nomenclature follows Svennerholm (1963) J. Neurochem. 10, 613-623] or as transfected cells expressing mostly GM2 and GD1a as the result of stable transfection of UDP-GalNAc:LacCer/GM3/GD3 N-acetylgalactosaminyltransferase. However, cells stably transfected with CMP-NeuAc:GM3 sialyltransferase and expressing GD3 at the cell surface showed both decreased EGFr phosphorylation and ERK2 activation after stimulation with EGF. Results suggest that changes in the ganglioside composition of cell membranes might be important in the regulation of the EGF signal transduction.

Insulin and phorbol ester regulation of gene 33 expression in CHO cells.

Rat gene 33 (g33) mRNA has a widespread tissue distribution. Insulin and various agents such as glucocorticoids, phorbol esters and plant lectins regulate G33 expression in rat hepatoma cells. The regulation of g33 by insulin and a phorbol ester was examined in two Chinese Hamster ovary (CHO) cell lines, CHO-T cells (which overexpress human insulin receptors (hIR)) and wild type CHOwt cells. These cell lines were used to determine how expression of the hIR influences the capacity of g33 to respond to insulin and phorbol myristate acetate (PMA). Treatment of CHOwt and CHO-T cells with insulin increased mRNAg33 levels three to four-fold, with a maximum effect reached after three hours of treatment. PMA treatment of CHOwt and CHO-T cells caused a similar elevation of mRNAg33 levels after three hours. Insulin had no effect on mRNAg33 stability in both CHO cell lines. Additionally, the effects of insulin and PMA on mRNAg33 levels were additive only in CHO-T cells. Insulin or PMA-pretreated CHO-T cells were able to respond to both agents, but elevation of mRNAg33 levels was submaximal. In contrast, when insulin and/or PMA-pretreated CHOwt cells were exposed to insulin or PMA, g33 was able to respond maximally. These results suggest that insulin and phorbol esters act through different signaling mechanisms in CHOwt cells. Additionally, insulin's ability to stimulate g33 expression in CHOwt cells suggests that this insulin effect may be independent of the insulin receptor. There are differences in the regulation pattern of g33 by insulin and PMA in rat hepatoma and among the two CHO cell lines used in this study.

Insulin and phorbol ester regulation of gene 33 expression in CHO cells

Rat gene 33 (g33) mRNA has a widespread tissue distribution. Insulin and various agents such as glucocorticoids, phorbol esters and plant lectins regulate G33 expression in rat hepatoma cells. The regulation of g33 by insulin and a phorbol ester was examined in two Chinese Hamster ovary (CHO) cell lines, CHO-T cells (which overexpress human insulin receptors (hIR)) and wild type CHOwt cells. These cell lines were used to determine how expression of the hIR influences the capacity of g33 to respond to insulin and phorbol myristate acetate (PMA). Treatment of CHOwt and CHO-T cells with insulin increased mRNAg33 levels three to four-fold, with a maximum effect reached after three hours of treatment. PMA treatment of CHOwt and CHO-T cells caused a similar elevation of mRNAg33 levels after three hours. Insulin had no effect on mRNAg33 stability in both CHO cell lines. Additionally, the effects of insulin and PMA on mRNAg33 levels were additive only in CHO-T cells. Insulin or PMA-pretreated CHO-T cells were able to respond to both agents, but elevation ofmRNAg33 levels was maximal. In contrast, when insulin and/or PMA-pretreated CHOwt cells were exposed to insulin or PMA, g33 was able to respond maximally. These results suggest that insulin and phorbol esters act through different signaling mechanisms in CHOwt cells. Additionally, insulin's ability to stimulate g33 expression in CHOwt cells suggests that this insulin effect may be independent of the insulin eceptor. There are differences in the regulation pattern of g33 by insulin and PMA in rat hepatoma and among the two CHO cell lines used in this study

The insulin receptor substrate 1 associates with phosphotyrosine phosphatase SHPTP2 in liver and muscle of rats.

