Indirect Regulation of Endogenous Glucose Production by Insulin: The Single Gateway Hypothesis Revisited

The Subcutaneous Adipose Microenvironment as a Determinant of Body Fat Development in Polycystic Ovary Syndrome

Context

Adipose steroid metabolism modifies body fat development in polycystic ovary syndrome (PCOS).

Objective

To determine whether subcutaneous (SC) abdominal adipose aldo-keto reductase 1C3 (AKR1C3; a marker of testosterone generation) is increased in normal-weight women with PCOS vs age- and body mass index (BMI)-matched normoandrogenic ovulatory women (controls) and is related to SC abdominal adipose activator protein-1 (AP-1; a marker of adipocyte differentiation) and/or androgen receptor (AR) protein expression in predicting fat accretion.

Design

Prospective cohort study.

Setting

Academic center.

Patients

Eighteen normal-weight PCOS women; 17 age- and BMI-matched controls.

Interventions

Circulating hormone/metabolic determinations, intravenous glucose tolerance testing, total body dual-energy x-ray absorptiometry, SC abdominal fat biopsy, immunohistochemistry.

Main Outcome Measures

Clinical characteristics, hormonal concentrations, body fat distribution, SC adipose AKR1C3, AR, and AP-1 protein expression.

Results

Women with PCOS had significantly higher serum androgen levels and greater android/gynoid fat mass ratios than controls. SC adipose AKR1C3, AR, and AP-1 protein expressions were comparable between the study groups, but groups differed in correlations. In PCOS women vs controls, SC adipose AKR1C3 protein expression correlated positively with android and gynoid fat masses and negatively with SC adipose AP-1 protein expression. SC adipose AR protein expression correlated negatively with fasting serum free fatty acid and high-density lipoprotein levels. In both study groups, SC adipose AKR1C3 protein expression negatively correlated with serum cortisol levels.

Conclusion

In normal-weight PCOS women, SC abdominal adipose AKR1C3 protein expression, in combination with intra-adipose AP-1 and AR-dependent events, predicts fat accretion in the presence of physiological cortisol levels.

MAD2-Dependent Insulin Receptor Endocytosis Regulates Metabolic Homeostasis

Insulin activates insulin receptor (IR) signaling and subsequently triggers IR endocytosis to attenuate signaling. Cell division regulators MAD2, BUBR1, and p31comet promote IR endocytosis on insulin stimulation. Here, we show that genetic ablation of the IR-MAD2 interaction in mice delays IR endocytosis, increases IR levels, and prolongs insulin action at the cell surface. This in turn causes a defect in insulin clearance and increases circulating insulin levels, unexpectedly increasing glucagon levels, which alters glucose metabolism modestly. Disruption of the IR-MAD2 interaction increases serum fatty acid concentrations and hepatic fat accumulation in fasted male mice. Furthermore, disruption of the IR-MAD2 interaction distinctly changes metabolic and transcriptomic profiles in the liver and adipose tissues. Our findings establish the function of cell division regulators in insulin signaling and provide insights into the metabolic functions of IR endocytosis.

ARTICLE HIGHLIGHTS

The physiological role of IR endocytosis in insulin sensitivity remains unclear. Disruption of the IR-MAD2 interaction delays IR endocytosis and prolongs insulin signaling. IR-MAD2 controls insulin clearance and glucose metabolism. IR-MAD2 maintains energy homeostasis.

Hepatic Insulin Resistance Model in the Male Wistar Rat Using Exogenous Insulin Glargine Administration

Metabolic diseases are a worldwide health problem. Insulin resistance (IR) is their distinctive hallmark. For their study, animal models that provide reliable information are necessary, permitting the analysis of the cluster of abnormalities that conform to it, its progression, and time-dependent molecular modifications. We aimed to develop an IR model by exogenous insulin administration. The effective dose of insulin glargine to generate hyperinsulinemia but without hypoglycemia was established. Then, two groups (control and insulin) of male Wistar rats of 100 g weight were formed. The selected dose (4 U/kg) was administered for 15, 30, 45, and 60 days. Zoometry, a glucose tolerance test, insulin response, IR, and the serum lipid profile were assessed. We evaluated insulin signaling, glycogenesis and lipogenesis, redox balance, and inflammation in the liver. Results showed an impairment of glucose tolerance, dyslipidemia, hyperinsulinemia, and peripheral and time-dependent selective IR. At the hepatic level, insulin signaling was impaired, resulting in reduced hepatic glycogen levels and triglyceride accumulation, an increase in the ROS level with MAPK-ERK1/2 response, and mild pro-oxidative microenvironmental sustained by MT, GSH, and GR activity. Hepatic IR coincides with additions in MAPK-p38, NF-κB, and zoometric changes. In conclusion, daily insulin glargine administration generated a progressive IR model. At the hepatic level, the IR was combined with oxidative conditions but without inflammation.

