The complex metabolic disturbances seen in diabetes are not just related to alterations in glucose homeostasis, but also include changes in fat and protein metabolism. Each change cannot be considered in isolation, since changes in one of these factors will lead to alterations in another. In the insulin-resistant state, glucose levels increase and there is also relative hyperinsulinaemia, which is exacerbated by a reduction in insulin degradation under these conditions. The process of insulin metabolism starts with binding of insulin to its receptor on the cell surface. The insulin receptor is then internalised into endosomes and degradation begins . Insulin degradation occurs by the action of insulin-degrading enzyme (IDE) , which is responsible for the majority of cellular insulin metabolism (reviewed in ). Although the majority of IDE is cytosolic, it is also found in endosomes, peroxisomes and mitochondria [3–5]. It is present in all cell types, not just those responsive to insulin, and in all organisms, from yeast to mammals [1, 6], suggesting that IDE has functions in addition to its degradative one. There is increasing evidence of a regulatory role for IDE in a variety of cell functions. Some of these include regulation of the proteasome [7–9], androgen and glucocorticoid receptors , peroxisome , myoblast differentiation , lymphocyte antigen presentation  and yeast bud site selection .
Insulin metabolism is altered in patients with type 2 diabetes, and human genetic studies have linked polymorphisms in the gene encoding IDE to an increased risk of type 2 diabetes  and of Alzheimer’s disease [15, 16]. Knockdown of Ide in mice causes insulin resistance and hyperglycaemia, and plaques similar to those seen in Alzheimer’s disease are also found . The Goto–Kakizaki (GK) rat has a mutation in IDE that causes altered cellular insulin degradation  in addition to other characteristics typical of type 2 diabetes.
Despite the obvious importance of IDE in the control of insulin metabolism and other metabolic processes, the metabolic control of IDE has not been extensively studied. It has been shown that certain NEFA inhibit IDE  and that other small molecules may interact with and inhibit IDE activity [20, 21]. But more information on the control of IDE is needed.
In this issue of Diabetologia, Pivovarova et al.  present the results of a study of the effects of glucose and insulin exposure on the activity and levels of IDE. They show that insulin increases the activity of IDE in extracts of cells that have been treated for 24 h with 10 mol/l of insulin. If the glucose concentration is increased from 1 g/l (5.55 mmol/l; ‘normal’ glucose) to 4.5 g/l (25 mmol/l; ‘high’ glucose), the increase in IDE activity promoted by insulin is abolished. This suggests that the reduction in insulin metabolism seen in cases of type 2 diabetes may be due to an effect of hyperglycaemia. How this insulin-induced increase in IDE activity occurs is unknown. The authors measured both mRNA and protein levels, and neither were altered by insulin under conditions of normal glucose concentrations. These findings suggest a change in the kinetic properties of the enzyme, such as the K m or V max, which could be measured in future experiments in enzyme preparations isolated from cells treated with glucose and/or insulin. Increasing the glucose levels per se did not change the expression of IDE mRNA. The addition of insulin to the high-glucose conditions increased levels of IDE mRNA by approximately 30% relative to the levels in the non-insulin-treated cells under high-glucose conditions. Since neither the levels nor the activity of the protein are altered, this difference does not appear to be biologically significant.
It has been shown that alternative splicing of IDE pre-mRNA can produce an inactive form of the enzyme . Pivovarova et al.  show that glucose levels do not affect the levels of either the 15a (active) or 15b (inactive) splice variants. However, insulin treatment of cells in the presence of either normal or high glucose levels increases the expression of the 15a splice variant; levels of the inactive variant are not altered. This correlates with the data on the activity of IDE under normal glucose conditions, but does not agree with the observations under high glucose. Incubation of adipocytes with high insulin concentrations causes insulin to be internalised and processed via a non-degradative pathway , and retroendocytosis is increased .
