The vascular endothelial growth factor-induced disruption of gap junctions is relayed by an autocrine communication via ATP release in coronary capillary endothelium

Gap26 inhibited angiogenesis through the β-catenin-VE-cadherin-VEGFR2-Erk signaling pathway

PURPOSE

To investigate the effect of connexin 43 (Cx43) on corneal neovascularization and its regulation of VEGFR2 on vascular endothelial cells.

METHODS

In vivo, we used mouse corneal suture model to induce corneal neovascularization and discovered the function of gap26 in corneal neovascularization. In vitro, the effect of gap26 on HUVEC was observed by cell proliferation, tube formation and scratch experiments. WB and PCR detected the changes in angiogenic protein and mRNA expression. Knockdown of key mRNA in neovascularization using siRNA confirmed that Cx43 regulates neovascularization through the β-catenin-VE-cadherin-VEGFR2-Erk signaling pathway.

RESULTS

In vivo, gap26 can reduce mouse corneal neovascularization. In vitro, we show that Cx43 expression is increased in the presence of VEGFA stimulation, and when we use gap26 to inhibit Cx43 can reduce vascular endothelial cell proliferation, tube formation and migration. We found that the expression of pVEGFR2 and pErk increased in response to VEGFA, while they decreased after using gap26. And the expression of β-catenin and VE-cadherin decreased in response to VEGFA, while they increased after using gap26. Furthermore, we found that Cx43 regulates angiogenesis through the β-catenin-VE-cadherin-VEGFR2-Erk pathway.

CONCLUSIONS

Gap26 can downregulate VEGFR2 phosphorylation by stabilizing the expression of β-catenin and VE-cadherin on the cell membrane, thereby inhibiting VEGFA-induced HUVECs proliferation, migration and tube formation and inhibiting corneal neovascularization.

The Role of Connexin in Ophthalmic Neovascularization and the Interaction between Connexin and Proangiogenic Factors

The formation of new blood vessels is an important physiological process that occurs during development. When the body is injured, new blood vessel formation helps the body recuperate by supplying more oxygen and nutrients. However, this mechanism can have a negative effect. In ophthalmologic diseases, such as corneal new blood vessels, neonatal vascular glaucoma, and diabetes retinopathy, the formation of new blood vessels has become a critical component in patient survival. Connexin is a protein that regulates the cellular and molecular material carried by cells. It has been demonstrated that it is widely expressed in vascular endothelial cells, where it forms a slit connection between adjacent cells to promote cell-cell communication via hemichannels, as well as substance exchange into intracellular environments. Numerous studies have demonstrated that connexin in vascular endothelial cells plays an important role in angiogenesis and vascular leakage. The purpose of this study was to investigate the effect between the angiogenesis-associated factor and the connexin. It also reveals the effect of connexin on ophthalmic neovascularization.

Normalizing Tumor Vasculature to Reduce Hypoxia, Enhance Perfusion, and Optimize Therapy Uptake

Simple Summary

In order for solid tumors to grow, they need to develop new blood vessels in order to support their increasing metabolic requirements. To facilitate the novel vessel formation, the tumor initiates an aggressive pro-angiogenic program. As a result of the aggressive angiogenesis, blood vessels form very rapidly and are often malformed and dysfunctional. There is a reduction in perfusion to the tumor, and often the tumors exhibit significant areas of tumor hypoxia. This review paper discusses the pro-tumorigenic environment induced by tumor hypoxia and how this can be targeted through normalization of the tumor vasculature. Here, we review tumor angiogenesis, the development of a hypoxic phenotype, and how this contributes to sustained tumorigenesis and resistance to therapy. We further discuss the potential of vascular normalization to reduce tumor hypoxia and facilitate uptake and efficacy of a variety of therapies.

