Investigating the Role of 17-Beta Estradiol in the Regulation of the Unfolded Protein Response (UPR) in Pancreatic Beta Cells.

Diabetes mellitus is clinically defined by chronic hyperglycemia. Sex differences in the presentation and outcome of diabetes exist with premenopausal women having a reduced risk of developing diabetes, relative to men, or women after menopause. Accumulating evidence shows a protective role of estrogens, specifically 17-beta estradiol, in the maintenance of pancreatic beta cell health; however, the mechanisms underlying this protection are still unknown. To elucidate these potential mechanisms, we used a pancreatic beta cell line (BTC6) and a mouse model of hyperglycemia-induced atherosclerosis, the ApoE-/-:Ins2+/Akita mouse, exhibiting sexual dimorphism in glucose regulation. In this study we hypothesize that 17-beta estradiol protects pancreatic beta cells by modulating the unfolded protein response (UPR) in response to endoplasmic reticulum (ER) stress. We observed that ovariectomized female and male ApoE-/-:Ins2+/Akita mice show significantly increased expression of apoptotic UPR markers. Sham operated female and ovariectomized female ApoE-/-:Ins2+/Akita mice supplemented with exogenous 17-beta estradiol increased the expression of adaptive UPR markers compared to non-supplemented ovariectomized female ApoE-/-:Ins2+/Akita mice. These findings were consistent to what was observed in cultured BTC6 cells, suggesting that 17-beta estradiol may protect pancreatic beta cells by repressing the apoptotic UPR and enhancing the adaptive UPR activation in response to pancreatic ER stress.

Investigating the Effects of Sex Hormones on Macrophage Polarization.

Sex differences in the development and progression of cardiovascular disease are well established, but the effects of sex hormones on macrophage polarization and pro-atherogenic functions are not well described. We hypothesize that sex hormones directly modulate macrophage polarization, and thereby regulate the progression of atherosclerosis. Bone marrow-derived monocytes from adult male and female C57BL/6 mice were differentiated into macrophages using macrophage colony-stimulating factor (20 ng/mL) and pre-treated with either 17ß-estradiol (100 nM), testosterone (100 nM), or a vehicle control for 24 h. Macrophages were polarized into pro- or anti-inflammatory phenotypes and the effects of sex hormone supplementation on the gene expression of macrophage phenotypic markers were assessed using RT-qPCR. Inflammatory markers, including IL-1ß, were quantified using an addressable laser bead immunoassay. A transwell migration assay was used to determine changes in macrophage migration. Sex differences were observed in macrophage polarization, inflammatory responses, and migration. Pre-treatment with 17ß-estradiol significantly impaired the gene expression of inflammatory markers and the production of IL-1ß in inflammatory macrophages. In anti-inflammatory macrophages, 17ß-estradiol significantly upregulated the expression of anti-inflammatory markers and enhanced migration. Pre-treatment with testosterone enhanced anti-inflammatory mRNA expression and impaired the production of IL-1ß. Our observations suggest a protective role of 17ß-estradiol in atherogenesis that may contribute to the sexual dimorphisms in cardiovascular disease observed in human patients.

Deletion of Macrophage-Specific Glycogen Synthase Kinase (GSK)-3α Promotes Atherosclerotic Regression in Ldlr-/- Mice.

Recent evidence from our laboratory suggests that impeding ER stress-GSK3α/ß signaling attenuates the progression and development of atherosclerosis in mouse model systems. The objective of this study was to determine if the tissue-specific genetic ablation of GSK3α/ß could promote the regression of established atherosclerotic plaques. Five-week-old low-density lipoprotein receptor knockout (Ldlr-/-) mice were fed a high-fat diet for 16 weeks to promote atherosclerotic lesion formation. Mice were then injected with tamoxifen to induce macrophage-specific GSK3α/ß deletion, and switched to standard diet for 12 weeks. All mice were sacrificed at 33 weeks of age and atherosclerosis was quantified and characterized. Female mice with induced macrophage-specific GSK3α deficiency, but not GSK3ß deficiency, had reduced plaque volume (~25%) and necrosis (~40%) in the aortic sinus, compared to baseline mice. Atherosclerosis was also significantly reduced (~60%) in the descending aorta. Macrophage-specific GSK3α-deficient mice showed indications of increased plaque stability and reduced inflammation in plaques, as well as increased CCR7 and ABCA1 expression in lesional macrophages, consistent with regressive plaques. These results suggest that GSK3α ablation promotes atherosclerotic plaque regression and identify GSK3α as a potential target for the development of new therapies to treat existing atherosclerotic lesions in patients with cardiovascular disease.

