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 Table of Contents  
REVIEW ARTICLE
Year : 2016  |  Volume : 3  |  Issue : 1  |  Page : 3-7

Molecular mechanisms of insulin signaling


1 Department of Human Anatomy, Faculty of Human Medicine, Ahmadu Bello University, Zaria-Kaduna State, Nigeria
2 Department of Veterinary Pathology, Faculty of Veterinary Medicine, Ahmadu Bello University, Zaria-Kaduna State, Nigeria
3 Department of Chemical Pathology, Ahmadu Bello University, Zaria-Kaduna State, Nigeria
4 Department of Biochemistry, Ahmadu Bello University, Zaria-Kaduna State, Nigeria
5 Department of Integrated Science (Biology), College of Education, Minna, Niger State, Nigeria

Date of Submission01-May-2015
Date of Acceptance08-Jan-2016
Date of Web Publication12-Feb-2016

Correspondence Address:
I A Iliya
Department of Human Anatomy, Ahmadu Bello University, Zaria, Kaduna State
Nigeria
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DOI: 10.4103/2384-5147.176267

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  Abstract 

Insulin signaling pathway is an important biochemical pathway that affects glucose homeostasis in the body. The pathway can be influenced by a variety of factors ranging from fed-fasting states, stress levels, diseases and even hormones. The pathway is a signal transduction pathway that comprises of trigger mechanisms which serve as signals throughout insulin targeted cells for the purpose of metabolic actions (glucose homeostasis) as well as mitogenic actions in the body. In pathological conditions, the signal-flow in this pathway can get disrupted resulting in serious metabolic and mitogenic disturbances in the body.

Keywords: Docking proteins, insulin, insulin receptor, insulin signaling, insulin substrates


How to cite this article:
Iliya I A, Mohammed B, Akuyam S A, Nok A J, Bauchi Z M, Tanko M, Timbuak J A, Yusuf B. Molecular mechanisms of insulin signaling. Sub-Saharan Afr J Med 2016;3:3-7

How to cite this URL:
Iliya I A, Mohammed B, Akuyam S A, Nok A J, Bauchi Z M, Tanko M, Timbuak J A, Yusuf B. Molecular mechanisms of insulin signaling. Sub-Saharan Afr J Med [serial online] 2016 [cited 2019 Oct 16];3:3-7. Available from: http://www.ssajm.org/text.asp?2016/3/1/3/176267


  Introduction Top


Insulin is a potent peptide hormone that is secreted from the endocrine portion of the pancreas. It plays a central role in the regulation of glucose homeostasis. It is also essential for the regulation of carbohydrate, lipid, and protein metabolisms. [1],[2] Except in the presence of metabolic disorders such as diabetes mellitus and metabolic syndrome, insulin is provided within the body in a constant proportion to remove excess glucose from the blood, and it also exerts an anabolic effect on the body.

On carbohydrates metabolism, insulin causes the oxidation of glucose by increasing the combustion of glucose in tissues and also helps in the transport of glucose into cells. It also increases the synthesis of glycogen from carbohydrate and lactate sources both in the liver and muscles. [1],[2]

On the protein metabolism, insulin prevents gluconeogenesis from proteins sources in the liver. In diabetes mellitus, this process is enhanced resulting in the overproduction of glucose. Insulin also stimulates proteogenesis and growth by increasing the incorporation of amino acids into peptides. Insulin also increases the synthesis of the mRNA and in the activities of many intracellular structures involved in protein biosynthesis. [2]

On fat metabolism, insulin exerts an antiketogenic effect in the liver. In addition, it also helps in lipogenesis in adipose tissue from glucose while at the same time decreases cholesterolemia and lipemia. It prevents the accumulation of excess lipid in the liver and the breakdown of lipids in the adipose tissue. [2],[3]


  Cellular Mechanisms of Insulin Signaling Top


Regulation of Carbohydrate Metabolism

Insulin promotes the transfer of glucose, amino acids, and electrolytes across cell membranes, the target tissues being liver, muscle, and adipose tissues.

The cellular events by which insulin initiates its stimulatory effects on glucose metabolism begins with the binding of the hormone to specific receptors referred to as insulin receptors (INSR) on the surface of the cell membrane of all insulin target tissues. [4],[5]

The normal INSR is a glycoprotein heterotetramer consisting of two α and two β subunits that are linked by disulfide bonds forming a heterotetrameric complex. [6] The two α-subunits are entirely extracellular and contain the insulin-binding domain. The β-subunits, on the other hand, have an extracellular domain, a transmembrane domain, and an intracellular domain (tyrosine kinase C-terminal domain) that expresses insulin-stimulated kinase activity directed toward its own tyrosine residues [Figure 1]. [6]
Figure 1: Basic structure of the insulin receptor: alpha (α) and beta (β) subunits of the receptor. Fundamentals of Biochemistry: Life at the molecular level (2nd ed.), John Wiley & Sons, NY., 2006[32]

