DNA Methylation and Type 2 Diabetes: Novel Biomarkers for Risk Assessment?

Diabetes is a severe threat to global health. Almost 500 million people live with diabetes worldwide. Most of them have type 2 diabetes (T2D). T2D patients are at risk of developing severe and life-threatening complications, leading to an increased need for medical care and reduced quality of life. Improved care for people with T2D is essential. Actions aiming at identifying undiagnosed diabetes and at preventing diabetes in those at high risk are needed as well. To this end, biomarker discovery and validation of risk assessment for T2D are critical. Alterations of DNA methylation have recently helped to better understand T2D pathophysiology by explaining differences among endophenotypes of diabetic patients in tissues. Recent evidence further suggests that variations of DNA methylation might contribute to the risk of T2D even more significantly than genetic variability and might represent a valuable tool to predict T2D risk. In this review, we focus on recent information on the contribution of DNA methylation to the risk and the pathogenesis of T2D. We discuss the limitations of these studies and provide evidence supporting the potential for clinical application of DNA methylation marks to predict the risk and progression of T2D.

Role of the HIF-1α/Nur77 axis in the regulation of the tyrosine hydroxylase expression by insulin in PC12 cells.

Tyrosine hydroxylase (TH), catalyzing the conversion of tyrosine into l-DOPA, is the rate-limiting enzyme in dopamine synthesis. Defects in insulin action contribute to alterations of TH expression and/or activity in the brain and insulin increases TH levels in 1-methyl-4-phenylpyridinium (MPP+)-treated neuronal cells. However, the molecular mechanisms underlying the regulation of TH by insulin have not been elucidated yet. Using PC12 cells, we show for the first time that insulin increases TH expression in a biphasic manner, with a transient peak at 2 hr and a delayed response at 16 hr, which persists for up to 24 hr. The use of a dominant negative hypoxia-inducible factor 1-alpha (HIF-1α) and its pharmacological inhibitor chetomin, together with chromatin immunoprecipitation (ChIP) experiments for the specific binding to TH promoter, demonstrate the direct role of HIF-1α in the early phase. Moreover, ChIP experiments and transfection of a dominant negative of the nerve growth factor IB (Nur77) indicate the involvement of Nur77 in the late phase insulin response, which is mediated by HIF-1α. In conclusion, the present study shows that insulin regulates TH expression through HIF-1α and Nur77 in PC12 cells, supporting the critical role of insulin signaling in maintaining an appropriate dopaminergic tone by regulating TH expression in the central nervous system.

Adipose Tissue Dysfunction as Determinant of Obesity-Associated Metabolic Complications.

Obesity is a critical risk factor for the development of type 2 diabetes (T2D), and its prevalence is rising worldwide. White adipose tissue (WAT) has a crucial role in regulating systemic energy homeostasis. Adipose tissue expands by a combination of an increase in adipocyte size (hypertrophy) and number (hyperplasia). The recruitment and differentiation of adipose precursor cells in the subcutaneous adipose tissue (SAT), rather than merely inflating the cells, would be protective from the obesity-associated metabolic complications. In metabolically unhealthy obesity, the storage capacity of SAT, the largest WAT depot, is limited, and further caloric overload leads to the fat accumulation in ectopic tissues (e.g., liver, skeletal muscle, and heart) and in the visceral adipose depots, an event commonly defined as "lipotoxicity." Excessive ectopic lipid accumulation leads to local inflammation and insulin resistance (IR). Indeed, overnutrition triggers uncontrolled inflammatory responses in WAT, leading to chronic low-grade inflammation, therefore fostering the progression of IR. This review summarizes the current knowledge on WAT dysfunction in obesity and its associated metabolic abnormalities, such as IR. A better understanding of the mechanisms regulating adipose tissue expansion in obesity is required for the development of future therapeutic approaches in obesity-associated metabolic complications.

Epigenetic modifications of the Zfp/ZNF423 gene control murine adipogenic commitment and are dysregulated in human hypertrophic obesity.