Molecular Genetics of Abnormal Redox Homeostasis in Type 2 Diabetes Mellitus

Numerous studies have shown that oxidative stress resulting from an imbalance between the production of free radicals and their neutralization by antioxidant enzymes is one of the major pathological disorders underlying the development and progression of type 2 diabetes (T2D). The present review summarizes the current state of the art advances in understanding the role of abnormal redox homeostasis in the molecular mechanisms of T2D and provides comprehensive information on the characteristics and biological functions of antioxidant and oxidative enzymes, as well as discusses genetic studies conducted so far in order to investigate the contribution of polymorphisms in genes encoding redox state-regulating enzymes to the disease pathogenesis.

Coronarin A modulated hepatic glycogen synthesis and gluconeogenesis via inhibiting mTORC1/S6K1 signaling and ameliorated glucose homeostasis of diabetic mice

Promotion of hepatic glycogen synthesis and inhibition of hepatic glucose production are effective strategies for controlling hyperglycemia in type 2 diabetes mellitus (T2DM), but agents with both properties were limited. Herein we report coronarin A, a natural compound isolated from rhizomes of Hedychium gardnerianum, which simultaneously stimulates glycogen synthesis and suppresses gluconeogenesis in rat primary hepatocytes. We showed that coronarin A (3, 10 μM) dose-dependently stimulated glycogen synthesis accompanied by increased Akt and GSK3β phosphorylation in rat primary hepatocytes. Pretreatment with Akt inhibitor MK-2206 (2 μM) or PI3K inhibitor LY294002 (10 μM) blocked coronarin A-induced glycogen synthesis. Meanwhile, coronarin A (10 μM) significantly suppressed gluconeogenesis accompanied by increased phosphorylation of MEK, ERK1/2, β-catenin and increased the gene expression of TCF7L2 in rat primary hepatocytes. Pretreatment with β-catenin inhibitor IWR-1-endo (10 μM) or ERK inhibitor SCH772984 (1 μM) abolished the coronarin A-suppressed gluconeogenesis. More importantly, we revealed that coronarin A activated PI3K/Akt/GSK3β and ERK/Wnt/β-catenin signaling via regulation of a key upstream molecule IRS1. Coronarin A (10, 30 μM) decreased the phosphorylation of mTOR and S6K1, the downstream target of mTORC1, which further inhibited the serine phosphorylation of IRS1, and subsequently increased the tyrosine phosphorylation of IRS1. In type 2 diabetic ob/ob mice, chronic administration of coronarin A significantly reduced the non-fasting and fasting blood glucose levels and improved glucose tolerance, accompanied by the inhibited hepatic mTOR/S6K1 signaling and activated IRS1 along with enhanced PI3K/Akt/GSK3β and ERK/Wnt/β-catenin pathways. These results demonstrate the anti-hyperglycemic effect of coronarin A with a novel mechanism by inhibiting mTORC1/S6K1 to increase IRS1 activity, and highlighted coronarin A as a valuable lead compound for the treatment of T2DM.

Reflections on the state of diabetes research and prospects for treatment

Research on the etiology and treatment of diabetes has made substantial progress. As a result, several new classes of anti-diabetic drugs have been introduced in clinical practice. Nonetheless, the number of patients achieving glycemic control targets has not increased for the past 20 years. Two areas of unmet medical need are the restoration of insulin sensitivity and the reversal of pancreatic beta cell failure. In this review, we integrate research advances in transcriptional regulation of insulin action and pathophysiology of beta cell dedifferentiation with their potential impact on prospects of a durable "cure" for patients suffering from type 2 diabetes.

Feeding desensitizes A1 adenosine receptors in adipose through FOXO1-mediated transcriptional regulation

OBJECTIVE

Adipose tissue is a critical regulator of energy balance that must rapidly shift its metabolism between fasting and feeding to maintain homeostasis. Adenosine has been characterized as an important regulator of adipocyte metabolism primarily through its actions on A₁ adenosine receptors (A1R). We sought to understand the role A1R plays specifically in adipocytes during fasting and feeding to regulate glucose and lipid metabolism.

METHODS

We used Adora1 floxed mice with an inducible, adiponectin-Cre to generate FAdora1^-/- mice, where F designates a fat-specific deletion of A1R. We used these FAdora1^-/- mice along with specific agonists and antagonists of A1R to investigate changes in adenosine signaling within adipocytes between the fasted and fed state.

