Relationship Between Serum Agouti-Related Peptide Levels and Metabolic Syndrome in Euthymic Bipolar Patients

Abstract

Introduction:

Agouti-Related Peptide (AgRP) is expressed primarily in the hypothalamic arcuate nucleus, stimulates appetite and decreases metabolism and energy expenditure. The aim of our study is to evaluate the relationship between serum Agouti-Related Peptide (AgRP) levels and metabolic syndrome in euthymic bipolar patients

Methods:

Forty euthymic bipolar patients who used only mood stabilizer for at least three months and 40 healthy volunteers as control group were included in the study. We measured fasting blood glucose levels and serum levels of AgRP, total cholesterol, triglyceride, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) of all participants. The main outcome measure was the difference between patients and control groups in terms of metabolic syndrome frequency and the relationship between serum AgRP level and metabolic syndrome is also investigated.

Results:

The metabolic syndrome was significantly more common in euthymic bipolar patients than in control group (p=0.039). Additionally, levels of blood glucose and triglyceride were significantly higher in the patient group than in the control group (p=0.006 and 0.01 respectively). Serum AgRP levels did not differ between the patient and control groups (p=0.35). Also, in euthymic bipolar patients, there was no significant difference in serum AgRP levels between patients with metabolic syndrome and those without (p=0.754).

Conclusion:

We found significantly higher frequency of metabolic syndrome in euthymic bipolar patients than in the control group. However, there was no significant difference in the levels of serum AgRP between bipolar patients with and without metabolic syndrome in either study groups.

INTRODUCTION

Metabolic syndrome is a condition that increase the risk of heart disease, stroke, diabetes mellitus, obesity and mortality (1). Metabolic syndrome is seen more common in bipolar disorder and the rate ranges between 16.7% and 67% across different studies compared to healthy control, and other disorders such as schizophrenia. (2). The main risk factors such as smoking, overeating, physical inactivity and dysregulation of neuro-endocrine system are almost similar for metabolic syndrome and bipolar disorder (3). Additionally, many pharmacological medications used for bipolar disorder especially psychotropic drugs cause weight gain and metabolic disturbances.

Agouti-related peptide (AgRP), is a neuropeptide acting in the regulation of food intake and energy balance, which is locally secreted by the arcuate nucleus of the hypothalamus and from peripheral tissues such as testis and adrenal gland (4). Agouti-related peptide release increases appetite and food intake. Previous studies conducted on transgenic mice revealed that the administration of AgRP into cerebroventricular tissue caused hyperphagia and obesity (5). It was also reported that AgRP decreased the energy expenditure and thus affected the energy homeostasis (6). The reduction of AgRP at RNA level with a rate of 50%, increased energy expenditure and resulted in weight loss, consecutively (7).

There is limited data available for the relationship between AgRP levels and metabolic syndrome in psychiatric patient groups (8–11). The role of AgRP for obesity in schizophrenia patients was previously reported (8). It was not found any relationship between obese and non-obese subjects in terms of AgRP levels. In another study investigated effect of olanzapine on AgRP, olanzapine did not change this peptid levels (9). Karagüzel, et al. evaluated AgRP levels in patients with euthymic bipolar disorder, and compared this marker to that of healty controls (10). They found AgRP levels were higher in euthymic bipolar disorder patients than in healty controls. In a study published by Parlak et al., serum AgRP was found to be lower in patients with manic bipolar disorder than those with euthymic bipolar disorder (11). However, there was no difference between patients with euthymic bipolar disorder and healty controls regarding AgRP levels. It is also suggested that AgRP is responsible for impaired energy homeostasis during manic episodes in bipolar patients.

In this study, we aimed to investigate the association of level of serum AgRP with metabolic syndrome in euthymic patients with bipolar disorder.

