MITOGENIC SIGNALING PATHWAY OF TUMOR-NECROSIS-FACTOR INVOLVES THE RAPID TYROSINE PHOSPHORYLATION OF 41 000-MR AND 43 000-MR CYTOSOL PROTEINS

Tumour necrosis factor (TNF) is a potent mitogen for some fibroblast cell lines. Here we have examined the TNF-mediated changes in protein phosphorylation in Swiss 3T3 and human FS-4 fibroblasts, and compared them with changes observed after the treatment of cells with other mitogens, such as platelet-derived growth factor (PDGF) and bombesin. TNF stimulated the rapid phosphorylation of two 41000-M(r) and two 43000-M(r) cytosol proteins on tyrosine, threonine and/or serine, as did PDGF, epidermal growth factor and fibroblast growth factor; the increased levels of this mitogen-induced protein-tyrosine phosphorylation correlated well with the extent of mitogen-induced DNA synthesis as determined by the percentage of labelled nuclei. In contrast, bombesin, which is an even better mitogen for Swiss 3T3 cells than TNF, stimulated the tyrosine phosphorylation of 41000-M(r) and 43000-M(r) proteins only to a limited extent. On the other hand, bombesin and PDGF stimulated the rapid serine phosphorylation of an 80000-M(r) acidic protein, a major substrate for protein kinase C; increased phosphorylation of the 80000-M(r) protein was not observed at all when cells were stimulated with TNF. These results suggest significant differences among the mitogenic signalling pathways of TNF, PDGF and bombesin as regards the involvement of protein kinases; the mitogenic signalling pathway of TNF involves the activation of tyrosine kinase, but not of protein kinase C, whereas bombesin seems to transduce its mitogenic signal mainly through the activation of protein kinase C, and the activation of both kinases seems to be involved in the mitogenic signalling pathway of PDGF.