Pharmacological and Molecular Perspectives on Diabetes

By Bentham Science Publishers

Pharmacological and Molecular Perspectives on Diabetes is a compilation of reviews on clinical and scientific aspects of diabetes mellitus. It presents 11 contributions by eminent scholars that give the reader rational pharmacological and genetic perspectives of the disease and its treatment. The reviews approach diabetes from different angles, and highlight research that has been done to understand some questions about the molecular biology of diabetes in experimental settings. Topics of clinical significance such as the use of different hypoglycemic agents, and diabetic complications in clinical settings are also covered.
Topics included in this book are:
· Epigenetic alterations and type 2 diabetes mellitus
· Responses to nutritional chromium supplements for type 2 diabetes mellitus
· Endocrine role of osteocalcin in homeostatic regulation of glucose metabolism
· Effect of diabetes on memory
· Osteoarthritis in relation to type 2 diabetes mellitus: prevalence, etiology, symptoms and molecular mechanism
· Infection of novel coronavirus in patients with diabetes mellitus
· Role of an anti-inflammatory agent in the management of type 2 diabetes mellitus
· Role of antidiabetic agents which helps regulates TCF7L2 variations in type 2 diabetes mellitus
· Relationship between type 2 diabetes mellitus, PCOD and neurological disorders: role of antidiabetic drugs
· Comparison of different types of insulin available for type 1 diabetes treatment
· Circadian rhythm disruption: special reference to type 2 diabetes mellitus
· Type 2 diabetes mellitus and its complications: pharmacogenetics based correlations and circulating microRNA as biomarkers

Pharmacological and Molecular Perspectives on Diabetes should prove to be of interest to all pharmaceutical and molecular biology scientists who are involved in research in anti-diabetic drug design and discovery, and practicing endocrinologists who wish to keep abreast of recent developments in the field.

• Language: English
• Publisher: Bentham Science Publishers
• Release date: Aug 15, 2001
• ISBN: 9789815040227

Book preview

Epigenetic Alterations and Type 2 Diabetes Mellitus

Shalini Singh¹, Ashwin Kumar Shukla¹, Kauser Usman², Monisha Banerjee¹, *

¹ Molecular and Human Genetics Laboratory, Department of Zoology, Faculty of Science, University of Lucknow, Lucknow, India

² Department of Medicine, King George’s Medical University, Lucknow, India

Abstract

Type 2 diabetes mellitus (T2DM) is one of the most challenging public health issues of the 21st century. T2DM, a complex polygenic metabolic disorder, is characterized by hyperglycemia and hyperinsulinemia resulting from the interplay of genetic/epigenetic and environmental factors. Epigenetic alterations present in T2DM patients and not in normal healthy individuals may give an insight into how environmental factors contribute to T2DM. Epigenetic mechanisms involve DNA methylation, histone modification, and gene expression alterations via micro RNAs (miRNA). These changes lead to glucose intolerance, insulin resistance, β-cell dysfunction, and ultimately T2DM. Extensive studies based on alterations in gene expression associated with DNA methylation/histone modifications are required to elucidate the relationship between vital environmental factors and T2DM progression. Candidate genes responsible for inter-individual differences in antidiabetic responses may also undergo epigenetic alterations. Identification and characterization of such epigenetic biomarkers may help in the prediction of T2DM risk as well as response to antidiabetic treatment and form an essential part of personalized medicine.

Keywords: T2DM, Epigenetic biomarkers, DNA methylation, Histone modification, Personalized medicine.

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* Corresponding author Monisha Banerjee: Molecular and Human Genetics Laboratory, Department of Zoology, Faculty of Science, University of Lucknow, Lucknow, India; Tel:[phone]; E-mail: [e-mail]

INTRODUCTION

Type 2 Diabetes mellitus (T2DM) is an incurable, progressive polygenic metabolic disorder characterized by both hyperglycemia as well as hyperinsulinemia. It is a major source of morbidity and mortality worldwide [1] and has increased dramatically over the past decades [2]. The global burden of

diabetes is presently 351.7 million affected people of working age (20-64 years), which is expected to rise to 486.1 million in 2045 [3]. A total of 77 million Indians were diagnosed as diabetics in the age group 20-79 years in 2019, and this figure is estimated to rise to 134.2 million in 2045 (IDF, 2019). It has been reported that subjects having T2DM-affected siblings are at two-to-three folds higher risk of developing T2DM compared to the general population [4]. Having one or both parents with diabetes increases the risk of developing T2DM by 30-40 and 70%, respectively [5].

