Sustained beta-cell dysfunction but normalized islet mass in aged thrombospondin-1 deficient mice

Thrombospondin 1 in Metabolic Diseases

The thrombospondin family comprises of five multifunctional glycoproteins, whose best-studied member is thrombospondin 1 (TSP1). This matricellular protein is a potent antiangiogenic agent that inhibits endothelial migration and proliferation, and induces endothelial apoptosis. Studies have demonstrated a regulatory role of TSP1 in cell migration and in activation of the latent transforming growth factor beta 1 (TGFβ1). These functions of TSP1 translate into its broad modulation of immune processes. Further, imbalances in immune regulation have been increasingly linked to pathological conditions such as obesity and diabetes mellitus. While most studies in the past have focused on the role of TSP1 in cancer and inflammation, recently published data have revealed new insights about the role of TSP1 in physiological and metabolic disorders. Here, we highlight recent findings that associate TSP1 and its receptors to obesity, diabetes, and cardiovascular diseases. TSP1 regulates nitric oxide, activates latent TGFβ1, and interacts with receptors CD36 and CD47, to play an important role in cell metabolism. Thus, TSP1 and its major receptors may be considered a potential therapeutic target for metabolic diseases.

The Role of Vascular Cells in Pancreatic Beta-Cell Function

Insulin-producing β-cells constitute the majority of the cells in the pancreatic islets. Dysfunction of these cells is a key factor in the loss of glucose regulation that characterizes type 2 diabetes. The regulation of many of the functions of β-cells relies on their close interaction with the intra-islet microvasculature, comprised of endothelial cells and pericytes. In addition to providing islet blood supply, cells of the islet vasculature directly regulate β-cell activity through the secretion of growth factors and other molecules. These factors come from capillary mural pericytes and endothelial cells, and have been shown to promote insulin gene expression, insulin secretion, and β-cell proliferation. This review focuses on the intimate crosstalk of the vascular cells and β-cells and its role in glucose homeostasis and diabetes.

Formation of βTC3 and MIN6 Pseudoislets Changes the Expression Pattern of Gpr40, Gpr55, and Gpr119 Receptors and Improves Lysophosphatidylcholines-Potentiated Glucose-Stimulated Insulin Secretion

The impaired spatial arrangement and connections between cells creating islets of Langerhans as well as altered expression of G protein-coupled receptors (GPCRs) often lead to dysfunction of insulin-secreting pancreatic β cells and can significantly contribute to the development of diabetes. Differences in glucose-stimulated insulin secretion (GSIS) are noticeable not only in diabetic individuals but also in model pancreatic β cells, e.g., βTC3 and MIN6 β cell lines with impaired and normal insulin secretion, respectively. Now, we compare the ability of GPCR agonists (lysophosphatidylcholines bearing fatty acid chains of different lengths) to potentiate GSIS in βTC3 and MIN6 β cell models, cultured as adherent monolayers and in a form of pseudoislets (PIs) with pancreatic MS1 endothelial cells. Our aim was also to investigate differences in expression of the GPCRs responsive to LPCs in these experimental systems. Aggregation of β cells into islet-like structures greatly enhanced the expression of Gpr40, Gpr55, and Gpr119 receptors. In contrast, the co-culture of βTC3 cells with endothelial cells converted the GPCR expression pattern closer to the pattern observed in MIN6 cells. Additionally, the efficiencies of various LPC species in βTC3-MS1 PIs also shifted toward the MIN6 cell model.

Thrombospondin-1 in maladaptive aging responses: a concept whose time has come

Numerous age-dependent alterations at the molecular, cellular, tissue and organ systems levels underlie the pathophysiology of aging. Herein, the focus is upon the secreted protein thrombospondin-1 (TSP1) as a promoter of aging and age-related diseases. TSP1 has several physiological functions in youth, including promoting neural synapse formation, mediating responses to ischemic and genotoxic stress, minimizing hemorrhage, limiting angiogenesis, and supporting wound healing. These acute functions of TSP1 generally require only transient expression of the protein. However, accumulating basic and clinical data reinforce the view that chronic diseases of aging are associated with accumulation of TSP1 in the extracellular matrix, which is a significant maladaptive contributor to the aging process. Identification of the relevant cell types that chronically produce and respond to TSP1 and the molecular mechanisms that mediate the resulting maladaptive responses could direct the development of therapeutic agents to delay or revert age-associated maladies.

