The Importance of the Mechanisms by Which Insulin Regulates Meal-Associated Liver Glucose Uptake in the Dog

Affordable oral proinsulin bioencapsulated in plant cells regulates blood sugar levels similar to natural insulin

Diabetes Mellitus is a silent epidemic affecting >500 million, which claimed 6.7 million lives in 2021, a projected increase of >670% in <20 years old in the next two decades but insulin is unaffordable for the large majority of the globe. Therefore, we engineered proinsulin in plant cells to facilitate oral delivery. Stability of the proinsulin gene and expression in subsequent generations, after removal of the antibiotic-resistance gene, was confirmed using PCR, Southern and western blots. Proinsulin expression was high (up to 12 mg/g DW or 47.5% of total leaf protein), stable up to one year after storage of freeze-dried plant cells at ambient temperature and met FDA regulatory requirements of uniformity, moisture content and bioburden. GM1 receptor binding, required for uptake via gut epithelial cells was confirmed by pentameric assembly of CTB-Proinsulin. IP insulin injections (without C peptide) in STZ mice rapidly decreased blood glucose level leading to transient hypoglycemia, followed by hepatic glucose compensation. On the other hand, other than the 15-min lag period of oral proinsulin (transit time required to reach the gut), the kinetics of blood sugar regulation of oral CTB-Proinsulin in STZ mice was very similar to naturally secreted insulin in healthy mice (both contain C-peptide), without rapid decrease or hypoglycemia. Elimination of expensive fermentation, purification and cold storage/transportation should reduce cost and increase other health benefits of plant fibers. The recent approval of plant cell delivery of therapeutic proteins by FDA and approval of CTB-ACE2 for phase I/II human clinical studies augur well for advancing oral proinsulin to the clinic.

Profound Sensitivity of the Liver to the Direct Effect of Insulin Allows Peripheral Insulin Delivery to Normalize Hepatic but Not Muscle Glucose Uptake in the Healthy Dog

Endogenous insulin secretion is a key regulator of postprandial hepatic glucose metabolism, but this process is dysregulated in diabetes. Subcutaneous insulin delivery alters normal insulin distribution, causing relative hepatic insulin deficiency and peripheral hyperinsulinemia, a major risk factor for metabolic disease. Our aim was to determine whether insulin's direct effect on the liver is preeminent even when insulin is given into a peripheral vein. Postprandial-like conditions were created (hyperinsulinemia, hyperglycemia, and a positive portal vein to arterial glucose gradient) in healthy dogs. Peripheral (leg vein) insulin infusion elevated arterial and hepatic levels 8.0-fold and 2.8-fold, respectively. In one group, insulin's full effects were allowed. In another, insulin's indirect hepatic effects were blocked with the infusion of triglyceride, glucagon, and inhibitors of brain insulin action (intracerebroventricular) to prevent decreases in plasma free fatty acids and glucagon, while blocking increased hypothalamic insulin signaling. Despite peripheral insulin delivery the liver retained its full ability to store glucose, even when insulin's peripheral effects were blocked, whereas muscle glucose uptake markedly increased, creating an aberrant distribution of glucose disposal between liver and muscle. Thus, the healthy liver's striking sensitivity to direct insulin action can overcome the effect of relative hepatic insulin deficiency, whereas excess insulin in the periphery produces metabolic abnormalities in nonhepatic tissues.

A physiologic increase in brain glucagon action alters the hepatic gluconeogenic/glycogenolytic ratio but not glucagon's overall effect on glucose production

It has been proposed that brain glucagon action inhibits glucagon-stimulated hepatic glucose production (HGP), which may explain, at least in part, why glucagon's effect on HGP is transient. However, the pharmacologic off-target effects of glucagon in the brain may have been responsible for previously observed effects. Therefore, the aim of this study was to determine if central glucagon action plays a physiologic role in the regulation of HGP. Insulin was maintained at baseline while glucagon was either infused into the carotid and vertebral arteries or into a peripheral (leg) vein at rates designed to increase glucagon in the head in one group, while keeping glucagon at the liver matched between groups. The extraction rate of glucagon across the head was high (double that of the liver), and hypothalamic cAMP increased twofold, in proportion to the exposure of the brain to increased glucagon, but HGP was not reduced by the increase in brain glucagon signaling, as had been suggested previously (the areas under the curve for HGP were 840 ± 14 vs. 871 ± 36 mg/kg/240 min in head vs. peripheral infusion groups, respectively). Central nervous system glucagon action reduced circulating free fatty acids and glycerol, and this was associated with a modest reduction in net hepatic gluconeogenic flux. However, offsetting autoregulation by the liver (i.e., a reciprocal increase in net hepatic glycogenolysis) prevented a change in HGP. Thus, while physiologic engagement of the brain by glucagon can alter hepatic carbon flux, it does not appear to be responsible for the transient fall in HGP that occurs following the stimulation of HGP during a square wave rise in glucagon.NEW & NOTEWORTHY Glucagon stimulates hepatic glucose production through its direct effects on the liver but may indirectly inhibit this process by acting on the brain. This was tested by delivering glucagon via the cerebral circulatory system. Central nervous system glucagon action reduced liver gluconeogenic flux, but glycogenolysis increased, resulting in no net change in hepatic glucose production. Surprisingly, brain glucagon also appeared to suppress lipolysis (plasma free fatty acid and glycerol levels were reduced).

