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Insulin resistance can be said to exist "whenever normal concentrations of hormone produce a less than normal biological response". 1 In the 1930s, Himsworth first differentiated patients with diabetes mellitus into "insulin sensitive" and "insulin insensitive" based on the ability of subcutaneous insulin administration to dispose of an oral glucose load. 2. He further suggested that this differentiation corresponded to the clinical presentation of diabetes: that of either young ketosis-prone insulin sensitive or middle aged, nonketotic, insulin insensitive patients. The former is now classified as type 1 diabetes mellitus with the latter "insulin insensitive" classified as type 2 diabetes mellitus. Upon the development of the radioimmunoassay technique in 1960, Yalow and Berson demonstrated that patients with the adult-onset form of diabetes had, on average, higher circulating insulin levels than nondiabetic subjects. 3 It was thus concluded that "the tissues of the maturity onset diabetic do not respond to insulin as well as the tissues of the nondiabetic subjects respond to insulin." [ABSTRACT FROM AUTHOR]

Insulin resistance plays a key role in the pathogenesis of several human diseases, including diabetes, obesity, hypertension and cardiovascular diseases. The predisposition to insulin resistance results from genetic and environmental factors. The search for gene variants that predispose to insulin resistance has been thwarted by its genetically heterogeneous pathogenesis. However, using techniques of targeted mutagenesis and transgenesis in rodents, investigators have developed mouse models to test critical hypotheses on the pathogenesis of insulin resistance. Moreover, experimental crosses among mutant mice have shed light onto the polygenic nature of the interactions underlying this complex metabolic condition. This review focuses on targeted mutations affecting the function of genes in the insulin signaling pathway in mice. [ABSTRACT FROM AUTHOR]

Insulin rapidly lowers blood glucose levels via inhibition of endogenous glucose production and stimulation of glucose uptake. The mechanisms by which insulin modulates hepatic glucose production involve either activation of insulin signaling in hepatocytes (direct efects) or activation of insulin receptors in extra-hepatic sites (indirect effects), which in turn leads to inhibition of glucose production via neural and/or humoral mediators. The direct effects can be further divided into acute insulin action leading to rapid decrease in glucose production and chronic insulin action modulating the gene expression of rate-limiting enzymes within the biochemical pathways leading to glucose production. Short-term effects of insulin on hepatic glucose fluxes may be divided into three major components: (a) direct effects on the liver, mostly leading to rapid inhibition of glycogenolysis;1,2 (b) indirect effects mediated via peripheral actions of insulin, mostly modulating lipolysis;3-6 (c) indirect effects mediated via activation of hypothalamic insulin signaling.7,8 Furthermore, insulin exerts potent long-term effects on liver gene expression and function.9. These more chronic actions of insulin can in turn markedly affect the acute responses to an increase in circulating insulin levels.10,11 [ABSTRACT FROM AUTHOR]

The techniques that allow spatio-temporal control of gene deletion or gene expression in transgenic and knockout animals have been useful to directly evaluate the roles of the insulin and IGF-1 receptors and proteins in their signaling pathway in islet cells. While a functional role for insulin signaling in islet β-cells is now well established, the precise pathways and proteins that mediate β-cell growth and function during development and adulthood are continuing to be unraveled. This chapter will focus on recent studies derived from using transgenic, knockout and siRNA techniques to directly examine the role of insulin and IGF-I in the regulation of development, growth and function of islet β-cells. [ABSTRACT FROM AUTHOR]

Insulin regulates metabolism by altering the concentration of critical proteins or by inducing post-translational modifications of preexisting molecules. The latter represents a well-recognized action of insulin, and it has been extensively studied for many years.1-3By contrast, it is only recently that considerable advances have been made in understanding several aspects of insulin-regulated gene expression. [ABSTRACT FROM AUTHOR]

It has been recognized for more than a century that the liver plays an important role in maintaining metabolic fuel homeostasis. The purpose of this chapter is to summarize mechanisms by which circulating glucose and lipid concentrations are controlled by hepatic metabolic activities. [ABSTRACT FROM AUTHOR]

The stimulation of protein synthesis is a classic action of insulin. 1-3 Loss of the stimulatory effect of insulin on protein synthesis contributes to the cessation of growth and weight loss, which are hallmarks of untreated Type 1 diabetes mellitus. The effect of insulin on protein metabolism is complex and involves changes in both synthesis and degradation.1-3 In some cell types an increase in rate of protein synthesis may be detected within minutes of insulin treatment. This response to insulin occurs within a timeframe comparable to that of other acute actions of the hormone, such as the activation of glucose transport and glycogen synthase activation. The rapid effects of insulin on protein synthesis involve increases in mRNA translation, the process through which the genetic code transcribed in the mRNA template is translated into protein. [ABSTRACT FROM AUTHOR]

