Abstract
Whilst the cataractogenic potential of ionizing radiation has been known for over the past 120 years, little is known about radiation responses of lens cells. Our previous work was the first to evaluate the radiosensitivity of lens cells with the clonogenic assay, documenting that the survival of HLEC1 human lens epithelial cells is comparable to that of WI-38 human lung fibroblasts. Moreover, HLEC1 cells were found to contain subsets where irradiation stimulates proliferation or facilitates formation of abortive colonies with fewer cells than human fibroblasts. This study aims to gain insights into these mechanisms. Irradiation of HLEC1 cells with 10% survival dose caused a growth delay but did not reduce viability. HLEC1 cells at high cumulative population doubling level were more susceptible to radiogenic premature senescence than WI-38 cells. Concerning p53 binding protein 1 (53BP1) foci, HLEC1 cells harbored less spontaneous foci but more radiogenic foci than in WI-38 cells, and the focus number returned to spontaneous levels within 48 h postirradiation both in HLEC1 and WI-38. The chemical inhibition of DNA repair kinases ataxia telangiectasia mutated, DNA-dependent protein kinase or both delayed and attenuated the appearance and disappearance of radiogenic 53BP1 foci, increased radiogenic premature senescence and enhanced clonogenic inactivation. The DNA microarray analysis suggested both radiogenic stimulation and inhibition of cell proliferation. Treatment with conditioned medium from irradiated cells did not change growth and the plating efficiency of nonirradiated cells. These results partially explain mechanisms of our previous observations, such that unrepaired or incompletely repaired DNA damage causes a growth delay in a subset of HLEC1 cells without changing viability through induction of premature senescence, thereby leading to clonogenic inactivation, but that growth is stimulated in another subset via as yet unidentified mechanisms, warranting further studies.
Highlights
Rontgen’s discovery of X-rays in 1896 was followed by observations of ionizing radiation cataracts, of which first case was reported in animals in 1897 and in humans in 1903 [1,2]
Treatment with conditioned medium from irradiated cells did not change growth and the plating efficiency of nonirradiated cells. These results partially explain mechanisms of our previous observations, such that unrepaired or incompletely repaired DNA damage causes a growth delay in a subset of HLEC1 cells without changing viability through induction of premature senescence, thereby leading to clonogenic inactivation, but that growth is stimulated in another subset via as yet unidentified mechanisms, warranting further studies
In HLEC1 (Fig 2A), a difference in SA-β-gal positivity at cumulative PDN (CPD) 11.4 ± 0.1 and 14.3 ± 0.1 was statistically insignificant at 0 Gy (t-test p = 0.51 between 3.3 ± 2.7% and 4.5 ± 0.9%), but SA-β-gal positivity at CPD 14.3 ± 0.1 (72.9 ± 22.5%) was significantly higher than that at CPD 11.4 ± 0.1 (16.8 ± 6.1%) at D10 (t-test p = 0.02). These results suggest that irradiation induces premature senescence in HLEC1 and WI-38, and indicate that HLEC1 at higher CPD is more vulnerable to such radiogenic premature senescence
Summary
Rontgen’s discovery of X-rays in 1896 was followed by observations of ionizing radiation cataracts, of which first case was reported in animals in 1897 and in humans in 1903 [1,2]. Irradiation response of primary normal human lens epithelial cells (ICRP) writes that gonads, bone marrow and the crystalline lens of the eye are among the most radiosensitive tissues in the body [3]. ICRP has classified radiation cataracts as tissue reactions (formerly called nonstochastic or deterministic effects) with a dose threshold below which no effect would occur, and has recommended an equivalent dose limit for the ocular lens of workers and public to prevent radiation cataracts [4,5]. The lens is considered much more radiosensitive than previously thought, but its mechanisms remain incompletely understood [8,9]
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