The recent molecular, biologic, and genetic understanding of the inflammasome has revolutionized the diagnosis of and therapy for the phenotypically heterogeneous group of rare oligogenic disorders, now recognized to have autoinflammatory origin. This article reviews the importance of inflammasome activation in the central and peripheral mechanisms underlying a common, multifactorial, lifestyle-related, and polygenetic disease (type 2 diabetes mellitus), and conceptualizes the notion that this health challenge should now be recognized to have an autoinflammatory cause. It is hoped that targeting these mechanisms will enable the introduction of novel therapies that attack the basic pathogenetic mechanisms of type 2 diabetes mellitus rather than the epiphenomena that are its consequences.
Key points
- •
Type 2 diabetes mellitus shares features with autoinflammatory disorders and is known for its recurrent inflammatory skin complications.
- •
Inhibitory treatments of aberrant inflammasome activation that dramatically cure the diverse rashes, erythemas, hives, pustoloses, and pyodermas of rare autoinflammatory disorders may have a place in the therapy for common disorders, such as type 2 diabetes mellitus, and thereby it is hoped also reduce its dermatologic complications.
The skin and the pancreatic islets as autoinflammatory targets: homage to Paul Langerhans
To the dermatologist the name of the German histopathologist Paul Langerhans (1847–1888) is as inseparably connected with the dendritic epidermal Langerhans cells as it is with the pancreatic islets of Langerhans to the diabetologist. Langerhans trained at the Friedrich Wilhelm Universität in Berlin with his main mentors Rudolf Virchow (1821–1902) and Julius Cohnheim (1839–1884). While applying the gold chloride staining developed by Julius Cohnheim to study cutaneous innervation in Virchow’s laboratory at the Charité Institute of Pathology in Berlin, Langerhans discovered already as an undergraduate student in 1868 the dendritic cells in the epidermis, which he erroneously classified as neuronal cells because of their stellate appearance. Only a year later in his doctorate thesis he described the pancreatic islets and suggested that they were small intrapancreatic lymph nodes.
Tragically, at the age of 41 Paul Langerhans succumbed to the inflammatory consequences of disseminated tuberculosis that he contracted as a 27-year-old Chair of Pathology at the University of Freiburg. He died unknowingly of the functions of the cells that to date bear his name. It would no doubt have been gratifying to him to realize that the Langerhans cells of the skin belong to the innate immune system, and that the pancreatic islets of Langerhans constitute the endocrine pancreas. Most certainly he would have been amazed if he had lived to take part in the progress that in the last decade has united the two anatomically remote and apparently functionally disparate cell types he discovered: that both the Langerhans cell of the skin and the pancreatic β-cell of the islets of Langerhans strongly express the protein complex that is the subject of this special issue of Dermatologic Clinics and caused much of the symptomatology that haunted him the last 14 years of his life: the inflammasome.
The skin and the pancreatic islets as autoinflammatory targets: homage to Paul Langerhans
To the dermatologist the name of the German histopathologist Paul Langerhans (1847–1888) is as inseparably connected with the dendritic epidermal Langerhans cells as it is with the pancreatic islets of Langerhans to the diabetologist. Langerhans trained at the Friedrich Wilhelm Universität in Berlin with his main mentors Rudolf Virchow (1821–1902) and Julius Cohnheim (1839–1884). While applying the gold chloride staining developed by Julius Cohnheim to study cutaneous innervation in Virchow’s laboratory at the Charité Institute of Pathology in Berlin, Langerhans discovered already as an undergraduate student in 1868 the dendritic cells in the epidermis, which he erroneously classified as neuronal cells because of their stellate appearance. Only a year later in his doctorate thesis he described the pancreatic islets and suggested that they were small intrapancreatic lymph nodes.
Tragically, at the age of 41 Paul Langerhans succumbed to the inflammatory consequences of disseminated tuberculosis that he contracted as a 27-year-old Chair of Pathology at the University of Freiburg. He died unknowingly of the functions of the cells that to date bear his name. It would no doubt have been gratifying to him to realize that the Langerhans cells of the skin belong to the innate immune system, and that the pancreatic islets of Langerhans constitute the endocrine pancreas. Most certainly he would have been amazed if he had lived to take part in the progress that in the last decade has united the two anatomically remote and apparently functionally disparate cell types he discovered: that both the Langerhans cell of the skin and the pancreatic β-cell of the islets of Langerhans strongly express the protein complex that is the subject of this special issue of Dermatologic Clinics and caused much of the symptomatology that haunted him the last 14 years of his life: the inflammasome.
