Glycation, the
nonenzymatic glycosylation of biomolecules, is commonly observed in diabetes
and ageing. Reactive dicarbonyl species such as methylglyoxal and glyoxal are
thought to be major physiological precursors of glycation. Because these
dicarbonyls tend to be formed intracellularly, the levels of advanced glycation
end products on cellular proteins are higher than on extracellular ones. The
formation of glycation adducts within cells can have severe functional
consequences such as inhibition of protein activity and promotion of DNA
mutations. Although several lines of evidence suggest that there are specific
mitochondrial targets of glycation, and mitochondrial dysfunction itself has
been implicated in disease and ageing, it is unclear if glycation of biomolecules
specifically within mitochondria induces dysfunction and contributes to disease
pathology. We discuss
here the possibility that mitochondrial glycation contributes to disease,
focussing on diabetes, ageing, cancer, and neurodegeneration, and highlight the
current limitations in our understanding of the pathological significance of
mitochondrial glycation.
1. Introduction
Glycation is a common feature of diabetic complications
[1, 2] and ageing
pathologies [3, 4]. This nonenzymatic
glycosylation
process stems from multiple reactions between reducing sugars or sugar
derivatives and amino groups on proteins, lipids and nucleotides. Glycation
involves multiple oxidative and nonoxidative reactions, collectively termed the
Maillard reaction, eventually generating advanced glycation end products (AGEs)
[5]. In clinical
and experimental models
of diabetes and ageing, the levels of intra- and extracellular AGEs have been
found to increase relative to healthy or young controls [3, 6–10]. Such correlative
data suggests that glycation can potentially contribute to disease progression
and ageing pathology. However, whether there is a causal relationship between
glycation and pathology is uncertain and the mechanistic details are unclear.
That the formation of AGEs can have significant
functional
consequences further supports this hypothesis. Glycation affects all major
classes of biomolecules (Figure 1), with damage
levels estimated at
0.1–1% of lysine and arginine residues on proteins, 1 in 107 nucleotides on DNA and 0.1% of basic phospholipids [11]. The consequences
of
such glycation damage can be severe. The formation and accumulation of protein
AGEs, for instance, can result in biochemical dysfunction. For example, some
AGEs like MOLD (methylglyoxal-derived lysine dimers) and pentosidine form
protein crosslinks, altering protein structure, generally causing proteins to
become more resistant to proteolysis [12]. Protein
structure
may also be modified as a result of charge neutralization when arginine and
lysine residues are glycated [13]. Consequently,
structural integrity of proteins becomes compromised. For instance, glycated
collagen is stiff and nonelastic relative to the non-glycated protein [14, 15]. The change
in charge
distribution could also promote protein aggregation, for instance, of lens
crystallins, leading to cataract formation in diabetes and in old age [16, 17]. Changes
in protein
conformation could also influence its function, as would glycation of amino
acid residues at sites for substrate binding and allosteric regulation on enzymes.
It has been shown, for instance, that methylglyoxal-induced glycation of lys
126 and arg 463 in glutamate dehydrogenase isolated from bovine liver affects
its ability to bind its substrate and allosteric activator (adenosine
diphosphate (ADP)), respectively, resulting in a decrease in glutamate
dehydrogenase activity [18]. Similarly,
glycation
of extracellular matrix (ECM) proteins affects cell-ECM interactions. The
reaction of methylglyoxal with arginine residues on the RGD and GFOGER motifs
in the integrin-binding sites of collagen, for instance, resulted in
endothelial cell detachment by interfering with cell-ECM interactions [7].
