Chapter Intech
Chapter Intech
Chapter Intech
1. Introduction
The major disease epidemics of modern society are not those of contagion, but are the result
of lifestyle imposed upon our genetic pre-disposition. Unrestricted access to calorie-dense
food, along with a reduction in physical activity, has resulted in a rapid rise in metabolic
disorders. One such condition, type 2 diabetes (T2D), has increased dramatically in recent
times, with the International Diabetes Foundation estimating that 371 million people world
wide have T2D, with this number expected to increase to greater than 550 million by 2030
(http://www.idf.org/diabetesatlas/5e/Update2012). T2D is characterized by fasting blood
glucose levels higher than 7.0 mM or two-hour blood glucose levels higher than 11.1 mM after
a glucose tolerance test. T2D rarely occurs in isolation and is frequently associated with a
number of comorbidities, including obesity, dyslipidemia, cardiovascular disease, and
inflammation, collectively referred to as the metabolic syndrome.
A central aspect of the disorders comprising the metabolic syndrome is insulin resistance;
defined as an impaired ability for insulin to regulate fuel metabolism in target tissues. With
respect to glucose homeostasis the main insulin-responsive tissues involved are skeletal
muscle, liver and adipose tissue. Under normal physiological conditions, insulin is released
into the circulation from the beta cells in the islets of Langerhans in the pancreas in response
to the ingestion of a meal. Upon binding to its receptor, insulin stimulates a well-described
signaling cascade [1] involving the phosphorylation, docking and translocation of a series of
signaling molecules, ultimately leading to alterations in specific endpoints of glucose and lipid
metabolism (Figure 1):
In skeletal muscle, insulin promotes the translocation of the glucose transporter GLUT4 to
the plasma membrane to increase glucose uptake and also stimulates glycogen synthesis.
The major hepatic actions of insulin are the promotion of glycogen and lipid synthesis and
the suppression of gluconeogenesis.
2013 Turner; licensee InTech. This is an open access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
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In adipose tissue, insulin stimulates GLUT4-mediated glucose uptake and lipid synthesis,
and additionally represses lipolysis, leading to net lipid accumulation.
IRS, insulin receptor substrate; SHC, Src Homology 2 domain; GRB2, growth factor receptor-bound protein 2; ERK, ex
tracellular-signal-regulated kinases or classical MAP kinases; PI3K Phosphoinositide 3-kinase; PDK1, phosphoinositidedependent protein kinase 1; mTORC mammalian target of rapamycin complex; FoxO1 Forkhead box protein O1;
SREBP1c sterol regulatory element binding protein 1c; GSK-3, glycogen synthase kinase 3; AS160, 160 kDa Akt sub
strate.
Figure 1. Insulin signaling pathway. Binding of insulin to the insulin receptor initiates a signaling cascade that involves
multiple phosphorylation events (green circles) and leads to alterations in glucose and lipid metabolism.
In the insulin resistant state, the effect of insulin on the above pathways is compromised,
leading to insufficient uptake of glucose into tissues and an impaired suppression of hepatic
glucose output. To overcome the diminished effectiveness of insulin, the pancreatic beta cells
secrete more insulin. The ensuing hyperinsulinemia can adequately compensate for the insulin
resistance in most of the population, however in genetically susceptible individuals, the beta
cells ultimately fail in the face of the increased workload and this leads to elevated blood
glucose levels and T2D. Thus insulin resistance can be considered a very early and important
player in the pathogenesis of T2D.
At the molecular level, the precise mechanisms responsible for insulin resistance are not fully
elucidated. Studies have reported overactivation of stress-related and inflammatory pathways
in tissues of insulin resistant humans and rodents. For example, ER stress was shown by the
Hotamisligil lab to be present in the liver of obese mice and subsequent studies using chap
erones that reduce ER stress revealed improvements in metabolic homeostatsis [2,3]. Oxidative
stress has also been implicated in the development of insulin resistance, with studies showing
elevated reactive oxygen species generation in insulin resistant cell models, rodents and
humans [4-6]. Finally, inflammation in adipose tissue and liver (and to some extent muscle)
has been reported in obese, insulin-resistant humans and rodents [7,8]. While the above factors
are often described as causative players in the development of insulin resistance, it still remains
unresolved whether they are the primary factors leading to diminished insulin action, or if
they arise as a consequence of insulin resistance.
