**5. Islet inflammation in T1DM and T2DM**

macrophages and/or cytokines produced by T-lymphocytes, as initially occurs in early stage T1DM. In fact, it also stems from local production of pro-inflammatory cytokines by the pancreatic β-cells themselves. The similarity between pancreatic β-cells and immune cells is an intriguing characteristic. Both can release and respond to cytokines; their function is dependent on changes in concentration of ROS/RNS and they both express high levels of proinflammatory proteins such as NFκB, iNOS, NOX and TLR's. Pancreatic β-cells have been shown to express biologically active cytokines like the pro-inflammatory cytokine IL-1β in hyperglycaemic conditions [77,78]. Due to their potent effects, cytokine production is strin‐ gently regulated. Control mechanisms include down-stream activation/processing (conver‐ sion of pro-IL-1β to IL-1β by inflammasomes), and co-expression of binding proteins/ antagonists (like the IL-1 receptor antagonist, IL-1Ra), that regulate cytokine bio-reactivity [76]. However, expression of the biologically active form of IL-1β was evident in pancreatic β-cells, indicating that similar to immune cells, these cells possess the necessary cellular machinery to allow expression of immunologically active cytokines [77]. Autocrine production of IL-1β, has been correlated with autoimmune destruction of β-cells in T1DM and is also associated with glucotoxicity in the pathogenesis of T2DM patients [76,79]. IL-1β elicits its potent cytotoxic effects through activation of NFκB, and a subsequent initiation of the extrinsic cell-death pathway [78]. Additionally, chronic exposure to IL-1β results in increased iNOS expression, and consequently excess NO production. High levels of NO inhibit mitochondrial ATP synthesis and up-regulate the expression of pro-inflammatory genes in β-cells, which may

Similar to macrophages and dendritic cells, β-cells also express TLR's that normally function to regulate the immune system [80]. TLR's interact with a wide variety of pathogenrelated molecules, including lipopolysaccharide (LPS), a component of bacterial cell walls. This allows phagocytosis of microbes before infection can be established. However, in βcells, it is believed that TLR's play a role in insulin-resistance and inflammation in T2DM. TLR2 and TLR4 have been suggested as receptors for fatty acids, and may alter insulin signalling during dyslipidaemia. We have shown that β-cells express a range of TLR's and could indeed respond to LPS via TLR's, and this interaction decreased insulin exocytosis accordingly [80]. However, glutamine restored insulin release. Glutamine can also regu‐ late pro-inflammatory gene expression in mononuclear cells [11,80]. Glutamine also upregulates nuclear factor of activated T cells (NFAT), and thus promotes β-cell growth, while suppressing β-cell death. Mutations in NFAT-dependent genes have been demonstrated to result in hereditary forms of T2DM [11]. Moreover, as discussed above, glutamine can enter HBP thus regulating GSK-3β activity and HSP70 expression which promotes anti-inflamma‐

Pancreatic β-cells are also reported to express other cytokines, including IL-6, IL-8, granulocyte colony-stimulating factor (G-CSP) and MIP-1 (macrophage inflammatory protein-1) that not only induce apoptotic β-cell death, but also signal patrolling macrophages, enhancing islet immune cell infiltration [76]. Macrophages, monocytes, neutrophils and dendritic cells perform their function by engulfing invading foreign matter including bacteria or dead cells,

) generated from plasma membrane-bound NOX [27].


potentiate β-cell failure [78].

