Thursday, November 30, 2023
The results of the United Kingdom Prospective Diabetes Study (UKPDS)[1] provided evidence that impairment of pancreatic islet beta-cell function plays a significant role in the pathogenesis of type 2 diabetes. Accordingly, attention has focused on understanding the cellular physiology of insulin secretion, and there is renewed excitement in the field of beta-cell biology. At the 62nd Scientific Sessions of the American Diabetes Association, Susumo Seino, MD, PhD, Chiba University, Chiba, Japan, and other investigators[2] discussed the scientific progress that has been made in deciphering the molecular mechanisms of insulin secretion. These studies offer the prospect of new therapeutic strategies for patients with type 2 diabetes.
Type 2 diabetes is a heterogeneous disorder with varying degrees of insulin resistance and insulin secretion. Traditionally, insulin resistance has been considered to play the more prominent role in the causation of type 2 diabetes. However, the UKPDS clinical trial revealed a progressive impairment in pancreatic islet function during the course of the disease, implicating an important role for beta-cell failure in the pathogenesis of type 2 diabetes.
Type 2 diabetes progresses from a stage of normal glucose tolerance through prediabetes to overt type 2 diabetes. This progression is associated with minimal changes in the degree of insulin resistance; however, insulin secretion is progressively blunted with transition from prediabetes to overt diabetes. Thus, beta-cell failure or dysfunction is inherently associated with type 2 diabetes and may precede the onset of hyperglycemia. There is indeed evidence that beta-cell mass is reduced in type 2 diabetes.[3]
Although the mechanisms that cause beta-cell dysfunction remain unclear, it is believed that metabolic perturbations -- including hyperglycemia itself -- may be important causative factors. Thus, therapeutic interventions to maintain euglycemia during the prediabetes stage may "protect" beta cells from failing. When the beta cells do fail, overt type 2 diabetes results.
These facts highlight the importance of studying the mechanisms that regulate beta-cell function, since knowledge of these processes may aid our understanding of the pathophysiology of beta-cell dysfunction and aid in the development of new therapeutic agents to prevent or reverse these defects. Further, successful genetic-engineering approaches to the development of new sources of islet cells for transplantation will require a better understanding of the biology of beta cells.
The secretion of insulin from pancreatic beta cells is a complex process involving the integration and interaction of multiple external and internal stimuli. Thus, nutrients, hormones, neurotransmitters, and drugs all activate -- or inhibit -- insulin release. The primary stimulus for insulin secretion is the beta-cell response to changes in ambient glucose. Normally, glucose induces a biphasic pattern of insulin release. First-phase insulin release occurs within the first few minutes after exposure to an elevated glucose level; this is followed by a more enduring second phase of insulin release. Of particular importance is the observation that first-phase insulin secretion is lost in patients with type 2 diabetes. Thus, molecular mechanisms involved in phasic insulin secretion are important. Dr. Seino discussed these processes.[2]
A widely accepted sequence of events involved in glucose-induced insulin secretion is as follows:
1. Glucose is transported into beta cells through facilitated diffusion of GLUT2 glucose transporters.
2. Intracellular glucose is metabolized to ATP.
3. Elevation in the ATP/ADP ratio induces closure of cell-surface ATP-sensitive K+ (KATP) channels, leading to cell membrane depolarization.
4. Cell-surface voltage-dependent Ca2+ channels (VDCC) are opened, facilitating extracellular Ca2+ influx into the beta cell.
5. A rise in free cytosolic Ca2+ triggers the exocytosis of insulin.
The KATP channels play an integral role in glucose-stimulated insulin secretion by serving as the transducer of a glucose-generated metabolic signal (ie, ATP) to cell electrical activity (membrane depolarization). Thus, like neurons, beta cells are electrically excitable and capable of generating Ca2+ action potentials that are important in synchronizing islet cell activity and insulin release. In addition to being signal targets for glucose, KATP channels are the targets for sulfonylureas, which are commonly prescribed oral agents in the treatment of type 2 diabetes. The sulfonylureas, like glucose, induce closure of KATP channels and stimulate insulin secretion.
The beta-cell KATP channel is a complex octameric unit of 2 different proteins: the sulfonylurea receptor (SUR-1) and an inward rectifier (Kir6.2). The sulfonylurea receptor belongs to a superfamily of ATP-binding cassette proteins and contains the binding site for sulfonylurea drugs and nucleotides. The inward rectifier represents the K+ conducting pore and is also regulated by ATP. It is interesting that KATP channels are present in other tissues of the body, including heart (SUR-2A/Kir 6.2), smooth muscle (SUR-2B/Kir 6.2), and brain (SUR-1/Kir 6.2). Recently, Mark L. Evans, MD, Yale University Medical School, New Haven, Connecticut, and colleagues[4] have suggested that glucose sensing in the brain during hypoglycemia may be mediated by KATP channels located in brain hypothalamic neurons. Thus, these molecules may also serve as new therapeutic targets for the restoration of impaired hypoglycemia awareness and glucose counterregulation in type 1 diabetes.