T2DM develops due to impaired insulin signaling, predominantly in genetically predisposed subjects exposed to lifestyle risk factors like obesity, physical inactivity, and aging [6]. Environmental factors viz. diet and sedentary lifestyle play crucial roles in the progression of T2DM and seem to alter the epigenome. The connecting link between environment and disease is epigenetics which influences gene transcription followed by organ functions [7]. However, all individuals do not respond to environmental conditions in an equal manner. Alterations in the epigenome are more frequent than mutations in DNA and may occur in response to environmental, psychological, and pathological stimuli [8].

Epigenetic alterations consist of DNA methylation, histone modifications, and miRNA dysregulation. Such changes can be passed from one generation to the next (mitotic inheritance) or between generations, i.e., meiotic inheritance. Several candidate genes are associated with glucose and lipid metabolism viz. insulin gene [9], peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) [10-12], etc. have been reported to be epigenetically misregulated. Epigenetic alterations may predispose the future development of disease or might increase in number once a disease has developed. Large numbers of human models are subsequently required to dissect the key role of epigenetics in the pathogenesis/onset of T2DM. These models may consist of case-control cohorts, cultures of human cells exposed to environmental risk factors, prospective cohorts, and intervention studies [13].

Identification of individuals having the risk of developing T2DM in the future could facilitate early intervention strategies to delay/prevent disease progression, therefore resulting in minimization of disease burden. Epigenetic alterations can be reversed; hence can be used as potential therapeutic targets. Epigenetic therapy has been strongly suggested as a forthcoming possibility for T2DM treatment on an individual basis.

Epigenetic Alterations Related To T2DM

DNA Methylation

DNA methylation is a regular physiological process that is involved in gene expression and regulation, therefore, playing a crucial role in disease progression and development [14]. It occurs in many key physiological processes, including X-chromosome inactivation, imprinting, and silencing of germline-specific genes and repetitive elements. DNA methylation has been reported to alter the expression of genes involved in glucose intolerance, insulin resistance, and β-cell dysfunction, which ultimately leads to the onset of T2DM [15]. Several enzymes are responsible for the attachment/removal of methyl groups to the CpG sites and histone modifications in the human genome. DNA methyltransferases (DNMTs) catalyze the addition of methyl groups from S-adenosyl-L-methionine (SAM) to the 5th position of so-called CpG sites in DNA moieties. Methylated cytosine at CpG sites represses transcription by inhibiting the binding of transcription factors or increases the binding of some transcriptional repressors, including histone deacetylases (HDACs).

In mammals, five types of DNMTs have been reported, viz. DNMT1, DNMT2, DNMT3A, DNMT3B (2methyl transferases), and DNMTL. Out of these DNMT1, DNMT2, and DNMT3B play a pivotal role in DNA methylation [16]. DNMT1 is responsible for the maintenance of methyltransferase and the pattern of DNA methylation during cell replication. DNMT3A/DNMT3B encodes de novo methyltransferases and hence is responsible for the transfer of methyl groups required to establish and conserve genomic methylation [17, 18]. It is the location of methylation which decides whether it will repress or activate gene expression. Usually, methylation at the transcription start site or enhancer region leads to suppression of gene expression.

DNA methylation can be categorized into two subgroups, i.e., hypermethylation and hypomethylation. DNA methylation in the enhancer region may change transcription levels (hyper/hypo) of distal promoters via binding of chromatin modulating proteins and transcription factors [19], leading to abnormal gene expression responsible for several diseases. In the peripheral blood mononuclear cells (PBMCs), hypomethylation in the promoter region of monocyte chemoattractant protein-1 (MCP-1) was found to be significantly associated with serum MCP-1 levels, fasting blood glucose and HbA1c in T2DM patients [20]. Hyperinsulinemia, one of the characteristics of T2DM, is directly regulated by pancreatic β cells.

