Activation of IGF-1 receptors and Akt signaling by systemic hyperinsulinemia contributes to cardiac hypertrophy but does not regulate cardiac autophagy in obese diabetic mice

Duodenogastric Intussusception in a 14-Week-Old Infant with Donohue Syndrome: Case Study

Donohue syndrome (DS) is a rare recessively inherited disorder characterized by severe insulin resistance caused by genetic defects affecting the insulin receptor. The classical clinical characteristics include severe intrauterine growth restriction, craniofacial dysmorphic features, body and skin features, and soft tissue overgrowth. Postnatal growth retardation, cardiac, gastrointestinal, and renal complications, and infection susceptibility develop within the first few months of life, leading to a short life expectancy (<2 years). The classical metabolic abnormalities vary from fasting hypoglycemia to postprandial hyperglycemia with severe hyperinsulinemia. We present the case of a 14-week-old infant with DS who developed cardiac, renal, hepatic, pancreatic, and gastrointestinal features, all of them previously reported in infants with DS. The gastrointestinal features started during the first week of life and included abdominal distension, feeding difficulties, intermittent vomiting, and two episodes of intestinal obstruction. The diagnosis of duodenogastric intussusception was made, and this previously unreported complication tragically resulted in mortality. We discuss how basic mechanisms of cross-talk between insulin and insulin-growth factor 1 receptors could be linked to hyperinsulinemia and its associated comorbidities.

Relevance of mitochondrial dysfunction in heart disease associated with insulin resistance conditions

Insulin resistance plays a key role in the development and progression of obesity, diabetes, and their complications. Moreover, insulin resistance is considered the principal link between metabolic diseases and cardiovascular diseases. Heart disease associated with insulin resistance is one of the most important consequences of both obesity and diabetes, and it is characterized by impaired cardiac energetics, diastolic dysfunction, and finally heart failure. Mitochondrion plays a key role in cell energy homeostasis and is the main source of reactive oxygen species. Obesity and diabetes are associated with alterations in mitochondrial function and dynamics. Mitochondrial dysfunction is characterized by changes in mitochondrial respiratory chain with reduced ATP production and elevated reactive oxygen species production. These mitochondrial alterations together with inflammation contribute to the development and progression of heart disease under insulin resistance conditions. Finally, numerous miRNAs participate in the regulation of energy substrate metabolism, reactive oxygen species production, and apoptotic pathways within the mitochondria. This notion supports the relevance of interactions between miRNAs and mitochondrial dysfunction in the pathophysiology of metabolic heart disease.

Late-in-life treadmill training rejuvenates autophagy, protein aggregate clearance, and function in mouse hearts

Protein quality control mechanisms decline during the process of cardiac aging. This enables the accumulation of protein aggregates and damaged organelles that contribute to age-associated cardiac dysfunction. Macroautophagy is the process by which post-mitotic cells such as cardiomyocytes clear defective proteins and organelles. We hypothesized that late-in-life exercise training improves autophagy, protein aggregate clearance, and function that is otherwise dysregulated in hearts from old vs. adult mice. As expected, 24-month-old male C57BL/6J mice (old) exhibited repressed autophagosome formation and protein aggregate accumulation in the heart, systolic and diastolic dysfunction, and reduced exercise capacity vs. 8-month-old (adult) mice (all p < 0.05). To investigate the influence of late-in-life exercise training, additional cohorts of 21-month-old mice did (old-ETR) or did not (old-SED) complete a 3-month progressive resistance treadmill running program. Body composition, exercise capacity, and soleus muscle citrate synthase activity improved in old-ETR vs. old-SED mice at 24 months (all p < 0.05). Importantly, protein expression of autophagy markers indicate trafficking of the autophagosome to the lysosome increased, protein aggregate clearance improved, and overall function was enhanced (all p < 0.05) in hearts from old-ETR vs. old-SED mice. These data provide the first evidence that a physiological intervention initiated late-in-life improves autophagic flux, protein aggregate clearance, and contractile performance in mouse hearts.

Insulin signaling in the heart

Insulin receptors are highly expressed in the heart and vasculature. Insulin signaling regulates cardiac growth, survival, substrate uptake, utilization, and mitochondrial metabolism. Insulin signaling modulates the cardiac responses to physiological and pathological stressors. Altered insulin signaling in the heart may contribute to the pathophysiology of ventricular remodeling and heart failure progression. Myocardial insulin signaling adapts rapidly to changes in the systemic metabolic milieu. What may initially represent an adaptation to protect the heart from carbotoxicity may contribute to amplifying the risk of heart failure in obesity and diabetes. This review article presents the multiple roles of insulin signaling in cardiac physiology and pathology and discusses the potential therapeutic consequences of modulating myocardial insulin signaling.