Investigating the protective effects of estrogen on ß-cell health and the progression of hyperglycemia-induced atherosclerosis.

Sex differences in the prevalence and development of diabetes and associated cardiometabolic complications are well established. The objective of this study was to analyze the effects of estrogen on the maintenance of ß-cell health/function and atherosclerosis progression, using a mouse model of hyperglycemia-induced atherosclerosis, the ApoE-/-:Ins2+/Akita mouse. ApoE-/-:Ins2+/Akita mice exhibit sexual dimorphism in the control of blood glucose levels. Male ApoE-/-:Ins2+/Akita mice are chronically hyperglycemic due to a significant reduction in pancreatic ß-cell mass. Female mice are only transiently hyperglycemic, maintain ß-cell mass, and blood glucose levels normalize at 35 ± 1 days of age. To determine the effects of estrogen on pancreatic ß-cell health and function, ovariectomies and estrogen supplementation experiments were performed, and pancreatic health and atherosclerosis were assessed at various time points. Ovariectomized ApoE-/-:Ins2+/Akita mice developed chronic hyperglycemia with significantly reduced ß-cell mass. To determine whether the observed effects on ovariectomized ApoE-/-:Ins2+/Akita mice were due to a lack of estrogens, slow-releasing estradiol pellets were inserted subcutaneously. Ovariectomized ApoE-/-:Ins2+/Akita mice treated with exogenous estradiol showed normalized blood glucose levels and maintained ß-cell mass. Exogenous estradiol significantly reduced atherosclerosis in both ovariectomized female and male ApoE-/-:Ins2+/Akita mice relative to controls. Together, these findings suggest that estradiol confers significant protection to pancreatic ß-cell health and can directly and indirectly slow the progression of atherosclerosis.NEW & NOTEWORTHY This study examines the effect(s) of estrogen on ß cell and cardiometabolic health/function in a novel mouse model of hyperglycemia-induced atherosclerosis (ApoE-/-:Ins2+/Akita). Using a combination of estrogen deprivation (ovariectomy) and supplementation strategies, we quantify effects on glucose homeostasis and atherogenesis. Our results clearly show a protective role for estrogen on pancreatic ß-cell health and function and glucose homeostasis. Furthermore, estrogen supplementation dramatically reduces atherosclerosis progression in both male and female mice.

The Role of Estrogen in Insulin Resistance: A Review of Clinical and Preclinical Data.

Insulin resistance results when peripheral tissues, including adipose, skeletal muscle, and liver, do not respond appropriately to insulin, causing the ineffective uptake of glucose. This represents a risk factor for the development of type 2 diabetes mellitus. Along with abdominal obesity, hypertension, high levels of triglycerides, and low levels of high-density lipoproteins, insulin resistance is a component of a condition known as the metabolic syndrome, which significantly increases the risk of developing cardiometabolic disorders. Accumulating evidence shows that biological sex has a major influence in the development of cardiometabolic disturbances, with females being more protected than males. This protection appears to be driven by female sex hormones (estrogens), as it tends to disappear with the onset of menopause but can be re-established with hormone replacement therapy. This review evaluates current knowledge on the protective role of estrogens in the relevant pathways associated with insulin resistance. The importance of increasing our understanding of sex as a biological variable in cardiometabolic research to promote the development of more effective preventative strategies is emphasized.

Macrophage Function and the Role of GSK3.

Macrophages are present in nearly all vertebrate tissues, where they respond to a complex variety of regulatory signals to coordinate immune functions involved in tissue development, metabolism, homeostasis, and repair. Glycogen synthase kinase 3 (GSK3) is a ubiquitously expressed protein kinase that plays important roles in multiple pathways involved in cell metabolism. Dysregulation of GSK3 has been implicated in several prevalent metabolic disorders, and recent findings have highlighted the importance of GSK3 activity in the regulation of macrophages, especially with respect to the initiation of specific pathologies. This makes GSK3 a potential therapeutic target for the development of novel drugs to modulate immunometabolic responses. Here, we summarize recent findings that have contributed to our understanding of how GSK3 regulates macrophage function, and we discuss the role of GSK3 in the development of metabolic disorders and diseases.

