Studying Adipose Endothelial Cell/Adipocyte Cross-Talk in Human Subcutaneous Adipose Tissue

Microvascular endothelial cells (MVECs) have many critical roles, including control of vascular tone, regulation of thrombosis, and angiogenesis. ...

University of Leeds

Vascular endothelial growth factor receptor 1 (VEGFR1) is an essential receptor tyrosine kinase that regulates mammalian vascular development and embryogenesis but its function is not well understood. Herein, we present evidence whereby endothelial VEGFR1 is largely resident within the Golgi apparatus but translocates to the plasma membrane via a calcium-regulated process. Primary human endothelial cells reveal differing VEGFR1 and VEGFR2 intracellular distribution and dynamics. The major proportion of the full-length VEGFR1 membrane protein was resident within the Golgi apparatus in primary endothelial cells. Whereas VEGFR2 displayed down-regulation in response to VEGF-A, VEGFR1 was not significantly affected arguing for a significant intracellular pool that was inaccessible to extracellular VEGF-A. This intracellular VEGFR1 pool showed significant co-distribution with key Golgi residents. Brefeldin A caused VEGFR1 Golgi fragmentation consistent with redistribution to the endoplasmic reticulum. Metabolic labeling experiments and microscopy using domain-specific VEGFR1 antibodies indicated that the mature processed VEGFR1 species and an integral membrane protein was resident within Golgi apparatus. Cytosolic calcium ions play a key role in VEGFR1 trafficking as treatment with either VEGF-A, histamine, thrombin, thapsigargin or A23187 ionophore caused VEGFR1 redistribution from the Golgi apparatus to small punctate vesicles and plasma membrane. We thus propose a model whereby the balance of VEGFR1 and VEGFR2 plasma membrane levels dictate either negative or positive endothelial signaling to influence vascular physiology.

VEGFR1 receptor tyrosine kinase localization to the Golgi apparatus is calcium-dependent.

Vascular endothelial growth factor A (VEGF-A)-induced signaling through VEGF receptor 2 (VEGFR2) regulates both physiological and pathological angiogenesis in mammals. However, the temporal and spatial mechanism underlying VEGFR2-mediated intracellular signaling is not clear. Here, we define a pathway for VEGFR2 trafficking and proteolysis that regulates VEGF-A-stimulated signaling and endothelial cell migration. Ligand-stimulated VEGFR2 activation and ubiquitination preceded proteolysis and cytoplasmic domain removal associated with endosomes. A soluble VEGFR2 cytoplasmic domain fragment displayed tyrosine phosphorylation and activation of downstream intracellular signaling. Perturbation of endocytosis by the depletion of either clathrin heavy chain or an ESCRT-0 subunit caused differential effects on ligand-stimulated VEGFR2 proteolysis and signaling. This novel VEGFR2 proteolysis was blocked by the inhibitors of 26S proteasome activity. Inhibition of proteasome activity prolonged VEGF-A-induced intracellular signaling to c-Akt and endothelial nitric oxide synthase (eNOS). VEGF-A-stimulated endothelial cell migration was dependent on VEGFR2 and VEGFR tyrosine kinase activity. Inhibition of proteasome activity in this assay stimulated VEGF-A-mediated endothelial cell migration. VEGFR2 endocytosis, ubiquitination and proteolysis could also be stimulated by a protein kinase C-dependent pathway. Thus, removal of the VEGFR2 carboxyl terminus linked to phosphorylation, ubiquitination and trafficking is necessary for VEGF-stimulated endothelial signaling and cell migration.

Ligand-stimulated VEGFR2 signaling is regulated by co-ordinated trafficking and proteolysis.