Figure 1: Reactive α,β-dicarbonyl-induced
glycation damage in
cells. Glycation by α,β-dicarbonyls
like methylglyoxal and glyoxal affects all classes of biological
macromolecules. Proteins, in particular arginine, lysine, and cysteine
residues, are highly susceptible to glycation, forming protein cross-links and
various AGE adducts, examples of which are shown here. Of these, the most
important protein-AGEs quantitatively are arginine-derived hydroimidazolones,
especially MG-H1. Formation of protein AGEs alters protein structure and
function, leading to biochemical dysfunction. Nucleotides and lipids may also
undergo glycation, with deoxyguanosine and basic phospholipids being
particularly vulnerable. Consequences of nucleotide and lipid glycation damage
are increased DNA mutations and compromised lipid membrane integrity,
respectively. Within mitochondria, it is expected that glycation affects matrix
and membrane proteins, phospholipids on the outer and inner mitochondrial
membranes and mtDNA.
Membrane interactions [19] may also
be affected
by the formation of lipid glycation adducts which increase membrane fluidity [20]. Lipids possessing
a
free amino group such as phosphatidylethanolamine are susceptible to glycation,
whereas those without, for example phosphatidylcholine, are not glycated [21]. It is thought
that
lipid glycation may promote lipid peroxidation, resulting in oxidative damage [22–24]. For example, in vitro incubation of
human low-density lipoprotein (LDL) with 200 mM glucose for up to 12 days
increased levels of both glycation and lipid oxidation products, which appears
to support the hypothesis that lipid glycation causes, or at least enhances,
lipid peroxidation [22]. However,
since lipid
glycation and peroxidation can both occur in parallel, it is difficult to
distinguish one from the other as being the primary initiating event versus a
secondary downstream event, particularly in vivo [22].
Likewise, glycation of DNA can have multiple
effects such as
strand breaks, unwinding of the double helix, mutations and formation of
DNA-protein and nucleotide-nucleotide cross-links [25–30]. The extent
of modification
appears to be dependent on the glycating agent used. For example, methylglyoxal
induced 10-fold more DNA-protein crosslinks than did glyoxal when
incubated in vitro with
DNA and DNA polymerase I derived from Escherichia
coli [31]. Such DNA-DNA
polymerase crosslinks could stall replication and subsequently promote
frameshift mutations [31]. The steric
hindrance
imposed by DNA glycation adducts may also impair transcription by preventing
transcription factors from binding [32]. That is,
DNA
glycation may not only influence genome integrity but could also alter gene
expression.
Besides direct glycation damage to biomolecules,
AGEs, especially
extracellular AGEs, could also contribute to disease pathology by binding to
cell surface receptors such as the receptor for AGEs (RAGE), thereby activating
the proinflammatory NF-κB
pathway
and downstream signalling molecules such as p21, MAP kinases, and JNK [33, 34].
The formation of AGEs arises from reactions between
reducing
sugars (e.g., glucose) or sugar derivatives (e.g. α,β-dicarbonyls) and amino groups on biological macromolecules
(Figure 2). Glycation
reactions can be broadly
classified as early and late stages of glycation. During early glycation, the
carbonyl group of acyclic glucose reacts with amino groups, then undergoes
dehydration to form a Schiff base, which subsequently undergoes further
rearrangement to form a more stable Amadori product, fructosamine [35] (Figure 2). Alternatively,
the Schiff base may
undergo spontaneous degradation to form reactive α,β-dicarbonyl species such as methylglyoxal and glyoxal [36] (Figure 3(a)). These dicarbonyls
may also be formed from glucose breakdown via the Wolff pathway [37] (Figure 3(a)), triose phosphate
fragmentation [38] (Figure 3(b)), acetone
[39] and threonine
metabolism
[40] (Figures 3(c) and 3(d)), lipid peroxidation
[41], and
fructose-3-phosphate decomposition [42]. Downstream
of
Amadori product formation, further oxidative and nonoxidative modifications may
occur, generating AGEs [5] (Figure 2). Where oxidative
processes are
involved, the term glycoxidation has also been used [43]. AGEs can
also be
formed by the direct modification of amino groups by α,β-dicarbonyls (Figure 2).