AIMS/HYPOTHESIS: Subcutaneous adipocyte hypertrophy is associated with insulin resistance and increased risk of type 2 diabetes, and predicts its future development independent of obesity. In humans, subcutaneous adipose tissue hypertrophy is a consequence of impaired adipocyte precursor cell recruitment into the adipogenic pathway rather than a lack of precursor cells. The zinc finger transcription factor known as zinc finger protein (ZFP) 423 has been identified as a major determinant of pre-adipocyte commitment and maintained white adipose cell function. Although its levels do not change during adipogenesis, ectopic expression of Zfp423 in non-adipogenic murine cells is sufficient to activate expression of the gene encoding peroxisome proliferator-activated receptor γ (Pparγ; also known as Pparg) and increase the adipogenic potential of these cells. We investigated whether the Zfp423 gene is under epigenetic regulation and whether this plays a role in the restricted adipogenesis associated with hypertrophic obesity. METHODS: Murine 3T3-L1 and NIH-3T3 cells were used as fibroblasts committed and uncommitted to the adipocyte lineage, respectively. Human pre-adipocytes were isolated from the stromal vascular fraction of subcutaneous adipose tissue of 20 lean non-diabetic individuals with a wide adipose cell size range. mRNA levels were measured by quantitative real-time PCR, while methylation levels were analysed by bisulphite sequencing. Chromatin structure was analysed by micrococcal nuclease protection assay, and DNA-methyltransferases were chemically inhibited by 5-azacytidine. Adipocyte differentiation rate was evaluated by Oil Red O staining. RESULTS: Comparison of uncommitted (NIH-3T3) and committed (3T3-L1) adipose precursor cells revealed that Zfp423 expression increased (p < 0.01) in parallel with the ability of the cells to differentiate into mature adipocytes owing to both decreased promoter DNA methylation (p < 0.001) and nucleosome occupancy (nucleosome [NUC] 1 p < 0.01; NUC2 p < 0.001) in the 3T3-L1 compared with NIH-3T3 cells. Interestingly, non-adipogenic epigenetic profiles can be reverted in NIH-3T3 cells as 5-azacytidine treatment increased Zfp423 mRNA levels (p < 0.01), reduced DNA methylation at a specific CpG site (p < 0.01), decreased nucleosome occupancy (NUC1, NUC2: p < 0.001) and induced adipocyte differentiation (p < 0.05). These epigenetic modifications can also be initiated in response to changes in the pre-adipose cell microenvironment, in which bone morphogenetic protein 4 (BMP4) plays a key role. We finally showed that, in human adipocyte precursor cells, impaired epigenetic regulation of zinc nuclear factor (ZNF)423 (the human orthologue of murine Zfp423) was associated with inappropriate subcutaneous adipose cell hypertrophy. As in NIH-3T3 cells, the normal ZNF423 epigenetic profile was rescued by 5-azacytidine exposure. CONCLUSIONS/INTERPRETATION: Our results show that epigenetic events regulate the ability of precursor cells to commit and differentiate into mature adipocytes by modulating ZNF423, and indicate that dysregulation of these mechanisms accompanies subcutaneous adipose tissue hypertrophy in humans.

Pathologic endoplasmic reticulum stress induced by glucotoxic insults inhibits adipocyte differentiation and induces an inflammatory phenotype.