Abstract

A basic requirement of tumorigenesis is the development of a vascular network to support the metabolic requirements of tumor growth and metastasis. Tumor vascular formation is regulated by a balance between promoters and inhibitors of angiogenesis. Typically, the pro-angiogenic environment created by the tumor is extremely aggressive, resulting in the rapid vessel formation with abnormal, dysfunctional morphology. The altered morphology and function of tumor blood and lymphatic vessels has numerous implications including poor perfusion, tissue hypoxia, and reduced therapy uptake. Targeting tumor angiogenesis as a therapeutic approach has been pursued in a host of different cancers. Although some preclinical success was seen, there has been a general lack of clinical success with traditional anti-angiogenic therapeutics as single agents. Typically, following anti-angiogenic therapy, there is remodeling of the tumor microenvironment and widespread tumor hypoxia, which is associated with development of therapy resistance. A more comprehensive understanding of the biology of tumor angiogenesis and insights into new clinical approaches, including combinations with immunotherapy, are needed to advance vascular targeting as a therapeutic area.

Increased expression of connexin 43 in a mouse model of spinal motoneuronal loss

Amyotrophic lateral sclerosis (ALS) is one of the most common motoneuronal disease, characterized by motoneuronal loss and progressive paralysis. Despite research efforts, ALS remains a fatal disease, with a survival of 2-5 years after disease onset. Numerous gene mutations have been correlated with both sporadic (sALS) and familiar forms of the disease, but the pathophysiological mechanisms of ALS onset and progression are still largely uncertain. However, a common profile is emerging in ALS pathological features, including misfolded protein accumulation and a cross-talk between neuroinflammatory and degenerative processes. In particular, astrocytes and microglial cells have been proposed as detrimental influencers of perineuronal microenvironment, and this role may be exerted via gap junctions (GJs)- and hemichannels (HCs)-mediated communications. Herein we investigated the role of the main astroglial GJs-forming connexin, Cx43, in human ALS and the effects of focal spinal cord motoneuronal depletion onto the resident glial cells and Cx43 levels. Our data support the hypothesis that motoneuronal depletion may affect glial activity, which in turn results in reactive Cx43 expression, further promoting neuronal suffering and degeneration.

Electrotonic transmission in the retinal vasculature: inhibitory role of the diabetes/VEGF/aPKC pathway

The deleterious impact of diabetes on the retina is a leading cause of vision loss. Ultimately, the hypoxic retinopathy caused by diabetes results in irreversible damage to vascular, neuronal, and glial cells. Less understood is how retinal physiology is altered early in the course of diabetes. We recently found that the electrotonic architecture of the retinovasculature becomes fundamentally altered soon after the onset of this disorder. Namely, the spread of voltage through the vascular endothelium is markedly inhibited. The goal of this study was to elucidate how diabetes inhibits electrotonic transmission. We hypothesized that vascular endothelial growth factor (VEGF) may play a role since its upregulation in hypoxic retinopathy is associated with sight-impairing complications. In this study, we quantified voltage transmission between pairs of perforated-patch pipettes sealed onto abluminal cells located on retinal microvascular complexes freshly isolated from diabetic and nondiabetic rats. We report that exposure of diabetic retinal microvessels to an anti-VEGF antibody or to a small-molecule inhibitor of atypical PKCs (aPKC) near-fully restored the efficacy of electrotonic transmission. Furthermore, exposure of nondiabetic microvessels to VEGF mimicked, via a mechanism sensitive to the aPKC inhibitor, the diabetes-induced inhibition of transmission. Thus, activation of the diabetes/VEGF/aPKC pathway switches the retinovasculature from a highly interactive operational unit to a functionally balkanized complex. By delimiting the dissemination of voltage-changing vasomotor inputs, this organizational fragmentation is likely to compromise effective regulation of retinal perfusion. Future pharmacological targeting of the diabetes/VEGF/aPKC pathway may serve to impede progression of vascular dysfunction to irreversible diabetic retinopathy.

The Functional Implications of Endothelial Gap Junctions and Cellular Mechanics in Vascular Angiogenesis

Angiogenesis—the sprouting and growth of new blood vessels from the existing vasculature—is an important contributor to tumor development, since it facilitates the supply of oxygen and nutrients to cancer cells. Endothelial cells are critically affected during the angiogenic process as their proliferation, motility, and morphology are modulated by pro-angiogenic and environmental factors associated with tumor tissues and cancer cells. Recent in vivo and in vitro studies have revealed that the gap junctions of endothelial cells also participate in the promotion of angiogenesis. Pro-angiogenic factors modulate gap junction function and connexin expression in endothelial cells, whereas endothelial connexins are involved in angiogenic tube formation and in the cell migration of endothelial cells. Several mechanisms, including gap junction function-dependent or -independent pathways, have been proposed. In particular, connexins might have the potential to regulate cell mechanics such as cell morphology, cell migration, and cellular stiffness that are dynamically changed during the angiogenic processes. Here, we review the implication for endothelial gap junctions and cellular mechanics in vascular angiogenesis.

