The β-cell/EC axis: how do islet cells talk to each other?

Innate immune priming of insulin secretion

Increasing evidence suggests a role for the immune system to finely tune metabolic homeostasis. The possibility that the immune system can likewise regulate islet endocrine function has only commenced drawing attention. Islet beta cells are the main producers of insulin and have to dynamically respond to fluctuating insulin demands of the body. While inflammation has long been considered as an important pathogenic feature of diabetes development, pioneer studies have shown that immune cells reside inside pancreatic islets under steady state and that components of the immune system can promote beta cell insulin production. The present review will thus highlight the recent research on specific immune pathways regulating beta cell function discussing the beneficial influence of innate immune cells.

A 1 week IGF-1 infusion decreases arterial insulin concentrations but increases pancreatic insulin content and islet vascularity in fetal sheep

Fetal insulin is critical for regulation of growth. Insulin concentrations are partly determined by the amount of β-cells present and their insulin content. Insulin-like growth factor-1 (IGF-1) is a fetal anabolic growth factor which also impacts β-cell mass in models of β-cell injury and diabetes. The extent to which circulating concentrations of IGF-1 impact fetal β-cell mass and pancreatic insulin content is unknown. We hypothesized that an infusion of an IGF-1 analog for 1 week into the late gestation fetal sheep circulation would increase β-cell mass, pancreatic islet size, and pancreatic insulin content. After the 1-week infusion, pancreatic insulin concentrations were 80% higher than control fetuses (P < 0.05), but there were no differences in β-cell area, β-cell mass, or pancreatic vascularity. However, pancreatic islet vascularity was 15% higher in IGF-1 fetuses and pancreatic VEGFA, HGF, IGF1, and IGF2 mRNA expressions were 70-90% higher in IGF-1 fetuses compared to control fetuses (P < 0.05). Plasma oxygen, glucose, and insulin concentrations were 25%, 22%, and 84% lower in IGF-1 fetuses, respectively (P < 0.05). The previously described role for IGF-1 as a β-cell growth factor may be more relevant for local paracrine signaling in the pancreas compared to circulating endocrine signaling.

Adhesion characteristics of porcine pancreatic islets and exocrine tissue to coating materials

Since the report of the Edmonton protocol in 2000, islet transplantation has been implemented worldwide, and xenotransplantation using porcine islets has also been reported. In addition, many basic experiments using pancreatic islets and exocrine tissue after isolation have been reported. Recently, exocrine cells have been found to be essential for inducing the differentiation of pancreatic islets. Therefore, the importance of the culture conditions for pancreatic tissue when conducting experiments using pancreatic tissue is also increasing. In this study, we focused on the coat material and examined the adhesive properties of porcine pancreatic islets and exocrine tissue after isolation. Porcine islet isolation was performed, and isolated islets (purity ≥95%) and exocrine tissue (purity ≥99%) were used to achieve adhesion to several extracellular matrixes, fibronectin, collagen type I, collagen type IV, laminin I, fibrinogen, and bovine serum albumin (BSA). DMEM with 0.5% FBS was used as the assay medium. For exocrine tissue, the adhesion was promoted in fibronectin, collagen type I, laminin I, and fibrinogen. The adhesive ability to fibronectin was more than twice that to BSA, while the adhesive ability to collagen type I, laminin I, and fibrinogen was less than twice that to BSA. For islets, the adhesive ability to fibronectin was weaker than that of exocrine tissue. Furthermore, the adhesion effect in fibronectin was obtained within 30 minutes and in medium containing little serum for both islets and exocrine tissues. These data suggest that fibronectin may be useful for the adhesion of pancreatic tissue.

Gene expression profiles of glucose toxicity-exposed islet microvascular endothelial cells

OBJECTIVE

Islet microcirculation is mainly composed by IMECs. The aim of the study was to investigate the differences in gene expression profiles of IMECs upon glucose toxicity exposure and insulin treatment.

METHODS

IMECs were treated with 5.6 mmol L^-1 glucose, 35 mmol L^-1 glucose, and 35 mmol L^-1 glucose plus 10^-8  mol L^-1 insulin, respectively. Gene expression profiles were determined by microarray and verified by qPCR. GO terms and KEGG analysis were performed to assess the potential roles of differentially expressed genes. The interaction and expression tendency of differentially expressed genes were analyzed by Path-Net algorithm.