Maintenance of glucose homeostasis requires ‘cross-talk' between pancreatic insulin secretion and insulin signaling in the peripheral tissues. Both insulin secretion and glucose uptake are regulated exocytotic processes mediated by SNARE protein complexes. SNARE core complexes are heterotrimeric, composed of syntaxin, SNAP-25/23 and VAMP2 proteins in a 1∶1∶1 ratio. It has become clear that we must thoroughly define the precise mechanisms underlying insulin-stimulated GLUT4 vesicle translocation by skeletal muscle and adipose cells as well as glucose-stimulated insulin secretion by pancreatic beta cells in order to develop therapeutic strategies to better treat and eventually cure diabetic patients. In this chapter, I will examine the molecular interactions responsible for these SNARE dependent events. Similarities and differences in molecular machinery, exocytosis mechanisms as well as the impact of accessory proteins such as Munc18 and Rab GTPases will be discussed. New findings on alterations of particular SNARE proteins upon glucose homeostasis are described in terms of potential for pharmacological targets for intervention. In 1957 Lacy and colleagues published immunofluorescent and later electron microscopy images of islet beta cells filled with darkened organelles, or insulin granules. These granules were later shown to fuse with the plasma membrane to release insulin.1-3Analogously, in 1990 the localization of the insulin-responsive glucose transporter ‘GLUT4' to intracellular vesicles of adipocytes was revealed, and these GLUT4-storage vesicles were later shown to fuse with the plasma membrane upon stimulation with insulin, to ultimately facilitate glucose disposal. 4,5 However, it was not until 1993 that the molecular basis for vesicle fusion and secretion, termed ‘vesicle exocytosis', was elucidated by the discovery of SNARE proteins (soluble N-ethylmaleimide sensitive factor attachment protein receptor)6,7. [ABSTRACT FROM AUTHOR]

Skeletal muscle and adipose tissue are the major sites of postprandial glucose disposal. The insulin-regulated transport of glucose into these tissues is a multi-step process that begins with the binding of insulin to its cell surface receptor. Once activated, the insulin receptor generates multiple intracellular signaling cascades, some of which induce the rapid redistribution of the GLUT4 facilitative glucose transporter from intracellular compartments to the plasma membrane. Although probably best known for its role in glucose homeostasis, insulin regulates a variety of metabolic, mitogenic, and anti-apoptotic processes in specific tissues. Moreover, in addition to insulin several other hormones and growth factors can also activate signaling targets that function downstream of the insulin receptor. For example, phosphatidylinositol-3′-kinase (PI3K), a key enzyme in the signaling pathway leading to insulin-stimulated glucose uptake, can be activated by many extracellular signals. However, only insulin and highly related hormones such as IGF-I efficiently stimulate acute glucose transport. These observations suggest that cellular mechanisms have evolved for maintaining specificity among signaling pathways mediated by various hormones and growth factors. Elucidating the underlying mechanisms for this specificity is a current challenge engaging the attention of many researchers, and will require a thorough understanding of the signaling pathway responsible for each cellular response. To this end, recent work suggests that the coordination of multiple pathways occurs in part through the intracellular compartmentalization of key signaling molecules. Indeed, subcellular compartmentalization plays a critical role in maintaining the specificity of insulin signaling and the fidelity of GLUT4 vesicle trafficking. [ABSTRACT FROM AUTHOR]

The insulin and IGF-I receptors are members of the superfamily of receptor tyrosine kinases (RTKs). Unlike most RTKs that are single-chain monomeric transmembrane polypeptides, the insulin and IGF-I receptors are covalent dimers composed of two extracellular α subunits and two transmembrane β subunits containing the tyrosine kinase domains. The α subunits contain the ligand binding sites, of which at least three subdomains have been defined by photoaffinity crosslinking, alanine-scanning mutagenesis or minimized receptor constructs. All RTKs are dimeric or oligomeric in the ligand-activated form. The residues of insulin involved in receptor binding have been mapped by alanine-scanning mutagenesis. They form at least two major epitopes that partially overlap with the dimer- and hexamer-forming surfaces of the insulin molecule, and we propose that insulin is using those surfaces to asymmetrically cross-link the two receptor α subunits. This mechanism provides a structural basis for high affinity binding and negative cooperativity, and probably also operates in the IGF-receptor interaction. It also provides a structural basis for the approximation and transphosphorylation of the kinase domains and triggering of the signalling cascade. [ABSTRACT FROM AUTHOR]

After cloning a gene of interest, many researchers wish to analyze its characteristics by overexpression analysis or by introduction of mutated forms of the gene of interest into various cell types. In the analysis of insulin-stimulated glucose transport, the most appropriate cell systems are striated muscle and adipocytes (1). However, the introduction of DNA or genes of interest into these insulin-responsive tissues by standard transfection protocols such as calcium phosphate, DEAE-dextran, and liposome-mediated transfection are very inefficient. Furthermore, although transgenes can be expressed in muscle using adenovirus infection systems, this is difficult to accomplish in adipocytes and is substantially more labor intensive. The production of recombinant adenoviruses to use in infection of insulin-responsive tissues can take several months and requires very high titers of adenovirus. Therefore, we have recently established electroporation conditions that consistently provide at least 50% transfection efficiency for cultured differentiated 3T3-L1 adipocytes (2). Although the electroporation is not 100% efficient, it provides an easy and fast method to introduce DNA into adipocytes. Using 600 μg of CMV-LacZ plasmid DNA, we consistently obtain an electroporation efficiency of 50-80% (see Fig. 1). [ABSTRACT FROM AUTHOR]