Connexin Channels at the Glio-Vascular Interface: Gatekeepers of the Brain

Neuronal survival, electrical signaling and synaptic activity require a well-balanced micro-environment in the central nervous system. This is achieved by the blood-brain barrier (BBB), an endothelial barrier situated in the brain capillaries, that controls near-to-all passage in and out of the brain. The endothelial barrier function is highly dependent on signaling interactions with surrounding glial, neuronal and vascular cells, together forming the neuro-glio-vascular unit. Within this functional unit, connexin (Cx) channels are of utmost importance for intercellular communication between the different cellular compartments. Connexins are best known as the building blocks of gap junction (GJ) channels that enable direct cell-cell transfer of metabolic, biochemical and electric signals. In addition, beyond their role in direct intercellular communication, Cxs also form unapposed, non-junctional hemichannels in the plasma membrane that allow the passage of several paracrine messengers, complementing direct GJ communication. Within the NGVU, Cxs are expressed in vascular endothelial cells, including those that form the BBB, and are eminent in astrocytes, especially at their endfoot processes that wrap around cerebral vessels. However, despite the density of Cx channels at this so-called gliovascular interface, it remains unclear as to how Cx-based signaling between astrocytes and BBB endothelial cells may converge control over BBB permeability in health and disease. In this review we describe available evidence that supports a role for astroglial as well as endothelial Cxs in the regulation of BBB permeability during development as well as in disease states.

Oncogenic extracellular HSP70 disrupts the gap-junctional coupling between capillary cells

High levels of circulating heat shock protein 70 (HSP70) are detected in many cancers. In order to explore the effects of extracellular HSP70 on human microvascular endothelial cells (HMEC), we initially used gap-FRAP technique. Extracellular human HSP70 (rhHSP70), but not rhHSP27, blocks the gap-junction intercellular communication (GJIC) between HMEC, disrupts the structural integrity of HMEC junction plaques, and decreases connexin43 (Cx43) expression, which correlates with the phosphorylation of Cx43 serine residues. Further exploration of these effects identified a rapid transactivation of the Epidermal Growth Factor Receptor in a Toll-Like Receptor 4-dependent manner, preceding its internalization. In turn, cytosolic Ca²⁺ oscillations are generated. Both GJIC blockade and Ca²⁺ mobilization partially depend on ATP release through Cx43 and pannexin (Panx-1) channels, as demonstrated by blocking activity or expression of channels, and inactivating extracellular ATP. By monitoring dye-spreading into adjacent cells, we show that HSP70 released from human monocytes in response to macrophage colony-stimulating factor, prevents the formation of GJIC between monocytes and HMEC. Therapeutic manipulation of this pathway could be of interest in inflammatory and tumor growth.

Connexin43 phosphorylation by PKC and MAPK signals VEGF-mediated gap junction internalization

Phosphorylation on well-recognized, regulatory connexin43 amino acid residues by a series of kinases serves as an early molecular signal that triggers not only inhibition of gap junction (GJ)–mediated cell-to-cell communication but also GJ internalization. The findings contribute to the newly evolving dynamic picture of GJs.

Gap junctions (GJs) exhibit a complex modus of assembly and degradation to maintain balanced intercellular communication (GJIC). Several growth factors, including vascular endothelial growth factor (VEGF), have been reported to disrupt cell–cell junctions and abolish GJIC. VEGF directly stimulates VEGF-receptor tyrosine kinases on endothelial cell surfaces. Exposing primary porcine pulmonary artery endothelial cells (PAECs) to VEGF for 15 min resulted in a rapid and almost complete loss of connexin43 (Cx43) GJs at cell–cell appositions and a concomitant increase in cytoplasmic, vesicular Cx43. After prolonged incubation periods (60 min), Cx43 GJs reformed and intracellular Cx43 were restored to levels observed before treatment. GJ internalization correlated with efficient inhibition of GJIC, up to 2.8-fold increased phosphorylation of Cx43 serine residues 255, 262, 279/282, and 368, and appeared to be clathrin driven. Phosphorylation of serines 255, 262, and 279/282 was mediated by MAPK, whereas serine 368 phosphorylation was mediated by PKC. Pharmacological inhibition of both signaling pathways significantly reduced Cx43 phosphorylation and GJ internalization. Together, our results indicate that growth factors such as VEGF activate a hierarchical kinase program—including PKC and MAPK—that induces GJ internalization via phosphorylation of well-known regulatory amino acid residues located in the Cx43 C-terminal tail.

