Download Serum total Osteocalcin level as a vascular marker in

Serum total Osteocalcin level as a vascular marker in
elderly patients with metabolic syndrome
BY
NANY HASAN
ASSISTANT PROFESSOR OF INTERNAL MEDICINE
GERIATERICS DEPARTMENT
ALEXANDRIA UNIVERSITY
EGYPT
Introduction
Metabolic syndrome (MetS) is a major public-health problem and clinical challenge
worldwide. Metabolic syndrome is a constellation of interconnected physiological,
biochemical, clinical and metabolic factors that directly increases the risk of
atherosclerotic cardiovascular diseases, type 2 diabetes mellitus and all-cause mortality.
Its main components are:
Dyslipidemia, elevation of arterial blood pressure, and dys-regulated glucose
homeostasis, while abdominal obesity and/or insulin resistance (IR) have
gained increasing attention as the core manifestations of the syndrome.
Recently, other abnormalities such as chronic pro-inflammatory and prothrombotic states, non-alcoholic fatty liver disease and sleep apnea have
been added to the entity of the syndrome, making its definition even more
complex(1).
Dysfunctional adipose tissue plays an important role in the pathogenesis of obesityrelated insulin resistance. Both adipose cell enlargement and infiltration of
macrophages into adipose tissue result in the release of pro-inflammatory cytokines
and promote insulin resistance . Insulin resistance appears to be the primary mediator
of metabolic syndrome .
The distribution of adipose tissue appears to affect its role in metabolic syndrome.
Visceral or intra-abdominal fat correlates with inflammation, whereas subcutaneous fat
does not. There are numbers of potential explanations for this, including experimental
observations that omental fat is more resistant to insulin and may result in a higher
concentration of toxic free fatty acids in the portal circulation.
Abdominal fat is known to produce potentially harmful levels of cytokines, such as
tumor necrosis factor, adiponectin, leptin, resistin, and plasminogen activator
inhibitor(2).
The most commonly used criteria for definition of metabolic syndrome at present are
from:
• The World Health Organization (WHO)
• The European Group for the study of Insulin Resistance (EGIR)
• The National Cholesterol Education Program Adult Treatment Panel III (NCEP
ATP III)
• American Association of Clinical Endocrinologists (AACE)
• The International Diabetes Federation (IDF)
Currently, the two most widely used definitions are those of the NCEP: ATP III and
IDF focusing specifically on waist circumference, which is a surrogate measure of
central obesity. In contrast, the AACE, WHO and the EGIR definitions are all largely
focused on insulin resistance.
Prevalence of metabolic syndrome
Clearly, the prevalence of MetS varies and depends on the criteria used in different definitions, as well
as the composition (sex, age, race and ethnicity) of the population studied. No matter which criteria are
used, the prevalence of MetS is high and rising in all western societies, probably as a result of the
obesity epidemic. According to National Health and Examination Survey (NHANES) 2003-2006 ,
approximately 34% of people studied met the NCEP: ATPIII revised criteria for MetS. The prevalence
of metabolic syndrome increases with age, with about 40% of people older than 60 years meeting the
criteria .
Several epidemiological studies have confirmed the increased risk of CVD in individuals with MetS,
independently of the diagnostic criteria used. Overall a range of 1.5-3 times greater risk of CVD and
CHD mortality has been found in several prospective studies, whereas a recent meta-analysis showed
that MetS was associated with a 2-fold increase in cardiovascular outcomes and a 1.5-fold increase in
all-cause mortality.(3-4)
The pathophysiology is very complex and has been only partially elucidated. Most
patients are :
Old
Obese
Sedentary
Have a degree of insulin resistance
Stress can also be a contributing factor.
