All MCF7Jun clones demonstrated high levels of AP-1 DNA binding activity compared to minimal activity of control clones. Transactivation of AP-1 regulated genes led to increased expression of vimentin mRNA. Overexpression of cJun also resulted in a decrease in the levels of junB, junD and c-fos mRNA and an increase in fra-1expression.
MCF7Jun cells were larger than control cells, grew less compactly in monolayer and had an increased ability to invade both collagen and Matrigel in Boyden chamber assays using fibroblast conditioned medium as a chemoattractant. Type IV collagenase was detected in cJun transfected cells.
Of 18 nude mice injected with MCF7Jun cells, 15/18 and 13/18 (with/without ovariectomy) rapidly developed tumours in the absence of oestrogen in comparison to none of the control group. In culture, cJun-transfected cells grew slower than control cells in complete media but, unlike control cells, their growth was not affected by the addition of oestrogen and Tamoxifen. All three MCF7Jun clones did not express ER protein or mRNA and demonstrated no oestrogen-inducible ER transcriptional activity. Comparison of MCF-7 cells transfected with a variety of deletion mutants of cJun showed that only the clone containing full length cJun was unresponsive to oestrogen.