To further examine the role of KATP channels, mouse models have been generated in which genes for either SUR-1 or Kir6.2 have been ablated. One would predict that this would lead to increased sensitivity of the beta cells to glucose. Indeed, beta cells in these SUR-1 or Kir6.2 knockout mice are constantly depolarized and exhibit constant bursts of Ca2+ action potentials, irrespective of ambient glucose levels. Surprisingly, though, these changes in cell electrical activity are not associated with increased insulin secretion, as would have been predicted. Rather, glucose- or sulfonylurea-induced insulin secretion is impaired or unaffected. By contrast, mutations in SUR-1 have been identified in humans and are associated with hyperinsulinism (persistent hyperinsulinemic hypoglycemia of infancy). It is unclear why the mouse phenotype does not resemble the human phenotype, and investigation in this area is ongoing. Possible explanations are that mice have redundant mechanisms or changes in feeding behavior to protect against hypoglycemia.
The incretins are another set of factors that are important hormonal regulators of insulin secretion.[5] The incretins are polypeptide hormones released in the gut after a meal that potentiate insulin secretion in a glucose-dependent manner. Due to their dependence on ambient glucose for action, they are emerging as important new therapeutic agents to promote insulin secretion without accompanying hypoglycemia (a common complication of sulfonylurea treatment).
Unlike sulfonylureas, incretins act by activating Gs (a G-protein that activates adenylyl cyclase) to increase cAMP in beta cells. cAmp, like ATP, is an important signal that regulates insulin release. Typically, the main mechanism of action of cAMP is by activation of an enzyme called protein kinase A (PKA) that, in turn, phosphorylates other substrates to turn on (or off) vital cell functions. Using a biochemical assay called the yeast hybrid screening method to identify and isolate new proteins, Yasushige Kashima, MD, PhD, Chiga University, Chiga, Japan, and colleagues[6] identified a novel protein, cAMP-GEF II, a cAMP sensor (cAMPS) that forms a complex with other intracellular proteins (Rim2 and Rab3) to directly regulate insulin exocytosis. Then, using molecular reagents that antagonize the effects of cAMPS, they observed that incretin-potentiated insulin secretion is attenuated. These results provide a mechanism whereby cAMP can directly promote exocytosis of insulin granules without activation of PKA (ie, a PKA-independent pathway), and thereby provide additional molecular targets for therapeutic intervention.
Extracellular Ca2+ influx through L-type voltage-dependent Ca2+ channels (VDCC) raises free cytoplasmic Ca2+ levels and triggers insulin secretion. The structure of the VDCC is complex and consists of 5 subunits: alpha1, alpha2, beta, gamma, and delta units. The alpha subunit constitutes the ion-conducting pore, whereas the other units serve a regulatory role. Previous work[7] has identified that isoforms of alpha1 subunits interact with exocytotic proteins. More recently, using the yeast hybrid screening method, a novel protein, Kir-GEM, interacting with the beta3 isoform of the VDCC, has been identified by Seino and colleagues.[8] Furthermore, it has been determined that Kir-GEM inhibits alpha ionic activity and prevents cell-surface expression of alpha subunits. The investigators have proposed that in the presence of Ca2+, Kir-GEM binds to the beta isoform, and this interaction interferes in the trafficking or translocation of alpha subunits to the plasma membrane. The relevance of Kir-GEM in insulin secretion was made evident by its attenuation of glucose-stimulated Ca2+ increases and C-peptide secretion in an insulin-secreting cell line.
The potential therapeutic role of Kir-GEM lies in the inhibitory effects on VDCC activity that may serve to protect beta cells from overstimulation and subsequent failure, which is part of the disease etiology of type 2 diabetes.
KATP channels have been identified in hypothalamic neurons and are believed to be important in glucose sensing[4]. Consistent with these observations, Takashi Miki, MD, PhD, Chiga University, Chiga, Japan, and colleagues[9] observed an attenuation of the glucagon-counterregulatory response to hypoglycemia in Kir 6.2 mice (containing ablated KATP channels). Since the glucagon response was preserved in in vitro studies of islets isolated from these animals, the investigators concluded that a functional interaction that is normally present between the hypothalamus and islet cells is destroyed in the mutant mice.
Thus, beta cells respond to an increase in glucose by inactivation (closure) of KATP channels, triggering insulin release. On the other hand, the brain (hypothalamus) responds to lowering of glucose by activation (opening) of KATP channels, which results in autonomic signaling to promote glucagon secretion from alpha cells. Glucose homeostasis is therefore achieved through an integrated response from multiple organ systems, including the pancreatic islets and the brain.