The most important factors are:
Genetics
Aging
Diet (particularly sugar-sweetened beverage consumption)
Sedentary behavior, or low physical activity
Disrupted sleep
Mood disorders
Psychotropic medication use
Excessive alcohol use
Osteocalcin
Osteocalcin is a non-collagenous, 49 amino acid glutamate-rich polypeptide bone
matrix protein with a molecular weight of about 5800 kDa. Osteoblasts produce
osteocalcin and incorporate it into the bone matrix. Osteocalcin is released into the
circulation from the matrix during bone resorption and, therefore, is considered a
marker of bone turnover rather than a specific marker of bone formation . Osteocalcin,
has raised much attention as a hormone regulating glucose metabolism and fat mass.
Recently, osteocalcin has been recognized as a bone-derived hormone to regulate
energy metabolism.
Osteocalcin knockout mice exhibited glucose intolerance, increased fat mass, insulin
resistance, decreased expression of insulin target genes in liver and muscle, and
decreased adiponectin gene expression in adipose tissue, while administration of
recombinant osteocalcin increased insulin secretion, decreased blood glycaemia and
weaken the development of obesity. (5)
Figure 1: Osteocalcin synthesis in osteoblasts. The BGLAP gene encoding osteocalcin is mainly expressed in
osteoblasts and to lesser extent odontoblasts. After transcription (which is stimulated by vitamin D) the
preproosteocalcin peptide undergoes proteolysis giving rise to a prepeptide (23 aa) and a proosteocalcin
peptide (75 aa). The latter can be carboxylated at Glu residues 17, 21, and 24, resulting in formation of Gla
residues in a vitamin K dependent process. Generally, this process only occurs in a proportion of newly
synthesized pro-osteocalcin. Then Gla and Glu pro-osteocalcin peptides are subjected to a final proteolytic
process that produces, respectively, carboxylated and undercarboxylated osteocalcins. Both forms are
released from osteoblasts in a process which is calcium dependent. While the carboxylated Gla residues are
involved in calcium and hydroxyapatite binding, allowing osteocalcin deposition on mineralized bone matrix,
undercarboxylated osteocalcin has a low affinity for hydroxyapatite and is more easily released into the
circulation.
Osteocalcin undergoes γ-carboxylation. The γ-carboxylated form binds hydroxyapatite
and is abundant in bone extracellular matrix. In contrast, the under-carboxylated
circulating form has been implicated as a novel hormone and positive regulator of
glucose homeostasis.
The uOC acts directly on the β-cells to increase their mass and proliferation and
therefore increases insulin secretion. Moreover, it influences white adipocytes to induce
the expression of genes involved in energy expenditure and to enhance the secretion of
adiponectin, thus increasing insulin sensitivity. Apart from its secretion from the
osteoblasts, OC is also secreted locally by adipocytes and megakaryocytes. Combining
these actions, OC is capable of enhancing the secretion of insulin and increasing insulin
sensitivity in both fat and muscle.
Endocrine actions of osteocalcin.
Circulating osteocalcin and particularly
its undercarboxylated fraction (released
during active bone resorption) exerts a
direct effect on β cells, stimulating
insulin production as well as on
adipocytes
enhancing
adiponectin
production. Adiponectin itself is able to
promote insulin sensitivity. In turn,
insulin also acts directly on osteoblast
and indirectly on osteoclast. Osteoclast
stimulates
bone
resorption
with
subsequent release of undercarboxylated
osteocalcin in blood circulation. Finally,
osteocalcin has a role also on Leydig
cells, increasing their activity and
testosterone production.
Importantly, osteocalcin expression has been described in calcifying vascular smooth
muscle cells (VSMCs), although the physiological significance of this observation has
remained unclear. Osteocalcin is considered as a novel regulator of osteochondrogenic differentiation of pathologically mineralizing VSMCs, which provides
the first evidence that osteocalcin may be an active player in vascular calcification,
with its presence in the calcified vasculature, and potentially the circulation,
activating novel signaling pathways that promote mineralization.
Pathological mineralization of the vasculature has a detrimental effect on
cardiovascular function and is associated with increased mortality in patients with
aging, atherosclerosis, type 2 diabetes, and chronic kidney disease.
