Heather S. Bevan

VEGF165b, an Inhibitory Vascular Endothelial Growth Factor Splice Variant Mechanism of Action, In vivo Effect On Angiogenesis and Endogenous Protein Expression

Growth of new blood vessels (angiogenesis), required for all tumor growth, is stimulated by the expression of vascular endothelial growth factor (VEGF). VEGF is up-regulated in all known solid tumors but also in atherosclerosis, diabetic retinopathy, arthritis, and many other conditions. Conventional VEGF isoforms have been universally described as proangiogenic cytokines. Here, we show that an endogenous splice variant, VEGF165b, is expressed as protein in normal cells and tissues and is circulating in human plasma. We also present evidence for a sister family of presumably inhibitory splice variants. Moreover, these isoforms are down-regulated in prostate cancer. We also show that VEGF165b binds VEGF receptor 2 with the same affinity as VEGF165 but does not activate it or stimulate downstream signaling pathways. Moreover, it prevents VEGF165-mediated VEGF receptor 2 phosphorylation and signaling in cultured cells. Furthermore, we show, with two different in vivo angiogenesis models, that VEGF165b is not angiogenic and that it inhibits VEGF165-mediated angiogenesis in rabbit cornea and rat mesentery. Finally, we show that VEGF165b expressing tumors grow significantly more slowly than VEGF165-expressing tumors, indicating that a switch in splicing from VEGF165 to VEGF165b can inhibit tumor growth. These results suggest that regulation of VEGF splicing may be a critical switch from an antiangiogenic to a proangiogenic phenotype.

Molecular diversity of VEGF-A as a regulator of its biological activity.

The vascular endothelial growth factor (VEGF) family of proteins regulates blood flow, growth, and function in both normal physiology and disease processes. VEGF‐A is alternatively spliced to form multiple isoforms, in two subfamilies, that have specific, novel functions. Alternative splicing of exons 5–7 of the VEGF gene generates forms with differing bioavailability and activities, whereas alternative splice‐site selection in exon 8 generates proangiogenic, termed VEGFxxx, or antiangiogenic proteins, termed VEGFxxxb. Despite its name, emerging roles for VEGF isoforms on cell types other than endothelium have now been identified. Although VEGF‐A has conventionally been considered to be a family of proangiogenic, propermeability vasodilators, the identification of effects on nonendothelial cells, and the discovery of the antiangiogenic subfamily of splice isoforms, has added further complexity to their regulation of microvascular function. The distally spliced antiangiogenic isoforms are expressed in normal human tissue, but downregulated in angiogenic diseases, such as cancer and proliferative retinopathy, and in developmental pathologies, such as Denys Drash syndrome and preeclampsia. Here, we examine the molecular diversity of VEGF‐A as a regulator of its biological activity and compare the role of the pro‐ and antiangiogenic VEGF‐A splice isoforms in both normal and pathophysiological processes.

The Alternatively Spliced Anti-Angiogenic Family of VEGF Isoforms VEGFxxxb in Human Kidney Development

Background/Aim: Vascular endothelial growth factor (VEGF), required for renal development, is generated by alternative splicing of 8 exons to produce two families, pro-angiogenic VEGFxxx, formed by proximal splicing in exon 8 (exon 8a), and anti-angiogenic VEGFxxxb, generated by distal splicing in exon 8 (exon 8b). VEGF165b, the first described exon 8b-containing isoform, antagonises VEGF165 and is anti-angiogenic in vivo. Methods: Using VEGFxxxb-specific antibodies, we investigated its expression quantitatively and qualitatively in developing kidney, and measured the effect of VEGF165b on renal endothelial and epithelial cells. Results: VEGFxxxb formed 45% of total VEGF protein in adult renal cortex, and VEGF165b does not increase glomerular endothelial cell permeability, it inhibits migration, and is cytoprotective for podocytes. During renal development, VEGFxxxb was expressed in the condensed vesicles of the metanephros, epithelial cells of the comma-shaped bodies, invading endothelial cells and epithelial cells of the S-shaped body, and in the immature podocytes. Expression reduced as the glomerulus matured. Conclusion: These results show that the anti-angiogenic VEGFxxxb isoforms are highly expressed in adult and developing renal cortex, and suggest that the VEGFxxxb family plays a role in glomerular maturation and podocyte protection by regulating the pro-angiogenic pro-permeability properties of VEGFxxx isoforms.