Figure 2: Formation of AGEs in
physiological
systems. In early glycation, glucose reacts reversibly with amine groups on macromolecules
to form a Schiff base and subsequently an Amadori product. This can undergo
further oxidative and nonoxidative modifications to form AGEs. Alternatively,
AGEs may also be formed by the direct reaction of reactive α,β-dicarbonyls with macromolecules.
Figure 3: Formation of methylglyoxal
and glyoxal
in biological systems. (a) The acyclic form of glucose may undergo
autooxidation via the Wolff pathway to form glyoxal or break down to
3-deoxyglucosone which in turn degrades to glyceraldehyde and methylglyoxal.
Glucose may also react with amine groups on proteins to form a Schiff base
which can undergo similar breakdown via the Namiki pathway to also generate
glyoxal and methylglyoxal. (b) The major pathway by which methylglyoxal is
formed in cells involves the breakdown of the triose phosphates,
glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate, via phosphate
elimination from an ene-diol intermediate. (c) Methylglyoxal may also be
generated during threonine catabolism. This involves oxidation of threonine by
threonine dehydrogenase to 2-aminoacetoacetate, followed by spontaneous
decarboxylation to aminoacetone. Monoamine oxidase then catalyses the
conversion of aminoacetone to methylglyoxal. (d) Another pathway by which
methylglyoxal is produced is from acetoacetate metabolism. This reaction
proceeds via acetone and acetol and is catalysed by acetone monooxygenase and
acetol monooxygenase.
Although glycation is traditionally thought of
as a reaction
between sugars and amino groups, other functional groups on macromolecules, for
example, protein thiols (-SH), may also react in analogous reactions [44]. Similar
to glycation
of lysine and arginine residues on proteins, glycation of thiol groups can
modulate enzyme activity. For example, glyceraldehyde-3-phosphate dehydrogenase
can be inhibited by methylglyoxal-mediated loss of thiol groups at its active
site [44]. Glycation
of
creatine kinase by glyoxal also led to enzyme inactivation [45]. Although
thiol
modification by α,β-dicarbonyls could be damaging
to
protein function, low molecular weight thiols such as N-acetylcysteine and GSH
have been suggested as therapeutic agents for the removal of α,β-dicarbonyls before they can react with proteins [45, 46].
Despite the common association of glycation with
glucose, this
reducing sugar is not itself highly reactive. Rather, it is reactive α,β-dicarbonyl species such as methylglyoxal and glyoxal, which are
up to 50,000-fold more reactive than glucose, that are thought to be the major
physiological precursors of glycation damage [47]. Because
these
dicarbonyls are mostly produced intracellularly through processes such as
triose phosphate fragmentation [38] and lipid
peroxidation [41], there is
a greater
likelihood for cellular proteins to be glycated relative to plasma proteins.
Therefore, levels of AGEs tend to be higher in cellular proteins than in plasma
proteins. For instance, levels of the methylglyoxal/arginine-derived
hydroimidazolone, MG-H1 [Nδ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine],
were found to be 1.22 mmol/mol arginine in human blood cells and 0.92 mmol/mol
arginine in plasma proteins [48]. Similarly,
levels of
another AGE, CML (Nε-carboxymethyl-lysine),
was at least three-fold higher in cellular proteins (0.068–0.233 mmol/mol
lysine) than in plasma proteins (0.021 mmol/mol lysine) [48].
To minimize glycation damage by reactive α,β-dicarbonyls, there exist various enzyme
systems that
catalytically remove these species (Figure 4). One of the
most important of these
defences is the glyoxalase enzyme system which functions primarily to remove
methylglyoxal and, to a lesser extent, other compounds such as glyoxal [49, 50]. Using glutathione
(GSH) as a cofactor, glyoxalase I catalyzes the formation of S-2-hydroxyacylglutathione from
methylglyoxal or glyoxal. Glyoxalase II then converts this intermediate compound
into an α-hydroxyacid
(either D-lactate from methylglyoxal or glycolate from glyoxal), regenerating
GSH in the process. Other enzymes involved in the detoxification of α,β-dicarbonyls include aldehyde dehydrogenases which oxidize
methylglyoxal and glyoxal to pyruvate and glyoxylate, respectively [51], and aldo-keto
reductases/aldose reductases which reduce them to form alcohols (e.g.,
methylglyoxal to acetol and lactaldehyde, glyoxal to glycoaldehyde and ethylene
glycol) [52].
