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Molecular Endocrinology, 2006: 87-108 ISBN: 81-308-0100-0
Editor: Patricia Joseph-Bravo
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Molecular basis of type-2
diabetes
Cristina Fernandez-Mejia
Nutritional Genetics Unit, Biomedical Research Institute
National University of Mexico
Abstract
Diabetes mellitus is taking its place as one of the
most important diseases in the world. Diabetes mellitus
is a group of metabolic diseases characterized by
hyperglycemia resulting from defects of insulin secretion,
insulin action, or both. There are two main types of
diabetes: type-1 and type-2, the worldwide diabetes
epidemic relates particularly to type-2 diabetes. Type2 diabetes is characterized by impaired insulin action
and/or abnormal insulin secretion. An early abnormality
in the disease is insulin resistance; a defective state in
which insulin is unable to exert its biological effects at
circulation concentrations that are effective in normal
subjects. Insulin resistance leads to profound
decreases in glucose uptake and glycogen synthesis in
Correspondence/Reprint request: Dr. Cristina Fernandez-Mejia, Unidad de Genética y Nutrición, Instituto de
Investigaciones Biomédicas, Universidad Nacional Autónoma de México/Instituto de Pediatría, Av. del Iman
#1, 4th floor, Mexico City, CP 04530 Mexico. E-mail: [email protected]
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Cristina Fernandez-Mejia
peripheral tissues. Insulin resistance yields to defective suppression of
hepatic glucose output. Resistance to the antilipolytic action of insulin also
favors triglyceride breakdown in adipose tissue and the generation of free
fatty acids, which interfere with insulin receptor signals. Changes in serum
adipokine concentrations are also part of the insulin resistant state. At the
pre-onset of type-2 diabetes, resistance to the glucose-lowering action of
insulin tends to lead a slight increase of blood glucose concentration, which
stimulates insulin secretion and causes hyperinsulinemia. Initially
hyperinsulinemia is able to overcome insulin resistance. The diabetic state
develops when insulin secretion cannot longer be sustained to compensate
insulin resistance, and it is at this stage that fasting and post-prandrial
hyperglycemia is apparent. Plentiful efforts have been made to understand
the molecular basis of type 2-diabetes. It is now well accepted that defective
postreceptor insulin signaling is the main feature involved in insulin
resistance of type-2 diabetes. As a result the metabolic insulin actions are
affected. Several mechanisms such as insulin receptor tyrosine
dephosphorylation, imbalance of serine/threonine phosphorylation, or
insulin receptor internalization impair insulin signaling. A number of
molecules associated to insulin resistance such as free fatty acids,
interleukin-6, TNF-alpha, affect insulin receptor signaling, interestingly
most of them are related to adipose tissue. Transcriptional factors such as
peroxisome proliferator activated receptor gamma (PPAR gamma) and
peroxisome proliferator activated receptor gamma coactivator 1 alpha
(PGC-1 alpha) have also been found to be associated with insulin resistance.
Beta-cell function plays a pivotal role in determining progress to type-2
diabetes. Beta-cell gene expression defects, as seen in the monogenic
diabetes forms (MODY) or secondary beta-cell defects, caused by
glucotoxicity, increased free fatty acids, cytokines and/or mitochondrial
disfunction may be implicated in the physiopathology of type-2 diabetes.
Classical physiology studies, biochemical methods, tissue culture
technology, the use of gene targeting approaches in mice, and naturally
occurring mutations in patients have shed some light on the molecular
causes of type-2 diabetes and have contributed to a deeper understanding of
the molecular mechanisms involved in the disease.
Molecular basis of type-2 diabetes
Diabetes mellitus is taking its place as one of the most important diseases
in the world. In the past two decades the disease has experienced an explosive
increase, now reaching epidemic proportions. Global estimates for the year
2010 predict a further growth of approximately 50%, with the greatest increase
in developing countries [1]. Pronounced changes in environment, food
Molecular basis of type-2 diabetes
89
availability and lifestyle have resulted in escalating rates of obesity and
diabetes.
1. History
The first document known to mention descriptions of a polyuric state
resembling diabetes is an Egyptian papyrus dating from approximately 1500
BC. The term “diabetes” was first used by Aretaeus of Cappadocia in the 2nd
century AD and described a condition showing increased urine production.