RESULTS

We found that the adipose tissue response to adenosine is not static, but changes dynamically according to nutrient conditions through the insulin-Akt-FOXO1 axis. We show that under fasted conditions, FAdora1^-/- mice had impairments in the suppression of lipolysis by insulin on normal chow and impaired glucose tolerance on high-fat diet. FAdora1^-/- mice also exhibited a higher lipolytic response to isoproterenol than WT controls when fasted, however this difference was lost after a 4-hour refeeding period. We demonstrate that FOXO1 binds to the A1R promoter, and refeeding leads to a rapid downregulation of A1R transcript and desensitization of adipocytes to A1R agonism. Obesity also desensitizes adipocyte A1R, and this is accompanied by a disruption of cyclical changes in A1R transcription between fasting and refeeding.

CONCLUSIONS

We propose that FOXO1 drives high A1R expression under fasted conditions to limit excess lipolysis during stress and augment insulin action upon feeding. Subsequent downregulation of A1R under fed conditions leads to desensitization of these receptors in adipose tissue. This regulation of A1R may facilitate reentrance into the catabolic state upon fasting.

Insulin: The master regulator of glucose metabolism

Insulin is the master regulator of glucose, lipid, and protein metabolism. Following ingestion of an oral glucose load or mixed meal, the plasma glucose concentration rises, insulin secretion by the beta cells is stimulated and the hyperinsulinemia, working in concert with hyperglycemia, causes: (i) suppression of endogenous (primarily reflects hepatic) glucose production, (ii) stimulation of glucose uptake by muscle, liver, and adipocytes, (iii) inhibition of lipolysis leading to a decline in plasma FFA concentration which contributes to the suppression of hepatic glucose production and augmentation of muscle glucose uptake, and (iv) vasodilation in muscle, which contributes to enhanced muscle glucose disposal. Herein, the integrated physiologic impact of insulin to maintain normal glucose homeostasis is reviewed and the molecular basis of insulin's diverse actions in muscle, liver, adipocytes, and vasculature are discussed.

An integrative transcriptional logic model of hepatic insulin resistance

Abnormalities of lipid/lipoprotein and glucose metabolism are hallmarks of hepatic insulin resistance in type 2 diabetes. The former antedate the latter, but the latter become progressively refractory to treatment and contribute to therapeutic failures. It's unclear whether the two processes share a common pathogenesis and what underlies their progressive nature. In this study, we investigated the hypothesis that genes in the lipid/lipoprotein pathway and those in the glucose metabolic pathway are governed by different transcriptional regulatory logics that affect their response to physiologic (fasting/refeeding) as well as pathophysiologic cues (insulin resistance and hyperglycemia). To this end, we obtained genomic and transcriptomic maps of the key insulin-regulated transcription factor, FoxO1, and integrated them with those of CREB, PPAR-α, and glucocorticoid receptor. We found that glucose metabolic genes are primarily regulated by promoter and intergenic enhancers in a fasting-dependent manner, while lipid genes are regulated through fasting-dependent intron enhancers and fasting-independent enhancerless introns. Glucose genes also showed a remarkable transcriptional resiliency (i.e., the ability to compensate following constitutive FoxO1 ablation through an enrichment of active marks at shared PPAR-α/FoxO1 regulatory elements). Unexpectedly, insulin resistance and hyperglycemia were associated with a "spreading" of FoxO1 binding to enhancers and the emergence of unique target sites. We surmise that this unusual pattern correlates with the progressively intractable nature of hepatic insulin resistance. This transcriptional logic provides an integrated model to interpret the combined lipid and glucose abnormalities of type 2 diabetes.

Serum Testosterone to Androstenedione Ratio Predicts Metabolic Health in Normal-Weight Polycystic Ovary Syndrome Women

Context

Increased aldo-keto reductase 1C3 (AKR1C3)-mediated conversion of androstenedione (A4) to testosterone (T) promotes lipid storage in subcutaneous (SC) abdominal adipose in overweight/obese polycystic ovary syndrome (PCOS) women.

Objective

This work examines whether an elevated serum T/A4 ratio, as a marker of enhanced AKR1C3 activity in SC abdominal adipose, predicts metabolic function in normal-weight PCOS women.

Methods

This prospective cohort study took place in an academic center and comprised 19 normal-weight PCOS women and 21 age- and body mass index–matched controls. Interventions included circulating hormone/metabolic determinations, intravenous glucose tolerance testing, total body dual-energy x-ray absorptiometry, and SC abdominal fat biopsy. Serum T/A4 ratios, hormone/metabolic measures, and AKR1C3 expression of adipocytes matured in vitro were compared between female types; serum T/A4 ratios were correlated with serum lipids, adipose insulin resistance (adipose-IR), homeostatic model assessment of insulin resistance (HOMA-IR) and insulin sensitivity (Si).

