CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 10, November 2012
AFRICA
565
•
Factors that maintain telomere length:
––
Telomerase: in addition to the level of telomerase within
a cell, telomere length is also dependent on the delivery
of 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 lengthen-
ing 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 anti-
atherosclerotic agent that protects vascular cells from apop-
tosis.
51-53
•
Inflammation: ROS increase the production of pro-inflamma-
tory 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 forma-
tion 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 pro-
atherogenic effects are exerted by influencing the phospho-
inositol-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.
