Complex regulation of fibronectin synthesis by cells in culture.

We have studied the biosynthesis of fibronectin by NIL8 hamster embryo cells in various stages of growth. Pulse labeling with [35S]methionine, immunoprecipitation, electrophoresis, and autoradiography were employed to compare fibronectin synthesis by cells in subconfluent monolayers, confluent monolayers, and "aged" postquiescent monolayers. We have determined that fibronectin synthesis is proportionally low while cells are subconfluent, rises to maximal levels at confluence but just prior to quiescence, and then declines with increasing culture age in quiescent cultures. Furthermore, when cells are stimulated to grow, the effects on rates of fibronectin synthesis depend on the prior history of the cells; freshly confluent cells stimulated to grow show reductions in rate of fibronectin synthesis, whereas aged postquiescent cultures show increases in this rate. These results are in contrast to those on the rates of synthesis of other proteins (including the precursor to the C3 component of complement), which strictly correlate with quiescence and do not decline significantly with culture age postconfluence. We have also determined that the microheterogeneity of pulse-labeled fibronectin differs between exponentially growing and quiescent cells and that it becomes less heterogeneous once cells are quiescent. We conclude that the microheterogeneity of pulse-labeled fibronectin and the proportionate amount of total fibronectin synthesized both depend on growth state prior to the entry of cells into quiescence and that they depend on culture age thereafter.