We report that human breast cancer cells secrete a growth factor that is biologically and immunologically similar to platelet-derived growth factor (PDGF). Serum-free medium conditioned by estrogen-independent MDA-MB-231 or estrogen-dependent MCF-7 cells contains a mitogenic or "competence" activity that is capable of inducing incorporation of [3H]thymidine into quiescent Swiss 3T3 cells in the presence of platelet-poor plasma. In addition, the conditioned medium contains an activity that competes with 1251-labeled PDGF for binding to PDGF receptors on normal human fibroblasts. The secretion of PDGF-like activity by the hormone-responsive cell line MCF-7 is stimulated by 17.3-estradiol. Like authentic PDGF, the PDGF-like activity produced by breast cancer cells is stable after acid and heat treatment (950C) and inhibited by reducing agents. The mitogenic activity comigrates with a material of "30 kDa on NaDodSO4/polyacrylamide gels. Immunoprecipitation with PDGF antiserum of proteins from metabolically labeled cell lysates and conditioned medium followed by analysis on nonreducing NaDod-S04/polyacrylamide gels identified proteins of 30 and 34 kDa.Upon reduction, the 30-and 34-kDa bands were converted to 15-and 16-kDa bands suggesting that the immunoprecipitated proteins were made up of two disulfide-linked polypeptides similar to PDGF. Hybridization studies with cDNA probes for the A chain of PDGF and the B chain of PDGF/S1S identified transcripts for both PDGF chains in the MCF-7 and MDA-MB-231 cells. The data summarized above provide conclusive evidence for the synthesis and hormonally regulated secretion of a PDGF-like mitogen by breast carcinoma cells. Production of a PDGF-like growth factor by breast cancer cell lines may be important in mediating paracrine stimulation of tumor growth.

No abstract

We have selected and cloned a stable variant of the MCF-7 human breast cancer cell line (LY 2) that is resistant to LY 117018 (LY), a potent antiestrogen that inhibits cell growth at concentrations as low as 10(-10) M. The cell line was selected by increasing the concentration of LY in the growth medium in a stepwise manner from 10(-8) to 10(-6) M as the cells become resistant. LY2 has been cloned in soft agar and carried for over 50 passages with no change in resistance. Other antiestrogens, such as tamoxifen and 40-hydroxytamoxifen no longer inhibit cell proliferation of LY 2. The cell line is still responsive to estrogen in a cell proliferation assay, but contains somewhat less estrogen receptors than MCF-7. The cytosolic estrogen receptor sediments to a 4S position on high salt sucrose density gradient centrifugation and is completely shifted to a denser gradient region when the receptor is incubated with a monoclonal antiestrophilin. The nuclear estrogen receptor when covalently labeled with [3H]tamoxifen aziridine has the same mol wt (62,000) in both MCF-7 and LY2 cells, when determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In a competitive binding assay, LY 117018 competes for [3H]estradiol binding to its cytosol receptor with the same Ki in both MCF-7 and LY2 cells. When the induction of estrogen-specific proteins was examined, no detectable progesterone receptor could be detected in either estrogen-induced or control LY2 cells, in contrast to MCF-7 cells. However, both 52,000- and 160,000-dalton proteins were estrogen inducible in the medium of LY2 and MCF-7 cells, as measured by labeling with [35S]methionine. The phenotypic stability of the antiestrogen resistance in LY2 cells coupled with the cross-resistance the antiestrogens of widely different structures make this cell line an ideal model system for the study of hormone resistance in human breast cancer. In addition, while the mechanism of resistance is currently not elucidated, the selective loss of estrogen-inducible functions in this cell line may provide powerful clues for future study.

The MCF-7 human breast cancer cell line responds to estrogen stimulation in vitro by increased secretion of growth factors and proliferation and in vivo by tumor formation in the nude mouse. To test a possible role of growth factor secretion in expression of the tumorigenic phenotype, we stably transfected MCF-7 cells with the v-Ha-ras oncogene to produce the MCF-7ras cell line. The MCF-7ras cell line was tumorigenic in the absence of estrogens and secreted 3-to 5-fold elevated levels of a high molecular weight form of a type a transforming growth factor-like growth factor, type ,B transforming growth factor, and insulin-like growth factor I. MCF-7ras cells, in contrast to MCF-7, were less sensitive to further growth stimulation by estrogen, type a transforming growth factor, and insulin-like growth factor I and showed little change in receptor levels for these hormones. Conditioned medium from MCF-7ras cells as well as two of its component growth factors (insulin-like growth factor I and type a transforming growth factor) replaced estrogen in stimulating MCF-7 colony formation in vitro. A coordinate increase in growth factor secretion by human breast cancer may contribute to its escape from estrogen dependence.

To better understand the possible roles and interactions of transforming growth factor-alpha (TGF alpha) and its receptor, the epidermal growth factor (EGF) receptor in human breast epithelium, we have studied the expression of TGF alpha and the EGF receptor in a series of normal human mammary epithelial cells derived from reduction mammoplasty before in vitro propagation, during short term proliferation in vitro, and after immortalization. Increased TGF alpha mRNA expression coincided with conversion of the cells to a proliferative state in vitro. After establishment, propagation, and proliferation in vitro, the cells expressed high levels of both TGF alpha and EGF receptor mRNAs. Addition of diverse growth inhibitory agents, including 12-O-tetradecanoylphorbol-13-acetate (TPA), TGF beta, and sodium butyrate, to one of these rapidly proliferating cell populations (no. 184) failed to reduce the expression of either TGF alpha or the EGF receptor. Likewise, cessation of growth associated with both senescence and confluence of the 184 cells did not result in reduced expression. However, regulation of TGF alpha mRNA could be demonstrated by withdrawal of EGF from the medium or by antibody-mediated blockade of the EGF receptor in 184 cells. Antibody-mediated EGF receptor blockade also results in inhibition of growth and [3H]thymidine labeling. An autoregulatory autocrine loop appears operant in proliferating breast epithelial cells. Both growth and levels of TGF alpha mRNA expression are controlled by binding of ligand to the EGF receptor. These studies suggest a role for the TGF alpha/EGF receptor pathway in normal breast cell physiology.