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S.L. Guérin, M.–Ê. Gingras, M. Gaudreault, C. Salesse; The activity of the 5 and 6 integrin subunit genes promoter is under the influence of the extracellular Matrix Components Fibronectin and Laminin in rabbit corneal epithelial cells . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3767.
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Purpose: Damages to the corneal epithelium results in the massive secretion of fibronectin (FN) within the first few hours following corneal injury. During the wounding process, FN secretion is progressively turned off while secretion of laminin (LM) increases. Both these extracellular matrix (ECM) components are recognized by membrane–bound receptors that belong to the integrin family. The integrin α5ß1 is recognized as the main FN–binding receptor whereas LM is bound by both the α6ß1 and α6ß4 integrins. In the present study, we investigated whether expression of both the α5 and α6 integrin subunit genes might be modulated by their corresponding ligands FN and LM, respectively. Methods: Plasmids bearing the CAT reporter gene fused to various lengths from both the α5 and α6 promoters were transiently transfected in primary cultures of rabbit corneal epithelial cells (RCECs) grown on FN or LM–coated culture plates. Western blot analyses and electrophoretic mobility shift assays (EMSAs) were used to follow both the expression and DNA binding ability of the positive transcription factor Sp1, respectively. Results: Expression of the α5 gene promoter was shown to be positively modulated by FN in a dose–dependent manner. FN responsiveness was found to be determined by the binding of hyperphosphorylated Sp1 to a target site from the α5 basal promoter that we designated the Fibronectin Responsive Element (FRE). However, LM had totally opposed effects on the activity driven by the α6 gene GC–rich promoter, which has been reported to be under the regulatory influence of Sp1. Indeed, transcription directed by the α6 promoter was shown to be repressed by LM in a dose–dependent manner in RCECs. We demonstrated that α6 repression results primarily from the disappearance of Sp1 when RCECs are grown on LM. Conclusions: Our results demonstrate that components from the ECM possess the ability to modulate the transcription of specific integrin subunits genes by altering the nuclear proteins, such as Sp1, that are required for their proper transcription.
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