Hypoxia Slows Lens Growth With Age

Abstract

Purpose:: Epidemilogical studies suggest that lens thickness is an important risk factor for age-related cataract formation. Thicker lenses are associated with nuclear caataract formation and thinner lenses with cortical cataract. However, the mechanisms that regulate lens epithelial cell proliferation and lens growth in vivo are not understood. We tested the prediction that the low oxygen level that is normally present in the eye is an important regulator of lens growth during adult life.

Methods:: One or eight month old rats and mice were exposed to 11% oxygen, room air (21% oxygen) or 60% oxygen. Intraocular oxygen levels were measured with a fiber optic oxygen sensor. Cell proliferation was measured by counting the percentage of germinative zone epithelial cells labeled after one hour exposure to BrdU. Some of the mice over expressed transgenes encoding forms of the oxygen-regulated transcription factor, HIF-1α, that were genetically modified to make the protein insensitive to oxygen. Western blots were performed for HIF-1α and for the cyclin-dependent kinase inhibitor, p27^KIP1.

Results:: Intraocular oxygen levels were <3% at all ages. Exposure to 11% oxygen decreased intraocular oxygen by 50%, while breathing 60% oxygen increased intraocular oxygen four to six fold. Lenses from one month old rats or mice had high rates of proliferation that were not affected by lowering or raising the level of oxygen in the eye. Lenses from older animals had decreased levels of proliferation that were increased more than three-fold by exposure to 60% oxygen, but were not altered when the eye was made more hypoxic. After animals were exposed to 60% oxygen, HIF-1α protein levels in lens epithelial cells were decreased in young mice and eliminated in old mice. Over expression of oxygen-insensitive forms of HIF-1alpha had no effect on basal levels of proliferation in old or young lenses, but significantly reduced the proliferation caused by treatment with 60% oxygen in older lenses. Treatment of old, but not young lenses with 60% oxygen reduced p27 levels by 60%. This effect was prevented by over expression of oxygen-insensitive forms of HIF-1α.

Conclusions:: The slowing of lens cell proliferation and lens growth that occurs with age requires the normal, hypoxic intraocular environment. Intraocular hypoxia maintains a high level of HIF-1α, which inhibits lens cell proliferation in older lenses. This inhibition may be due, in part, to the ability of HIF-1α to maintain high levels of p27.

Keywords: hypoxia • proliferation • aging