Pathobiochemistry of
atherosclerosis, ischemic heart
disease, cardiac markers
Vascular structure
- Blood vessels consist of 3 layers:
1. Tunica intima (formed by
endothelial cells).
2. Tunica media (formed by smooth
muscle cells and elastic fibers).
3. Tunica adventitia (formed by
collagen fibers).
Atherosclerosis
= chronic progressive disease of a vascular wall.
- It is characterized by the accumulation of lipids and other components of blood
and fibrous tissue in the intima of the arteries, accompanied by changes in a media
of vascular wall.
.
- Atherosclerosis develops as a chronic inflammation with an excessive proliferative
response of the intima and the media to various stimuli, mainly modified LDL (low
density lipoproteins).
- The development of atherosclerosis is the result of interactions between
lipoproteins and cells of the immune system (mainly macrophages and Tlymphocytes)
and other cell types (endothelial cells, smooth muscle cells and
platelets).
Development of atherosclerosis
- The development of atherosclerotic lesion (damage) is a long-term process.
- The first step is probably endothelial cell damage.
- Monocytes and T lymphocytes are adhered to the damaged endothelial cells. These then
penetrate into the space of intima.
- Conversion of monocytes and T-lymphocytes into macrophages in the intima space. Macrophages
= major cells involved in atherogenesis.
- Increased permeability of endothelial lining → increased penetration of low-density lipoprotein
particles into the subendothelial space.
- Lipoperoxidation in lipoprotein particles by reactive forms of oxygen and nitrogen.
Absorption of lipoprotein particles by macrophages. Lipoproteins are taken up by macrophages
by scavenger receptors → accumulation of lipoprotein particles in macrophages and their
transformation into foam cells = basis for the formation of atheroma plaques.
- Scavenger receptors recognize oxidized LDL particles due to structural modification in Apo B100.
LDL particles that have undergone lipoperoxidation cannot be removed by LDL receptors (do not
recognize modified Apo B100).
Development of atherosclerosis
- Oxidized LDL stimulates cells of smooth muscle, endothelial cells and
monocytes to produce chemotactic and growth factors such as: PDFG
(platelet growth factor), FGF (fibroblast growth factor), IL 1β, TNFα,
and heparin-binding epidermal growth factor. → increased smooth
muscle cell replication → accelerated atherogenesis process.
- Endothelial cells, due to oxidized LDL, induce formation of tissue
factor and reduce formation of plasminogen activator inhibitor →
conditions to accelerate platelet aggregation and thrombus
formation, especially where the atheroma plaque is calcified and thus
more susceptible to rupture.
Atheroma
= atherosclerotic plaque, atheroma plaque.
- Composed of a central lipid core (mainly cholesterol) covered with a fibrin cap
(smooth muscle of vascular cells, collagen and elastic fibers).
- The composition of atheroma affects its stability, we distinguish:
Stable atheroma: characterized by a strong and intact cap.
Unstable atheroma: characterized by a higher content of lipids, foam cells, Tlymphocytes
in the nucleus. The fibrous cap is thin and contains less collagen.
- Unstable plaques are more susceptible to rupture due to ongoing inflammation.
Macrophages produce proteolytic enzymes (metalloproteinases such as
collagenases, elastases, stromelysin) that weaken the fibrous cover. Tlymphocytes
produce interferon gamma suppressing collagen production and
CD40 promoting metalloproteinase synthesis.
- Plaque rupture → plaque hemorrhage + thrombus formation → acute coronary
syndrom.
Areas affected by atherosclerosis + its clinical manifestations
- Atherosclerosis occurs most frequently in the artery branching areas turbulent
flow (hemodynamic stress, inflammatory response).
- Clinical manifestation:
in coronary arteries → ischemic heart disease (IHD).
In cerebral arteries → stroke.
In lower extremity arteries → ischemic disease of the lower
extremities (leg ischemia).
Risk factors for the development of atherosclerosis:
Uncontrollable risk factors:
- Older age
- Male gender
- Positive family history
- Genetic predisposition (DM, hypertension, but also hereditary disorders of lipid metabolism such as familial hypercholesterolaemia).
Controllable risk factors:
- Obesity
- Wrong lifestyle (physical inactivity, high intake of sugars, cholesterol and trans unsaturated fatty acids).
- Hypertension (untreated hypertension contributes to vascular endothelial damage).
- Diabetes mellitus
- Smoking (contributes to vascular endothelial damage).
- Dyslipidemia
Other risk factors:
- Estrogen deficiency in postmenopausal women.
- Infections (contributing to epithelial dysfunction, e.g. Chlamydia pneumoniae, Cytomegalovirus, Haemophilus influenzae, Helicobacetr pylori, EpsteinBarr
virus).
