Potential roles for transforming growth factor-B during terminal differentiation of ocular lens cells /

During terminal differentiation, lens fiber cells permanently withdraw from the cell cycle, migrate posteriorly along the lens capsule, become greatly elongated and eliminate all cellular organelles. Lens fiber differentiation may be controlled by various factors within the ocular environment. The lens capsule may influence lens fibers, since regions of the capsule differ in components and architecture. TGF-β is known as a regulator of both cell division and differentiation as well as a modulator of ECM production. Since TGF-β is found in the ocular environment and within lens cells, these functions of TGF-β in the lens were examined. In the avian lens, post-mitotic cells destined to become lens fibers are located in a structure called the annular pad. TGF-β type I and type II receptors were identified by enhanced chemiluminescence in freshly isolated and cultured chicken lens annular pad (CLAP) cells. Immunoblot and metabolic labelling showed that TGF-β stimulation resulted in decreased synthesis of lens crystallins and no accumulation of a differentiation marker protein, phakinin. Measures of tritiated thymidine incorporation demonstrated that TGF-β stimulated, in a dose-dependent manner, cellular growth and spreading. The production of collagen type IV and fibronectin were detected in TGF-β stimulated CLAP cells using immunoblotting techniques and polyacrylamide gel electrophoresis. Substrates of collagen type IV, laminin or fibronectin stimulated increased thymidine incorporation and cellular spreading. TGF-β stimulation of cells cultured on an ECM substratum resulted in monolayer growth and a synergistic increase in thymidine incorporation. Whether TGF-β modifies the lens capsule by stimulating CLAP cells to produce matrix degrading enzymes (MMPs) was examined by zymography. TGF-β induced MMP activity corresponding to MMP2, MMP9 and an unknown MMP at approximate 77 kD molecular weight. Thus, TGF-β does not function to keep CLAP cells withdrawn from the cell cycle or to promote fiber cell differentiation. TGF-β does induce CLAP cells to produce ECM proteins and matrix degrading enzymes. Thus, TGF-β may inhibit mitosis in vivo, but more importantly, it may influence the composition of the lens capsule