EphA4 is highly expressed in the atria of heart and its deletion leads to atrial hypertrophy and electrocardiographic abnormalities in rats

AIMS

EphA4 is a member of the Eph receptor family, and expressed mainly in central nervous system (CNS), which is involved in CNS development and multiple diseases. Due to the variability in EphA4 expression, we wondered if EphA4 is expressed in other tissues, and what role does EphA4 play?

MATERIALS AND METHODS

We generated an EphA4 knockout (KO) rat line with red fluorescent marker protein encoded by the mCherry cassette inserted downstream of the EphA4 promoter as a reporter. Using this system, we observed high expression of EphA4 in the heart atria and in the brain.

KEY FINDINGS

EphaA4 KO rats (EphA4^-/-) developed obvious atrial hypertrophy with an increased atria-to-heart weight ratio and atrial cardiomyocyte cross-sectional area at six months of age. EphA4^-/- rats had reduced atrial end diastolic volume (EDV), atrial ejection fraction (EF) and left ventricular EF. They also exhibited increased amplitude of QRS complexes and QT intervals, with invisible p waves. RNA sequencing revealed that EphA4 KO altered the transcription of multiple genes involved in regulation of transcription and translation, ion binding, metabolism and cell adhesion. Deletion of EphA4 reduced IGF1 mRNA and protein expression, which is involved in cardiac remodeling.

SIGNIFICANCE

Our data demonstrated that EphA4 was highly expressed in the atria and its deletion caused atrial dysfunction. Our findings also suggested that the EphA4 KO rat could be a potential model for studies on atrial remodeling.

Obesity cardiomyopathy: evidence, mechanisms, and therapeutic implications

The prevalence of heart failure is on the rise and imposes a major health threat, in part, due to the rapidly increased prevalence of overweight and obesity. To this point, epidemiological, clinical, and experimental evidence supports the existence of a unique disease entity termed “obesity cardiomyopathy,” which develops independent of hypertension, coronary heart disease, and other heart diseases. Our contemporary review evaluates the evidence for this pathological condition, examines putative responsible mechanisms, and discusses therapeutic options for this disorder. Clinical findings have consolidated the presence of left ventricular dysfunction in obesity. Experimental investigations have uncovered pathophysiological changes in myocardial structure and function in genetically predisposed and diet-induced obesity. Indeed, contemporary evidence consolidates a wide array of cellular and molecular mechanisms underlying the etiology of obesity cardiomyopathy including adipose tissue dysfunction, systemic inflammation, metabolic disturbances (insulin resistance, abnormal glucose transport, spillover of free fatty acids, lipotoxicity, and amino acid derangement), altered intracellular especially mitochondrial Ca²⁺ homeostasis, oxidative stress, autophagy/mitophagy defect, myocardial fibrosis, dampened coronary flow reserve, coronary microvascular disease (microangiopathy), and endothelial impairment. Given the important role of obesity in the increased risk of heart failure, especially that with preserved systolic function and the recent rises in COVID-19-associated cardiovascular mortality, this review should provide compelling evidence for the presence of obesity cardiomyopathy, independent of various comorbid conditions, underlying mechanisms, and offer new insights into potential therapeutic approaches (pharmacological and lifestyle modification) for the clinical management of obesity cardiomyopathy.

Concurrent diabetes and heart failure: interplay and novel therapeutic approaches

Diabetes mellitus increases the risk of developing heart failure, and the co-existence of both diseases worsens cardiovascular outcomes, hospitalization and the progression of heart failure. Despite current advancements on therapeutic strategies to manage hyperglycemia, the likelihood of developing diabetes-induced heart failure is still significant, especially with the accelerating global prevalence of diabetes and an ageing population. This raises the likelihood of other contributing mechanisms beyond hyperglycemia in predisposing diabetic patients to cardiovascular disease risk. There has been considerable interest in understanding the alterations in cardiac structure and function in the diabetic patients, collectively termed as "diabetic cardiomyopathy". However, the factors that contribute to the development of diabetic cardiomyopathies is not fully understood. This review summarizes the main characteristics of diabetic cardiomyopathies, and the basic mechanisms that contribute to its occurrence. This includes perturbations in insulin resistance, fuel preference, reactive oxygen species generation, inflammation, cell death pathways, neurohormonal mechanisms, advanced glycated end-products accumulation, lipotoxicity, glucotoxicity, and posttranslational modifications in the heart of the diabetic. This review also discusses the impact of antihyperglycemic therapies on the development of heart failure, as well as how current heart failure therapies influence glycemic control in diabetic patients. We also highlight the current knowledge gaps in understanding how diabetes induces heart failure.

