Apolipoprotein B levels and its relation to lipid profile in type II - Diabetes Mellitus; Comparative study

Introduction

Type 2 diabetes mellitus is a metabolic disorder characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency.¹ Over the recent years, it has been observed that the rate of diabetes has increased markedly in parallel with obesity. As per the global statistics in 2010, it is estimated that there are 285 million people with type 2 diabetes making up about 90% of diabetes cases.² Hence, type 2 diabetes mellitus is now been recognized as a global epidemic by the World Health Organization.³ Type 2 diabetes mellitus is not only associated with carbohydrate metabolic disturbances, but also been associated with dyslipidaemia comprising of multiple lipoprotein disorders, which increases the risk of cardiovascular morbidity and mortality. Apolipoprotein A-1 (Apo A1) and apolipoprotein B (Apo B) are the main structural proteins of the circulating HDL particles and atherogenic lipoproteins, respectively.⁴ Apo B identifies highrisk dyslipidaemic phenotypes that are not detected by standard lipid profile in type 2 diabetic patients. Since apo B is the major apoprotein content of the atherogenic lipoprotein, study of apo B levels may help in identifying diabetics who are at risk of developing cardiovascular morbidity, even before there is alteration in the levels of LDL and HDL cholesterol. However, estimation of apo B is not a parameter of choice during the routine workup for a case involving dyslipidemia. Hence, this study of apolipoprotein B levels in type 2 diabetes mellitus patients and its relation with lipid profile was undertaken to note the frequency and pattern of dyslipidemias in type 2 diabetes mellitus patients and to compare it with apolipoprotein B levels.

Material and Methods

Subjects for the study were from the patients attending Medical OPD & Diabetology OPD of R.L.Jalappa Hospital and Research Centre, Kolar. The subjects of either sex within the age group of 40 to 65 years were included for the study. Sample size was calculated taking prevalence of diabetes mellitus as 12.4% at 95% confidence interval with confidence limits of 10%. Sample size of 42 as suggested by the formula was rounded off to 45 and divisions of following groups were made:

Group A - Proven cases of Type 2 diabetes mellitus under poor control

Group B - Diabetic Controls: Type 2 diabetes mellitus patients under control based on glycated hemoglobin levels

Group C - Normal Healthy Controls

All the controls in Group B and Group C were age and gender matched. Any subject who had complications of diabetes like diabetic ulcers and conditions known to influence lipid profile levels like familial hypercholesterolemia was excluded from the study.

Sample collection: After taking the informed consent, 4 ml venous sample was collected under aseptic precautions, centrifuged at 3000 rpm for 10 minutes and the serum was obtained. Serum was analysed for the following parameters:

1. Total Cholesterol by Cholesterol OxidasePeroxidase method⁵

2. Triglycerides by Glycerol Peroxidase method⁶

3. HDL-Cholesterol by precipitation method⁷

4. Calculation of LDL by Friedwald’s formula^{8 5}. Apo B by immunoturbidimetric method⁹

All the samples were analyzed in batches with the inclusion of Bio-Rad internal quality control using auto-analyzer.

The values of the parameters collected were tabulated. Mean and standard deviation was obtained and comparison between the groups were done using ANOVA and post-hoc test. Correlation of Apo B levels with cholesterol, triglycerides, HDL and LDL levels was done using Pearson’s coefficient.

Results

The mean values of the age and the biochemical parameters are indicated in the table 1. The levels of apo B is found to be increased in uncontrolled diabetes patients compared to the controlled diabetics and healthy controls (103.60±30.61 Vs 81.6±18.45 & 92.56±20.7).

Group A and Group B comparison of the various parameters showed an increase in total cholesterol and triglycerides. Triglycerides also showed an increased in Group A when compared with Group C. LDL Cholesterol is increased highly significantly in Group A when compared with Group B. Apo B levels were found to be increased highly significantly when compared between Group A and Group B as well as between Group A and Group C, as depicted in table 2.

From Table 2, it can be derived that Apo B is positively correlated with total cholesterol (p < 0.05) and also it correlates positively with LDL Cholesterol (p <0.001) in group A. Apo B correlates positively with total cholesterol (p < 0.001) and also with LDL Cholesterol (p <0.05) in group B. Apo B correlates positively with total cholesterol and LDL Cholesterol (p < 0.001) in group C.