RESULTS

Compared with glucose toxicity-exposed IMECs, 1574 mRNAs in control group and 2870 mRNAs in insulin-treated IMECs were identified with differential expression, respectively. GO and KEGG pathway analysis revealed that these genes conferred roles in regulation of apoptosis, proliferation, migration, adhesion, and metabolic process etc. Additionally, MAPK signaling pathway and apoptosis were the dominant nodes in Path-Net. IMECs survival and function pathways were significantly changed, and the expression tendency of genes from euglycemia and glucose toxicity exposure to insulin treatment was revealed and enriched in 7 patterns.

CONCLUSIONS

Our study provides a microcirculatory framework for gene expression profiles of glucose toxicity-exposed IMECs.

Sprouting angiogenesis in engineered pseudo islets

Despite the recent achievements in cell-based therapies for curing type-1 diabetes (T1D), capillarization in beta (β)-cell clusters is still a major roadblock as it is essential for long-term viability and function of β-cells in vivo. In this research, we report sprouting angiogenesis in engineered pseudo islets (EPIs) made of mouse insulinoma βTC3 cells and rat heart microvascular endothelial cells (RHMVECs). Upon culturing in three-dimensional (3D) constructs under angiogenic conditions, EPIs sprouted extensive capillaries into the surrounding matrix. Ultra-morphological analysis through histological sections also revealed presence of capillarization within EPIs. EPIs cultured in 3D constructs maintained their viability and functionality over time while non-vascularized EPIs, without the presence of RHMVECs, could not retain their viability nor functionality. Here we demonstrate angiogenesis in engineered islets, where patient specific stem cell-derived human beta cells can be combined with microvascular endothelial cells in the near future for long-term graft survival in T1D patients.

Endothelial and beta cell composite aggregates for improved function of a bioartificial pancreas encapsulation device

INTRODUCTION

Encapsulation of pancreatic islets or beta cells is a promising strategy for treatment of type 1 diabetes by providing an immune isolated environment and allowing for transplantation in a different location than the liver. However, islets used for encapsulation often show lower functionality due to the damaging of islet endothelial cells during the isolation procedure. Factors produced by endothelial cells have great impact on beta cell insulin secretion. Therefore, mutual signaling between endothelial cells and beta cells should be considered for the development of encapsulation systems to achieve high insulin secretion and maintain beta cell viability. Here, we investigate whether co-culture of beta cells with endothelial cells could improve beta cell function within encapsulation devices.

MATERIALS AND METHODS

Mouse insulinoma MIN6 cells and human umbilical vein endothelial cells were used for creating composite aggregates on agarose microwell platform. The composite aggregates were encapsulated within flat poly(ether sulfone)/polyvinylpyrrolidone device. Their functionality was assessed by glucose-induced insulin secretion test and compared to non-encapsulated free-floating aggregates.

RESULTS

We created composite aggregates of 80-100 µm in diameter, closely mimicking pancreatic islets. Upon glucose stimulation, their insulin secretion is improved in comparison to aggregates consisting of only MIN6 cells. Moreover, the composite aggregates encapsulated within a device secrete more insulin than aggregates consisting of only MIN6 cells.

CONCLUSION

Composite aggregates of MIN6 cells with human umbilical vein endothelial cells have improved insulin secretion in comparison to MIN6 aggregates showing that the interaction of beta cell and endothelial cell is crucial for a functional encapsulation system.

Quercetin improves endothelial function in diabetic rats through inhibition of endoplasmic reticulum stress-mediated oxidative stress

