Introduction

Diabetes mellitus (DM) is a syndrome of chronic hyperglycaemia due to relative insulin deficiency, resistance or both. It affects more than 220 million people worldwide, and it is estimated that it will affect 440 million by the year 2030. Diabetes is usually irreversible and, although patients can lead a reasonably normal lifestyle, its late complications result in reduced life expectancy and major health costs. These include macrovascular disease, leading to an increased prevalence of coronary artery disease, peripheral vascular disease and stroke, and microvascular damage causing diabetic retinopathy and nephropathy. Neuropathy is another major complication.

Insulin structure and secretion

Insulin is the key hormone involved in the storage and controlled release within the body of the chemical energy available from food. It is coded for on chromosome 11 and synthesized in the beta cells of the pancreatic islets (Fig. 20.1). The synthesis, intracellular processing and secretion of insulin by the beta cell is typical of the way that the body produces and manipulates many peptide hormones. Figure 20.2 illustrates the cellular events triggering the release of insulin-containing granules. After secretion, insulin enters the portal circulation and is carried to the liver, its prime target organ. About 50% of secreted insulin is extracted and degraded in the liver; the residue is broken down by the kidneys. C-peptide is only partially extracted by the liver (and hence provides a useful index of the rate of insulin secretion) but is mainly degraded by the kidneys.

Figure 20.1 Part of a beta cell. The ribosomes manufacture pre-proinsulin from insulin mRNA. The hydrophobic ‘pre’ portion of pre-proinsulin allows it to transfer to the Golgi apparatus, and is subsequently enzymatically cleaved off. Proinsulin is parcelled into secretory granules in the Golgi apparatus. These mature and pass towards the cell membrane where they are stored before release. The proinsulin molecule folds back on itself and is stabilized by disulphide bonds. The biochemically inert peptide fragment known as connecting (C) peptide splits off from proinsulin in the secretory process, leaving insulin as a complex of two linked peptide chains. Equimolar quantities of insulin and C-peptide are released into the circulation via the ‘regulated pathway’. A small amount of insulin is secreted by the beta cell directly via the ‘constitutive pathway’, which bypasses the secretory granules.

Figure 20.2 Local forces regulating insulin secretion from beta cells. Glucose enters the beta cell via the GLUT-2 transporter protein, which is closely associated with the glycolytic enzyme glucokinase. Metabolism of glucose within the beta cell generates ATP. ATP closes potassium channels in the cell membrane (a). If a sulfonylurea binds to its receptor, this also closes potassium channels. Closure of potassium channels predisposes to cell membrane depolarization, allowing calcium ions to enter the cell via calcium channels in the cell membrane (b). The rise in intracellular calcium triggers activation of calcium-dependent phospholipid protein kinase which, via intermediary phosphorylation steps, leads to fusion of the insulin-containing granules with the cell membrane and exocytosis of the insulin-rich granule contents. Similar mechanisms produce hormone-granule secretion in many other endocrine cells.

An outline of glucose metabolism

Blood glucose levels are closely regulated in health and rarely stray outside the range of 3.5–8.0 mmol/L (63–144 mg/dL), despite the varying demands of food, fasting and exercise. The principal organ of glucose homeostasis is the liver, which absorbs and stores glucose (as glycogen) in the post-absorptive state and releases it into the circulation between meals to match the rate of glucose utilization by peripheral tissues. The liver also combines 3-carbon molecules derived from breakdown of fat (glycerol), muscle glycogen (lactate) and protein (e.g. alanine) into the 6-carbon glucose molecule by the process of gluconeogenesis.

Glucose production

About 200 g of glucose is produced and utilized each day. More than 90% is derived from liver glycogen and hepatic gluconeogenesis, and the remainder from renal gluconeogenesis.