Evolution of the diagnostic value of "the sugar of the blood": hitting the sweet spot to identify alterations in glucose dynamics

In this paper, we provide an overview of the evolution of the definition of hyperglycemia during the past century and the alterations in glucose dynamics that cause fasting and postprandial hyperglycemia. We discuss how extensive mechanistic, physiological research into the factors and pathways that regulate the appearance of glucose in the circulation and its uptake and metabolism by tissues and organs has contributed knowledge that has advanced our understanding of different types of hyperglycemia, namely prediabetes and diabetes and their subtypes (impaired fasting plasma glucose, impaired glucose tolerance, combined impaired fasting plasma glucose, impaired glucose tolerance, type 1 diabetes, type 2 diabetes, gestational diabetes mellitus), their relationships with medical complications, and how to prevent and treat hyperglycemia.

α1-Antitrypsin A treatment attenuates neutrophil elastase accumulation and enhances insulin sensitivity in adipose tissue of mice fed a high-fat diet

Neutrophils accumulate in insulin-sensitive tissues during obesity and may play a role in impairing insulin sensitivity. The major serine protease expressed by neutrophils is neutrophil elastase (NE), which is inhibited endogenously by α1-antitrypsin A (A1AT). We investigated the effect of exogenous (A1AT) treatment on diet-induced metabolic dysfunction. Male C57Bl/6j mice fed a chow or a high-fat diet (HFD) were randomized to receive intraperitoneal injections three times weekly of either Prolastin (human A1AT; 2 mg) or vehicle (PBS) for 10 wk. Prolastin treatment did not affect plasma NE concentration, body weight, glucose tolerance, or insulin sensitivity in chow-fed mice. In contrast, Prolastin treatment attenuated HFD-induced increases in plasma and white adipose tissue (WAT) NE without affecting circulatory neutrophil levels or increases in body weight. Prolastin-treated mice fed a HFD had improved insulin sensitivity, as assessed by insulin tolerance test, and this was associated with higher insulin-dependent IRS-1 (insulin receptor substrate) and Akt^Ser473 phosphorylation, and reduced inflammation markers in WAT but not liver or muscle. In 3T3-L1 adipocytes, Prolastin reversed recombinant NE-induced impairment of insulin-stimulated glucose uptake and IRS-1 phosphorylation. Furthermore, PDGF mediated p-Akt^Ser473 activation and glucose uptake (which is independent of IRS-1) was not affected by recombinant NE treatment. Collectively, our findings suggest that NE infiltration of WAT during metabolic overload contributes to insulin resistance by impairing insulin-induced IRS-1 signaling.NEW & NOTEWORTHY Neutrophils accumulate in peripheral tissues during obesity and are critical coordinators of tissue inflammatory responses. Here, we provide evidence that inhibition of the primary neutrophil protease, neutrophil elastase, with α1-antitrypsin A (A1AT) can improve insulin sensitivity and glucose homeostasis of mice fed a high-fat diet. This was attributed to improved insulin-induced IRS-1 phosphorylation in white adipose tissue and provides further support for a role of neutrophils in mediating diet-induced peripheral tissue insulin resistance.

Importance of the route of insulin delivery to its control of glucose metabolism

Pancreatic insulin secretion produces an insulin gradient at the liver compared with the rest of the body (approximately 3:1). This physiological distribution is lost when insulin is injected subcutaneously, causing impaired regulation of hepatic glucose production and whole body glucose uptake, as well as arterial hyperinsulinemia. Thus, the hepatoportal insulin gradient is essential to the normal control of glucose metabolism during both fasting and feeding. Insulin can regulate hepatic glucose production and uptake through multiple mechanisms, but its direct effects on the liver are dominant under physiological conditions. Given the complications associated with iatrogenic hyperinsulinemia in patients treated with insulin, insulin designed to preferentially target the liver may have therapeutic advantages.