A previous study
23 suggested that foveolar choroidal blood flow decreased in patients with AMD and large drusen and reported a systematic decrease in choroidal circulatory parameters and an increase in the severity of AMD features associated with risk for CNV. Ross et al.
24 showed an association between the locations of the macular choroidal watershed vascular filling zones detected by FA and CNV membranes. During angiography, watershed filling zones corresponded to the last areas of the choroid that fill with the dye. CNV occurs in proximity to these areas that are the most prone to the development of ischemia and hypoxia in cases of decreased choroidal blood flow. Furthermore, CNV always associated with the deposition of materials and the thickening of the RPE-Bruch membrane complex, which may impede the diffusion of substances and increase the distance oxygen must travel from the choriocapillaris to the photoreceptors, further reducing the availability of oxygen in the outer retina.
25 26 HIF-1α, the main reactor of ischemia, plays a pivotal role in angiogenesis.
27 Previous results showed that HIF-1α is expressed in human CNV membranes,
28 human retinal angiomatous proliferation specimens,
29 and retinal neovascularization.
30 The HIF-1α inhibitor YC-1 could also prohibit the formation of laser-induced CNV.
31 Our previous in vitro results also showed that mRNA and protein levels of HIF-1α in the RPE increased in response to hypoxia, followed by increasing expression of VEGF. mRNA and protein levels of HIF-1α and VEGF in the RPE were decreased dramatically after transfection with an HIF-1α-specific small interference RNA vector.
32 The proliferation, migration, and tube formation of choroidal endothelial cells were significantly inhibited by the HIF-1α knocked-down RPE compared with the control in the coculture system.
33 Based on these results, we presumed that ischemia/hypoxia may mediate CNV through HIF-1α-regulated VEGF. To further elucidate the mechanism for this, we investigated whether PI3K/Akt and MEK/ERK signaling pathways were involved in regulating HIF-1α/VEGF. Our in vivo results illustrated that HIF-1α and VEGF were upregulated in laser-induced CNV lesions. We also concluded that the PI3K/Akt pathway was required for the expression of HIF-1α and VEGF, whereas MEK/ERK was needed only for VEGF expression. PI3K- and MEK-specific inhibitors could significantly suppress the development of CNV. Given that PI3K has several different isoforms (p110α, p110β, and p110δ) and only p110α was selectively required for angiogenesis,
34 we proved that p110α was involved in the development of laser-induced CNV. Our group also found that hypoxia could stimulate the expression of p110α at mRNA and protein levels in cultured hRPE and that LY294002 could downregulate the expression of VEGF by hRPE under normoxia and hypoxia (data not shown). Further study is needed to determine whether p110β or p110δ also functions in laser-induced CNV.