Plenary Lecture Energy sensing by the AMP-activated protein kinase and its effects on muscle metabolism

Article (PDF Available)inProceedings of The Nutrition Society 70(1):92-9 · November 2010with39 Reads
DOI: 10.1017/S0029665110003915 · Source: PubMed
Abstract
The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status, and a regulator of energy balance at both the cellular and whole body levels. Although ubiquitously expressed, its function is best understood in skeletal muscle. AMPK contains sites that reversibly bind AMP or ATP, with an increase in cellular AMP:ATP ratio (signalling a fall in cellular energy status) switching on the kinase. In muscle, AMPK activation is therefore triggered by sustained contraction, and appears to be particularly important in the metabolic changes that occur in the transition from resistance to endurance exercise. Once activated, AMPK switches on catabolic processes that generate ATP, while switching off energy-requiring processes not essential in the short term. Thus, it acutely activates glucose uptake (by promoting translocation of the transporter GLUT4 to the membrane) and fatty acid oxidation, while switching off glycogen synthesis and protein synthesis (the later via inactivation of the mammalian target-of-rapamycin pathway). Prolonged AMPK activation also causes some of the chronic adaptations to endurance exercise, such as increased GLUT4 expression and mitochondrial biogenesis. AMPK contains a glycogen-binding domain that causes a sub-fraction to bind to the surface of the glycogen particle, and it can inhibit glycogen synthesis by phosphorylating glycogen synthase. We have shown that AMPK is inhibited by exposed non-reducing ends in glycogen. We are working on the hypothesis that this ensures that glycogen synthesis is rapidly activated when glycogen becomes depleted after exercise, but is switched off again as soon as glycogen stores are replenished.

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Proceedings of the Nutrition Society
The Summer Meeting of the Nutrition Society hosted by the Scottish Section was held at Heriot-Watt University, Edinburgh on
28 June–1 July 2010
Conference on ‘Nutrition and health: cell to community’
Plenary Lecture
Energy sensing by the AMP-activated protein kinase and its effects
on muscle metabolism
D. Grahame Hardie
College of Life Sciences, University of Dundee, Dundee DD1 5EH, UK
The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status, and a regulator
of energy balance at both the cellular and whole body levels. Although ubiquitously expressed,
its function is best understood in skeletal muscle. AMPK contains sites that reversibly bind
AMP or ATP, with an increase in cellular AMP:ATP ratio (signalling a fall in cellular energy
status) switching on the kinase. In muscle, AMPK activation is therefore triggered by sustained
contraction, and appears to be particularly important in the metabolic changes that occur in the
transition from resistance to endurance exercise. Once activated, AMPK switches on catabolic
processes that generate ATP, while switching off energy-requiring processes not essential in
the short term. Thus, it acutely activates glucose uptake (by promoting translocation of the
transporter GLUT4 to the membrane) and fatty acid oxidation, while switching off glycogen
synthesis and protein synthesis (the later via inactivation of the mammalian target-of-
rapamycin pathway). Prolonged AMPK activation also causes some of the chronic adaptations
to endurance exercise, such as increased GLUT4 expression and mitochondrial biogenesis.
AMPK contains a glycogen-binding domain that causes a sub-fraction to bind to the surface
of the glycogen particle, and it can inhibit glycogen synthesis by phosphorylating glycogen
synthase. We have shown that AMPK is inhibited by exposed non-reducing ends in glycogen.
We are working on the hypothesis that this ensures that glycogen synthesis is rapidly activated
when glycogen becomes depleted after exercise, but is switched off again as soon as glycogen
stores are replenished.
Muscle: Exercise: Type 2 diabetes: Nutraceuticals
Animal cells take up fuel molecules such as glucose or
fatty acids, and oxidise them to CO
2
via the process of
catabolism. Much of the energy released during this pro-
cess is used to convert ADP to ATP, which can be likened
to the chemicals in a rechargeable battery. Extending this
analogy, catabolism charges up the battery by converting
ADP to ATP, whereas most other cellular activities (e.g.
growth, division, secretion and movement) require energy
and are driven by the conversion of ATP back to ADP,
thus draining the battery. Just as machines that utilise
rechargeable batteries (such as laptop computers or electric
cars) require systems to monitor the state of the battery,
cells require systems to monitor their ATP: ADP ratio and
match the rates of uptake and consumption of carbon
nutrients to the rate of ATP utilisation. The topic of this
review is the AMP-activated protein kinase (AMPK),
which is the major system responsible for achieving this
task in eukaryotes. Researchers who wish to understand
disorders of energy balance, such as obesity and type-2
diabetes, have been particularly interested in the system.