Results

Increased serum T/A4 ratios (P = .040) and log adipose-IR values (P = .002) in PCOS women vs controls were accompanied by AKR1C3 messenger RNA overexpression of PCOS adipocytes matured in vitro (P = .016). Serum T/A4 ratios in PCOS women, but not controls, negatively correlated with log triglycerides (TGs: R = –0.65, P = .002) and the TG index (R = –0.57, P = .011). Adjusting for serum free T, serum T/A4 ratios in PCOS women remained negatively correlated with log TG (R = –0.57, P = .013) and TG index (R = –0.50, P = .036), respectively, without significant relationships with other metabolic measures.

Conclusion

An elevated serum T/A4 ratio, as a marker of enhanced AKR1C3 activity in SC abdominal adipose, predicts healthy metabolic function in normal-weight PCOS women.

Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis

The reactions of the tricarboxylic acid (TCA) cycle allow the controlled combustion of fat and carbohydrate. In principle, TCA cycle intermediates are regenerated on every turn and can facilitate the oxidation of an infinite number of nutrient molecules. However, TCA cycle intermediates can be lost to cataplerotic pathways that provide precursors for biosynthesis, and they must be replaced by anaplerotic pathways that regenerate these intermediates. Together, anaplerosis and cataplerosis help regulate rates of biosynthesis by dictating precursor supply, and they play underappreciated roles in catabolism and cellular energy status. They facilitate recycling pathways and nitrogen trafficking necessary for catabolism, and they influence redox state and oxidative capacity by altering TCA cycle intermediate concentrations. These functions vary widely by tissue and play emerging roles in disease. This article reviews the roles of anaplerosis and cataplerosis in various tissues and discusses how they alter carbon transitions, and highlights their contribution to mechanisms of disease. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Direct and indirect control of hepatic glucose production by insulin

There is general agreement that the acute suppression of hepatic glucose production by insulin is mediated by both a direct and an indirect effect on the liver. There is, however, no consensus regarding the relative magnitude of these effects under physiological conditions. Extensive research over the past three decades in humans and animal models has provided discordant results between these two modes of insulin action. Here, we review the field to make the case that physiologically direct hepatic insulin action dominates acute suppression of glucose production, but that there is also a delayed, second order regulation of this process via extrahepatic effects. We further provide our views regarding the timing, dominance, and physiological relevance of these effects and discuss novel concepts regarding insulin regulation of adipose tissue fatty acid metabolism and central nervous system (CNS) signaling to the liver, as regulators of insulin's extrahepatic effects on glucose production.

Transcriptome and Differential Methylation Integration Analysis Identified Important Differential Methylation Annotation Genes and Functional Epigenetic Modules Related to Vitiligo

Vitiligo is an pigmentation disorder caused by a variety of pathogenic factors; its main pathophysiological conditions include oxidative stress, immune activation, and genetic background. Additionally, DNA methylation is often associated with the pathogenesis of vitiligo; however, the underlying mechanism remains unknown. In the present study, we used the Human Methylation 850K BeadChip platform to detect DNA methylation changes in the vitiligo melanocytes. We then integrated the results with the transcriptome data of vitiligo melanocytes and lesions to analyse the correlation between differentially methylated levels and differentially expressed genes. The results showed that there was a significant negative correlation between methylation levels and differentially expressed genes. Subsequently, we enriched GO and KEGG based on methylated differentially expressed genes (MDEGs) using R package ClusterProfiler, and the results were closely related to the pathogenesis of vitiligo. In addition, we also constructed a PPI network of MDEGs and excavated three important functional epigenetic modules, involving a total of 12 (BCL2L1, CDK1, ECT2, HELLS, HSP90AA1, KIF23, MC1R, MLANA, PBK, PTGS2, SOX10, and TYRP1) genes. These genes affect melanocyte melanogenesis, cellular oxidative stress and other important biological processes. Our comprehensive analysis results support the significant contribution of the status of DNA methylation modification to vitiligo, which will help us to better understand the molecular mechanism of vitiligo and explore new therapeutic strategies.

Accelerated subcutaneous abdominal stem cell adipogenesis predicts insulin sensitivity in normal-weight women with polycystic ovary syndrome

OBJECTIVE

To examine whether subcutaneous (SC) abdominal adipose stem cell differentiation into adipocytes in vitro predicts insulin sensitivity (Si) in vivo in normal-weight women with polycystic ovary syndrome (PCOS) and controls.

DESIGN

Prospective cohort study.

SETTING

Academic medical center.

PATIENT(S)

Eight normal-weight women with PCOS and 8 age- and body mass index-matched controls.

INTERVENTION(S)

Women underwent circulating hormone/metabolic determinations, intravenous glucose tolerance testing, total-body dual-energy x-ray absorptiometry, and SC abdominal fat biopsy.

MAIN OUTCOME MEASURE(S)

PPARγ and CEBPa gene expression and lipid content of adipocytes matured in vitro were compared between women with PCOS and control women, and correlated with patient characteristics, systemic Si, and adipose insulin resistance (adipose-IR).

