Steroid induction of therapy-resistant cytokeratin-5-positive cells in estrogen receptor-positive breast cancer through a BCL6-dependent mechanism

Therapy resistance remains a major problem in estrogen receptor-α (ERα)-positive breast cancer. A subgroup of ERα-positive breast cancer is characterized by mosaic presence of a minor population of ERα-negative cancer cells expressing the basal cytokeratin-5 (CK5). These CK5-positive cells are therapy resistant and have increased tumor-initiating potential. Although a series of reports document induction of the CK5-positive cells by progestins, it is unknown if other 3-ketosteroids share this ability. We now report that glucocorticoids and mineralocorticoids effectively expand the CK5-positive cell population. CK5-positive cells induced by 3-ketosteroids lacked ERα and progesterone receptors, expressed stem cell marker, CD44, and displayed increased clonogenicity in soft agar and broad drug-resistance in vitro and in vivo. Upregulation of CK5-positive cells by 3-ketosteroids required induction of the transcriptional repressor BCL6 based on suppression of BCL6 by two independent BCL6 small hairpin RNAs or by prolactin. Prolactin also suppressed 3-ketosteroid induction of CK5+ cells in T47D xenografts in vivo. Survival analysis with recursive partitioning in node-negative ERα-positive breast cancer using quantitative CK5 and BCL6 mRNA or protein expression data identified patients at high or low risk for tumor recurrence in two independent patient cohorts. The data provide a mechanism by which common pathophysiological or pharmacologic elevations in glucocorticoids or other 3-ketosteroids may adversely affect patients with mixed ERα+/CK5+ breast cancer. The observations further suggest a cooperative diagnostic utility of CK5 and BCL6 expression levels and justify exploring efficacy of inhibitors of BCL6 and 3-ketosteroid receptors for a subset of ERα-positive breast cancers.

Abstract P5-04-05: Preclinical modeling of luminal breast cancer: Recapitulating progression to lethal and tamoxifen-resistant lung metastases in novel patient-derived xenotransplant models in prolactin-humanized mice

Seventy to eighty percent of newly diagnosed breast cancer cases are estrogen receptor(ER)-positive and are classified as luminal. Despite hormone therapy, 25-30% of luminal breast cancers will recur within 15 years of surgical removal of the primary tumor, and many of these patients will die from currently incurable distant metastases. Lung and bone are the most common organ sites for distant breast cancer metastases, with either site affected in approximately 70% of patients based on autopsy studies. A major hurdle for therapeutic progress with luminal breast cancer is the historical difficulty to establish xenograft models of human luminal breast cancer in mice. In particular, there is a need for experimental ER-positive breast cancer models that recapitulate distant metastasis formation from orthotopic tumor implants in mammary glands. Such experimental models will allow surgical resection of the primary tumor followed by clinically relevant testing of targeted adjuvant agents against distant residual disease.

We have engineered prolactin-humanized mice that have been backcrossed for ten generations into the Nod-SCOD-IL2Rgamma (NSG) immunodeficient background. These prolactin-humanized mice display improved take rates of patient-derived luminal breast cancer. Using prolactin-humanized mice we have established patient-derived serially transplantable luminal breast cancer models that metastasize to distant sites when grown as primary tumors in the mammary gland. One of the ER-positive lines, PDX2, effectively metastasizes to lungs in 100% of animals within 55 days of grafting into mammary glands. PDX2 lung metastases retain ER and progesterone receptor (PR) expression as well as expression of the luminal marker, GATA3, and display high degree of Ki67 positivity indicating rapidly proliferative lesions. Importantly, metastatic PDX2 lesions show only limited dormancy. In fact, when primary PDX2 tumors are surgically removed at Day 55, mice will die from lung metastases around Day 150. Estrogen supplementation is required for establishment of PDX2 tumors in mice. After primary PDX2 tumors are established in mammary glands in the presence of estradiol, primary tumors respond to tamoxifen with growth suppression but do not undergo tumor regression, and develop resistance to tamoxifen. Treatment of mice with adjuvant tamoxifen following surgical removal of primary PDX2 tumors led to extensive regression of existing lung metastases to barely detectable levels within 30 days of surgery. However, tamoxifen-refractory PDX2 lung metastases regrew during the next 30 day-period in the continued exposure to tamoxifen. Molecular phenotyping of the PDX2 model and other new luminal breast cancer models are ongoing, with the goal of characterizing tamoxifen-responsive and tamoxifen-refractory primary and metastatic lesions. Long-term, our intent is to use the PDX2 and other preclinical xenograft models in prolactin-humanized mice to systematically explore agents for synergy with anti-estrogens to establish curative combination treatments for metastatic luminal breast cancer.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-04-05.

