Purpose: The goals of this study were to determine whether MAP kinase signaling pathways play a role in the formation of lens cataracts and to examine the potential signaling relationship between Src and MAP kinases in signaling the induction of lens opacities.
Methods: Embryonic day (E)10 chick lenses were cultured in Medium 199 containing 10% fetal bovine serum. The activation state of Src kinases and the MAP kinases extracellular signal-regulated protein kinase (ERK), c-jun N-terminal kinase (JNK), and p38 in the lens epithelium was determined over a time course from 10 minutes to 10 days in culture by immunoblot analysis. Src kinase activation was suppressed by exposure to the Src family kinase-specific inhibitor PP1. To examine the role of specific MAP kinases in the development of lens opacities, lenses were grown for 10 days in the presence or absence of inhibitors of ERK (U0126), JNK (SP600125), and p38 (SB203580). Lenses were observed and photographed daily, and the degree of opacification was quantified by using image-analysis software.
Results: Within a short time after placing embryonic lenses in culture conditions that induce the formation of cataracts, there occurred a great increase in the activation state of the MAP kinase ERK. Activation of ERK was both rapid and transient. No activation of the MAP kinase JNK was observed in the cataract cultures beyond that which occurred in normal lens epithelium, even though JNK activation is often linked to the cellular response to stress. In contrast, although p38 activation was barely detected in the normal embryonic lens, this stress-activated protein kinase exhibited a robust activation in cataract cultures that was sustained throughout the culture period. Studies conducted to map the cataract signaling pathways indicate that the p38 MAP kinase functions upstream of the Src kinase. To analyze the potential role of ERK, JNK, and p38 in cataract induction, lenses were cultured in the presence of specific MAP kinase inhibitors. Although the inhibitors of ERK and JNK did not interfere with the formation of cataract, p38 inhibitors blocked the development of lens opacities with an efficacy similar to that of the Src kinase inhibitor PP1.
Conclusions: Activation of both Src and p38 kinases lead to the induction of cataract.