Fibroblast Growth Factor 4 Transfection of MCF-7 Cells Produces Cell Lines That Are Tumorigenic and Metastatic in Ovariectomized or Tamoxifen-treated Athymic Nude Mice 1

Successful antiestrogen treatment in patients with tamoxifen-responsive breast tumors is often followed by an outgrowth of tumors cells that are antiestrogen resistant, implying that estrogen-dependent tumors can be come estrogen-independent. In an effort to mimic this progression, we have transfected fibroblast growth factor 4 into MCF-7 cells, a human breast carcinoma cell line that is estrogen-dependent for growth in nude mice. This transfection results in cell lines that form progressively grow ing, metastatic tumors when injected s.c. into untreated or tamoxifentreated ovariectomized nude mice. In contrast to the parental cell line, growth of transfected cells in ovariectomized nude mice is stimulated by tamoxifen treatment and inhibited by estrogen treatment of the mice. Parental MCF-7 cells were transfected with an expression vector for ß-galactosidase, conferring the ability to convert the chromogenic substrate, 5-bromo-4-chloro-3-indoyl-ß-galactoside, to a blue color and allowing the detection of their presence within tumors developing after coinoculation with fibroblast growth factor 4-transfected cells. The fibroblast growth factor 4-transfected cells could support growth and metastasis of the ß-galactosidase-expressing parental cell line when both lines were coinjected into the same site in untreated or tamoxifen-treated, ovariectomized mice. These data suggest a possible role for fibroblast growth factors in the progression of breast tumors to an estrogen-independent, antiestrogenresistant, metastatic phenotype. They also support a role for paracrine factors in mixed populations of tumor cells of differing states of malignant progression.