RESULT(S)

Serum androgen levels, adipose-IR, and percentage of android fat were greater in women with PCOS than control women. Stem cell PPARγ and CEBPa gene expression increased maximally by day 12 without a female-type effect. In control cells, gene expression positively correlated with fasting serum insulin levels (both genes) and adipose-IR (CEBPa) and negatively correlated with Si (CEBPa). Conversely, CEBPa gene expression in PCOS cells negatively correlated with adipose-IR and serum free testosterone, whereas total lipid accumulation in these cells positively corelated with Si.

CONCLUSION

In normal-weight women with PCOS, accelerated SC abdominal adipose stem cell differentiation into adipocytes in vitro favors Si in vivo, suggesting a role for hyperandrogenism in the evolution of metabolic thrift to enhance fat storage through increased cellular glucose uptake.

Reappraisal of the optimal fasting time for insulin tolerance tests in mice

Objective

Most studies routinely use overnight or 6 h of fasting before testing metabolic glucose homeostasis in mice. Other studies used empirically shorter fasting times (<6 h). We attempted to determine the shortest fasting time required for optimal insulin responsiveness while minimizing metabolic stress.

Methods

A course of fasting for up to 24 h (0, 2, 4, 6, 12, and 24 h) was conducted in C57Bl/6J male mice. Body weight, metabolic parameters, and insulin tolerance were measured in each experimental group. The organs were collected at the same time on separate occasions and glycogen and metabolic gene expression were measured in the liver and skeletal muscle.

Results

Our data show that blood glucose levels do not significantly change during a 6 h fast, while plasma insulin levels decrease to similar levels between 2 h and 6 h of fasting. During overnight (12 h) and 24 h fasts, a robust decrease in blood glucose and plasma insulin was observed along with a profound depletion in liver glycogen content. Insulin tolerance was comparable between baseline and 6 h fasts while 4 h and 6 h fasts were associated with a greater depletion of liver glycogen than 2 h fasts, impacting the glucose counter-regulatory response. Fasting induced progressive weight loss that was attenuated at thermoneutrality. Fasting longer than 4 h induced major body weight loss (>5%) and significant changes in catabolic gene expression in the liver and skeletal muscle.

Conclusion

Collectively, these data suggest that 2 h of fasting appears optimal for the assessment of insulin tolerance in mice as this duration minimizes major metabolic stress and weight loss.

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Fasting in mice induces time-dependent metabolic stress and weight loss.

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Fasting promotes profound changes in catabolic gene expression in liver and muscles.

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Fasting-induced weight loss is attenuated at thermoneutrality.

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Two hours fasting appears to be optimal prior to assessing insulin tolerance in mice.

Metabolically healthy obesity: facts and fantasies

Although obesity is typically associated with metabolic dysfunction and cardiometabolic diseases, some people with obesity are protected from many of the adverse metabolic effects of excess body fat and are considered "metabolically healthy." However, there is no universally accepted definition of metabolically healthy obesity (MHO). Most studies define MHO as having either 0, 1, or 2 metabolic syndrome components, whereas many others define MHO using the homeostasis model assessment of insulin resistance (HOMA-IR). Therefore, numerous people reported as having MHO are not metabolically healthy, but simply have fewer metabolic abnormalities than those with metabolically unhealthy obesity (MUO). Nonetheless, a small subset of people with obesity have a normal HOMA-IR and no metabolic syndrome components. The mechanism(s) responsible for the divergent effects of obesity on metabolic health is not clear, but studies conducted in rodent models suggest that differences in adipose tissue biology in response to weight gain can cause or prevent systemic metabolic dysfunction. In this article, we review the definition, stability over time, and clinical outcomes of MHO, and discuss the potential factors that could explain differences in metabolic health in people with MHO and MUO - specifically, modifiable lifestyle factors and adipose tissue biology. Better understanding of the factors that distinguish people with MHO and MUO can produce new insights into mechanism(s) responsible for obesity-related metabolic dysfunction and disease.

Diabetic‑induced alterations in hepatic glucose and lipid metabolism: The role of type 1 and type 2 diabetes mellitus (Review)

Diabetes mellitus (DM) is a growing health concern in society. Type 1 and type 2 DM are the two main types of diabetes; both types are chronic diseases that affect glucose metabolism in the body and the impaired regulation of glucose and lipid metabolism promotes the development and progression of DM. During the physiological metabolism process, the liver serves a unique role in glucose and lipid metabolism. The present article aimed to review the association between DM and glucose metabolism in the liver and discuss the changes of the following hepatic glucose fluxes: Gluconeogenesis, glucose/glucose 6-phosphate cycling, glycogenolysis, glycogenesis and the pentose phosphate pathway. Moreover, the incidence of fatty liver in DM was also investigated.

