Studies on the tumour promoter-induced release of fibronectin from human lung fibroblasts, and its counteraction by retinoic acid.

The purpose of the experiments reported here is to improve our understanding of the mechanism whereby tumour promoters (e.g., 12-O-tetradecanoylphorbol-13-acetate, TPA) stimulate increased release of fibronectin (FN) from human lung fibroblasts (HLF) in culture. We investigated whether pretreatment of these cultures with a brief pulse of TPA would be sufficient to cause this effect, or whether continuous presence of TPA is required. We found that a pretreatment of 5-15 min with TPA caused the increased FN release when followed by a 2-h TPA-free incubation. The increased release did not cease upon removal of TPA. Longer exposure to TPA (1-2 h) also gave higher FN release than control cultures, but the effect was less pronounced. Pretreatment with retinoic (RA) for up to 30 min did not counteract the subsequent TPA effect after the RA was removed, even though we could show with labeled RA that it had entered the cells. Therefore, RA must be present simultaneously with TPA; it presumably acts on the cell surface when antagonizing the effect of TPA on FN release. When all cell-surface FN was removed by trypsinization and the cells were incubated with TPA, an increased release of FN into the medium occurred and a decreased accumulation of cell-associated FN compared to controls. These results suggest that FN is released even if the pericellular matrix is absent and that TPA interferes with the subsequent build-up of the matrix.