A Novel Anti-angiogenic VEGF-A Isoform is Expressed in Human Ocular Tissue

Abstract

Abstract: : Background / Purpose: We have recently identified a new splice variant of VEGF-A formed by splicing from exon 7 into the previously assumed 3' UTR of the VEGF-A mRNA. The new isoform (isolated from human renal cortex) contains 18 bases coded for by exon 9, in place of 18 bases of exon 8. We have termed this isoform VEGF₁₆₅b to distinguish it from the existing VEGF₁₆₅ isoform. Further experiments have shown that VEGF₁₆₅b appears to be anti-angiogenic, inhibiting VEGF₁₆₅ mediated endothelial cell migration, proliferation and vasodilation ex vivo. The purpose of this study was to determine tissue expression profiles for VEGF₁₆₅b to elucidate a potential role of this isoform in the regulation of angiogenesis. Methods: We have designed exon 9 specific primers and optimised a RT-PCR technique that identifies VEGF₁₆₅b cDNA template even in the presence of 1000 fold excess VEGF₁₆₅ cDNA template. We screened a range of tissues for VEGF₁₆₅b expression. Ocular samples were obtained from patients undergoing vitrectomy and penetrating keratoplasty at Bristol Eye Hospital, UK. Results: VEGF₁₆₅b is clearly expressed in a variety of tissues including vitreous, cornea, pituitary, cerebrum, lung, skeletal muscle, kidney, prostate, liver, umbilical cord and synovium. Fainter expression was seen in colon, skin, bladder and spinal cord. It is not expressed in the hypothalamus or vena cava. Discussion: We have identified a new anti-angiogenic form of VEGF-A, VEGF₁₆₅b, and demonstrated that it is expressed in a range of tissues including the human eye. We have also shown (in a separate study) that, unlike all other VEGF isoforms, VEGF₁₆₅b is down-regulated in both renal cell carcinomata and malignant prostate tissue. These results highlight the potential role of this new isoform in maintaining angiogenic equilibrium. It has been well established that VEGF plays a key role in vascular eye disease, such as diabetic retinopathy, but until now it has not been possible to differentiate between the two isoforms of VEGF₁₆₅. This study demonstrates that VEGF₁₆₅b is expressed in both cornea and vitreous, placing it in a key position to regulate ocular angiogenesis. We are currently developing this work to compare normal levels of VEGF₁₆₅b expression with expression levels in a range of vascular eye diseases.

Keywords: growth factors/growth factor receptors • vascular cells • molecular biology