Ionizing Irradiation Not Only Inactivates Clonogenic Potential in Primary Normal Human Diploid Lens Epithelial Cells but Also Stimulates Cell Proliferation in a Subset of This Population

Abstract

Over the past century, ionizing radiation has been known to induce cataracts in the crystalline lens of the eye, but its mechanistic underpinnings remain incompletely understood. This study is the first to report the clonogenic survival of irradiated primary normal human lens epithelial cells and stimulation of its proliferation. Here we used two primary normal human cell strains: HLEC1 lens epithelial cells and WI-38 lung fibroblasts. Both strains were diploid, and a replicative lifespan was shorter in HLEC1 cells. The colony formation assay demonstrated that the clonogenic survival of both strains decreases similarly with increasing doses of X-rays. A difference in the survival between two strains was actually insignificant, although HLEC1 cells had the lower plating efficiency. This indicates that the same dose inactivates the same fraction of clonogenic cells in both strains. Intriguingly, irradiation enlarged the size of clonogenic colonies arising from HLEC1 cells in marked contrast to those from WI-38 cells. Such enhanced proliferation of clonogenic HLEC1 cells was significant at ≥2 Gy, and manifested as increments of ≤2.6 population doublings besides sham-irradiated controls. These results suggest that irradiation of HLEC1 cells not only inactivates clonogenic potential but also stimulates proliferation of surviving uniactivated clonogenic cells. Given that the lens is a closed system, the stimulated proliferation of lens epithelial cells may not be a homeostatic mechanism to compensate for their cell loss, but rather should be regarded as abnormal. This is because these findings are consistent with the early in vivo evidence documenting that irradiation induces excessive proliferation of rabbit lens epithelial cells and that suppression of lens epithelial cell divisions inhibits radiation cataractogenesis in frogs and rats. Thus, our in vitro model will be useful to evaluate the excessive proliferation of primary normal human lens epithelial cells that may underlie radiation cataractogenesis, warranting further investigations.

Nonlinear ionizing radiation-induced changes in eye lens cell proliferation, cyclin D1 expression and lens shape

“…Using primary human [ 52 ] and mouse LECs [ 53 ], others have reported a linear dose-response to low-dose IR. The data presented here extend these previous studies in terms of the use of an established human LEC line (FHL124) alongside additional markers and by providing additional evidence at low doses.…”

Inverse dose-rate effect of ionising radiation on residual 53BP1 foci in the eye lens

“…The role of DNA damage and its potential impact on cataractogenesis in the lens is documented 16–18,22,27,28 . Notable examples include; cataractous lenses showing a high frequency of single strand breaks 28 , base and nucleotide excision repair genes having a suggested role in cataractogenesis 29 and mutated Xpd/Ercc2 genes (involved in DNA repair) result in sensitivity to IR 18 . Lens epithelial cells appear to employ both non-homologous end joining and homologous recombination to repair DNA damage, observed in the dose responses of both 53BP1 and Rad51 for both pathways respectively 17 .…”

Ionizing radiation response of primary normal human lens epithelial cells

“…Lens epithelial cells (LECs) are the only proliferative population among the lenticular structures and have long been regarded as target cells for radiation cataractogenesis [ 6 , 10 ]. Our previous work was the first to evaluate the radiosensitivity of lens cells with the clonogenic assay, and demonstrated that the survival of HLEC1 human LECs and WI-38 human lung fibroblasts following irradiation is similar [ 11 ]. Furthermore, HLEC1 cells were found to contain various subsets with differing vulnerability to radiogenic inactivation of clonogenic potential, such that while some cells irradiated at ≥2 Gy form clonogenic colonies with more cells than those arising from sham-irradiated cells, other irradiated cells form abortive colonies with less cells than those arising from irradiated fibroblasts [ 11 ].…”