Figure 4: Physiological defences
against α,β-dicarbonyl precursors of glycation. α,β-dicarbonyls
like methylglyoxal (R:CH3) and
glyoxal (R:H) can be removed by
aldose reductase-catalysed reduction to the corresponding alcohol or by
oxidation to pyruvate and glycolate. respectively, catalysed by aldehyde
dehydrogenase. Reactive α,β-dicarbonyls may also combine reversibly with
reduced glutathione
to form a hemithioacetal which can then be reduced by aldose reductases,
forming lactaldehyde and glycoaldehyde from methylglyoxal and glyoxal,
respectively. Alternatively, the hemithioacetal may undergo a two-step
conversion to an α-hydroxyacid, catalysed by glyoxalase I
and glyoxalase II. The glyoxalase enzyme system represents the major pathway by
which methylglyoxal is removed.
Under normal physiological conditions, these
anti-glycation
defences are sufficient to prevent significant glycation damage. Any damage
that does occur is dealt with by damage repair and removal systems. For
example, proteasomes and lysosomes prevent the accumulation of glycated proteins
[53]; the nucleotide
excision repair system removes glycated nucleotides [54]; lipid turnover
helps
clear glycated lipids [19]. When there
is an
imbalance between the production of glycation precursors such as α,β-dicarbonyls and the removal of these species such that the former
is favoured, carbonyl stress occurs [55] and glycation
damage
accumulates.
Because α,β-dicarbonyls are the
major precursors of AGEs and their production is largely intracellular, it is
likely that there would be significant functional consequences of AGEs
formation and accumulation at the organellar level. In this review, we focus on
glycation in mitochondria and its association with mitochondrial dysfunction in
disease and ageing.
2. Glycation
in Mitochondria: Involvement in Diabetes and
Ageing
The levels of both intra- and extracellular AGEs
have been found
to increase in diabetic or old animals relative to healthy or young controls [3, 6–10]. Indeed,
a glycation
adduct of glucose to haemoglobin, HbA1c, is used
clinically as an indicator of glycaemic control [56, 57], and levels
of
glycation adducts like CML and pentosidine have been suggested as prognostic
tools in diabetics [58]. The accumulation
of
AGEs arises from an increase in their production and/or a decrease in their
removal. For instance, low protein turnover is responsible for the accumulation
of extracellular AGEs on collagen with age [59]; in diabetics,
plasma
levels of HbA1c increase as a result
of high
levels of blood glucose which forms an adduct to the N-terminal valine on
the β-chain
of
haemoglobin [56, 57].
Within cells, the levels of AGEs also increase
with age or in
diabetes, and this is likely to reflect changes in intracellular α,β-dicarbonyl levels (Figure 5). In the case
of diabetes, the levels
of intracellular AGEs rise extremely quickly compared to that of extracellular
AGEs and are thought to be a result of the intracellular formation of the
reactive dicarbonyl species, methylglyoxal and glyoxal [60]. According
to the
unifying hypothesis proposed by Brownlee [1], mitochondrial
superoxide production
increases during hyperglycaemia, resulting in DNA damage and poly(ADP-ribose)
polymerase (PARP) activation. ADP-ribose polymers generated by PARP may then
bind glyceraldehyde-3-phosphate (G3P) dehydrogenase, inhibiting it.