The complex network of mTOR signaling in the heart

The mechanistic target of rapamycin (mTOR) integrates several intracellular and extracellular signals involved in the regulation of anabolic and catabolic processes. mTOR assembles into two macromolecular complexes, named mTORC1 and mTORC2, which have different regulators, substrates and functions. Studies of gain- and loss-of-function animal models of mTOR signaling revealed that mTORC1/2 elicit both adaptive and maladaptive functions in the cardiovascular system. Both mTORC1 and mTORC2 are indispensable for driving cardiac development and cardiac adaption to stress, such as pressure overload. However, persistent and deregulated mTORC1 activation in the heart is detrimental during stress and contributes to the development and progression of cardiac remodeling and genetic and metabolic cardiomyopathies. In this review, we discuss the latest findings regarding the role of mTOR in the cardiovascular system, both under basal conditions and during stress, such as pressure overload, ischemia and metabolic stress. Current data suggest that mTOR modulation may represent a potential therapeutic strategy for the treatment of cardiac diseases.

Echocardiographic parameters according to insulin dose in young patients affected by type 1 diabetes

Insulin dose has been found to associate to several cardiometabolic risk factors in type 1 diabetes. Changes over time in body weight and composition may partly explain this association. However, no data are available on the relationship between insulin dose and echocardiographic parameters of both systolic and diastolic function in type 1 diabetes. Therefore, the aim of the present study was to examine systolic and diastolic echocardiographic parameters in relation to insulin dose in young patients with type 1 diabetes. The study was carried out on 93 consecutive outpatients with type 1 diabetes with a mean age of 32.8 ± 9.8 years. All patients were examined with a transthoracic echocardiography. Clinical and laboratory data were collected. The median value of daily insulin dose was used to categorized patients in two groups: high and low insulin dose group. Patients belonging to the high insulin dose group showed higher levels of cardiometabolic risk factors such as BMI, triglycerides and TG/HDL cholesterol ratio. Indexes of both systolic and diastolic function were similar in both groups except isovolumetric relaxation time (IVRT), that was significantly prolonged in patients of the high insulin group (94.4 ± 15.0 vs 86.7 ± 13.1 ms, p = 0.008). In the multivariate regression analysis, insulin dose was positively and significantly associated with IVRT. In this study we report an association between insulin dose and impaired active diastolic myocardial relaxation. Future studies are needed to further explore this observation.

The endocrinological component and signaling pathways associated to cardiac hypertrophy

Although myocardial growth corresponds to an adaptive response to maintain cardiac contractile function, the cardiac hypertrophy is a condition that occurs in many cardiovascular diseases and typically precedes the onset of heart failure. Different endocrine factors such as thyroid hormones, insulin, insulin-like growth factor 1 (IGF-1), angiotensin II (Ang II), endothelin (ET-1), catecholamines, estrogen, among others represent important stimuli to cardiomyocyte hypertrophy. Thus, numerous endocrine disorders manifested as changes in the local environment or multiple organ systems are especially important in the context of progression from cardiac hypertrophy to heart failure. Based on that information, this review summarizes experimental findings regarding the influence of such hormones upon signalling pathways associated with cardiac hypertrophy. Understanding mechanisms through which hormones differentially regulate cardiac hypertrophy could open ways to obtain therapeutic approaches that contribute to prevent or delay the onset of heart failure related to endocrine diseases.

Phosphorylation Modifications Regulating Cardiac Protein Quality Control Mechanisms

Many forms of cardiac disease, including heart failure, present with inadequate protein quality control (PQC). Pathological conditions often involve impaired removal of terminally misfolded proteins. This results in the formation of large protein aggregates, which further reduce cellular viability and cardiac function. Cardiomyocytes have an intricately collaborative PQC system to minimize cellular proteotoxicity. Increased expression of chaperones or enhanced clearance of misfolded proteins either by the proteasome or lysosome has been demonstrated to attenuate disease pathogenesis, whereas reduced PQC exacerbates pathogenesis. Recent studies have revealed that phosphorylation of key proteins has a potent regulatory role, both promoting and hindering the PQC machinery. This review highlights the recent advances in phosphorylations regulating PQC, the impact in cardiac pathology, and the therapeutic opportunities presented by harnessing these modifications.

