Maintenance of hormone responsiveness in luminal breast cancers by suppression of Notch.

Luminal breast cancers express estrogen (ER) and/or progesterone (PR) receptors and respond to hormone therapies. Basal-like "triple negative" cancers lack steroid receptors but are cytokeratin (CK) 5-positive and require chemotherapy. Here we show that more than half of primary ER(+)PR(+) breast cancers contain an ER(-)PR(-)CK5(+) "luminobasal" subpopulation exceeding 1% of cells. Starting from ER(+)PR(+) luminal cell lines, we generated lines with varying luminal to luminobasal cell ratios and studied their molecular and biological properties. In luminal disease, luminobasal cells expand in response to antiestrogen or estrogen withdrawal therapies. The phenotype and gene signature of the hormone-resistant cells matches that of clinical triple negative basal-like and claudin-low disease. Luminobasal cell expansion in response to hormone therapies is regulated by Notch1 signaling and can be blocked by γ-secretase inhibitors. Our data establish a previously unrecognized plasticity of ER(+)PR(+) luminal breast cancers that, without genetic manipulation, mobilizes outgrowth of hormone-resistant basal-like disease in response to treatment. This undesirable outcome can be prevented by combining endocrine therapies with Notch inhibition.

Modeling luminal breast cancer heterogeneity: combination therapy to suppress a hormone receptor-negative, cytokeratin 5-positive subpopulation in luminal disease.

INTRODUCTION: Many Luminal breast cancers are heterogeneous, containing substantial numbers of estrogen (ER) and progesterone (PR) receptor-negative cells among the ER+ PR+ ones. One such subpopulation we call "Luminobasal" is ER-, PR- and cytokeratin 5 (CK5)-positive. It is not targeted for treatment. METHODS: To address the relationships between ER+PR+CK5- and ER-PR-CK5+ cells in Luminal cancers and tightly control their ratios we generated isogenic pure Luminal (pLUM) and pure Luminobasal (pLB) cells from the same parental Luminal human breast cancer cell line. We used high-throughput screening to identify pLB-specific drugs and examined their efficacy alone and in combination with hormone therapy in mixed-cell tumor models. RESULTS: We show that pLUM and MCF7 cells suppress proliferation of pLB cells in mixed-cell 3D colonies in vitro and that pLUM cells suppress growth of pLB cells in mixed-cell xenografts in vivo. High-throughput screening of 89 FDA-approved oncology drugs shows that pLB cells are sensitive to monotherapy with the epidermal growth factor receptor (EGFR) inhibitors gefitinib and erlotinib. By exploiting mixed-cell 3D colonies and mixed-cell solid mouse tumors models we demonstrate that combination therapy with gefitinib plus the anti-estrogen fulvestrant constitutes a robust treatment strategy. CONCLUSIONS: We propose that response to combination endocrine/EGFR inhibitor therapies in heterogeneous Luminal cancers may improve long-term survival in patients whose primary tumors have been preselected for appropriate biomarkers, including ER, PR, CK5 and EGFR.

Luminal breast cancer metastases and tumor arousal from dormancy are promoted by direct actions of estradiol and progesterone on the malignant cells.

INTRODUCTION: Luminal, estrogen receptor-positive (ER(+)) breast cancers can metastasize but lie dormant for years before recurrences prove lethal. Understanding the roles of estrogen (E) or progestin (P) in development of luminal metastases or in arousal from dormancy is hindered by few preclinical models. We have developed such models. METHODS: Immunocompromised, ovariectomized (ovx'd) mice were intracardiac-injected with luminal or basal human breast cancer cells. Four lines were tested: luminal ER(+)PR(+) cytokeratin 5-negative (CK5(-)) E3 and MCF-7 cells, basal ER(-)PR(-)CK5(+) estrogen withdrawn-line 8 (EWD8) cells, and basal ER(-)PR(-)CK5(-) MDA-MB-231 cells. Development of micrometastases or macrometastases was quantified in ovx'd mice and in mice supplemented with E or P or both. Metastatic deposits were analyzed by immunohistochemistry for luminal, basal, and proliferation markers. RESULTS: ER(-)PR(-) cells generated macrometastases in multiple organs in the absence or presence of hormones. By contrast, ovx'd mice injected with ER(+)PR(+) cells appeared to be metastases-free until they were supplemented with E or E+P. Furthermore, unlike parental ER(+)PR(+)CK5(-) cells, luminal metastases were heterogeneous, containing a significant (6% to 30%) proportion of non-proliferative ER(-)PR(-)CK5(+) cells that would be chemotherapy-resistant. Additionally, because these cells lack receptors, they would also be endocrine therapy-resistant. With regard to ovx'd control mice injected with ER(+)PR(+) cells that appeared to be metastases-free, systematic pathologic analysis of organs showed that some harbor a reservoir of dormant micrometastases that are ER(+) but PR(-). Such cells may also be endocrine therapy- and chemotherapy-resistant. Their emergence as macrometastases can be triggered by E or E+P restoration. CONCLUSIONS: We conclude that hormones promote development of multi-organ macrometastases in luminal disease. The metastases display a disturbing heterogeneity, containing newly emergent ER(-)PR(-) subpopulations that would be resistant to endocrine therapy and chemotherapy. Similar cells are found in luminal metastases of patients. Furthermore, lack of hormones is not protective. While no overt metastases form in ovx'd mice, luminal tumor cells can seed distant organs, where they remain dormant as micrometastases and sheltered from therapies but arousable by hormone repletion. This has implications for breast cancer survivors or women with occult disease who are prescribed hormones for contraception or replacement purposes.