Adipocyte differentiation is critical in obesity. By controlling new adipocyte recruitment, adipogenesis contrasts adipocyte hypertrophy and its adverse consequences, such as insulin resistance. Contrasting data are present in literature on the effect of endoplasmic reticulum (ER) stress and subsequent unfolded protein response (UPR) on adipocyte differentiation, being reported to be either necessary or inhibitory. In this study, we sought to clarify the effect of ER stress and UPR on adipocyte differentiation. We have used two different cell lines, the widely used pre-adipocyte 3T3-L1 cells and a murine multipotent mesenchymal cell line, W20-17 cells. A strong ER stress activator, thapsigargin, and a pathologically relevant inducer of ER stress, glucosamine (GlcN), induced ER stress and UPR above those occurring in the absence of perturbation and inhibited adipocyte differentiation. Very low concentrations of 4-phenyl butyric acid (PBA, a chemical chaperone) inhibited only the overactivation of ER stress and UPR elicited by GlcN, leaving unaltered the part physiologically activated during differentiation, and reversed the inhibitory effect of GlcN on differentiation. In addition, GlcN stimulated proinflammatory cytokine release and PBA prevented these effects. An inhibitor of NF-kB also reversed the effects of GlcN on cytokine release. These results indicate that while ER stress and UPR activation is "physiologically" activated during adipocyte differentiation, the "pathologic" part of ER stress activation, secondary to a glucotoxic insult, inhibits differentiation. In addition, such a metabolic insult, causes a shift of the preadipocyte/adipocyte population towards a proinflammatory phenotype.

The Transcription Factor HOXA5: Novel Insights into Metabolic Diseases and Adipose Tissue Dysfunction.

The transcription factor HOXA5, from the HOX gene family, has long been studied due to its critical role in physiological activities in normal cells, such as organ development and body patterning, and pathological activities in cancer cells. Nonetheless, recent evidence supports the hypothesis of a role for HOXA5 in metabolic diseases, particularly in obesity and type 2 diabetes (T2D). In line with the current opinion that adipocyte and adipose tissue (AT) dysfunction belong to the group of primary defects in obesity, linking this condition to an increased risk of insulin resistance (IR) and T2D, the HOXA5 gene has been shown to regulate adipocyte function and AT remodeling both in humans and mice. Epigenetics adds complexity to HOXA5 gene regulation in metabolic diseases. Indeed, epigenetic mechanisms, specifically DNA methylation, influence the dynamic HOXA5 expression profile. In human AT, the DNA methylation profile at the HOXA5 gene is associated with hypertrophic obesity and an increased risk of developing T2D. Thus, an inappropriate HOXA5 gene expression may be a mechanism causing or maintaining an impaired AT function in obesity and potentially linking obesity to its associated disorders. In this review, we integrate the current evidence about the involvement of HOXA5 in regulating AT function, as well as its association with the pathogenesis of obesity and T2D. We also summarize the current knowledge on the role of DNA methylation in controlling HOXA5 expression. Moreover, considering the susceptibility of epigenetic changes to reversal through targeted interventions, we discuss the potential therapeutic value of targeting HOXA5 DNA methylation changes in the treatment of metabolic diseases.

Methylglyoxal Impairs the Pro-Angiogenic Ability of Mouse Adipose-Derived Stem Cells (mADSCs) via a Senescence-Associated Mechanism.

Adipose-derived stem cells (ADSCs) play a crucial role in angiogenesis and repair of damaged tissues. However, in pathological conditions including diabetes, ADSC function is compromised. This work aims at evaluating the effect of Methylglyoxal (MGO), a product of chronic hyperglycemia, on mouse ADSCs' (mADSCs) pro-angiogenic function and the molecular mediators involved. The mADSCs were isolated from C57bl6 mice. MGO-adducts and p-p38 MAPK protein levels were evaluated by Western Blot. Human retinal endothelial cell (hREC) migration was analyzed by transwell assays. Gene expression was measured by qRT-PCR, and SA-ßGal activity by cytofluorimetry. Soluble factor release was evaluated by multiplex assay. MGO treatment does not impair mADSC viability and induces MGO-adduct accumulation. hREC migration is reduced in response to both MGO-treated mADSCs and conditioned media from MGO-treated mADSCs, compared to untreated cells. This is associated with an increase of SA-ßGal activity, SASP factor release and p53 and p21 expression, together with a VEGF- and PDGF-reduced release from MGO-treated mADSCs and a reduced p38-MAPK activation in hRECs. The MGO-induced impairment of mADSC function is reverted by senolytics. In conclusion, MGO impairs mADSCs' pro-angiogenic function through the induction of a senescent phenotype, associated with the reduced secretion of growth factors crucial for hREC migration.

