Macrophage Contact Dependent and Independent TLR4 Mechanisms Induce β-Cell Dysfunction and Apoptosis in a Mouse Model of Type 2 Diabetes

Abstract

Type 2 diabetes (T2D) is evolving into a global disease and patients have a systemic low-grade inflammation, yet the role of this inflammation is still not established. One plausible mechanism is enhanced expression and activity of the innate immune system. Therefore, we evaluated the expression and the function of the toll-like receptor 4 (TLR4) on pancreatic β-cells in primary mouse islets and on the murine β-cell line MIN6 in the presence or absence of macrophages. Diabetic islets have 40% fewer TLR4 positive β-cells, but twice the number of TLR4 positive macrophages as compared to healthy islets. Healthy and diabetic islets respond to a TLR4 challenge with enhanced production of cytokines (5–10-fold), while the TLR4 negative β-cell line MIN6 fails to produce cytokines. TLR4 stimulation induces β-cell dysfunction in mouse islets, measured as reduced glucose stimulated insulin secretion. Diabetic macrophages from 4-months old mice have acquired a transient enhanced capacity to produce cytokines when stimulated with LPS. Interestingly, this is lost in 6-months old diabetic mice. TLR4 activation alone does not induce apoptosis in islets or MIN-6 cells. In contrast, macrophages mediate TLR4-dependent cell-contact dependent (3-fold) as well as cell-contact independent (2-fold) apoptosis of both islets and MIN-6 cells. Importantly, diabetic macrophages have a significantly enhanced capacity to induce β-cell apoptosis compared to healthy macrophages. Taken together, the TLR4 responsiveness is elevated in the diabetic islets and mainly mediated by newly recruited macrophages. The TLR4 positive macrophages, in both a cell-contact dependent and independent manner, induce apoptosis of β-cells in a TLR4 dependent fashion and TLR4 activation directly induces β-cell dysfunction. Thus, targeting either the TLR4 pathway or the macrophages provides a novel attractive treatment regime for T2D.

mTOR: A double-edged sword for diabetes

“…mTOR is involved in the regulation of metabolism and metabolism‐dependent cellular processes in macrophages. A key component may be TLR4 activation, which has been implicated in the pathogenesis of diabetes . Activation of M1 macrophages by the TLR4 ligand LPS involves B‐cell adapter for PI3K‐mediated regulation of TLR4 signaling, initiating activation of PI3K, Akt, and mTORC1.…”

“…A key component may be TLR4 activation, which has been implicated in the pathogenesis of diabetes. 89 Activation of M1 macrophages by the TLR4 ligand LPS involves B-cell adapter for PI3K-mediated regulation of TLR4 signaling, initiating activation of PI3K, 90 Akt, and mTORC1. Moreover, LPS is also capable of inducing HIF1 stability in myeloid-derived macrophages.…”

The Role of the Microbial Metabolites Including Tryptophan Catabolites and Short Chain Fatty Acids in the Pathophysiology of Immune-Inflammatory and Neuroimmune Disease

“…For example, elevated levels of TNF-α, IL-1β, IL-6, and NFkB, produced by LPS-induced activation of TLR2 and TLR4, are upregulated in the liver, as well as in adipose tissue and striated muscles, and make a major contribution to the development of increased insulin resistance [184,187,188]. LPS activation of TLR4 and the subsequent increased production of proinflammatory cytokines in macrophages and beta cells conspire to decrease the function, and ultimately the viability, of the latter cells; this is a critical element in the development of insulin resistance and TD2 [189,190]. The Th17/Treg balance is disturbed in patients with TD2, with elevated numbers of circulating Th17 T cells and reduced numbers and function of Tregs [191,192].…”

Inhibition of TLR4 protects rat islets against lipopolysaccharide-induced dysfunction

“…TLR4 is expressed in pancreatic β-cells and insulin-sensitive tissues, including adipocytes and muscle cells (7)(8)(9). TLR4 has been directly associated with proinflammatory signaling in β-cell dysfunction (10,11). The effect of TLR4 signaling in insulin resistance is a well-established concept; however, the specific function of TLR4 during LPS-induced oxidative stress on pancreatic β-cells remains to be elucidated.…”