Glucose utilization

The brain is the major consumer of glucose, and its function depends upon an uninterrupted supply of this substrate. Its requirement is 1 mg/kg bodyweight per minute, or 100 g daily in a 70 kg man. Glucose uptake by the brain is obligatory and is not dependent on insulin, and the glucose used is oxidized to carbon dioxide and water. Other tissues, such as muscle and fat, are facultative glucose consumers. The effect of insulin peaks associated with meals is to lower the threshold for glucose entry into cells; at other times, energy requirements are largely met by fatty-acid oxidation. Glucose taken up by muscle is stored as glycogen or metabolized to lactate or carbon dioxide and water. Fat uses glucose as a source of energy and as a substrate for triglyceride synthesis; lipolysis releases fatty acids from triglyceride together with glycerol, a substrate for hepatic gluconeogenesis.

Hormonal regulation

Insulin is a major regulator of intermediary metabolism, although its actions are modified in many respects by other hormones. Its actions in the fasting and postprandial states differ (Fig. 20.3). In the fasting state, its main action is to regulate glucose release by the liver, and in the postprandial state, it additionally promotes glucose uptake by fat and muscle. The effect of counter-regulatory hormones (glucagon, epinephrine (adrenaline), cortisol and growth hormone) is to cause greater production of glucose from the liver and less utilization of glucose in fat and muscle for a given level of insulin.

Figure 20.3 Fasting and postprandial effects of insulin. In the fasting state, insulin concentrations are low and it acts mainly as a hepatic hormone, modulating glucose production (via glycogenolysis and gluconeogenesis) from the liver. Hepatic glucose production rises as insulin levels fall. In the postprandial state insulin concentrations are high and it then suppresses glucose production from the liver and promotes the entry of glucose into peripheral tissues (increased glucose utilization).

Glucose transport

Cell membranes are not inherently permeable to glucose. A family of specialized glucose-transporter (GLUT) proteins carry glucose through the membrane into cells.

GLUT-1 – enables basal non-insulin-stimulated glucose uptake into many cells (see Fig. 6.29).

GLUT-2 – transports glucose into the beta cell, a prerequisite for glucose sensing, and is also present in the renal tubules and hepatocytes.

GLUT-3 – enables non-insulin-mediated glucose uptake into brain neurones and placenta.

GLUT-4 – enables much of the peripheral action of insulin. It is the channel through which glucose is taken up into muscle and adipose tissue cells following stimulation of the insulin receptor (Fig. 20.4).

Figure 20.4 Insulin signalling in peripheral cells (e.g. muscle and adipose tissue). The insulin receptor consists of α- and β-subunits linked by disulphide bridges (top right of figure). The β-subunits straddle the cell membrane. The transporter protein GLUT-4 (bottom left of figure) is stored in intracellular vesicles. The binding of insulin to its receptor initiates many intracellular actions including translocation of these vesicles to the cell membrane, carrying GLUT-4 with them; this allows glucose transport into the cell.

The insulin receptor

This is a glycoprotein (400 kDa), coded for on the short arm of chromosome 19, which straddles the cell membrane of many cells (Fig. 20.4). It consists of a dimer with two α-subunits, which include the binding sites for insulin, and two β-subunits, which traverse the cell membrane. When insulin binds to the α-subunits it induces a conformational change in the β-subunits, resulting in activation of tyrosine kinase and initiation of a cascade response involving a host of other intracellular substrates. One consequence of this is migration of the GLUT-4 glucose transporter to the cell surface and increased transport of glucose into the cell. The insulin-receptor complex is then internalized by the cell, insulin is degraded, and the receptor is recycled to the cell surface.

Genetic susceptibility and inheritance

Increased susceptibility to type 1 diabetes is inherited, but the disease is not genetically predetermined. The identical twin of a patient with type 1 diabetes has a 30–50% chance of developing the disease, which implies that non-genetic factors must also be involved. The risk of developing diabetes by age 20, curiously, is greater with a diabetic father (3–7%) than with a diabetic mother (2–3%). If one child in a family has type 1 diabetes, each sibling has a ~6% risk of developing diabetes by age 20. This risk rises to about 20% in HLA-identical siblings who have the same HLA type as the proband. Since type 1 diabetes can present at any age, the lifetime risk for a sibling or child is at least double the risk by age 20.

