Hyperglycemia promotes overexpression of SR-BII isoform of the scavenger receptor class B type I in type 2 diabetes mellitus: A study in Juana Koslay City, San Luis, Argentina

Gisela V. Mendoza, Susana Siewert, Irma González, María C. Della Vedova, Gustavo Fernandez, Marta S. Ojeda

Laboratory of Diabetes, Department of Biochemistry and Biological Sciences, School of Chemistry, Biochemistry and Pharmacy, National University of San Luis, San Luis, Argentina.

DOI: 10.4236/jdm.2013.34027                

Abstract

The scavenger receptor class B type I (SR-BI) is a high-density lipoprotein (HDL) receptor involved in reverse cholesterol transport. Some studies reported the association to be stronger in the presence of diabetes. The full length gene encoding SR-BI is comprised in 13 exons that are alternatively spliced to produce two major transcripts: the full length SR-BI and the splice variant SR-BII, in which exon 12 is skipped. Considering that type 2 diabetes status is characterized by changes in the concentration of plasma lipids, modifications in lipoprotein size and composition, which may be important modulators of the SR-BI expression; the aims of the study were to examine the influence of SR-BI polymorphism (rs838895) on lipid profile and SR-BI mRNA expression in a population of diabetic patients living in Juana Koslay City. Blood samples were drawn from controls (n = 40) and Type 2 diabetic patients (n = 66) and DNA and total RNA were obtained. SR-BI mRNA expression was measured by RT-PCR and SR-BI polymorphism was detected by Tetra Primer ARMSPCR. Compared to controls, diabetic patients had higher fasting serum glucose, glycated hemoglobin, triglycerides, total cholesterol, lowdensity lipoprotein cholesterol, and lower highdensity lipoprotein cholesterol. SR-BI mRNA expression was lower in T2DM when compared to controls, suggesting that the hyperglycemia presents in T2DM patients down-regulates SR-BI mRNA expression. Interestingly, we found that decreased SR-BI expression resulted in markedly increased plasma LDL concentrations in T2DM subjects, and the overexpression of SRBII isoform is responsible for the markedly increased plasma LDL-c concentrations. The polymorphism (rs838895) did not modify the mRNA level of SR-BI in leucocytes from control and diabetic patients. This study provides novel evidence suggesting that hyperglycemia may affect reverse cholesterol transport by controlling SRBI expression in diabetic patients. LDL cholesterol levels are associated with low SR-BI mRNA expression in T2DM. 

Keywords

Type 2 Diabetes Mellitus; Hyperglycemia; Scavenger Receptor Class B Type I and Polymorphism

1. INTRODUCTION

Type 2 diabetes mellitus (T2DM) has emerged as one of the most prevalent chronic diseases worldwide. Decreased plasma high-density lipoprotein cholesterol (HDLc) is one of the most common lipid disorders in diabetic subjects [1-3]. Moreover, low HDL-c concentrations have been defined as a key sign of the insulin resistance syndrome, a common metabolic condition linked to a higher cardiovascular risk, which has been estimated to be present in one fourth of all Americans [4,5]. HDL-c is believed to exert its antiatherogenic role mainly through the process of delivering cholesterol from peripheral tissues back to the liver for removal from the body, which is called reverse cholesterol transport [6,7]. 

Scavenger receptor class B, type I (SR-BI), a major HDL receptor [8,9], plays an important role in reverse cholesterol transport, a major pathway for the clearance of excess cholesterol from the body. In this process, peripheral cholesterol is packaged into HDL from which it is subsequently removed in the liver and excreted into bile. SR-BI mediates the uptake and biliary secretion of HDL-c by the liver [10,11].  

Besides its role as a functional HDL receptor involved in reverse cholesterol transport, SR-BI also participates in the metabolism of Apolipoprotein B-containing lipoproteins, including low density lipoprotein (LDL) [12,13] and very low density lipoprotein (VLDL) [14,15]. 

The full length gene encoding SR-BI (gene symbol SCARB1) is comprised of 13 exons that are alternatively spliced to produce two major transcripts: the full length SR-BI and the splice variant SR-BII, in which exon 12 is skipped. SR-BI and SR-BII splice forms, conserved in both mouse and rat genomes, have different tissue distributions and may influence cellular cholesterol trafficking and homeostasis in different ways [16]. SR-BII is reported to be a minor splice variant in human liver and has shown to be less efficient at reverse cholesterol transport [17]. 