Mammary alveolar development during lactation is inhibited by the endogenous antiangiogenic growth factor isoform, VEGF165b

Extensive tissue remodeling occurs in breast tissue during pregnancy, resulting in growth and development of the mammary gland associated with extensive vascular remodeling, which is thought to be dependent on vascular endothelial growth factor (VEGF). We show here that the endogenous antiangio‐genic splice isoform of VEGF, VEGF165b, is normally expressed in nonlactating human and mouse breast, and is down‐regulated in WT mice during lactation. To demonstrate the physiological role of VEGF165b in mammary tissue, we generated transgenic (TG) mice expressing VEGF165b, under the control of the mouse mammary tumor virus (MMTV) enhancer/promoter. These mice increase expression of VEGF165b in mam‐mary tissue during mammary development. The offspring of TG mothers, but not TG fathers, die shortly after birth. The female TG mice have fewer blood vessels, less blood in the mammary tissue, and impaired alveolar coverage of the fat pad, and do not produce sufficient milk for nourishment of their pups. These findings demonstrate that endogenous overexpression of VEGF165b in the mammary gland inhibits physiological angiogenesis and that the regulation of the balance of VEGF isoforms is a requirement for mammary alveolar development and milk production. This study provides the first evidence for the role of endogenous antiangiogenic VEGF isoforms in normal physiology— their down‐regulation is required for effective milk production. Qiu Y., Bevan, H., Weeraperuma, S., Wratting, D., Murphy, D., Neal, C. R., Bates, D. O., Harper S. J. Mammary alveolar development during lactation is inhibited by the endogenous antiangiogenic growth factor isoform, VEGF165b. FASEB J. 22, 1104–1112 (2008)

The endogenous anti-angiogenic family of splice variants of VEGF, VEGFxxxb, are down-regulated in pre-eclamptic placentae at term

PET (pre-eclamptic toxaemia) has recently been linked with alterations in production of a VEGFR1 [VEGF (vascular endothelial growth factor) receptor 1] splice variant that acts as a circulating inhibitor. We have recently described a family of naturally occurring splice variants of VEGF, termed VEGFxxxb, that also appear to act as inhibitors of conventional VEGFxxx-mediated angiogenesis. To determine whether alteration in splicing of VEGF-VEGFR family members extended beyond VEGFR1, we investigated the effect of pre-eclampsia on placental VEGFxxxb mRNA and protein expression. VEGFxxx and VEGFxxxb mRNA and protein were both found in normal human term placentae. VEGFxxx protein formed the majority of the total VEGF protein (980+/-195 pg/mg), whereas VEGFxxxb (11.5 pg/mg) was found to form a small part of the total VEGF protein expression (1.5+/-0.24%). Evidence for VEGF165b, VEGF121b and VEGF145b expression was found. In pre-eclamptic placentae, there was a significant down-regulation of VEGFxxxb isoforms, but a small up-regulation of VEGFxxx isoforms. In normal placenta VEGFxxxb and VEGFxxx concentrations were positively correlated (r=0.69, P<0.02), whereas in pre-eclamptic placentae, there was a significant negative correlation between VEGFxxxb and VEGFxxx protein expression (r=-0.8, P<0.02), indicating that there was a significant uncoupling of the splicing regulation of the VEGF isoforms. Combined with previous studies showing increased soluble VEGFR1 isoforms in human pre-eclampsia, these data suggest that there may be a common mechanism in pre-eclampsia that involves dysregulation of mRNA splicing of members of the VEGF-VEGFR axis.