In pancreatic β islets of T2DM individuals, hypermethylation of insulin promoter was reported to have a negative correlation with insulin gene expression and positive correlation with HbA1c levels [21]. Potassium voltage-gated channel subfamily Q member 1 (KCNQ1) gene was reported for statistically significant alteration in pancreatic-β islet functions [22, 23].

In 2015, Chambers and colleagues [24] reported five methylation markers associated with the risk of T2DM development, including phosphocholine phosphatase (PHOSPHO1), ATP binding cassette subfamily G member 1 (ABCG1), sterol regulatory element binding transcription factor 1 (SREBF1), suppressor of cytokine signaling 3 (SOCS3), and thioredoxin interacting protein (TXNIP), thus throwing new insights into the pathogenesis underlying T2DM.

T2DM is a type of vital metabolic disorder that is affected by both genetic and environmental factors. In recent studies, it has been noticed that dietary substrates affect the regulation of DNA methylation through various pathways viz. alteration in cofactors which are necessary for proper DNA methylation, by altering the activity of DNA methyl transferase (DNMT) enzymes.

Histone Modification

Along with DNA methylation, post-translational modifications of histone proteins also participate in the progression of T2DM and its vascular complications [25]. Histone modification is an important post-translational event that plays a key role in gene expression. It includes methylation, acetylation, phosphorylation, ubiquitylation, and SUMOylation.

It has been reported that under diabetic environment, monocyte cells display changes in histone lysine modifications in inflammatory genes [26]. Modifications in chromatin have been reported in white blood cells (WBCs) of Type 1 diabetic subjects as compared to normal healthy controls [27]. El-Osta et al. [28] reported that the transient hyperglycemic exposure causes altered gene expression and persistent epigenetic alterations during subsequent normoglycemia.

Histone Methylation

Histone methylation (Addition of –CH3 group) occurs at the lysine and arginine side chains. One, two or three methyl groups may be added to lysine, while arginine may be mono- or di-methylated. Methylation of histone proteins is catalyzed by histone methyltransferases (HMTs) and is a reversible process. In the eukaryotic cell nucleus, whenever histone methylation occurs, related genes within the DNA complexed with histone proteins may be activated or silenced [29]. Jufvas et al. [30] reported 40% higher trimethylation at lysine 4 of H3 in adipocytes of T2DM overweight patients than in normal weight and non-diabetic patients.

Histone Acetylation/Deacetylation

Histone acetylation/deacetylation is the addition/deletion of acetyl group(s) from acetyl coenzyme irrespective of the ε-amino group on the target lysine. Histone acetylation process is involved in the regulation and control of several cellular activities, including transcription, chromatin dynamics, cell cycle progression, gene silencing, apoptosis, DNA replication, nuclear import, DNA repair, and neuronal repression. Enzymes responsible for histone acetylation are histone acetyl transferases (HATs). The hydrolytic removal of acetyl groups from histone lysine residues is catalyzed by the enzymes histone deacetylases (HDACs). Glucose homeostasis is maintained by the pancreas and concomitant increase in glucose concentration can cause insulin resistance in β-islets. Inhibitors of histone deacetylase (HDACs) promote β-cell function in animal diabetic models and insulin resistance [31]. A study reported elevated levels of histone acetylation/H3 acetylation in the promoter region of cyclooxygenase-2 (COX2) and tumour necrosis factor-alpha (TNFα) in blood mononuclear cells of T2DM patients as compared to control samples [32].