During the same period, Galen, the Roman physician, spoke of two cases of a
rare disease characterized by polyuria and polydipsia. In the 5th and 6th
centuries, two notable Indian physicians, Susruta and Sharuka, described for
the first time the association of polyuria with a sweet-tasting substance in the
urine. In the 17th century, Thomas Willis made several clever observations
about the disease, which still hold true today. He wrote that diabetes had been
rare in classical times (Galen) “but in our age, given to good fellowship and
gusling down chiefly of unallayed wine, we meet with examples and instances
enough, I may say daily, of this disease..... As to what belongs the cure, it
seems a most hard thing of this disease to draw propositions for curing, for this
cause lies so deeply hid, and hath its origin so deep and remote” [2]. Since
then, plentiful efforts have been made to unravel the deep causes of this
frequent disease.
2. Definition
Diabetes mellitus is a group of metabolic diseases characterized by
hyperglycemia resulting from defects of insulin secretion, insulin action, or
both [3].
3. Diabetes classification
There are two main types of diabetes: type-1 and type-2. Type-1 diabetes
is due to the autoimmune-mediated destruction of pancreatic beta cells,
resulting in insulin deficiency. Patients with type-1-diabetes require exogenous
insulin for survival. Its frequency amounts to nearly 10% of all diabetes cases.
There is marked geographical variation in its prevalence, Scandinavian
countries showing the highest rate of this illness [4].
Type-2 diabetes accounts for approximately 90% of diabetes cases, and is
characterized by impaired insulin action and/or abnormal insulin secretion. The
worldwide diabetes epidemic relates particularly to type-2 diabetes. Besides
type-1 and type-2 diabetes, there are other specific types of diabetes as
described on Table 1.
To understand the metabolic and molecular mechanisms responsible for
type-2 diabetes, it is necessary to understand the regulation of fuel metabolism
in the human body.
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Table 1. Etiologic classification of diabetes mellitus [3].
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4. Basic principles of metabolism
In the fed state, fuels in excess are stored as glycogen and triglycerides.
During the fasting state these reservoirs are broken down to provide fuels.
Energy reservoirs are built up and broken down in response of hormonal messages.
In the fed state, coordination of insulin secretion by the pancreatic beta
cells along with the responsiveness to insulin of major glucose metabolic
tissues such as muscle, liver and fat, control plasma glucose. Insulin promotes
glucose uptake, glycogen synthesis in the liver and muscle, lipid formation to
be stored in the adipose tissue, and protein synthesis in most cells. The rate
limit step in whole body glucose uptake is the transport of glucose into skeletal
muscle cells, this accounts for more that 75% of glucose uptake [5]. Alongside
the insulin stimulatory effect on fuel reservoir synthesis, in the fed state, the
hormone has restrained functions on glucose output and lipolysis [6, 7].
In the fasting state, decreased plasma insulin concentration and
increased counterinsulin hormones, such as glucagons, glucocorticoids, and
catecholamines, contribute to glucose output via glycogen breakdown and
gluconeogenesis, and via lipolysis as well as decreased synthesis and increased
protein degradation.
Beside the classical regulating hormones, a considerable piece of evidence
indicates that adipose tissue hormones, adipokines, as well as free fatty acids,
influence metabolism and fuel expenditure.
Below, we describe the basic knowledge of the molecular mechanisms
involved in insulin secretion and of responsiveness to insulin in normal
conditions. We also describe the role of adipose tissue in fuel metabolism.
5. Molecular mechanisms involved in the regulation of
metabolism under normal conditions
Molecular mechanisms of insulin secretion
Insulin secretion in response to glucose is a complex, multistep process
that requires transport and oxidation of glucose, electrophysiological changes
and fusion of insulin-containing secretory granules with the beta-cell plasma
membrane (Fig.1). Glucose enters the cell by facilitated diffusion mediated
by a group of structurally related glucose transport proteins (GLUT),
characterized by 12 hydrophobic helical domains. To date, at least 12
GLUTs have been described [8]. In the pancreatic beta cell, glucose is
transported by the glucose transporter 2 isoform (GLUT2). Glucose is
phosphorylated to form glucose-6-phosphate by glucokinase. This enzyme
plays a critical role in glucose-induced insulin secretion and is considered the
glucosensor of the pancreatic beta cell. Due to its kinetic characteristics,
glucokinase is a determining factor for glucose phosphorylation [9] and
hence for its metabolism through glycolysis and oxidation.
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Figure 1. Insulin secretion in response to glucose. Glucose enters the cell by
facilitated diffusion mediated the glucose transporter 2 isoform (GLUT2) and it is
phosphorylated to form glucose-6-phosphate by glucokinase. The generation of ATP by
glucose oxidation leads to closure of the ATP-sensitive K+ channel, a hetero-octamer
comprised of four subunits of the sulphonylurea 1 receptor (SUR1) and four subunits of
the inwardly rectifying K+ channel Kir6.2. The closing of the ATP-sensitive K+
channel leads to depolarization of the plasma membrane and influx of extracellular
calcium. This leads to fusion of insulin-containing secretory granules with the plasma
membrane and the release of insulin into the circulation.