Biochemical risk factors for the development of atherosclerosis:
- ↑ total cholesterol
- ↑ LDL-cholesterol
- ↓ HDL-cholesterol
- ↑ TAG (triacylglycerols)
- Hyperglycemia
- Hyperinsulinemia
- ↑ level of homocysteine
Gender and estrogen deficiency in postmenopausal women
- Women in reproductive age are protected from atherosclerosis and
its consequences against men of the same age.
- This protection in women decreases during menopause, when
estrogen decreases and as a result LDL cholesterol and triglyceride
levels increase and protective HDL cholesterol levels are stagnating or
decreasing.
↑ total cholesterol, ↑ LDL-cholesterol, ↓ HDL-cholesterol
- Cholesterol is always present in the atherosclerotic plaque.
- Atherogenicity of cholesterol is due multiplication lipoproteins
containing apolipoprotein B100 that transport cholesterol to
peripheral tissues = atherogenic lipoproteins.
- HDL is a lipoprotein that transports cholesterol into other lipoproteins
and to the liver = anti-atherogenic lipoprotein.
- ↑ pro-atherogenic lipoproteins (VLDL, IDL, LDL, Lp (a)) + ↓ HDL =
higher risk development of atherosclerosis.
↑ level of homocysteine
- Homocysteine is a non-essential amino acid that is produced by the
metabolism of methionine.
- ↑ level of homocysteine support the process of atherosclerosis by
directly damaging the vascular endothelium, stimulating oxidative
stress and vascular myocyte proliferation, and reducing local nitric
oxide production.
- Vitamins B6, B13 and folic acid play an important role in the
metabolism of homocysteine → a diet with a higher content of these
vitamins or their supplementation can help reduce homocysteine
levels.
Diabetes mellitus, hyperglycemia, hyperinsulinemia
- Long-term hyperglycaemia contributes to oxidative stress. Hyperglycaemia
affects the formation of free radicals, which can lead to lower levels of
antioxidants and increase lipid peroxidase.
- In hyperglycemia, a non-enzymatic protein glycation process occurs,
resulting in advanced glycation end products (AGEs). AGEs react with
specific receptors (RAGE). Binding of AGE to RAGE leads to activation of NFκB
and oxidative stress → damage of the vascular endothelium.
- Hyperinsulinemia accelerates atherosclerosis by several mechanisms: it
stimulates lipogenesis leading to higher VLDL synthesis/secretion, affects
the growth and proliferation of vascular smooth muscle cells, activates
genes involved in the anti-inflammatory response, and stimulates collagen
synthesis. Furthermore, hyperinsulinemia causes sodium retention, arterial
hypertension and weight gain.
Ischemic heart disease (IHD)
- IHD is defined as myocardial ischemia due to a pathological process in the coronary
arteries.
- Causes of disorder of myocardial perfusion:
A) Causes of organic origin (atherosclerosis, thrombus, embolism, arteriitis, dissection of
the coronary artery).
B) Causes of functional origin (spasm of the coronary artery).
- Very common is combination of causes of organic origin with cause of functional origin.
- The most common cause of IHD is unstable atherosclerotic plaque in coronary artery →
thrombus formation above plaque → coronary artery occlusion → acute MI
- Basic classification of IHD:
- Acute forms of IHD (unstable angina pectoris, acute myocardial infarction (MI), sudden death)
- Chronic forms of IHD (angina pectoris, vasospastic angina pectoris, silent ischemia, syndrome X,
IHD with arrhythmias, IHD with heart failure).
IHD
- Ischemia occurs when oxygen supply demands outweigh the
possibilities of perfusion.
- Increased oxygen demand for the myocardium can be induced by
physical exertion, increase in systolic pressure or tachycardia.
Myocardium responds to chronic ischemia by developing a collateral
vascular bed that helps blood to circumvent stenosis, allowing the
supply of myocardium.
- Risk factors contributing to IHD: hypertension, lipid metabolism
disorders, smoking, diabetes mellitus, obesity, lack of physical activity,
stress, positive family history, male gender, etc.
Symptoms of IHD
- The main symptom is chest pain (astringent, burning) = stenocardia,
chest pressure, feeling of shortness of breath. The pain response to
nitroglycerin administration is important. The patient may also be
pain free (silent ischemia, e.g. in diabetics). Patients with DM may
have damaged nerve fibers transmitting painful stimuli → they do not
feel stenocardia, but myocardium is not supplied with oxygen).
- Other symptoms of IHD include dyspnea (after exertion, at rest,
nighttime), swelling of the lower limbs, syncope, palpitations.
- The basic examination method is ECG + determination of cardiac
markers.