Cellular Protein Quality Control in Diabetic Cardiomyopathy: From Bench to Bedside

Heart failure is a serious comorbidity and the most common cause of mortality in diabetes patients. Diabetic cardiomyopathy (DCM) features impaired cellular structure and function, culminating in heart failure; however, there is a dearth of specific clinical therapy for treating DCM. Protein homeostasis is pivotal for the maintenance of cellular viability under physiological and pathological conditions, particularly in the irreplaceable cardiomyocytes; therefore, it is tightly regulated by a protein quality control (PQC) system. Three evolutionarily conserved molecular processes, the unfolded protein response (UPR), the ubiquitin-proteasome system (UPS), and autophagy, enhance protein turnover and preserve protein homeostasis by suppressing protein translation, degrading misfolded or unfolded proteins in cytosol or organelles, disposing of damaged and toxic proteins, recycling essential amino acids, and eliminating insoluble protein aggregates. In response to increased cellular protein demand under pathological insults, including the diabetic condition, a coordinated PQC system retains cardiac protein homeostasis and heart performance, on the contrary, inappropriate PQC function exaggerates cardiac proteotoxicity with subsequent heart dysfunction. Further investigation of the PQC mechanisms in diabetes propels a more comprehensive understanding of the molecular pathogenesis of DCM and opens new prospective treatment strategies for heart disease and heart failure in diabetes patients. In this review, the function and regulation of cardiac PQC machinery in diabetes mellitus, and the therapeutic potential for the diabetic heart are discussed.

Dihydrotanshinone I Ameliorates Cardiac Hypertrophy in Diabetic Mice Induced by Chronic High-Fat Feeding

Salvia miltiorrhiza Bge. (Danshen) is widely used to improve blood circulation and the dredge meridian in traditional Chinese medicine. In the present study, we evaluated the effects of dihydrotanshinone I (DHTS), a natural product from Danshen, on chronic high-fat feeding-induced cardiac remodeling and dysfunction. DHTS (25 mg/kg, intraperitoneal) did not affect blood glucose, insulin levels, and glucose intolerance. However, it alleviated diastolic dysfunction induced by the high-fat diet, as indicated by the increase in the ratio of peak early filling velocity to peak late filling velocity of the mitral and suppression of the extension of the isovolumic relaxation phase of the left ventricle. Further investigations revealed that DHTS ameliorated high-fat induced cardiac hypertrophy in mice and suppressed insulin-induced enlargement of cardiomyocytes in vitro. In neonatal cardiomyocytes, DHTS restored insulin-induced suppression of CCAAT/enhancer-binding protein beta-2 isoform (CEBPβ) and the phosphorylation of glycogen synthase kinase-3β (GSK3β) and extracellular signal-regulated kinase (ERK). Taken together, our results indicated that DHTS ameliorated cardiac hypertrophy and diastolic dysfunction in high-fat-fed mice, probably through the inhibition of insulin-induced suppression of CEBPβ and phosphorylation of GSK3β and ERK in cardiomyocytes.

Basic Mechanisms of Diabetic Heart Disease

Diabetes mellitus predisposes affected individuals to a significant spectrum of cardiovascular complications, one of the most debilitating in terms of prognosis is heart failure. Indeed, the increasing global prevalence of diabetes mellitus and an aging population has given rise to an epidemic of diabetes mellitus-induced heart failure. Despite the significant research attention this phenomenon, termed diabetic cardiomyopathy, has received over several decades, understanding of the full spectrum of potential contributing mechanisms, and their relative contribution to this heart failure phenotype in the specific context of diabetes mellitus, has not yet been fully resolved. Key recent preclinical discoveries that comprise the current state-of-the-art understanding of the basic mechanisms of the complex phenotype, that is, the diabetic heart, form the basis of this review. Abnormalities in each of cardiac metabolism, physiological and pathophysiological signaling, and the mitochondrial compartment, in addition to oxidative stress, inflammation, myocardial cell death pathways, and neurohumoral mechanisms, are addressed. Further, the interactions between each of these contributing mechanisms and how they align to the functional, morphological, and structural impairments that characterize the diabetic heart are considered in light of the clinical context: from the disease burden, its current management in the clinic, and where the knowledge gaps remain. The need for continued interrogation of these mechanisms (both known and those yet to be identified) is essential to not only decipher the how and why of diabetes mellitus-induced heart failure but also to facilitate improved inroads into the clinical management of this pervasive clinical challenge.