The pathogenesis of type 2 diabetes mellitus
Before reviewing the growing evidence that type 2 diabetes mellitus (T2D) has an autoinflammatory origin, the following list summarizes inflammation in its metabolic context.
- •
T2D is the metabolic consequence of failure of the insulin-producing pancreatic β-cell to compensate for increased insulin needs.
- •
Most commonly, insulin resistance caused by obesity is the reason for increased insulin needs; puberty, pregnancy, and certain drugs are additional causes.
- •
Sedentary lifestyle and inappropriate quality and quantity of foods mediate inflammatory and neurohumoral alterations in appetite regulation, thermogenesis, satiety, and food choices believed to instigate a vicious cycle that contribute to obesity.
- •
The accumulation of fats, particularly in visceral depots, alters adipocyte differentiation and size that leads to alterations in adipose tissue blood flow, hypoxia, and shear stress activating the transcription, translation, and processing of proinflammatory cytokines and adipokines.
- •
Adipocytokines elicit a systemic low-grade inflammatory response characterized by discretely elevated C-reactive protein (CRP) driven in particular by circulating interleukin (IL)-1 and IL-6.
- •
Intrahepatic fat deposition contributes to local inflammation that may progress into nonalcoholic steatohepatitis and potentiate the systemic inflammatory response.
- •
Circulating proinflammatory cytokines may amplify insulin resistance by interfering directly with the insulin signaling cascade in liver, skeletal muscle, fat, and pancreatic β-cells and stimulate proinflammatory gene transcription in these tissues and in the hypothalamus, further contributing to neurohumoral dysregulation of metabolism; however, clinical proof-of-principle is lacking.
- •
The increased insulin need caused by insulin resistance is initially compensated by expansion of the functional β-cell mass and secretory hyperactivity leading to hyperinsulinemia.
- •
Insulin is a potent macrophage chemoattractant and compensatory hypersecretion may be a primary cause for increased recruitment of islet macrophages.
- •
With insulin, islet amyloid polypeptide (IAPP) and extracellular danger-associated molecular patterns (DAMPs), such as ATP, are secreted. IAPP and ATP are believed to activate the intraislet macrophage and β-cell inflammasomes leading to local secretion of IL-1, known for long to signal β-cell apoptosis.
- •
Once β-cell functional mass starts to decline, insulin secretory decompensation follows, leading to impaired glucose and lipid homeostasis and eventually overt T2D.
- •
Elevated extracellular glucose and lipids (glucolipotoxicity) in turn enhance insulin resistance and β-cell dysfunction, believed in part to involve inflammatory pathways and inflammasome activation in insulin-responsive and insulin-secreting cells. This accelerating process leads to the progressive metabolic deterioration of T2D.
- •
Blockade of IL-1 signaling improves glycemia and β-cell function, but not insulin resistance, in particular in patients genetically deficient in endogenous production of the naturally occurring IL-1 receptor antagonist. Thus, T2D shares properties with the genetic deficiency of IL-1Ra syndrome.
- •
T2D and Alzheimer’s disease share genetic susceptibility genes, cooccur more frequently than expected, and in both diseases inflammasome activation by IAPP and β-amyloid has been implicated in β-cell and neuronal failure, respectively.
Thus, accumulating genetic, preclinical, and clinical evidence supports a primary role of inflammasome activation in T2D, justifying the inclusion of T2D to the group of autoinflammatory diseases. This appreciation may provide novel therapeutic options for the treatment of T2D.
T2D and the definitions of autoinflammation
Autoinflammatory diseases are clinical disorders marked by abnormally increased sterile inflammation, mediated predominantly by the cells and molecules of the innate immune system, with a significant genetic or epigenetic host predisposition.