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Once insulin binds to the α-subunit, it transduces signals across the plasma membrane and unsuppresses the β-subunits activity thereby promoting auto-phosphorylation of the β-subunit and subsequent phosphorylation of the tyrosine residues leading to amplification of the tyrosine kinase activity. [4],[5],[6],[7]

Following its activation, the INSR tyrosine kinase affects a series of intramolecular transphosphorylation reactions in which one β-subunit phosphorylates its adjacent partner on specific tyrosine residues [8] which subsequently phosphorylates specific intracellular proteins referred to as docking proteins and undergoes tyrosine phosphorylation in regions containing specific amino acid sequence motifs that when phosphorylated, serve as recognition sites for proteins containing the src-homology 2 (SH2) domains. [9] Autophosphorylation of these tyrosine residues stimulates the catalytic activity of the receptor tyrosine kinase to recruit docking proteins. [8] Of these intracellular docking proteins, the insulin-receptor-substrate proteins (IRS) and two proteins exhibiting src-homology (SH2 domains) namely Shc and Grb2 have been classically identified [Figure 2]. [8],[9]
Figure 2: The Kegg Pathways: Insulin signaling pathway: Kenkisa Laboratories, http://www.genome.jp [33]

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In the muscle tissue, IRS-1 serves as the major docking protein that interacts with the IR tyrosine kinase domain, [10] while IRS-2 serves as the major docking protein in the liver. Both of these docking proteins undergo tyrosine phosphorylation and mediate the effect of insulin on hepatic glucose production, gluconeogenesis, and glycogen formation by activating the intracellular enzyme-phosphatidylinositol-3-kinase (Pi3K) [Figure 2]. [8],[9]

The phosphorylated tyrosine residues of the IRS proteins mediate the association with the regulatory subunit of Pi3K (85 kDa) leading to the activation of the enzyme. [1],[3],[11] The Pi3K enzyme is composed of a regulatory unit (85 kDa) and a catalytic unit (110 kDa). The catalytic unit catalyzes the 3' phosphorylation of specific phosphoinositides in the plasma membrane to form phosphatidylinositol-4,5-biphosphate and phosphatidylinositol-3, 4, 5-triphosphate (PIP 3 ). [1]

Phosphatidylinositol-3-Kinase Pathway

The formation of PIP 3 and activation of Pi3K by the tyrosine residues of the IRS proteins stimulate the phosphoinositide to activate a serine/threonine kinase known as phosphoinositide-dependent kinase-1 (PDK-1) and activated PDK-1 phosphorylates of another serine/threonine kinase also known as protein kinase B (also referred to as PKB or AKT). PKB contains a PH domain that interacts directly with PIP3. [12] Once activated, PKB moves to link the glucose transporter (GLUT2 or GLUT4) to the insulin pathway. The linking of these insulin-dependent GLUT proteins to the signaling pathway activates the transporter to translocate to the cell membrane and import glucose into the intracellular compartment of the cell. [3],[12]

Once inside the cell, glucose gets handled by a variety of enzymes, of these, three have been documented to be classical in its intracellular utilization. One of such enzymes is glucokinase, a type of hexokinase found in the liver. It controls the phosphorylation of glucose inside the cell to glucose-6-phosphate (G-6-P), after which G-6-P is further phosphorylated to fructose-1,6-biphosphate by a key regulatory enzyme called phosphofructokinase that controls the rate-limiting step into the glycolytic pathway. Excess glucose is handled by glycogen synthetase which converts it to stored form of glucose in the body (glycogen) [Figure 2]. [13]

Cbl/Cbl Associated Protein Pathway

This is a compartmentalized pathway that operates parallel to the Pi3K/PKB pathway and thus can lead to GLUT2 or GLUT4 translocation to the cell membrane. [14],[15]

Some studies have shown that insulin signaling operating via this pathway begins from specialized regions in the plasma membrane that are enriched in cholesterol, sphingolipids, glycosylphosphatidylinositol-anchored proteins and glycolipids, and lipid-modified signaling proteins. These enriched specialized regions are known as lipid rafts or caveolin. [15] At least some part of the INSR has been supported by other studies to reside in these lipid rafts. [16],[17] Activation of the INSR in these plasma membrane sub-domains stimulates the tyrosine phosphorylation of the proto-oncogenes Cbl-c and Cbl-b. This phosphorylation step requires recruitment of Cbl to the adapter protein APS, which contains SH2 and PH domains. [4],[18],[19] The Cbl associated protein (CAP) is recruited with Cbl to the INSR/APS complex. [19] CAP is a bifunctional adapter protein with 3 SH3 domains in its COOH-terminus, and an NH2-terminal region of similarity to the gut peptide sorbin called the sorbin homology (SoHo) domain. [16] CAP expression correlates well with insulin sensitivity. The protein is found predominantly in insulin-sensitive tissues, and expression is increased upon activation of the nuclear receptor peroxisome proliferator-activated receptor γ, the receptor for the thiazolidinedione class of insulin-sensitizing drugs. [20]

