Loss of ß-Cell Identity Occurs in Type 2 Diabetes and Is Associated With Islet Amyloid Deposits.

Loss of pancreatic islet ß-cell mass and ß-cell dysfunction are central in the development of type 2 diabetes (T2DM). We recently showed that mature human insulin-containing ß-cells can convert into glucagon-containing α-cells ex vivo. This loss of ß-cell identity was characterized by the presence of ß-cell transcription factors (Nkx6.1, Pdx1) in glucagon(+) cells. Here, we investigated whether the loss of ß-cell identity also occurs in vivo, and whether it is related to the presence of (pre)diabetes in humans and nonhuman primates. We observed an eight times increased frequency of insulin(+) cells coexpressing glucagon in donors with diabetes. Up to 5% of the cells that were Nkx6.1(+) but insulin(-) coexpressed glucagon, which represents a five times increased frequency compared with the control group. This increase in bihormonal and Nkx6.1(+)glucagon(+)insulin(-) cells was also found in islets of diabetic macaques. The higher proportion of bihormonal cells and Nkx6.1(+)glucagon(+)insulin(-) cells in macaques and humans with diabetes was correlated with the presence and extent of islet amyloidosis. These data indicate that the loss of ß-cell identity occurs in T2DM and could contribute to the decrease of functional ß-cell mass. Maintenance of ß-cell identity is a potential novel strategy to preserve ß-cell function in diabetes.

Structure-guided design of selective Epac1 and Epac2 agonists.

The second messenger cAMP is known to augment glucose-induced insulin secretion. However, its downstream targets in pancreatic ß-cells have not been unequivocally determined. Therefore, we designed cAMP analogues by a structure-guided approach that act as Epac2-selective agonists both in vitro and in vivo. These analogues activate Epac2 about two orders of magnitude more potently than cAMP. The high potency arises from increased affinity as well as increased maximal activation. Crystallographic studies demonstrate that this is due to unique interactions. At least one of the Epac2-specific agonists, Sp-8-BnT-cAMPS (S-220), enhances glucose-induced insulin secretion in human pancreatic cells. Selective targeting of Epac2 is thus proven possible and may be an option in diabetes treatment.

PKA Enhances the Acute Insulin Response Leading to the Restoration of Glucose Control.

Diabetes arises from insufficient insulin secretion and failure of the ß-cell mass to persist and expand. These deficits can be treated with ligands to Gs-coupled G-protein-coupled receptors that raise ß-cell cAMP. Here we studied the therapeutic potential of ß-cell cAMP-dependent protein kinase (PKA) activity in restoring glucose control using ß-caPKA mice. PKA activity enhanced the acute insulin response (AIR) to glucose, which is a primary determinant of the efficacy of glucose clearance. Enhanced AIR improved peripheral insulin action, leading to more rapid muscle glucose uptake. In the setting of pre-established glucose intolerance caused by diet-induced insulin resistance or streptozotocin-mediated ß-cell mass depletion, PKA activation enhanced ß-cell secretory function to restore glucose control, primarily through augmentation of the AIR. Enhanced AIR and improved glucose control were maintained through 16 weeks of a high-fat diet and aging to 1 year. Importantly, improved glucose tolerance did not increase the risk for hypoglycemia, nor did it rely upon hyperinsulinemia or ß-cell hyperplasia, although PKA activity was protective for ß-cell mass. These data highlight that improving ß-cell function through the activation of PKA has a large and underappreciated capacity to restore glucose control with minimal risk for adverse side effects.

Long-term ketogenic diet causes glucose intolerance and reduced ß- and α-cell mass but no weight loss in mice.

High-fat, low-carbohydrate ketogenic diets (KD) are used for weight loss and for treatment of refractory epilepsy. Recently, short-time studies in rodents have shown that, besides their beneficial effect on body weight, KD lead to glucose intolerance and insulin resistance. However, the long-term effects on pancreatic endocrine cells are unknown. In this study we investigate the effects of long-term KD on glucose tolerance and ß- and α-cell mass in mice. Despite an initial weight loss, KD did not result in weight loss after 22 wk. Plasma markers associated with dyslipidemia and inflammation (cholesterol, triglycerides, leptin, monocyte chemotactic protein-1, IL-1ß, and IL-6) were increased, and KD-fed mice showed signs of hepatic steatosis after 22 wk of diet. Long-term KD resulted in glucose intolerance that was associated with insufficient insulin secretion from ß-cells. After 22 wk, insulin-stimulated glucose uptake was reduced. A reduction in ß-cell mass was observed in KD-fed mice together with an increased number of smaller islets. Also α-cell mass was markedly decreased, resulting in a lower α- to ß-cell ratio. Our data show that long-term KD causes dyslipidemia, a proinflammatory state, signs of hepatic steatosis, glucose intolerance, and a reduction in ß- and α-cell mass, but no weight loss. This indicates that long-term high-fat, low-carbohydrate KD lead to features that are also associated with the metabolic syndrome and an increased risk for type 2 diabetes in humans.