Protective effect of pharmacological castration on metabolic perturbations and cardiovascular disease in the hyperglycemic male ApoE-/-:Ins2+/Akita mouse model.

BACKGROUND: Unlike in other mouse models of atherogenesis, it has recently been suggested that orchiectomy plays a role in accelerating atherosclerosis and inhibiting the progression of cardiovascular disease in the ApoE-/-:Ins2+/Akita mouse model of hyperglycemia. Androgen-deprivation therapy (ADT) is a common treatment for prostate cancer, a population with high prevalence of cardiovascular disease and its risk factors. Our objectives were to test and further characterize the effects of pharmacological castration which is currently the acceptable modality to deliver ADT in the clinic. METHODS: Male ApoE-/-:Ins2+/Akita mice received one of three modes of ADT (gonadotropin-releasing hormone (GnRH)-antagonist (degarelix), GnRH-agonist (leuprolide), or bilateral orchiectomy) and were compared to corresponding untreated control mice (n = 9-13/group). Mice were followed for 5 months. Body weight, fasting blood glucose, glucose tolerance, serum C-peptide, leptin, and testosterone levels along with atherosclerotic aortic plaque size and characteristics were determined. In a separate experiment, the survival of mice, untreated and on ADT, was determined. RESULTS: Castration was achieved for all three modes of ADT. However, degarelix-treated mice gained significantly less weight, had lower serum leptin levels and systolic blood pressure compared to orchiectomy and leuprolide-treated mice. ADT improved dysglycemia and atherosclerotic burden. GnRH-antagonist significantly improved survival compared to GnRH-agonist but not compared to orchiectomy. CONCLUSIONS: Further characterization of the ApoE-/-:Ins2+/Akita mouse model confirms that pharmacological ADT ameliorated metabolic syndrome and cardiovascular complications. Improved dysglycemia and atherosclerosis associated with increased survival which was longest after degarelix followed by orchiectomy.

Role of Estrogen in Type 1 and Type 2 Diabetes Mellitus: A Review of Clinical and Preclinical Data.

The incidence and prevalence of diabetes mellitus, and the cardiovascular complications associated with this disease, are rapidly increasing worldwide. Individuals with diabetes have a higher mortality rate due to cardiovascular diseases and a reduced life expectancy compared to those without diabetes. This poses a significant economic burden on health-care systems worldwide, making the diabetes epidemic a global health crisis. Sex differences in the presentation and outcome of diabetes do exist. Premenopausal women are protected from developing diabetes and its cardiovascular complications relative to males and postmenopausal women. However, women with diabetes tend to have a higher mortality as a result of cardiovascular complications than age-matched men. Despite this evidence, preclinical and clinical research looking at sex as a biologic variable in metabolic disorders and their cardiovascular complications is very limited. The aim of this review is to highlight the current knowledge of the potential protective role of estrogens in humans as well as rodent models of diabetes mellitus, and the possible pathways by which this protection is conferred. We stress the importance of increasing knowledge of sex-specific differences to facilitate the development of more targeted prevention strategies.

Deletion of Myeloid GSK3α Attenuates Atherosclerosis and Promotes an M2 Macrophage Phenotype.

OBJECTIVE: Glycogen synthase kinase (GSK)-3α/ß has been implicated in the pathogenesis of diabetes mellitus, cancer, Alzheimer, and atherosclerosis. The tissue- and homolog-specific functions of GSK3α and ß in atherosclerosis are unknown. This study examines the effect of hepatocyte or myeloid cell deletion of GSK3α or GSK3ß on atherosclerosis in low-density lipoprotein receptor (LDLR)(-/-) mice. APPROACH AND RESULTS: We ablated GSK3α or GSK3ß expression in hepatic or myeloid cells of LDLR(-/-) mice, and mice were fed a high-fat diet for 10 weeks. GSK3α or GSK3ß deficiency in hepatic or myeloid cells did not affect metabolic parameters, including plasma lipid levels. Hepatic deletion of GSK3α or GSK3ß did not affect the development of atherosclerosis or hepatic lipid content. Myeloid deletion of GSK3α, but not of GSK3ß, reduced atherosclerotic lesion volume and lesion complexity. Mice lacking GSK3α in myeloid cells had a less inflammatory and more anti-inflammatory plasma cytokine profile. Macrophages within atherosclerotic lesions of myeloid GSK3α-deficient mice, but not of GSK3ß-deficient mice, displayed reduced expression of markers associated with M1 macrophage polarization and enhanced expression of the M2 markers. Finally, bone marrow-derived macrophages were isolated and differentiated into classical M1 macrophages or alternative M2 macrophages in vitro. GSK3α deletion, but not GSK3ß deletion, attenuated the expression of genes associated with M1 polarization while promoting the expression of genes associated with M2 polarization by modulating STAT3 and STAT6 activation. CONCLUSIONS: Our findings suggest that deletion of myeloid GSK3α attenuates the progression of atherosclerosis by promoting an M2 macrophage phenotype.

