The outcome of heregulin-induced activation of ovarian cancer cells depends on the relative levels of HER-2 and HER-3 expression

Members of the epidermal growth factor receptor family of tyrosine kinases, including epidermal growth factor receptor, c-erbE-2 (HER-2), c-erbB-3 (HE R-3), and c-erbB-4 (HER-4), can be coexpressed at different levels in nonhe matopoietic tissues. Amplification and overexpression of HER-2 is found in approximately one-third of cancers that arise in the breast and ovary. In o ur previous studies, heregulin (HRG) and anti-HER-2 antibodies inhibited pr oliferation, increased invasiveness, and enhanced tyrosine autophosphorylat ion of SKBr3 breast cancer cells that overexpressed HER-2, In the present r eport, the effects of HRG and anti-HER-2 antibody have been compared in six ovarian cancer cell lines. HRG inhibited anchorage-independent growth of S KOv3 cells that overexpressed HER-2 (10(5) receptors/cell) but stimulated t he growth of OVCA420, OVCA429, OVCA432, OVCA433, and OVCAR-3 cells that exp ressed lower levels of the receptor (10(4) receptors/cell), Thus, cell line s with a high level of HER-2 relative to HER-3 or HER-4 were growth inhibit ed, whereas cell lines with lower levels of HER-2 were growth stimulated by HRG, Stimulation or inhibition of clonogenic growth did not correlate with endogenous expression of HRG or with the impact of exogenous HRG on phosph orylation of HER-2, HER-3, or HER-4, Anti-HER-2 antibodies inhibited the gr owth of SKOv3 cells but failed to affect the growth of the other cell lines . In OVCAR-3 cells that had been transfected with HER-2 cDNA to increase ex pression to 10(5) receptors/cell, HRG inhibited rather than stimulated grow th. Conversely, when HER-2 expression by SKOv3 cells was downregulated by t ransfection of the viral E1A gene, HRG stimulated rather than inhibited gro wth. To evaluate the relative importance of HER-3 and HER-4, NIH 3T3 cells were cotransfected with HER-2 and HER-3 or with HER-2 and HER-4. HRG inhibi ted the growth of cells with a high ratio of HER-2:HER-3, whereas HRG stimu lated the growth of cells with low levels of the two receptors, In cells th at express only HER-2 and HER-4, HRG stimulated the growth of cells that ex pressed HER-4 independent of HER-2 levels. Anti-HER-2 antibodies inhibited the growth of transfectants with high levels of HER-2 expression independen t of HER-3 or HER-4 expression. In ovarian cancer cells that express all th ree receptors, the relative levels of HER-2 and HER-3 appear to determine t he response to HRG, Taken together, these studies support the concept that the level of HER-2 expression can modulate response to HRG, determining whe ther the response is stimulatory or inhibitory. In contrast, agonistic anti bodies that bind to HER-2 alone inhibit anchorage-independent growth but fa il to mimic HRG's ability to stimulate growth of cells with low HER-2: HER- 3 ratios.