Protein kinases, including AMPK, are signalling en-
zymes that modify the function of target proteins by
transferring phosphate groups from ATP to side chains of
specific amino acids, usually serine or, less frequently,
Abbreviations: ACC, acetyl-CoA carboxylase; AMPK, AMP-activated protein kinase; mTOR, mammalian target-of-rapamycin.
Corresponding author: Professor D.G. Hardie, fax + 44 1382 385507, email d.g.hardie@dundee.ac.uk
Proceedings of the Nutrition Society, Page 1 of 8 doi:10.1017/S0029665110003915
g
The Author 2010
Proceedings of the Nutrition Society
threonine or tyrosine. This modification, which is reversed
by a different reaction catalysed by protein phosphatases,
often triggers a marked change in the function of the target
protein. For example, if the target is a metabolic enzyme it
can switch enzyme activity on or off, or change the
response to other regulators. In other cases, phosphory-
lation targets proteins for degradation, changes their as-
sociation with other proteins, or affects their targeting to
particular subcellular compartments. The importance of
protein kinases was emphasised when sequencing of the
human genome revealed that it encoded over 500 protein
kinase catalytic subunits, accounting for nearly 2% of all
genes
(1)
. This great diversity reflects the likelihood that
protein phosphorylation is the most important mechanism
by which cell function is modulated in response to changes
in local conditions, and to molecules like hormones and
cytokines that carry messages about conditions in other
locations.
The AMPK was first defined in the late 1980s by the
author when he recognised that protein kinases previously
found to phosphorylate and inactivate the key regulatory
enzymes of fatty acid and cholesterol synthesis, i.e. acetyl-
CoA carboxylase (ACC) and 3-hydroxy-3-methylglutaryl-
CoA reductase, were identical
(2)
. We named it AMPK after
the nucleotide 5
0
-AMP, which causes its activation
(3)
. Why
should cells express a protein kinase activated by AMP? A
high ATP:ADP ratio, which is maintained by catabolism
in cells that have adequate supplies of glucose and oxygen,
drives the reversible reaction catalysed by adenylate kinase
(ATP + AMP$2ADP) from left to right, keeping the AMP
concentration low (up to 100-fold lower than ATP). How-
ever, any metabolic stress that causes a drop in the ATP:
ADP ratio will cause some displacement of the adenylate
kinase reaction to the left, producing AMP (Fig. 1). A rise
in AMP is therefore a signal that the energy status of the
cell has been compromised. Since deprivation for glucose
or oxygen are two of the metabolic stresses that can cause
an increase in AMP, the AMPK system acts as a nutrient
sensor and is a key player in cellular sensing mechanisms
for both glucose and oxygen.
Regulation, structure and evolution of AMP-activated
protein kinase
Addition of AMP causes almost instant activation of
AMPK by up to 10-fold
(2,4)
. Since this is caused by bind-
ing of AMP at a site distinct from the active site, it is
referred to as allosteric activation. However, it is not the
whole story: the author discovered that phosphorylation
of AMPK at a specific site within the kinase domain
(Thr-172), catalysed by distinct upstream protein kinases,
causes >100-fold activation of AMPK
(5)
(Fig. 1). This
multiplies with the allosteric effect to give up to 1000-fold
activation overall. Phospho-specific antibodies recognising
AMPK phosphorylated at Thr-172, and antibodies recog-
nising the downstream target ACC phosphorylated at the
AMPK site (Ser-79) are now widely used as biomarkers to
monitor AMPK activation. We and others identified the
major upstream kinase phosphorylating Thr-172 to be a
complex containing the protein kinase LKB1
(6,7)
. This was
an exciting discovery, because LKB1 had been previously
identified as a tumour suppressor, i.e. a gene product that
inhibits the development of cancer. It now seems clear that
AMPK mediates many of the tumour suppressor effects of
LKB1, although discussion of this is beyond the scope of
this review. The LKB1 complex appears to phosphorylate
Thr-172 continuously under basal conditions, although
the phosphate is normally immediately removed by protein
phosphatases. However, the binding of AMP to AMPK
causes a structural change that prevents dephosphorylation
of Thr-172, thus triggering a net switch to the phos-
phorylated form, causing an activation that is further am-
plified by the allosteric mechanism. Thr-172 can also be
phosphorylated by the Ca
2 +
-activated kinase calmodulin-
dependent kinase kinase-b, which can trigger the ac-
tivation of AMPK in response to agents that increase
intracellular Ca
2 +
(Fig. 1), even in the absence of a rise in
AMP
(8,9)
.