METHODS

Study groups

We determined a total sample size of 80 patients (n=40 in each group) with the Type I error rate (α=0.05) and power of 80%. From June 2016 to November 2016, 40 euthymic bipolar patients who were followed up by Affective Disorders Unit in the Department of Psychiatry in Ege University, fulfilled the diagnostic criteria for Bipolar Disorder according to DSM-IV and 40 healthy volunteers as control group (80 in total) were included in the study. The inclusion criteria for the study was as followed: bipolar disorder patients (type I) with euthymic period at least for the last three months and used only mood stabilizers (lithium, valproic acid, lamotrigine and carbamazepine). We defined euthymia as scoring 12 or less according to the Young Mania Rating Scale (YMRS), less than 7 by Hamilton Depression Rating Scale (HDRS), and less than 12 on the Montgomery–Asberg Depression Rating Scale (MADRS) using Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) The Structured Clinical Interview for DSM-IV (SCID) (12–17).

The patients were excluded from the study if they had diabetes mellitus, hypertension, and other major psychiatric disorder diagnoses (e.g. psychotic disorders, obsessive compulsive disorder and generalized anxiety disorder). The patients who were using psychotropic agents other than mood stabilizers, antilipidemic, antihypertensive, antidiabetic drugs and alcohol or illicit drugs were also excluded from the study.

After giving information about the study, control group was participated into the study with verbal invitation. The control group consisted of 40 people, taking into account the similarity in terms of gender and age and persons who met the criteria for participation in the study underwent a psychiatric examination. Inclusion criteria’s of volunteer for the study are 1- volunteers with the ages of 18–65 who accept to be control group and with education level who can read and fill the scales, 2- no active or chronic psychiatric disease is detected in the patient’s psychiatric examination, 3- not taking any psychotropic, antilipidemic or antidiabetic medication for at least three months. The exclusion criteria’s are 1- persons who couldn’t fill the assessment forms and scales truly or completely, 2- persons with hypertension, diabetes mellitus and obesity.

The study is approved by the local ethical committee (ID: 2016-TIP-034) and written informed consent was obtained from all the individuals included in the study, accordingly. Socio-demographic data, biochemical values and serum AgRP levels of all the attendants were recorded on the case report forms. Metabolic syndrome was diagnosed based on the criteria of National Cholesterol Education Program Adult Treatment Panel (NCEP-ATPIII) (16). The difference of serum AgRP levels and metabolic syndrome frequency between bipolar patients and control group were primary endpoints of the study.

Biochemical Analysis

Venous blood samples were obtained from antecubital vein of the patients and the control group between 8 AM and 10AM after a 12-hour fasting. Levels of serum AgRP, fasting blood glucoselevel, total cholesterol, triglyceride, high-density lipoprotein (HDL), low-density lipoprotein (LDL) of all the attendants were measured. Serum AgRP level was measured with ELISA (Enzyme-Linked Immunosorbent Assay) kit and other measurements were carried out in autoanalyzer in a biochemical laboratory. Standard curve concentrations used for ELISA range from 31.25pg/mL to 2000 pg/mL.

Statistical Analysis

All the collected data were evaluated with SPSS (Statistical Package for the Social Sciences) 20 software. Numerical variables with normal distribution were expressed with mean and standard deviations and non-normal distribution were expressed as median value with inter quartile range. While comparing the quantitative data in order to determine whether there was a difference between the groups, student t test was used for normally distributed variables and Mann-Whitney U test was used for not-normally distributed variables. χ² test was used in order to evaluate categorical data such as gender and axis I diagnosis. The relationship between numerical variables was evaluated with Spearman correlation analysis for non-normally distributed variables. In case the result was p<0.05, the difference between the groups were considered statistically significant.

RESULTS

Socio-demographic characteristics of the patient and control groups were given in Table 1. The number of male and female were equal in both patient and control groups and there were 20 (50%) females and 20 (50%) males in each group. There was no statistically significant difference between the mean ages of patient and control groups (49.38±11.18 and 45.80±6.88, p=0.09, respectively). The frequency of metabolic syndrome was significantly higher in euthymic bipolar patients than in control group (p=0.039).

Table 1.

Sociodemographic data of the study population. Data are expressed as mean ± standard deviation

Variable	Patient group (n=40)	Control group (n=40)	χ2 p-value
Gender/Female, n	20 (50)	20 (50)	1.00
Cigarette smoking n (%)	12 (30)	18 (45)	0.37
Alcohol use n (%)	11 (27.5)	13 (32.5)	0.06
Metabolic Syndrome n (%)	20 (50)	11 (27.5)	0.039
			t-test p-value
Age (years), mean ± SD	49.38±11.18	45.80±6.88	0.09

χ², chi-square; t, Independent Student’s t-test.