One factor that is one of the earliest defects associated with insulin resistance and T2D is lipid
accumulation in non-adipose tissues [9-13]. Under conditions of excess nutrient supply, fatty
acids and their metabolites inappropriately spillover into tissues such as skeletal muscle, liver
and the heart, precipitating defects in insulin action. More specifically, while elevated trigly
cerides are frequently reported in tissues of insulin resistant humans and rodents, the accu
mulation of metabolically active long chain acyl-CoAs (LCACoAs) and other cytosolic lipid
metabolites, such as ceramides and diacylglycerol (DAG), are considered to be more directly
linked with insulin resistance [9,10]. In support of this, the above lipid metabolites can activate
many pathways and factors (e.g. protein kinase C, c-jun N-terminal kinase (JNK), reactive
oxygen species, the nuclear factor B (NFB) pathway, protein phosphatase A2 (PPA2) and
cytokines) that directly antagonize insulin signal transduction and glucose metabolism
pathways [9,10].
The extent of lipid accumulation within any given tissue is determined by several factors.
Under conditions of elevated lipid availability, enhanced uptake of fat into tissues contributes
to greater lipid deposition [14,15]. This increased uptake is associated with greater expression
and/or translocation of fatty acid transport proteins (e.g. CD36). Any impairment in the
utilization (oxidation) of lipids would also be predicted to increase partitioning of lipids into
storage pools. Indeed, over the last decade a popular theory has emerged suggesting that
defects in mitochondrial oxidative metabolism, particularly in skeletal muscle, lead to obesity
and lipid accumulation and thus may play an important role in the pathogenesis of insulin
resistance and T2D [16].
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or FADH2) are generated from glycolysis, the TCA cycle and -oxidation. When NADH and
FADH2 are oxidized to NAD+ or FAD, electrons pass along the mitochondrial electron
transport chain coupled to the pumping of protons into the intermembrane space through
complex I, III and IV. The electrons are transferred to oxygen at complex IV to produce H2O.
The pumped protons generate an electrochemical gradient across the inner mitochondrial
membrane, which is used as the driving force for the ATP synthase (complex V) to produce
ATP. The electrochemical gradient may also dissipate through uncoupling proteins (UCP),
producing heat in a process referred to as thermogenesis.
TAG Triacylglycerol; DAG Diacylglycerol; PDH Pyruvate dehydrogenase; CPT Carnitine palmitoyltransferase; UCP Un
coupling Protein
Figure 2. During the oxidative metabolism of glucose and fatty acids, reducing equivalents (NADH or FADH2) are gen
erated from glycolysis, the TCA cycle and -oxidation. When NADH and FADH2 are oxidized to NAD+ or FAD, electrons
pass along the mitochondrial respiratory chain while protons are pumped into the intermembrane space through
complex I, III and IV. The electrons are transferred to oxygen at complex IV to produce H2O. The pumped protons gen
erate an electrochemical gradient across the inner mitochondrial membrane, which is used as the driving force for ATP
synthase (complex V) to produce ATP. Protons can also enter the matrix through uncoupling proteins. Deficiencies in
mitochondrial fatty acid oxidation can lead to the buildup of bioactive lipid intermediates (red circle) that can cause
insulin resistance.
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have been shown to be acetylated, and reversible lysine acetylation/deacetylation has been
shown to impact on the activity of a large range of mitochondrial enzymes involved in virtually
all metabolic pathways within this organelle [61]. In addition to phosphorylation and acety
lation, numerous other PTMs (e.g. glutathionylation, nitrosylation, succinylation) have been
described as being present in mitochondria [62-64], and their functional importance remains
to be determined.
2.5. Mitochondrial dynamics
Mitochondria are not static organelles, but exist largely as a reticular network. Mitochondria
are constantly engaged in the process of fusion and fission, providing morphological plasticity
to allow adjustments in response to the prevailing cellular stresses and metabolic requirements
[65]. Mitochondrial fusion is mediated by the mammalian GTPases mitofusin 1 and mitofusin
2, as well as optic atrophy protein 1 (Opa1). Fusion occurs in a two-step process, which initially
involves fusion of the outer membrane (mediated by mitofusins), followed by subsequent
fusion of the inner membrane (driven by Opa1) [66,67]. Fission is regulated by another GTPase,
dynamin-related protein 1 (Drp1), which resides in the cytosol and is recruited to the mito
chondrial surface to engage other key components of the fission machinery (e.g. Fis 1) [68,69].