Myocardium Metabolism in Physiological and Pathophysiological States: Implications of Epicardial Adipose Tissue and Potential Therapeutic Targets

The main energy substrate of adult cardiomyocytes for their contractility are the fatty acids. Its metabolism generates high ATP levels at the expense of high oxygen consumption in the mitochondria. Under low oxygen supply, they can get energy from other substrates, mainly glucose, lactate, ketone bodies, etc., but the mitochondrial dysfunction, in pathological conditions, reduces the oxidative metabolism. In consequence, fatty acids are stored into epicardial fat and its accumulation provokes inflammation, insulin resistance, and oxidative stress, which enhance the myocardium dysfunction. Some therapies focused on improvement the fatty acids entry into mitochondria have failed to demonstrate benefits on cardiovascular disorders. Oppositely, those therapies with effects on epicardial fat volume and inflammation might improve the oxidative metabolism of myocardium and might reduce the cardiovascular disease progression. This review aims at explain (a) the energy substrate adaptation of myocardium in physiological conditions, (b) the reduction of oxidative metabolism in pathological conditions and consequences on epicardial fat accumulation and insulin resistance, and (c) the reduction of cardiovascular outcomes after regulation by some therapies.

Suppression of Cardiac Autophagy by Hyperinsulinemia in Insulin Receptor-Deficient Hearts Is Mediated by Insulin-Like Growth Factor Receptor Signaling

Aims: Autophagy is a catabolic process required for the maintenance of cardiac health. Insulin and insulin-like growth factor 1 (IGF-1) are potent inhibitors of autophagy and as such, one would predict that autophagy will be increased in the insulin-resistant/diabetic heart. However, autophagy is rather decreased in the hearts of diabetic/insulin-resistant mice. The aim of this study is to determine the contribution of IGF-1 receptor signaling to autophagy suppression in insulin receptor (IR)-deficient hearts. Results: Absence of IRs in the heart was associated with reduced autophagic flux, and further inhibition of autophagosome clearance reduced survival, impaired contractile function, and enhanced myocyte loss. Contrary to the in vivo setting, isolated cardiomyocytes from IR-deficient hearts exhibited unrestrained autophagy in the absence of insulin, whereas addition of insulin was able to suppress autophagy. To investigate the mechanisms involved in the maintenance of the responsiveness to insulin in IR-deficient hearts, we generated mice lacking both IRs and one copy of the IGF-1 receptor (IGF-1R) in cardiac cells and showed that these mice had increased autophagy. Innovation and Conclusion: This study unveils a new mechanism by which IR-deficient hearts can still respond to insulin to suppress autophagy, in part, through activation of IGF-1R signaling. This is a highly significant observation because it is the first to show that systemic hyperinsulinemia can suppress autophagy in IR-deficient hearts through IGF-1R signaling.

Autophagy in Adipose Tissue Physiology and Pathophysiology

Significance: Alterations in adipose tissue function have profound consequences on whole body energy homeostasis because this tissue is central for fat accumulation, energy expenditure, glucose and insulin metabolism, and hormonal regulation. With the obesity reaching epidemic proportions globally, it is important to understand the mechanisms leading to adipose tissue malfunction. Recent Advances: Autophagy has originally been viewed as an adaptive response to cellular stress, but in recent years this process was shown to regulate important cellular processes. In adipose tissue, autophagy is a key regulator of white adipose tissue (WAT) and brown adipose tissue (BAT) adipogenesis, and dysregulated autophagy impairs fat accumulation both in vitro and in vivo. Animal studies have also suggested an important role for autophagy and mitophagy during the transition from beige to white fat. Human studies have provided evidence for altered autophagy in WAT, and these alterations correlated with the degree of insulin resistance. Critical Issues: Despite these important advances in the study of autophagy in adipose tissue, we still do not understand the physiological role of autophagy in mature white and brown adipocytes. Furthermore, several human studies involving autophagy assessment were performed on whole adipose tissue, which complicates the interpretation of the results considering the cellular heterogeneity of this tissue. Future Directions: Future studies will undoubtedly expand our understanding of the role of autophagy in fully differentiated adipocytes, and uncover novel cross-talks between this tissue and other organs in regulating lipid metabolism, redox signaling, energy homeostasis, and insulin sensitivity.