HLA system

The HLA genes on chromosome 6 are highly polymorphic and modulate the immune defence system of the body. More than 90% of patients with type 1 diabetes carry HLA-DR3-DQ2, HLA-DR4-DQ8 or both, as compared with some 35% of the background population. All DQB1 alleles with an aspartic acid at residue 57 confer neutral to protective effects with the strongest effect from DQB1*0602 (DQ6), while DQB1 alleles with an alanine at the same position (i.e. DQ2 and DQ8) confer strong susceptibility. Genotypic combinations have a major influence upon risk of disease. For example, HLA DR3-DQ2/HLA DR4-DQ8 heterozygotes have a considerably increased risk of disease, and some HLA class I alleles also modify the risk conferred by class II susceptibility genes.

Autoimmunity and type 1 diabetes

Pancreatic islet showing infiltration by chronic inflammatory cells (insulitis).

Type 1 diabetes is associated with other organ-specific autoimmune diseases including autoimmune thyroid disease, coeliac disease, Addison’s disease and pernicious anaemia. Autopsies of patients who died following diagnosis of type 1 diabetes show infiltration of the pancreatic islets by mononuclear cells. This appearance, known as insulitis, resembles that in other autoimmune diseases such as thyroiditis. Several islet antigens have been characterized, and these include insulin itself, the enzyme glutamic acid decarboxylase (GAD), protein tyrosine phosphatase (IA-2) (Fig. 20.6) and the cation transporter ZnT8. Recent studies have shown that GAD immunotherapy has no benefit. The observation that treatment with immunosuppressive agents such as ciclosporin prolongs beta-cell survival in newly diagnosed patients has confirmed that the disease is immune-mediated.

Figure 20.6 Islet autoantibodies. Islet cell antibodies are detected by a fluorescent antibody technique which detects binding of autoantibodies to islet cells. Much of this staining reaction is due to antibodies specific for glutamic acid decarboxylase (GAD) and protein tyrosine phosphatase – IA-2 (also known as ICA512). Not all the staining seen with ICA is due to these two autoantibodies, so it is assumed that other islet autoantibodies are also involved. Insulin autoantibodies also appear in the circulation but do not contribute to the ICA reaction.

Environmental factors

The incidence of childhood type 1 diabetes is rising across Europe at the rate of 2–3% each year, suggesting that environmental factor(s) are involved in its pathogenesis. Islet autoantibodies (see above) appear in the first few years of life, indicating prenatal or early postnatal interactions with the environment. Exposures to dietary constituents, enteroviruses such as Coxsackie B4 and relative deficiency of vitamin D are possible candidates, but their role in the causation of the disease has yet to be confirmed. A cleaner environment with less early stimulation of the immune system in childhood may increase susceptibility for type 1 diabetes, as for atopic/allergic conditions (the hygiene hypothesis) (see p. 824), and more rapid weight gain in childhood and adolescence leading to increased insulin resistance might accelerate clinical onset (the accelerator hypothesis).

Pre-type 1 diabetes and prevention of type 1 diabetes

Children who test positive for two or more autoantibodies have a >80% risk of progression to diabetes, and the risk approaches 100% in those who additionally lose their first phase insulin response to intravenous glucose and/or develop glucose intolerance. The ability to predict type 1 diabetes with this degree of precision has opened the way to trials of disease prevention, but intervention before clinical onset of diabetes has so far proved unsuccessful.