AMPK occurs universally as heterotrimeric complexes,
i.e. complexes formed from three non-identical protein
subunits, these being the catalytic a subunit (which con-
tains Thr-172) and the regulatory b and g subunits
(10)
.In
human subjects each subunit is encoded by multiple genes,
giving rise to up to twelve heterotrimeric combinations
whose expression varies between cell types. The b subunits
Fig. 1. Regulation of the AMP-activated protein kinase (AMPK)
system. Numbers below the three forms of AMPK indicate their
relative kinase activity. The upstream kinase (LKB1) continually con-
verts AMPK to its phosphorylated form, increasing the activity 100-
fold. However, in the absence of AMP it is rapidly converted back
to the dephosphorylated, inactive form by protein phosphatases.
Binding of AMP causes a conformational change that increases the
activity a further 10-fold via an allosteric effect, and also prevents
dephosphorylation. The activating signal, AMP, rises during meta-
bolic stress because a rise in ADP causes displacement of the
adenylate kinase reaction towards AMP. The system can also be
activated by a rise in intracellular Ca
2 +
, due to phosphorylation
catalysed by the Ca
2 +
-activated kinase calmodulin-dependent
kinase kinase-b (CaMKKb).
2 D. G. Hardie
Proceedings of the Nutrition Society
contain carbohydrate-binding modules whose regulatory
function will be discussed later, whereas the g subunits
contain two sites that bind the regulatory nucleotide,
AMP
(11,12)
. Why there should be two sites is not yet clear,
but one explanation is that one is required for the allosteric
effect of AMP and the other for the effect on depho-
sphorylation. Both sites also bind ATP in competition with
AMP, but this fails to trigger the activating effects. Thus,
AMPK is activated by an increase in the cellular
AMP:ATP ratio, a signal that the energy status of the cell
is compromised.
Genes encoding the a, b and g subunits of AMPK are
readily found in the genomes of all eukaryotes, ranging
from unicellular organisms such as protozoa (e.g. Giardia
lamblia) and fungi (e.g. budding yeast, Saccharomyces
cerevisae), to multicellular organisms such as nematodes,
insects, plants and mammals. In lower eukaryotes, the
function of the AMPK relatives (orthologues) has been
studied by genetics, and the results give interesting clues as
to the probable function of AMPK during early evolution.
Budding yeast lacking the complex are viable if always
kept in a medium containing high glucose, but cannot
mount any of the normal responses to glucose removal,
showing that it is involved in nutrient sensing and the
response to starvation
(13)
. In the nematode worm Caeno-
rhabditis elegans, the AMPK orthologue is involved in re-
sponse to dietary energy restriction. This organism was one
of the model systems where the effect of dietary energy
restriction to enhance longevity, which is now thought to
occur also in mammals, was first defined. Restricting the
diet during early development delays sexual maturation of
the worms but also considerably extends their lifespan, and
the AMPK orthologue is required for this response
(14,15)
.In
the plant Physcomitrella patens (a moss), the AMPK
orthologue is not required for growth in continuous light,
but is required for growth in alternate light–dark cycles
(16)
.
Since darkness for a plant is the equivalent of a period of
starvation or fasting, this reinforces the view that the
ancestral role of the AMPK system was in response to
starvation for carbon nutrients.
Activation of mammalian AMP-activated protein
kinase by metabolic stresses
If mammalian cells are deprived of glucose in culture,
AMPK is rapidly activated (as in yeast) due to a reduced
rate of glucose catabolism and a consequent increase in
the AMP:ATP ratio
(17)
. Most mammalian cells express
GLUT1 or GLUT4, and the first step in subsequent glucose
metabolism is catalysed by hexokinase. GLUT1, GLUT4
and hexokinase all exhibit high K
m
values for glucose, and
are thus saturated at normal plasma glucose concentrations.
In these cells, the activation of AMPK therefore only
occurs at very low glucose concentrations that may not be
physiologically relevant. However, specialised ‘glucose-
sensing’ cells, such as the b cells of the pancreas and
specific neurones in the hypothalamus, express high K
m
transporters and glucose phosphorylating enzymes, i.e.
GLUT2 and glucokinase, and fluctuations in glucose
within the normal physiological range alter the rate of
glucose metabolism and consequently modulate the AMPK
activity. In these cells, the activation of AMPK appears to
play important roles in the physiological responses to
hypoglycaemia, such as reduced secretion of insulin and
increased secretion of glucagon and adrenaline by the
pancreas and adrenal medulla
(17–20)
.