138 Type 1 Diabetes

tion and cytoprotection [53,54].

and degrade them using super oxide (O2

T1DM is exclusively an autoimmune form of DM, and islet inflammation is characterised by the presence of leukocyte infiltrates that include B-cells, T-cells, macrophages and Natural Killer (NK) cells [81]. Macrophages play a critical role since they phagocytose apoptotic and necrotic β-cells, as well as produce ROS and cytokines (TNFα, INF-γ and IL-1β), that can promote β-cell death, which leads to patient insulin-dependence. However, effector CD4 helper and CD8-cytotoxic T-cells represent the predominant pancreatic infiltrate for this disease, and recent evidence has suggested that T1DM progression may be dependent on a precarious equilibrium between migration and activation of effector and regulatory T-cells (Treg) [82]. An important element in T1DM disease development is the generation of autor‐ eactive effector T-cells that kill pancreatic β-cells through expression of Fas, lytic granules and cytokines such as INF-γ [82]. Research into formation of these autoimmune cell types is still at an early stage, and it was only definitively shown in 2012, that autoreactive effector cytotoxic-CD8 T-cells were indeed present in T1DM human pancreatic islets [81]. Furthermore, the means by which these "homicidal" immune cells are generated and go on to attack β-cells is still not fully understood. However, part of the process is believed to involve dendritic cell migration to draining lymph nodes following antigen presentation, and stimulation of autoreactive T-cell differentiation [82,83]. T-cells sub-sets such as Th1, Th2 and Th17 are thus formed and they express the necessary weaponry that is responsible for β-cell death in T1DM [82], this being exacerbated by strong psychological stress [84], one of the possible triggering factors for the onset of T1DM (for review, please see [85]). Additionally, T-cell–mediated release of INF-γ and TNFα can up-regulate expression of pro-apoptotic proteins (Bim and PUMA) leading to β-cell death, along with promoting recruitment and clearance of damagedcells by macrophages [77,86]. On the other hand, in normal individuals, activity of these autoimmune cells is normally controlled by Treg cells and it is the failure to control the action of effector T-cells that result in autoimmune disease. The mechanisms by which Treg cells prevent autoimmune attack is also not fully elucidated, but they are thought to prevent cytotoxic action of T-cells by use of contact inhibition and release of soluble signalling factors, such as IL-10 and TGFβ (transforming growth factorβ) [82]. It is also unclear whether the precise causes of inflammation in T1DM are a consequence of T-cell failure to respond to Treg, or whether defective or low Treg numbers are to blame for disease progression. Nonetheless, the interplay between these cell populations offers a potential therapeutic strategy for T1DM treatment [82].

influenced by nutrient availability, such as in hyperglycaemia and dyslipidaemia conditions. Further to this, there is significant crossover between the molecules in these categories, and several can significantly impact on the others, indicating a complex role in both T1 and T2DM.

The Impact of Inflammation on Pancreatic β-Cell Metabolism, Function and Failure in T1DM and T2DM…

http://dx.doi.org/10.5772/55349

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T1DM is an autoimmune disease and it comes as no surprise that cytokine expression is elevated in these patients [79]. Interestingly, it is becoming more evident that cytokines also play a critical role in T2DM progression, and increased levels have been reported in T2DM patients [76,87]. The most obvious source of cytokine production is from islet invading immune cells, although other researchers have illustrated that islet β-cells could also express cytokines [76,79]. Cytokine and adipokine release also occurs from adipose tissue since it expands rapidly in obese patients. Here, hypoxia also plays a key part in cytokine release due to an inflamma‐ tory response to lack of vasculature in rapidly growing adipose tissue [90,91]. Recent evidence has suggested that adipocyte invading macrophages are a significant supplier of TNFα to the circulation in obese T2DM patients, and this could be a contributing-factor that modulates inflammation in disease progression [92,93]. It is likely that all sources contribute in some way or another to elevate cytokine levels, and consequently compound inflammation in DM

