Stimulation of proliferation of a human osteosarcoma cell line by exogenous acidic fibroblast growth factor requires both activation of receptor tyrosine kinase and growth factor internalization.

U2OS Dr1 cells, originating from a human osteosarcoma, are resistant to the intracellular action of diphtheria toxin but contain toxin receptors on their surfaces. These cells do not have detectable amounts of fibroblast growth factor receptors. When these cells were transfected with fibroblast growth factor receptor 4, the addition of acidic fibroblast growth factor to the medium induced tyrosine phosphorylation, DNA synthesis, and cell proliferation. A considerable fraction of the cell-associated growth factor was found in the nuclear fraction. When the growth factor was fused to the diphtheria toxin A fragment, it was still bound to the growth factor receptor and induced tyrosine phosphorylation but did not induce DNA synthesis or cell proliferation, nor was any fusion protein recovered in the nuclear fraction. On the other hand, when the fusion protein was associated with the diphtheria toxin B fragment to allow translocation to the cytosol by the toxin pathway, the fusion protein was targeted to the nucleus and stimulated both DNA synthesis and cell proliferation. In untransfected cells containing toxin receptors but not fibroblast growth factor receptors, the fusion protein was translocated to the cytosol and targeted to the nucleus, but in this case, it stimulated only DNA synthesis. These data indicate that the following two signals are required to stimulate cell proliferation in transfected U2OS Dr1 cells: the tyrosine kinase signal from the activated fibroblast growth factor receptor and translocation of the growth factor into the cell.