Data are expressed as mean ± standard deviation and as number (percentage).

Biochemical parameters of the patient and control groups were shown in Table 2. All biochemical parameters showed the normal distribution. Blood glucose and serum triglyceride levels were significantly higher in the patients compared to the controls (p=0.006 and 0.01 respectively). The levels of serum AgRP and HOMA-IR between euthymic bipolar patients and the controls did not differ.

Table 2.

Biochemical parameters of the study population

Variable	Patient group (n=40)	Control group (n=40)	t-test p-value
Serum AgRP (pg/mL), mean ± SD	30.46±3.81	30.56±3.25	0.35
Total Cholesterol (mg/dL), mean ± SD	201.63±36.77	206.45±39.68	0.91
HDL (mg/dL), mean ± SD	48.60±15.14	49.90±15.49	0.76
LDL (mg/dL), mean ± SD	116.30±30.60	129.50±35.82	0.81
Triglyceride (mg/dL), mean ± SD	166.50±17	111.50±93	0.01
Pre-prandial blood glucose (mg/dL), mean ± SD	89.00±17	85.00±13	0.006

AgRP, agouti related peptide; HDL, high-density lipoprotein, LDL, low-density lipoprotein; t, independent Student’s t-test; SD, standart deviation.

Table 3 show the relationship between metabolic syndrome and serum AgRP level in euthymic bipolar patients and in the control group. In euthymic bipolar patients, there was no significant difference in serum AgRP levels between patients with metabolic syndrome and those without (p=0.754).

Table 3.

Comparison of Serum AgRP levels in Patient and Control Group according to the presence of Metab.

Variable		Patient AgRP (pg/mL)			Control AgRP (pg/mL)			Statistics Z	Statistics Mann-Whitney U test
		N	Median	IQR	N	Median	IQR
									p-value
Metabolic Syndrome	Yes	20	30.4650	10.97	11	30.7600	15.86	-1.091	0.275
	No	20	30.9350	8.47	29	30.2700	5.21	-0.352	0.725

AgRP, agouti related peptide; IQR, inter quartile range.

Data are expressed as median with inter quartile range.

Table 4 show the comparison of socio-demographic and clinical characteristics in bipolar patients with or without metabolic syndrome. According to the table 4, no significant difference was found on duration of illness, first episode ages and using of mood stabilizer drugs between two groups (p=0.201, p=0.612 and p=0.495 respectively).

Table 4.

Socio-demographic and clinical features of bipolar patients. Data are expressed as mean ± standard deviation and as number (percentage)

Variable	Metabolic syndrome (-) (n=20)	Metabolic syndrome (+) (n=20)	t-test p-value
Age (years), mean ± SD	52.1±9.4	46.7±12.4	0.125
Duration (years) of illness, median with IQR	23 (11–29)	16 (7–21)	0.201
First episode age (years), mean ± SD	29.7±10.5	31.4±11.1	0.612
			χ2 p-value
Female gender, n (%)	9 (45)	11 (55)	0.527
Smoking, n (%)	8 (40)	5 (25)	0.311
Alchool, n (%)	5 (25)	6 (30)	0.832
Episode types			1.000
Mania, n (%)	12 (60)	12 (60)
Hypomania, n (%)	0 (0)	0 (0)
Depressive, n (%)	8 (40)	8 (40)
Mix, n (%)	0 (0)	0 (0)
Drug use, n (%)			0.495
Lithium	10 (50)	13 (65)
Valproate	1 (5)	1 (5)
Lithium+valproate	3 (15)	3 (15)
Carbamazepine + Lithium	1 (5)	2 (10)
Lithium + Lamotrigine	5 (25)	1 (5)

χ², chi-square; t, Independent Student’s t-test.

DISCUSSION

According to the results of this study, metabolic syndrome in euthymic bipolar patients was seen significantly more common than in normal population. However, there was no significant relationship between serum AgRP levels and metabolic syndrome.

