Vascular endothelial PDPK1 plays a pivotal role in the maintenance of pancreatic beta cell mass and function in adult male mice

The aim of this study was to elucidate the impact of 3′-phosphoinositide-dependent protein kinase-1 (PDPK1) in vascular endothelial cells on the maintenance of pancreatic beta cell mass and function.

Male vascular endothelial cell-specific Pdpk1-knockout mice (Tie2^+/−/Pdpk1^flox/flox mice) and their wild-type littermates (Tie2^−/−/Pdpk1^flox/flox mice; control) were used for this study. At 12 weeks of age, an IPGTT and OGTT were conducted. Pancreatic blood flow was measured under anaesthesia. Thereafter, islet blood flow was measured by the microsphere method. Mice were killed for islet isolation and further functional study and mRNA was extracted from islets. Pancreases were sampled for immunohistochemical analyses.

During the IPGTT, the blood glucose level was comparable between knockout mice and control flox mice, although serum insulin level was significantly lower in knockout mice. During the OGTT, glucose tolerance deteriorated slightly in knockout mice, accompanied by a decreased serum insulin level. During an IPGTT after pre-treatment with exendin-4 (Ex-4), glucose tolerance was significantly impaired in knockout mice. In fact, glucose-stimulated insulin secretion of isolated islets from knockout mice was significantly reduced compared with control flox mice, and addition of Ex-4 revealed impaired sensitivity to incretin hormones in islets of knockout mice. In immunohistochemical analyses, both alpha and beta cell masses were significantly reduced in knockout mice. In addition, the CD31-positive area was significantly decreased in islets of knockout mice. The proportion of pimonidazole-positive islets was significantly increased in knockout mice. mRNA expression levels related to insulin biosynthesis (Ins1, Ins2, Mafa, Pdx1 and Neurod [also known as Neurod1]) and beta cell function (such as Gck and Slc2a2) were significantly decreased in islets of knockout mice. Microsphere experiments revealed remarkably reduced islet blood flow. In addition, mRNA expression levels of Hif1α (also known as Hif1a) and its downstream factors such as Adm, Eno1, Tpi1 (also known as Ets1), Hmox1 and Vegfa, were significantly increased in islets of knockout mice, indicating that islets of knockout mice were in a more hypoxic state than those of control flox mice. As a result, mRNA expression levels related to adaptive unfolded protein response and endoplasmic reticulum stress-related apoptotic genes were significantly elevated in islets of knockout mice. In addition, inflammatory cytokine levels were increased in islets of knockout mice. Electron microscopy revealed reduced endothelial fenestration and thickening of basal membrane of vascular endothelium in islets of knockout mice.

Vascular endothelial PDPK1 plays an important role in the maintenance of pancreatic beta cell mass and function by maintaining vascularity of pancreas and islets and protecting them from hypoxia, hypoxia-related endoplasmic reticulum stress, inflammation and distortion of capillary structure.