Activation and inhibition of the AP-1 complex in human breast cancer cells

Abstract

We have studied the expression and activity of the jun and fos families of transcription factors in a panel of human breast cancer cells. Numerous breast‐cancer cell lines showed variable levels of expression of jun and fos family‐member RNA, activator protein‐1 (AP‐1) DNA binding, and transcriptional‐activating activities during exponential growth. In all of the breast‐cancer lines tested, c‐jun RNA and AP‐1 DNA‐binding activity correlated. In addition, in most breast cancer cell lines AP‐1 DNA‐binding activity also correlates with AP‐1‐transactivating activity. However, some breast cancer cell lines have high c‐jun RNA expression, high AP‐1 DNA‐binding activity, and low AP‐1‐transactivating activity. Such results suggest that in these breast cancer cell lines there exist AP‐1 complexes that can bind DNA but cannot activate transcription. Multiple peptide growth factors as well as the tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate induced the expression of jun and fos family‐member RNAs and also increased AP‐1 DNA‐binding activity and functional AP‐1‐transcriptional activating activity in MCF7 breast cancer cells. However, treatment with estrogen, a steroid growth factor, failed to increase jun and fos RNA expression and induced minimal increases in AP‐1 DNA binding and AP‐1‐induced transcriptional‐activating activity in comparison with that seen after peptide hormone treatment. Thus, mitogenic peptide hormones and the tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate, but not estrogen, strongly activate the AP‐1 transcription factor in breast cancer cells. A dominant‐negative mutant of c‐jun that specifically inhibits AP‐1‐transactivating activity in rat fibroblasts inhibited AP‐1‐transactivating activity in breast‐cancer cells and blocked the increase in AP‐1‐mediated transcription induced by serum or specific growth factors. This dominant‐negative mutant also inhibited MCF7 colony formation, indicating that expression of this AP‐1 inhibitor suppressed the proliferation of these breast cancer cells. Such results suggest that growth factor‐induced proliferation of breast cancer cells can possibly be blocked by inhibiting AP‐1‐transactivating activity.