Bipolar disorder is a chronic disease associated with cardiovascular diseases, cerebrovascular events, metabolic and endocrinological problems (18). It is well known that metabolic disorders such as obesity and metabolic syndrome are seen more common in bipolar patients than in normal population (19). In reported data the rates of MetS in bipolar patients vary widely from 16.7% to 67% compared to healthy control, and other disorders such as schizophrenia (2). It is more likely due to share common risk factors including neuro-endocrine disturbances and behaviour patterns, such as physical inactivity, smoking, and overeating. Similarly, the rate of metabolic syndrome was 50% in euthymic bipolar disorder patients and this syndrome was significantly more common in bipolar patients than in the control group in our study.

It was revealed that some adipokines such as leptin, ghrelin and resistin were responsible for metabolic syndrome. In addition, some endocrinopathologies such as excessive glucocorticoid release and insulin resistance were also held responsible for metabolic syndrome. AgRP is an orexygenic peptide synthesized from arcuate nucleus of hypothalamus and increases the food intake. It is known that AgRP is a molecule predicting obesity in the gene level (20). In studies conducted with mice, it was revealed that increase in AgRP expression has an impact on the development of metabolic syndrome (21).

At clinical level, it was determined that AgRP neurons were responsible for desire to eat and regulation of food need and distribution to peripheral tissues. It was emphasized that they were also responsible for obesity and metabolic syndrome (22).

The relationship between bipolar disorder and serum AgRP level was investigated in a study by Parlak et al., and serum AgRP level was significantly lower in manic group than in the euthymic and control group and they claimed that lower level of this peptide could be a predictor for manic attacks (11). The different results may be attributed to actual period of the disease. It has been shown that AgRP reduces energy expenditure levels, thus contributing to the anobolism. Also, patients with manic disorders expend more energy than controls. Due to both the effect on AgRP on energy expenditure and increased physical activity in manic patients, serum AgRP levels may be lower in manic patients compared with those with bipolar disoreders and healty controls. Additionally, they found that no significant differences in serum AgRP levels could be detected between the euthymic bipolar disorder and control groups, and this result was congruent with our findings, as manic patients were excluded from our study population and serum AgRP levels were similar in euthymic bipolar patients and control group. However, another study evaluated serum AgRP levels in patients with euthymic bipolar disorder showed its level was higher in these patients than in healty controls unlike our results (10).

The role of this peptide on obesity in schizophrenia patients treated with antipsychotics were investigated before, but its role on obesity and metabolic syndrome in bipolar patients is unknown (8.9). The relationship between some adipokines and metabolic syndrome in bipolar patients was investigated previously. For instance, there was no relationship between resistin level and metabolic syndrome in euthymic bipolar patient group (23). Kalenderoglu et al. was reported that there was no relationship between ghrelin level and metabolic syndrome in bipolar patients (24).

Metabolic side effects of psychotropic drugs are frequently seen in bipolar patients. The pathophysiology of metabolic side effects of psychotropic drugs is extensively studied in the past years. Bipolar disorder patients using antipsychotic medication were at a significantly 1.72 times greater risk of metbolic syndrome relative to antipsychotic-free patients (25). However, Parlak et al. showed the use of antipsychotics did not have any effect on blood levels of AgRP in bipolar diorders patients (11). Similarly, we did not find any significant relationship between the presence of metabolic syndrome and the use of mood stabilizers in euthymic bipolar patients.

It has been shown that illness duration, number of episodes, the presence of depression at onset and during acute episodes, age of onset of first epidose were associated with the development of metabolic syndromes in bipolar disorders (26–29). Although we did not evaluated episode numbers, we did not any association between metabolic syndrome and duration of illness and age of onset of first episode in euthymic bipolar patients in presented study.

There was some limitation of this study. We did not have the data about episode number and severity of illness which they could be associated with the development of metabolic syndrome and AgRP levels. The prospective studies are needed to show the effect of treatment modalities and different episodes on metabolic syndrome and serum agouti-related peptide.

In conclusion, metabolic syndrome was more common in euthymic bipolar patients than in normal population, but serum AgRP levels were similar in both group.