Deprivation of oxygen (hypoxia) is another stress that
can limit catabolism, thus causing increases in AMP:ATP
and AMPK activation. This occurs, for example, in cardiac
muscle in response to ischaemia, an interruption in the
blood supply
(21–23)
. Similar to what has just been described
for hypoglycaemia, in most cells the oxygen tension has to
drop to pathologically low levels (such as occurs during
ischaemia) before AMPK is activated. However, just as
there are specialised glucose-sensing cells, there are also
specialised oxygen-sensing cells, such as the Type 1 cells
in the carotid body, in which AMPK is activated by more
physiological levels of hypoxia. Type 1 cells release
neurotransmitters onto afferent nerves, signalling to the
brain to regulate breathing in response to the oxygen con-
tent of the blood reaching it via the carotid artery. Other
oxygen-sensing cells include the smooth muscle cells lin-
ing pulmonary arteries, which (opposite to what happens in
most arteries) contract in response to hypoxia, in order to
divert blood flow to more oxygenated regions of the lung.
In both cell types, mitochondrial catabolism appears to be
particularly sensitive to decreasing oxygen tension so that
AMPK is activated, triggering changes in Ca
2 +
movement
that promote either neurotransmitter release (Type 1 cells)
or contraction (pulmonary smooth muscle)
(24–26)
.
The stresses discussed thus far activate AMPK by inhi-
biting ATP synthesis. A metabolic stress that activates
AMPK by accelerating ATP consumption, and which oc-
curs under normal physiological conditions, is muscle
contraction. With Will Winder, the author found that
AMPK is activated in rodent skeletal muscle during
treadmill exercise
(27)
, and this has been confirmed many
times using biopsies of human muscle. In a later section,
I will focus on the metabolic effects that occur downstream
of AMPK in contracting muscle.
Regulation of AMP-activated protein kinase by
adipokines and other cytokines
As discussed earlier, the ancestral role of AMPK in uni-
cellular eukaryotes appears to have been in response
to starvation for a carbon source. In mammals, plasma
glucose is maintained within tight limits by hormonal
homoeostasis, and so glucose deprivation is less of an
issue. Nevertheless, AMPK is still ubiquitously expressed.
It appears that during the evolution of multicellular
organisms, adaptations occurred to allow other types of
input to interact with the system. This is illustrated by the
effects of the adipokines, cytokine-like substances released
by adipocytes, of which leptin is the classical example.
Leptin activates AMPK in skeletal muscle, accounting for
its ability to stimulate fatty acid oxidation (see the next
section) and hence energy expenditure
(28)
. The more well-
known role of leptin is to repress appetite (and hence
energy intake) via effects on the hypothalamus, and Kahn
AMPK and muscle metabolism 3
Proceedings of the Nutrition Society
and co-workers have suggested that this is mediated by
inhibition of AMPK
(29)
. Results with knock-out mice that
lack AMPK in hypothalamic neurones have not fully sup-
ported this model, but do support the idea that AMPK
is involved in glucose sensing in the hypothalamus
(19)
.
There is also widespread agreement that treatments which
activate AMPK in the hypothalamus, such as infusion with
the nucleoside 5-aminoimidazole-4-carboxamide riboside,
treatment with the gut hormone ghrelin or cannabinoids, or
hypoglycaemia, all increase food intake in rodents
(30–32)
.
The other well-known adipokine is adiponectin, whose
plasma concentration (in contrast to leptin) is paradoxically
reduced in obese or insulin-resistant individuals. Adipo-
nectin, acting via the AdipoRI receptor
(33)
, activates
AMPK in the liver and other tissues, and this appears to be
responsible for many of its effects, including suppression
of gluconeogenesis in the liver
(34)
and enhancement of
food intake via effects on the hypothalamus
(35)
. Finally,
a number of cytokines, hormones and nutrients have
been reported to modulate AMPK activity, accounting for
diverse effects in different cell types, including IL-6
(36)
,
ciliary neurotrophic factor
(37,38)
, macrophage inhibitory
factor
(39)
and lipoic acid
(40)
. In most of these cases, the
exact mechanisms by which these agents activate AMPK,
whether through changes in AMP :ATP or some other
mechanism, remain unclear.
Metabolic effects of AMP-activated protein kinase
activation during muscle contraction
The metabolic consequences of AMPK activation have
been particularly well studied in skeletal muscle. AMPK
is responsible not only for acute responses to exercise,
occurring within seconds or minutes, but also for longer-
term adaptations to repeated exercise caused by changes in
gene expression. AMPK is mainly activated during pro-
longed exercise and may not be involved in responses to
short bouts of resistance exercise, such as weight-lifting. In
this case, much of the ATP is generated by the conversion
of phosphocreatine to ATP, so that there is little change in
the content of ATP, ADP or AMP. As phosphocreatine
becomes depleted during an event such as a sprint, glyco-
gen breakdown and anaerobic glycolysis then become
important for the generation of ATP. However, once again
these effects do not require AMPK, because the key reg-
ulatory enzymes involved (phosphorylase and phospho-
fructokinase) directly bind AMP and are activated by
increases in the AMP :ATP ratio independent of AMPK.
