Abstract
Using a sensitive method to determine the amount of fibro-nectin (FN), we investigated the release of cellular FN in C3H mouse fibro-blasts, which depends upon the progress of the cell cycle. Cells first labeled with [14C]leucine and arrested in confluent culture were trypsinized and re-plated in fresh medium containing [14C]leucine with or without 1 mM hydroxy-urea (HU). The cell number in the untreated culture increased 1.3-fold 36 h after replating, whereas HU-treated cells neither entered the S phase nor divided. The synthesis, turnover and rebinding of FN to cells were almost the same for both cultures. The amounts of FN released into the medium also were the same for untreated and HU-treated cultures. These results indicate that FN was not released into the medium specifically before or during the rounding up of mitotic cells. Immunofluorescence observations using a specific antibody directed against FN showed the presence of FN networks on the surface of the substratum, which surrounded the point at which mitotic cells were attached. In addition, for about half the population of mitotic cells no FN was detected on the cell surfaces. Because of these results, we suggest that when mitotic cells become round, they leave FN in networks on the substratum.