New Insights Into the Role of mTOR Signaling in the Cardiovascular System

The mTOR (mechanistic target of rapamycin) is a master regulator of several crucial cellular processes, including protein synthesis, cellular growth, proliferation, autophagy, lysosomal function, and cell metabolism. mTOR interacts with specific adaptor proteins to form 2 multiprotein complexes, called mTORC1 (mTOR complex 1) and mTORC2 (mTOR complex 2). In the cardiovascular system, the mTOR pathway regulates both physiological and pathological processes in the heart. It is needed for embryonic cardiovascular development and for maintaining cardiac homeostasis in postnatal life. Studies involving mTOR loss-of-function models revealed that mTORC1 activation is indispensable for the development of adaptive cardiac hypertrophy in response to mechanical overload. mTORC2 is also required for normal cardiac physiology and ensures cardiomyocyte survival in response to pressure overload. However, partial genetic or pharmacological inhibition of mTORC1 reduces cardiac remodeling and heart failure in response to pressure overload and chronic myocardial infarction. In addition, mTORC1 blockade reduces cardiac derangements induced by genetic and metabolic disorders and has been reported to extend life span in mice. These studies suggest that pharmacological targeting of mTOR may represent a therapeutic strategy to confer cardioprotection, although clinical evidence in support of this notion is still scarce. This review summarizes and discusses the new evidence on the pathophysiological role of mTOR signaling in the cardiovascular system.

ERK: A Key Player in the Pathophysiology of Cardiac Hypertrophy

Cardiac hypertrophy is an adaptive and compensatory mechanism preserving cardiac output during detrimental stimuli. Nevertheless, long-term stimuli incite chronic hypertrophy and may lead to heart failure. In this review, we analyze the recent literature regarding the role of ERK (extracellular signal-regulated kinase) activity in cardiac hypertrophy. ERK signaling produces beneficial effects during the early phase of chronic pressure overload in response to G protein-coupled receptors (GPCRs) and integrin stimulation. These functions comprise (i) adaptive concentric hypertrophy and (ii) cell death prevention. On the other hand, ERK participates in maladaptive hypertrophy during hypertension and chemotherapy-mediated cardiac side effects. Specific ERK-associated scaffold proteins are implicated in either cardioprotective or detrimental hypertrophic functions. Interestingly, ERK phosphorylated at threonine 188 and activated ERK5 (the big MAPK 1) are associated with pathological forms of hypertrophy. Finally, we examine the connection between ERK activation and hypertrophy in (i) transgenic mice overexpressing constitutively activated RTKs (receptor tyrosine kinases), (ii) animal models with mutated sarcomeric proteins characteristic of inherited hypertrophic cardiomyopathies (HCMs), and (iii) mice reproducing syndromic genetic RASopathies. Overall, the scientific literature suggests that during cardiac hypertrophy, ERK could be a “good” player to be stimulated or a “bad” actor to be mitigated, depending on the pathophysiological context.

Postprandial Insulin Response and Clearance Among Black and White Women: The Federal Women's Study

CONTEXT

Postprandial hyperinsulinemia might be an important cardiometabolic risk determinant in black compared with white women. However, the contributions of insulin clearance and β-cell function to racial differences in postprandial insulin response are unknown.

OBJECTIVE

To compare, by race and menopause, early insulin response to oral and intravenous glucose and to measure postprandial intact glucagon-like peptide 1 (GLP-1) concentrations, insulin clearance, and β-cell function.

DESIGN AND PARTICIPANTS

119 federally employed women without diabetes [87 premenopausal (52 black, 35 white) and 32 postmenopausal (19 black, 13 white)] underwent an oral glucose tolerance test, insulin-modified frequently sampled intravenous glucose test (IM-FSIGT), and mixed meal tolerance test (MMTT).

OUTCOME MEASURES

Early insulin response was measured as follows: (i) insulinogenic index (oral glucose tolerance test); (ii) acute insulin response to glucose (IM-FSIGT); and (iii) ratio of incremental insulin/glucose area under the curve in the first 30 minutes of the MMTT. Insulin clearance was assessed during the IM-FSIGT and MMTT. During the MMTT, intact GLP-1 was measured and β-cell function assessed using the insulin secretion rate and β-cell responsivity indexes.