Endoplasmic reticulum (ER) stress attributes a crucial role in diabetes-induced endothelial dysfunction. The present study investigated the effects of quercetin, a potent antioxidant on the attenuation of ER stress-modulated endothelial dysfunction in streptozotocin (STZ)-induced diabetic rats. Oral administration of quercetin for six weeks to diabetic rats dose-dependently reduced the blood glucose levels and improved insulin secretion. Histopathological examination of pancreatic tissues in diabetic rats showed pathological changes such as shrunken islets, reduction in islet area and distorted β-cells, which were found to be restored by quercetin treatment. In addition, quercetin reduced the pancreatic ER stress-induced endothelial dysfunction as assessed by immunohistochemical analysis of C/ERB homologous protein (CHOP) and endothelin-1 (ET-1). Moreover, quercetin administration progressively increased the expression of vascular endothelial growth factor (VEGF) and its receptor, VEGFR2 in diabetes rats. Quercetin-mediated decrease in the nitric oxide (NO∙) and cyclic 3',5'- guanosine monophosphate (cGMP) levels were also observed in the diabetic rats. Quercetin treatment reduced the lipid peroxidation in the diabetic rats, meanwhile increased the total antioxidant capacity in the pancreas from diabetic rats. Altogether, these results demonstrated the vasoprotective effect of quercetin against STZ-induced ER stress in the pancreas of diabetic rats.

Connexins and microRNAs: Interlinked players in regulating islet function?

Pancreatic β-cells are connected to neighboring endocrine cells through the adherin proteins and gap junctions. Connexin 36 (Cx36) is one of the most well-studied and abundantly expressed gap-junction proteins within rodent islets, which is important in coordinated insulin secretion. The expression of connexins is regulated at various levels and by several mechanisms; one of which is via microRNAs. In past 2 decades, microRNAs (miRNAs) have emerged as key molecules in developmental, physiologic and pathological processes. However, very few studies have demonstrated miRNA-mediated regulation of connexins. Even though there are no reports yet on miRNAs and Cx36; we envisage that considering the important role of connexins and microRNAs in insulin secretion, there would be common pathways interlinking these biomolecules. Here, we discuss the current literature on connexins and miRNAs specifically with reference to islet function.

The islet endothelial cell: a novel contributor to beta cell secretory dysfunction in diabetes

The pancreatic islet is highly vascularised, with an extensive capillary network. In addition to providing nutrients and oxygen to islet endocrine cells and transporting hormones to the peripheral circulation, islet capillaries (comprised primarily of islet endothelial cells) are an important source of signals that enhance survival and function of the islet beta cell. In type 2 diabetes, and animal models thereof, evidence exists of morphological and functional abnormalities in these islet endothelial cells. In diabetes, islet capillaries are thickened, dilated and fragmented, and islet endothelial cells express markers of inflammation and activation. In vitro data suggest that this dysfunctional islet endothelial phenotype may contribute to impaired insulin release from the beta cell. This review examines potential candidate molecules that may mediate the positive effects of islet endothelial cells on beta cell survival and function under normal conditions. Further, it explores possible mechanisms underlying the development of islet endothelial dysfunction in diabetes and reviews therapeutic options for ameliorating this aspect of the islet lesion in type 2 diabetes. Finally, considerations regarding differences between human and rodent islet vasculature and the potentially unforeseen negative consequences of strategies to expand the islet vasculature, particularly under diabetic conditions, are discussed.

Local Sphingosine Kinase 1 Activity Improves Islet Transplantation

Pancreatic islet transplantation is a promising clinical treatment for type 1 diabetes, but success is limited by extensive β-cell death in the immediate posttransplant period and impaired islet function in the longer term. Following transplantation, appropriate vascular remodeling is crucial to ensure the survival and function of engrafted islets. The sphingosine kinase (SK) pathway is an important regulator of vascular beds, but its role in the survival and function of transplanted islets is unknown. We observed that donor islets from mice deficient in SK1 (Sphk1 knockout) contain a reduced number of resident intraislet vascular endothelial cells. Furthermore, we demonstrate that the main product of SK1, sphingosine-1-phosphate, controls the migration of intraislet endothelial cells in vitro. We reveal in vivo that Sphk1 knockout islets have an impaired ability to cure diabetes compared with wild-type controls. Thus, SK1-deficient islets not only contain fewer resident vascular cells that participate in revascularization, but likely also a reduced ability to recruit new vessels into the transplanted islet. Together, our data suggest that SK1 is important for islet revascularization following transplantation and represents a novel clinical target for improving transplant outcomes.

