Pathogenesis Of Glucocorticoid-induced Cataract: An Invivo And Invitro Study

Prolonged use of glucocorticoids (GC) is a significant risk factor for the development of posterior subcapsular cataract (PSC). The PSC presents a different etiology from the more common cortical and nuclear cataracts. The mechanism of glucocorticoid-induced cataract (GIC) is somewhat obscure. Several hypotheses have been offered for the mechanism of GIC formation. Some investigations have concentrated on the metabolic impact of GC in the lens. Because both increases and decreases have been reported for the same enzymes and metabolic intermediates and the effects vary with the steroid, it is difficult to come to any clear conclusions about the effects of GC on lens metabolism. Some have suggested that steroids cause an inhibition of the cation pump and the resulting electrolyte/water imbalance is responsible for cataract formation, however, there are several reports indicating steroids do not influence lens ion levels. It seems unlikely that this is responsible for cataract. The oxidation explanation has been invoked for GIC, based on the idea that GC may affect the activities of mechanisms involved in protection of the lens from oxidative stress. However, again, the data are less than convincing. Furthermore, the morphology of the GIC, with its localised light scatter, is unlike that of oxidative cataracts, which are characterized by the light scatter throughout the lens. The most prominent hypothesis holds that lens protein adducts have been implicated in cataracts associated with long-term GC treatment. The covalently attached steroid is postulated to destabilize the protein's conformation,'opening up'its structure, allowing the oxidation and cross-linking of the protein SH groups and result in the formation of high molecular weight aggregates that become insoluble and produce light scatter. However, these observations and conclusions have been questioned as some authors showed that non-glucocorticoids also bound to the proteins but did not cause opacities. They suggest that a mechanism other than covalent binding of steroids to lens proteins is responsible for GIC.The above inconclusive observations on the effects of steroids on the lens have led to the idea that steroids may not act directly on the lens to generate cataract. One of the features of GIC appears to be the accumulation of undifferentiated epithelial cells just under the posterior capsule. Such cells should be found only on the anterior surface of the lens mass. This suggests that aberrant cell behavior may be involved in cataract formation. Progeny of epithelial cells proliferate and migrate towards the equatorial regions and differentiate into fibre cells, losing their nuclei and all other intracellular organelles. The proliferation and differentiation of the epithelial cells are under the control of growth factors present in the ocular media that bathe the lens. Alterations in the complement of growth factors impacting on the lens could lead to aberrant cell behavior and cataract development. Basic fibroblast growth factor (bFGF) is a major affector of lens growth and critical for controlling lens cells behavior. Jobling et al. suggest that GIC is due to an alteration in growth factors reaching the lens and that this follows steroid-induced alterations in the production of these growth factors in other ocular tissues. It is proposed that steroids cause a reduction in the growth factors such as FGF, which stimulate differentiation of lens epithelial cells (LECs) or an increase in growth factors such as TGF-β, which inhibit this differentiation. When aqueous humor (AqH) containing the altered complement of growth factors acts on the lens, proliferation and migration of anterior epithelial cells occurs, as normal, but no differentiation takes place in the equatorial zone. As a result, cells are not incorporated into the fibre cell mass but, instead, continue their migration until they reach the posterior pole of the lens, where they accumulate forming clumps which scatter light.The Jobling hypothesis makes several assumptions. First, there is a reduction of bFGF and an increase in growth factors such as TGF-βin AqH with long-term use of GC. Second, GC inhibits the differentiation of LECs. To properly test this hypothesis, the two points should be addressed. The work described in this study evaluates these assumptions using both in vivo and in vitro methods to investigate expression of bFGF and TGF-βin AqH, FGF and TGF-βreceptors and on LECs, andβ-crystallin in LECs, after GC treatment.The main results and conclusions are as follows:1. Some lens of all rats throughout period of topical DEX therapy exhibited posterior subcapsular opacity or accumulation of cells under posterior capsules, which meet the features of GIC. 2. It was difficult to determine whether these cells were normal LECs. However, it was reasonable to presume these cells derived of LECs.3. Data from in vivo study showed that DEX had no contribution to expression of bFGF and TGF-βin AqH of rats. However, DEX reduced expression of FGFR1, TGF-βR1 and TGF-βR2 in rat LECs at protein and transcription level.4. Data from rat LECs culture also demonstrated that DEX reduced expression of FGFR1, TGF-βR1 and TGF-βR2 in rat LECs at protein and transcription level.5. RU486 blocked the effects of DEX on FGFR1, TGF-βR1 and TGF-βR2, supporting the idea that GR is expressed in rat lens epithelium and mediates glucocorticoid to regulate cellular physiology.6. Data from in vitro study indicated that DEX inhibited LECs differentiate into fiber cells, and stimulated proliferation of LECs. RU486 blocked the effects of DEX on differentiation of LECs.7. Downregulation of FGFR inhibited differentiation of LECs. As a result, LECs continue their migration until they reach the posterior pole of the lens.8. Downregulation of TGF-βR protected LECs from apoptosis. As a result, LECs continue their proliferation and migration and accumulate under posterior capsule.9. Differentiation inhibition of LECs treated by DEX may play an important role in the development of glucocorticoid-induced posterior subcapsular cataract. Aberrant LECs at equator region continue to proliferate and migrate to the posterior pole of the lens. As a result, accumulation of LECs under posterior capsule develops to PSC.10.It is reasonable to suppose FGFR and TGF-βR in the LECs, not FGF in AqH, may play an important role in the formation of PSC induced by DEX.