Transcript END2.4 - Diabetes mellitus 4 Biochemistry of diabetic

END2.4 - Diabetes mellitus 4
Biochemistry of diabetic
complications
©Dr S Nussey
Hyperglycaemic hypothesis
• DM is associated with 2 types of complication – Macrovascular - i.e. accelerated atherosclerosis
– Microvascular - affecting predominantly the eye,
nerves and kidneys
• Microvascular complications are specific to
diabetes
• Although subject to genetic influences,
microvascular complications are related to the
duration and quality of glucose control
Protein structure
• Protein structure
determines function
• Changes in structure
alter function
• In these examples,
lens crystal is rendered
opaque and collagen
inflexible
Cataract
Cheiroarthropathy
Cell metabolism
• Alterations in cellular
metabolism affect cell
function
• In this example,
disturbances in
axoplasmic transport
affect nerve function
Neuropathic ulcer on heel
Extracellular matrix structural
changes
• Deposition of
interstitial materials
may affect function
• In this case,
glomerular function is
severely affected
leading to renal failure
Normal
Diabetic
Cellular proliferation
• Changes in cell
signalling may lead to
cell division
• In this example,
proliferation of new
blood vessels in the
retina lead to blindness
Proliferative retinopathy
Vascular function
• Changes may lead to
an increase in vascular
permeability
• In this example,
production of exudates
or haemorrhages in the
retina lead to blindness
Four major hypotheses*
• Increased activity of aldose reductase
(sorbitol pathway)
• Formation of reactive oxygen species (‘freeradicals’)
• Increased production of advanced glycation
end-products (AGE)
• Activation of protein kinase C (PKC)
*Note - these are potentially linked in complex ways
Principle of hyperglycaemic
‘memory’
• Experimentally, the damage done to tissue
during periods of hyperglycaemia is
‘remembered’.
• Thus, after a period of poor diabetic control
complications occur at an accelerated rate
even though subsequent glycaemic control
is excellent.
Q - Why is an understanding of the
biochemistry of diabetic
complications important?
A - Because it leads to therapeutic
opportunities.
Sorbitol pathway
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Increased fructose leads to: osmotic changes;
non-enzymic fructosylation and AGE (via 3deoxyglucasone)
Decreased NADPH/NADP+ leads to: alteration
in redox state(decreased ability to deal with
oxidative stress); increased activity of pentose
phosphate shunt (PPP)
Increased NADH/NAD+ leads to: increased
activity of PPP
Increased triose phosphate intermediates leads to
increased second messenger diacylglycerol
(DAG) and, thus, Protein Kinase C activity.
Increase PKC activity leads to a wide variety of
changes (see later)
Arachidonate in DAG may be a substrate for the
synthesis of eicosanoids including
prostaglandins, prostacyclins, thromboxanes and
leukotrienes all of which have potent vascular
(and other) actions
Pentose phosphate pathway
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Important in the generation of:
– NADPH as source of reducing
power in biosynthesis
– ribose phosphate to synthesize
RNA, DNA, FAD, CoA
The red loop indicates the
reversibility via transketolase and
transaldolase and the intermediates
fructose 6-phosphate and
glyceraldehyde 3-phosphate thus
linking the PPP with glycolysis
Aldose reductase inhibitors
• Good results in animal models
• Less good in human trials
• Why?
– AR not expressed in all tissues affected by DM
e.g. endothelial cell
– most trials short-term but complications accrue
over many years
– other pathways more important?
Free radicals in DM - ‘Oxidative
stress’
• Increased production e.g. by auto-oxidation
of glucose, superoxide production from
mitochondrial oxidation of NADH to NAD+
• Decreased clearance via action of catalase
or glutathione peroxidase. Regeneration of
reduced glutathione requires NADPH,
levels of which are decreased (in tissues
containing aldose reductase)
‘Carbonyl stress’
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The idea of carbonyl stress arose from the
recognition that not all damaging processes
required oxidation.
Carbonyl products (e.g. methylglyoxal and
3-deoxyglucasone) are obtained nonoxidatively via the PPP and inhibit
glutathione reductase.
These may form AGE or, in the presence
of membrane lipids, form lipid dialdehydes
that form glycoxidated adducts.
Note that AR has apparently deleterious
effects at 1 but potentially beneficially
ones at 2 and 3
AGE
• Maillard reaction
‘browning’ of food
described in 1912
• Non-enzymatic
glycation of amines
AGE
CML
Pentosidine
MOLD
AGE
AGE
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Receptors for AGE include:
macrophage scavenger receptor for
AGE (types I and II); oligosaccharyl
transferase-48; 80-K H phosphoprotein
and galectin
The scavenger receptors belong to the
immunoglobulin superfamily
Expressed on wide variety of cells and
expression is increased in animal
models of DM
AGE-RAGE interactions are important
in a number of inflammatory conditions
They also occur in atherosclerosis and
this draws both micro- and macrovascular complications together
pathophysiologically
AGE directed therapies
• Aminoguanidine an inhibitor of AGE
formation has good results in animal
nephropathy models
• Undergoing phase III clinical trials
• Others include phenacylthiazolium
Protein kinase C
PKC
X = processes inhibited by LY333531
PKC directed therapy
• LY333531- an orally active inhibitor of
PKC-bII isoform- good results in animal
models of both retinal and renal disease
Overview