Inheritance

Identical twins of patients with type 2 diabetes have >50% chance of developing diabetes; the risk to non-identical twins or siblings is of the order of 25%, confirming a strong inherited component to the disease. Type 2 diabetes is a polygenic disorder, and, as with type 1 diabetes, genome-wide studies of associations between common DNA variants and disease have allowed identification of numerous susceptibility loci. Several of these loci subserve beta-cell development or function, and there is no overlap with the immune function genes identified for type 1 diabetes. There is no major gene susceptibility, involving the HLA region. However, transcription factor-7-like (TCF7-L2) is the most common variant observed in type 2 diabetes in Europeans, and KCNQ1 (a potassium voltage-gated channel) in Asians. TCF7-L2 carries an increased risk of around 35%, while other common variants account for no more than 10–20%. TCF7-L2 has now been shown to modulate pancreatic islet cell function. Paradoxically, the genes for type 2 diabetes account for a relatively small fraction of its observed heritability. They do not allow subtypes of the condition to be identified with any confidence, or provide useful disease prediction.

Environmental factors: early and late

An association has been noted between low weight at birth and at 12 months of age and glucose intolerance later in life, particularly in those who gain excess weight as adults. The concept is that poor nutrition early in life impairs beta-cell development and function, predisposing to diabetes in later life. Low birthweight has also been shown to predispose to heart disease and hypertension.

Inflammation

Subclinical inflammatory changes are characteristic of both type 2 diabetes and obesity, and in diabetes, high-sensitivity C-reactive protein (CRP) levels are modestly elevated in association with raised fibrinogen and increased plasminogen activator inhibitor-1 (PAI-1), and contribute to cardiovascular risk. Circulating levels of the pro-inflammatory cytokines TNF-α and IL-6 are elevated in both diabetes and obesity.

Abnormalities of insulin secretion and action

The relative role of secretory failure versus insulin resistance in the pathogenesis of type 2 diabetes has been much debated, but even massively obese individuals with a fully functioning beta-cell mass do not necessarily develop diabetes, which implies that some degree of beta-cell dysfunction is necessary. Insulin binds normally to its receptor on the surface of cells in type 2 diabetes, and the mechanisms of ‘insulin resistance’ are still poorly understood. Insulin resistance is, however, associated with central obesity and accumulation of intracellular triglyceride in muscle and liver in type 2 diabetes, and a high proportion of patients have non-alcoholic fatty liver disease (NAFLD), see page 303. It has long been stated that patients with type 2 diabetes retain up to 50% of their beta-cell mass at the time of diagnosis, as compared with healthy controls, but the shortfall is greater than this when they are matched with healthy individuals who are equally obese. In addition, patients with type 2 diabetes almost all show islet amyloid deposition at autopsy, derived from a peptide known as amylin or islet amyloid polypeptide (IAPP), which is co-secreted with insulin. It is not known if this is a cause or consequence of beta-cell secretory failure.

Abnormalities of insulin secretion manifest early in the course of type 2 diabetes. An early sign is loss of the first phase of the normal biphasic response to intravenous insulin. Established diabetes is associated with hypersecretion of insulin by a depleted beta-cell mass. Circulating insulin levels are therefore higher than in healthy controls, although still inadequate to restore glucose homeostasis. Relative insulin lack is associated with increased glucose production from the liver (owing to inadequate suppression of gluconeogenesis) and reduced glucose uptake by peripheral tissues. Hyperglycaemia and lipid excess are toxic to beta cells, at least in vitro, a phenomenon known as glucotoxicity, and this is thought to result in further beta-cell loss and further deterioration of glucose homeostasis. Circulating insulin levels are typically higher than in non-diabetics following diagnosis and tend to rise further, only to decline again after months or years due to secretory failure, an observation sometimes referred to as the ‘Starling curve’ of the pancreas. Type 2 diabetes is thus a condition in which insulin deficiency relative to increased demand leads to hypersecretion of insulin by a depleted beta-cell mass and progression towards absolute insulin deficiency requiring insulin therapy. Its time course varies widely between individuals.

Physical examination at diagnosis

Evidence of weight loss and dehydration may be present, and the breath may smell of ketones. Older patients may present with established complications, and the presence of the characteristic retinopathy is diagnostic of diabetes. In occasional patients, there will be physical signs of an illness causing secondary diabetes (see Table 20.1). Patients with severe insulin resistance may have acanthosis nigricans, which is characterized by blackish pigmentation at the nape of the neck and in the axillae (p. 1217).