Studies using animal models have demonstrated that SR-BI has an atheroprotective role. SR-BI overexpression in the liver is associated with reduction of atherosclerosis in LDL receptor-deficient mice under a high fat and cholesterol diet [18]. On the other hand, SR-BI deficiency results in lipid deposition in the aorta and atherosclerosis in mice [19]. In contrast, much less is known about the role of SR-BI in humans.

Genome-wide association studies have uncovered a number of loci associated with type 2 diabetes, but together they account for little of the overall variance and provide little predictive power over traditional risk factors [20]. It is likely that many loci still remain to be discovered. Genes located in chromosomal regions showing linkage to type-2 diabetes in family-based studies are rational candidates for more detailed investigation. SCARB1 lies in a region on chromosome 12q24 that has been linked to type-2 diabetes [21] and more recently to abdominal obesity, a risk factor for type-2 diabetes, in three studies [22-24]. Acton et al. [25] were the first to identify single nucleotide polymorphisms (SNPs) of the SCARB1 in a white European population and associated some of these common variants with plasma lipid levels and body mass index. Genetic variation in the SCARB1 has also been associated with increased risk of coronary artery disease [26], obesity [25], triglycerides [27,28], and HDL-c [29-33], all facets of the metabolic syndrome. Furthermore, there is evidence that diabetes status may modify the SCARB1 association with HDL-c [34].

A poorly studied polymorphism is intron 11 (IVS11) SCARB1 where G is changed by C (rs838896). ChibaFalek et al. [35] showed that in liver tissue of young adult females (but not men) genotypes that carried the mutant allele (C) were associated with generally lower level of SR-BI, a non-significant lower level of SR-BII and a lower level of overall SCARB1, compared to young women not carrying this allele.

Considering that type 2 diabetes status is characterized by changes in the concentration of plasma lipids, modifications in lipoprotein size, and composition [2,36,37] that may be important modulators of the SCARB1 expression; the aims of the current work were: 1) to assess the phenotypic effect on lipid and lipoprotein concentrations of the genetic variant (rs838896) at SCARB1 in T2DM patients who reside in Juana Koslay, San Luis, Argentina, 2) to examine the association of the genetic variation at the SCARB1 locus with the type 2 diabetes status in these patients, 3) to estimate whether diabetes status modifies the association between the SCARB1 variation and the examined phenotypic traits, and 4) to examine the influence of this genetic variant (rs838896) on mRNA expression in peripheral leucocytes.

2. RESEARCH DESIGN AND METHODS

2.1. Subjects

The present study was carried out in accordance to the guidelines of the Helsinki Declaration. A total of 106 volunteers (66 patients with type 2 diabetes and 40 healthy age-matched controls) participated in this investigation. Criteria published by the American Diabetes Association were used to diagnose T2DM [38]. These patients reside in Juana Koslay, San Luis, Argentina. The protocol for this study was approved by the local Institutional Review Board, and a written informed consent was obtained from each patient to be enrolled. During an initial interview with each patient, they were asked for diseases, medication and smoking histories. Exclusion criteria included liver, kidney and thyroid diseases, as well as the use of anti-lipemic drugs. 

2.2. Anthropometric and Clinical Data

For each subject enrolled, height (meters) and weight (Kg) measurements were acquired. Height and weight were measured to the nearest 0.5 cm and 0.1 kg, respectively. The body mass index (BMI) was calculated as weight divided by height squared (Kg/m²).

2.3. Blood Sampling

Venous blood samples for glucose, lipid, and lipoprotein analysis were collected into EDTA-containing (1 g/liter) tubes from all subjects after a 12-h overnight fast at the beginning of the study. Blood was also collected to extract DNA and RNA (see below).

2.4. Biochemical Measurement

Fasting plasma glucose (FPG) was measured by using a glucose oxidase method with a commercial enzymatic kit (Wiener Lab, Rosario, Argentina) and Glycated hemoglobin (HbA1c) concentration was measured with a coupled ionic-exchange chromatography/spectrophotometric assay (BioSystems, Barcelona, Spain) in a Bayer Express Plus Chemistry Analyzer (Bayer Diagnostics, Siemens, Germany). Total cholesterol (TC), triglycerides (TG) and HDL-c concentrations were measured using commercial kits by following manufacturer’s instructions ((Wiener Lab, Rosario, Argentina) in a Bayer Express Plus Chemistry Analyzer (Bayer Diagnostics, Siemens, Germany). Low density lipoprotein-cholesterol (LDL-c) was calculated with the Friedewald formula: LDL-c= total cholesterol (mg/dL) – HDL-c (mg/dL) – triglycerides (mg/dL)/5 [39].