Exploring inter-organ crosstalk to uncover mechanisms that regulate β-cell function and mass

Impaired β-cell function and insufficient β-cell mass compensation are twin pathogenic features that underlie type 2 diabetes (T2D). Current therapeutic strategies continue to evolve to improve treatment outcomes in different ethnic populations and include approaches to counter insulin resistance and improve β-cell function. Although the effects of insulin secretion on metabolic organs such as liver, skeletal muscle and adipose is directly relevant for improving glucose uptake and reduce hyperglycemia, the ability of pancreatic β-cells to crosstalk with multiple non-metabolic tissues is providing novel insights into potential opportunities for improving β-cell function and/or mass that could have beneficial effects in patients with diabetes. For example, the role of the gastrointestinal system in the regulation of β-cell biology is well recognized and has been exploited clinically to develop incretin-related antidiabetic agents. The microbiome and the immune system are emerging as important players in regulating β-cell function and mass. The rich innervation of islet cells indicates it is a prime organ for regulation by the nervous system. In this review, we discuss the potential implications of signals from these organ systems as well as those from bone, placenta, kidney, thyroid, endothelial cells, reproductive organs and adrenal and pituitary glands that can directly impact β-cell biology. An added layer of complexity is the limited data regarding the relative relevance of one or more of these systems in different ethnic populations. It is evident that better understanding of this paradigm would provide clues to enhance β-cell function and/or mass in vivo in the long-term goal of treating or curing patients with diabetes.

Role of TRPV channels in regulating various pancreatic β-cell functions: Lessons from in vitro studies

Pancreatic β-cell functions are regulated by a variety of endogenous and exogenous factors. Calcium is one of the most potent triggers of β-cell growth, insulin production and exocytosis. Recently, others and we showed that TRPV channels are expressed in insulin producing cell lines and/or primary β-cells. These channels modulate calcium ions, insulin secretion and cell proliferation. Besides the classical roles of TRPV channels in the sensory system, there are also novel functions described in non-excitable cells such as in insulin-producing β-cells. This review summarises the current knowledge about the expression and the role of TRPV channels in controlling β-cell functions based upon studies performed in isolated primary β-cells as well as permanent β-cell models.

Impact of islet architecture on β-cell heterogeneity, plasticity and function

Although β-cell heterogeneity was discovered more than 50 years ago, the underlying principles have been explored only during the past decade. Islet-cell heterogeneity arises during pancreatic development and might reflect the existence of distinct populations of progenitor cells and the developmental pathways of endocrine cells. Heterogeneity can also be acquired in the postnatal period owing to β-cell plasticity or changes in islet architecture. Furthermore, β-cell neogenesis, replication and dedifferentiation represent alternative sources of β-cell heterogeneity. In addition to a physiological role, β-cell heterogeneity influences the development of diabetes mellitus and its response to treatment. Identifying phenotypic and functional markers to discriminate distinct β-cell subpopulations and the mechanisms underpinning their regulation is warranted to advance current knowledge of β-cell function and to design novel regenerative strategies that target subpopulations of β cells. In this context, the Wnt/planar cell polarity (PCP) effector molecule Flattop can distinguish two unique β-cell subpopulations with specific transcriptional signatures, functional properties and differential responses to environmental stimuli. In vivo targeting of these β-cell subpopulations might, therefore, represent an alternative strategy for the future treatment of diabetes mellitus.

Immune and Pancreatic β Cell Interactions in Type 1 Diabetes

The autoimmune destruction of the pancreatic islet β cells is due to a targeted lymphocyte attack. Different T cell subsets communicate with each other and with the insulin-producing β cells in this process, with evidence not only of damage to the tissue cells but also of lymphocyte regulation. Here we explore the various components of the immune response as well as the cellular interactions that are involved in causing or reducing immune damage to the β cells. We consider these in the light of the possibility that understanding them may help us identify therapeutic targets to reduce the damage and destruction leading to type 1 diabetes.

Effective endothelial cell and human pluripotent stem cell interactions generate functional insulin-producing beta cells

AIMS/HYPOTHESIS

Endothelial cells (ECs) play an essential role in pancreatic organogenesis. We hypothesise that effective in vitro interactions between human microvascular endothelial cells (HMECs) and human pluripotent stem cells (hPSCs) results in the generation of functional pancreatic beta cells.