With the recognition that T2D is characterized by sterile low-grade systemic inflammation, discrete but significant inflammatory cell infiltrates in fat, liver, and islets of Langerhans and in most organs affected by the late diabetic complications (ie, the vascular wall, the glomerulus, and the retina), a polygenetic predisposition, and significant and dynamic epigenetic changes, such as gene methylation/demethylation induced by inactivity, metabolic, and inflammatory factors, it is clear that T2D fulfills the definition of and should long have been recognized as an autoinflammatory disease. Reluctance to broadly accept this concept is probably more related to the nature of the definition than to professional conservatism; the current definition is, with purpose, indiscriminately inclusive to accommodate the many heterogeneous disorders that it attempts to cover.
If the definition is narrowed to diseases associated with an aberrant activity of the inflammasome, the criteria become more stringent by focusing on the subset designated inflammasomopathies. Accordingly, the autoinflammatory diseases caused by mutations in the NLRP3 inflammasome are designated intrinsic and those caused by genetic variation in other activators or by aberrant activation of the inflammasome are named extrinsic inflammasomopathies. Evidence to support the notion that T2D fulfills the requirements for a metabolically activated extrinsic inflammasomopathy is presented next.
The role of the inflammasome in the pathogenesis of T2D and its complications
The evidence of IL-1 as a type 2 diabetokine, listed in Box 1 , suggests that IL-1 is the link between dysnutrition and obesity, obesity and insulin resistance, dysmetabolism and progressive β-cell failure, and dysmetabolism and late diabetic vascular complications.
IL-1: a type 2 diabetokine
- •
Insulin-resistance and obesity
- ○
IL-1 is an adipokine
- ○
Regulates appetite and body weight homostasis
- ○
- •
Low-grade inflammation
- ○
Elevated circulating IL-1 levels in T2D
- ○
Drives IL-6, CRP, and IL-1Ra
- ○
Elevated IL-1 and IL-1 Ra predict development of T2D
- ○
- •
Diabetic macroangiopathy
- ○
Foam cell formation
- ○
Fibrous cap generation
- ○
- •
Progressive β-cell failure and destruction
- ○
Mediates glucose-induced human β-cell apoptosis
- ○
Expressed by β cells and islet macrophages in those with T2D
- ○
IL-1 Ra expression by β cells is reduced in those with T2D
- ○
Imbalance between IL-1 and IL-1Ra in the T2D islet
- ○
The Inflammasome in Dysnutrition and Obesity
The laws of mass constancy and thermodynamics define that obesity arises from an imbalance between caloric intake and expenditure, but the pathophysiologic basis of this imbalance is debated. Hypothalamic neuron leptin and insulin resistance and dysnutrition-induced hypothalamic neuronal dysfunction by endoplasmic reticulum (ER) stress and the canonical inflammatory nuclear factor kappa B (NFκB) pathway have been implicated to disrupt the energy balance and lead to obesity in animal models. Defective autophagic removal of dysfunctional mitochondria associated with increased formation of reactive oxygen species (ROS) contributes to the activation of the hypothalamic NFκB signaling pathway leading to obesity. Of note, defective autophagy and ROS are activators of the inflammasome, and the NLRP1 inflammasome is abundantly expressed in the brain in neurons and oligodendrocytes. Because the inflammasome is activated by free fatty acids it is tempting to speculate that dysnutrition may instigate a vicious cycle of energy imbalance leading to obesity by hypothalamic inflammasome activation ( Fig. 1 ). Formal experimental proof and direct evidence from human research of this hypothesis is lacking.
IL-1 in Obesity and Insulin Resistance
The concept of T2D as an inflammatory disorder long rested on epidemiologic associations between the disease and inflammatory biomarkers and on associations between the antidiabetic effects of drugs with anti-inflammatory properties as bonus to their specific actions, such as angiotensin-converting enzyme inhibitors, insulin sensitizers, or cholesterol-lowering statins. The discovery of the expression of adipose tissue tumor necrosis factor (TNF)-α and the protective effects of TNF-α neutralization on glucose uptake in obese fa/fa rats provided a mechanistic link in the inflammatory pathogenesis of insulin resistance.