Aim of the work
We aimed in the that study to analyze the correlation between serum levels of
osteocalcin and vascular calcification in elderly persons with metabolic
syndrome.
Materials and methods
The current study included 74 elderly males, 65 years and older who were recruited
either from the geriatric outpatient clinic, or hospitalized patients.
Participants were divided into two groups;
Group (I); 40 elderly male patients satisfied at least three criteria of the metabolic
syndrome (MetS),
Group (II); 34 healthy elderly males serving as a control group.
We used the definition of MetS according to the NCEP-ATP III ;
Fasting blood glucose level ≥110 mg/dl,
Blood pressure ≥130/85 mmHg,
Triglycerides ≥ 150 mg/dl,
HDL cholesterol <40 mg/dl , and
Waist circumference >102 cm for men.
The exclusion criteria included:
Presence of any significant liver disease.
Renal failure requiring renal-replacement therapy.
Patients taking lipid-lowering drugs or any drugs that could
influence bone metabolism.
Anthropometric measurements (including height and weight) were taken, and the body
mass index (BMI) was calculated.
Blood samples obtained from all participants after overnight fast for
determination of :
Total serum cholesterol
HDL-cholesterol
Serum triglycerides
Fasting blood glucose.
Serum total osteocalcin (TOC) was measured using an N-MID Osteocalcin ELISA
kit (Elecsys, Roche diagnostic Ltd., Switzerland).
Ultrasound on the carotid arteries for measuring intima-media thickness.
Results
74 elderly males participated in the current study divided into two groups according to
the presence or absence of MetS, according to the NCEP-ATP III criteria. Group I; 40
males (62.5%) with MetS, their mean age was 70.70 ± 4.98 years, compared to 34 agematched males (37.5%) without MetS, served as a control group, their mean age
was70.76 ± 4.59 years, with no statistical significant difference between both groups.
(P =0.954)
Group I patients exhibited significantly higher BMIs, compared to group II, with a
mean of 30.90 ± 1.68 Kg/m2 in group I , and 26.33 ± 2.0 Kg/m2 in group II (p<0.001).
Group I patients had significantly higher waist circumference, fasting blood sugar,
Triglycerides, blood pressure, and lower HDL-ch , compared to group II subjects.
Also, total cholesterol was significantly higher in group I patients than group II
subjects.
Patients of group I had significantly lower levels of total Osteocalcin (TOC),
compared with subjects in group II .
Results of carotid Doppler revealed mean intima-media thickness of 1.07 ± 0.16 mm
in group I, and 0.70 ± 0.07 mm in group II, with a statistical significant difference
between both groups (p<0.001). Also group I patients had significantly higher
plaques number compared to group II subjects (p<0.001).
Age
Systolic
Diastolic
FBG
Normal
Abnormal
TG
Normal
Abnormal
HDL
Normal
Abnormal
Total ch.
Normal
Abnormal
WC
Normal
Abnormal
Group I(MetS)
Group II(Controls)
(n = 40)
(n = 34)
70.70 ± 4.98
70.76 ± 4.59
154.37 ± 7.78
132.79 ± 6.30
92.50 ± 4.24
78.09 ± 4.27
140.82 ± 32.54
103.18 ± 4.12
6(15.0%)
34(100.0%)
34(85.0%)
0(0.0%)
153.87 ± 21.75
107.35 ± 14.86
13(32.5%)
34(100.0%)
27(67.5%)
0(0.0%)
47.30 ± 7.25
60.65± 6.28
34(85.0%)
34(100.0%)
6(15.0%)
0(0.0%)
200.0(88.0 – 240.0)
102.0 (89.0 –
188.0)
18(45.0%)
34(100.0%)
22(55.0%)
0(0.0%)
88.94 ± 2.81
103.78 ± 6.60
34(100.0%)
16(40.0%)
0(0.0%)
24(60.0%)
P
0.954
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
<0.001*
BMI
Normal
Over weight
Obese
OCN
Normal
Abnormal
IMT
Carotid plaques
No
1
2
30.90 ± 1.68
0(0.0%)
14(35.0%)
26(65.0%)
26.33 ± 2.0
10(29.4%)
23(67.6%)
1(2.9%)
<0.001*
5.79 ± 1.77
0(0.0%)
40(100.0%)
1.07 ± 0.16
30.20 ± 8.69
33(97.1%)
1(2.9%)
0.70 ± 0.07
<0.001*
5(12.5%)
23(57.5%)
12(30.0%)
31(91.2%)
3(8.8%)
0(0.0%)
Statistically significant at p ≤ 0.05
<0.001*
<0.001*
<0.001*
<0.001*
OCN
Group I(MetS)
Group II(Controls)
r
p
r
p
Systolic
-0.127
0.434
-0.196
0.266
Diastolic
-0.102
0.532
-0.120
0.501
FBG
0.050
0.760
-0.128
0.470
TG
0.057
0.725
0.042
0.815
HDL
0.116
0.474
0.322
0.064
WC
-0.029
0.858
-0.273
0.118
OCN
Group I(MetS)
Group II(Controls)
r
p
r
p
TG
-0.469*
0.005
0.042
0.815
Total Ch.