Consequently, there is a build-up of glycolytic intermediates upstream of this
enzyme. Among these intermediates are the triose phosphates, G3P and
dihydroxyacetone phosphate (DHAP), which break down to form methylglyoxal [38]. In addition,
oxidative stress resulting from hyperglycaemia promotes lipid peroxidation [61, 62], another
source of a
major α,β-dicarbonyl species, glyoxal
[41]. As such,
there is
increased formation of reactive dicarbonyls in diabetes. With respect to
ageing, decreases in glyoxalase I activity have been reported in Caenorhabditis elegans [3] and rats
[63]. The less
efficient
removal of α,β-dicarbonyls consequently
leads to
elevation of the levels of AGEs in older animals [3].
Figure 5: Intra- versus extracellular
glycation—involvement in diabetes and ageing. Glycation can take place in the
extracellular environment or within cells in the cytosol and in organelles like
mitochondria. Extracellular AGEs may arise from oxidative and nonoxidative
modifications of the Amadori product or from direct reaction of α,β-dicarbonyls
with proteins.
Extracellular AGEs may bind to cell surface receptors such as RAGE, thereby
activating cell signalling pathways. The formation of lipid-AGEs on the cell
surface membrane can further generate reactive α,β-dicarbonyls such as
glyoxal. These
reactive carbonyl species can diffuse through lipid membranes and enter cells
where they react with cellular biomolecules to form intracellular AGEs. They
can also diffuse further into mitochondria and similarly cause glycation damage
within these organelles. Intracellular glycation may also arise from α,β-dicarbonyls
produced during the
breakdown of triose phosphates generated during glycolysis. Normally, glycation
damage is kept under control by defences such as the glyoxalase enzyme system
and aldehyde dehydrogenases. However, in diabetes, glycation damage increases
due to elevated formation of α,β-dicarbonyls, arising from high glucose levels
and consequent
inhibition of the glycolytic enzyme, G3P dehydrogenase, both of which lead to a
rise in triose phosphate levels. These in turn break down nonenzymatically to
form methylglyoxal, a major precursor of glycation. In addition, activation of
RAGE is associated with decreased glyoxalase I expression, which is expected to
further raise methylglyoxal levels by preventing its removal. During ageing, glycation
damage also increases, but mainly as a result of a loss of glyoxalase activity
with age.
Once formed
within the
cell, these reactive dicarbonyl species may then diffuse across membranes [47, 64] and access
mitochondrial targets (Figure 5). It is estimated that
there is 1-2 mmol CML/mol lysine in rat heart mitochondria [65], 0.5–1 mmol CML/mol
lysine in rat liver mitochondria [66] and
0.7–1 mmol CML/mol lysine in rat brain mitochondria [67]. Corresponding values
for carboxyethyl-lysine (CEL) are 0.5, 0.2–0.5, and 0.1–0.2 mmol CEL/mol lysine
[65–67]. It is striking that
levels of CML and CEL in mitochondria from the three tissues (heart, liver, and
brain) are kept within narrow range (<2 mmol AGE/mol lysine). This suggests
that excessive glycation of mitochondrial proteins is highly damaging and that
levels of mitochondrial AGEs have to be minimized to prevent mitochondrial
dysfunction. Likewise, mitochondrial DNA (mtDNA) [68] and lipids [69] are also targets for
glycation. Indeed, measurements of CEdG [N2-(1-carboxyethylguanine], a
nucleotide glycation adduct, were found up to three-fold more CEdG in mtDNA of
cultured fibroblasts than in nuclear DNA [68], suggesting that
mtDNA is more vulnerable to glycation than nuclear DNA due to the absence of
protective histones in the former [70]. The presence of
glycated phosphatidylethanolamine in mitochondrial membranes has also prompted
suggestions that mitochondrial lipid glycation could affect electron transport
chain activity and cause mitochondrial dysfunction [69].