Non-coding RNA

Recent studies have revealed that most of the RNAs do not play a role in coding for proteins and these non-coding RNAs help us in studying several human diseases (https://ncbi.nlm.nih.gov). Non-coding RNAs can be classified according to their size into two broad groups- long non-coding RNAs (>200 nucleotides) and small non-coding RNAs (<200 nucleotides), both of them play a role in regulating gene expression. Small non-coding RNAs include transfer-RNAs (tRNA), microRNA (miRNA), PIWI-interacting-RNAs (pi-RNA), small-nuclear- RNA (snRNA) and small nucleolar-RNAs (snoRNA).

miRNA

Recently, microRNAs (miRNAs) received considerable recognition as new potential biomarkers (evolutionally conserved) in several diseases. MicroRNAs are small noncoding, endogenous RNAs of 19-22 nucleotides, which regulate gene expression negatively via binding to the 3’untranslated region (3’UTR) of mRNA targets [33]. miRNAs are actively involved in various biological processes viz. cell differentiation, apoptosis and immune functions [34]. Day-by-day increasing number of evidence advocated the pivotal role of miRNAs in the development of insulin resistance and/or β cell dysfunction. In addition to environmental factors and genetic impairment, it has been shown that miRNA plays a role in β-cell processes, which may include β-cell proliferation, differentiation, as well as synthesis and secretion of insulin.

It has been demonstrated that the expression of different miRNAs is altered in respective tissues during the development and progression of diabetes. Various miRNAs have been implicated in regulating both glucose metabolism and insulin signaling [35]. Deletion of Dicer1 (encoding pre-miRNA processing enzyme) in adult mice led to a decreased insulin secretion, leading to the pathophysiology of a diabetic phenotype [36]. In order to examine the specific role of miRNA in β-cell function, Kalis et al. [37] made β-cell-specific disruption of Dicer1 in mice, which then showed reduced expression of insulin gene and thus decreased secretion of insulin. In another study, Yan et al. [38] observed reduced expression of Dicer1 in diabetic cases, which corresponded to the impaired/altered expression patterns of specific miRNAs viz. miR-125a-5p and miR-146a. Hence, it may be assumed that re-establishing/restoring Dicer1 activity may reduce the damaging effects of diabetes. In MIN6 cells, knockdown of miR375 led to increased glucose-stimulated insulin secretion [39, 40]. Hypomethylation of the promoter of human miR-375 was observed to contribute to the onset of T2DM [41].

piRNA

PIWI-interacting RNAs)/P-element induced Wimpy testis (PIWI)-interacting RNAs (piRNAs) discovered in 2006 is a class of small non-coding RNAs. They are regulatory RNAs and are slightly longer than miRNAs [42-45]. piRNAs interact with the PIWI-proteins and guide them to silence the transposable elements. Imène et al. [46] in their study showed that piRNAs and their associated PIWI-proteins controlled the functions of beta-cells and suggested their possible involvement in the development of T2DM.

lncRNA

Long non-coding RNAs play a role in many biological functions and are involved in causing many diseases, including T2DM. It is now known that around 70-90% of genome is transcribed to produce long non-coding RNAs (lnc-RNAs). Glucose induced/stimulated secretion of insulin is positively regulated by LncRNA-p3134 through the key regulators in β-cell (Pdx-1, MafA, GLUT2 and Tcf7l2). Elevated levels of lncRNA-p3134 were found in diabetic patients [47]. These studies showed the association of impaired expression of lncRNA with elevated glucose level/hyperglycaemia and insulin resistance.

Effect of DNA Methylation on Different Genes Involved in T2DM

Insulin Signaling

T2DM is the resultant of both impaired secretion of insulin from pancreatic-β cells and resistance of target tissue to insulin signaling. The effect of elevated glucose concentration differs according to the duration of exposure. Short duration of exposure stimulates insulin secretion, but if exposed for a longer duration, it impairs glucose stimulated insulin secretion both in-vivo and in-vitro [48-50].