In the present study, it was observed that LDL cholesterol and Apo B levels were significantly elevated in Group A compared with group B (p < 0.001). Correlation of Apo B showed a positive significant correlation with total cholesterol and LDL cholesterol in all the groups, and thus indicating its association. The details of comparison have been depicted in Table 3.

Discussion

Lipid derangements commonly occurs in diabetes mellitus, and is more pronounced with poor glucose homeostasis. Altered lipid profile does not always reflect the pathophysiology & adverse effects produced by them. Apolipoprotein estimations give more insight to on-going metabolic changes occurring with circulating lipids. Estimation of Apo B, a component of LDL, reflects its metabolism as well as predicts complications associated with it, particularly atherosclerosis & coronary artery disease.

In the present study as presented in Table 1, the levels of Apo B was found to be increased in uncontrolled diabetic patients compared to controlled diabetics and healthy controls. Similarly, LDL cholesterol was also found to be highly significantly increased in uncontrolled diabetic group. Total cholesterol levels and triglycerides also show the same trend of increase in subjects with poor glycemic control. Diabetic dyslipidemia reflects an increased Very Low Density Lipoprotein (VLDL) production, which has consequences on the metabolism of other lipoproteins such as Low Density Lipoproteins (LDL) and High Density Lipoproteins (HDL).¹⁰ In patients with type 2 diabetes, an increased production of VLDL particles are potentially detrimental in generating atherogenic remnants, small dense LDL particles and triglyceride-rich HDL particles. Further, existence of insulin resistance causes defective deactivation of PI3 kinase, which in turn is responsible for defective degradation of Apo B in hepatocytes.

The central pathophysiologic feature of abnormal lipid profile due to insulin resistance in type 2 diabetes mellitus is overproduction of VLDL leading to increased plasma levels of triglycerides which, via an exchange process mediated by cholesterol ester transfer protein (CETP). Posttranscriptional regulation of apolipoprotein B (apoB) metabolism in the liver results in increased assembly and secretion of VLDL by the liver. This results in low levels of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I, and the generation of small, dense, cholesterol ester depleted low density lipoproteins (LDL).¹¹

Amongst apolipoproteins, Apolipoprotein B is most often contributing to dyslipidemia in type 2 diabetes.⁴ This study strongly supports our observations in which they had conducted the study on 120 diabetic subjects. In a study done in Canada by Sylvia H and their co-workers reflects the association of Apolipoprotein B with incident Type 2 Diabetes in an Aboriginal Canadian Population.¹²

In Table 3, correlation of Apo B shows a positive trend with total cholesterol and LDL cholesterol in uncontrolled diabetic group and in the control groups. This clearly indicates the association of Apo B with LDL metabolism and good glycemic control aids in lowering the LDL particles and its clearance by effective metabolism.

Albers JJ and his fellow investigators in their study observed that the youth with type 1 diabetes, elevated apoB and dense LDL were not highly prevalent, whereas elevated apoB and dense LDL were common lipoprotein abnormalities in youth with type 2 diabetes. The prevalence of these risk factors substantially increased with poor glycemic control in both groups, stressing the importance of achieving and maintaining an optimal glucose control.¹³

Hua Xi Yi and his associates observed in their study that abnormal changes of apo(s) in type 2 diabetes mellitus may be a cause of type 2 diabetes associated with high triglycerides and Coronary Heart Disease (CHD). The levels of fasting plasma glucose (FPG), insulin, triglycerides (TG), total cholesterol (TC), apo A I, An II, B100, C II, C III, E were all measured in 127 non-diabetic subjects and 143 type 2 diabetic patients.¹⁴

Conclusion

The present study implicates that lipid alterations are reflected better by Apo B estimation in type 2 diabetes mellitus and predicts the risk of associated complications with it. A positive association exists between total cholesterol and LDL cholesterol with Apolipoprotein B, which states that they are interrelated circulating lipid components. Good control of glucose homeostasis in diabetes is extremely necessary to keep a check on abnormalities in lipid profile & apolipoprotein levels.

However, the study can be further extended which would involve the estimations and finding associations of other apolipoproteins such as Apo A and Apo C in such diabetic population on a larger scale, which would contribute to strategies for the effective management in type 2 diabetes mellitus patients.

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