Intriguingly, specific mitochondrial protein
targets of glycation
have been identified in kidneys of streptozotocin-induced diabetic rats [71] and in livers
of aged
rats [18]. Increasing
the
removal of α,β-dicarbonyls either by administering
the prototypical dicarbonyl scavenger, aminoguanidine, or by overexpression of
glyoxalase I decreased both glycation and oxidative damage, restored complex
III activity, and improved respiration in experimental models of diabetes [71, 72]. Likewise,
increasing
glyoxalase I expression in nematodes decreased mitochondrial levels of
methylglyoxal and AGEs, and extended lifespan, while inhibiting the enzyme
increased methylglyoxal levels and reduced lifespan [3]. These results
suggest that glycation
of mitochondrial proteins could account, at least in part, for the
mitochondrial dysfunction and oxidative damage observed in hyperglycaemia and
ageing. Separately, changes in the expression of mitochondrial proteins have
been observed in Schwann cells grown under hyperglycaemic conditions [73] and in Akita
mice, a
mouse model of type 1 diabetes [74]. While the
molecular
mechanisms inducing these changes have not been elucidated, one study in
cultured endothelial cells found that glycation could cause epigenetic changes
in expression of nuclear-encoded genes [75], highlighting
the
potential for glycation to do likewise in mitochondrial genes, thereby
prompting the “remodelling of the mitochondrial proteome” as is observed in
Akita mice [74].
The hypothesis
that
glycation of mitochondrial targets profoundly influences mitochondrial function
is supported by studies in which exogenous dicarbonyls were administered and
mitochondrial parameters studied. For example, methylglyoxal treatment of
isolated mitochondria from rat kidney [76] and from several
carcinoma cell lines [77–79] decreased oxygen
consumption by mitochondria. Similarly, treatment of cultured cells with
methylglyoxal or glyoxal decreased mitochondrial membrane potential reduced the
activities of the respiratory chain complexes, reduced ATP synthesis, and increased reactive
oxygen species
(ROS) levels [80–83]. While these
studies
all point towards a glycation-induced mitochondrial dysfunction, the
physiological relevance of such experiments is uncertain. Some have suggested
that the use of millimolar concentrations of dicarbonyls in such experiments is
physiologically irrelevant [47, 84], especially
since
cellular levels of methylglyoxal have been estimated to be in the low
micromolar range. Others, however, have argued that these values, being
measures of steady state concentrations of dicarbonyls, do not accurately
reflect the actual dicarbonyl flux within cells where α,β-dicarbonyls are produced and that millimolar concentrations are
reasonable estimates of the actual flux in vivo [85]. [what about the effect in
humans over decades and the synergistic effect of reduce ATP???] That only a
very small proportion of exogenous dicarbonyl becomes incorporated into cells
has also been used to justify the use of high concentrations of exogenous
dicarbonyls [83, 86]. It should
also be
noted that in treating cultured cells with exogenous methylglyoxal or glyoxal,
extracellular, cytosolic and mitochondrial levels of these dicarbonyls and
their associated AGEs are all increased. Therefore it is unclear whether any
mitochondrial dysfunction observed is a direct result of the glycation of
mitochondrial targets or if it is a downstream consequence following the
glycation of extracellular or cytosolic targets especially since incubation of
cells with exogenous AGEs can similarly induce mitochondrial dysfunction [87–89].
3. Mitochondrial
Glycation in Cancer and Neurodegeneration:
A Hypothesis
As Experimental uncertainties notwithstanding,
the issue of
mitochondrial glycation and its links to disease is scientifically and
clinically interesting. Should mitochondrial glycation be a major cause of disease, then a
broad strategy of limiting glycation damage in mitochondria may ameliorate
disease initiation and progression across multiple clinical conditions. Such a strategy would be akin to that of targeting antioxidants
to mitochondria for the treatment of conditions as disparate as Parkinson’s disease,
diabetes and ischaemia-reperfusion injury [90]. In this
context, it
is intriguing to consider whether mitochondrial glycation could contribute to
conditions apart from diabetes, and ageing. We discuss here the possibility of
this hypothesis in cancer and neurodegeneration.
3.1. Cancer
The Warburg phenotype in which aerobic tumour
cells are largely
dependent on glycolysis, rather than oxidative phosphorylation, for their
energy supply has been well-described in many cancer cell lines [91]. Since triose
phosphate intermediates of the glycolytic pathway are an important source of
the major glycation precursor, methylglyoxal [38], the levels
of
methylglyoxal and consequently glycation should be increased in tumour cells.
