Unliganded estrogen receptor inhibits breast cancer cell growth through interaction with a cyclin

Estrogens are mitogenic in human breast cancer cells, but the presence of estrogen receptor (ER) is associated with a favorable prognosis in pri- mary tumors and the molecular basis for this paradox- ical relationship remains unknown. Here we show that ER and ER mutants devoid of ligand and DNA- binding domains inhibit cell growth in three-dimen- sional matrix as well as tumor formation in nude mice. Using in vitro and intracellular approaches, we have found that ER, via its amino acids 184 -283, interacts with cyclin-dependent kinase inhibitor p21WAF1. Both proteins exhibit mutual interactions in the absence of estrogens or in the presence of pure antiestrogen ICI182,780, whereas estradiol treatment disrupts their interactions. Cross-linking experiments reveal that these proteins are present in a larger complex of 200 kDa that also contains cdk2 and cyclin E. We further demonstrate that the unliganded full-length ER or the variant having the p21WAF1 interaction region signifi- cantly increases p21WAF1 expression, whereas ER si- lencing reduces p21WAF1 levels and silencing of p21WAF1 is sufficient to prevent ER-induced growth inhibition. Taken together, our results point to an antiproliferative function of the unliganded ER through its physical interactions with p21WAF1 that may also explain the favorable prognosis of ER-positive breast cancers.—Maynadier, M., Ramirez, J.-M., Cath- iard, A.-M., Platet, N., Gras, D., Gleizes, M., Sheikh, M. S., Nirde, P., Garcia, M. Unliganded estrogen receptor alpha inhibits breast cancer cell growth through interaction with a cyclin-dependent kinase in- hibitor (p21WAF1). FASEB J. 22, 671- 681 (2007)