The generation of ATP by glycolysis and the Krebs cycle leads to closure
of the ATP-sensitive K+ channel — a hetero-octamer comprised of four
subunits of the sulphonylurea 1 receptor (SUR1) and four subunits of the
inwardly rectifying K+ channel Kir6.2 [10]. The closing of the ATP-sensitive
K+ channel leads to depolarization of the plasma membrane and influx of
extracellular calcium [11]. Together with calcium mobilized from intracellular
stores, this leads to fusion of insulin-containing secretory granules with the
plasma membrane and the release of insulin into the circulation [12]. These
pathways are stimulated to varying degrees by neurotransmitters and hormones
via activation of receptors on the pancreatic beta-cell. For instance,
acetylcholine, acting via the muscarinic receptor type 3 activates proteine
kinase C: Similarly, glucagon-like peptide-1 (GLP-1) promotes a rise in cyclic
AMP and therefore activation of protein kinase A [13].
Molecular basis of type-2 diabetes
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Molecular mechanisms of insulin signaling
Insulin starts its action by binding to the insulin receptor; this leads to a
cascade of events that involves protein and membrane phospholipid
phosphorylation, scaffold and docking proteins, and cytoeskeleton activity
(Fig. 2).
Figure 2. Molecular mechanisms of insulin signaling. The insulin interaction with its
receptor promotes insulin receptor autophosphorylation, and catalyses the
phosphorylation of cellular proteins such as members of the IRS family and Cbl. Upon
tyrosine phosphorylation, these proteins interact with signalling molecules, resulting in
a diverse series of signalling pathways, including activation of PI3K and the activation
of TC10. These pathways act in a concerted fashion to coordinate the regulation of
glucose transporter 4 (GLUT4) vesicle trafficking, protein synthesis, enzyme activation
and inactivation, and gene expression, which results in the regulation of glucose, lipid
and protein metabolism.
Insulin receptor. The insulin receptor belongs to a subfamily of receptors,
all with protein tyrosine kinase activity. The insulin receptor is a heterotetrameric
membrane glycoprotein consisting of two alfa-subunits and two beta-subunits.
Insulin binds to the extracellular alpha-subunit of the receptor and induces a
conformational change that brings the alpha-subunits closer together. This
leads to a rapid auto-phosphorylation of the receptor, and catalyzes the
phosphorylation of intracelular proteins such as: a) members of the insulin
receptor substrate family, b) Shc and c) CbI. Upon tyrosine phosphorylation,
these proteins act as docking sites for proteins that contain SH2 (Srchomology-2) domains, such as phosphoinositide 3-kinase, Grb-2 and SHP-2,
resulting in a diverse series of signaling pathways. These pathways act
concertedly to transmit the signal from the insulin receptor to biological
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endpoints, such as glucose transport, protein, lipid and glycogen synthesis,
mitogenesis and gene expression [14].
Insulin receptor substrate family (IRS). At least 12 substrates of the
insulin receptor have been identified: IRS-1, IRS-2, IRS-3, IRS-4, IRS-5, IRS-6,
Gab-1, three isoforms of Shc, p62dok and APS (adapter protein containing a
PH and SH2 domain) [15]. Studies in knockout mice suggest that IRS proteins
serve complementary, rather that redundant, roles in insulin signaling. IRS-1
knockout mice demonstrate defective action of insulin in peripheral tissues,
and reduced growth [16]. IRS-2 knockout mice have a progressive
development of type-2 diabetes, increased adiposity and female infertility [17],
with abnormal insulin action in the liver and skeletal muscle, together with a
lack of beta-cell mass compensation for the impaired actions of insulin in
peripheral tissues [18]. These animal models have provided clues for the
pathogenesis of type-2 diabetes.
Phosphoinositide 3-kinase (PI3K). PI3K is a heterodimeric lipid kinase
that plays a critical role in the actions of insulin and other growth factors
[reviewed in [19]. PI3K consists of a regulatory subunit (p85) that associates
with a catalytic subunit (p110). The regulatory subunit binds the IRSs, whereas
the catalytic subunit phosphorylates phosphatidylinositols in the membrane to
generate the lipid second messenger: phosphatidylinositol 3,4,5-trisphosphate,
which binds to the plecstrin homology (PH) domains of a variety of signaling
molecules, thereby altering their activity or subcellular localization. [20].
Insulin signaling downstream of PI3K
Downstream PI3K intracellular serine/threonine kinases, Akt and atypical
PKC (aPKC), are activated by the formation of lipid products of PI3K. These
kinases require the involvement of IRS to transmit the insulin signal to
downstream biological events [21].
