Characterization of the roles of STAT1 and STAT3 signal transduction pathways in mammalian lens development

IGF-1 and PDGF are implicated in regulating lens proliferation and/or providing spatial cues that restrict lens proliferation to germinative and transition zones of the lens. However, very little is known about how IGF-1- or PDGF-induced signals are transduced and coupled to gene transcription in lens cells. Therefore, we examined whether these growth factors mediate their effects in the lens through the evolutionarily conserved JAK/STAT signal transduction pathway and if STAT signaling is essential for mammalian lens development.

Expression of STAT1 and STAT3 was analyzed in mouse lens and lens epithelial cells by RT-PCR and western blot analysis. Activation of the STAT signaling pathway was examined by a combination of gel-shift, super-shift, and western blotting assays. Regulation of lens proliferation and gene transcription by STAT pathways was assessed by 3H-Thymidine incorporation or RT-PCR assays with lens explants treated or untreated with Genistein or the JAK2 and STAT3 inhibitor, AG-490. Mice with targeted deletion of STAT3 in the lens were generated by Cre/lox recombination and STAT1-/-, STAT3-/- deficient as well as normal lenses were characterized by histology.

We show that PDGF and IGF1 induce proliferation in 1AMLE6 lens cells and couple their extracellular signals to gene transcription, in part through activation of STAT3 and to a lesser extent STAT1 signal transduction pathways. We further show that targeted deletion of STAT3 in E10.5 lens does not produce overt developmental lens defects. STAT1 knockout mice also exhibit a normal lens phenotype.

Our results showing that deletion of either STAT1 or STAT3 does not affect the normal development of the lens is surprising in view of the fact that STAT pathways are activated in developing chick or mouse lens and inappropriate activation of STAT1 pathway in the lens by ectopic lens expression of IFN? inhibits lens differentiation and induces cataract in transgenic mice. Our data thus suggest that although STAT-signaling pathways may contribute to activation of gene transcription in the lens, it may not be essential for normal lens development or STAT proteins may be functionally redundant during lens development. However, because several growth factors and cytokines present in the lens activate STATs in mouse lens explants and 1AMLE6 lens epithelial cells, it may well be that this evolutionarily conserved signaling pathway is under stringent control in the mammalian lens. Whereas deficiency in any particular STAT pathway can be compensated for by any of the functionally redundant STAT proteins induced by a wide array of growth factors in the lens, chronic or prolonged activation of a particular STAT protein may perturb homeostatic balance in STAT-dependent growth factor signaling, culminating in pathologic lens changes.