2.5. Genotyping and SNP Analysis

Genomic DNA was isolated from diabetic patients and healthy volunteers using conventional protocol by Qiagen kits (Qiagen, Inc., Valencia, CA). DNA concentration was detected by UV-VIS spectroscopy and diluted to a final concentration of 20 ng/μL. The SCARB1 polymorphism (rs838896) was analyzed by Tetra Primer AMRSPCR [40]. Two pairs of primers were used, one which amplifies a fragment of 431 bp, common to both alleles (outer primers: forward 5’ GTAGATAGTGAGCTTTGACATGGAGGCCAG 3’ and reverse 5’ ACTTCCGTTCCCTCCTGATAGCAGCT 3’) and another pair specific for the SNP (inner primers): forward 5’ TTGGAACTGATTCTGGGAGTGACAGTGG 3’ for the G allele and reverse 5’ GATTCCCCTCTTTAAAAGCCTCCAAAGG 3’ for the C allele. Nucleotide sequence and SNP details were obtained from NCBI website (http://www.ncbi.nlm. nih.gob). Primers were designed ‘‘in silico’’ in a free access web (http://cedar.genetics.soton.ac.uk) and then checked for specificity (http://blast.ncbi.nlm.nih.gov/ Blast.cgi). Each PCR reaction was performed containing 200 ng of genomic DNA, 1 pmol of each outer primer and 5 pmol of each inner primer (1:5 ratio of outer to inner primer concentration) , 200 μM dNTPs, 2.5 mM MgCl₂, 1X buffer, and 1 unit of AmpliTaq Gold DNA polymerase (Perkin-Elmer). PCRs were carried out in 35 μL reactions. The template DNA was denatured for 3 minutes at 95˚C before undergoing 35 cycles of denaturation for 1 minute at 95˚C, primer annealing for 1 minute at 68˚C, and extension for 1 minute at 72˚C, and final extension at 72˚C for 3 minutes. The resultant products obtained after PCR were separated by electrophoresis on 2.5% agarose gel containing GelRed. This resulted in 3 DNA fragments of: 431 bp common to both alleles (outer primers amplification), 292 bp for the C allele and 195 bp for the G allele. The image was visualized and photographed under UV transillumination. Allele frequencies for this SNP were calculated by allele counting. Randomly selected 20% of samples were re-genotyped for cross validating initial genotypes. In case of unclear genotyping results, the samples were repeated again in duplicates until clear genotype was available. Unclear genotyping results, even after repetition was excluded from the study. No genotyping error was observed during cross validation.

2.6. Measurement of Gene Expression

Total RNA was isolated from leucocytes using the TRIzol reagent following manufacturer’s instructions (Life Technologies, Carlsbad, CA). Agarose gel electrophoresis and GelRed staining confirmed the purity and integrity of isolated RNA. RNA concentration was assessed by spectrophotometric measurements at 260/280 nm. Then, ten micrograms of total RNA were reversetranscribed with 200 units of Moloney-murine leukemia virus (MMLV) reverse transcriptase (Promega) using random hexamers as primers in 20-μL reaction mixture following the instructions provided by the manufacturer. The reverse transcription reactions were performed using a GeneAmp PCR system 2400 (PerkinElmer, Wellesley, MA) with the following conditions: 65˚C for 10 min, 37˚C for 60 min, and 90˚C for 5 min. 

Given the existence of alternative splice forms of SCARB1 involving inclusion/exclusion of exon 12, primers able to detect both isoforms were used. Primers were constructed from the published cDNA (http://www. ncbi.nlm.nih.gob) sequence of SCARB1 (Forward 5’ CTGCGTCCTGCTGCTGGTCC 3’ and Reverse 5’ GGCTCACGGTGTCCTCAGGA 3’) and the house-keeping gene beta-Globin (Forward 5’ CAACTTCATCCACGTTCACC 3’ and Reverse 5’ GAAGAGCCAAGGACAGGTAC 3’), used as internal control. SCARB1 primers were designed to generate a 197 bp fragment for the SR-BI transcript (exon 12 including) and a 68 bp fragment for the SR-BII transcript (exclusion of exon 12) (Figure 1).

PCR was used to quantify human overall SCARB1 products (SR-BI and SR-BII isoforms) and beta-Globin expression from the cDNA samples. PCRs were carried out in 35 μL reactions. Each amplification reaction included 20 ng of cDNA, 20 pmol of each PCR primer and 1 unit of AmpliTaq Gold DNA polymerase (Perkin-Elmer,

Figure 1. Schematic illustration of the location of primers (arrows) used for RT-PCR.