Akt. The generation of phosphatidylinositol 3,4,5-trisphosphate, by PI3K
activates Akt (a serine/threonine kinase also called PKB). Increasing levels of
phosphatidylinositol 3,4,5-trisphosphate: a) recruit Akt to the plasma membrane
through specific interactions with its amino-terminal PH domain, and b)
promote the activation of PI-dependent kinases (PDK1 y PDK2). In turn, PDK
activate Akt by phosphorylation on threonine 308 and serine 473 [14].
Three ubiquitously expressed isoforms of Akt have been identified: Akt1,
Akt2 and Akt3. Both Akt1 and Akt2 are involved in insulin signal transduction
in skeletal muscle and adipose tissue. In contrast, Akt3 is not activated by
insulin in liver, muscle or adipose tissue [22].
Atypical protein kinase C (aPKC). In addition to Akt, the aPKC ζ and λ
have also been implicated in insulin signaling. Unlike the conventional PKCs,
aPKCs are not activated by diacylglycerol. However, although aPKCs lack PH
domains, phosphatidylinositol 3,4,5-trisphosphate bind to the regulatory domains
Molecular basis of type-2 diabetes
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of these kinases and apparently induce conformational changes that lead to
aPKC activation. [23]. Activation of aPKC activity has also been reported to
occur through autophosphorylation, phosphorylation by PDK1, and relief from
pseudosubstrate inhibition [23].
Insulin signaling downstream Grb2
Insulin stimulates the mitogen-activated protein (MAP) kinase/
extracellular signal-regulated kinase (ERK). This pathway involves the
tyrosine phosphorylation of IRS proteins and/or Shc, which in turn interact
with the adapter protein Grb2, recruiting the Son-of-sevenless (SOS) exchange
protein to the plasma membrane for activation of Ras. The activation of Ras
also requires stimulation of the tyrosine phosphatase SHP2, through its
interaction with receptor substrates such as Gab-1 or IRS1/2. Once activated,
Ras operates as a switch, stimulating a serine kinase cascade through the
activation of Raf, MEK and ERK. Activated ERK can translocate into the
nucleus, where it catalyzes the phosphorylation of transcription factors, that
leads to cellular proliferation or differentiation [21].
Insulin signaling downstream CbI
This insulin signaling pathway involves tyrosine phosphorylation of the
Cbl protooncogene. Cbl is associated with the adapter protein CAP. Upon
phosphorylation, the Cbl−CAP complex translocates to lipid raft domains in
the plasma membrane. Translocation of phosphorylated Cbl recruits the
adapter protein CrkII to the lipid raft via interaction of the SH2 domain of
CrkII with phospho-Cbl. CrkII also forms a constitutive complex with the
guanyl nucleotide-exchange protein C3G. Once translocated into lipid rafts,
C3G comes into proximity with the G protein TC10, and catalyzes the
exchange of GTP for GDP, resulting in the activation of the protein. Once
activated, TC10 seems to provide a second signal to the GLUT4 protein that
functions in parallel with the activation of the PI3K pathway. This may involve
the stabilization of cortical actin, which seems to be important in GLUT4
vesicle translocation to the plasma membrane [21].
Inhibition of insulin signaling
Termination of the insulin signal is critical for the maintenance of
metabolic control. Signaling of the insulin receptor cascade is terminated
by specific phosphatases. They act to terminate insulin’s effects through
activation of lipid or protein phosphatases and through the induction of
serine/threonine kinases that phosphorylate and uncouple various elements
along the insulin signaling pathways [24]. Because Ser/Thr phosphorylation of
IRS proteins is stimulated by insulin treatment and by inducers of insulin
resistance, it suggests that the same Ser kinases might be utilized to
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phosphorylate the IRS proteins under both physiological and pathological
conditions [24]. Protein-tyrosine phosphatase IB (PTP1B), phosphatase and
tensin homologue protein (PTEN) and SHIP2, which inactivate the lipid
products of PK3I, are involved in the termination of insulin signaling. Other
negative feedback control mechanisms operate on a longer time scale, and
involve a reduction in the cellular content of insulin receptor and its substrates.
Inhibition of insulin signal represents a new area of research that might shed
light to the molecular mechanisms involved in the physiopathology of type-2
diabetes [24].