Soluble (pro)renin receptor treats metabolic syndrome in mice with diet-induced obesity via interaction with PPARγ

The therapies available for management of obesity and associated conditions are limited, because they are often directed toward an individual component of metabolic syndrome and are associated with adverse effects. Here, we report the multifaceted therapeutic potential of histidine-tagged recombinant soluble (pro)renin receptor (sPRR), termed sPRR-His, in a mouse model of diet-induced obesity (DIO). In the DIO model, 2-week administration of sPRR-His lowered body weight and remarkably improved multiple metabolic parameters in the absence of fluid retention. Conversely, inhibition of endogenous sPRR production by PF429242 induced diabetes and insulin resistance, both of which were reversed by the sPRR-His supplement. At the cellular level, sPRR-His enhanced insulin-induced increases in glucose uptake via upregulation of phosphorylated AKT and protein abundance of glucose transporter 4. Promoter and gene expression analysis revealed PRR as a direct target gene of PPARγ. Adipocyte-specific PPARγ deletion induced severe diabetes and insulin resistance associated with reduced adipose PRR expression and circulating sPRR. The sPRR-His supplement in the null mice nearly normalized blood glucose and insulin levels. Additionally, sPRR-His treatment suppressed DIO-induced renal sodium-glucose cotransporter-2 (SGLT2) expression. Overall, sPRR-His exhibits a therapeutic potential in management of metabolic syndrome via interaction with PPARγ.

Pathophysiology of Type 2 Diabetes in Children and Adolescents

BACKGROUND

The prevalence of type 2 diabetes (DM) in children is disturbingly increasing in parallel with the increasing childhood obesity. Better knowledge regarding the pathophysiology of type 2 DM in children is paramount to devise an effective management plan.

OBJECTIVE

Discuss the pathophysiology of type 2 DM in children and adolescents.

METHODS AND RESULTS

This is a comprehensive review of the literature on this topic. Type 2 DM in childhood is viewed as a continuum of insulin resistance (IR) which is determined by an underlying genetic predisposition, intrauterine environment, excessive food consumption, continued rapid weight gain, and poor lifestyle. Besides IR, this is compounded by multiple metabolic defects including β-cell dysfunction and inadequate insulin secretion, α-cell dysfunction, hyperglucagonemia and increased hepatic glucose production, lipotoxicity, inflammation, deficiencies in incretin production and action, and increased renal glucose reabsorption. The confluence of genetic and environmental factors underscores the complexity in disease progression.

CONCLUSION

A consistent single risk factor for type 2 DM is obesity and related IR and therefore it is essential to curtail the progression of obesity. It is important to investigate the role of stringent dietary and nutritional approaches, medications that enhance β-cell function and insulin sensitivity.

The vascular endothelium plays a role in insulin action

The endocrine system relies on the vasculature for delivery of hormones throughout the body, and the capillary microvasculature is the site where the hormones cross from the blood into the target tissue. Once considered an inert wall, various studies have now highlighted the functions of the capillary endothelium to regulate transport and therefore affect or maintain the interstitial environment. The role of the capillary may be clear in areas where there is a continuous endothelium, yet there also appears to be a role of endothelial cells in tissues with a sinusoidal structure. Here we focused on the most common endocrine disorder, diabetes, and several of the target organs associated with the disease, including skeletal muscle, liver and pancreas. However, it is important to note that the ability of hormones to cross the endothelium to reach their target tissue is a component of all endocrine functions. It is also a consideration in organs throughout the body and may have greater impact for larger hormones with target tissues containing a continuous endothelium. We noted that the blood levels do not always equal interstitial levels, which is what the cells are exposed to, and discussed how this may change in diseases such as obesity and insulin resistance. The capillary endothelium is, therefore, an essential and understudied aspect of endocrinology and metabolism that can be altered in disease, which may be an appropriate target for treatment.

Hyperinsulinemia: An Early Indicator of Metabolic Dysfunction

Hyperinsulinemia is strongly associated with type 2 diabetes. Racial and ethnic minority populations are disproportionately affected by diabetes and obesity-related complications. This mini-review provides an overview of the genetic and environmental factors associated with hyperinsulinemia with a focus on racial and ethnic differences and its metabolic consequences. The data used in this narrative review were collected through research in PubMed and reference review of relevant retrieved articles. Insulin secretion and clearance are regulated processes that influence the development and progression of hyperinsulinemia. Environmental, genetic, and dietary factors are associated with hyperinsulinemia. Certain pharmacotherapies for obesity and bariatric surgery are effective at mitigating hyperinsulinemia and are associated with improved metabolic health. Hyperinsulinemia is associated with many environmental and genetic factors that interact with a wide network of hormones. Recent studies have advanced our understanding of the factors affecting insulin secretion and clearance. Further basic and translational work on hyperinsulinemia may allow for earlier and more personalized treatments for obesity and metabolic diseases.

