II – Regulation and production of insulin

This session was opened by Donald F Steiner (Chicago, USA) who first gave a review of the protein structure of insulin, highlighting the importance of the disulfide bonds that join the A and B chains of the protein.
The identification of the prohormone precursor to insulin, proinsulin, completely revised previous models of insulin biosynthesis and enabled a better understanding of the role of glucose in this process.

Proinsulin is synthesised in the endoplasmic reticulum, and is then transported to the Golgi apparatus to be packaged into secretory vesicles. In these vesicles proinsulin is cleaved to yield C-peptide and mature insulin, and both are secreted in equimolar amounts. Thus Cpeptide can be used as a functional marker of pancreatic beta cells in vivo including in patients with type 2 diabetes. Other studies of patients with familial hyperinsulinemia have demonstrated that the base-pair residues joining the A and B chains to the C-peptide have an essential role in proinsulin processing. In patients suffering from neonatal diabetes, studies have also identified other key residues that affect the structure of proinsulin and its subsequent processing. These findings highlight that defects in insulin processing and secretion are one of the major abnormalities seen in the pathophysiology of diabetes mellitus.

Michele Solimena (Dresden, Germany) discussed recent advances in the study of the biogenesis of insulin secretory granules. The new method of histological preparation using High Pressure Freezing (HPF) enables a more accurate resolution of insulin secretory granules and exocytosis under electron microscopy. In comparison, the older technique of chemical fixation overestimated granule size due to poorer resolution, and insulin secretory granules are smaller than previously thought. Studies using HPF have established a dynamic model of insulin secretory granules with the most recently formed granules showing the greatest mobility. However, at present, no factor has been revealed that specifically influences the greater mobility and exocytosis of recently formed granules compared with others.

Continuing with studies on insulin secretory granules, Susumo Seino (Kobe, Japan), focused on the cellular mechanisms of their recruitment and exocytosis. The secretion of insulin is regulated by a number of complex intracellular signals, among which cyclic adenosine monophosphate (cAMP) is particularly important for amplifying insulin secretion. The principal stimulus of insulin secretion is glucose. However, at low concentrations, glucose alone is not sufficient to stimulate insulin secretion. This is compensated by the gastrointestinal incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulotropic peptide (GIP), which both potentiate insulin production at a given glucose concentration through increasing cAMP production in the beta-cell.

Jean Claude Henquin (Brussels, Belgium) explained that the old “consensus” model of insulin secretion is now probably obsolete. This model consists of the following chain of events: an acceleration of glucose metabolism, closure of ATP-dependent potassium channels in the cell membrane, an influx of intracellular calcium across the voltage-dependent calcium channels, and an increase in intracellular calcium, which in turn triggers excytosis of insulin secretory granules. However, recent experimental models using pharmacological inactivation of potassium channels have identified an amplification pathway, also activated by glucose, which increases the secretory response to the intracellular calcium signal under physiological conditions (Figure 2). Therefore, as well as triggering insulin secretion, glucose metabolism also amplifies the effect of intracellular calcium on exocytosis of insulin secretory granules.

Figure 2. The complex model of insulin secretion. SUR1, sulfonylurea receptor 1; Kir, inwardly rectifying K channel; ACh, acetylcholine; Epac-2, exchange protein directly activated by cAMP 2; GLP-1, glucagon-like peptide 1; PKA, protein kinase A; PKC, protein kinase C

Figure 2. The complex model of insulin secretion. SUR1, sulfonylurea receptor 1; Kir,inwardly
rectifying K channel; ACh, acetylcholine; Epac-2, exchange protein directly activated by cAMP 2;
GLP-1, glucagon-like peptide 1; PKA, protein kinase A; PKC, protein kinase C

FOCUS: Pancreatic beta cell–endothelial cell interaction

During embryonic development, beta cells initially develop as scattered, isolated cells. As development progresses, they aggregate with other beta cells and endocrine cells to form pancreatic islets. This aggregation is essential for beta-cell communication, and also involves the regulation of insulin secretion.

While glucose metabolism autonomously triggers insulin secretion, communication between beta cells reduces basal insulin secretion but increases glucose-stimulated insulin secretion. This ensures that beta cells secrete low amounts of insulin during times of starvation but sufficient amounts of insulin following intake of food. The regulation of insulin secretion through beta-cell communication involves the receptor tyrosine kinases, EphA, and their ligands, ephrin-A, both of which are coexpressed on the beta-cell membrane. Ephrin-A5-deficient mice show increased basal insulin secretion, reduced glucose-stimulated insulin secretion, and impaired glucose tolerance.4 Thus, EphA forward signalling inhibits basal insulin secretion in response to starvation, whereas glucose stimulation attenuates EphA forward signalling and allows predominant ephrin-A reverse signalling leading to increased insulin secretion.(Lammert lecture)

I- Regulation and maintenance of beta-cell mass
II – Regulation and production of insulin
III- Factors underlying beta-cell dysfunction in type 2 diabetes
IV- New tools in research and their clinical interest
Conclusions
References