Molecular regulation of vascular endothelial growth factor expression in the retinal pigment epithelium.

PURPOSE: Vascular endothelial growth factor (VEGF) plays an important role in homeostasis and diseases of the retinal pigment epithelium (RPE), choriocapillaris, and, most notably, age-related macular degeneration (AMD). Although much is known about VEGF regulation in pathologies, little is known about the control of VEGF expression under normal conditions. VEGF expression has been previously shown to be regulated in coordination with cell differentiation in the muscle and kidney. We therefore tested the hypothesis that VEGF in the adult RPE would similarly be regulated in conjunction with differentiation. METHODS: A human retinal pigment epithelium cell line (ARPE-19), a line of immortalized human RPE cells, was used for all experiments. RPE cells were polarized in culture for 4 weeks on laminin-coated Transwells. Levels of VEGF mRNA and protein were determined with real-time PCR and enzyme-linked immunosorbent assay, respectively. VEGF-luciferase reporter constructs were used to identify regions of the VEGF promoter that control VEGF expression in the RPE. Microphthalmia-associated transcription factor (MITF)-Tfe transcription factors were blocked using either a pan MITF-Tfe dominant negative or specific small interfering RNA (siRNA). RESULTS: VEGF mRNA and protein secretion increased over time in the RPE cells cultured on Transwells, with protein secretion occurring in a polarized fashion primarily toward the basolateral side. Overexpression of a dominant negative that targets the MITF-Tfe family resulted in a 50% reduction in VEGF expression. The role of the MITF-Tfe family in VEGF regulation in the RPE was corroborated in studies with the VEGF-luciferase reporter constructs, where deletion of the distal VEGF promoter region containing putative binding sites for the MITF-Tfe family resulted in a 50% reduction in VEGF promoter activity. siRNA knockdown of the MITF-Tfe family individually, and in combination, revealed that downregulation of Tfe3 resulted in reduced VEGF expression. CONCLUSIONS: Our results indicate that Tfe3, in conjunction with other MITF-Tfe members, regulates VEGF expression in the RPE and are consistent with the hypothesis that VEGF expression in RPE cells is regulated as part of their differentiation.

Expression and role of VEGF--a in the ciliary body.

PURPOSE: The role of VEGF-A in the normal ciliary body is largely unexplored. The ciliary body is similar in many respects to the choroid plexus of the brain, and we demonstrated previously the importance of VEGF-A in maintenance of choroid plexus vasculature and ependymal cells. Therefore, the role of VEGF-A in ciliary body homeostasis was explored. METHODS: Swiss-Webster mice (VEGF-LacZ) were used to determine VEGF-A expression during ciliary body development and in the adult. VEGFR2 expression was determined in adult wild type C56BL/6J mice. Systemic VEGF-A neutralization in vivo was achieved with adenovirus-mediated overexpression of soluble VEGFR1 (sFlt1). Following VEGF-A neutralization, the ciliary epithelium was analyzed by light microscopy and transmission electron microscopy (TEM). The effect of VEGF-A blockade on ciliary body function also was assessed by measuring intraocular pressure. RESULTS: VEGF-A expression was detected at embryonic day 18.5 (E18.5), the onset of ciliary process formation. In the adult ciliary body, VEGF-A was expressed by the pigmented epithelium, whereas VEGFR2 was localized primarily to the capillary endothelium and nonpigmented epithelium. Systemic VEGF-A neutralization led to a thinning of the nonpigmented epithelium, vacuolization of the pigmented epithelium, loss of capillary fenestrations, and thrombosis. These changes were associated with impaired ciliary body function, as evidenced by decreased intraocular pressure in sFlt1-overexpressing animals (15.31 ± 2.06 mm Hg) relative to controls (18.69 ± 1.49 mm Hg). CONCLUSIONS: VEGF-A has an important role in ciliary body homeostasis. Potential for undesired off-target effects should be considered with the chronic use of anti-VEGF-A therapies.

Expression and role of VEGF in the adult retinal pigment epithelium.

PURPOSE: Despite a lack of active angiogenesis, VEGF is expressed in nearly every adult tissue, and recent evidence suggests that VEGF may serve as a survival factor for both vascular and nonvascular tissues. VEGF blockade is a widely used treatment for neovascular diseases such as wet age-related macular degeneration (AMD). Therefore, it was sought in this study to evaluate the expression and role of endogenous VEGF in RPE. METHODS: VEGF and VEGFR2 expression in the murine retina were assessed during development. Bevacizumab was used to neutralize VEGF in ARPE-19 cells, and the effects on cell survival and apical microvill were assessed by TUNEL and SEM, respectively. VEGF was systemically neutralized in vivo by adenoviral-mediated overexpression of soluble VEGFR1 (sFlt). RPE and choriocapillaris were analyzed by transmission electron microscopy (TEM). Changes in gene expression were evaluated by quantitative real-time PCR. RESULTS: VEGF expression was detected in the developing RPE as early as embryonic day (E) 9.5, whereas VEGFR2 expression by RPE began nonuniformly between postnatal (P) day 6.5 and P8.5. VEGF neutralization in vitro led to increased apoptosis and reduced microvilli density and length. Systemic VEGF neutralization led to transient degenerative changes; RPE were vacuolated and separated from photoreceptor outer segments, and choriocapillaris fenestrations were decreased. VEGF levels were elevated in RPE of Ad-sFlt1 mice at day 4 postinfection, and there was increased expression of the neurotrophic factor CD59a at day 14. CONCLUSIONS: These results indicate that VEGF plays a critical role in survival and maintenance of RPE integrity. Potential undesired off-target effects should be considered with chronic use of anti-VEGF agents.