Axl and Tyro3 modulate female reproduction by influencing gonadotropin-releasing hormone neuron survival and migration.

GnRH neurons must undergo a complex and precise pattern of neuronal migration to appropriately target their projections to the median eminence to trigger gonadotropin secretion and thereby control reproduction. Using NLT GnRH cells as a model of early GnRH neuronal development, we identified the potential importance of Axl and Tyro3, members of the TAM (Tyro3, Axl, and Mer) family of receptor tyrosine kinases in GnRH neuronal cell survival and migration. Silencing studies evaluated the role of Tyro3 and Axl in NLT GnRH neuronal cells and suggest that both play a role in Gas6 stimulation of GnRH neuronal survival and migration. Analysis of mice null for both Axl and Tyro3 showed normal onset of vaginal opening but delayed first estrus and persistently abnormal estrous cyclicity compared with wild-type controls. Analysis of GnRH neuronal numbers and positioning in the adult revealed a total loss of 24% of the neuronal network that was more striking (34%) when considered within specific anatomical compartments, with the largest deficit surrounding the organum vasculosum of the lamina terminalis. Analysis of GnRH neurons during embryogenesis identified a striking loss of immunoreactive cells within the context of the ventral forebrain compartment (36%) and not more rostrally. Studies using caspase 3 cleavage as a marker of apoptosis showed that Axl(-/-), Tyro3(-/-) double-knockout mice had increased cell death in the nose and dorsal forebrain, supporting the underlying mechanism of cell loss. Together these data suggest that Axl and Tyro3 mediate the survival and appropriate targeting of GnRH neurons to the ventral forebrain, thereby contributing to normal reproductive function and cyclicity in the female.

Loss of Growth arrest specific gene 6 (Gas6) results in altered GnRH neuron migration, delayed vaginal opening and sexual maturation in mice.

Prior work has shown the importance of TAM (Tyro3, Axl, Mer) receptor tyrosine kinases in GnRH neuronal development and reproductive function. It is unclear if TAM receptor actions are dependent on ligand activation for their functional effects; thus, we characterized reproductive phenotype of ligand Growth arrest specific gene (Gas6) null mice. Gas6 null mice showed delayed vaginal opening and delayed first estrus. Animals eventually attained normal estrous cycles as adults. The GnRH neuronal population was significantly decreased in Gas6 null adults and embryos, but the final positioning of cell bodies in the hypothalamus was normal. Vaginal tissue showed up-regulation of TAM receptor mRNAs in the absence of the ligand. These data confirm that Gas6 plays a role in early GnRH neuronal development and during vaginal opening. The phenotype of Gas6 KO mice suggests that TAMs function in a ligand-dependent and independent manner to control GnRH neuron development to modulate normal reproductive function.

Hypothalamic but not pituitary or ovarian defects underlie the reproductive abnormalities in Axl/Tyro3 null mice.

AXL and TYRO3, members of the TYRO3, AXL and MER (TAM) family of tyrosine kinase receptors, modulate GnRH neuronal cell migration, survival and gene expression. Axl/Tyro3 null mice exhibit a selective loss of GnRH neurons, delayed sexual maturation and irregular estrous cycles. Here we determined whether the defects were due to direct ovarian defects, altered pituitary sensitivity to GnRH and/or an impaired LH surge mechanism. Ovarian histology and markers of folliculogenesis and atresia as well as corpora luteal development and ovarian response to superovulation were not impaired. Axl/Tryo3 null mice exhibited a robust LH response to exogenous GnRH, suggesting no altered pituitary sensitivity. Ovariectomized Axl/Tyro3 null mice, however, demonstrated an impaired ability to mount a steroid-induced LH surge. Loss of GnRH neurons in Axl/Tyro3 null mice impairs the sex hormone-induced gonadotropin surge resulting in estrous cycle abnormalities confirming that TAM family members contribute to normal female reproductive function.