title: The in vitro influences of epidermal growth factor and heregulin-β1 on the efficacy of trastuzumab used in Her-2 positive breast adenocarcinoma.
abstract: BACKGROUND
  Human epidermal growth factor receptor-2 (Her-2) is over expressed in approximately 25-30% of all primary breast tumors resulting in a distinctive breast cancer subtype associated with a poor prognosis and a decrease in overall survival. Trastuzumab (Herceptin®), an anti-Her-2 monoclonal antibody, has dramatically altered the prognosis of Her-2 positive breast cancer. Trastuzumab is, however, associated with primary and acquired resistance.
  
  
  OBJECTIVE
  To investigate the in-vitro effects of trastuzumab on cell viability (tetrazolium conversion assay), cell cycling (propidium iodide staining), apoptosis (executioner caspases and annexin-V) and relative surface Her-2 receptor expression (anti-Her-2 affibody molecule) in Her-2-positive (SK-Br-3) and oestrogen receptor positive (MCF-7) breast adenocarcinoma cells and to determine potential augmentation of these effects by two endogenous ligands, epidermal growth factor (EGF) and heregulin-β1 (HRG- β1).
  
  
  RESULTS
  Cell viability was decreased in SK-Br-3 cells by exposure to trastuzumab. This was associated with G1 accumulation and decreased relative surface Her-2 receptor density, supporting the cytostatic nature of trastuzumab in vitro. SK-Br-3 cells exposed to EGF and heregulin-β1 produced differential cell responses alone and in combination with trastuzumab, in some instances augmenting cell viability and cell cycling. Relative surface Her-2 receptor density was reduced substantially by trastuzumab, EGF and heregulin-β1. These reductions were amplified when ligands were used in combination with trastuzumab.
  
  
  CONCLUSIONS
  Cell type specific interactions of endogenous ligands appear to be dependent on absolute Her-receptor expression and cross activation of signaling pathways. This supports the notion that receptor density of Her-family members and multiplicity of growth ligands are of mutual importance in breast cancer cell proliferation and therefore also in resistance associated with trastuzumab.
authors.affiliation: Department of Pharmacology, Faculty of Health Sciences, School of Medicine, University of Pretoria, Private Bag X323, Pretoria 0007, South Africa. tracey.hurrell@up.ac.za.
authors.forename: Tracey
authors.initials: T
authors.lastname: Hurrell
authors.forename: Kim
authors.initials: K
authors.lastname: Outhoff