Autocrine production of insulin-like growth factor II using an inducible expression system results in reduced estrogen sensitivity of MCF-7 human breast cancer cells

Abstract

An inducible expression vector, utilizing the metal response elements from the human metallothionein IIA promoter and encoding human prepro-insulin-like growth factor II (IGF-II), was transfected into MCF-7 McG cells, an MCF-7 subline which exhibits an estrogen dependent phenotype in vitro and does not express detectable levels of IGF-II mRNA. Two stably transfected clones, designated MI5 and MI7, which expressed IGF-II mRNA in a Zn2+ regulated manner, were isolated. Clone MI5 did not secrete detectable levels of IGF-II activity, as determined by radioimmunoassay of conditioned medium, but clone MI7 secreted high levels of IGF-II activity in a Zn2+ inducible fashion. Clone MI5 and control clones transfected with the selection plasmid pSV2 neo and the control plasmid pSP65 continued to display an estrogen dependent phenotype in vitro. However, under both anchorage dependent and anchorage independent growth conditions, clone MI7 cells exhibited an estrogen responsive, rather than dependent, phenotype. Moreover, when grown in the presence of inducing concentrations of Zn2+, MI7 cells were either virtually (for anchorage dependent growth) or completely (anchorage independent growth) unresponsive to estradiol. Both the basal growth rate in the absence of metal ions and the Zn2+ induced increases in cell proliferation could be inhibited by the monoclonal antibody alpha-IR3, which blocks the binding site of the IGF-I receptor. The antiestrogens tamoxifen and 4-hydroxytamoxifen were found to enhance the growth stimulation resulting from Zn2+ induced IGF-II production in MI7 cells.(ABSTRACT TRUNCATED AT 250 WORDS)