AMPK does become crucial in the switch from anaerobic
to aerobic metabolism that is required for endurance
exercise, for example, in running events of more than
200 m. During prolonged exercise, the use of aerobic
metabolism and blood-borne fuels such as glucose and
fatty acids becomes increasingly important, and this is
where AMPK plays a key role. Intriguingly, one of the
roles of the AMPK orthologue in budding yeast is to
trigger the switch from fermentation (anaerobic meta-
bolism) to the more efficient oxidative metabolism when
the glucose in the medium starts to become depleted
(13)
.
The acute increase in glucose uptake in skeletal muscle
in response to contraction is caused by a translocation of
the GLUT4 from intracellular storage vesicles to the
plasma membrane. The first evidence that this might be
mediated by AMPK came from experiments by Winder
and the author using the nucleoside, 5-aminoimidazole-4-
carboxamide riboside. This is taken up by cells and con-
verted to the nucleotide 5-aminoimidazole-4-carboxamide
riboside monophosphate, which mimics the activating
effects of AMP on the AMPK system. When rat skeletal
muscle was perfused with 5-aminoimidazole-4-carboxamide
riboside, AMPK was activated and this was associated
with increased glucose uptake
(41)
. Subsequent studies with
GM mice, in which AMPK activation during contraction
was rendered defective by various means, confirmed that
AMPK was involved in this effect
(42,43)
. There may be
multiple mechanisms by which AMPK promotes GLUT4
translocation to the plasma membrane, but one appears to
be the phosphorylation of a protein called TBC1D1 (TRE2/
BUB2/CDC16 domain family member 1), which enhances
GLUT4 translocation by promoting the conversion of
small guanine nucleotide-binding proteins called Rabs
from their inactive (Rab :GDP) to their active (Rab :GTP)
forms
(44,45)
. This is similar to the mechanism by which
insulin stimulates glucose uptake in resting muscle, when
the insulin-activated protein kinase B phosphorylates
TBC1D1 or other members of the same family. In insulin-
resistant individuals, insulin-dependent activation of pro-
tein kinase B is defective, but the mechanism by which
contraction stimulates glucose uptake via AMPK remains
unaffected
(46)
. This is one reason why regular exercise is
particularly beneficial in insulin-resistant subjects.
Another catabolic pathway activated during low-
intensity exercise is fatty acid oxidation. Muscle expresses
the ACC2 isoform of ACC which, like the ACC1 isoform
expressed in most other cells, is phosphorylated and in-
activated by AMPK
(47)
. ACC2 is associated with mito-
chondria and produces malonyl-CoA, an inhibitor of the
enzyme carnitine-palmitoyl transferase-1 located on the
outer mitochondrial membrane. Carnitine-palmitoyl trans-
ferase-1 is required for the uptake of fatty acids into
mitochondria, and is inhibited in resting muscle by the
malonyl-CoA produced by ACC2
(48)
. Inhibition of ACC2
by AMPK during contraction therefore promotes fatty acid
uptake into mitochondria and hence oxidation
(41)
. Inter-
estingly, excessive storage of fatty acids as TAG in mus-
cle, as in other tissues, is associated with insulin resistance.
Stimulation of fat oxidation by AMPK is therefore another
reason why exercise is beneficial in insulin-resistant
subjects.
As well as these acute effects on muscle metabolism,
AMPK is also responsible for some of the longer-term
adaptations induced by repeated endurance exercise train-
ing. This includes increased expression of GLUT4
(49)
,
which may be mediated in part by phosphorylation of a
histone deacetylase that modifies the chromatin structure
at the GLUT4 promoter
(50)
. AMPK activation also up-
regulates mitochondrial biogenesis by up-regulating the
transcriptional co-activator PPARg co-activator-1a, which
promotes the expression of mitochondrial genes encoded in
both nuclear and mitochondrial DNA. AMPK has been
reported to directly phosphorylate PPARg co-activator-
1a
(51)
, but the mechanism may also involve deacetylation
4 D. G. Hardie
Proceedings of the Nutrition Society
of PPARg co-activator-1a by Sirtuin-1, which is activated
downstream of AMPK
(52)
.
These longer-term adaptations to endurance exercise
would ensure that, as exercise is repeated regularly, it
becomes easier because glucose is taken up more rapidly
and mitochondrial ATP production is faster. Because
both glucose uptake and mitochondrial function tend to
be reduced in individuals at risk of developing type-2
diabetes
(53)
, AMPK can also help to explain the ability of
regular exercise to provide protection against the develop-
ment of diabetes.
