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SA JOURNAL OF DIABETES & VASCULAR DISEASE
REVIEW
VOLUME 12 NUMBER 2 • NOVEMBER 2015
55
telomerase to the telomere by Cajal bodies, telomerase access
to the DNA terminus and the presence of molecules that
stimulate or inhibit telomerase activity.
31
– A recombination process known as alternative lengthening
of telomeres or ALT (10% of cancers maintain their telomere
length by ALT).
35,37
The two major mechanisms responsible for telomere shortening are
the end-replication problem, and more importantly, the oxidative
DNA damage induced by environmental risk factors. Telomere
shortening due to the end-replication problem is relatively small
and constant in each cell, irrespective of telomere length, whereas
telomere shortening induced by oxidative stress is proportional to
telomere length, as longer telomeres are larger targets for free
radicals.
38-40
Variability in telomere length is also noted at birth and is
influenced by heredity, race and gender. Telomere length has been
shown to be shorter in healthy offspring of patients with coronary
artery disease (CAD).
16,17
This finding offers some explanation for
the increased familial risk of CAD and also implies that shorter
telomeres are likely a primary abnormality in the pathogenesis of
the disease.
41
African-Americans have longer telomeres than whites
and Indians,
42-44
and females have longer telomeres than their male
counterparts.
45
Mechanisms of disease: a balance between injury and
repair
Mechanism of injury: oxidative stress
Oxidative stress is the unifying pathophysiological mechanism
responsible for ageing and age-related disorders.
46-49
It is defined
as an increase in the intra-cellular concentration of reactive oxygen
species (ROS). ROS are generated during regular metabolism
because of incomplete oxygen reduction in the mitochondrial
electron transport chain – a one-electron reduction of oxygen forms
superoxide (O
2
-
), a two-electron reduction forms hydrogen peroxide
(H
2
O
2
), and a three-electron reduction forms the hydroxyl radical
(OH). Many other ROS species can be derived from superoxide and
hydrogen peroxide.
These ROS initiate processes involved in atherogenesis through
several enzyme systems including xanthine oxidase, NADPH
(nicotinamide adenine dinucleotide phosphate) oxidases and
nitric oxide synthase.
50
The ROS damage all components of the
cell including proteins, lipids and DNA. The exact mechanism of
damage is via:
• Decreased availability of nitric oxide (NO), which results
in defective endothelial vasodilation. Nitric oxide is an
antiatherosclerotic agent that protects vascular cells from
apoptosis.
51-53
• Inflammation: ROS increase the production of pro-inflammatory
cytokines such as tumour necrosis factor alpha (TNF-
α
), which
in turn can also increase the production of ROS. TNF-
α
activates
two transcription factors: nuclear factor kappa-
β
(NF-
κβ
) and
activator protein-1 (AP-1), which increase the expression of
pro-inflammatory genes. Cytokines stimulate the synthesis
of acute-phase reactants such as C-reactive protein (CRP) by
the liver. ROS also increase the expression of cellular adhesion
molecules on the endothelial cell surface. These molecules,
intercellular adhesion molecule 1 (ICAM-1) and vascular cell
adhesion molecule 1 (VCAM-1), enhance monocyte adhesion
to endothelial cells and lead to the formation of atherosclerotic
plaques.
54-58
• Modification of lipoproteins and lipids: ROS contribute to the
formation of lipid peroxides, which bind to proteins to form
advanced lipoxidation end products (ALEs).
59
Oxidised LDL and
ALE-containing LDL are pro-atherogenic.
In vitro
studies have
shown that LDL cholesterol (LDL-C) is not atherogenic in itself
but it is the oxidative modification of LDL-C that plays a critical
role in the pathogenesis of atherosclerosis.
60,61
In the early
phase of atherosclerosis, oxidised-LDL (ox-LDL) contributes to
inflammation by enhancing expression of chemokines such as
the monocyte chemo-attractant protein-1. Ox-LDL decreases
the bioavailability of nitric oxide. The proatherogenic effects are
exerted by influencing the phosphoinositol-3 (PI3) kinase/Akt
signalling pathway.
62
This pathway has an important regulatory role in cellular
proliferation and survival. Of the three known isoforms of Akt,
Akt 1 is most relevant in regulating cardiovascular cell growth
and survival and Akt 2, which is highly expressed in muscle and
adipocytes, contributes to regulation of glucose homeostasis.
These isoforms are activated by growth factors, extra-cellular
stimuli such as pro-atherogenic factors and by oncogenic
mutations in upstream regulatory proteins. Akt mediates
downstream signalling pathways through phosphorylation of
a host of substrates. Thus far, more than a hundred substrates
for Akt have been identified, indicating that it has widespread
biological effects. Dysregulation of Akt is associated with
cardiovascular disease, diabetes, cancer and neurological
disorders.
Our current understanding of its role in cardiovascular
disease is incomplete and studies explaining its effects describe
conflicting mechanisms. Breitschopf
et al
. have demonstrated
that pro-atherogenic factors such as ox-LDL, TNF-
α
and
hydrogen peroxide promoted endothelial cell senescence by
inactivation of the PI3/Akt pathway. Akt was shown to maintain
telomerase activity by phosphorylation of its TERT subunit,
and inactivating Akt reduced telomerase activity, leading to
accelerated endothelial cell senescence.
63
On the other hand, Miyauchi
et al
. demonstrated that
activation of Akt promotes senescence and arrests cell growth
via the p53/p21-dependent pathway and that inhibition of Akt
extends the lifespan of primary cultured human endothelial cells.
Akt achieved growth arrest by phosphorylating and inhibiting a
forkhead transcription factor (FOXO 3a), which influences p53
activity by regulating levels of ROS.
64
Rosso
et al
. confirmed the
latter mechanism by demonstrating that endothelial progenitor
cells cultured in the presence of ox-LDL in a diabetic milieu
underwent senescence and growth arrest by activation of the
Akt pathway via accumulation of p53/p21.
65
Miyauchi
et al
.
commented that the divergent observations may be explained
by the different cell types used in studies. They used primary
human endothelial cells, whereas most other studies examined
immortal cells in which the normal cell cycle machinery may have
been impaired. In addition, Akt may promote cell proliferation
or senescence depending on other factors such as the duration
and extent of its activation. It has been noted that activation of
Akt in itself is insufficient to cause cancer unless combined with
other oncogenic stimuli.
There is currently much interest in the development of Akt
inhibitors for the treatment of cancer and it remains to be seen
what effects such therapy would have on the cardiovascular
system. In addition to Akt signalling, mitogenic stimuli