The fusion process is thought to allow two mitochondria to functionally complement each
other through the exchange and repartitioning of their respective components (e.g. copies of
the genome, metabolic enzymes and metabolites). Fission on the other hand is important both
in the separation of the organelle into daughter cells during cell division and also in isolating
and targeting damaged mitochondria for degradation. Collectively the balance of fusion and
fission allows mitochondria to form a spectrum of shapes from small individual units to
elongated interconnected networks.
In muscle cells, the mitochondrial network is arranged into two discrete, but interconnected
pools the subsarcolemmal (SS) pool near the cell surface, and the intermyofibrillar (IMF) pool
in the interior of the cell between myofibres [70-72]. These two pools of mitochondria have
been reported to display some differences in their metabolic characteristics, with SS mito
chondria appearing to be more responsive to increase their oxidative capacity following an
exercise stimuli than IMF mitochondria [57,73]. Despite the differences between mitochondrial
pools, it has been proposed that the arrangement of mitochondria may important for efficient
mitochondrial function; SS mitochondria have greater access to oxygen and metabolic
substrates, and the proton gradient generated through substrate oxidation in the SS pool may
potentially contribute fuel ATP synthesis in the IMF pool, where energy demands are highest
during contraction [71].
mitochondrial theory of insulin resistance has developed over the last 10-15 years and is based
on the notion that defective mitochondrial metabolism will result in inadequate substrate
oxidation, leading to a buildup of lipid metabolites and the subsequent development of insulin
resistance. Support for this theory comes from many studies in humans and rodents, which
have largely examined skeletal muscle and are reviewed below.
In the late 1990s and early part of last decade, several groups published studies showing that
muscle from obese and insulin resistant subjects displayed reduced oxidative enzyme activity
[74-76]. Some of these studies also examined lipid oxidation either in muscle homogenates, or
by making RQ measurements across the leg, and it was shown that fatty acid oxidation was
also decreased in obese, insulin resistant subjects compared to age-matched controls, poten
tially suggesting that defects in mitochondrial metabolism may be involved in the develop
ment of obesity and insulin resistance [74,75]. In 2002, Kelleys group showed that there was
lower NADH:O2 oxidoreductase activity and reduced mitochondrial size, as determined by
electron microscopy, in muscle of obese subjects with insulin resistance and/or T2D compared
to controls [77]. A year later, two influential microarray studies were published, reporting a
coordinated downregulation of genes involved in mitochondrial biogenesis and oxidative
phosphorylation in subjects with T2D and non-diabetic individuals with a family history (FH
+) of T2D [78,79]. These microarray studies were considered particularly important, as they
documented a reduction in the master regulator of mitochondrial biogenesis, PGC-1, and
thus they provided a mechanism for the reduced oxidative gene expression. They were also
important, as they showed that abnormal mitochondrial gene expression could be observed
in insulin resistant relatives of patients with T2D and thus may be a pathogenic factor in the
pre-diabetic state. Overall the conclusion from these studies was that depressed PGC-1
levels, due to genetic predisposition, physical inactivity, or excessive caloric intake, could lead
to a reduction in mitochondrial content, predisposing individuals to develop insulin resistance
and T2D.
In the ensuing decade since these landmark studies were published, there has been intense
research into this field and one issue that has arisen is how to best measure mitochondrial
function/dysfunction. Numerous approaches have been employed, including measurements
of parameters in frozen muscle samples (e.g. mRNA, protein content, oxidative enzyme
activity and mtDNA), functional assessment of substrate oxidation in fresh samples (e.g.
radiolabelled fatty acid oxidation, mitochondrial respiration measurements) and non-invasive
magnetic resonance spectroscopy (MRS) with 31P or 13C to determine in vivo ATP synthesis
rates, phosphocreatine resynthesis rates or TCA cycle activity as an index of mitochondrial
function. All these assays provide some indication of mitochondrial function, however they
may not always correlate with each other and this needs to be considered when interpreting
studies in this area. Details of a number of key studies in this area are presented in the following
sections.