Insulin stimulates the tyrosine kinase activity of its receptor resulting in the phosphorylation of its cytosolic substrate, insulin receptor substrate-1 (IRS-1) which, in turn, associates with proteins containing SH2 domains. It has been shown that IRS-1 associates with the tyrosine phosphatase SHPTP2 in cell cultures. While the effect of the IRS-1/SHPTP2 association on insulin signal transduction is not completely known, this association may dephosphorylate IRS-1 and may play a critical role in the mitogenic actions of insulin. However, there is no physiological demonstration of this pathway of insulin action in animal tissues. In the present study we investigated the ability of insulin to induce association between IRS-1 and SHPTP2 in liver and muscle of intact rats, by co-immunoprecipitation with anti-IRS-1 antibody and anti-SHPTP2 antibody. In both tissues there was an increase in IRS-1 association with SHPTP2 after insulin stimulation. This association occurred when IRS-1 had the highest level of tyrosine phosphorylation and the decrease in this association was more rapid than the decrease in IRS-1 phosphorylation levels. The data provide evidence against the participation of SHPTP2 in IRS-1 dephosphorylation in rat tissues, and suggest that the insulin signal transduction pathway in rat tissues is related mainly to the mitogenic effects of the hormone.

Tissue-specific regulation of IRS-1 in unilaterally nephrectomized rats

Insulin stimulates the tyrosine kinase activity of its receptor, resulting in the phosphorylation of its cytosolic substrate, insulin receptor substrate 1 (IRS-1). IRS-1 is also a substrate for different peptides and growth factors, and a transgenic mouse "knockout" for this protein does not have normal growth. However, the role of IRS-1 in kidney hypertrophy and/or hyperplasia was not investigated. In the present study we investigated IRS-1 protein and tyrosine phosphorylation levels in the remnant kidney after unilateral nephrectomy (UNX) in 6-week-old male Wistar ratas. After insulin stimulation the levels of insulin receptor and IRS-1 tyrosine phosphorylation were reduced to 79 + 5 percent (P<0.005) and 58 + 6 percent (P<0.0001), respectively, of the control (C) levels, in the remnant kidney. It is possible that a circulating factor and/or a local (paracrine) factor playing a role in kidney growth can influence the early steps of insulin action in parallel. To investigate the hypothesis of a circulating factor, we studied the early steps of insulin action in liver and muscle of unilateral nephrectomized rats. There was no change in pp185 tyrosine phosphorylation levels in liver (C 100 + 12 percent vs UNX 89 + 9 percent, NS) and muscle (C 100 + 22 percent vs UNX 91 + 17 percent, NS), and also there was no change in IRS-1 phosphorylation levels in both tissues. These data demonstrate that after unilateral nephrectomy there is a decrease in insulin-induced insulin receptor and IRS-1 tyrosine phosphorylation levels in kidney but not in liver and muscle. It will be of interest to investigate which factors, probably paracrine ones, regulate these early steps of insulin action in the contralateral kidney of unilaterally nephrectomized rats.

Growth-hormone signal transduction.

Growth hormone (GH) has long been recognized as one of the principal factors that control postnatal growth. Advances made in the last 5 years have increased our understanding of the intracellular signaling mechanisms subsequent to GH binding. The earliest event in GH signaling appears to be the binding of a single GH molecule by a pair of GH receptors (GHRs). The dimerization of GHRs leads to the activation of Janus kinase 2 (JAK2), a nonreceptor tyrosine kinase that associates with the cytoplasmic domain of GHR. It is thought that all signaling downstream from GHR depends on this initial activation of JAK2. Once activated, JAK2 tyrosyl-phosphorylates both itself and the cytoplasmic domain of GHR. These phosphorylated tyrosine residues act as docking sites for various signaling molecules that contain Src homology 2 (SH-2) or other phosphotyrosyl-binding domains. The signaling molecules that are recruited and activated by the GHR-JAK2 complex include signal transducers and activators of transcription (Stat) factors, the adapter protein Shc, and the insulin receptor substrates (IRSs) 1 and 2. The recruitment and activation of these signaling intermediates leads to the activation of enzymes such as MAP kinase, phosphatidylinositol-3'-kinase, protein kinase C, and phospholipase A2 and to the release of various second messengers such as diacylglycerol, calcium, and nitric oxide. Ultimately, these pathways modulate cellular functions such as gene transcription, metabolite transport, and enzymatic activities that affect the GH-dependent control of growth and metabolism.