Abstract P3-05-10: Glucocorticoid and aldosterone mimic progestin-induction of a therapy-resistant cytokeratin-5 positive cell population in estrogen receptor-positive breast cancer through a Bcl6-dependent mechanism

Resistance to anti-estrogen therapy remains a significant problem in patients diagnosed with estrogen receptor-α (ERα) positive breast cancer. Recent progress has defined a "lumino-basal" subclass of ERα-positive breast cancer characterized by mosaic presence of a minor population of ERα-negative cells expressing the basal cytokeratin-5 (CK5). The CK5-positive cells are therapy-resistant and have increased tumorigenic potential. Initial studies have suggested that progestins but not other steroids expand this CK5+ cell population. Unexpectedly, we discovered that at least two 3-ketosteroids other than progestins, glucocorticoids and mineralocorticoids, are capable of inducing the CK5+/ERα- cell population. CK5+ cells induced by glucocorticoid or aldosterone showed increased clonogenicity in soft agar, expressed the stem cell marker CD44, showed loss of ERα and PR expression, and demonstrated therapy-resistance with reduced apoptosis in response to chemotherapy, and were further enriched following adjuvant antiestrogen or chemotherapies. Induction of CK5+ cells by 3-ketosteroids was consistently preceded by induction of Bcl6, a transcriptional repressor implicated in breast cancer progression. Suppression of Bcl6 by shRNA or the Bcl6 suppressor, prolactin, abolished 3-ketosteroid-induction of CK5+ cells. Prolactin also blocked 3-ketosteroid-induced colony formation in vitro and suppressed progestin-induction of the CK5+ cell population in T47D xenograft tumors in vivo. Survival analyses with recursive partitioning revealed that CK5 and Bcl6 transcripts or protein levels in ERα+ breast cancer identify patients at high or low risk for tumor in two independent cohorts. The observations provide a mechanism by which stress-related or pharmacologic elevation of glucocorticoids may adversely affect patients with ERα+/CK5+ breast cancer, and may justify further exploring of inhibitors to 3-ketosteroid receptors or Bcl6 for therapeutic benefit. Citation Format: Hallgeir Rui, Takahiro Sato, Amy Peck, Melanie A Girondo, Chengbao Liu, Albert J Kovatich, Jeffery A Hooke, Craig D Shriver, Edith Mitchell, Terry Hyslop, Chelain Goodman. Glucocorticoid and aldosterone mimic progestin-induction of a therapy-resistant cytokeratin-5 positive cell population in estrogen receptor-positive breast cancer through a Bcl6-dependent mechanism [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-10.