In line with the microarray studies noted above, mRNA levels for a variety of mitochondrial
genes have been shown to be reduced in muscle biopsies obtained from various insulin
resistant populations, including lean insulin-resistant offspring of patients with T2D [80],
obese subjects [81], patients with polycystic ovarian syndrome [82] and subjects with estab
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lished T2DM [83,84]. The level of mtDNA was also shown to be lower in both obese, insulin
resistant subjects and obese subjects with T2D [85,86]. Heilbronn et al. [81] demonstrated
reduced protein expression of respiratory chain subunits in obese insulin-resistant subjects
and consistent with these findings, a recent proteomics study comparing lean, obese and T2D
subjects, showed patterns of reduced mitochondrial proteins in the insulin-resistant subjects
[87]. The activity of specific enzymes involved in oxidative pathways have been reported to
be lower in various insulin-resistant populations [81,86,88,89] and additionally electron
microscopy studies have reported reduced mitochondrial size and density in insulin-resistant
muscle [77,80,86]. Interestingly, in the studies reporting mitochondrial deficiencies, there has
been disparate results regarding which population of mitochondria may underlie the func
tional defects. Ritov et al. [86] reported that the number and functional activity of subsarco
lemmal mitochondria was reduced in obesity and T2D, while a more recent study found similar
subsarcolemmel mitochondrial content in lean controls, lean insulin-resistant non-diabetic
subjects and insulin-resistant T2D subjects, however intermyofibrillar mitochondrial content
was reduced in the latter two groups [90]. Differences in mitochondrial function may not only
be present within different intramuscular populations, but also between different muscles
across the body. Rabol and colleagues used high resolution respirometry to measure mito
chondrial function in saponin-permeabilised fibres from m. deltoideus and m. vastus lateralis
and observed reduced respiratory capacity only in the leg muscles of type 2 diabetic subjects
compared to lean controls [91].
In addition to the above studies, several investigators have also measured in vivo mitochondrial
function using MRS. Petersen et al. [92] studied lean, healthy elderly subjects using hyperin
sulinemic-euglycemic clamps and MRS measurements and observed marked insulin resist
ance in skeletal muscle of the elderly subjects compared to weight-matched controls. This
impairment in insulin action was associated with a 40% reduction in ATP synthesis capacity,
and a pronounced accumulation of intramuscular fat. The same group published a paper the
following year in which they studied lean insulin-resistant offspring of patients with T2D using
the same methods. The insulin-resistant offspring displayed a 60% reduction in insulinstimulated glucose uptake into muscle and this was again associated with increased intra
myocellular lipid and reduced basal mitochondrial ATP synthesis capacity [93]. A subsequent
study by Petersen et al. [94] also reported reduced-insulin-stimulated ATP synthesis in firstdegree relatives of subjects with T2D and in later work it was shown that family history of T2D
was associated with reduced TCA cycle flux [95]. In several other studies, patients with T2D
have been shown to have reduced ATP synthesis capacity or phosphocreatine recovery rates,
indicative of reduced mitochondrial function in these populations [96-99]. A further interesting
case report using MRS showed that a MELAS patient with mtDNA mutations, displayed
insulin resistance in muscle association with reduced baseline and insulin-stimulated ATP
synthesis capacity [100].
A number of investigations have sought to determine if there is an intrinsic difference in the
functional capacity per mitochondrion that may underlie the reductions in mitochondrial
function reported with MRS. Some studies examining respiration or fatty acid oxidation in
isolated mitochondria or permeabilised muscle fibres, have reported that the functional
capacity per mitochondrion in insulin resistant and/or type 2 diabetic subjects is similar or only
very mildly reduced [85,88,91,101-103] in insulin-resistant individuals, but when normalized
to muscle mass, a substantial reduction is seen in insulin-resistant subjects [85,88,101]. These
studies therefore only see marked differences when mitochondrial capacity is expressed per
unit mass of skeletal muscle and thus indicate that in vivo mitochondrial defects observed with
MRS may be more strongly related to reductions in mitochondrial number, than to substantial
intrinsic mitochondrial defects. However, an elegant study by Phielix et al. [97] measured both
in vivo mitochondrial function (with MRS) and ex vivo mitochondrial respiration in muscle
from the same patients with T2D and they reported that in this population of subjects, the in
vivo defects in mitochondrial function could be attributed to impairments in intrinsic mito
chondrial substrate oxidation. Another study from this group also observed similar differences
in intrinsic mitochondrial function in T2D patients compared to BMI-matched controls [96].