6. Molecular mechanisms of metabolic insulin actions
Insulin-mediated glucose transport
In fed normoglycemic conditions the transport of glucose into the skeletal
muscle cells represents the main pathway for glucose uptake [5]. In skeletal
muscle and adipose tissue, GLUT1 mediates basal glucose transport, whereas
GLUT4 is responsible for insulin-stimulated glucose uptake. In the basal state,
GLUT4 is primarily intracellular. Insulin stimulates the translocation of
GLUT4-vesicles to the plasma membrane, where they facilitate glucose
uptake. [25]. The molecular mechanism by which insulin causes redistribution
of GLUT4 has been extensively studied. Two pathways appear to be involved:
The first, and more established, pathway is PI3K/Akt. However, activation of
PI3K is insufficient for insulin-stimulated glucose transport. A second pathway
indicates that the CAP (Cbl-associated protein)/Cbl complex, may play a role
in glucose uptake [26].
Recent studies have revealed the importance of the actin cytoskeleton in
insulin-induced GLUT4 translocation [27]. Actin is an exceedingly complex
molecule in terms of its dynamics and interactions with other proteins. The
insulin-induced dynamic actin rearrangements necessary for GLUT4
translocation apparently result from a combination of the PI3K signal and
spatially compartmentalized TC10 signal since both are activated by insulin
stimulation [28].
Insulin-mediated glycogen synthesis
Insulin stimulates glycogen accumulation through a coordinated increase
in glucose transport and glycogen synthesis. The hormone activates glycogen
synthase by promoting its dephosphorylation, through the inhibition of
kinases such as PKA or GSK-3 [29], and activation of protein phosphatase 1
(PP1). [30]. Insulin, via PI3K/Akt, phosphorylates and inactivates GSK-3,
decreasing the rate of phosphorylation of glycogen synthase, thus increasing
its activity state. On the other hand, the mechanism by which insulin
activates glycogen-associated PP1 remains unknown, inhibitors of PI3K block
Molecular basis of type-2 diabetes
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this effect, suggesting that PtdIns(3,4,5)P3-dependent protein kinases are
involved.
Insulin-mediated lipid synthesis
Insulin induces the transcription of critical enzymes of lipid synthesis. The
transcriptional factor, sterol regulatory element binding protein I c (SREBP1c), has a pivotal role on lipogenic gene expression, and has been proposed as
a key mediator of insulin transcriptional effects [31]. Insulin stimulates the
transcription of SREBP-1c [32], this effect of insulin on SREBP-1c appears to
involve PI3K/Akt pathway [33]. Dominant negative forms of SREBP-1c can
block expression of lipogenic genes, whereas overexpression can increase their
transcription effects [31].
Alongside the anabolic actions of insulin (i.e. glycogen synthesis,
lipogenesis) in the fed state, the hormone has restricted functions on glucose
output and lipolysis [6,7].
Insulin repression of glucose output
Insulin inhibits the production and release of glucose by the liver by
blocking gluconeogenesis and glycogenolysis. This occurs through a direct
effect of insulin on the liver, as well as by indirect effects of insulin on
substrate availability, such as free fatty acids, lactate and aminoacids [21].
Insulin inhibits the transcription of the gene encoding phosphoenolpyruvate
carboxylase, the rate-limiting step in gluconeogenesis [34]. The hormone also
decreases transcription of the genes encoding fructose-1, 6-bisphosphatase and
glucose-6-phosphatase. The mechanism is produced via phosphatidylinositol
3-kinase (PI3K) and Akt through phosphorylation of the transcription factor
Foxo 1 [6]. In the fasting state, Foxo1 is largely dephosphorylated and is localized
in the nucleus, where it binds the peroxisome proliferator–activated receptorcoactivator (PGC)-1 [35] and Cbp/p300 [36] to promote phosphoenolpyruvate
carboxylase and glucose-6-phosphatase transcription. Insulin-dependent
phosphorylation via AKT dissociates the Foxo1/PGC-1 complex and
causes Foxo1 nuclear export and consequently inhibition of target gene
expression [6].
Insulin repression of lipolysis
The hormone strongly inhibits lipolysis in adipocytes, through the
inhibition of the hormone-sensitive lipase [37]. Insulin inhibits the activity of
the lipase primarily through reductions in cAMP levels, owing to the activation
of a cAMP-specific phosphodiesterase 3B by the serine-threonine kinase Akt
[38]. The low concentration of insulin required to inhibit the hormone-sensitive
lipase may explain why patients with mild type-2 diabetes and glucose
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intolerance are hyperglycemic in the absence of significant elevations of
plasma free fatty acids or ketone bodies.
7. Adipose tissue
Adipose tissue is a unique tissue in fuel metabolism in several ways: a) it
stores and releases excess energy in the form of lipids, modifying its size and
number in accordance with changing metabolic needs; b) it releases free fatty
acids that inhibit glucose uptake, glucose oxidation and glycogen synthesis and
stimulate hepatic glucose output [39-42]; c) adipose metabolism, specifically
glucose transport mediated by GLUT4, is capable of directing insulin action in
peripheral tissues [43]; d) moreover, adipose tissue is capable of integrating
signals from other organs and responds by regulating secretion of multiple
proteins, termed adipokines [44] that interfere with insulin action. Cytokines
such as, TNF-alpha, IL-6, and resistin decrease insulin action. On the contrary,
adiponectin and visfatin possess a positive effect on insulin action [For review
see 44].