Identification of Insulin-Responsive Transcription Factors That Regulate Glucose Production by Hepatocytes

Hepatocyte glucose production is a complex process that integrates cell-autonomous mechanisms with cellular signaling, enzyme activity modulation, and gene transcription. Transcriptional mechanisms controlling glucose production are redundant and involve nuclear hormone receptors and unliganded transcription factors (TFs). Our knowledge of this circuitry is incomplete. Here we used DNA affinity purification followed by mass spectrometry to probe the network of hormone-regulated TFs by using phosphoenolpyruvate carboxykinase (Pck1) and glucose-6-phosphatase (G6pc) in liver and primary hepatocytes as model systems. The repertoire of insulin-regulated TFs is unexpectedly broad and diverse. Whereas in liver the two test promoters are regulated by largely overlapping sets of TFs, in primary hepatocytes Pck1 and G6pc regulation diverges. Insulin treatment preferentially results in increased occupancy by the two promoters, consistent with a model in which the hormone's primary role is to recruit corepressors rather than to clear activators. Nine insulin-responsive TFs are present in both models, but only FoxK1, FoxA2, ZFP91, and ZHX3 require an intact Pck1p insulin response sequence for binding. Knockdown of FoxK1 in primary hepatocytes decreased both glucose production and insulin's ability to suppress it. The findings expand the repertoire of insulin-dependent TFs and identify FoxK1 as a contributor to insulin signaling.

Impaired ketogenesis and increased acetyl-CoA oxidation promote hyperglycemia in human fatty liver

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent, and potentially morbid, disease that affects one-third of the U.S. population. Normal liver safely accommodates lipid excess during fasting or carbohydrate restriction by increasing their oxidation to acetyl-CoA and ketones, yet lipid excess during NAFLD leads to hyperglycemia and, in some, steatohepatitis. To examine potential mechanisms, flux through pathways of hepatic oxidative metabolism and gluconeogenesis were studied using five simultaneous stable isotope tracers in ketotic (24-hour fast) individuals with a wide range of hepatic triglyceride contents (0-52%). Ketogenesis was progressively impaired as hepatic steatosis and glycemia worsened. Conversely, the alternative pathway for acetyl-CoA metabolism, oxidation in the tricarboxylic (TCA) cycle, was upregulated in NAFLD as ketone production diminished and positively correlated with rates of gluconeogenesis and plasma glucose concentrations. Increased respiration and energy generation that occurred in liver when β-oxidation and TCA cycle activity were coupled may explain these findings, inasmuch as oxygen consumption was higher during fatty liver and highly correlated with gluconeogenesis. These findings demonstrate that increased glucose production and hyperglycemia in NAFLD is not a consequence of acetyl-CoA production per se, but how acetyl-CoA is further metabolized in liver.

The role of kidney in the inter-organ coordination of endogenous glucose production during fasting

Objective

The respective contributions to endogenous glucose production (EGP) of the liver, kidney and intestine vary during fasting. We previously reported that the deficiency in either hepatic or intestinal gluconeogenesis modulates the repartition of EGP via glucagon secretion (humoral factor) and gut–brain–liver axis (neural factor), respectively. Considering renal gluconeogenesis reportedly accounted for approximately 50% of EGP during fasting, we examined whether a reduction in renal gluconeogenesis could promote alterations in the repartition of EGP in this situation.

Methods

We studied mice whose glucose-6-phosphatase (G6Pase) catalytic subunit (G6PC) is specifically knocked down in the kidneys (K-G6pc^-/- mice) during fasting. We also examined the additional effects of intestinal G6pc deletion, renal denervation and vitamin D administration on the altered glucose metabolism in K-G6pc^-/- mice.

Results

Compared with WT mice, K-G6pc^-/- mice exhibited (1) lower glycemia, (2) enhanced intestinal but not hepatic G6Pase activity, (3) enhanced hepatic glucokinase (GK encoded by Gck) activity, (4) increased hepatic glucose-6-phosphate and (5) hepatic glycogen spared from exhaustion during fasting. Increased hepatic Gck expression in the post-absorptive state could be dependent on the enhancement of insulin signal (AKT phosphorylation) in K-G6pc^-/- mice. In contrast, the increase in hepatic GK activity was not observed in mice with both kidney- and intestine-knockout (KI-G6pc^-/- mice). Hepatic Gck gene expression and hepatic AKT phosphorylation were reduced in KI-G6pc^-/- mice. Renal denervation by capsaicin did not induce any effect on glucose metabolism in K-G6pc^-/- mice. Plasma level of 1,25 (OH)₂ D₃, an active form of vitamin D, was decreased in K-G6pc^-/- mice. Interestingly, the administration of 1,25 (OH)₂ D₃ prevented the enhancement of intestinal gluconeogenesis and hepatic GK activity and blocked the accumulation of hepatic glycogen otherwise observed in K-G6pc^-/- mice during fasting.