Waltham, MA). These reactions were performed in a buffer 1X containing 1.5 mmol/L MgCl₂, 50 mmol/L KCl, 20 mmol/L Tris-HCl (pH 8.4), and 200 pmol/L of each deoxynucleotide triphosphate. cDNA and control preparations were amplified using the following conditions: 95˚C for 3 minutes followed by 35 cycles of: denaturation for 1 minute at 95˚C, primer annealing for 1 minute (at 62˚C for SCARB1 and 55˚C for beta-Globin), and extension for 1 minute at 72˚C; followed by a final extension at 72˚C for 3 minutes. We controlled for DNA contamination by running three randomly selected, RNA control samples that were not converted to cDNA and no-cDNA/RNA sample in each plate. No observable amplification was detected. The amplification is predicted to generate a 197 bp fragment for the SR-BI transcript and a 68 bp fragment for the SR-BII transcript. PCR products were separated on a 2.5% agarose gel containing GelRed. The image was visualized and photographed under UV transillumination. The intensity of each band was measured using NIH Scion Image 1.6.3 software and reported as the values of band intensity units. 

2.7. Statistical Analysis

Chi square test was used to check adjustment of the data to the Hardy-Weinberg equilibrium and to compare the allelic frequencies between controls and diabetic subjects. Comparison of allele frequencies and genotype distributions between case and control samples were done by Pearson’s chi-square test. To analyze the association between SCARB1 genotypes, clinical and biochemical parameters a Student t-test was used when variables were continuous, whereas a Fisher’s exact test was used for the categorical variables. A p < 0.05 was considered to be statically significant.

3. RESULTS

3.1. Subjects Characteristics

One hundred and six individuals were included in this study, 66 of them were diabetic and 40 were healthy controls. Table 1 shows the demographic characteristics of both groups. 51.4% were women and 48.6% were men, age distribution was not different between the groups with a media age of 56.3 years old. The average value of BMI was 25.3 kg/m² for the control group and 31.2 kg/m² for the diabetic one. There was a significant difference in weight and BMI between both groups. Diabetic female subjects were more obese than diabetic male subjects.

As shown in Table 2 FPG and HbA_1c concentrations were higher in the diabetic group when compared with age-matched control subjects. Total cholesterol, TG and LDL-c were increased in the diabetic group, while HDL-c was significantly lower in these patients, when

Table 1. Anthropometric characteristics in control and T2DM patients.

Data are shown as mean ± SD. Abbreviations used: BMI, body mass index.

Table 2. Biochemical characteristics in control and T2DM patients.

Data are shown as mean ± SD. Abbreviations used: TG, triglycerides; TC, total cholesterol; LDL-c, low density lipoprotein; HDL-c, high density lipoprotein; FPG, fasting plasma glucose; HbA1c, glycated hemoglobin; AI: TC/HDL-c.

compared to controls. LDL-c was lower in diabetic female than in diabetic male subjects. The atherogenic index (AI) was higher in the diabetic patients, what suggests a risk for cardiovascular diseases in these patients.

3.2. Genotype Frequencies

During the amplification reaction, a 431 bp region of SCARB1 was amplified with Outer forward primer and Outer reverse primer that served as internal control for the quality of the PCR amplification. The allele-specific amplification (Forward inner/Reverse outer and Forward outer/Reverse inner) resulted in the 195 bp PCR product specific for the G allele and the 292 bp PCR products specific for the C allele (Figure 2).

Table 3 shows the absolute number and frequency (%) of controls and T2DM patients having each genotype. The distribution of genotypes was as expected from the Hardy-Weinberg equilibrium because no statistically significant differences (p > 0.05) were detected between the observed and the expected frequencies in any case. We did find statistically significant differences between diabetes status and controls in the genotype and allele frequencies of the SCARB1 variants. The frequency of the C allele was significantly higher in diabetic subjects than that in controls (0.5 vs. 0.30), and this SNP showed strong association with T2DM [OR = 2.429 with 95% CI (1.359 - 4.339), p = 0.004].

3.3. Genotype Associations with Lipid Traits

Table 4 shows plasma concentrations of lipids and lipoproteins by the SCARB1 polymorphism in controls and T2DM. Because of the small number of subjects, homozygotes for the minor C allele and heterozygotes were grouped as carriers of the C allele (GC and CC). In controls, no differences in lipids concentrations were

Conflicts of Interest

The authors declare no conflicts of interest.

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