Synthesis and secretion of platelet-derived growth factor by human breast cancer cell lines

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 ¹²⁵I-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 17β-estradiol. Like authentic PDGF, the PDGF-like activity produced by breast cancer cells is stable after acid and heat treatment (95°) and inhibited by reducing agents. The mitogenic activity comigrates with a material of ~30kDa on NaDodSO₄/polyacrylamide gels. Immunoprecipitation with PDGF antiserum of proteins from metabolically labeled cell lysates and conditioned medium followed by analysis on nonreducing NaDodSO₄/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/SIS 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.