Background: In previous studies we have shown that epidermal growth factor (EGF) at concentrations between 50 and 100 ng/mL induced apoptosis in wild-type SK-N-SH neuroblastoma cells. We hypothesize that this apoptotic event separates EGF-induced neuroblastoma cell growth into a biphasic concentration-dependent process, due to activation of different signaling cascades.
Methods: Cells were incubated in concentrations of EGF ranging from 5 to 250 ng/mL for 3 days, and cell proliferation was determined by the MTT assay. Cells incubated with EGF 5, 100, or 250 ng/mL for 17 h were also assayed for apoptosis by DNA laddering. Western immunoblots were performed on whole cell lysates prepared from cells incubated with EGF (5-250 ng/mL) for 17 h. Antibodies against cleaved caspase3, p-AKT, p-GSK-3beta, p-BAD, p-RAF, p-ERK, and p-P38 were used as probes.
Results: A triphasic, concentration-dependent response was observed following incubation of cells with EGF. Cell proliferation was increased by EGF 5 ng/mL (P < 0.05), decreased by EGF 100 ng/mL, and increased when incubated with EGF 250 ng/mL (P < 0.05). DNA laddering only occurred after treatment with EGF 100 ng/mL. The expressions of p-ERK, p-RAF, p-BAD, and p-GSK-3beta were increased at EGF concentrations of 5-10 ng/mL. At 50-100 ng/mL EGF, the expression of cleaved caspase3 was increased. Maximal p-P38 expression was at 50 ng/mL EGF. At EGF concentrations of 150-250 ng/mL, the expressions of p-AKT and p-GSK-3beta were elevated.
Conclusions: Neuroblastoma cell growth induced by EGF exhibited a triphasic pattern; cell growth was increased at EGF concentrations 5-20 and 150-250 ng/mL, but decreased at 50-100 mg/mL. Apoptosis was induced at 50-100 ng/mL EGF. Each growth phase activated different signaling molecules.