Transcription Factor 7like 2 (TCF7L2)

Transcription factor 7 like 2 (TCF7L2) gene found to be associated with T2DM [51] encodes a transcription factor implicated in Wnt/β-catenin signaling. This is an important pathway that negatively regulates adipogenesis [52]. A 5-folds increased expression of TCF7L2 gene was reported in human pancreatic islets followed by a reduced glucose-stimulated insulin secretion [53]. Methylation of TCF7L2 promoter in peripheral blood has been found to be significantly associated with fasting plasma glucose, total cholesterol and LDL-cholesterol [54]. Hu et al. [55] cultured β-cells in high-glucose-lipid environment and found aberrant DNA methylation in the TCF7L2 gene promoter. It was further observed that hypermethylated TCF7L2 promoter led to increased TCF7L2 mRNA expression, while unexpectedly, the protein expression of TCF7L2 was down regulated in pancreatic β-cells. It has been reported earlier that silencing of TCF7L2 gene via miRNA resulted in suppression of insulin secretion in pancreatic β-islets in both humans and mice [56].

Hu et al. [55] studied the effect of high fat diet (HFD) on the pattern of methylation on mouse islets and concluded that HFD induced aberrant methylation of TCF7L2 promoter leading to a diminished gene expression.

Pancreatic and Duodenal Homeobox 1(PDX-1)

Pancreatic duodenal homeobox 1 (PDX-1) is a transcription factor that plays a crucial role in the pancreatic development and function of mature islets [57]. Mutational changes in PDX-1 gene were reported to cause MODY (Maturity Onset Diabetes of the Young) in humans [58] while silencing the gene in islet-β cells of mice caused diabetes [59]. Epigenetic alterations in the PDX-1 gene are significantly associated with decreased mRNA expression, pancreatic-β cell dysfunction and diabetes onset in rodents [60]. In rodents, exposure to elevated glucose concentrations leads to increased DNA methylation of PDX-1 promoter and insulin gene, which negatively regulates the transcriptional activity [70]. Yang and colleagues [61] further examined DNA methylation and mRNA expression of PDX-1 gene in human pancreatic islets and established a negative correlation of glycosylated haemoglobin (HbA1c) with mRNA expression and a positive correlation with DNA methylation.

Tyrosine-protein Phosphatase Non-receptor Type 1 (PTPN1)

This gene plays an important role in the insulin signaling pathway. CpG hypermethylation of the PTPN1 promoter often down regulates its transcription. In a Chinese population, it was observed that elevated CpG methylation of PTPN1 promoter is a risk factor for T2DM [62]. Further, polymorphisms in PTPN1 gene alters its expression leading to insulin resistance and the onset of T2DM [63, 64].

Insulin-like Growth Factor-binding Protein 7 (IGFBP-7)

Insulin-like growth factor-binding protein-7 encoded by the IGFBP7 gene plays a key role in the signaling of insulin-like growth factors (IGFs). Gu et al. [65] observed enhanced IGFBP-7 promoter methylation level in newly diagnosed T2DM males, but not in females, as compared with non-diabetic individuals.

Glucose Homeostasis and Bisphenol A (BPA) Exposure

Nowadays, the pivotal role of Bisphenol A (BPA) on glucose homeostasis via insulin signaling has attained much attention. However, its exact molecular mechanism is still unknown [66]. BPA, an environmental pollutant is a major contributor for T2DM pathogenesis and progression by affecting insulin signaling in skeletal muscle cells [67]. Acute exposure to BPA leads to a significantly increased leakage of proton in mitochondrial, which affects ROS production. Oxidative stress and mitochondrial dysfunction are directly related to altered glucose metabolism in skeletal muscles.

Studies suggested that glucose metabolism is affected by BPA exposure through different molecular mechanisms, which include pancreatic β-cell dysfunction, insulin resistance, oxidative stress and inflammation [68, 69]. A reduction in insulin secretion from pancreatic β-cells was seen in rodents exposed prenatally to BPA. Furthermore, it was found that BPA can lead to persistent postpartum hyperinsulinemia and insulin levels. In one study, it was found that in pregnant rodents, misexpression of some key genes, Pdx-1, Ccnd-2 and p16 gradually led to high apoptosis rate and β-cell dysfunction [70, 71]. The increase in insulin resistance by oxidative stress and mitochondrial dysfunction was one of the several mechanisms that were suggested for the role of BPA in glucose intolerance [72]. Furthermore, urinary BPA concentrations, inflammation and markers of oxidative stress were found to