Abstract P1-06-10: Characterization of novel activated human mammary fibroblast lines and their protumorigenic effect on human breast cancer xenotransplants in mice

Human breast cancer is typically characterized by an extensive stromal compartment enriched in fibroblasts. In contrast, xenotransplants of human breast cancer cell lines are typically epithelial-rich and characterized by sparse in-growth of murine fibroblast stroma. Stromal fibroblasts are important for structural integrity of normal tissue and constitute a major element of the stromal microenvironment of invasive cancer. Cancer associated fibroblasts (CAFs) frequently undergo activation as they co-evolve with cancer cells, serving to promote tumor growth and angiogenesis through secretion of multiple paracrine factors. Presence of activated CAFs in solid malignancies is generally associated with higher grade tumors and poor prognosis. Activated CAFs are resistant to apoptosis and display a myofibroblastic phenotype, including expression of a-smooth muscle actin (α-SMA). To more accurately model human tumor-stroma interactions in human breast cancer xenograft lines in mice, we aimed to develop immortalized activated human mammary fibroblasts for admixture xenografting with human breast cancer cells. Human mammary fibroblasts (HMFs) were isolated from multiple surgically excised tissues of reduction mammoplasties or mastectomies. Five isolated primary HMF lines were screened for α-SMA expression. HMF1 expressed the highest levels of α-SMA and was immortalized by stable lentiviral-delivered hTERT. Quantitative real-time PCR, anchorage-independent growth assay, and in vivo studies were used to further characterize selected HMFs. hTERT-HMF1 and HMF2, despite lower level of α-SMA expression in HMF2, displayed significantly elevated levels of mRNA of proteins commonly associated with activated fibroblasts, including SDF-α, SDF-β, HGF, IL-6, VEGF and podoplanin, as well as promoted a 10-fold increase in anchorage-independent growth of MCF7 breast cancer cells in vitro. To evaluate the ability of these HMF lines to promote tumor growth in vivo, MCF7 human breast cancer cells were orthotopically injected into murine mammary fat pads in the presence or absence of hTERT-HMF1 or HMF2. Both HMF lines facilitated in vivo MCF7-xenograft growth and induced histological changes, including higher grade and greater stromal development, when compared to pure MCF7 cell xenografts. IHC for SMA and collagen confirmed the presence of activated fibroblasts in all tumors; however HMF/MCF7 xenografts showed a distinct distribution of fibroblast stroma throughout the tumor in contrast to sparser stroma between larger epithelial aggregates in tumors grafted from MCF7 cells alone. hTERT-HFM1 cells promoted tumor growth in vivo more robustly than HFM2 cells. By 60 days post xenografting, hTERT-HMF1/MCF7 tumors were significantly larger (∼4-fold) than control MCF7 tumors. In addition, admixture with hTERT-HMF1 cells promoted increased tumor angiogenesis and cancer cell proliferation as measured by Ki67 expression. hTERT-HMF1 represents a novel mammary myofibroblast line that may be useful for improved preclinical xenotransplant modeling and tumor drug response testing of human breast cancer in vivo.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-06-10.

Abstract S1-8: Prolactin-humanized mice: an improved animal recipient for therapy response-testing of patient-derived breast cancer xenotransplants

Eighty percent of newly diagnosed breast cancer represents estrogen receptor(ER)-positive luminal subtypes. Many patients with luminal breast cancer develop antiestrogen resistant disease. It has historically been particularly difficult to establish ER-positive breast cancer lines from primary breast cancer in the laboratory or in mice. Murine and bovine prolactins, the major lactogens in current laboratory experimental in vivo and in vitro conditions, fail to activate human prolactin receptors because of species incompatibility. In fact, murine prolactin is a potent antagonist for human prolactin receptors. Because ER-positive, luminal breast cancers also express prolactin receptors, we hypothesized that lack of human lactogenic activity under experimental conditions selected against establishment of ER-positive breast cancer in the laboratory. We therefore genetically engineered mice to express physiological levels of human prolactin in place of mouse prolactin and backcrossed the mice for ten generations into the immunodeficient NSG strain. The resulting hPRL.NSG mice have a greatly improved take rate for ER positive, luminal type of breast cancer, suggesting key tumor-promoting roles for prolactin in luminal breast cancer. A panel of novel transplantable human breast cancer lines has been established in hPrl.NSG mice, the majority of which are ER-positive. The transplantable lines maintain key histopathological characteristics and expression of major marker proteins of the primary patient tumors. Intriguingly, initial tumor establishment and growth rates of breast cancer xenografts were consistently greater in the hPrl.NSG mice than in wildtype NSG mice. Furthermore, tumors grown in hPrl.NSG were more responsive to tamoxifen than size-matched tumors grown in wildtype NSG mice. At least two new tumor lines examined so far develop spontaneous distant metastases in hPrl.NSG mice, with evidence of prolactin-dependent progression of ER-positive disease. Collectively, these observations validate the hPrl.NSG mice as an improved recipient for preclinical modeling of human breast cancer in vivo, both for therapeutic targeting of prolactin-pathways and other growth and survival pathways, as well as overcoming anti-estrogen resistance.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S1-8.