Glycogen synthase kinase 3α deficiency attenuates atherosclerosis and hepatic steatosis in high fat diet-fed low density lipoprotein receptor-deficient mice.

Studies have implicated signaling through glycogen synthase kinase (GSK) 3α/ß in the activation of pro-atherogenic pathways and the accelerated development of atherosclerosis. By using a mouse model, we examined the role of GSK3α in the development and progression of accelerated atherosclerosis. We crossed Gsk3a/GSK3α-knockout mice with low-density lipoprotein receptor (Ldlr) knockout mice. Five-week-old Ldlr(-/-);Gsk3a(+/+), Ldlr(-/-);Gsk3a(+/-), and Ldlr(-/-);Gsk3a(-/-) mice were fed a chow diet or a high-fat diet for 10 weeks and then sacrificed. GSK3α deficiency had no detectible effect on any measured parameters in chow-fed mice. High-fat-diet fed Ldlr(-/-) mice that were deficient for GSK3α had significantly less hepatic lipid accumulation and smaller atherosclerotic lesions (60% smaller in Ldlr(-/-);Gsk3a(+/-) mice, 80% smaller in Ldlr(-/-);Gsk3a(-/-) mice; P < 0.05), compared with Ldlr(-/-);Gsk3a(+/+) controls. GSK3α deficiency was associated with a significant increase in plasma IL-10 concentration and IL-10 expression in isolated macrophages. A twofold to threefold enhancement in endoplasmic reticulum stress-induced IL-10 expression was observed in Thp-1-derived macrophages that were pretreated with the GSK3α/ß inhibitor CT99021. Together, these results suggest that GSK3α plays a pro-atherogenic role, possibly by mediating the effects of endoplasmic reticulum stress in the activation of pro-atherogenic pathways.

GnRH antagonist associates with less adiposity and reduced characteristics of metabolic syndrome and atherosclerosis compared with orchiectomy and GnRH agonist in a preclinical mouse model.

OBJECTIVES: Observational studies relate androgen deprivation therapy (ADT) to metabolic syndrome (MS) and cardiovascular disease, an association potentially subject to uncontrollable confounding factors, especially diet and genetic/metabolic risk factors. In the absence of prospective randomized clinical trials, causality remains unproven. We comparatively investigated the effects of different ADT modalities on the development of MS and atherosclerosis in a mouse model. MATERIALS AND METHODS: Low-density lipoprotein receptor knockout mice underwent orchiectomy plus vehicle (2.5% mannitol), sham surgery plus vehicle (control), sham surgery plus gonadotropin-releasing hormone (GnRH) antagonist (degarelix), or sham surgery plus GnRH agonist (leuprolide) (n = 9-13/group) and were followed for 4 months. Visceral fat accumulation, lean body mass, adipocyte size, fasting blood glucose, glucose tolerance, serum levels of leptin, follicle-stimulating hormone, luteinizing hormone, and testosterone, along with atherosclerotic plaque size and characteristics were measured. RESULTS: All 3 modes of ADT decreased circulating testosterone levels in mice, although leuprolide treatment reached nadir levels of testosterone later. Orchiectomized and leuprolide-treated mice gained significantly more visceral fat compared with degarelix-treated mice. Improved glucose tolerance tests were recorded in degarelix-treated mice. The aortic atherosclerotic plaque area in leuprolide-treated and orchiectomized mice was larger than in control mice (P<0.005 and P = 0.002, respectively), but it was not significantly different from control in degarelix-treated mice. The necrotic core area in degarelix-treated mice was smaller compared with leuprolide-treated and orchiectomized mice (P = 0.011 and P = 0.002, respectively). CONCLUSIONS: Our results suggest that ADT induced MS and atherosclerosis in a preclinical mouse model to a mode-specific extent. GnRH antagonist generated the least atherosclerosis and characteristics of MS compared with orchiectomy and GnRH agonist.