The mammalian endothelium expresses two related but distinct receptor tyrosine kinases, VEGFR1 and VEGFR2 [VEGF (vascular endothelial growth factor) receptor 1 and 2], that regulate the vascular response to a key cytokine, VEGF-A. In the present review, we suggest a model for integrating the signals from these receptor tyrosine kinases by co-ordinating the spatial and temporal segregation of these membrane proteins linked to distinct signalling outputs associated with each intracellular location. Activation of pro-angiogenic VEGFR2 stimulates a programme of tyrosine phosphorylation, ubiquitination and proteolysis. This is linked to ESCRT (endosomal sorting complex required for transport)-mediated recognition of activated VEGFR2 and sorting in endosomes before arrival in lysosomes for terminal degradation. In addition, Rab GTPases regulate key events in VEGFR2 trafficking between the plasma membrane, early and late endosomes, with distinct roles for Rab4a, Rab5a and Rab7a. Manipulation of GTPase levels affects not only VEGFR2 activation and intracellular signalling, but also functional outputs such as VEGF-A-stimulated endothelial cell migration. In contrast, VEGFR1 displays stable Golgi localization that can be perturbed by cell stimuli that elevate cytosolic Ca(2+) ion levels. One model is that VEGFR1 translocates from the trans-Golgi network to the plasma membrane via a calcium-sensitive trafficking step. This allows rapid and preferential sequestration of VEGF-A by the higher-affinity VEGFR1, thus blocking further VEGFR2 activation. Recycling or degradation of VEGFR1 allows resensitization of the VEGFR2-dependent signalling pathway. Thus a dual VEGFR system with a built-in negative-feedback loop is utilized by endothelial cells to sense a key cytokine in vascular tissues.

VEGF-A-stimulated signalling in endothelial cells via a dual receptor tyrosine kinase system is dependent on co-ordinated trafficking and proteolysis.

Vascular endothelial growth factor A (VEGF-A) binds to the VEGFR2 receptor tyrosine kinase, regulating endothelial function, vascular physiology and angiogenesis. However, the mechanism underlying VEGFR2 turnover and degradation in this response is unclear. Here, we tested a role for heat-shock proteins in regulating the presentation of VEGFR2 to a degradative pathway. Pharmacological inhibition of HSP90 stimulated VEGFR2 degradation in primary endothelial cells and blocked VEGF-A-stimulated intracellular signaling via VEGFR2. HSP90 inhibition stimulated the formation of a VEGFR2-HSP70 complex. Clathrin-mediated VEGFR2 endocytosis is required for this HSP-linked degradative pathway for targeting VEGFR2 to the endosome-lysosome system. HSP90 perturbation selectively inhibited VEGF-A-stimulated human endothelial cell migration in vitro. A mouse femoral artery model showed that HSP90 inhibition also blocked blood vessel repair in vivo consistent with decreased endothelial regeneration. Depletion of either HSP70 or HSP90 caused defects in blood vessel formation in a transgenic zebrafish model. We conclude that perturbation of the HSP70-HSP90 heat-shock protein axis stimulates degradation of endothelial VEGFR2 and modulates VEGF-A-stimulated intracellular signaling, endothelial cell migration, blood vessel development and repair.

A heat-shock protein axis regulates VEGFR2 proteolysis, blood vessel development and repair.

Vascular endothelial growth factor A (VEGF-A) regulates many aspects of vascular physiology. VEGF-A stimulates signal transduction pathways that modulate endothelial outputs such as cell migration, proliferation, tubulogenesis, and cell-cell interactions. Multiple VEGF-A isoforms exist, but the biological significance of this is unclear. Here we analyzed VEGF-A isoform-specific stimulation of VCAM-1 gene expression, which controls endothelial-leukocyte interactions, and show that this is dependent on both ERK1/2 and activating transcription factor-2 (ATF-2). VEGF-A isoforms showed differential ERK1/2 and p38 MAPK phosphorylation kinetics. A key feature of VEGF-A isoform-specific ERK1/2 activation and nuclear translocation was increased phosphorylation of ATF-2 on threonine residue 71 (T71). Using reverse genetics, we showed ATF-2 to be functionally required for VEGF-A-stimulated endothelial VCAM-1 gene expression. ATF-2 knockdown blocked VEGF-A-stimulated VCAM-1 expression and endothelial-leukocyte interactions. ATF-2 was also required for other endothelial cell outputs, such as cell migration and tubulogenesis. In contrast, VCAM-1 was essential only for promoting endothelial-leukocyte interactions. This work presents a new paradigm for understanding how soluble growth factor isoforms program complex cellular outputs and responses by modulating signal transduction pathways.

