Caveolae are flask-shaped invaginations of the plasma membrane found in many cell types. Caveolae play a role in lipid transport, endocytosis, signal transduction, and cell transformation. Expression of caveolin-1, the principal component of caveolae and a regulator of caveolae-dependent signaling and endocytosis, was investigated in lens epithelial cells and lens fiber cells in wild-type (wt) and SPARC-null mice and normal human donors in vivo and in vitro. RT-PCR, immunofluorescence and immunoblot analyses of human and murine ocular tissues revealed that caveolin-1 was expressed in the corneal epithelium, corneal endothelial cells, and blood vessels of iris, ciliary body and retina, but minimal in the normal lens epithelia or fiber cells of murine and human lens. In contrast, caveolin-1 was significantly up-regulated in mesenchymal-transdifferentiated lens epithelia in SPARC-null cataract lenses. In addition, lens epithelial cells from primary culture or from cultures of immortalized lens epithelial cell lines expressed significant amounts of caveolin-1. The lens epithelial cells expressed epidermal growth factor (EGF) receptor and were responsive to EGF-mediated cell proliferation, but they did not show EGF-dependent caveolin-1 tyrosine phosphorylation. Caveolin-1 might have a role in the process of epithelial-mesenchymal transdifferentiation (EMT) in the lens, the most common cause of vision loss in human secondary cataracts.