Impact of vegf on astrocytes: analysis of gap junctional intercellular communication, proliferation, and motility

The purpose of the present study was to investigate the effects of vascular endothelial growth factor (VEGF) on gap junctional intercellular communication (GJIC), cell proliferation, and cell dynamics in primary astrocytes. VEGF is known as a dimeric polypeptide that potentially binds to two receptors, VEGFR-1 and VEGFR-2, however many effects are mediated by VEGFR-2, for example, actin polymerization, forced cell migration, angiogenesis, and cell proliferation. Recently it has been shown that in case of hypoxia, ischemia or injury VEGF is upregulated to stimulate angiogenesis and cell proliferation. Besides this, VEGF reveals a potent therapeutical target for averting tumor vascularization, emerging in bevacizumab, the first humanized anti-VEGF-A antibody for treating recurrent Glioblastoma multiforme. To expand our knowledge about VEGF effects in glial cells, we cultivated rat astrocytes in medium containing VEGF for 1 and 2 days. To investigate the effects of VEGF on GJIC, we microinjected neurobiotin into a single cell and monitored dye-spreading into adjacent cells. These experiments showed that VEGF significantly enhances astrocytic GJIC compared with controls. Cell proliferation measured by BrdU-labeling also revealed a significant increase of astrocytic mitose rates subsequent to 1 day of VEGF exposure, whereas longer VEGF treatment for 2 days did not have additive effects. To study cell-dynamics of astrocytes subsequent to VEGF treatment, we additionally transfected astrocytes with LifeAct-RFP. Live-cell imaging and quantitative analysis of these cells with aid of confocal laser scanning microscopy revealed higher process movement of VEGF-treated astrocytes. In conclusion, VEGF strongly affects cell proliferation, GJIC, and motility in astrocytes.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

The CpG island methylation regulated expression of endothelial proangiogenic genes in response to β-carotene and arachidonic acid

β-carotene (BC) and arachidonic acid (AA) were demonstrated to modulate carcinogenesis by influencing angiogenesis. DNA methylation is an epigenetic mechanism regulating gene expression. The aim of this study was to investigate whether BC and AA change DNA methylation and expression of the proangiogenic genes, which might help explain their impact on carcinogenesis. Human umbilical vein endothelial cells (HUVECs) and endothelial progenitors (EPCs) were incubated with BC or AA for 24 h. Based on microarray results, we selected 18 genes for DNA methylation analysis. CpG island methylation was quantified using the combined bisulphite restriction analysis method and methylation sensitive restriction enzymes. Relative gene expression was quantified using a quantitative real-time PCR (qRT-PCR) method. Incubation with AA significantly decreased methylation of the promoters of both KDR (P = 0.048) and Notch4 (P = 0.027) genes in HUVECs. In EPCs, BC increased methylation of the connexin 43 gene (P = 0.036). qRT-PCR showed that AA (P = 0.059) and BC (P = 0.044) upregulated the KDR gene expression in HUVECs. Connexin 43 gene expression was induced in the presence of 1 μM (P = 0.039) and 3 μM (P = 0.043) BC in EPCs. No significant changes in the Notch4 gene expression were found. The impact of BC and AA on carcinogenesis may be due, at least in part, to changes in expression of angiogenic genes and these transcriptional effects may be mediated by changes in methylation of CpG islands in the gene promoters. However more research is necessary to confirm this hypothesis.

