Chapter 2 Inflammation as a precursor of diabetes
The evidence for obesity as a proximal cause of insulin resistance and diabetes (DM) is compelling. Similarly the relationship between diabetes and cardiovascular disease (CVD) is also well established, with CVD occurring one to two decades earlier in people with DM., Research on unraveling the mechanisms underlying the development of diabetes in a substrate of obesity have pointed out the activation of the innate immune system by nutrient excess as a plausible cause.
Inflammation represents an integrated response to harmful stimuli that aims to return the system to a normal baseline. It is now clear that inflammation is associated with obesity and mediates the consequences of obesity such as diabetes. Several distinct features characterize the inflammatory response in obesity. First, the inflammation is chronic and low grade with effects on metabolic homeostasis. Superimposed on this, is acute inflammation accompanying acute events of nutrition excess.4 Second, the inflammation occurs in multiple organs.
Adipose tissue inflammation as a precursor to diabetes
Much of our knowledge about inflammation as a proximate contributor to insulin resistance, diabetes and CVD arises from the study of adipose tissue (AT). Inflammation is evident in other organs as well, including the liver, pancreas and hypothalamus in obesity and the metabolic syndrome. Our view of adipose tissue from an inert fat store has evolved with the recognition of its role as a secretory cell with prominent role in energy homeostasis and immune response.
Increase in waist size has been correlated with both a reduced disposal of glucose and an increase in the risk of diabetes. A similar relationship exists for adipocytes (AC). The expansion of AC occurs early in the progression of obesity. This expansion, known as adipocyte hypertrophy, is accompanied by a sequence of events that culminates in loss of the integrity of the cell resulting in dissemination of fat from AT into other tissue compartments. There is considerable evidence to show that adipose tissue macrophages (ATM) accumulate in visceral adipose tissue, which correlates with the development of insulin resistance.
New evidence suggests that the increased demand for lipid storage by nutritional excess leads to the conversion of the AC from a small multilocular cell with intact glucose transporter type 4 (GLUT4) to a unilocular cell with a large lipid droplet and decreased GLUT4 translocation. Thus, insulin resistance at the AC level seems to be an early event in the meta-inflammatory response. Adipocyte hypertrophy and dysfunction have two other consequences – a change in the secretory pattern of the AC and an increase in cytokine production from the resident (and subsequently recruited) macrophages.
To a large extent, metabolic homeostasis, nutrient utilization and immune pathways appear to be intertwined. AT contains multiple sets of immune cells whose primary function appears to be maintenance of the integrity and hormonal sensitivity of the AC. In the lean state, these cells are in a Th2 or type 2 state which facilitates a subset of T cell lymphocytes to secrete cytokines that will maintain the resident macrophages in an alternately active (M2) state. Overall, M2 macrophages appear to block the inflammatory response and promote tissue repair. M2 polarized macrophages secrete cytokines including interleukin 10 (IL-10) that help maintain insulin sensitivity in the AT. The M2 activation state has a fundamental link with the activation of peroxisome proliferator-activated receptor (PPAR)-? and -?, both well known regulators of lipid metabolism and mitochondrial activity. When M2 pathways are enhanced by eosinophilic recruitment (during parasitic infestations, for instance), there is a reduction in inflammation and enhancement in insulin sensitivity.
As obesity progresses, the immune environment shifts to a Th1 (Type 1) state. The Th1 state is generally viewed as a response to infection and indeed a role for gut microbiota as trigger of this inflammatory state in obesity has been suggested., In obesity, the inflammation is chronic and results in the activation of effector T cells, NK cells and B cell subtypes that produce cytokines, which in turn, results in the activation and accumulation of proinflammatory M1 macrophages.
AC hypertrophy is accompanied by micro hypoxia of the AT leading to the upregulation of chemokines secreted by AT such as migration inhibition factor (MIF), matrix metalloproteinases (MMP) 2 and 9, IL-6, VEGF, leptin etc., The hypertrophy and necrosis of the AC appears to be a potent phagocytic stimulus. Indeed macrophages accumulate around necrotic adipocyte forming crown-like structures (CLS). AC and/or ATM produce monocyte chemo-attractant protein (MCP1) which activates pathways that lead to the influx of CCR2+ monocytes into the AT. Dissemination of fatty acids from necrosed adipose tissue appears to be an important contributor to progression of the inflammatory process in other organs.
The increase in the number of macrophages and the ratio of M1 to M2 macrophages results in a switch of the AT to a proinflammatory state with increased secretion of cytokines such as tumor necrosis factor-alpha (TNF-?), resulting in insulin resistance and metabolic disease.