-0.366*
0.020
0.057
0.725
0.116
0.474
0.322
0.064
HDL
r: Pearson coefficient
*: Statistically significant at p ≤ 0.05
Australian study showed that serum osteocalcin levels predicted all-cause
and CVD-related mortality in community-dwelling older men.(6)
Saleem et al. determined that serum TOC is negatively associated
with MetS in both blacks and non-Hispanic whites .(7)
Oosterwerff et al. found that plasma TOC was inversely
associated with MetS in a community-dwelling cohort of older
persons in the Netherlands, and reported that the subjects with
the lowest quartile of TOC concentrations had an approximately
3.7-fold higher risk of MetS than did the subjects with the highest
quartile.(8)
Also, Yeap et al. reported that men with lower serum TOC
concentrations have a higher risk of MetS. (9)
In a study by Bezerra dos Santos et al, the mean osteocalcin was significantly lower in
patients with MetS and decreased significantly with the rise in the number of criteria
for diagnosis of MetS. Serum osteocalcin was lower in patients with body mass index
(BMI) ≥25 and FPG ≥100 mg/dl, and in hypertensive and diabetic patients, and was
inversely associated with BMI, waist circumference, FPG, and systolic blood
pressure.(10)
There is evidence to show the influence of bone proteins on cardiovascular
disease. During atherogenesis, bone matrix proteins, including osteocalcin,
may have a regulatory role in the atherosclerotic calcification process.
Recent evidence suggests that osteoblast-like cells are present in the
vasculature and capable of calcifying vascular cells. Furthermore, paracrine
regulators of bone metabolism such as osteocalcin is also present in
atherosclerotic arteries. Thus, the vascular microenvironment possesses
mechanisms similar to those in bone tissues to maintain mineral homeostasis.
Osteocalcin-knockout mice develop extensive calcification of arteries that
rapidly becomes lethal, suggesting that osteocalcin has an anti-mineralization
role in the artery.
In humans, osteocalcin is detected in human carotid arteries from endarterectomy samples.
Thus, osteocalcin could play a pivotal role in not only bone mineralization but also vascular
wall calcification.
However, at present, little is known about whether serum osteocalcin secreted from osteoblasts
in bone or osteoblast-like cells in vessels actually could modulate atherosclerosis.
Thus, further studies are needed to clarify the pathophysiological processes underlying the
relationship between serum osteocalcin level and atherosclerosis parameters.
Conclusion
Our study indicated that serum osteocalcin levels were significantly associated with carotid
atherosclerosis in patients with metabolic syndrome. This may reflect the role of osteocalcin as
a circulating endocrine factor which regulates glucose metabolism and thereby cardiovascular
risk in patients with metabolic syndrome.
Prospective studies are needed to assess the time course and relevance of serum osteocalcin in
the development of atherosclerosis in patients with metabolic syndrome.
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