State of differentiation of bovine epithelial lens cells in vitro: Modulation of the synthesis and of the polymerization of specific proteins (crystallins) and non-specific proteins in relation to cell divisions

Abstract

Maintenance of the state of differentiation in serially cultured bovine epithelial lens cells has been investigated. The radioactive labelled soluble proteins were studied by gel filtration and gel electrophoresis. 1. 1. In the lens epithelium on its capsule, preferential synthesis of αB 2 vs αA 2 crystallin subunits and synthesis of β-crystallins (mainly βBp) were observed. 2. 2. Epithelial lens cells cultured on plastic Petri dishes for up to 35 divisions still synthesized αB 2 and βB p, but no longer αA 2. Conversely, the same cells injected into nude mice synthesized αB and αA, but no β-crystallin could be detected. 3. 3. The ratio of non-crystallin proteins to crystallin polypeptides increased drastically with the number of cell divisions. Among these proteins, both Mr 45 000 and Mr 57 000 proteins are probably constituents of the water-soluble cytoskeletal proteins, respectively actin and vimentin. A Mr 17 000 polypeptide was observed and its relationship with a metabolic product of α-crystallin is proposed. 4. 4. The polymerization process of crystallin polypeptides in these cells was studied and compared with crystallin aggregates found in the lens. Newly synthesized α crystallins were readily involved in high molecular aggregates. This process does not seem to require αA, since only αB was detected. Interestingly, non-crystallin-soluble proteins form the bulk of proteins found in high molecular weight (HMW) polymers. The time course of crystallin aggregate formation, in long-term culture cells, seems to be different for α- vs β-polypeptides. These results allowed us to conclude that bovine epithelial lens cells in vitro, although they do not undergo terminal differentiation into fibers, are not dedifferentiated, since they still express specific features of the epithelium in situ.