METHODS

Embryoid bodies (EBs) derived from hPSCs were cultured alone (controls) or with ECs in collagen gels. Subsequently, cells were analysed for pancreatic beta cell markers, and then isolated and expanded. Insulin secretion in response to glucose was evaluated in vitro by static and dynamic (perifusion) assays, and in vivo by EB transplantation into immunodeficient mice.

RESULTS

Co-cultured EBs had a higher expression of mature beta cells markers and enhanced insulin secretion in vitro, compared with controls. In mice, transplanted EBs had higher levels of human C-peptide secretion with a significant reduction in hyperglycaemia after the selective destruction of native pancreatic beta cells. In addition, there was significant in vitro upregulation of bone morphogenetic proteins 2 and 4 (BMP-2, 4) in co-cultured cells, compared with controls.

CONCLUSIONS/INTERPRETATION

ECs provide essential signalling in vitro, such as activation of the BMP pathway, for derivation of functional insulin-producing beta cells from hPSCs.

Islet-like organoids derived from human pluripotent stem cells efficiently function in the glucose responsiveness in vitro and in vivo

Insulin secretion is elaborately modulated in pancreatic ß cells within islets of three-dimensional (3D) structures. Using human pluripotent stem cells (hPSCs) to develop islet-like structures with insulin-producing ß cells for the treatment of diabetes is challenging. Here, we report that pancreatic islet-like clusters derived from hESCs are functionally capable of glucose-responsive insulin secretion as well as therapeutic effects. Pancreatic hormone-expressing endocrine cells (ECs) were differentiated from hESCs using a step-wise protocol. The hESC-derived ECs expressed pancreatic endocrine hormones, such as insulin, somatostatin, and pancreatic polypeptide. Notably, dissociated ECs autonomously aggregated to form islet-like, 3D structures of consistent sizes (100–150 μm in diameter). These EC clusters (ECCs) enhanced insulin secretion in response to glucose stimulus and potassium channel inhibition in vitro. Furthermore, ß cell-deficient mice transplanted with ECCs survived for more than 40 d while retaining a normal blood glucose level to some extent. The expression of pancreatic endocrine hormones was observed in tissues transplanted with ECCs. In addition, ECCs could be generated from human induced pluripotent stem cells. These results suggest that hPSC-derived, islet-like clusters may be alternative therapeutic cell sources for treating diabetes.

Bilateral oophorectomy may have an unfavorable effect on glucose metabolism compared with natural menopause

The incidence of diabetes mellitus is rising worldwide. The aim of this prospective epidemiological study was to compare the effects of natural and surgical menopause on parameters of glucose metabolism. In a group of 587 repeatedly examined women, with a baseline age of 45-55 years, the following subgroups of women were separated: those after bilateral oophorectomy (BO, n=37) and those in natural menopause (NAT, n=380) including women menopausal already at baseline (POST, n=89). The study parameters including glycemia, insulinemia, HOMA-IR and beta-cell function using HOMA-beta were determined at baseline and 6 years later. Over the study period, there was a marked rise in prediabetic and diabetic values of fasting glycemia; the percentage of women with diabetic values increased in the NAT (from 0.8 % to 3.9 %) and POST (from 2.2 % to 9.0 %) subgroups, with the highest prevalence in the BO subgroup (from 8.1 % to 10.8 %). While, among women with non-diabetic fasting glycemia, an increase in fasting glycemia was observed in all study subgroups, it was more marked in the BO subgroup than in the NAT and POST ones (p=0.02 both). This difference between NAT and BO was also found in the long-term trend of development of glycemia in non-diabetic women (p=0.014). Compared with natural menopause, bilateral oophorectomy may have an adverse effect on glucose metabolism.