Detection and quantification of endoplasmic reticulum stress in living cells using the fluorescent compound, Thioflavin T.

The endoplasmic reticulum (ER) plays a central role in the co- and post-translational modification of many proteins. Disruption of these processes can lead to the accumulation of misfolded proteins in the endoplasmic reticulum - a condition known as endoplasmic reticulum stress. In recent years, the association of endoplasmic reticulum stress with a number of disease pathologies has increased interest in the study of this condition. Current methods to detect endoplasmic reticulum stress are indirect and retrospective. Here we describe a new method to detect and quantify endoplasmic reticulum stress in live cells using Thioflavin T (ThT), a small molecule that exhibits enhanced fluorescence when it binds to protein aggregates. We show that enhanced ThT-fluorescence correlates directly with established indicators of unfolded protein response activation. Furthermore, enhanced ThT-fluorescence can be detected in living cells within 20 min of application of an endoplasmic reticulum stress-inducing agent. ThT is capable of detecting endoplasmic reticulum stress induced by distinctly different conditions and compounds, in different cultured cell types as well as in mouse tissue samples. Pre-treatment with a potent endoplasmic reticulum stress-reducing agent, 4-phenylbutyric acid, mitigates the enhanced ThT signal. This new tool will be useful in future research investigating the role of protein misfolding in the development and/or progression of human diseases.

Leptin promotes osteoblast differentiation and mineralization of primary cultures of vascular smooth muscle cells by inhibiting glycogen synthase kinase (GSK)-3ß.

In this study, we begin to investigate the underlying mechanism of leptin-induced vascular calcification. We found that treatment of cultured bovine aortic smooth muscle cells (BASMCs) with leptin (0.5-4 µg/ml) induced osteoblast differentiation in a dose-dependent manner. Furthermore, we found that leptin significantly increased the mRNA expression of osteopontin and bone sialoprotein, while down-regulating matrix gla protein (MGP) expression in BASMCs. Key factors implicated in osteoblast differentiation, including members of the Wnt signaling pathway, were examined. Exposure to leptin enhanced phosphorylation of GSK-3ß on serine-9 thereby inhibiting activity and promoting the nuclear accumulation of ß-catenin. Transfection of BASMCs with an adenovirus that expressed constitutively active GSK-3ß (Ad-GSK-3ß S9A) resulted in a >2-fold increase in GSK-3ß activity and a significant decrease in leptin-induced alkaline phosphatase (ALP) activity. In addition, qRT-PCR analysis showed that GSK-3ß activation resulted in a significant decrease in the expression of osteopontin and bone sialoprotein, but a marked increase in MGP mRNA expression. When taken together, our results suggest a mechanism by which leptin promotes osteoblast differentiation and vascular calcification in vivo.

Metabolic syndrome and acute hyperglycemia are associated with endoplasmic reticulum stress in human mononuclear cells.

Endoplasmic reticulum (ER) stress and the activation of the unfolded protein response (UPR) have been implicated in a number of complications associated with diabetes mellitus including micro- and macrovascular dysfunction. In this study we examine ER stress levels in blood cells isolated from human subjects with metabolic syndrome and in healthy controls. Total RNA and protein were isolated from leukocytes and the levels of specific ER stress markers were quantified by real-time-PCR and immunoblot analysis. Our results indicate that, compared to healthy controls, individuals with metabolic syndrome have elevated mRNA levels of genes indicative of ER stress; including spliced XBP-1 (sXBP-1), Grp78, and CHOP. Induced ER stress levels correlate with blood glucose but not plasma lipid concentration. Furthermore, in healthy individuals, a standard 75 g oral glucose challenge produced a significant elevation in spliced XBP-1 (1.3 fold), Grp78 (2.0 fold), and calreticulin (3.5 fold) mRNA 60 min post challenge and a significant increase in Grp78 (2.0 fold), calreticulin (2.7 fold) protein levels 2 h postchallenge, relative to fasting levels. The UPR was also activated ex vivo, in human leukocytes cultured in the presence of 15 mmol/l glucose, supporting a specific role for glucose. The oral glucose challenge was associated with a significant increase in the expression of inflammatory cytokines, including interleukin (IL)-1α/ß, IL-6, and IL-8, that may result from ER stress. These findings suggest that there is an association between both acute and chronic dysglycemia and ER stress in humans.

