ANTIESTROGEN STIMULATION OF ERBB2 ECTODOMAIN SHEDDING FROM BT-474 HUMAN BREAST-CANCER CELLS WITH ERBB2 GENE AMPLIFICATION

Oestrogen has previously been shown to downregulate the expression of ERBB2 oncogene in human breast cancer cells, which contain a normal no n-amplified ERBB2 gene. However, amplified ERBB2 to escape from hormon al regulation. We studied shedding of the extracellular domain (ectodo main, ECD) of the ERBB2 encoded protein in BT-474 human breast cancer cells treated with oestrogen or anti-oestrogen. Oestrogen -responsiven ess of these cells has been previously demonstrated by stimulation of cell growth and expression of pS2, a marker gene known to be regulated by oestrogen receptor at transcriptional level. The concentration of the soluble ECD in the culture medium was increased by the anti-oestro gen toremifene as a function of time. In contrast, the level of ERBB2 mRNA and protein in cell lysates was not stimulated, but was transient ly suppressed by toremifene. In the presence of oestrogen, the level o f ECD remained low. The increased shedding of ECD in the presence of t oremifene, without parallel change in ERBB2 transcripts (4.8 and 2.3 k b) and in cellular ERBB2 protein level, suggests that toremifene speci fically contributes to the shedding of the ERBB2 ectodomain. These res ults show that shedding of ECD is an additional level of regulation of ERBB2 by the anti-oestrogen toremifene. This may contribute to resist ance to growth inhibition by anti-oestrogens of breast cancers which o verexpress ERBB2.