It appears that the type and distribution of such glycation damage varies
between tumour types, and not all tumours necessarily display equal extents of
glycation [92]. In addition,
overexpression of glyoxalase I and glyoxalase II has been correlated with
multidrug resistance in tumours [93]. The upregulation
of
the glyoxalase enzyme system is thought to allow tumour cell growth by
counteracting the rise in methylglyoxal production [93]. However,
tumour
cells with high glyoxalase I expression have higher levels of DNA glycation
adducts than those with relatively lower expression [94] suggesting
that
glyoxalase I expression increases to try to counteract methylglyoxal-induced
cytotoxicity [94]. It is therefore
not
surprising that glyoxalase I inhibitors are being evaluated for use in cancer
chemotherapy [95] as they render
multidrug-resistant tumours vulnerable to apoptosis [96]. Glyoxalase
I
inhibitors may potentiate apoptosis by increasing DNA glycation that results in
PARP activation which depletes cellular nicotinamide adenine dinucleotide (NAD+) and thereby inhibits G3P dehydrogenase, leading to further
increases in methylglyoxal formation [97]. Methylglyoxal
also
induces cytotoxicity in a variety of other ways such as promoting the
mitochondrial permeability transition pore, activation of protein kinase
C-delta [93] and inhibition
of
STAT3-associated signaling [98].
As in diabetes and ageing, mitochondria are profoundly
affected in
cancers; the upregulation of glycolysis in tumour cells is accompanied by
mitochondrial dysfunction and decreased oxidative phosphorylation [99, 100]. For example,
a
recent review [101] argues that
such
mitochondrial dysfunction is critical for tumour growth, citing a strong
correlation between tumour progression and increased mitochondrial DNA (mtDNA)
mutations. Although the role of mitochondrial glycation in cancers has not been
explored, it is
possible that elevated glycolysis in cancer cells leads to increased glycation
and mutation of mtDNA [75]. Thus up-regulation of glycolysis
in tumour cells may contribute to increased mitochondrial damage and thereby
establish a vicious cycle that enhances the Warburg effect.
3.2.
Neurodegeneration
As triose phosphates are a major source of methylglyoxal
[38], any disturbance
in
triose phosphate metabolism will influence methylglyoxal formation. For
example, triose phosphate isomerase (TPI) deficiency results in an inherited
neurological disorder [102]. This occurs
because
TPI catalyzes the interconversion of DHAP and G3P, and its inhibition results in
an accumulation of DHAP which breaks down to methylglyoxal [103]. Methylglyoxal-induced
protein and nucleotide glycation might result in paralysis and
neurodegeneration as is observed in flies expressing a mutant form of TPI [104]. Nitrotyrosinated TPI
has been detected in brains from mouse models of, and human patients with,
Alzheimer’s disease [105], and this nitration of
TPI decreases its enzyme activity and increases methylglyoxal production [105]. Tau aggregation and
neurofibrillary tangle formation can also be promoted by methylglyoxal [106, 107]. These
suggest that methylglyoxal and glycation can contribute to
the progression of neurodegeneration, and this is supported by observations
that glyoxalase I expression increases in the brains of early and middle stage
Alzheimer’s patients [108], indicating
an
attempt to scavenge excessive α,β-dicarbonyls.
That type II diabetics are at 2–2.5 fold
greater risk of developing dementia also lends weight to a contribution of
glycation in neurodegenerative diseases [109]. It has also been
proposed that intra- and extracellular AGEs contribute to neurodegeneration by
two main pathways, the former by promoting protein aggregation and thus
inhibiting their proper function, and the latter by accumulating on senile
plaques and inducing oxidative stress and inflammation [110].