The AC has pattern recognition receptors (PRR), like the toll like receptor 4 (TLR4), which can be activated resulting in activation of inflammatory pathways particularly NF-kB and JNK pathways. A higher circulating level of lipopolysaccaride (LPS) from bacteria, which activates TLRs, is seen in obesity and correlates with diabetes. Circulating free fatty acids may also activate the PRRs contributing to inflammation. The NOD-like receptor (NLR) family of PRR is activated by stressed or dying cells and act by preparing leukocytes to contain tissue damage in various cells.4 In adipose tissue macrophages (ATM), NLR activation results in the production of IL-1? and IL-18 by the cryptopyrin/NLRP3 inflammasome through capase 1. Similar activation of this pathway may be responsible for pancreatic beta cell dysfunction and death and impact diabetes progression. Local release of FFA may contribute to activation of PRR through the TLR4 complex.4 This mechanism is more important in other organs where inflammation may be exacerbated by lipotoxicity. The activation of inflammatory gene transcription that is mediated by NF-kB seems to be the critical convergence of these activation pathways.
Besides NF-kB, there is evidence that obesity activates JNK1. This is associated with molecular pathways that govern ER stress and the unfolded protein response. Extensive activation of ER stress signaling components and cascades has been noted in obesity.
Evidence linking the amplification of insulin resistance (IR) by inflammation is considerable. Disruption of inflammatory pathways in obese mice leads to reduction in IR. Many of these pathways including leukotriene B4 (LTB4) and galectin attenuate insulin signaling., Secretory products of activated macrophages (M1), such as TNF-?? initiate NF-kB and JNK pathways. A common mechanism of IR appears to be the serine or threonine phosphorylation of insulin receptor substrates, reducing insulin-induced tyrosine phosphorylation and consequent downstream effects. Activation of PRRs leads to downstream effects that affect insulin action. Some of these include regulation of ceramides and sphingolipids.
Other organ involvement
There is considerable evidence to propose a role for pancreatic inflammation in reduction in beta cell function and stimulation of apoptosis which are important events in the progression to diabetes. Accumulation of macrophages that produce cytokines may inhibit ? cell function. Cytokines produced by the beta cells themselves may also be involved.-23
Non-alcoholic fatty liver disease (NAFLD) is strong risk factor for insulin resistance, independent of obesity.24 The same pathways outlined above appear to be activated in NAFLD.
Inflammatory cytokines are increased in skeletal muscle in obesity; the mechanism appears to be TLR 4 mediated.25
Hypothalamic inflammation has been demonstrated in diet induced obesity. At least part of this effect seems to be through activation of JNK and NF-kB signaling pathways. The consequence is the disruption of insulin and leptin signaling.4,26 Brain inflammation appears to impact peripheral insulin signaling through the sympathetic nervous system.27
Inflammation of the placenta is a hallmark of maternal obesity and independently predicts obesity in the offspring. It may alter nutrient set points that affect eating, obesity and future metabolic disease.4
Anti-inflammatory therapies as a therapeutic strategy in diabetes
Several of the current therapies for diabetes in vogue, at least in part, reduce inflammation. Weight loss, both physiologic and pharmacologic, reduces inflammatory markers and increases protective lipokines such as adiponectin.28,29 It is unclear if the benefits seen through weight loss, such as improvement in control of diabetes or improvement in CV mortality, are mediated through reduction in inflammation.2
Insulin reduces NF-kB levels in peripheral monocytes;30 insulin administration in acute myocardial infarction (MI) reduces inflammatory markers.31 Metformin appears to have anti-inflammatory effects independent of glucose lowering. This may be related through its principal mode of action which is the activation of AMP kinase. Activation of AMP kinase results in the suppression of NF-kB activity.32 Thiazolidenediones (TZD) are potent reducers of IR. The principal mechanism of action of TZDs lies in the activation of PPAR-? which is intricately linked to adipocyte differentiation, glucose uptake, fat uptake and storage.33 TZDs may act through suppressed activation of NF-kB and its targets,34 and lower circulating inflammatory cytokines to a greater extent than sulphonylureas.35
Dipeptidyl peptidase-4 inhibitors (DPP-4i) appear to have anti inflammatory activity in part by inhibition of protein kinase C (PKC) activity, leading to suppression of the NLRP3 inflammasome.36 Interestingly, a lower risk of rheumatic disease has been reported with DPP-4i use. Both the glucagon-like peptide-1 (GLP-1) receptor agonists and sodium-glucose co-transporter 2 (SGLT2) inhibitors are associated with improved clinical cardiovascular outcomes. The contribution of the anti-inflammatory effects demonstrated with these drugs to the benefit seen is unclear.
Lastly statin use is associated with improvement in inflammatory markers; again it is unclear if the obvious beneficial outcomes of statin therapy are related to reduction in inflammation.37
The enthusiasm to use anti-inflammatory drugs to prevent and treat diabetes and CV risk has been tempered with downside of their potential effects on immunity. From the discussion above, it will be apparent that complex pathways are involved and our knowledge of downstream and off-stream effects is anything but complete. Several anti-inflammatory agents, both specific and non specific, such as salsalates, TNF-? inhibitors, IL-1? antagonists, etc., have been used in clinical trials and are outlined in greater length in a later chapter. Of note, the disease modulator hydroxychloroquine has been approved as an antidiabetic by the DCGI in India.