RESULTS

Black pre-menopausal and postmenopausal women had a greater insulin response and lower insulin clearance and greater dynamic β-cell responsivity (P ≤ 0.05 for all). No differences were found in the total insulin secretion rates or intact GLP-1 concentrations.

CONCLUSIONS

Greater postprandial hyperinsulinemia in black pre-menopausal and postmenopausal women was associated with lower hepatic insulin clearance and heightened β-cell capacity to rapid changes in glucose, but not to higher insulin secretion. The relationship of increased β-cell secretory capacity, reduced insulin clearance, and ambient hyperinsulinemia to the development of cardiometabolic disease requires further investigation.

Cardiovascular sexual dimorphism in a diet-induced type 2 diabetes rodent model, the Nile rat (Arvicanthis niloticus)

Background

The Nile rat (Arvicanthis niloticus) is an emerging laboratory model of type 2 diabetes. When fed standard rodent chow, the majority of males progress from hyperinsulinemia by 2 months to hyperglycemia by 6 months, while most females remain at the hyperinsulinemia-only stage (prediabetic) from 2 months onward. Since diabetic cardiomyopathy is the major cause of type-2 diabetes mellitus (T2DM)-related mortality, we examined whether sexual dimorphism might entail cardiac functional changes. Our ultimate goal was to isolate the effect of diet as a modifiable lifestyle factor.

Materials and methods

Nile rats were fed either standard rodent chow (Chow group) or a high-fiber diet previously established to prevent type 2 diabetes (Fiber group). Cardiac function was determined with echocardiography at 12 months of age. To isolate the effect of diet alone, only the small subset of animals resistant to both hyperinsulinemia and hyperglycemia were included in this study.

Results

In males, Chow (compared to Fiber) was associated with elevated heart rate and mitral E/A velocity ratio, and with lower e’-wave velocity, isovolumetric relaxation time, and ejection time. Of note, these clinically atypical types of diastolic dysfunction occurred independently of body weight. In contrast, females did not exhibit changes in cardiovascular function between diets.

Conclusions

The higher prevalence of T2DM in males correlates with their susceptibility to develop subtle diastolic cardiac dysfunction when fed a Western style diet (throughout most of their lifespan) despite no systemic evidence of metabolic syndrome, let alone T2DM.

L-carnitine ameliorates peripheral neuropathy in diabetic mice with a corresponding increase in insulin‑like growth factor‑1 level

Diabetic peripheral neuropathy (DPN) is one of the common complications in diabetes, affecting more than half of patients with diabetes. L‑carnitine (LC) was recently demonstrated to serve a positive role in ameliorating DPN. Therefore, the aim of the present study was to investigate the underlying mechanisms of LC in ameliorating DPN. Male Kunming mice were randomly assigned into five groups, including the control group, diabetes mellitus group, pre‑treatment group, treatment group and post‑treatment group. Type 2 diabetes was induced in mice using a combination of high‑fat diet and streptozotocin injection. Subsequently, peripheral neuropathy was measured and the levels of LC, insulin and insulin‑like growth factor‑1 (IGF‑1) were detected. When diabetic mice were treated with LC, the levels of IGF‑1 in the plasma and pancreas were increased. In addition, hyperalgesia, as determined by the tail‑flick test as well as food intake, body weight and blood glucose levels were decreased. An amelioration of demyelination, axonal atrophy and mitochondria swelling in the nerve fibres of diabetic mice was also observed. The present study demonstrated that LC ameliorated peripheral neuropathy in type 2 diabetic mice and the effect of LC may in part be mediated by an increase in local and circulatory IGF‑1 levels.

Up regulated Tmbim1 activation promotes high fat diet (HFD)-induced cardiomyopathy by enhancement of inflammation and oxidative stress