Vascular endothelial growth factor over-expressed mesenchymal stem cells-conditioned media ameliorate palmitate-induced diabetic endothelial dysfunction through PI-3K/AKT/m-TOR/eNOS and p38/MAPK signaling pathway

In the pathogenesis of diabetes mellitus (DM), islet microvasculares are severely damaged due to glucolipotoxicity and other reasons. Vascular endothelial growth factor (VEGF) is an indispensable and specific angiogenic factor in the pathogenesis and treatment of diabetic islet microvascular disease. Mesenchymal stem cells (MSCs) are regarded as a promising treatment of diabetes because of their immunosuppressive effect and multipotential differentiation potency. In this study, we tested whether MSCs over-expressing VEGF conditioned medium (MSC-VEGF-CM) could ameliorate pancreatic islet endothelial cells (MS-1) dysfunction induced by a common diabetic inducer palmitate (PA). We found that cell survival and migration were restrained by PA and partly repaired by the pro-protected of MSC-VEGF-CM. Meanwhile, PI-3K/AKT/m-TOR/eNOS and p38/MAPK signaling pathways were also up-regulated. Though apoptosis-related proteins, caspase-3 and caspase-9, had no significantly suppressed between MSC-VEGF-CM and MSC-CM alone, the expression levels of vascular surface factors such as CD31, VE-cadherin, occludin and ICAM-1, were remarkably up-regulated by the pro-protected of MSC-VEGF-CM. Our data suggested that MSC-VEGF-CM had therapeutic effect on the PA-induced dysfunction through the re-activation of PI-3K/AKT/m-TOR/eNOS and p38/MAPK signaling pathways.

Pancreatic β-cell protection from inflammatory stress by the endoplasmic reticulum proteins thrombospondin 1 and mesencephalic astrocyte-derived neutrotrophic factor (MANF)

Cytokine-induced endoplasmic reticulum (ER) stress is one of the molecular mechanisms underlying pancreatic β-cell demise in type 1 diabetes. Thrombospondin 1 (THBS1) was recently shown to promote β-cell survival during lipotoxic stress. Here we show that ER-localized THBS1 is cytoprotective to rat, mouse, and human β-cells exposed to cytokines or thapsigargin-induced ER stress. THBS1 confers cytoprotection by maintaining expression of mesencephalic astrocyte-derived neutrotrophic factor (MANF) in β-cells and thereby prevents the BH3-only protein BIM (BCL2-interacting mediator of cell death)-dependent triggering of the mitochondrial pathway of apoptosis. Prolonged exposure of β-cells to cytokines or thapsigargin leads to THBS1 and MANF degradation and loss of this prosurvival mechanism. Approaches that sustain intracellular THBS1 and MANF expression in β-cells should be explored as a cytoprotective strategy in type 1 diabetes.

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.

Thrombospondin 1 protects pancreatic β-cells from lipotoxicity via the PERK-NRF2 pathway

The failure of β-cells has a central role in the pathogenesis of type 2 diabetes, and the identification of novel approaches to improve functional β-cell mass is essential to prevent/revert the disease. Here we show a critical novel role for thrombospondin 1 (THBS1) in β-cell survival during lipotoxic stress in rat, mouse and human models. THBS1 acts from within the endoplasmic reticulum to activate PERK and NRF2 and induce a protective antioxidant defense response against palmitate. Prolonged palmitate exposure causes THBS1 degradation, oxidative stress, activation of JNK and upregulation of PUMA, culminating in β-cell death. These findings shed light on the mechanisms leading to β-cell failure during metabolic stress and point to THBS1 as an interesting therapeutic target to prevent oxidative stress in type 2 diabetes.

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.

Revisiting the matricellular concept

The concept of a matricellular protein was first proposed by Paul Bornstein in the mid-1990s to account for the non-lethal phenotypes of mice with inactivated genes encoding thrombospondin-1, tenascin-C, or SPARC. It was also recognized that these extracellular matrix proteins were primarily counter or de-adhesive. This review reappraises the matricellular concept after nearly two decades of continuous investigation. The expanded matricellular family as well as the diverse and often unexpected functions, cellular location, and interacting partners/receptors of matricellular proteins are considered. Development of therapeutic strategies that target matricellular proteins are discussed in the context of pathology and regenerative medicine.