Altered H3K4me3 profile at the TFAM promoter causes mitochondrial alterations in preadipocytes from first-degree relatives of type 2 diabetics.

BACKGROUND: First-degree relatives of type 2 diabetics (FDR) exhibit a high risk of developing type 2 diabetes (T2D) and feature subcutaneous adipocyte hypertrophy, independent of obesity. In FDR, adipose cell abnormalities contribute to early insulin-resistance and are determined by adipocyte precursor cells (APCs) early senescence and impaired recruitment into the adipogenic pathway. Epigenetic mechanisms signal adipocyte differentiation, leading us to hypothesize that abnormal epigenetic modifications cause adipocyte dysfunction and enhance T2D risk. To test this hypothesis, we examined the genome-wide histone profile in APCs from the subcutaneous adipose tissue of healthy FDR. RESULTS: Sequencing-data analysis revealed 2644 regions differentially enriched in lysine 4 tri-methylated H3-histone (H3K4me3) in FDR compared to controls (CTRL) with significant enrichment in mitochondrial-related genes. These included TFAM, which regulates mitochondrial DNA (mtDNA) content and stability. In FDR APCs, a significant reduction in H3K4me3 abundance at the TFAM promoter was accompanied by a reduction in TFAM mRNA and protein levels. FDR APCs also exhibited reduced mtDNA content and mitochondrial-genome transcription. In parallel, FDR APCs exhibited impaired differentiation and TFAM induction during adipogenesis. In CTRL APCs, TFAM-siRNA reduced mtDNA content, mitochondrial transcription and adipocyte differentiation in parallel with upregulation of the CDKN1A and ZMAT3 senescence genes. Furthermore, TFAM-siRNA significantly expanded hydrogen peroxide (H2O2)-induced senescence, while H2O2 did not affect TFAM expression. CONCLUSIONS: Histone modifications regulate APCs ability to differentiate in mature cells, at least in part by modulating TFAM expression and affecting mitochondrial function. Reduced H3K4me3 enrichment at the TFAM promoter renders human APCs senescent and dysfunctional, increasing T2D risk.

Epigenetic Reprogramming of the Inflammatory Response in Obesity and Type 2 Diabetes.

For the past several decades, the prevalence of obesity and type 2 diabetes (T2D) has continued to rise on a global level. The risk contributing to this pandemic implicates both genetic and environmental factors, which are functionally integrated by epigenetic mechanisms. While these conditions are accompanied by major abnormalities in fuel metabolism, evidence indicates that altered immune cell functions also play an important role in shaping of obesity and T2D phenotypes. Interestingly, these events have been shown to be determined by epigenetic mechanisms. Consistently, recent epigenome-wide association studies have demonstrated that immune cells from obese and T2D individuals feature specific epigenetic profiles when compared to those from healthy subjects. In this work, we have reviewed recent literature reporting epigenetic changes affecting the immune cell phenotype and function in obesity and T2D. We will further discuss therapeutic strategies targeting epigenetic marks for treating obesity and T2D-associated inflammation.

ZMAT3 hypomethylation contributes to early senescence of preadipocytes from healthy first-degree relatives of type 2 diabetics.

Senescence of adipose precursor cells (APC) impairs adipogenesis, contributes to the age-related subcutaneous adipose tissue (SAT) dysfunction, and increases risk of type 2 diabetes (T2D). First-degree relatives of T2D individuals (FDR) feature restricted adipogenesis, reflecting the detrimental effects of APC senescence earlier in life and rendering FDR more vulnerable to T2D. Epigenetics may contribute to these abnormalities but the underlying mechanisms remain unclear. In previous methylome comparison in APC from FDR and individuals with no diabetes familiarity (CTRL), ZMAT3 emerged as one of the top-ranked senescence-related genes featuring hypomethylation in FDR and associated with T2D risk. Here, we investigated whether and how DNA methylation changes at ZMAT3 promote early APC senescence. APC from FDR individuals revealed increases in multiple senescence markers compared to CTRL. Senescence in these cells was accompanied by ZMAT3 hypomethylation, which caused ZMAT3 upregulation. Demethylation at this gene in CTRL APC led to increased ZMAT3 expression and premature senescence, which were reverted by ZMAT3 siRNA. Furthermore, ZMAT3 overexpression in APC determined senescence and activation of the p53/p21 pathway, as observed in FDR APC. Adipogenesis was also inhibited in ZMAT3-overexpressing APC. In FDR APC, rescue of ZMAT3 methylation through senolytic exposure simultaneously downregulated ZMAT3 expression and improved adipogenesis. Interestingly, in human SAT, aging and T2D were associated with significantly increased expression of both ZMAT3 and the P53 senescence marker. Thus, DNA hypomethylation causes ZMAT3 upregulation in FDR APC accompanied by acquisition of the senescence phenotype and impaired adipogenesis, which may contribute to FDR predisposition for T2D.

