Epidermal growth factor receptors in clonal lines of a rat osteogenic sarcoma and in osteoblast-rich rat bone cells

Studies were carried out to identify and characterize the receptors for epidermal growth factor (EGF) in osteoblast-rich newborn rat calvarial cells and in 4 clonal lines derived from a transplantable rat osteogenic sarcoma with a well-characterized osteoblast-like phenotype. The cells were grown in monolayer culture in replicate wells; 40,000-50,000 cpm¹²⁵I-labeled mouse EGF with a specific activity of 100-120 μCi/μg was added to each well. Binding studies were carried out at 37°C. Binding of¹²⁵I-labeled EGF was specific, saturable, reversible, and pH dependent. Maximum binding occurred 2 h after addition of the tracer. Thereafter, cell-bound radioactivity decreased to reach a plateau of 15-20% of maximum binding at 24 h. This observation is consistent with internalization and processing of the receptor-hormone complex as has been shown with other EGF target cells. Scatchard analyses revealed a single class of high-affinity binding sites in the normal and malignant osteoblast-like cells. Dissociation constants (K_D) in the clonal lines ranged from 2.3 × 10^-10M to 4.7 × 10^-10M with receptor number per cell ranging from 25,000 to 33,000. The calvarial cells had a K_D of 2.0 × 10^-10M with 14,000 receptors per cell. In both the normal and malignant cell strains, EGF was found to increase incorporation of³H-labeled thymidine into acid-precipitable macromolecules. EGF has been shown to stimulate bone resorption; however, studies in organ cultures have not identified the target cell for EGF. The present results point to an interaction of EGF with osteoblasts.