Chronic forms of IHD
- Angina pectoris (AP): is characterized by seizure stenocardia due to
transient myocardial ischemia due to physical stress, emotional stress ...
Due to atherosclerotic coronary artery damage, there is a disproportion
between the supply and consumption of oxygen in the myocardium.
Stenocardia disappears after finishing of physical stress, when demand of
myocardium for oxygen decreases.
- Vasospastic AP: arises due to spasm of coronary artery, stenocardia not
manifesting in physical activity. Factors causing vasospastic AP include
smoking, emotional stress, rapid cooling…
- Syndrome X: The cause of the syndrome is damage to small arterioles
(microvascular AP). Stenocardia after exercise and normal coronarography
is present.
Acute forms of IHD
- Unstable AP: is defined as a newly formed AP or an existing AP that has
deteriorated in the last 4 weeks. The deterioration may relate to a higher
frequency of complaints, increased pain intensity or prolonged pain.
- Sudden death: is defined as death within 1 hour of the onset of symptoms.
70% is caused by IHD.
- Acute MI = acute necrosis of the area of cardiomyocyties due to prolonged
ischemia. The cause of ischemia is a sudden coronary artery occlusion or
extreme progressive stenosis. Stenocardia occurs regardless of physical
stress. Pain persists after nitrate administration. Coronary artery occlusion
is most often caused by a thrombus formed after the rupture of an
unstable atherosclerotic plaque. Coronary artery occlusion may also be due
to the following causes: arteriitis, trauma, aortic dissection or embolism in
the coronary artery. Necrotic tissue is replaced by connective tissue (infarct
scar).
Cardiac markers
- Cardiac markers = biochemical indicators of ischemia, necrosis and hemodynamic
changes affecting the heart. It is used to detect and evaluate the severity of acute forms
of IHD (acute MI, unstable AP) and to determine the prognosis of people who have had
MI. Appropriate treatment may be initiated according to cardiac marker levels.
- cardiac markers = enzymes, proteins and hormones found in cardiomyocytes. Normally,
their blood concentration is very low. When the heart musce cells are damaged (mainly
when they die), these enzymes, proteins and hormones begin to release → ↑ their
blood concentration.
- Cardiac markers are examined from a blood sample.
- Different cardiac markers have different times of their rise, peak and decrease → we can
determine the progression of MI, the time of onset of IM or predict its possible
recurrence.
- Specific cardiac markers (their levels are increased only when the heart is damaged) x
non-specific cardiac markers (their levels may also be increased when other tissues, such
as skeletal muscle, are damaged).
Major cardiac markers - troponin
- Troponins are proteins, which in the form of the so-called troponin
complex, together with actin and tropomyosin, are part of the thin muscle
fibers.
- Troponin exists in several molecular isoforms.
- Tropononin T (TnT) and troponin I (TnI) are used as cardiac markers.
TnT and TnI occur in skeletal muscle and myocardium. Cardiac isoforms
(cTnT and cTnI) have a unique amino acid composition and are therefore
specific for the myocardium.
- Troponin levels remain elevated for up to 2 weeks → the possibility of
revealing myocardial damage.
- Examination: immunochemical methods.
- Troponin = cardiac marker of the first choice.
Major cardiac markers – creatinine kinase
- Creatine kinase (CK) catalyzes the reversible phosphorylation of creatine to
creatine phosphate by ATP
- CK consists of 2 subunits: B (brain), M (muscle).
- According to the representation of individual subunits we can distinguish 3
isoforms of CK: CK-BB (brain isoenzyme), CK-MM (muscle isoenzyme) and
CK-MB (myocardial isoenzyme).
- CK-MB is not completely cardiospecific (an increase may also be caused by
skeletal muscle damage).
- CK-MB can be determined as an enzyme activity or preferably
immunochemically as a protein in the form of a mass concentration (CKMB
mass).
- Determination of CK-MB mass is more sensitive and specific → we also
show partially degraded molecules that have lost enzymatic activity.
Major cardiac markers – lactate dehydrogenase
- Lactate dehydrogenase (LDH, LD) = oxidative-reducing enzyme catalysing the conversion of lactate to pyruvate.
- LDH consists of 4 subunits. Each of these subunits can be either M (muscle) or H (heart) → we recognize 5
isoenzymes:
- Increased catalytic LDH concentration in serum accompanies many diseases. It is currently used as a non-specific marker of
cell breakdown, e.g. in cancer. It is characterized by an increase in total LDH after MI, which may persist for up to 15 days.
Hemolysis (high LDH content in the erytrocytes) may falsely increase serum LDH concentrations. The use of LDH and its
isoenzymes for the diagnosis of acute coronary syndrome is currently obsolete.
