OBJECTIVETo examine the effect of trastuzumab on cell proliferation, colony formation and changes of HER-2 proteins in human breast cancer cell line SKBR3 and human ovarian cancer cell line SKOV3 cells which overexpress p185 HER-2 but shed high or low HER-2 extracellular domain (ECD) levels.

METHODSSKBR3 cells and SKOV3 cells were treated with or without trastuzumab. Cell number and the rate of colony formation were calculated. Western blot analysis was used to detect p185 HER-2, HER-2 ECD and phospho-HER-2. Two-site ELISA assay was used for the detection of HER-2 ECD.

RESULTSTrastuzumab inhibited cell proliferation, colony formation, and decreased or eliminated the levels of two uncharacterized phospho-proteins (molar weight about 90 000 and 40 000) in SKBR3 cells shedding high level of HER-2 ECD expression. These responses were not observed in SKOV3 cells shedding low level of HER-2 ECD expression. But total p185, phospho-p185 and phospho-p95 proteins did not appear to change in SKBR3 and SKOV3 cells after treatment with trastuzumab. Trastuzumab reacts not only with proteolytic cleavage HER-2 ECD containing HER-2 ECD I , II , III and IV subdomains of p185 HER-2 extracellular domain, but also with the secreted autoinhibitor p68/ECD III a specifying 340 residues, identical to subdomains I and II from the extracellular domain of p185 HER-2, followed by a unique C-terminal sequence of 79 aa encoded by intron 8, which suggested that there may be a trastuzumab binding site on p68/ECD III a protein. Comparing with HER-2 ECD levels of the same number of SKBR3 cells, there was no significant decrease of HER-2 ECD shedding level after treatment with or without trastuzumab for 4 days in serum-free medium.

CONCLUSIONAntitumor effects of trastuzumab may be related to the two uncharacterized phospho-p90 and/or phospho-p40 proteins. There is probably a trastuzumab epitope on p68/ECD III a. The decrease of HER-2 ECD levels may be positively correlated with the number of SKBR3 cells.