Endoplasmic reticulum stress and glycogen synthase kinase-3ß activation in apolipoprotein E-deficient mouse models of accelerated atherosclerosis.

OBJECTIVE: The goal of this study was to examine the role of endoplasmic reticulum (ER) stress signaling and the contribution of glycogen synthase kinase (GSK)-3ß activation in hyperglycemic, hyperhomocysteinemic, and high-fat-fed apolipoprotein E-deficient (apoE(-/-)) mouse models of accelerated atherosclerosis. METHODS AND RESULTS: Female apoE(-/-) mice received multiple low-dose injections of streptozotocin (40 µg/kg) to induce hyperglycemia, methionine-supplemented drinking water (0.5% wt/vol) to induce hyperhomocysteinemia, or a high-fat (21% milk fat+0.2% cholesterol) diet to induce relative dyslipidemia. A subset of mice from each group was supplemented with sodium valproate (625 mg/kg), a compound with GSK3 inhibitory activity. At 15 and 24 weeks of age, markers of ER stress, lipid accumulation, GSK3ß phosphorylation, and GSK3ß activity were analyzed in liver and aorta. Atherosclerotic lesions were examined and quantified. Hyperglycemia, hyperhomocysteinemia, and high-fat diet significantly enhanced GSK3ß activity and also increased hepatic steatosis and atherosclerotic lesion volume compared with controls. Valproate supplementation blocked GSK3ß activation and attenuated the development of atherosclerosis and the accumulation of hepatic lipids in each of the models examined. The mechanism by which GSK3ß activity is regulated in these models likely involves alterations in phosphorylation at serine 9 and tyrosine 216. CONCLUSIONS: These findings support the existence of a common mechanism of accelerated atherosclerosis involving ER stress signaling through activation of GSK3ß. Furthermore, our results suggest that atherosclerosis can be attenuated by modulating GSK3ß phosphorylation.

Diabetes, hyperglycemia and accelerated atherosclerosis: evidence supporting a role for endoplasmic reticulum (ER) stress signaling.

Diabetes mellitus is associated with both micro- and macrovascular complications that can lead to significantly elevated incidence of retinopathy, nephropathy, neuropathy, myocardial infarction and stroke. The diabetic cardiovascular mortality rate exceeds 70% and individuals with diabetes are 2-3 times more likely to die from myocardial infarction and stroke than those with no history of diabetes even after controlling for other cardiovascular risk factors. Despite the profound clinical importance of vascular disease in patients with diabetes mellitus, our understanding of the molecular and cellular mechanisms by which diabetes promotes these vascular complications is incomplete. Endoplasmic reticulum (ER) stress and the unfolded protein response pathways have been previously associated with the development of several different diseases, including neurodegenerative disorders, cancer, and obesity. In addition, ER stress has been directly implicated in complications that are associated with diabetes, including pancreatic b cell dysfunction and insulin resistance. In this review we examine the potential role of endoplasmic reticulum stress in the initiation and progression of hyperglycemia-associated atherosclerosis.

Hexosamine biosynthesis pathway flux promotes endoplasmic reticulum stress, lipid accumulation, and inflammatory gene expression in hepatic cells.

There is increasing evidence that endoplasmic reticulum (ER) stress contributes to the development of atherosclerosis in diabetes mellitus. The purpose of this study was to determine the effects of increased hexosamine biosynthesis pathway (HBP) flux on ER stress levels and the complications of ER stress associated with diabetes and atherosclerosis in hepatic cells. Glutamine:fructose-6-phosphate amidotransferase (GFAT), the rate-limiting enzyme of the HBP, was overexpressed in HepG2 cells by use of an adenoviral expression system. The ER stress response and downstream effects, including activation of lipid and inflammatory pathways, were determined using real-time PCR, immunoblot analysis, and cell staining techniques. GFAT overexpression resulted in increased expression of ER stress markers, including Grp78, Grp94, calreticulin, and GADD153, relative to cells infected with an empty adenoviral vector. In addition, GFAT overexpression promoted lipid, but not cholesterol, accumulation in hepatic cells as well as inflammatory pathway activation. Treatment with 6-diazo-5-oxo-norleucine, a GFAT antagonist, blocked the effects of GFAT overexpression. Consistent with our in vitro data, hyperglycemic mice presented with elevated markers of hepatic ER stress, glucosamine and lipid accumulation. Together, these data suggest that HBP flux-induced ER stress plays a role in the development of hepatic steatosis and atherosclerosis under conditions of hyperglycemia.