8. Metabolism in type-2 diabetes
Type-2 diabetes is characterized by impaired insulin action and/or
abnormal insulin secretion. An early abnormality in the disease is insulin
resistance; a defective state in which insulin is unable to exert its biological
effects at circulation concentrations that are effective in normal subjects.
Insulin resistance has been proposed as the key linking factor for the metabolic
syndrome disease cluster of glucose intolerance, hypertension and
dyslipidemia.
Insulin resistance leads to profound decreases in glucose uptake and
glycogen synthesis in peripheral tissues [45, 46]. Impaired hepatic glycogen
stores and glycogen synthase activity are also observed in insulin resistance [42,
47]. Insulin resistance yields to defective suppression of hepatic glucose output,
under the fasting as well as the fed state [6]. Resistance to the antilipolytic action
of insulin also favors triglyceride breakdown in adipose tissue and the generation
of free fatty acids, which inhibit insulin-stimulated glucose uptake and
metabolism in skeletal muscle, stimulate hepatic gluconeogenesis [41,42] and
interfere with insulin receptor signals [21]. Changes in serum adipokine
concentrations are also part of the insulin resistant state [44].
At the pre-onset of type-2 diabetes, resistance to the glucose-lowering
action of insulin, tends to lead a slight increase of blood glucose concentration,
which stimulates insulin secretion and causes hyperinsulinemia. Initially
hyperinsulinemia is able to overcome insulin resistance. The diabetic state
develops when insulin secretion cannot longer be sustained to compensate
insulin resistance, and it is at this stage that fasting and post-prandrial
hyperglycemia is apparent.
Molecular basis of type-2 diabetes
99
9. Molecular mechanisms affected in type-2 diabetes
Insulin resistance
It is now well accepted that defective postreceptor insulin signaling is the
main feature involved in insulin resistance of type-2 diabetes. As a result the
metabolic insulin actions are affected. Several mechanisms such as tyrosine
dephosphorylation, imbalance of serine/threonine phosphorylation, or insulin
receptor internalization impair insulin signaling [24]. A number of molecules
associated to insulin resistance affect these mechanisms, and most of them are
related to adipose tissue. Transcriptional factors have also been found to be
associated with insulin resistance.
Adipose tissue
Dysfunction of adipose tissue plays a crucial role in the development of
insulin resistance in type-2 diabetes. Both obesity and lipodystrophy lead to
insulin resistance in muscle. Impaired glucose transport by GLUT4 in adipose
tissue results in insulin resistance in the muscle and liver [43]. Emerging data
suggest that molecules released from adipocytes, such as free fatty acids, TNF
alpha, interleukin 6, inhibit insulin signaling and induce insulin resistance, and
activate serine/threonine kinases that phosphorylate the IRS proteins and
inhibit their function [48, 44].
Free fatty acids. At molecular level, elevated FFAs are associated with a
reduction in insulin-stimulated IRS-1 phosphorylation and IRS-1-associated
PI3K activity [21]. Free fatty acids activate cellular kinases, including atypical
protein kinase C isoforms by increasing cellular diacylglycerol concentrations,
which can activate the inflammatory kinase inhibitor kB (IKK) and c.jun Nterminal kinases, increasing serine/threonine phosphorylation of IRS-1 and
reducing downstream IRS-1 signalling [49].
Interleukin 6 (IL-6). In insulin resistance states, a two- to threefold
elevation of circulating IL-6 has been observed. IL-6 decreases tyrosine
phosphorylation of the IR substrate (IRS)1, and decreases association of the
p85 subunit of phosphatidylinositol 3-kinase with IRS-1 in response to
physiologic insulin levels. In addition, insulin-dependent activation of Akt, is
markedly inhibited by IL-6 treatment [50]. These events are mediated through
increases in the expression of the suppressor of cytokine signaling-3 (SOCS-3)
protein. [51].
Tumor necrosis alpha (TNF-alpha). TNF-alpha is a cytokine produced
by adipocytes that has been widely implicated as a causative factor in obesityassociated insulin resistance and the pathogenesis of type-2 diabetes [revised in
52]. Multiple mechanisms have been suggested to account for the metabolic
effects of TNF-alpha. These include the induction of elevated free fatty acids
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via stimulation of lipolysis, downregulation of genes that are required for
normal insulin action, such as GLUT4, direct effects on insulin signaling, and
negative regulation of PPAR gamma [revised in 52].