VEGF-A165b is cytoprotective and antiangiogenic in the retina

PURPOSE A number of key ocular diseases, including diabetic retinopathy and age-related macular degeneration, are characterized by localized areas of epithelial or endothelial damage, which can ultimately result in the growth of fragile new blood vessels, vitreous hemorrhage, and retinal detachment. VEGF-A(165), the principal neovascular agent in ocular angiogenic conditions, is formed by proximal splice site selection in its terminal exon 8. Alternative splicing of this exon results in an antiangiogenic isoform, VEGF-A(165)b, which is downregulated in diabetic retinopathy. Here the authors investigate the antiangiogenic activity of VEGF(165)b and its effect on retinal epithelial and endothelial cell survival. METHODS VEGF-A(165)b was injected intraocularly in a mouse model of retinal neovascularization (oxygen-induced retinopathy [OIR]). Cytotoxicity and cell migration assays were used to determine the effect of VEGF-A(165)b. RESULTS VEGF-A(165)b dose dependently inhibited angiogenesis (IC(50), 12.6 pg/eye) and retinal endothelial migration induced by 1 nM VEGF-A(165) across monolayers in culture (IC(50), 1 nM). However, it also acts as a survival factor for endothelial cells and retinal epithelial cells through VEGFR2 and can stimulate downstream signaling. Furthermore, VEGF-A(165)b injection, while inhibiting neovascular proliferation in the eye, reduced the ischemic insult in OIR (IC(50), 2.6 pg/eye). Unlike bevacizumab, pegaptanib did not interact directly with VEGF-A(165)b. CONCLUSIONS The survival effects of VEGF-A(165)b signaling can protect the retina from ischemic damage. These results suggest that VEGF-A(165)b may be a useful therapeutic agent in ischemia-induced angiogenesis and a cytoprotective agent for retinal pigment epithelial cells.

Overexpression of VEGF165b in Podocytes Reduces Glomerular Permeability

The observation that therapeutic agents targeting vascular endothelial growth factor-A (VEGF-A) associate with renal toxicity suggests that VEGF plays a role in the maintenance of the glomerular filtration barrier. Alternative mRNA splicing produces the VEGF(xxx)b family, which consists of antiangiogenic peptides that reduce permeability and inhibit tumor growth; the contribution of these peptides to normal glomerular function is unknown. Here, we established and characterized heterozygous and homozygous transgenic mice that overexpress VEGF(165)b specifically in podocytes. We confirmed excess production of glomerular VEGF(165)b by reverse transcriptase-PCR, immunohistochemistry, and ELISA in both heterozygous and homozygous animals. Macroscopically, the mice seemed normal up to 18 months of age, unlike the phenotype of transgenic podocyte-specific VEGF(164)-overexpressing mice. Animals overexpressing VEGF(165)b, however, had a significantly reduced normalized glomerular ultrafiltration fraction with accompanying changes in ultrastructure of the glomerular filtration barrier on the vascular side of the glomerular basement membrane. These data highlight the contrasting properties of VEGF splice variants and their impact on glomerular function and phenotype.

Regulation of alternative VEGF-A mRNA splicing is a therapeutic target for analgesia.

