Age-related UV absorption of the human eye lens and its molecular background

The ultraviolet (UV) absorption of various sections of the human lens was studied and compared with protein expression paralleling differential UV absorbance in anterior and posterior lenticular tissue. The UV absorbance of serial lens cryostat sections (60 lm) and that of lens capsules was determined using a Shimadzu scanning spectrophotometer, and the absorption coefficients were calculated. Two-dimensional gel electrophoresis was performed using two pooled lenticular protein extracts (anterior and posterior sections). Protein spots were quantified and significantly different spots were identified by mass spectrometry following in-gel digestion with trypsin and chymotrypsin. The UV-C and UV-B absorption of the human lens increased toward the posterior parts of the lens. The anterior and posterior lens capsules also effectively absorbed UV radiation. Levels of molecular chaperone proteins Beta-crystallin B2 (UniProtKB ID:P43320), A3 (UniProtKB ID:P05813) and of glyceraldehyde 3-phosphate dehydrogenase (UniProtKB ID:P04406) were significantly higher in the anterior part of the lens, whereas lens proteins Beta-crystallin B1 (UniProtKB ID:P53674) and Alpha-crystallin A chain (UniProtKB ID:P02489) were higher in the posterior sections. These results provide evidence that differential UV absorption in the anterior and posterior lens is accompanied by differential protein expression. INTRODUCTION Ultraviolet (UV) radiation poses an increasing risk on the tissues and organs exposed to radiation. In the human eye, the cornea absorbs most of the UV radiation (1), but still significant radiation may reach the lens. It is well established that UV radiation, especially within the range of UV-B (280–330 nm), may cause ocular damage (2,3). Exposure to near UV light (315–400 nm) has been reported to lead to pathological changes in the lens of albino mice and gray squirrels (4,5). Several studies provided evidence for the relationship between the risk of cataract and sun exposure (2,6–9). Brown type of human cataracts develops much more frequently in outdoor-active individuals than in indoor workers (9) and an increased risk of posterior subcapsular cataract is associated with high exposure to UV-B radiation (6). The primate lens absorbs most of the UV-A (330–400 nm) and UV-B (280–330 nm) radiation (10). Accordingly, it is thought that under normal circumstances no considerable amounts of UV radiation reach the retina. Therefore, it is of critical importance to determine to what degree various sections of the lens and its capsule contribute to the capacity of the lens to absorb UV radiation. An earlier study has shown that the various layers of the cornea are able to absorb UV radiation to different extent (1). There are only few studies concerning the absorption properties of the different layers of the lens. Gaillard et al. (11) found, that in primates, the posterior layers of the lens show higher UV absorbance than the anterior ones in the 300–440 nm range. Another study by Dillon et al. (12) showed that the nucleus of an old human lens reveals higher absorbance than the outer cortex. Moreover, no UV absorption data are available on the lens capsules. The most abundant known low–molecular-weight UV-filter compounds in the human lens are kynurenine, 3-hydroxykynurenine glucoside (3OHKG), and 4-(2-amino-3-hydroxyphenyl)-4oxobutanoic acid O-glucoside (13). The UV spectra of these molecules show higher absorbance in the UV-B and UV-C ranges than in the UV-A range. Results suggest that these tryptophan-derived UV filters are responsible for UV radiation absorbance in the lens. Levels of these UV filters and protein-bound kynurenine and 3-hydroxykynurenine (3OHKyn) are higher in the nucleus than in the cortex (14). Accordingly, the nuclear concentration of free UV filters 3OHKG and 3OHKyn has been shown to be lower than that in the cortex (15). Apart from these low-molecular-weight compounds proteins may significantly contribute to UV absorption capacity. The human lens has the highest protein content within the body and all proteins are absorbing in the UV range (16). Key proteins, i.e., crystallins of the lens have earlier been described in detail as major constituents and recently identified by mass spectrometry, too (17). These specific lenticular proteins are main determinants for optical properties including UV absorbance. To the best of our knowledge, however, the presence of these proteins was not localized to anterior or posterior parts of the lens. †These authors have equally contributed to this study §Current address: Department of Anatomy, Histology and Embryology H-6724 Szeged Kossuth L. Sgt. 40. Hungary *Corresponding author email: nogradi@opht.szote. u-szeged.hu (Antal N ogr adi) © 2013 Wiley Periodicals, Inc. Photochemistry and Photobiology © 2013 The American Society of Photobiology 0031-8655/13