VEGF-A isoforms differentially regulate ATF-2-dependent VCAM-1 gene expression and endothelial-leukocyte interactions.

Vascular endothelial growth factor A (VEGF-A) regulates many aspects of vascular physiology such as cell migration, proliferation, tubulogenesis and cell-cell interactions. Numerous isoforms of VEGF-A exist but their physiological significance is unclear. Here we evaluated two different VEGF-A isoforms and discovered differential regulation of cytosolic calcium ion flux, transcription factor localisation and endothelial cell response. Analysis of VEGF-A isoform-specific stimulation of VEGFR2-dependent signal transduction revealed differential capabilities for isoform activation of multiple signal transduction pathways. VEGF-A165 treatment promoted increased phospholipase Cγ1 phosphorylation, which was proportional to the subsequent rise in cytosolic calcium ions, in comparison to cells treated with VEGF-A121. A major consequence of this VEGF-A isoform-specific calcium ion flux in endothelial cells is differential dephosphorylation and subsequent nuclear translocation of the transcription factor NFATc2. Using reverse genetics, we discovered that NFATc2 is functionally required for VEGF-A-stimulated endothelial cell migration but not tubulogenesis. This work presents a new mechanism for understanding how VEGF-A isoforms program complex cellular outputs by converting signal transduction pathways into transcription factor redistribution to the nucleus, as well as defining a novel role for NFATc2 in regulating the endothelial cell response.

VEGF-A isoform-specific regulation of calcium ion flux, transcriptional activation and endothelial cell migration.

Low circulating levels of insulin-like growth factor binding protein 1 (IGFBP-1) are associated with insulin resistance and predict the development of type 2 diabetes. IGFBP-1 can affect cellular functions independently of IGF binding through an Arg-Gly-Asp (RGD) integrin-binding motif. Whether causal mechanisms underlie the favorable association of high IGFBP-1 levels with insulin sensitivity and whether these could be exploited therapeutically remain unexplored. We used recombinant IGFBP-1 and a synthetic RGD-containing hexapeptide in complementary in vitro signaling assays and in vivo metabolic profiling in obese mice to investigate the effects of IGFBP-1 and its RGD domain on insulin sensitivity, insulin secretion, and whole-body glucose regulation. The RGD integrin-binding domain of IGFBP-1, through integrin engagement, focal adhesion kinase, and integrin-linked kinase, enhanced insulin sensitivity and insulin secretion in C2C12 myotubes and INS-1 832/13 pancreatic β-cells. Both acute administration and chronic infusion of an RGD synthetic peptide to obese C57BL/6 mice improved glucose clearance and insulin sensitivity. These favorable effects on metabolic homeostasis suggest that the RGD integrin-binding domain of IGFBP-1 may be a promising candidate for therapeutic development in the field of insulin resistance.

Insulin-Like Growth Factor Binding Protein 1 Could Improve Glucose Regulation and Insulin Sensitivity Through Its RGD Domain.

Surgical resection of colorectal cancer liver metastases (CLM) can be curative, yet 80% of patients are unsuitable for this treatment. As angiogenesis is a determinant of CLM progression we isolated endothelial cells from CLM and sought a mechanism which is upregulated, essential for angiogenic properties of these cells and relevant to emerging therapeutic options. Matched CLM endothelial cells (CLMECs) and endothelial cells of normal adjacent liver (LiECs) were superficially similar but transcriptome sequencing revealed molecular differences, one of which was unexpected upregulation and functional significance of the checkpoint kinase WEE1. Western blotting confirmed that WEE1 protein was upregulated in CLMECs. Knockdown of WEE1 by targeted short interfering RNA or the WEE1 inhibitor AZD1775 suppressed proliferation and migration of CLMECs. Investigation of the underlying mechanism suggested induction of double-stranded DNA breaks due to nucleotide shortage which then led to caspase 3-dependent apoptosis. The implication for CLMEC tube formation was striking with AZD1775 inhibiting tube branch points by 83%. WEE1 inhibitors might therefore be a therapeutic option for CLM and could be considered more broadly as anti-angiogenic agents in cancer treatment.