Oxidative and endoplasmic reticulum stress in β-cell dysfunction in diabetes

The inability of pancreatic β-cells to make sufficient insulin to control blood sugar is a central feature of the aetiology of most forms of diabetes. In this review we focus on the deleterious effects of oxidative stress and endoplasmic reticulum (ER) stress on β-cell insulin biosynthesis and secretion and on inflammatory signalling and apoptosis with a particular emphasis on type 2 diabetes (T2D). We argue that oxidative stress and ER stress are closely entwined phenomena fundamentally involved in β-cell dysfunction by direct effects on insulin biosynthesis and due to consequences of the ER stress-induced unfolded protein response. We summarise evidence that, although these phenomenon can be driven by intrinsic β-cell defects in rare forms of diabetes, in T2D β-cell stress is driven by a range of local environmental factors including increased drivers of insulin biosynthesis, glucolipotoxicity and inflammatory cytokines. We describe our recent findings that a range of inflammatory cytokines contribute to β-cell stress in diabetes and our discovery that interleukin 22 protects β-cells from oxidative stress regardless of the environmental triggers and can correct much of diabetes pathophysiology in animal models. Finally we summarise evidence that β-cell dysfunction is reversible in T2D and discuss therapeutic opportunities for relieving oxidative and ER stress and restoring glycaemic control.

Adenosine signalling in diabetes mellitus--pathophysiology and therapeutic considerations

Adenosine is a key extracellular signalling molecule that regulates several aspects of tissue function by activating four G-protein-coupled receptors, A1, A2A, A2B and A1 adenosine receptors. Accumulating evidence highlights a critical role for the adenosine system in the regulation of glucose homeostasis and the pathophysiology of type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). Although adenosine signalling is known to affect insulin secretion, new data indicate that adenosine signalling also contributes to the regulation of β-cell homeostasis and activity by controlling the proliferation and regeneration of these cells as well as the survival of β cells in inflammatory microenvironments. Furthermore, adenosine is emerging as a major regulator of insulin responsiveness by controlling insulin signalling in adipose tissue, muscle and liver; adenosine also indirectly mediates effects on inflammatory and/or immune cells in these tissues. This Review critically discusses the role of the adenosine-adenosine receptor system in regulating both the onset and progression of T1DM and T2DM, and the potential of pharmacological manipulation of the adenosinergic system as an approach to manage T1DM, T2DM and their associated complications.

Skin deep: from dermal fibroblasts to pancreatic beta cells

Type I diabetes (T1D) is a chronic autoimmune disease caused by pancreatic β-cell destruction induced by autoantibodies and autoreactive T cells. After significant reduction of the β-cell mass, diabetes sets in and can cause significant complications. It is estimated that more than 3 million Americans have T1D, and its prevalence among young individuals is progressively rising; however, the reasons for this increase are not known. Islet transplantation is recognized as the ultimate cure for T1D, but unfortunately, the severe scarcity of available islets makes it necessary to establish alternative sources of β-cells. Our lab seeks to establish human-induced pluripotent stem cells as an unlimited, novel source of insulin-producing cells (IPCs) that are patient-specific, obviating the requirement for immunosuppression. Although several reports have emerged demonstrating successful derivation of IPCs from human pluripotent stem cells, the efficiencies of derivation are inadequate and these IPCs do not respond to glucose stimulation in vitro. We reasoned that the use of a growth factor sequestering bioscaffold and promotion of cell-cell signaling through 3D clustering would enhance the generation of functionally superior IPCs compared to those derived by 2D differentiation. Here, we discuss a novel 3D platform for the generation of highly efficient human IPCs.

Matrix metalloproteinase-9 is essential for physiological Beta cell function and islet vascularization in adult mice

The availability of paracrine factors in the islets of Langerhans, and the constitution of the beta cell basement membrane can both be affected by proteolytic enzymes. This study aimed to investigate the effects of the extracellular matrix-degrading enzyme gelatinase B/matrix metalloproteinase-9 (Mmp-9) on islet function in mice. Islet function of Mmp9-deficient (Mmp9(-/-)) mice and their wild-type littermates was evaluated both in vivo and in vitro. The pancreata of Mmp9(-/-) mice did not differ from wild type in islet mass or distribution. However, Mmp9(-/-) mice had an impaired response to a glucose load in vivo, with lower serum insulin levels. The glucose-stimulated insulin secretion was reduced also in vitro in isolated Mmp9(-/-) islets. The vascular density of Mmp9(-/-) islets was lower, and the capillaries had fewer fenestrations, whereas the islet blood flow was threefold higher. These alterations could partly be explained by compensatory changes in the expression of matrix-related proteins. This in-depth investigation of the effects of the loss of MMP-9 function on pancreatic islets uncovers a deteriorated beta cell function that is primarily due to a shift in the beta cell phenotype, but also due to islet vascular aberrations. This likely reflects the importance of a normal islet matrix turnover exerted by MMP-9, and concomitant release of paracrine factors sequestered on the matrix.