Automated quantification of pancreatic β-cell mass

β-Cell mass is a parameter commonly measured in studies of islet biology and diabetes. However, the rigorous quantification of pancreatic β-cell mass using conventional histological methods is a time-consuming process. Rapidly evolving virtual slide technology with high-resolution slide scanners and newly developed image analysis tools has the potential to transform β-cell mass measurement. To test the effectiveness and accuracy of this new approach, we assessed pancreata from normal C57Bl/6J mice and from mouse models of β-cell ablation (streptozotocin-treated mice) and β-cell hyperplasia (leptin-deficient mice), using a standardized systematic sampling of pancreatic specimens. Our data indicate that automated analysis of virtual pancreatic slides is highly reliable and yields results consistent with those obtained by conventional morphometric analysis. This new methodology will allow investigators to dramatically reduce the time required for β-cell mass measurement by automating high-resolution image capture and analysis of entire pancreatic sections.

The role of thrombospondin (TSP)-1 in obesity and diabetes

Matricellular proteins are extracellular macromolecules that do not serve a structural role, but when incorporated into the matrix, modulate cell:cell and cell:matrix interactions. The matricellular protein thrombospondin (TSP)-1, a potent angiostatic mediator and activator of transforming growth factor (TGF)-β, is upregulated in diabetes and obesity and may be involved in the pathogenesis of metabolic dysregulation and organ dysfunction. This manuscript discusses recently published observations on the role of TSP-1 in metabolic disease. In obesity models induced by a high-fat diet, adipose tissue TSP-1 upregulation induces inflammation and promotes weight gain and metabolic dysfunction. TSP-1 may have direct effects on adipocyte proliferation and fatty acid uptake. In diabetic subjects, TSP-1 upregulation in kidney, myocardium, and vascular tissue may promote dysfunction. In the myocardium, TSP-1 upregulation may transduce angiostatic signals inducing vascular rarefaction. Dissection of the functional domains involved in TSP-1 actions may lead to the development of peptide-based strategies for treatment of diabetes and its complications.

Thrombospondin-1 regulates adiposity and metabolic dysfunction in diet-induced obesity enhancing adipose inflammation and stimulating adipocyte proliferation

As a typical matricellular protein, thrombospondin (TSP)-1, binds to the structural matrix and regulates cellular behavior by modulating growth factor and cytokine signaling. Obesity and diabetes are associated with marked upregulation of TSP-1 in adipose tissue. We hypothesized that endogenous TSP-1 may play an important role in the pathogenesis of diet-induced obesity and metabolic dysfunction. Accordingly, we examined the effects of TSP-1 gene disruption on weight gain, adiposity, and adipose tissue inflammation in mice receiving a high-fat diet (HFD: 60% fat, 20% carbohydrate) or a high-carbohydrate low-fat diet (HCLFD: 10% fat, 70% carbohydrate). HFD mice had significantly higher TSP-1 expression in perigonadal adipose tissue; TSP-1 was predominantly localized in the adipose interstitium. TSP-1 loss attenuated weight gain and fat accumulation in HFD and HCLFD groups. Compared with corresponding wild-type animals, TSP-1-null mice had decreased insulin levels but exhibited elevated free fatty acid and triglyceride levels, suggesting impaired fatty acid uptake. TSP-1 loss did not affect adipocyte size and had no effect on adipose vascular density. However, TSP-1-null mice exhibited attenuated tumor necrosis factor-α mRNA expression and reduced macrophage infiltration, suggesting a role for TSP-1 in mediating obesity-associated inflammation. In vitro, TSP-1 enhanced proliferation of 3T3-L1 preadipocytes but did not modulate inflammatory cytokine and chemokine synthesis. In conclusion, TSP-1 upregulation contributes to weight gain, adipose growth, and the pathogenesis of metabolic dysfunction. The effects of TSP-1 may involve stimulation of adipocyte proliferation, activation of inflammatory signaling, and facilitated fatty acid uptake by adipocytes.