Heterologous regulation of EGF receptors in fibroblastic cells

AbstractPlatelet‐derived growth factor (PDGF) increases the mitogenic activity of epidermal growth factor (EGF) in several cells lines, including BALB/C‐3T3. PDGF‐treated BALB/C‐3T3 cells manifest a reduced capacity to bind 125I‐labeled EGF due to a loss of high affinity EGF receptors. Cholera toxin potentiates the ability of PDGF to both decrease EGF binding and initiate mitogenesis. Whether PDGF increases EGF sensitivity via its effects on EGF receptors is not known and requires a more complete understanding of the mechanism by which PDGF decreases EGF binding.12‐0‐tetradecanoylphorbol 13‐acetate (TPA) also reduces EGF binding in BALB/C‐3T3 and other cells, presumably by activating protein kinase C and, consequently, inducing the phosphorylation of EGF receptors at threonine–654. PDGF indirectly activates protein kinase C, and EGF receptors in PDGF‐treated WI‐38 cells are phosphorylated at threonine‐654. Thus, the effects of PDGF on EGF binding may also be mediated by protein kinase C. We investigated this hypothesis by comparing the actions of PDGF and TPA on EGF binding in density‐arrested BALB/C‐3T3 cells.Both PDGF and TPA caused a rapid, transient, cycloheximide‐independent l