One limitation of the aforementioned studies is that they only provide static measurements of
different populations at a given time and are unable to delineate whether the observed defects
in mitochondrial metabolism are primary drivers of insulin resistance or arise as a consequence
of decreases in insulin action. In this regard, intervention studies in rodents and humans have
provided some experimental evidence that manipulations which result in declines in insulin
action, are also associated with mitochondrial dysfunction. For example, infusion of fatty acids
into humans for 648h to mimic the effects of chronic lipid overload resulted in a robust
induction of whole-body insulin resistance and reduced insulin-stimulated ATP synthesis
rates and expression of mRNA encoding PGC1 and other mitochondrial genes in muscle
[104-106]. In healthy male subjects, high-fat feeding for 3 days was sufficient to reduce mRNA
levels of PGC1, PGC-1 and several other mitochondrial genes in skeletal muscle [107].
Similarly, genetic, or high-fat diet-induced obesity and insulin resistance in rodents has been
reported by several groups to reduce mitochondrial gene expression, protein expression and
mitochondrial respiration in skeletal muscle [107-111]. Providing additional evidence of a link
between mitochondrial dysfunction and insulin resistance is the fact that antiretroviral therapy
used to suppress human immunodeficiency virus infection causes insulin resistance in
association with mtDNA copy number [112]. Collectively, the above studies illustrate that
there are many instances where defects in mitochondrial metabolism and impairments in
insulin action occur in conjunction with each other in skeletal muscle.
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A range of different parameters have been studied in rodents and humans with respect to liver
mitochondrial metabolism. The collective findings indicate that the liver appears to be able to
adapt to an excess of lipid by upregulating fatty acid oxidative capacity and TCA cycle activity,
but this is not always coupled to a concomitant increase in electron transport chain activity,
and as a consequence reactive oxygen species are produced (see [113] for an excellent review
on the topic). There are also some in vivo MRS studies that have examined indices of mito
chondrial metabolism in individuals with NAFLD and T2D, with the findings generally
indicating mild abnormalities in mitochondrial function in these populations [114-116].
4.2. Adipose tissue
4.2.1. White adipose tissue
White adipose tissue (WAT) serves a principal role as the most important energy store in the
body. However it has become increasingly clear over the last decade that WAT is also an active
endocrine organ, releasing adipokines that influence whole-body energy homeostasis and
insulin action. Mitochondrial content in WAT is low compared to other tissues, however the
diversity of mitochondrial proteins in WAT has been shown to be greater than in muscle and
heart [117]. Intact mitochondrial metabolism is critical for maintaining normal WAT functions,
such as the appropriate synthesis and secretion of adipokines and cycling reactions involved
in lipid synthesis [118].
WAT mitochondrial content has been reported to be reduced in insulin-resistant humans and
rodents. In women with T2D, electron transport chain genes were shown to be downregulated
in visceral WAT independently of obesity and perhaps as a consequence of TNFalpha-induced
inflammation [119]. In obese humans, mtDNA copy number was reported to be lower than in
control subjects and was directly correlated with basal and insulin-stimulated lipogenesis
[120]. In rodent models of genetic or dietary-induced obesity and insulin resistance, there are
reductions in mtDNA copy number, mitochondrial density and mitochondrial OXPHOS
activity [121-123]. Administration of thiazolidinediones promotes mitochondrial biogenesis in
WAT in animals and humans, in conjunction with improved whole-body insulin sensitivity
[46,123], suggesting that specific changes in WAT mitochondrial metabolism in obesity and
T2D, may be imparting whole-body metabolic consequences. Indeed, recent work has shown
adipose-restricted alterations in mitochondrial activity can have profound effects on global
glucose and lipid homeostasis [124,125].