The role of mitochondrial dysfunction in neurological
disorders has
been extensively reviewed [111–113]. While mitochondrial
glycation was not specifically investigated in these studies, it may be that
glycation damage of proteins involved in oxidative phosphorylation and of mtDNA
may contribute to the mitochondrial dysfunction observed. Specific protein targets
of carbonylation, oxidation, and nitration have been identified in mitochondria
in neurodegenerative disorders [114, 115]. Since glycation
and
oxidative damage are closely correlated, with both types of damage markers
often increasing in parallel with ageing and disease [22–24], it is plausible
that
glycation of specific mitochondrial targets also occurs. A recent study in a mouse model of Alzheimer’s disease further found
that mitochondrial dysfunction precedes the presentation of any
neurodegenerative pathology [116]. Particularly
striking was the observation in the same study of increased glycolysis and
decreased oxidative phosphorylation in neurons from these mice, features
reminiscent of the Warburg phenotype of tumour cells. The many similarities between
the conditions discussed above—diabetes, ageing, and cancer—with
neurodegeneration suggest a potential for mitochondrial glycation to contribute
to these conditions.
4. Conclusion
In the four conditions discussed above—hyperglycaemia,
ageing,
cancer, and neurodegeneration—there is increased production of glycation
precursors, namely, reactive α,β-dicarbonyls
such as methylglyoxal and glyoxal, leading to elevated molecular glycation
damage as evidenced by the rise in levels of AGEs. However, despite the
correlation between glycation, mitochondrial dysfunction, and disease, the
relative importance of mitochondrial glycation as opposed to extracellular or
cytosolic glycation is still unclear. This is in part because it is the whole
cell levels of α,β-dicarbonyls that are altered
by
experimental manipulation with aminoguanidine or glyoxalase I overexpression,
and also because treatment of cell cultures with exogenous methylglyoxal or
glyoxal raises both extra- and intracellular dicarbonyl levels. As such, the
effects of any changes are not isolated to mitochondria. Besides, the amount of
exogenous dicarbonyl that is physiologically relevant is a further point of
contention. Unfortunately, it is technically difficult to accurately measure
dicarbonyl levels within mitochondria in cells and in vivo as existing
methods are prone to artefacts arising from variations in mitochondria
isolation, sample preparation, and derivatization [84, 117, 118]. There is
also a lack
of experimental tools for manipulating dicarbonyl levels within mitochondria
alone without modifying cytosolic and extracellular levels. Therefore, it is
difficult to isolate the effects of glycation observed in mitochondria from
more general consequences on the whole cell. Consequently, to divorce the
contribution of mitochondrial glycation from cytosolic or extracellular
glycation to disease is experimentally challenging. Nonetheless, the prospect
of mitochondrial glycation contributing as a common damaging agent across a
broad spectrum of diseases is an intriguing possibility and is also a novel
potential therapeutic target.
Abbreviations
ADP:
|
Adenosine diphosphate
|
AGEs:
|
Advanced
glycation end products
|
ATP:
|
Adenosine
triphosphate
|
CEdG:
|
N2-(1-carboxyethylguanine)
|
CEL:
|
Carboxyethyl-lysine
|
CML:
|
-carboxymethyl-lysine
|
DHAP:
|
Dihydroxyacetone
phosphate
|
ECM:
|
Extracellular
matrix
|
G3P:
|
Glyceraldehyde-3-phosphate
|
GSH:
|
Reduced
glutathione
|
LDL:
|
Low-density
lipoprotein
|
MG-H1:
|
[-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine]
|
MOLD:
|
Methylglyoxal-derived
lysine dimers
|
mtDNA:
|
Mitochondrial
DNA
|
:
|
Nicotinamide
adenine dinucleotide
|
PARP:
|
Poly(ADP-ribose)
polymerase
|
RAGE:
|
Receptor
for AGEs
|
ROS:
|
Reactive
oxygen species
|
TPI:
|
Triose
phosphate isomerase.
|
Acknowledgments
The authors would like to thank the Medical
Research Council and
the Cambridge Commonwealth Trust for funding our work.
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