Insulin-degrading neutral cysteine proteinase activity of adipose tissue and liver of nondiabetic, streptozotocin-diabetic, and insulin-treated diabetic rats

Insulin-degrading enzyme: an ally against metabolic and neurodegenerative diseases

Insulin-degrading enzyme (IDE) function goes far beyond its known proteolytic role as a regulator of insulin levels. IDE has a wide substrate promiscuity, degrading several proteins such as amyloid-β peptide, glucagon, islet amyloid polypeptide (IAPP) and insulin-like growth factors, that have diverse physiological and pathophysiological functions. Importantly, IDE plays other non-proteolytical functions such as a chaperone/dead-end chaperone, an E1-ubiquitin activating enzyme, and a proteasome modulator. It also responds as a heat shock protein, regulating cellular proteostasis. Notably, amyloidogenic proteins such as IAPP, amyloid-β and α-synuclein have been reported as substrates for IDE chaperone activity. This is of utmost importance as failure of IDE may result in increased protein aggregation, a key hallmark in the pathogenesis of beta cells in type 2 diabetes mellitus and of neurons in neurodegenerative diseases such as Alzheimer's and Parkinson's disease. In this review, we focus on the biochemical and biophysical properties of IDE and the regulation of its physiological functions. We further raise the hypothesis that IDE plays a central role in the pathological context of dysmetabolic and neurodegenerative diseases and discuss its potential as a therapeutic target. This article is protected by copyright. All rights reserved.

Insulin-degrading enzyme: new therapeutic target for diabetes and Alzheimer's disease?

Insulin-degrading enzyme (IDE) is a major enzyme responsible for insulin degradation. In addition to insulin, IDE degrades many targets including glucagon, atrial natriuretic peptide, and beta-amyloid peptide, regulates proteasomal degradation and other cell functions. IDE represents a pathophysiological link between type 2 diabetes (T2DM) and late onset Alzheimer's disease (AD). Potent and selective modulators of IDE activity are potential drugs for therapies of both diseases. Acute treatment with a novel IDE inhibitor was recently tested in a mouse study as a therapeutic approach for the treatment of T2DM. In contrast, effective IDE activators can be used for the AD treatment. However, because of the pleiotropic IDE action, the sustained treatment with systemic IDE modulators should be carefully tested in animal studies. Development of substrate-selective IDE modulators could overcome possible adverse effects of IDE modulators associated with multiplicity of IDE targets. KEY MESSAGES Insulin-degrading enzyme (IDE) represents a pathophysiological link between type 2 diabetes (T2DM) and Alzheimer's disease (AD). Selective modulators of IDE activity are potential drugs for both T2DM and AD treatment. Development of substrate-selective IDE modulators could overcome possible adverse effects of IDE modulators associated with multiplicity of IDE targets.

Glucose inhibits the insulin-induced activation of the insulin-degrading enzyme in HepG2 cells

AIMS/HYPOTHESIS

Hepatic insulin degradation decreases in type 2 diabetes. Insulin-degrading enzyme (IDE) plays a key role in insulin degradation and its gene is located in a diabetes-associated chromosomal region. We hypothesised that IDE may be regulated by insulin and/or glucose in a liver cell model. To validate the observed regulation of IDE in vivo, we analysed biopsies of human adipose tissue during different clamp experiments in men.

METHODS

Human hepatoma HepG2 cells were incubated in normal (1 g/l) or high (4.5 g/l) glucose medium and treated with insulin for 24 h. Catalytic activity, mRNA and protein levels of IDE were assessed. IDE mRNA levels were measured in biopsies of human subcutaneous adipose tissue before and at 240 min of hyperinsulinaemic, euglycaemic and hyperglycaemic clamps.

RESULTS

In HepG2 cells, insulin increased IDE activity under normal glucose conditions with no change in IDE mRNA or protein levels. Under conditions of high glucose, insulin increased mRNA levels of IDE without changes in IDE activity. Both in normal and high glucose medium, insulin increased levels of the catalytically more active 15a IDE isoform compared with the 15b isoform. In subcutaneous adipose tissue, IDE mRNA levels were not significantly upregulated after euglycaemic or hyperglycaemic clamps.

CONCLUSIONS/INTERPRETATION

Insulin increases IDE activity in HepG2 cells in normal but not in high glucose conditions. This disturbance cannot be explained by corresponding alterations in IDE protein levels or IDE splicing. The loss of insulin-induced regulation of IDE activity under hyperglycaemia may contribute to the reduced insulin extraction and peripheral hyperinsulinaemia in type 2 diabetes.

Partial characterisation of human and porcine adipose acidic protease activity

1. An aspartic protease was isolated from human and porcine white adipose tissue and from isolated human adipocytes. The three preparations appeared to represent the same enzyme. 2. Electrofocusing of all three preparations revealed two bands corresponding to a pI of 3.6 and 4.4. respectively. On PAGE a single band in the same position was obtained in all three cases. 3. Both the porcine and human fractions were optimally active at pH 3.4, using acid denatured haemoglobin as substrate, and both activities were strongly inhibited by pepstatin and iodoacetate. 4. The Km values for haemoglobin for the porcine and human proteases were 0.16 and 0.14 mM respectively, whereas Vmax values of 30 and 33 units.nmol-1, respectively, were obtained.

Characterization of a cDNA for human glutathione-insulin transhydrogenase (protein-disulfide isomerase/oxidoreductase)

A human liver cDNA expression library in lambda-phage gt11 was screened with monoclonal antibodies to rat liver protein-disulfide isomerase/oxidoreductase (EC 5.3.4.1/1.8.4.2), also known as glutathione-insulin transhydrogenase (GIT). The nucleotide sequence of the largest cDNA insert (hgit-1) was determined. It contained approx. 1500 basepairs, representing an estimated 65% of the glutathione-insulin transhydrogenase message. The amino-acid sequence deduced from this cDNA insert contains a 7-amino-acid long polypeptide determined by sequencing the active-site fragment isolated from the rat GIT protein. A comparison of the nucleotide sequence of hgit-1 and a previously reported nucleotide sequence of rat glutathione-insulin transhydrogenase cDNA shows that the human hgit-1 clone corresponds to the middle of the transhydrogenase message at amino-acid residue number 275 of the rat protein, and codes for 206 amino-acid residues, including one of the two active-site regions of glutathione-insulin transhydrogenase, a stop codon (TAA), a long 3'-noncoding region of over 800 bases, a polyadenylation signal (AATAA), and a 29 base poly(A) tail. There exists high homology between the human and rat enzymes (94% in the overall amino-acid sequence, with 100% in the active site region and 81% in the nucleotide sequence within the coding portion of hgit-1). As with the rat enzyme, the human enzyme shows some identity with another dithiol-disulfide-exchange protein, Escherichia coli thioredoxin. Like rat cDNA, the human hgit-1 cDNA hybridized to rat mRNA of 2500 bases on a Northern blot. The relative quantitative abundance of GIT mRNA in nine rat tissues studied using hgit-1 as a hybridization probe was found to be in the same order as previously found with the rat cDNA. Thus, the above studies indicate that glutathione-insulin transhydrogenase is a highly conserved protein and that the human hgit-1 cDNA is suitable for use as a probe for further studies on gene regulation of this enzyme.