Vascular endothelial growth factor expression and signaling in the lens

Previous studies have identified sequences encoding vascular endothelial growth factor (VEGF)-A and one of the VEGF receptors (VEGFR2, Flk-1, KDR) in lens fiber cells. The current study was undertaken to determine the distribution of VEGF-A protein in the lens, whether signaling through VEGF receptors occurs in lens cells, the pattern of VEGF-A expression during lens development, and the effect of hypoxia on VEGF-A expression.

VEGF-A and VEGFR2 were localized using immunocytochemistry. VEGF-A and VEGFR2 protein were identified and quantified by Western blot analysis. Activated (tyrosine phosphorylated) VEGFR2 was detected by immunoprecipitation with an anti-phosphotyrosine antibody followed by Western blot analysis with antibody to VEGFR2. Levels of VEGF-A mRNA were measured by quantitative PCR. Suturing the lids of adult mouse or rabbit eyes for 3 days was used to induce lens hypoxia.

VEGFR2 sequences were present in adult human lens epithelial cells, and VEGF-A transcripts were detected in chicken embryo, adult human, and mouse lens epithelial cells. VEGF-A protein localized to the ends of mouse embryo lens fiber cells at developmental stages when the fetal vasculature was forming. At later stages, VEGF-A was distributed uniformly throughout the cytoplasm of cortical fiber cells. VEGFR2 was present in mouse lens epithelial and fiber cells and was tyrosine phosphorylated at all stages examined. VEGF-A protein was barely detectable in lens epithelial cells during the first postnatal week, but increased as the capillaries of the anterior pupillary membrane regressed. VEGF-A levels were highest in adult lenses. Suturing the eyelid caused an increase in VEGF-A mRNA and protein in lens epithelial and fiber cells.

VEGF-A secreted by lens cells may stimulate the formation of the fetal vasculature, but regression of these vessels is not likely to be caused by a reduction in VEGF-A production by the lens. An active VEGF-A signaling system of unknown function appears to be active in the lens. It is likely that VEGF-A expression is regulated by tissue hypoxia at all stages of lens development.