Abstract
AimsChanges in the islet vasculature have been implicated in the regulation of β-cell survival and function during the progression to type 2 diabetes (T2D). Failure of the β-cell to compensate for the increased insulin demand in obesity eventually leads to diabetes; as a result of the complex interplay of genetic and environmental factors (e.g. ongoing inflammation within the islets) and impaired vascular function. The Angiopoietin/Tie (Ang/Tie) angiogenic system maintains vasculature and is closely related to organ inflammation and angiogenesis. In this study we aimed to identify whether the vessel area within the islets changes in diabetes and whether such changes would be triggered by the Tie-antagonist Ang-2.MethodsImmunohistochemical and qPCR analyses to follow islet vascularization and Ang/Tie levels were performed in human pancreatic autopsies and isolated human and mouse islets. The effect of Ang-2 was assessed in β-cell-specific Ang-2 overexpressing mice during high fat diet (HFD) feeding.ResultsIslet vessel area was increased in autopsy pancreases from patients with T2D. The vessel markers Tie-1, Tie-2 and CD31 were upregulated in mouse islets upon HFD feeding from 8 to 24 weeks. Ang-2 was transiently upregulated in mouse islets at 8 weeks of HFD and under glucolipotoxic conditions (22.2 mM glucose/ 0.5 mM palmitate) in vitro in human and mouse islets, in contrast to its downregulation by cytokines (IL-1β, IFN-ɣ and TNF-α). Ang-1 on the other hand was oppositely regulated, with a significant loss under glucolipotoxic condition, a trend to reduce in islets from patients with T2D and an upregulation by cytokines. Modulation of such changes in Ang-2 by its overexpression or the inhibition of its receptor Tie-2 impaired β-cell function at basal conditions but protected islets from cytokine induced apoptosis. In vivo, β-cell-specific Ang-2 overexpression in mice induced hypervascularization under normal diet but contrastingly led to hypovascularized islets in response to HFD together with increased apoptosis and reduced β-cell mass.ConclusionsIslet hypervascularization occurs in T2D. A balanced expression of the Ang1/Ang2 system is important for islet physiology. Ang-2 prevents β-cell mass and islet vascular adaptation in response to HFD feeding with no major influence on glucose homeostasis.
Highlights
Pancreatic islets are densely vascularized mini-organs constituting 1–2% of the pancreatic mass but supplied by 5–15% of the pancreatic blood flow [1, 2]
We found no correlation in islet vessel area and BMI, neither in non-diabetic nor in diabetic donors (Fig 1C), suggesting that vessel area, normalized to islet area, was not affected by the BMI
Increased islet vessel area but reduced intraislet vessel density was shown in several mouse models of obesity and insulin resistance, including high fat diet (HFD) feeding [7]
Summary
Pancreatic islets are densely vascularized mini-organs constituting 1–2% of the pancreatic mass but supplied by 5–15% of the pancreatic blood flow [1, 2]. The laminin isoforms are known to directly influence β-cell function and proliferation [4] In their remarkable study Lammert et al showed that the endothelium sends inductive signals to direct islet development, beyond merely serving the metabolic demand [5]. A recent study shows that human islets present a higher vessel density in T2D, which occurs together with amyloid depositions in areas of thicker capillaries [9] Such higher vessel area in T2D suggests that vessels, they bring nutrients, which are especially important in the β-cell compensation state, may be detrimental through the delivery of inflammatory products from the circulation to the proximity of the highly sensitive β-cells. Human islets were found to have lower vessel density than mouse islets [9] and whether islet capillaries favor proximity to β-cells is yet an evolving debate [10, 11]
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