The adipocyte as immunoendocrine cell
Evolutionarily, fat-storing cells have developed from the phagocytic monocyte lineage; macrophage precursors can develop into adipocytes and adipocytes can dedifferentiate into macrophages. The adipocyte, traditionally viewed to have only a passive function as a fat depot, is now recognized to respond to the degree of fat storage by the synthesizing and releasing numerous humoral signals, including the growing list of adipocytokines. As the adipocyte differentiates and expands, adipocytokine gene expression is initiated by membrane stress or tissue hypoxia induced by compromised fat tissue microcirculation that is not compensated by angiogenesis. Apart from secreting signals reflecting fat mass, such as leptin, a negative feedback regulator on appetite and caloric intake, the adipocyte secretes proinflammatory cytokines, such as IL-1, TNF, and IL-6, which interferes with insulin signaling in insulin-sensitive tissues (eg, by inducing the expression of suppressors of cytokine signaling, directly interfering with tyrosine kinase activity of the insulin receptor, and by mediating the ubiquitination and proteasomal breakdown of the insulin receptor substrates IRS1/2).
Furthermore, stressed adipocytes release factors that recruit monocytes, such as macrophage chemotactic protein 1; increase adipose tissue endothelial adhesion molecules; increase capillary permeability; and recruit inflammatory cells that build up as so-called crown-like structures around expanded adipocytes.
These cells phagocytose necrotic adipocytes and are further activated to produce inflammatory cytokines and chemokines, leading to adipositis constituted by cells of the innate and adaptive immune systems.
The adipocyte inflammasome
The inflammasome is expressed by adipocytes and apart from processing pro–IL-1β is an important regulator of adipocyte differentiation, insulin sensitivity, and metabolism by mechanisms that are incompletely understood. NLRP3 or caspase 1 deletion reduces diet-induced obesity ; caspase 1 inhibitors, or genetic deletion, improve insulin sensitivity ; in obese mice NLRP3 deficiency reduces IL-18 and interferon-γ expression and effector T-cell numbers, and increases naive T-cell numbers in adipose tissue ; in obese humans with T2D, weight loss–induced improved insulin sensitivity is associated with reduced NLRP3 expression and adipositis ; free fatty acids activate the inflammasome in adipose tissue macrophages probably by increasing intracellular ceramide, but the precise molecular mechanisms are unclear; mitoNEET, an iron-containing mitochondrial outer membrane protein that enhances lipid uptake and storage and yet preserves insulin sensitivity in adiposity, was described to inhibit iron transport into the mitochondrial matrix, electron transport, β-oxidation, and thereby ROS production ; and reduced mitoNEET expression or deficient function may contribute to adipocyte inflammasome activation. Interestingly, mitoNEET expression is upregulated by catecholamines and downregulated by the antidiabetic drug glibenclamide, and binds the thiazolidinedione-class of insulin sensitizers.
Taken together this evidence suggests that the adipocyte inflammasome may have a primary role upstream to its metabolic activation and an amplifying action once circulating metabolite levels are elevated and cause secondary inflammasome activation ( Fig. 2 ).
Counteracting effects of IL-18 and IL-33
Inflammasome processing by caspase-1 cleaves and activates pro–IL-18 and pro–IL-1β, but inactivates pro–IL-33, which is active in its full-length form, or even more so after cleavage by the neutrophil serine proteases cathepsin G and elastase. Circulating levels of IL-18 are reduced after weight loss in accordance with the reduced adipose tissue NLRP3 expression. Surprisingly, mice deficient for IL-18 or the IL-18 receptor and mice overexpressing the neutralizing IL-18 binding protein displayed hyperphagia, obesity, and insulin resistance associated with defective phosphorylation of STAT3 enhancing expression of genes associated with hepatic gluconeogenesis. Intracerebral administration of IL-18 inhibited food intake and reversed hyperglycemia in IL-18 null mice through activation of STAT3 phosphorylation.
IL-33 and its receptor are expressed in adipocytes and expression is increased in severe obesity, in particular in adipose tissue endothelium. IL-33 reduced genetic adiposity, fasting glucose, and glucose and insulin intolerance in ob/ob mice, associated with adipose tissue Th2 cell accumulation and M2 polarization of adipose tissue macrophages that are known to protect against obesity-related dysmetabolism. High fat diet fed mice lacking the IL-33 receptor exhibited higher body weight and fat mass and impaired insulin secretion and glucose homeostasis than their wild-type counterparts. It may be anticipated that the beneficial metabolic effects of reducing IL-1 and IL-33 processing by therapeutic targeting of the inflammasome could in part be counteracted by reduced processing of IL-18; there is currently no experimental evidence to support this concern.