The prevalence of cardiomyopathy due to metabolic stress has up-regulated dramatically; nevertheless, its molecular mechanisms remain unclear. Here we suggested that transmembrane BAX inhibitor motif-containing 1 (Tmbim1) is down-regulated in the hearts of mice fed with high fat diet (HFD). We provided evidence that Tmbim1 knockout (KO) accelerated HFD-induced metabolic disorders in mice, as supported by the remarkable increase of fasting serum glucose and insulin levels. HFD-induced cardiac dysfunctions were greatly intensified by the loss of Tmbim1, along with higher levels of lactate dehydrogenase (LDH) and creatine kinase (CK) in serum. In addition, Tmbim1 deletion significantly enhanced lipid accumulation in heart of mice administrated with HFD. Furthermore, Tmbim1 knockout reinforced myocardial inflammation, evidenced by increasing the expression of pro-inflammatory cytokines (interleukin 1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α)), and the activation of nuclear factor-κB (NF-κB) signaling pathway. Tmbim1 deficiency strengthened oxidative damage in hearts of HFD-fed mice, accompanied with a significant reduction of nuclear factor-erythroid 2 related factor 2 (Nrf-2) pathway. In palmitate (PA)-treated primary cardiomyocytes, Tmbim1 ablation markedly enhanced cell inflammation and oxidative stress, which were abolished by the suppression of ROS generation and NF-κB activation. Taken together, these findings suggested that Tmbim1 might be a key suppressor of metabolic stress-induced cardiomyopathy, which could be a promising target for the treatment of metabolic syndrome-triggered myocardial damage and heart failure.

Deletion of the insulin receptor in sensory neurons increases pancreatic insulin levels

Insulin is known to have neurotrophic properties and loss of insulin support to sensory neurons may contribute to peripheral diabetic neuropathy (PDN). Here, genetically-modified mice were generated in which peripheral sensory neurons lacked the insulin receptor (SNIRKO mice) to determine whether disrupted sensory neuron insulin signaling plays a crucial role in the development of PDN and whether SNIRKO mice develop symptoms of PDN due to reduced insulin neurotrophic support. Our results revealed that SNIRKO mice were euglycemic and never displayed significant changes in a wide range of sensorimotor behaviors, nerve conduction velocity or intraepidermal nerve fiber density. However, SNIRKO mice displayed elevated serum insulin levels, glucose intolerance, and increased insulin content in the islets of Langerhans of the pancreas. These results contribute to the growing idea that sensory innervation of pancreatic islets is key to regulating islet function and that a negative feedback loop of sensory neuron insulin signaling keeps this regulation in balance. Our results suggest that a loss of insulin receptors in sensory neurons does not lead to peripheral nerve dysfunction. The SNIRKO mice will be a powerful tool to investigate sensory neuron insulin signaling and may give a unique insight into the role that sensory neurons play in modifying islet physiology.

Inhibition of Nogo-B promotes cardiac hypertrophy via endoplasmic reticulum stress

AIMS

Nogo-B is a key endoplasmic reticulum (ER) protein that regulates ER stress signaling. However, its role in cardiac hypertrophy remains poorly understood. ER stress is interrelated with autophagy in the process of cardiac hypertrophy. Therefore, we aimed to test the hypothesis that both ER stress and autophagy signaling mediate the function of Nogo-B in cardiac hypertrophy.

MAIN METHODS

Rat models of transverse aortic constriction (TAC), neonatal rat cardiomyocytes (NRCMs) stimulated with norepinephrine (Ne) and primary cardiac fibroblasts treated with transforming growth factor β1 (TGF-β1) were used in this study. The expression of Nogo-B and markers of ER stress were determined by quantitative RT-PCR, western blotting and immunofluorescence. Autophagy was measured by monitoring autophagic flux. Specific small interfering RNA (siRNA) of Nogo-B was transfected to investigate the role of Nogo-B in regulating cardiac hypertrophy.

KEY FINDINGS

In TAC-induced hypertrophic heart tissues, Ne-treated hypertrophic cardiomyocytes and TGF-β1-stimulated cardiac fibroblasts, the expression of Nogo-B, and markers of ER stress were significantly elevated. Impairment of autophagic flux was observed in the activated cardiac fibroblasts. Down-regulation of Nogo-B by siRNA further exacerbated Ne-induced cardiomyocyte hypertrophy and TGF-β1-induced cardiac fibroblast activation. Gene silencing of Nogo-B promoted the activation of the ER stress pathway and the impairment of autophagic flux. Moreover, inhibition of Nogo-B activated the protein kinase RNA-like ER kinase (PERK)/activating transcriptional factor 4 (ATF4) and activating transcriptional factor 6 (ATF6) branches of ER stress pathways.

SIGNIFICANCE

These findings suggest that inhibition of Nogo-B promotes cardiomyocyte hypertrophy and cardiac fibroblast activation by activating the PERK/ATF4 signaling pathway and defects of autophagic flux.