Low-dose Bisphenol-A Promotes Epigenetic Changes at Pparγ Promoter in Adipose Precursor Cells.

Exposure to endocrine-disrupting chemicals such as Bisphenol-A (BPA) is associated with an increase in obesity prevalence. Diet is the primary cause of human exposure to this contaminant. BPA promotes obesity by inducing adipocyte dysfunction and altering adipogenesis. Contradictory evidence and unanswered questions are reported in the literature concerning the BPA effects on adipogenesis. To clarify this issue, we tested the effects of prolonged low-dose BPA exposure on different phases of adipogenesis in committed 3T3L1 and uncommitted NIH3T3 preadipocytes. Our findings show that BPA effects on the adipogenesis are mediated by epigenetic mechanisms by reducing peroxisome proliferator-activated receptor gamma (Pparγ) promoter methylation in preadipocytes. Nevertheless, in BPA-exposed 3T3L1, Pparγ expression only transiently increases as lipid accumulation at day 4 of differentiation, without altering the adipogenic potential of the precursor cells. In the absence of differentiation mix, BPA does not make the 3T3L1 an in vitro model of spontaneous adipogenesis and the effects on the Pparγ expression are still limited at day 4 of differentiation. Furthermore, BPA exposure does not commit the NIH3T3 to the adipocyte lineage, although Pparγ overexpression is more evident both in preadipocytes and during the adipocyte differentiation. Interestingly, termination of the BPA exposure restores the Pparγ promoter methylation and inflammatory profile of the 3T3L1 cells. This study shows that BPA induces epigenetic changes in a key adipogenic gene. These modifications are reversible and do not affect preadipocyte commitment and/or differentiation. We identify an alternative transcriptional mechanism by which BPA affects gene expression and demonstrate how the challenge of preventing exposure is fundamental for human health.

Chronic Adipose Tissue Inflammation Linking Obesity to Insulin Resistance and Type 2 Diabetes.

Obesity is one of the major health burdens of the 21st century as it contributes to the growing prevalence of its related comorbidities, including insulin resistance and type 2 diabetes. Growing evidence suggests a critical role for overnutrition in the development of low-grade inflammation. Specifically, chronic inflammation in adipose tissue is considered a crucial risk factor for the development of insulin resistance and type 2 diabetes in obese individuals. The triggers for adipose tissue inflammation are still poorly defined. However, obesity-induced adipose tissue expansion provides a plethora of intrinsic signals (e.g., adipocyte death, hypoxia, and mechanical stress) capable of initiating the inflammatory response. Immune dysregulation in adipose tissue of obese subjects results in a chronic low-grade inflammation characterized by increased infiltration and activation of innate and adaptive immune cells. Macrophages are the most abundant innate immune cells infiltrating and accumulating into adipose tissue of obese individuals; they constitute up to 40% of all adipose tissue cells in obesity. In obesity, adipose tissue macrophages are polarized into pro-inflammatory M1 macrophages and secrete many pro-inflammatory cytokines capable of impairing insulin signaling, therefore promoting the progression of insulin resistance. Besides macrophages, many other immune cells (e.g., dendritic cells, mast cells, neutrophils, B cells, and T cells) reside in adipose tissue during obesity, playing a key role in the development of adipose tissue inflammation and insulin resistance. The association of obesity, adipose tissue inflammation, and metabolic diseases makes inflammatory pathways an appealing target for the treatment of obesity-related metabolic complications. In this review, we summarize the molecular mechanisms responsible for the obesity-induced adipose tissue inflammation and progression toward obesity-associated comorbidities and highlight the current therapeutic strategies.

