Trapping of genes induced upon growth arrest after treatment with antiestrogen or retinoids using retroviral promoter trap

Although chemopreventive anti‐steroids such as the antiestrogens are thought to act through competitive inhibition of agonist binding to estrogen receptors, it has been postulated that the estrogen receptor changes its conformation when bound to a strong antiestrogen such as ICI‐164,384. We hypothesized that such conformationally changed receptors could bind specific recognition sequences in the genome and activate specific genes that might be involved in growth arrest. In order to identify such genes with a functional assay, we used a retroviral gene trap U3lacZ. We have now isolated MCF‐7 breast cancer cell line clones in which the lacZ reporter gene is inserted into the genes activated by either ICI‐164,384 or retinoic acid. One such clone, B4, was further characterized. In B4, lacZ activity is induced by ICI‐164,384 or trans‐retinoic acid, and repressed after treatment with estradiol. Cloning of the 5′‐flanking genomic sequence in this clone will be possible using polymerase chain reaction

Clinical significance of erbB2 protein overexpression

Originally described as a homologue of the EGF receptor gene amplified in breast cancer [1], erbB-2 protein is now thought to be a cell membrane receptor for a newly described ligand-gp30 [2]. There is some confusion in the literature about the terminology for erbB-2. In brief, erbB-2, c-erbB-2, and HER-2 refer to the same human gene residing in the long arm of the chromosome 17, which has homology to human EGF receptor gene (c-erbB) [3]. erbB-2 protein, p185erbB-2, or p185HER-2 refer to the transmembrane receptor protein translated from the erbB-2 gene, which has a molecular weight of 185 kDa. c-neu refers to the rat counterpart of the erbB-2 gene [4]

Progression of human breast cancer cells from hormone-dependent to hormone-independent growth both in vitro and in vivo

We have isolated a series of sublines of the hormone-dependent MCF-7 human breast cancer cell line after selection both in vivo and in vitro for growth in the presence of subphysiological concentrations of estrogens. These sublines represent a model system for study of the processes leading to hormonal autonomy. The cells form growing tumors in ovariectomized athymic nude mice in the absence of estrogen supplementation but retain some responsivity to estrogen as determined by stimulation of the rate of tumor growth in vivo and by induction of progesterone receptor. An ovarian-independent but hormone-responsive phenotype may occur early in the natural progression to hormone-independent and unresponsive growth in breast cancer. We observed no change in the affinity or decrease in the level of expression of estrogen receptors and progesterone receptors among the sublines and the parental cells. Epidermal growth factor receptors are not overexpressed in ovarian-independent cells. Thus, altered hormone receptor expression may be a late event in the acquisition of a hormone-independent and unresponsive phenotype. Sublines isolated by in vivo but not in vitro selection are more invasive than the parental cells both in vivo and across an artificial basement membrane in vitro. Thus, as yet unknown tumor-host interactions may be important in the development of an invasive phenotype. Furthermore, acquisition of the ovarian-independent and invasive phenotypes can occur independently