Peroxisome proliferator activated receptor gamma (PPAR gamma).
PPAR’s are nuclear receptors that heterodimerize with the retinoic X receptor
and regulate transcription of a number of genes. PPAR-gamma, in particular,
has been shown to be involved in regulating genes involved in insulin action,
such as, SREBP-1c and PEPCK [53,54]. In addition, PPAR activation inhibits
leptin gene expression, as well as the expression of TNF-alpha, which, in turn,
is an inhibitor of PPAR gene expression [55]. The notion that PPAR’s play an
important role of type-2 diabetes is supported by the recent description of
families whose mutations in PPAR caused severe insulin resistance and
diabetes [56].
Fuel oxidation
Several investigations have found that mitochondrial function might be
involved in the pathogenesis of insulin resistance and type-2 diabetes [revised
in 57]. Microarray studies in obese and type-2 diabetic subjects found
decreased gene expression of genes regulated by the transcriptional factor
peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC-1
alpha) [58], a transcriptional factor that plays an important role in mitochondria
biogenesis. Furthermore, the activity of mitochondrial oxidative enzymes was
found to be lower in type-2 diabetic patients [59]. These data support the
concept that insulin resistance arises from defects in mitochondrial fatty acid
oxidation, which, in turn, lead to increases in fatty acid metabolites, such as
fatty acyl CoA and diacylglycerol) that disrupt insulin signaling [57].
Additional evidence of the importance of PGC-1 in metabolic regulation is its
role in the regulation of hepatic gluconeogenesis genes such as PEPCK and
glucose-6-phosphate phosphatase, and its molecular interaction with diabetogenic
agents such as glucocorticoids [6, 60]. Defects in fuel oxidation are also involved
in impaired insulin secretion and in diabetic genetic defects (see below).
Insulin secretion
Type-2 diabetes arises when pancreatic beta cells fail to secrete sufficient
insulin to andle with the insulin resistance demand, because of acquired betacell secretory dysfunction and/or decreased beta-cell mass. Recent studies have
presented evidence that the beta-cell mass plays a pivotal role in determining
whether an individual will progress to type-2 diabetes [61]. These defects may
be caused by primary beta-cell defects, such as seen in the monogenic diabetes
forms of MODY (see below in Genetic Factors in Diabetes Section), or by
secondary beta-cell defects, caused by glucotoxicity, increased free fatty acids,
cytokines, mitochondrial disfunction and/or metabolic stress.
Molecular basis of type-2 diabetes
101
Beta-cell mass
Pancreatic beta mass is regulated by: a) beta-cell replication, b) betacell size, c) beta-cell neogenesis, and d) beta-cell apoptosis. The beta-cell
mass adapts to an increased metabolic load caused by insulin resistance.
The onset of type-2 diabetes is accompanied by a progressive decrease in
the beta-cell mass that arises from a marked increase of beta-cell apoptosis
which prevails over beta-cell replication and neogenesis [61]. Studies in
IRS-2 knockout mice have provided clues for the important role of IRS-2
on beta-cell mass regulation [18]. Since then, several lines of evidence
support a role of IRS-2 in the pathogenesis of type diabetes [revised in 61].
Similar mechanisms to those produced by cytokines and free fatty acids on
insulin signaling in the peripheral tissues (see above, Insulin Resistance)
affect IRS-2 activity in beta cells and could account for the acquired betacell secretory dysfunction.
Glucotoxicity
Deterioration of insulin secretion over time is the usual course in most
type-2 diabetic patients, and many patients will end with more or less severe
insulin deficiency after about 10 years of diabetes [62]. Hyperglycemia itself
decreases insulin secretion and is implicated in the damage of beta cells [63].
Chronic hyperglycemia impairs insulin gene expression of two major beta-cell
transcription factors, pancreatic-duodenum homeobox-1 (PDX-1) and the
activator of the rat insulin promoter element 3b1 [64]. The molecular
mechanisms of glucotoxicity have been proposed to involve generation of
chronic oxidative stress [63]. Furthermore, a new study demonstrates that
hyperglycemia-induced mitochondrial superoxide production activates
uncoupling protein 2, which decreases the ATP/ADP ratio and thus reduces the
insulin-secretory response [65]. Additionally, reactive oxygen species are
known to enhance NFk-Beta activity, which potentially induces beta-cell
apoptosis, and might account for glucotoxicity [61].
