ERbB-2 expression is rate-limiting for epidermal growth factor-mediated stimulation of ovarian cancer cell proliferation.

Over-expression of the ErbB-2 proto-oncogene frequently coincides with an aggressive clinical course of certain human adenocarcinomas. The ErbB-2 receptor is a member of the ErbB family of growth factor receptors, and within this complex signaling network, ErbB-2-containing heterodimers are preferentially formed. To assess whether ErbB-2 is a critical component in epidermal growth factor (EGF)-mediated stimulation of tumor cell proliferation, we used as a model SK-OV-3 ovarian cancer cells, which over-express EGF receptor (EGFR) and ErbB-2 receptors. In these cells, we reduced ErbB-2 mRNA and protein expression by transfection with ErbB-2-targeted hammerhead ribozymes and generated cell lines expressing different levels of ErbB-2. In SK-OV-3 cells, ErbB-2 expression conferred a growth advantage and soft agar experiments revealed that ErbB-2 was rate-limiting for anchorage-independent growth. The induction of colony formation by EGF was completely abrogated in ErbB-2-depleted cells, despite unchanged expression levels and tyrosine phosphorylation of the EGFR. The duration of EGF-mediated c-Fos mRNA up-regulation was decreased in parallel with loss of ErbB-2 expression. Furthermore, the rate of spontaneous apoptosis was increased in ErbB-2-depleted cells. Our results demonstrate that in human ovarian cancer cells the EGFR-ErbB-2 heterodimer, and not the EGFR homodimer, can be rate-limiting for EGF-mediated proliferation, thus suggesting that the oncogenic activity of ErbB-2 in human tumors is due in part to its ability to increase the growth response to stroma-derived EGF-like growth factors.