4.2.2. Brown adipose tissue
Unlike WAT, the principal function of brown adipose tissue (BAT) is energy dissipation, rather
than energy storage. BAT has a high mitochondrial density per gram of tissue, and the unique
presence of uncoupling protein 1 (UCP1) allows brown adipocytes to couple the oxidation of
lipids, not to ATP synthesis, but to heat generation via proton leak across the mitochondrial
inner membrane. Interest in brown adipose tissue has recently soared on the back of 3
important papers published in 2009 that unequivocally demonstrated the presence of func
tional BAT in humans [126-128]. There is an inverse correlation between BAT activity (as
assessed by fluorodeoxyglucose PET) and obesity, suggesting that individuals with low BAT
mitochondrial activity, may be prone to obesity and other metabolic diseases [126,127,129-131].
4.3. Heart
Like skeletal muscle, translocation of GLUT4 in response to insulin occurs in myocardium.
This process is blunted in insulin-resistant humans and animals in association with other
abnormalities in fuel metabolism ([132-134]. With respect to mitochondrial metabolism,
genetic and diet-induced obesity and type 2 diabetes in rodents is associated impaired
mitochondrial function [135-137]. MRS studies in individuals with T1DM, T2DM, obesity and/
or NAFLD have also reported that that there is a decreased ratio of phosphocreatine:ATP in
myocardium, potentially indicating derangements in mitochondrial substrate metabolism in
these populations [138-142].
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of mitofusin 2 (MFN2), which appears to have additional pleitropic effects in cells beyond the
maintenance of the mitochondrial network [148-152], is reduced in the skeletal muscle of obese
insulin-resistant humans, type 2 diabetic humans and diabetic Zucker rats [149,153] and
correlates with the capacity for glucose oxidation [154]. Repression of MFN2 in L6E9 muscle
cells and 10T/2 fibroblasts results in decreased glucose oxidation, cellular respiration, mito
chondrial membrane potential, and causes fragmentation of the mitochondrial network [149]
and liver-specific deletion of MFN2 results in glucose intolerance and impairments in insulin
signaling [155]. Recent work has also shown that mice deficient in the mitochondrial protease
OMA1, display obesity and altered metabolic homeostasis, due to altered processing of the
inner membrane fusion protein OPA1 and disruptions in mitochondrial morphology and fuel
metabolism [156]. It has also been reported that abnormalitieis in mitochondrial fission events
may play a role lipid-induced insulin resistance. In C2C12 muscle cells, palmitic acid (but not
other long-chain fatty acids) was shown to induce mitochondrial fragmentation in conjunction
with insulin resistance and this effect could be blocked by genetic or pharmacological inhibi
tion of Drp1 [157]. Analysis of tissues from ob/ob mice and high-fat fed mice in this study
revealed increased Drp1 and Fis1 levels and pre-treatment of ob/ob mice with the Drp1 inhibitor
Mdivi-1 resulted in a mild improvement in insulin action in these animals. Collectively these
studies suggest that alterations in the equilibrium of mitochondrial fission and fusions events
may play some role in the pathogenesis of insulin resistance.
5.3. Reduced physical activity
Physical inactivity has recently been reported to be as big a risk factor for non-communicable
diseases as smoking, stressing the importance of exercise in metabolic health [158]. Exercise is
one of the major stimuli for mitochondrial biogenesis and chronic inactivity results in decreases
in mitochondrial number in muscle [159]. A number of studies have shown that obesity and
other metabolic disorders are characterised by decreased physical activity levels and elevations
in sedentary behaviour [160-162]. Interestingly the sedentary behaviours (e.g. sitting time) do
not seem to be influenced by changes in weight and have been suggested to be biologically
determined [161]. Given these differences, it is likely that some of the mitochondrial defects
reported in overweight or obese insulin-resistant subjects may be explained, in part, by low
levels of physical activity.
5.4. Genetic and epigenetic factors
There is evidence in the literature that the metabolic phenotype of skeletal muscle may be
strongly influenced by genetic programming. For example, despite being cultured under
similar conditions for several weeks, studies have shown that primary human skeletal muscle
cells in culture display a similar metabolic phenotype (e.g. gene expression and lipid parti
tioning) to that of the donor subject from which they originated [163,164]. Mutations in nuclearencoded genes involved in mitochondrial function (e.g. PGC-1, NDUFB6) have been linked
with insulin action and T2D, as have mtDNA deletions [165,166]. An emerging area of research
is also the regulation of mitochondrial function by epigenetic factors. Barres et al showed that
the promoter of PGC-1a is methylated at non-CpG sites and exposure of primary human
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syndrome in humans [181]. The above studies suggest that altered acetylation of mitochondrial
proteins may associate with insulin resistance and impaired mitochondrial function, and while
further study in this field is required, there is some evidence that other mitochondrial PTMs
may also be altered in insulin resistance and T2D [184,185].
