A TLR4-triggered complex inflammation in pancreatic islets causes beta-cell failure in diabetes

Abstract

Type 2 Diabetes (T2D) is strongly associated with obesity and characterized by chronic insulin resistance, progressive failure of pancreatic beta-cells, and ultimately hyperglycaemia. The association of T2D with chronic sterile inflammation has been extensively demonstrated, and the elevation of inflammatory mediators can predict type 2 diabetes progression. Pro-inflammatory cytokines and chemokines can cause insulin resistance in peripheral insulin-sensing tissues like fat, liver and muscle, and also lead to progressive beta-cell failure. This eventually shifts metabolism from relative insulin insufficiency - due to the greater insulin demand in obesity - to definite insulin deficiency, while on the beta-cell level - from compensation to decompensation. Toll-like receptor (TLR)-4 signaling is one of the major pro-inflammatory pathways activated by exogenous pathogen-related or endogenous danger-related molecules. Its ligands, including the classical ligand LPS as well as saturated fatty acids and CXCL10, among others, are increased systemically in patients with T2D as well as in at-risk individuals. TLR4-deficiency or its pharmacological inhibition have been shown to ameliorate obesity- or lipid-induced tissue inflammation and insulin resistance in humans and in mouse models. Increasing evidence also connects TLR4 to islet inflammation and beta-cell dysfunction in the context of the pathogenesis of T2D, but many underlying mechanisms remain unknown. In the first part of this thesis, I aimed to uncover the role of TLR4 activation by lipopolysaccharide (LPS), the classical TLR4 ligand, in islet inflammation and beta-cell function in human islets. My special focus was to identify the inflammatory mechanism in the intercellular level the possible interplay among different cells in human islets. I found that LPS-triggered TLR4 activation in cultured human islets induced beta-cell dysfunction, apoptosis and a pro-inflammatory profile with markedly increased IL-1beta, IL-6, TNFalpha and IL-8 production. Macrophage-depletion demonstrates that islet resident macrophages are responsible for the production of IL-1beta, while all the other cyto-/chemokines are predominantly produced from islet endocrine cells. IL-6 is partially responsible for the LPS-induced beta-cell dysfunction, while IL-8 produced from alpha-cells is responsible for monocyte migration to islets during TLR4-activation-induced islet inflammation. This complex inflammatory response in islets is further potentiated in obese individuals, with more IL-1beta, IL-6 and IL-8 expression and a tendency to more islet macrophage accumulation, suggesting a possibly self-augmented inflammatory cycle involving alpha-cells, beta-cells and islet macrophages, which may explain the higher susceptibility of obese individuals to the development of beta-cell damage und eventually T2D. Ageing is known to be associated to elevated T2D risk, though the underlying mechanism remains largely undiscovered. In the second part of this thesis, I aimed to find out if ageing could aggravate obesity-induced T2D, and further focus on the effects on beta-cell function and the role of inflammation in such processes. In a mouse model of high fat diet induced obesity, I found an adverse potentiation of impaired glucose homeostasis, beta-cell dysfunction and chronic tissue inflammation by the combination of obesity and aging. In contrast, TLR4-deficiency exhibited a protection against those deleterious effects through inhibiting pro-inflammatory cytokine expression and switching tissue macrophage activation to a more anti-inflammatory phenotype. In both parts of this thesis, I provide further evidence that TLR4 and inflammation play a causative role in the development of T2D, and thereby support the concept of TLR4- or inflammation-targeted therapeutic strategies.