As the global prevalence of diabetes continues to increase dramatically, with world-wide rates expected to nearly double by 2030, the need for new therapies is becoming more critical. Scientists have hypothesized for decades that inhibiting insulin degradation could be a potential therapeutic target for diabetes. Additionally, insulin-degrading enzyme (IDE) has been identified as a diabetes susceptibility gene. Ide-deficient mice show increased insulin levels but display glucose intolerance rather than improved glucose regulation. A recent publication in Nature (Maianti et al. 2014) describes the identification of an IDE inhibitor that improves glucose tolerance in vivo and identifies new roles for IDE in glucose homeostatsis.
Six candidate molecules with the ability to bind immobilized mouse IDE in vitro were identified from a library of nearly 14,000 synthetic macrocycles. One of these molecules, macrocycle 6b, strongly inhibited IDE in three different assays. The authors then synthesized and assayed 6b analogues in which each of the building blocks was varied, and identified 6bK as the best candidate. The selectivity of 6bK for IDE inhibition was greater than 1,000 fold over all other metalloproteases tested, which is significantly improved over the previously identified inhibitor Ii1. High levels of 6bK were found in peripheral circulation as well as the liver and kidneys (main insulin-degrading organs). Mice injected with 6bK showed lower glucose and higher insulin levels in an insulin-tolerance test, as compared to vehicle controls. These data lead the research team to examine the effects of 6bK on glucose tolerance.
C57BL/6J diet-induced obese (DIO) mice are obese and glucose intolerant but do not become hyperglycemic. Therefore, they serve as a model for pre type-2 diabetes. Glucose-tolerance tests were performed in DIO and lean C576BL/6J mice-- using either oral gavage (mimics the intake during a meal) or i.p. injections. DIO and lean C57BL/6J mice treated with 6bK displayed significantly improved glucose tolerance following oral administration of glucose (OGTT), as compared to vehicle and non-IDE inhibiting stereoisomer (bisepi-6bK) controls. The blood glucose profiles of the vehicle and bisepi-6bK controls were quite similar, which shows that effects of 6bK are due to IDE inhibition.
Curiously, when glucose was administered via i.p. injection (IPGTT) IDE inhibition with 6bK led to impaired glucose tolerance in both DIO and lean C57BL/6J mice, as compared to vehicle and bisepi-6bK controls. The DIO mice treated with 6bK showed a biphasic response, with significantly lower glucose levels over the first 30 minutes, and higher glucose levels one hour after glucose injection. The authors were unable to explain these results if IDE only degrades insulin and hypothesized that in vivo, IDE modulates other glucose-regulating hormones. They chose glucagon and amylin to further examine since IDE can also cleave those peptides in vitro.
Amylin is co-secreted with insulin. Amylin regulates glucose by inhibiting gastric emptying and promoting satiety. Glucagon raises blood glucose levels (an opposite effect to insulin). Low blood glucose levels stimulate glucagon release, which causes the liver to convert stored glycogen into glucose. Following an IPGTT, DIO mice treated with 6bK showed considerably elevated levels of amylin, insulin and glucagon compared to controls. When mice were injected with 6bK or vehicle, followed by amylin or glucagon, the 6bK-treated mice displayed significantly higher glucose levels than the control-treated mice. 6bK treatment also resulted in two-fold slower gastric emptying than controls, and those results were blocked when an amylin receptor antagonist was co-administered with 6bK.
The increased glucagon levels following 6bK treatment offers a possible explanation for the observed impaired glucose tolerance following the IPGTT. Higher insulin levels were observed during OGTTs than in the IPGTTs. During an OGTT, inhibition of IDE leads to increased insulin signaling, and lower blood glucose. During an IPGTT, less insulin is secreted and thus, proportionally, IDE is able to process more glucagon. Inhibition of IDE leads to higher glucagon levels, and ultimately higher glucose levels. To test this hypothesis, the glucose-tolerance tests were repeated using Gcgr (G-protein-coupled glucagon receptor) knockout mice. As expected, Gcgr knockout mice treated with 6bK exhibited improved glucose tolerance in the OGTT, when secreted insulin levels were high. Further, glucose tolerance was unaffected in the IPGTT, when less insulin is secreted, and 6bK action on glucagon could not be sensed due to the absence of Gcgr.
These data presented by Maianti et al. represent the first discovery of an IDE inhibitor that can improve glucose tolerance in meal-like conditions and identifies new physiological roles for IDE in glucose homeostasis, which represents a new target for diabetes therapeutics.