obese subjects enhanced insulin sensitivity, without altering mtDNA, cardiolipin content or
NADH-oxidase activity [192]. Improved insulin sensitivity was reported in insulin-resistant
subjects with a family history of T2D following 7 days of treatment with the anti-lipolytic agent
acipimox, yet mitochondrial gene expression in muscle actually declined in these subjects
[193]. Treatment of diabetic patients with rosiglitazone improved insulin sensitivity, without
altering in vivo mitochondrial function or markers of mitochondrial content [194,195]. Recently
Samocha-Bonet also showed that 28 days of high-fat overfeeding was sufficient to induce
insulin resistance in health humans, without any detectable defects in various markers of
mitochondrial function [6]. Shorter-term overfeeding studies in low birth-weight subjects also
revealed a disconnect between the induction of insulin resistance and the response of mito
chondrial metabolism [196].
6.2. Rodent studies
To complement the studies in humans, a number of investigators have used gene-manipu
lated mice to more directly test whether specifically targeting mitochondrial metabolism,
can induce changes in insulin sensitivity. Mitochondrial oxidative capacity was shown to
be compromised in muscle-specific TFAM knockout mice, however these animals exhibit
ed improved glucose clearance during a glucose tolerance test and maintained insulinstimulated glucose uptake in muscle [197]. TFAM knockout in adipose tissue was recently
shown to protect against diet-induced obesity and insulin resistance, despite causing
abnormalities in mitochondrial function [125]. Similar findings were reported in mice with
liver or muscle-specific deletion of apoptosis-inducing factor. These animals exhibited a
gene expression pattern of mitochondrial oxidative phosphorylation deficiency similar to
that observed in human insulin resistance [78,79], however they were lean and insulinsensitive and did not manifest the usual deleterious effects of a high-fat diet [198]. A number
of groups have also targeted other regulators of mitochondrial function in mice. Due to
their key role in mitochondrial biogenesis, muscle-specific knockout of PGC-1 or loss-offunction mutation of PGC-1 produced the expected decline in markers of mitochondrial
function yet insulin sensitivity in muscle was preserved or in fact slightly enhanced in these
animals compared to wild-type counterparts [26,199]. Two separately generated lines of
muscle-specific PGC-1 transgenic mice have shown predictable increases in many
mitochondrial parameters, but these animals are insulin resistant, potentially due to
excessive fatty acid delivery into muscle [200] or decreased GLUT4 expression [201]. In
other examples of a dissociation between insulin resistance and mitochondrial dysfunc
tion, it has been shown that Zucker diabetic fatty (ZDF) rats display normal in vivo muscle
oxidative capacity and improved activity of enzymes involved in lipid oxidation during the
progressions to insulin resistance and T2D [202], while db/db mice and ob/ob mice have been
shown to exhibit higher mitochondrial oxidative capacity in liver compared to lean control
animals [203,204]. Collectively the above studies clearly demonstrate that targeted manipu
lation of mitochondrial function, does not produce predictable alterations in insulin action.
A caveat to these studies is that in genetically manipulated mice, there is a complete lack
or substantial increase in the content/function of a specific protein and thus caution must
be exercise when interpreting the findings, as the phenotype (or lack thereof) may be
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significant literature showing that alterations in mitochondrial function in muscle and insulin
action are not always correlated. There are a myriad of differences between studies that may
explain these discrepancies, including the particular population of individuals studied and
their ethnic background and physical fitness level, the muscle group examined, the dietary
regime employed in rodent and human studies (e.g. duration of feeding, fat content and
composition) and the particular assay/technique used to assess mitochondrial function.
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action in rodent models of insulin resistance and T2D [226-229]. Recently it was also shown
that 30 days of resveratrol supplementation improved some markers of mitochondrial function
in muscle of obese subjects, in parallel with improved HOMA-IR and reduced hepatic lipid
levels [230]. These effects seem to be limited to metabolically compromised subjects, as
resveratrol did not improve markers of glycaemic control in non-obese women with normal
glucose tolerance [231]. An alternative approach to using direct SIRT1 activators to mimic
calorie restriction, is to increase the intracellular levels of NAD+, the obligate co-factor for the
sirtuin reaction. Indeed two recent studies using different NAD+ precursors, nicotinamide
mononucleotide or nicotinamide riboside, have both reported beneficial metabolic effects on
metabolic homeostasis in animals models of insulin resistance and T2D [232,233].
