Previous studies have demonstrated that binding of peptide growth factors such as epidermal growth factor (EGF) decreases as cell density increases. We now report that binding of EGF to MDA 468 breast carcinoma cells is reduced as cells increase in density with time in culture. Cells at low density bound more EGF per cell than cells at higher density. The reduction of EGF binding was due to a reduction in receptor number. Metabolic labeling of MDA 468 cells with [³⁵S]-methionine followed by immunoprecipitation of the EGF receptor (EGFR) indicated that the amount of total receptor protein was decreased. Using RNA blot hybridization, we found that high-density cells contained decreased amounts of EGFR transcripts. Northern analysis revealed that both the 10- and 5.6-kilobase mRNA transcripts encoding the EGFR were decreased. These data suggest that increasing cell density with time in culture results in modulation of EGFR content, with changes at the level of both protein and mRNA expression.

We have previously observed that a human adrenal carcinoma cell line (SW-13) produces autocrine growth factors that enhance the ability of these cells to clone in soft agar. We now show that a basic FGF-like protein is found in SW-13 lysates and stimulates SW-13 colony growth. Basic FGF (bFGF) was identified in SW-13 lysates by both its chromatographic behavior on heparin-Sepharose and its reactivity with an antibody specific for bFGF. Incubation of lysates with antibodies specific for bFGF abolished their ability to support anchorage-independent growth. Northern analysis demonstrated that the cells produced multiple bFGF mRNA transcripts. SW-13 cells also expressed bFGF cell-surface receptors. These data suggest that SW-13 cells produce and respond to a factor structurally and functionally related to bFGF. Such findings support recent evidence indicating that FGF secreted by human tumor cell lines plays a role in the maintenance of the transformed phenotype.