10. Genetic factors
After the elucidation of Mendelian disorders with diabetes as a major
phenotypic feature it has become clear that type-2 diabetes is heterogeneous
and may result from defects in one or more molecular pathways. Genetic
defects of the beta cell, usually referred to as maturity-onset diabetes of the
young (MODY), can result from mutations in any of at least six different genes
(see Table 1). Most of the MODY subtypes are caused by mutations in
transcription factors, which are involved in the tissue-specific regulation of
gene expression in the liver and in pancreatic beta-cells. Other related genetic
factors are due to insulin receptor mutations [66].
102
Cristina Fernandez-Mejia
Genetic defects of the beta cell
HNFs. HNF-1 alpha and HNF-1 Beta are members of the homeodomaincontaining family of transcription factors and HNF-4 alpha is an orphan
nuclear receptor. They involve part of a network of transcription factors that
controls gene expression during embryonic development and in the adult
tissues in which they are co-expressed. In the pancreatic beta cell, they regulate
the expression of insulin as well as proteins involved in glucose transport,
glycolysis and mitochondrial metabolism, all of which are important in the
regulation of insulin secretion [67].
IPF-1. is a homeodomain-containing transcription factor involved in
pancreatic development [68] transcriptional regulation of a number of critical
beta-cell genes including insulin, glucokinase and glucose transporter 2
(GLUT2), all involved in glucose-induced insulin secretion.
NeuroD1/BETA2. The basic helix−loop−helix transcription factor
NeuroD1/BETA2 participates in the regulation of transcription of the insulin
gene and is required for normal pancreatic islet development [69]. Mutations in
NeuroD1 are a rare cause of MODY and result in reduced serum insulin
concentrations [70].
Glucokinase. MODY-2 is due to mutations in the glucokinase gene.
Glucokinase is a tissue-specific enzyme expressed in liver, in pancreatic beta
cells and in certain neuroendocrine cells of the brain and gut [71] and it plays a
key-role in glucose homeostasis. In the liver, its activity is critical for glucose
uptake and glycogen synthesis in the post-prandrial state [72]. In beta cells,
glucokinase plays a key-role regulating insulin secretion in response to glucose
[9]. The combination of reduced glucose-induced insulin secretion from the
pancreatic beta cell and reduced glycogen storage in the liver leads to an
increase in plasma glucose concentrations.
Mitochondrial DNA. Abnormal mitochondrial function resulting from
mutations in the mitochondrial genome can lead to diabetes [73]. The most
common diabetes-associated mutation is an A-to-G transition in the
mitochondrial tRNALeu(UUR) gene at base pair 3,243. This results in defects
in insulin secretion including failure of glucose to prime the insulin secretory
response and abnormal insulin secretory oscillations [74]
Genetic factors in polygenic type-2 diabetes
Considerable efforts aiming to identify genes responsible for more
common, polygenic forms of type-2 diabetes have been underway for the last
two decades, however, the result of these investigations has been largely
disappointing. Sequence differences in a few genes have been associated, so
far, with complex, polygenic forms of type-2 diabetes [49]. Among the
variants identified are: PPAR gamma, glycogen synthase, IRS-1, proinsulin,
sulfonylurea receptor1, potassium inward rectifier 6.2, PGC1 alpha, and
Molecular basis of type-2 diabetes
103
calpain 10 a ubiquitously expressed protein protease which plays roles in
membrane fusion [75] and hydrolyzes various proteins that participate in
cellular signaling, such as kinases, receptors, and transcription factors. Further,
calpain is also important for differentiation of preadipocytes into adipocytes.
These facts raise the possibility that calpain modulates both insulin secretion
and action. Because many steps could be modulated by calpain, further precise
experiments are essential to clarify the molecular and physiological mechanism
explaining the association of calpain with type-2 diabetes. [75].
Conclusions
Classical physiology studies, biochemical methods, tissue culture
technology, the use of gene targeting approaches in mice, and naturally
occurring mutations in patients have shed some light on the molecular causes
of type-2 diabetes and have contributed to a deeper understanding of the
underlying mechanisms involved in the disease. With some variations, the
observations of Thomas Willis [2] remain true until today. Now we can write
that diabetes is rare in rural life, but in our age, given sedentary life style and
gusling down chiefly of soda and junk food, we meet with examples and
instances enough, I may say daily, of this disease. As to what belongs the cure,
it seems a most hard thing of this disease to draw propositions for curing, for
this cause lies hid inside the molecular mechanisms of insulin signaling and
insulin secretion.
Acknowledgments
We would like to thank Isabel Pérez Montfort for correcting the English
version of the manuscript and to Dr. Silvestre Frenk for critical reading of the
manuscript. Supported by research grants from the Consejo Nacional de
Ciencia y Tecnologia 44266M and the Direccion General de Asuntos del
Personal Academico IN208605.
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