The main cytokines that are responsible for inflammation in T1 and T2DM, include IL-1β, TNFα, INF-γ, IL-6 and IL-8. Central to the inflammatory role of each is stimulation of stressinduced kinases, IKKβ (inhibitor of nuclear factor kappaB kinase subunit β) and/or c-JNK (c-Jun-N-terminal kinase) [90]. Activation of IKKβ leads to translocation and activation of the NFκB. This factor targets transcription of genes associated with inflammation, and can cause subsequent up-regulation and release of IL-1β, TNFα, IL-6 and IL-8 [94,95]. Therefore, the aforementioned cytokines can initiate an auto-stimulatory or feed-forward inflammatory effect through NFκB-signalling in β-cells, resulting in amplification of inflammation. IL-1β and TNFα initiate NFκB-signalling directly via association with their relative receptors (IL-1R and TNFR) [96] and can also activate the apoptotic JNK pathway indirectly by intracellular interaction of TNF receptor associated factor (TRAF) with the cytoplasmic portion of IL-1R or TNFR [97]. NFκB can play either a pro-survival or pro-death role given the correct circum‐ stances [98]. Both NFκB and JNK are intrinsically connected, and NFκB can prevent JNKmediated cell death, the regulatory interactions of which have been reviewed expertly elsewhere [99]. An important component of NFκB activation and function is the presence/ absence of ROS/RNS. Therefore, ROS/RNS can influence NFκB-dependent cytokine expression

ROS/RNS can activate nuclear translocation of NFκB which promotes gene expression. However, ROS/RNS can also have an inhibitory effect when NFκB has already translocated to the nucleus [98]. The process by which ROS/RNS affects NFκB function is not entirely known but is believed to involve alteration of the NFκB catalytic site through interaction with cysteine residues, or by inhibiting specific kinase enzymes such as IκBα, that results in phosphorylation of NFκB [98]. Cellular ROS can be generated from Electron Transport Chain (ETC) respiratory complexes or from specific enzymes (e.g. NOX-mediated production for phagocytosis) [27,98]. Most notably, in hyperglycaemic/glucotoxic conditions (in T1 or T2DM), mitochondria are the major source of ROS/RNS primarily because of high oxidative phosphorylation and ATP

patients.

and consequently immune response [98].

Interestingly, an autoimmune element has also been reported in patients with T2DM, along with the accepted thesis of insulin-resistance [76,87]. Hyperglycaemia, dyslipidaemia and lowgrade inflammation (consisting of circulating inflammatory cytokines or adipokines released by adipocyte expansion), are considered important factors in the progression of T2DM and are generally present in obese individuals who are at risk of T2DM development [77]. These conditions lead to β-cell stress through a variety of processes that mainly include uncontrolled generation of ROS/RNS and cytokine-dependent initiation of death signals. Both processes combine to reduce β-cell function and decrease β-cell mass by inducing apoptotic cell death, leading to further hyperglycaemic and dyslipidaemic complications, and causing amplifica‐ tion of ROS/RNS generation, cytokine release and cytokine-mediated recruitment of the immune system (i.e. inflammation). These inflammatory factors are all detrimental for β-cell survival. As mentioned previously, IL-1β is elevated in the hyperglycaemic state, is increased in T1DM and is also expressed by β-cells in T2DM [77-79,88]. Moreover, concomitant downregulation of the receptor antagonist IL-1Ra was also observed in β-cells cultured in hyper‐ glycaemic conditions [76]. β-Cells are similar to immune cells and dysregulated expression of IL-1β in islets can cause auto-stimulation and subsequent release of IL-1β by other β-cells, via NFκB activation [76,88]. In addition, IL-1β can promote the local expression of other cytokines, for example IL-6 and IL-8. These cytokines aid in the recruitment of patrolling macrophages, which may subsequently become activated by high microenvironment levels of IL-1β and amplify IL-1β content in their own right [76]. In terms of islet inflammation, IL-1β expression and its effects on β-cell death appears to be a uniting factor, in both T1 and T2DM and is being considered a possible therapeutic target [77,89].

While inflammation is essential to maintain tissue homeostasis, it is also beneficial and allows repair of damaged organs. However, it is the presence of chronic, out of control and unchecked inflammatory factors that contribute to β-cell death and ensuing DM. Ultimately, increased local microenvironment cytokine production in islets is detrimental and understanding the mechanisms of cytokine release and regulation, and also suppression of β-cell function, will allow the development of new treatment regimens.