The COOH-terminal SH3 domain of CAP associates with a PXXP motif in the Cbl, such that these proteins are constitutively associated. Upon recruitment to the INSR, CAP interacts with the lipid raft domain protein flotillin, via its SoHo domain. [21] Upon tyrosine phosphorylation, Cbl interacts with the protein Crk-II, an SH2/SH3-containing adapter protein. [22] Crk-II binds to specific phosphorylation sites on Cbl via its SH2 domain [23] and is constitutively associated with the nucleotide exchange factor C3G or GRF2 via its SH3 domain. [24] C3G/Crk-II complex then catalyzes the activation of the small G proteins TC10. [25] Upon its activation, TC10 interacts with a number of potential effector molecules. One of these is a splice variant of the adapter protein, CIP4, and Exo70, a component of the exocyst complex. [26] The exocyst complex has been implicated in the tethering or docking of secretory vesicles. [26] Once activated, these two proteins involved in vesicular trafficking and membrane fusion will activate the GLUT and cause it to translocate to the plasma membrane to import glucose. [22]

Mitogen Activated Protein Kinase Pathway

Other signal transduction proteins interact with IRS including Grb2, an adaptor protein that contains SH2 domains, which in turn associates with the guanine nucleotide exchange factor son-of-sevenless and elicits activation of the mitogen-activated protein kinase (MAPK) cascade leading to mitogenic responses. [6],[27] Shc is another substrate for insulin. Upon phosphorylation, Shc associates with Grb2 and can, therefore, activate the MAPK pathway independent of IRS. The MAPK pathway plays an important role in the generation of transcription factors, promotion of cell growth, proliferation, and differentiation. Thus, inhibition of this pathway has been shown to prevent the stimulation of cell growth by insulin, but has no effect on the metabolic actions of insulin. [6],[27],[28]

Regulation of Protein Metabolism

Insulin stimulates amino acid uptake into cells, inhibits protein degradation (through an unknown mechanism), and promotes protein synthesis. [2] Under basal conditions, the constitutive activity of GSK3 leads to the phosphorylation and inhibition of a guanine nucleotide exchange factor eIF2B, which regulates the initiation of protein translation. Therefore, upon receipt of an insulin signal, inactivation of GSK3 by AKT leads to the dephosphorylation of eIF2B thereby promoting protein synthesis and the storage of amino acids. [2] AKT also activates mammalian target of rapamycin, which promotes protein synthesis through P70 ribosomal S6 kinase and inhibition of eIF-4E binding protein. [27]

Regulation of Lipid Metabolism

Insulin promotes the uptake of fatty acids and the synthesis of lipids from blood into tissues, while inhibiting the rate of lipolysis in adipose tissues hence lowering plasma fatty acid levels. Recent studies indicate that lipid synthesis requires an increase in the transcription factor steroid regulatory element-binding protein-c (SREBP-1c). [16],[29],[30],[31] SREBP-1c is a protein that regulates the genes required for glucose metabolism and fatty acid/lipid production, and its expression is controlled by insulin. [29] However, the pathway leading to changes in SREBP expression is unknown. Insulin inhibits lipid metabolism through decreasing cellular concentrations of cAMP by activating a cAMP-specific phosphodiesterase in adipocytes. [20],[31]


  Conclusion Top


Diabetes mellitus, especially type 2 is fast emerging as one of the great challenges of the 21 st century with most authors already linking molecular defects within the insulin signal transduction machinery to type 2 diabetes mellitus. Therefore, uncovering the normal biochemical events in this review that define the signaling cascades in the pathway can go a long way in helping us to understand the molecular alterations that have been reported by most literatures to occur in relation to type 2 diabetes mellitus and grant us an insight on how to develop novel strategies at the molecular level to combat this diabetes menace. [33]

Acknowledgments

I am most grateful to the Professor Andrew J. Nok of the Department of Biochemistry, Ahmadu Bello University and member of the Order of the Federal Republic of Nigeria (MFR), Dr. S.A. Akuyam of the Department of Chemical Pathology, Ahmadu Bello University Teaching Hospital Zaria-Nigeria, and Dr. B. Mohammed of the Department of Veterinary Pathology, Ahmadu Bello University, Zaria-Nigeria, for their help and contribution in the preparation of this review.

Financial Support and Sponsorship

Nil.

Conflicts of Interest

There are no conflicts of interest.

 
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