Vascular endothelial growth factor-A (VEGF-A) is best known as a key regulator of the formation of new blood vessels. Neutralization of VEGF-A with anti-VEGF therapy e.g. bevacizumab, can be painful, and this is hypothesized to result from a loss of VEGF-A-mediated neuroprotection. The multiple vegf-a gene products consist of two alternatively spliced families, typified by VEGF-A165a and VEGF-A165b (both contain 165 amino acids), both of which are neuroprotective. Under pathological conditions, such as in inflammation and cancer, the pro-angiogenic VEGF-A165a is upregulated and predominates over the VEGF-A165b isoform. We show here that in rats and mice VEGF-A165a and VEGF-A165b have opposing effects on pain, and that blocking the proximal splicing event – leading to the preferential expression of VEGF-A165b over VEGF165a – prevents pain in vivo. VEGF-A165a sensitizes peripheral nociceptive neurons through actions on VEGFR2 and a TRPV1-dependent mechanism, thus enhancing nociceptive signaling. VEGF-A165b blocks the effect of VEGF-A165a. After nerve injury, the endogenous balance of VEGF-A isoforms switches to greater expression of VEGF-Axxxa compared to VEGF-Axxxb, through an SRPK1-dependent pre-mRNA splicing mechanism. Pharmacological inhibition of SRPK1 after traumatic nerve injury selectively reduced VEGF-Axxxa expression and reversed associated neuropathic pain. Exogenous VEGF-A165b also ameliorated neuropathic pain. We conclude that the relative levels of alternatively spliced VEGF-A isoforms are critical for pain modulation under both normal conditions and in sensory neuropathy. Altering VEGF-Axxxa/VEGF-Axxxb balance by targeting alternative RNA splicing may be a new analgesic strategy.

Ovarian VEGF(165)b expression regulates follicular development, corpus luteum function and fertility.

Angiogenesis and vascular regression are critical for the female ovulatory cycle. They enable progression and regression of follicular development, and corpora lutea formation and regression. Angiogenesis in the ovary occurs under the control of the vascular endothelial growth factor-A (VEGFA) family of proteins, which are generated as both pro-(VEGF165) and anti(VEGF165b)-angiogenic isoforms by alternative splicing. To determine the role of the VEGF165b isoforms in the ovulatory cycle, we measured VEGF165b expression in marmoset ovaries by immunohistochemistry and ELISA, and used transgenic mice over-expressing VEGF165b in the ovary. VEGF165b was expressed in the marmoset ovaries in granulosa cells and theca, and the balance of VEGF165b:VEGF165 was regulated during luteogenesis. Mice over-expressing VEGF165b in the ovary were less fertile than wild-type littermates, had reduced secondary and tertiary follicles after mating, increased atretic follicles, fewer corpora lutea and generated fewer embryos in the oviduct after mating, and these were more likely not to retain the corona radiata. These results indicate that the balance of VEGFA isoforms controls follicle progression and luteogenesis, and that control of isoform expression may regulate fertility in mammals, including in primates.

Acute laminar shear stress reversibly increases human glomerular endothelial cell permeability via activation of endothelial nitric oxide synthase.

Laminar shear stress is a key determinant of systemic vascular behavior, including through activation of endothelial nitric oxide synthase (eNOS), but little is known of its role in the glomerulus. We confirmed eNOS expression by glomerular endothelial cells (GEnC) in tissue sections and examined effects of acute exposure (up to 24 h) to physiologically relevant levels of laminar shear stress (10–20 dyn/cm2) in conditionally immortalized human GEnC. Laminar shear stress caused an orientation of GEnC and stress fibers parallel to the direction of flow and induced Akt and eNOS phosphorylation along with NO production. Inhibition of the phophatidylinositol (PI)3-kinase/Akt pathway attenuated laminar shear stress-induced eNOS phosphorylation and NO production. Laminar shear stress of 10 dyn/cm2 had a dramatic effect on GEnC permeability, reversibly decreasing the electrical resistance across GEnC monolayers. Finally, the laminar shear stress-induced reduction in electrical resistance was attenuated by the NOS inhibitors l-NG-monomethyl arginine (l-NMMA) and l-NG-nitroarginine methyl ester (l-NAME) and also by inhibition of the PI3-kinase/Akt pathway. Hence we have shown for GEnC in vitro that acute permeability responses to laminar shear stress are dependent on NO, produced via activation of the PI3-kinase/Akt pathway and increased eNOS phosphorylation. These results suggest the importance of laminar shear stress and NO in regulating the contribution of GEnC to the permeability properties of the glomerular capillary wall.