Clinical studies
Despite promising preclinical evidence in favor of an inflammatory pathogenesis of insulin resistance, clinical trials of specific biologics targeting TNF-α, IL-1, and IL-6 in subjects who are insulin-resistant nondiabetic, or who have T2D have generally been disappointing. There are several nonrandomized, open reports of improved insulin sensitivity in patients treated with TNF blockers for rheumatologic diseases, whereas randomized open or placebo-controlled studies in healthy lean or obese or in subjects with T2D have largely been negative, possibly by type II error caused by small sample sizes and low statistical power. Alternatively, in insulin-resistant humans TNF-α could be a biomarker of another inflammatory signal (eg, IL-1), causally related to insulin resistance and, at the same time, inducing TNF-α as an epiphenomenon. IL-1 does indeed induce adipocyte and hepatocyte insulin resistance in vitro by direct interference at multiple levels in the insulin-signaling cascade. In T2D animal models IL-1 antagonism improves insulin resistance, but in analogy to TNF-α antagonism, blocking IL-1 signaling in patients who are obese, insulin-resistant, nondiabetic, or who have T2D has failed to consistently improve insulin sensitivity.
Expression and activation of the inflammasome seems to be an early and primary event in adipocyte differentiation, obesity, and insulin resistance, which introduces the difficulty that insulin-sensitizing therapies targeting the inflammasome or its products may have to be instituted very early in individuals at risk for the development of obesity and insulin resistance. It is possible that adipocyte hypertrophic stress and necrosis provokes the release of extracellular danger-associated molecular patterns that activate adipocyte inflammasome-dependent processing of IL-1β, which in turn stimulates lipoprotein lipase to liberate free fatty acids to induce insulin resistance by lipotoxicity. Once insulin resistance is established, elevated circulating IL-1β may contribute to systemic inflammation; however, because systemic inflammatory markers, such as IL-6 or CRP, do not correlate with improved glycemia and β-cell function that follow IL-1 antagonism in T2D, local rather than systemic inflammation may be more important for the inflammatory pathogenesis of T2D. This may explain the failure of blocking IL-1 (or other inflammatory cytokines) on overt insulin resistance in human studies, and also pertain to the role of inflammasome activation in the pancreatic islet, as reviewed in the next section.
IL-1, Dysmetabolism, and β-cell Failure
A unifying hypothesis implicating glucolipotoxicity as a common denominator of insulin resistance, β-cell dysfunction, and late diabetic complications suggested that chronic elevations of glucose and free fatty acids cause substrate-mediated mitochondrial ROS generation, activate the NFκB, p38, and JNK stress signaling pathways, eventually triggering the polyol-sorbitol, advanced glycation endproduct receptor and diacyl-glycerol/protein kinase C pathways in insulin-sensitive tissues, and causing cytokine and prostanoid production in pancreatic islets.
In search for the inflammatory effector mechanisms mediating glucose-induced β-cell dysfunction and apoptosis, Donath and colleagues discovered that high glucose induces Fas expression otherwise not detectable in pancreatic islets, raising the intriguing possibility of cis – or trans- ligation of this proapoptotic receptor by the Fas ligand, constitutively expressed on β-cells. Because IL-1 was known to be a potent inducer of Fas in β cells, these investigators next asked if IL-1Ra abrogated high glucose induced Fas expression and apoptosis in human islet cells and found that this was indeed the case. The following logical question was: what is the source of IL-1 in the T2 diabetic islet? High glucose induced release of mature IL-1β from human islets and IL-1β mRNA and protein expression in β cells, providing the first indirect demonstration of the presence of the inflammasome in the pancreatic islet. Subsequently, increased numbers of intraislet macrophages in animals and patients with T2D were described, providing an additional (and perhaps dominating) source of IL-1 in the endocrine pancreas. These findings were confirmed and extended by the finding that high glucose activates the β cell NLRP3 inflammasome. Later, other activators typical of the T2D state of the islet macrophage inflammasome (eg, IAPP and unsaturated free fatty acids) have been described, as well as unfolded protein response-independent ER stress ( Fig. 3 A).