Upregulated WEE1 protects endothelial cells of colorectal cancer liver metastases.

The evolutionary processes that drive universal therapeutic resistance in adult patients with diffuse glioma remain unclear. Here we analysed temporally separated DNA-sequencing data and matched clinical annotation from 222 adult patients with glioma. By analysing mutations and copy numbers across the three major subtypes of diffuse glioma, we found that driver genes detected at the initial stage of disease were retained at recurrence, whereas there was little evidence of recurrence-specific gene alterations. Treatment with alkylating agents resulted in a hypermutator phenotype at different rates across the glioma subtypes, and hypermutation was not associated with differences in overall survival. Acquired aneuploidy was frequently detected in recurrent gliomas and was characterized by IDH mutation but without co-deletion of chromosome arms 1p/19q, and further converged with acquired alterations in the cell cycle and poor outcomes. The clonal architecture of each tumour remained similar over time, but the presence of subclonal selection was associated with decreased survival. Finally, there were no differences in the levels of immunoediting between initial and recurrent gliomas. Collectively, our results suggest that the strongest selective pressures occur during early glioma development and that current therapies shape this evolution in a largely stochastic manner.

Longitudinal molecular trajectories of diffuse glioma in adults.

Pericytes regulate vascular development, stability, and quiescence; their dysfunction contributes to diabetic retinopathy. To explore the role of insulin receptors in pericyte biology, we created pericyte insulin receptor knockout mice (PIRKO) by crossing PDGFRβ-Cre mice with insulin receptor (Insr) floxed mice. Their neonatal retinal vasculature exhibited perivenous hypervascularity with venular dilatation, plus increased angiogenic sprouting in superficial and deep layers. Pericyte coverage of capillaries was unaltered in perivenous and periarterial plexi, and no differences in vascular regression or endothelial proliferation were apparent. Isolated brain pericytes from PIRKO had decreased angiopoietin-1 mRNA, whereas retinal and lung angiopoietin-2 mRNA was increased. Endothelial phospho-Tie2 staining was diminished and FoxO1 was more frequently nuclear localized in the perivenous plexus of PIRKO, in keeping with reduced angiopoietin-Tie2 signaling. Silencing of Insr in human brain pericytes led to reduced insulin-stimulated angiopoietin-1 secretion, and conditioned media from these cells was less able to induce Tie2 phosphorylation in human endothelial cells. Hence, insulin signaling in pericytes promotes angiopoietin-1 secretion and endothelial Tie2 signaling and perturbation of this leads to excessive vascular sprouting and venous plexus abnormalities. This phenotype mimics elements of diabetic retinopathy, and future work should evaluate pericyte insulin signaling in this disease.

Pericyte Insulin Receptors Modulate Retinal Vascular Remodeling and Endothelial Angiopoietin Signaling.

Glioblastoma (GBM) brain tumors lacking IDH1 mutations (IDHwt) have the worst prognosis of all brain neoplasms. Patients receive surgery and chemoradiotherapy but tumors almost always fatally recur.

Alexander F. Bruns

Leeds Institute of Cardiovascular and Metabolic Medicine

University of Leeds

Studying Adipose Endothelial Cell/Adipocyte Cross-Talk in Human Subcutaneous Adipose Tissue

Alexander F. Bruns

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University of Leeds

Studying Adipose Endothelial Cell/Adipocyte Cross-Talk in Human Subcutaneous Adipose Tissue

Leeds Institute of Cardiovascular and Metabolic Medicine