Muscle contraction causes very rapid ATP consumption
and it therefore makes sense that other ATP-requiring
processes, such as biosynthesis, should be switched off.
Another target for AMPK in skeletal muscle is glycogen
synthase, the enzyme catalysing the last step in glycogen
synthesis that also controls its rate
(54,55)
. AMPK phos-
phorylates glycogen synthase at site 2, reducing its activ-
ity
(54)
. This may help to ensure that when glucose uptake
increases during contraction, the increased flux of glucose
into the cell is diverted towards catabolic breakdown of
glucose rather than glycogen synthesis. Paradoxically,
however, immediately after exercise glycogen synthase
is found to be in a highly dephosphorylated and active
state
(56)
. The b subunits of AMPK contain carbohydrate-
binding modules that cause the complex to associate with
glycogen particles in intact cells and in vivo
(57–59)
. While
the role of the carbohydrate-binding module may in part
be simply to localise AMPK in close proximity to its
target, glycogen synthase, the author has recently obtained
evidence that glycogen also inhibits AMPK
(60)
. Synthetic
oligosaccharides that mimic the non-reducing ends of
glycogen at the surface of the glycogen particle inhibit the
kinase activity
(60)
, and recent unpublished evidence shows
that the inhibition reaches a maximum using a chain of
six glucose units lacking a non-reducing end (C Chevtzoff
and DG Hardie, unpublished results). Our current hypoth-
esis is that, in a full-sized glycogen particle that has up
to 2000 non-reducing ends packed around the surface of a
glycogen particle that is only 20–30 nm in diameter
(61)
, the
non-reducing ends would be so tightly packed that they
would be unable to inhibit AMPK, although it may still
bind to the particle via the carbohydrate-binding module.
In a full-sized glycogen particle AMPK would thus phos-
phorylate glycogen synthase, preventing it from further ex-
tending the outer chains of glycogen. However, as glycogen
becomes degraded during exercise the outer branches would
be removed and the structure would become more open.
AMPK would now become inhibited due to the greater
accessibility of the non-reducing ends, and would no longer
phosphorylate glycogen synthase, which would instead be
dephosphorylated by protein phosphatases bound to the
glycogen particle
(62)
. This model provides a potential
explanation for the paradoxical observation that, although
AMPK is activated during exercise, its substrate glycogen
synthase is found to be dephosphorylated immediately fol-
lowing exercise
(56,62)
. This mechanism would ensure that, if
glycogen became significantly depleted, it would be rapidly
replenished as soon as exercise ceased. If this model is cor-
rect, AMPK not only monitors the short-term availability of
energy in the form of ATP and ADP, but also the medium
term reserves in the form of glycogen. It also means that
AMPK is involved in the rapid glucose uptake that occurs
following exercise to replenish glycogen, another of the ben-
eficial outcomes of exercise in individuals who are insulin
resistant or hyperglycaemic.
Another biosynthetic pathway that is switched off, at
least temporarily, during exercise is translation of mRNA
into protein. An important signalling pathway that stimu-
lates translation has as its central component the mamma-
lian target-of-rapamycin (mTOR). mTOR is a protein
kinase that is stimulated by insulin and by amino acids,
and it promotes protein translation by regulating its initia-
tion
(63)
. AMPK activation, on the other hand, inhibits mTOR
by phosphorylating multiple upstream regulators
(64,65)
, and
thus inhibits protein synthesis. There is evidence that this
mechanism does partly account for a temporary cessation
of translation in muscle during contraction
(66,67)
. Interest-
ingly, mTOR is also activated during resistance exercise
training
(68)
as well as by insulin and by feeding amino
acids, and this may account for the protein synthesis and
hence muscle hypertrophy induced by all of these inputs.
Since AMPK opposes mTOR activation, this may help to
explain why endurance exercise training does not cause
muscle hypertrophy, and why endurance athletes have less
bulky muscles compared to weight-lifters or sprinters.
Regulation of AMP-activated protein kinase by drugs
and ‘nutraceuticals’
In the previous section, I stressed the numerous metabolic
changes caused by AMPK activation in muscle, which are
particularly beneficial in individuals at risk of developing
insulin resistance and/or type-2 diabetes. AMPK activation
in the liver, which occurs in response to adiponectin, is
also beneficial by repressing production of glucose by
gluconeogenesis
(34,69)
, which is at least partly responsible
for the tendency of insulin-resistant individuals to hyper-
glycaemia. AMPK activation in the liver also promotes fat
oxidation
(34)
, thus reducing TAG accumulation, a meta-
bolic state that is strongly associated with insulin resis-
tance. It is therefore not surprising that AMPK has become
an exciting target for new drugs aimed at the treatment of
type-2 diabetes
(70)
. In fact, the biguanide metformin, which
was developed from an ancient herbal remedy and is now
the first choice drug for treatment of type-2 diabetes, acti-
vates AMPK
(71)
, and there is good evidence from rodents
that this accounts for its ability to lower plasma glucose
(72)
.