Conclusions

A diminution in renal gluconeogenesis that is accompanied by a decrease in blood vitamin D promotes a novel repartition of EGP among glucose producing organs during fasting, featured by increased intestinal gluconeogenesis that leads to sparing glycogen stores in the liver. Our data suggest a possible involvement of a crosstalk between the kidneys and intestine (via the vitamin D system) and the intestine and liver (via a neural gut-brain axis), which might take place in the situations of deficient renal glucose production, such as chronic kidney disease.

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Reduced renal G6Pase activity promotes increased hepatic glycogen during fasting.

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Reduced renal G6Pase activity enhances intestinal but not hepatic G6Pase activity.

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Reduced renal G6Pase activity results in low vitamin D level.

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Vitamin D injection restores metabolism in mice with reduced renal G6Pase activity.

Effects of Preceding Ethanol Intake on Glucose Response to Low-Dose Glucagon in Individuals With Type 1 Diabetes: A Randomized, Placebo-Controlled, Crossover Study

OBJECTIVE

This study investigated whether preceding ethanol intake impairs glucose response to low-dose glucagon in individuals with type 1 diabetes.

RESEARCH DESIGN AND METHODS

This was a randomized, crossover, placebo-controlled study in 12 insulin pump-treated individuals (median [interquartile range] age, 37 [31-51] years; HbA_1c, 57 [51-59] mmol/mol or 7.3% [6.8-7.5]; and BMI, 23.9 [22-25] kg/m²). During two overnight study visits, a 6 p.m. dinner (1 g carbohydrates/kg) was served with diet drink (placebo) or diet drink and ethanol (0.8 g/kg). After 8-9 h, ethanol was estimated to be metabolized, and a subcutaneous (s.c.) insulin bolus was given to induce mild hypoglycemia. When plasma glucose (PG) was ≤3.9 mmol/L, 100 µg glucagon was given s.c., followed by another s.c. 100 µg glucagon 2 h later. Primary end point was incremental peak PG induced by the first glucagon bolus.

RESULTS

Ethanol was undetectable before insulin administration at both visits. The insulin doses (mean ± SEM: 2.5 ± 0.4 vs. 2.7 ± 0.4 IU) to induce hypoglycemia (3.7 ± 0.1 vs. 3.9 ± 0.1 mmol/L) did not differ and caused similar insulin levels (28.3 ± 4.6 vs. 26.1 ± 4.0 mU/L) before glucagon administration on ethanol and placebo visits (all, P > 0.05). The first glucagon bolus tended to cause lower incremental peak PG (2.0 ± 0.5 vs. 2.9 ± 0.3 mmol/L, P = 0.06), lower incremental area under the curve (87 ± 40 vs. 191 ± 37 mmol/L × min, P = 0.08), and lower 2-h PG level (3.6 ± 1.0 vs. 4.8 ± 0.4 mmol/L, P = 0.05) after ethanol compared with placebo. The second glucagon bolus had similar responses between visits, but PG remained 1.8 ± 0.7 mmol/L lower after ethanol compared with placebo.

CONCLUSIONS

The ability of low-dose glucagon to treat mild hypoglycemia persisted with preceding ethanol intake, although it tended to be attenuated.

Hepatic but Not Extrahepatic Insulin Clearance Is Lower in African American Than in European American Women

African Americans (AAs) tend to have higher plasma insulin concentrations than European Americans (EAs); the increased insulin concentrations have been attributed to increased secretion and/or decreased insulin clearance by liver or other tissues. This work characterizes the contributions of hepatic versus extrahepatic insulin degradation related to ethnic differences between AAs and EAs. By using a recently developed mathematical model that uses insulin and C-peptide measurements from the insulin-modified, frequently sampled intravenous glucose tolerance test (FSIGT), we estimated hepatic versus extrahepatic insulin clearance in 29 EA and 18 AA healthy women. During the first 20 min of the FSIGT, plasma insulin was approximately twice as high in AAs as in EAs. In contrast, insulin was similar in AAs and EAs after the 20-25 min intravenous insulin infusion. Hepatic insulin first-pass extraction was two-thirds lower in AAs versus EAs in the overnight-fasted state. In contrast, extrahepatic insulin clearance was not lower in AAs than in EAs. The difference in insulin degradation between AAs and EAs can be attributed totally to liver clearance. The mechanism underlying reduced insulin degradation in AAs remains to be clarified, as does the relative importance of reduced liver clearance to increased risk for type 2 diabetes.