Epigenetic silencing of the ANKRD26 gene correlates to the pro-inflammatory profile and increased cardio-metabolic risk factors in human obesity.

BACKGROUND: Obesity is a major worldwide threat to human health. Increasing evidence indicates that epigenetic modifications have a major impact on the natural history of this disorder. Ankyrin Repeat Domain 26 (Ankrd26) is involved in the development of both obesity and diabetes in mice and is modulated by environmentally induced epigenetic modifications. This study aims at investigating whether impaired ANKRD26 gene expression and methylation occur in human obesity and whether they correlate to the phenotype of these subjects. RESULTS: We found that downregulation of ANKRD26 mRNA and hyper-methylation of a specific region of the ANKRD26 promoter, embedding the CpG dinucleotides - 689, - 659, and - 651 bp, occur in peripheral blood leukocytes from obese compared with the lean subjects. ANKRD26 gene expression correlates inversely to the percentage of DNA methylation at these 3 CpG sites. Luciferase assays reveal a cause-effect relationship between DNA methylation at the 3 CpG sites and ANKRD26 gene expression. Finally, both ANKRD26 mRNA levels and CpG methylation correlate to body mass index and to the pro-inflammatory status and the increased cardio-metabolic risk factors of these same subjects. CONCLUSION: Downregulation of the ANKRD26 gene and hyper-methylation at specific CpGs of its promoter are common abnormalities in obese patients. These changes correlate to the pro-inflammatory profile and the cardio-metabolic risk factors of the obese individuals, indicating that, in humans, they mark adverse health outcomes.

PED/PEA-15 inhibits hydrogen peroxide-induced apoptosis in Ins-1E pancreatic beta-cells via PLD-1.

The small scaffold protein PED/PEA-15 is involved in several different physiologic and pathologic processes, such as cell proliferation and survival, diabetes and cancer. PED/PEA-15 exerts an anti-apoptotic function due to its ability to interfere with both extrinsic and intrinsic apoptotic pathways in different cell types. Recent evidence shows that mice overexpressing PED/PEA-15 present larger pancreatic islets and increased beta-cells mass. In the present work we investigated PED/PEA-15 role in hydrogen peroxide-induced apoptosis in Ins-1E beta-cells. In pancreatic islets isolated from Tg(PED/PEA-15) mice hydrogen peroxide-induced DNA fragmentation was lower compared to WT islets. TUNEL analysis showed that PED/PEA-15 overexpression increases the viability of Ins-1E beta-cells and enhances their resistance to apoptosis induced by hydrogen peroxide exposure. The activity of caspase-3 and the cleavage of PARP-1 were markedly reduced in Ins-1E cells overexpressing PED/PEA-15 (Ins-1E(PED/PEA-15)). In parallel, we observed a decrease of the mRNA levels of pro-apoptotic genes Bcl-xS and Bad. In contrast, the expression of the anti-apoptotic gene Bcl-xL was enhanced. Accordingly, DNA fragmentation was higher in control cells compared to Ins-1E(PED/PEA-15) cells. Interestingly, the preincubation with propranolol, an inhibitor of the pathway of PLD-1, a known interactor of PED/PEA-15, responsible for its deleterious effects on glucose tolerance, abolishes the antiapoptotic effects of PED/PEA-15 overexpression in Ins-1E beta-cells. The same results have been obtained by inhibiting PED/PEA-15 interaction with PLD-1 in Ins-1E(PED/PEA-15). These results show that PED/PEA-15 overexpression is sufficient to block hydrogen peroxide-induced apoptosis in Ins-1E cells through a PLD-1 mediated mechanism.