The author has recently shown that metformin activates
AMPK indirectly by causing a mild inhibition of complex I
of the respiratory chain, thus increasing cellular AMP:
ATP
(73)
. Despite this rather crude mechanism of action,
metformin is a safe drug that is now prescribed to >100
million patients worldwide. A number of other xenobiotics
or ‘nutraceuticals’ derived from foods and beverages,
or from traditional herbal medicines, also activate AMPK
and have been claimed to have beneficial effects in obesity,
type-2 diabetes and even cancer. These include resveratrol,
present in small quantities in red wine, quercetin present
in many fruit and vegetables and berberine, a yellow
dye derived from plants of the genus Berberis used in
AMPK and muscle metabolism 5
Proceedings of the Nutrition Society
traditional Chinese medicine. We have recently reported
that all of these activate AMPK due to their ability to
cause inhibition of mitochondrial ATP production
(73)
. They
are all secondary metabolites of plants, and an interesting
speculation is that they may be produced by the plant to
prevent or discourage grazing by animals, or infection by
micro-organisms. In support of this, the natural product
galegine (from which metformin is derived and which
itself a potent AMPK activator
(73)
), is produced by the
plant Galega officinalis, which also goes by the name of
Goat’s Rue and is classed as a noxious weed in USA
because it is poisonous to herbivores. Also, resveratrol is
produced by grapes in response to fungal infection, which
is why its concentration is often higher in organic wines
where the grapes have not been treated with fungicides.
Conclusions
AMPK is an ancient system that arose during early eukar-
yotic evolution, where its role may have been in response
to starvation for a carbon source. In mammals, by moni-
toring the rate of catabolism, it can act in specialised cells
as a sensor of either glucose or oxygen. In skeletal muscle
it is activated by contraction and appears to be responsible
for many of the acute metabolic changes and the longer-
term adaptations induced by exercise. Through its regula-
tion by adipokines and other signal molecules, AMPK is
also a regulator of energy balance at the whole body level,
and drugs that are used to treat disorders of energy balance,
such as metformin, now appear to work by activating
AMPK.
Acknowledgements
The author declares no conflicts of interest. Studies in the
author’s laboratory were supported by Programme Grants
(080982; 081195) from the Wellcome Trust, and by the
companies (AstraZeneca, Boehringer-Ingelheim, Glaxo-
SmithKline, Merck KGaA and Pfizer) that support the
Division of Signal Transduction Therapy at the University
of Dundee.
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    • "AMPK is an energy sensor that regulates both lipid and carbohydrate homeostasis, and impairments in its function have been linked with the progression of metabolic disorders [37]. AMPK is activated both in response to exercise and muscle contraction through an increase in AMP concentration [38] . AMP activates AMPK by binding to the two CBS domains on the γ-subunit, which activates AMPK directly by an allosteric mechanism and indirectly activating phosphorylation on Thr172 on the α-subunit by upstream kinase(s), including LKB1, resulting in promotion of GLUT4 translocation [. "
    [Show abstract] [Hide abstract] ABSTRACT: Procyanidins are the oligomeric or polymeric forms of epicatechin and catechin. In this study, we isolated and purified dimer to tetramer procyanidins from black soybean seed coat and investigated the anti-hyperglycemic effects by focusing on glucose transporter 4 (GLUT4) translocation and the underlying molecular mechanism in skeletal muscle of mice. The anti-hyperglycemic effects of procyanidins were also compared with those of monomer (-)-epicatechin (EC) and major anthocyanin, cyanidin-3-O-β-glucoside (C3G). To investigate GLUT4 translocation and its related signaling pathways, ICR mice were orally given procyanidins, EC and C3G in water at 10 μg/kg body weight. The mice were sacrificed 60 min after the dose of polyphenols, and soleus muscle was extracted from the hind legs. The results showed that trimeric and tetrameric procyanidins activated both insulin- and AMPK-signaling pathways to induce GLUT4 translocation in muscle of ICR mice. We confirmed that procyanidins suppressed acute hyperglycemia with an oral glucose tolerance test in a dose-dependent manner. Of these beneficial effects, cinnamtannin A2, one of the tetramers, was the most effective. In conclusion, procyanidins, especially cinnamtannin A2, significantly ameliorate postprandial hyperglycemia at least in part by promoting GLUT4 translocation to the plasma membrane by activating both insulin- and AMPK-signaling pathways.