Hyperhomocysteinemia induced by methionine supplementation does not independently cause atherosclerosis in C57BL/6J mice.

A causal relationship between diet-induced hyperhomocysteinemia (HHcy) and accelerated atherosclerosis has been established in apolipoprotein E-deficient (apoE(-/-)) mice. However, it is not known whether the proatherogenic effect of HHcy in apoE(-/-) mice is independent of hyperlipidemia and/or deficiency of apoE. In this study, a comprehensive dietary approach using C57BL/6J mice was used to investigate whether HHcy is an independent risk factor for accelerated atherosclerosis or dependent on additional dietary factors that increase plasma lipids and/or inflammation. C57BL/6J mice at 4 wk of age were divided into 6 dietary groups: chow diet (C), chow diet + methionine (C+M), western-type diet (W), western-type diet + methionine (W+M), atherogenic diet (A), or atherogenic diet + methionine (A+M). After 2, 10, 20, or 40 wk on the diets, mice were sacrificed, and the levels of total plasma homocysteine, cysteine, and glutathione, as well as total plasma cholesterol and triglycerides were analyzed. Aortic root sections were examined for atherosclerotic lesions. HHcy was induced in all groups supplemented with methionine, compared to diet-matched control groups. Plasma total cholesterol was significantly increased in mice fed the W or A diet. However, the W diet increased LDL/IDL and HDL levels, while the A diet significantly elevated plasma VLDL and LDL/IDL levels without increasing HDL. No differences in plasma total cholesterol levels or lipid profiles were observed between methionine-supplemented groups and the diet-matched control groups. Early atherosclerotic lesions containing macrophage foam cells were only observed in mice fed the A or A + M diet. Furthermore, lesion size was significantly larger in the A + M group compared to the A group at 10 and 20 wk; however, mature lesions were never observed even after 40 wk on these diets. The presence of lymphocytes, increased hyaluronan staining, and the expression of endoplasmic reticulum (ER) stress markers were also increased in atherosclerotic lesions from the A + M group. Taken together, these results suggest that HHcy does not independently cause atherosclerosis in C57BL/6J mice even in the presence of increased total plasma lipids induced by the W diet. However, HHcy can accelerate atherosclerotic lesion development under dietary conditions that increase plasma VLDL levels and/or inflammation.

Endoplasmic reticulum stress causes the activation of sterol regulatory element binding protein-2.

BACKGROUND: Sterol regulatory element binding protein-2 (SREBP-2) is a membrane-bound transcription factor that upon proteolytic processing can activate the expression of genes involved in cholesterol biosynthesis and uptake. We as well as others have demonstrated that the accumulation of misfolded proteins within the endoplasmic reticulum (ER), a condition known as ER stress, can dysregulate lipid metabolism by activating the SREBPs. The purpose of this study was to determine the mechanism by which ER stress induces SREBP-2 activation. METHODS AND RESULTS: HeLa and MCF7 cells were treated with ER stress-inducing agents to determine the effect of ER stress on SREBP-2 cleavage and subsequent cholesterol accumulation. Cells treated with thapsigargin (Tg) exhibit proteolytic cleavage of SREBP-2. Proteolytic cleavage of SREBP-2 induced by Tg occurred independently of caspase activation and was inhibited by the site-1 protease inhibitor AEBSF, suggesting that Tg-induced SREBP-2 cleavage occurs through the conventional site-1/-2 pathway. Treatment of HeLa cells with Tg also led to the accumulation of free cholesterol as measured by Filipin staining. CONCLUSIONS: These results imply that ER stress-induced SREBP-2 activation occurs through the conventional pathway that normally regulates SREBP in accordance with intracellular sterol concentration.