7.3. Mitochondrial uncoupling
As noted above, energy dissipation or wasting can occur in a process known as mitochondrial
uncoupling. While this occurs naturally through uncoupling proteins, there are also a phar
macological agents that can induce mitochondrial uncoupling, such as DNP (2,4-dintirophe
nol). These uncoupling agents are generally lipophilic weak acids, that cause mitochondrial
uncoupling by transporting protons across the mitochondrial inner membrane into the matrix,
deprotonating and then exiting as anions before repeating the catalytic cycle. Uncoupling
agents have been successfully used in the past as obesity treatments. In the 1930s DNP was
shown to be an effective weight-loss drug in humans, providing an important proof-of-concept
that the stimulation of energy expenditure by uncoupling is not necessarily compensated for
by an increase in caloric intake [234]. Despite its success as an anti-obesity therapy, DNP was
withdrawn from the market in 1938 as it (like most uncouplers) has a narrow therapeutic
window, with overdoses causing serious complications (even death) by compromising cellular
energy homeostasis. Due to the current obesity epidemic, and illicit sales via the internet, it is
alarming to see that DNP has made a comeback as a weight loss agent, with predictable lethal
results [235,236]. Thus, while mitochondrial uncoupling with DNP does not appears to be a
safe weight-loss therapy, other more recently described uncoupling agents may potentially
have a safer profile for use in humans [237-239]. An additional approach may to be to upre
gulate physiological uncoupling in brown adipose tissue, via sympathomimetic agents or
agonists for thyroid hormone or bile acid receptors.
7.4. Natural compounds
Natural compounds are another rich source of potential therapeutics for obesity and type
diabetes, as there is often a long history of use of these compounds in the treatment of metabolic
diseases states. One such compound is berberine, a natural plant alkaloid that has been used
as a traditional medicine in many Asian countries. Berberine was shown to improve insulin
sensitivity in a range of animal models [240] and there is also evidence of beneficial effects in
humans [241]. Although enhancing mitochondrial function appears to be an effective treat
ment for insulin resistance, we showed that berberine acted through inhibition of Complex 1
of the electron transport chain [242]. This mild mitochondrial inhibition led to the activation
of AMPK and subsequent metabolic benefits. Interestingly this pattern of mild inhibition of
8. Concluding remarks
The mitochondrial dysfunction theory of insulin resistance, proposes that defects in mito
chondrial metabolism are key events involved in the pathogenesis of insulin resistance. At
present, the available literature does not provide strong evidence for this relationship, and
there is mounting evidence that mitochondrial defects observed in insulin-resistant individ
uals are likely acquired (e.g. due to low physical activity or caloric excess), or develop
secondary to the insulin resistance itself. Furthermore, with respect to muscle, another
important issue that needs to be considered is whether the ~30% reduction in mitochondrial
function that has been observed in some insulin-resistant humans would limit the oxidation
of fatty acids, leading to lipid accumulation as proposed [16]. At rest the rate of oxygen
utilization in muscle is low; and the fact that muscle has enormous capacity to increase
substrate oxidation over normal levels, means that there is substantial spare capacity in this
system to maintain fatty acid utilization under normal free-living conditions when energy
requirements are low. Despite the unanswered questions about the precise role that mito
chondria play in insulin resistance and T2D, therapies targeting this important organelle,
should be explored for the the treatment of insulin resistance and its associated metabolic
disorders.
Acknowledgements
Research on mitochondrial metabolism in the laboratory of NT is funded by the National
Health and Medical Research Council of Australia, the Australian Research Council, the
Diabetes Australia Research Trust and the Rebecca Cooper Medical Research Foundation.
Author details
Nigel Turner1,2*
Address all correspondence to: n.turner@unsw.edu.au
1 Department of Pharmacology, School of Medical Sciences, University of New South Wales,
Sydney, Australia
2 Diabetes and Obesity Research Program, Garvan Institute of Medical Research, Darling
hurst, Australia
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