    Full-text · Article · Sep 2016
    • "AMPK is considered a master switch in muscle metabolism and key in the regulation of transport of fuel into the mitochondria for oxidation. During exercise increased AMPK activity stimulates FA utilization (Fentz et al., 2015) and inhibits other energy-consuming processes (Jorgensen et al., 2004; Jensen et al., 2009; Richter & Ruderman, 2009; Hardie, 2011). Furthermore, AMPK may partly favor FA oxidation through inhibition of Acetyl-CoA carboxylase 2 (ACC2) (Stephens et al., 2002) although this may not be a limiting factor (Dzamko et al., 2008). "
    [Show abstract] [Hide abstract] ABSTRACT: Age and inactivity have been associated with intramuscular triglyceride (IMTG) accumulation. Here, we attempt to disentangle these factors by studying the effect of 2 weeks’ unilateral leg immobilization on substrate utilization across the legs during moderate intensity exercise in young (n = 17; 23 ± 1 years) and older (n = 15; 68 ± 1 years) men, while the contralateral leg served as control. After immobilization, the participants performed two-legged isolated knee-extensor exercise at 20 ± 1 Watt (∼50% Wattmax) for 45 min with catheters inserted in the brachial artery and both femoral veins. Biopsy samples obtained from vastus lateralis muscles of both legs before and after exercise were used for analysis of substrates, protein content and enzyme activities. During exercise, leg substrate utilization (RQ) did not differ between groups or legs. Leg fatty acid (FA) uptake was greater in older than in young men, and while young men demonstrated net leg glycerol release during exercise, older men showed net glycerol uptake. At baseline, IMTG, muscle pyruvate dehydrogenase complex activity, protein content of adipose triglyceride lipase (ATGL), acetyl-CoA carboxylase 2, AMP-activated protein kinase (AMPK)γ3 were higher in young than in older men. Furthermore, ATGL, plasma membrane-associated FA binding protein, and AMPKγ3 subunit protein content were lower and IMTG being higher in the immobilized than the contralateral leg in young and older men. Thus, immobilization and age did not affect substrate choice (RQ) during moderate exercise, but the whole-leg and molecular differences in FA mobilization could explain the age and immobilization induced IMTG accumulation. This article is protected by copyright. All rights reserved
    Full-text · Article · Jan 2016
    • "The signaling proteins that regulate exercise-and contraction-stimulated glucose uptake are still not clearly understood, and there is considerable evidence that redundant signaling mechanisms may control this important physiological process (Rockl et al. 2008). AMP-activated protein kinase (AMPK), a master regulator of cellular energy homeostasis, has been proposed to be the central node regulating glucose transport in response to insulin-independent stimuli such as exercise, muscle contraction, hypoxia, metformin, and the AMPK activator AICAR (Merrill et al. 1997; Mu et al. 2001; Sajan et al. 2010; Hardie 2011; Richter and Hargreaves 2013). A number of studies using different animal models have convincingly demonstrated that AMPK is necessary for AICAR-and metformin-stimulated glucose transport (Zhou et al. 2001; Fryer et al. 2002; Sajan et al. 2010). "
    [Show abstract] [Hide abstract] ABSTRACT: Exercise increases skeletal muscle glucose uptake, but the underlying mechanisms are only partially understood. The atypical protein kinase C (PKC) isoforms λ and ζ (PKC-λ/ζ) have been shown to be necessary for insulin-, AICAR-, and metformin-stimulated glucose uptake in skeletal muscle, but not for treadmill exercise-stimulated muscle glucose uptake. To investigate if PKC-λ/ζ activity is required for contraction-stimulated muscle glucose uptake, we used mice with tibialis anterior muscle-specific overexpression of an empty vector (WT), wild-type PKC-ζ (PKC-ζ(WT)), or an enzymatically inactive T410A-PKC-ζ mutant (PKC-ζ(T410A)). We also studied skeletal muscle-specific PKC-λ knockout (MλKO) mice. Basal glucose uptake was similar between WT, PKC-ζ(WT), and PKC-ζ(T410A) tibialis anterior muscles. In contrast, in situ contraction-stimulated glucose uptake was increased in PKC-ζ(T410A) tibialis anterior muscles compared to WT or PKC-ζ(WT) tibialis anterior muscles. Furthermore, in vitro contraction-stimulated glucose uptake was greater in soleus muscles of MλKO mice than WT controls. Thus, loss of PKC-λ/ζ activity increases contraction-stimulated muscle glucose uptake. These data clearly demonstrate that PKC-λζ activity is not necessary for contraction-stimulated glucose uptake.
    Full-text · Article · Nov 2015
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