- LDH determination: optical tests, electrophoretic determination of LDH isoforms.
Isoenzyme Subunits Occurrence
LD1 H4 Myocardium + erytrocytes
LD2 H3M1 Myocardium + erytrocytes
LD3 H2M2 Skeletal muscle
LD4 H1M3 Liver + skeletal muscles
LD5 M4 Liver + skeletal muscles
Major cardiac markers – aspartate aminotransferase
- Aspartate aminotransferase (AST) catalyzes the reversible transfer of
the amino group from aspartate to 2-oxoglutarate.
- AST is highly present in the myocardium, but it can also be found in
the liver, skeletal muscle, erythrocytes, kidneys, pancreas…
- AST is currently not recommended for the diagnosis of acute coronary
syndrome.
- The determination of AST is used in liver assays to determine
hepatocyte damage.
Major cardiac markers - myoglobin
- Myoglobin = hemoprotein, consisting of a single polypeptide chain
and one heme group.
- Reversibly binds and carries oxygen in muscle cells.
- Skeletal muscle and myocardial myoglobin is identical.
- After myocardial damage, it is rapidly released into the blood →
suitable for early diagnosis of acute MI.
- Disadvantage: insufficient cardiospecificity. It may also be increased
due to skeletal muscle damage, after muscle load, in renal
insufficiency.
Enzyme Mr
[Da]
Biological half-life Localization in the cell
CK 86
000
17 h cytoplasm
CK-MB 86
000
13 h
LD
(mainly LD1)
135
000
110 h
Myoglobin 17
800
15 min.
cTnT (cytoplasmic fraction) 37
000
2 – 4 h
cTnI (cytoplasmic fraction) 22
500
2 – 4 h
cTnT 37
000
2 – 4 h fibrillar contractile
complex
cTnI 22
500
2 – 4 h
AST (mitochondrial isoenzyme) 93
000
34 h mitochondria
Markers Increase Peak level Normalization Physiological value
Myoglobin 0,5 – 2 h 4 – 10 h 0,5 – 1 day
M 19 – 92 μg/l
F 12 – 76 μg/l
CK-mass 2 – 6 h 12 – 24 h 2 – 3 days 0,0 – 5,0 μg/l
CK-MB 3 – 6 h 16 – 36 h 3 – 5 days
M 0,2 – 3,6 μkat/l
F 0,2 – 3,1 μkat/l
cTnT 3 – 8 h
12 – 18 h (1. peak)
72 – 96 h (2. peak)
7 – 14 days 0,00 – 0,05 μg/l
cTnI 3 – 12 h 12 – 24 h 5 – 10 days 0,0 – 0,1 μg/l
AST 4 – 8 h 16 – 48 h 3 – 6 days
0,05 – 0,72 μkat/l
LD 6 – 12 h 24 – 60 h 7 – 15 days
3,5 – 7,7 μkat/l
Basic characterization of biochemical markers of myocardial infarction Levels of biochemical markers in acute myocardial infarction
Major cardiac markers – natriuretic peptides
- Natriuretic peptides = tissue hormones whose function is to stimulate the secretion of
Na+ (and water) in response to an increase in blood pressure or cardiac muscle tension,
thereby reducing circulating volume.
- There are several natriuretic peptides:
1. Natriuretic peptide A (ANP, atrial): is secreted by cardiomyocytes in the form of
prohormone, which is cleaved into 2 fragments → self ANP (biologically active) and Nterminal
fragment (N-BNP, biologically inactive). Its secretion is increased by tension in
the wall of the atrium.
2. Natriuretic peptide B (BNP, brain): it was first described in pig brain (hence its name).
In humans, it is secreted by ventricular cardiomyocytes in response to increased tension
in wall of ventricle or ventricular dilatation. It is secreted in the form of prohormone,
which is cleaved into 2 fragments → self BNP (biologically active) and N-terminal
fragment (NT-proBNP biologically inactive).
3. Natriuretic peptide C (CNP): is secreted by the vessel endothelium in response to
endothelial stress.
- BNP or its N-terminal propeptide NT-proBNP is most suitable for diagnosis of cardiac
insufficiency.
References:
SOVOVÁ, Eliška a Jarmila ŘEHOŘOVÁ. Kardiologie pro obor ošetřovatelství. Praha: Grada, 2004. ISBN 80-247-1009-9.
MASOPUST, Jaroslav. Patobiochemie metabolickcýh drah. Univerzita Karlova, 1999. ISBN 8023845896.
KALOUSOVÁ, Marta. Patobiochemie ve schématech. Praha: Grada, 2006. ISBN 80-247-1522-8.
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