Placental insufficiency decreases pancreatic vascularity and disrupts hepatocyte growth factor signaling in the pancreatic islet endothelial cell in fetal sheep

Hepatocyte growth factor (HGF) and vascular endothelial growth factor A (VEGFA) are paracrine hormones that mediate communication between pancreatic islet endothelial cells (ECs) and β-cells. Our objective was to determine the impact of intrauterine growth restriction (IUGR) on pancreatic vascularity and paracrine signaling between the EC and β-cell. Vessel density was less in IUGR pancreata than in controls. HGF concentrations were also lower in islet EC-conditioned media (ECCM) from IUGR, and islets incubated with control islet ECCM responded by increasing insulin content, which was absent with IUGR ECCM. The effect of ECCM on islet insulin content was blocked with an inhibitory anti-HGF antibody. The HGF receptor was not different between control and IUGR islets, but VEGFA was lower and the high-affinity VEGF receptor was higher in IUGR islets and ECs, respectively. These findings show that paracrine actions from ECs increase islet insulin content, and in IUGR ECs, secretion of HGF was diminished. Given the potential feed-forward regulation of β-cell VEGFA and islet EC HGF, these two growth factors are highly integrated in normal pancreatic islet development, and this regulation is decreased in IUGR fetuses, resulting in lower pancreatic islet insulin concentrations and insulin secretion.

β-Cell function in type 2 diabetes

Different in vivo tests explore different aspects of β-cell function. Because intercorrelation of insulin secretion indices is modest, no single in vivo test allows β-cell function to be assessed with accuracy and specificity comparable to insulin sensitivity. Physiologically-based mathematical modeling is necessary to interpret insulin secretory responses in terms of relevant parameters of β-cell function. Models can be used to analyze intravenous glucose tests, but secretory responses to intravenous glucose may be paradoxical in subjects with diabetes. Use of oral glucose (or mixed meal) data may be preferable not only for simplicity but also for physiological interpretation. While the disposition index focuses on the relationship between insulin secretion and insulin resistance, secretion parameters reflecting the dynamic response to changing glucose levels over a time frame of minutes or hours--such as β-cell glucose sensitivity--are key to explain changes in glucose tolerance and are largely independent of insulin sensitivity. Pathognomonic of the β-cell defect of type 2 diabetes is a reduced glucose sensitivity, which is accompanied by normal or raised absolute insulin secretion rates--compensatory to the attendant insulin resistance--and impaired incretin-induced potentiation. As β-cell mass is frequently within the range of nondiabetic individuals, these defects are predominantly functional and potentially reversible. Any intervention, on lifestyle or with drugs, that improves glucose tolerance does so primarily through increased β-cell glucose sensitivity. So far, however, no intervention has proven unequivocally capable of modifying the natural course of β-cell dysfunction.

The endocrine role between β cells and intra-islet endothelial cells

The islets of Langerhans is the endocrine function region of pancreas, which exist in five cell types. The majority of endocrine cells are insulin-secreting β cells, mixed up with glucagon-secreting α-cells. The islets of Langerhans are highly vascularized, and the capillary network around the islet is about five times denser than that in the exocrine tissues. It guarantees endocrine cells adequately contact with the capillary networks. Above mentioned is the basis of deep study the interaction between β cells and capillary. Increasing number of studies contribute to the consensus that endothelial cells have positive effects in the islet microenvironment. Endothelial cells can act as endocrine cells which release many active substances, such as hepatocyte growth factors (HGF), thrombospondin-1(TSP-1), laminins, and collagens by means of different molecule pathways, inducing β cells differentiation, proliferation, survivor, and insulin release next to the vessels. Apart from the effect of endothelial cells on β cells by paracrine fashion, the islets can utilize VEGF-A, angiopoietin-1 and insulin signaling to increase the interaction with endothelial cells. As the endocrine role of endothelial cells to β cells, it may be a novel target to stimulate β cells regeneration, promote vascularization post islet transplantation strategy in the treatment of diabetes mellitus.