DiVerential induction of proto-oncogene expression and cell death in ocular tissues following ultraviolet irradiation of the rat eye

Background/aims—Ultraviolet (UV) irradiation of mammalian cells in culture evokes the transcriptional activation of diVerent proto-oncogenes, among them members of the fos/jun family which are known to play an important role in cell proliferation and diVerentiation. To investigate in vivo UV induced proto-oncogene expression of irradiated ocular cells, the expression of JunB, JunD, and Egr-1 was analysed in the cornea, lens, and retina. Furthermore, UV radiation is known to induce pleiotrophic events in irradiated cells which include growth arrest, inflammation, and even cell death. In order to determine the type of cell death—for example, apoptosis versus necrosis, sections of UV irradiated rat eyes were further examined for distinct ultrastructural morphology of cell death and DNA fragmentation. Methods—Eyes of anaesthetised rats were exposed to 1.5 J/cm of ultraviolet radiation (280–380 nm). Animals were perfused 6 and 16 hours after irradiation and tissue sections of enucleated bulbi were processed for light and electron microscopy. Results—Under control conditions, Jun B was constitutively expressed in numerous superficial cells but also in scattered basal cells of the corneal epithelium. After UV exposure JunB expression was massively upregulated in many cells of the basal cell layers of the corneal epithelium, although during the entire experiment, both the corneal stroma and endothelium were JunB negative. In contrast, Egr-1 was expressed exclusively in lens epithelium showing only a faint expression pattern under control conditions. However, Egr-1 expression increased after UV exposure, so that many Egr-1 positive cells of the lens epithelium could be found several hours after UV illumination. JunD was expressed in single cells of both the ganglion cell layer and the inner nuclear layer of the retina, a pattern of expression which did not change after UV exposure. Regarding the type of cell death, features of apoptosis were only occasionally present in scattered superficial cells of the corneal epithelium of control eyes. After UV exposure, however, morphological signs of apoptosis and TUNEL positive cells were visible both in the stroma and epithelium of the rat cornea. In contrast, UV irradiated lens epithelial cells exhibited features typical of necrosis. The corneal endothelium and the retina did not show any indications of morphological changes indicative of cell death after UV irradiation. Conclusion—Each proto-oncogene encoded protein was found to be expressed in a tissue specific manner and UV irradiation diVerentially modulates the expression pattern of these transcriptional regulatory proteins. This temporospatial expression pattern of these proteins is accompanied by two morphologically distinct types of cell death in the cornea and lens after UV irradiation. (Br J Ophthalmol 1999;83:225–230) In recent years it has become evident that excess ultraviolet radiation (UV) can lead to severe abnormalities within radiation exposed tissues. Beside promotion of solar elastosis with wrinkling and coarsening of mammalian skin surface and induction of melanoma skin cancer, UV radiation also damages ocular tissues. Normally, short wavelength UV radiation does not cause any abnormalities in irradiated tissues because atmospheric chemicals (for example, ozone) eVectively absorb light below approximately 290 nm. However, when eyes are exposed to solar radiation—for example, at high altitudes or in snow covered areas, the initial response is macroscopic visible inflammation such as photokeratitis or pinguecula. In addition, chronic exposure of eyes to UV is heavily implicated in the development of cataracts (for example, opacities of the lens) and can also cause phototoxic eVects to the retina. Although the histopathological signs of ultraviolet damage have been known for many years, the response at the molecular level to UV radiation is still unclear. Oxidative DNA damage and damage to cell membranes are thought to play key roles in eliciting the UV response and specific UV responsive cis acting elements have been identified. 6 UV irradiated mammalian cells rapidly respond with the induction of various genes, both in vitro and in vivo, among them members of the fos, jun, and egr-1 family of transcription factors. 8 The expression of proteins represents a cellular stress response which transactivates specific target genes. In UV irradiated ocular tissues little is known about the temporospatial Br J Ophthalmol 1999;83:225–230 225 Department of Electron Microscopy, Würzburg, Germany