One limitation of this study  is the use of cell extracts to determine IDE activity. It would be interesting to see what the effect of glucose and insulin concentrations were on whole cell IDE activity (i.e. insulin degradation). If changes in IDE activity are seen in whole cells, the effects on the regulatory actions of IDE should be investigated. This could show that changes in glucose and/or insulin levels alter IDE activity and then alter, for example, proteasome activity, changing total cellular protein degradation. As stated at the beginning of this Commentary, each symptom of diabetes is not seen in isolation—many must interact.
In the clinical situation, alterations in glucose and/or insulin levels would alter the activity of IDE, causing changes in the rate of removal of circulating insulin. Whether the changes reported by Pivovarova et al.  are localised to the liver or occur in other insulin-sensitive tissues (e.g. muscle, fat, and kidney) remains to be determined. The investigators did not show an effect of hyperglycaemic and/or hyperinsulinaemic clamps in humans on the expression of IDE mRNA in subcutaneous fat . This could be due to the relatively short duration of the clamp studies compared with the in vitro experiments or may instead be the result of the investigation of a different cell/tissue type.
Insulin metabolism is an important part of insulin action. Insulin metabolism should be considered very carefully in the clinical situation. Altered metabolism of insulin will cause the action of externally administered insulins (including insulin analogues) to be changed. In some cases, insulin analogues are metabolised differently from native insulin even in the ‘normal’ state .
The cellular response to insulin is controlled by the removal of insulin from the circulation and its degradation. Figure 1 shows a schematic representation of insulin degradation and its interaction with insulin action on both tyrosine kinase cascades and protein degradation. Insulin binds to the insulin receptor on the cell surface, causing receptor autophosphorylation and activating the receptor tyrosine kinase . This activates other molecules within the cascade, starting with IRS proteins and Src homology 2 domain-containing (SHC) proteins [27, 28]. Multiple other proteins are phosphorylated, leading to an increase in glucose uptake (in muscle and fat), glycogen synthesis and mitogenesis. These actions of insulin will be terminated by insulin degradation and subsequent dephosphorylation of the receptor . Following insulin binding, the receptor is internalised into endosomes, where degradation begins . Insulin receptor is recycled to the cell surface along with degraded and intact insulin. Intact and degraded insulin also enter the cytosol, where insulin binds to cytosolic IDE, some of which is associated with the proteasome. The interaction of IDE with the proteasome increases proteasome activity . Binding of insulin to IDE causes the dissociation of the IDE–proteasome complex and, thus, the inhibition of proteasome activity . Following insulin degradation, IDE will reassociate with the proteasome. As shown by Pivovarova et al. , when insulin levels are increased (under ‘normal’ glucose conditions), insulin degradation increases, which will cause the activity of insulin on glucose metabolism and growth to be abrogated. However, the degradative process is also involved in certain actions of insulin, including inhibition of protein degradation [30, 31]. An increase in insulin degradation by an increase in insulin levels, as seen in the study by Pivovarova and colleagues under normal glucose conditions , would increase the inhibition of protein degradation. The abolishment of this increased insulin degradation by hyperglycaemia would reduce the inhibition of protein degradation by insulin. This could lead to serious protein wasting if left unchecked. In patients with type 2 diabetes, who tend to have hyperinsulinaemia and hyperglycaemia, the activity of IDE (at least in the liver) will be reduced, pushing insulin levels in the blood even higher.
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W. C. Duckworth is financially supported by the Office of Research and Development, Medical Research Service, US Department of Veterans Affairs. The contents of this manuscript do not represent the views of the Department of Veterans Affairs or the US Government.
Duality of interest
The authors declare that there is no duality of interest associated with this manuscript.
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Fawcett, J., Duckworth, W.C. Hyperglycaemia and hyperinsulinaemia: is insulin-degrading enzyme the missing link?. Diabetologia 52, 1457 (2009). https://doi.org/10.1007/s00125-009-1414-8
- Insulin action
- Insulin degradation
- Insulin-degrading enzyme
- Protein metabolism