ATP and P2X purinoceptors in urinary tract disorders

The pharmacological concept of specifically targeting purinoceptors (receptors for ATP and related nucleotides) has emerged over the last two decades in the quest for novel, differentiated therapeutics. Investigations from many laboratories have established a prominent role for ATP in the functional regulation of most tissue and organ systems, including the urinary tract, under normal and pathophysiological conditions. In the particular case of the urinary tract, ATP signaling via P2X1 receptors participates in the efferent control of detrusor smooth muscle excitability, and this function may be heightened in disease and aging. Perhaps of greater interest, ATP also appears to be involved in bladder sensation, operating via activation of P2X3-containing receptors on sensory afferent neurones, both on peripheral terminals within the urinary tract tissues (e.g., ureters, bladder) and on central synapses in the dorsal horn of the spinal cord. Such findings are based on results from classical pharmacological and localization studies in nonhuman and human tissues, gene knockout mice, and studies using recently identified pharmacological antagonists - some of which have progressed as candidate drug molecules. Based on recent advances in this field, it is apparent that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of storage, voiding, and sensory symptoms for patients, while minimizing the systemic side effects that curb the clinical effectiveness of current urologic medicines.

MicroRNAs show mutually exclusive expression patterns in the brain of adult male rats

Background

The brain is a major site of microRNA (miRNA) gene expression, but the spatial expression patterns of miRNAs within the brain have not yet been fully covered.

Methodology/Principal Findings

We have characterized the regional expression profiles of miRNAs in five distinct regions of the adult rat brain: amygdala, cerebellum, hippocampus, hypothalamus and substantia nigra. Microarray profiling uncovered 48 miRNAs displaying more than three-fold enrichment between two or more brain regions. Notably, we found reciprocal expression profiles for a subset of the miRNAs predominantly found (> ten times) in either the cerebellum (miR-206 and miR-497) or the forebrain regions (miR-132, miR-212, miR-221 and miR-222).

Conclusions/Significance

The results indicate that some miRNAs could be important for area-specific functions in the brain. Our data, combined with previous studies in mice, provides additional guidance for future investigations of miRNA functions in the brain.

Intercellular Communication in Atherosclerosis

Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.

Changes in intracellular Ca2+ levels induced by cytokines and P2 agonists differentially modulate proliferation or commitment with macrophage differentiation in murine hematopoietic cells

The role of intracellular Ca2+ (Ca2+i) on hematopoiesis was investigated in long term bone marrow cultures using cytokines and agonists of P2 receptors. Cytokines interleukin 3 and granulocyte/macrophage colony stimulator factor promoted a modest increase in Ca2+i concentration ([Ca2+]i) with activation of phospholipase Cgamma, MEK1/2, and Ca2+/calmodulin kinase II. Involvement of protein kinase C was restricted to stimulation with interleukin 3. In addition, these cytokines promoted proliferation (20 times) and an increase in the Gr-1(-)Mac-1+ population with participation of gap junctions (GJ). Nevertheless ATP, ADP, and UTP promoted a large increase in [Ca2+]i, moderate proliferation (6 times), a reduction in the primitive Gr-1(-)Mac-1(-)c-Kit+ population, and differentiation into macrophages without participation of GJ. It is likely that Ca2+i participates as a regulator of hematopoietic signaling: moderate increases in [Ca2+]i would be related to cytokine-dependent proliferation with participation of GJ, whereas high increases in [Ca2+]i would be related to macrophage differentiation without maintenance of the primitive population.

Roles of Gap Junctions and Connexins in Non-Neoplastic Pathological Processes in which Cell Proliferation Is Involved

Cell proliferation is an important process for reproduction, growth and renewal of living cells and occurs in several situations during life. Cell proliferation is present in all the steps of carcinogenesis, initiation, promotion and progression. Gap junctions are the only specialization of cell membranes that allows communication between adjacent cells. They are known to contribute to tissue homeostasis and are composed of transmembrane proteins called "connexins." These junctions are also known to be involved in cell proliferation control. The roles of gap junctions and connexins in cell proliferation are complex and still under investigation. Since pioneer studies by Loewenstein, it is known that neoplastic cells lack communicating junctions. They do not communicate with their neighbors or with non-neoplastic cells from the surrounding area. There are many studies and review articles dedicated to neoplastic tissues. The aim of this review is to present evidence on the roles of gap junctions and connexins in non-neoplastic processes in which cell proliferation is involved.