The tumor-promoter-induced release of phosphorylated fibronectin from human lung fibroblasts.

The role of fibronectin (FN) phosphorylation was examined for its involvement in tumor-promoter-induced FN release from normal fibroblasts. We investigated phosphorylated FN in spent media and in cell layers of human lung fibroblasts (HLF) cultured in the presence and absence of the tumor-promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Our data, obtained by metabolic labeling of HLF with [32P]orthophosphate, revealed that 32P-labeled FN accumulated rapidly in the cell layer in the absence of TPA. With TPA present, accumulation of 32P-labeled FN at the cell layer of HLF was much slower than under control conditions. On the other hand, treatment of the cells with TPA caused a marked increase in the amount of medium-released FN. This increase in released FN, however, was caused by unlabeled FN and was not paralleled by an increase in 32P-labeled FN. We investigated the ability of FN from normal and TPA-treated cells to bind to normal HLF monolayers. We found no difference in re-binding ability, regardless of whether FN was derived from cell extracts of control cultures (showing highly phosphorylated FN) or from the medium of TPA-treated cultures with low phosphorylation of FN.