Cancer-induced immune responses affect tumor progression and therapeutic response. In multiple murine models and clinical datasets, we identified large variations of neutrophils and macrophages, which define “immune subtypes” of triple negative breast cancer (TNBC) including neutrophil-enriched (NES) and macrophage-enriched subtypes (MES). Different tumor-intrinsic pathways and mutual regulation between macrophages/monocytes and neutrophils contribute to the development of dichotomous myeloid compartment. MES contains predominantly macrophages that are CCR2-dependent and exhibit variable responses to immune checkpoint blockade (ICB). NES exhibits systemic and local accumulation of immunosuppressive neutrophils (or granulocytic myeloid-derived suppressor cells (gMDSCs), is resistant to ICB, and contains a minority of macrophages that appear to be unaffected by CCR2 knockout. A MES-to-NES conversion mediated acquired ICB resistance of initially sensitive MES models. Our results demonstrate diverse myeloid cell frequencies, functionality, and potential roles in immunotherapies, and highlight the need to better understand the inter-patient heterogeneity of the myeloid compartment.

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While a first pregnancy before age 22 lowers breast cancer risk, a pregnancy after age 35 significantly increases life-long breast cancer risk. Pregnancy causes several changes to the normal breast that raise barriers to transformation, but how pregnancy can also increase cancer risk remains unclear. We show in mice that pregnancy has different effects on the few early lesions that have already developed in the otherwise normal breast—it causes apoptosis evasion and accelerated progression to cancer. The apoptosis evasion is due to the normally tightly controlled STAT5 signaling going astray—these precancerous cells activate STAT5 in response to pregnancy/lactation hormones and maintain STAT5 activation even during involution, thus preventing the apoptosis normally initiated by oncoprotein and involution. Short-term anti-STAT5 treatment of lactation-completed mice bearing early lesions eliminates the increased risk after a pregnancy. This chemoprevention strategy has important implications for preventing increased human breast cancer risk caused by pregnancy.DOI: http://dx.doi.org/10.7554/eLife.00996.001

The Hedgehog (Hh) signaling network is critical for patterning and organogenesis in mammals, and has been implicated in a variety of cancers. Smoothened (Smo), the gene encoding the principal signal transducer, is overexpressed frequently in breast cancer, and constitutive activation in MMTV-SmoM2 transgenic mice caused alterations in mammary gland morphology, increased proliferation, and changes in stem/progenitor cell number. Both in transgenic mice and in clinical specimens, proliferative cells did not usually express detectable Smo, suggesting the hypothesis that Smo functioned in a non-cell autonomous manner to stimulate proliferation. Here, we employed a genetically tagged mouse model carrying a Cre-recombinase-dependent conditional allele of constitutively active Smo (SmoM2) to test this hypothesis. MMTV-Cre- or adenoviral-Cre-mediated SmoM2 expression in the luminal epithelium, but not in the myoepithelium, was required for the hyper-proliferative phenotypes. High levels of proliferation were observed in cells adjacent or in close-proximity to Smo expressing cells demonstrating that SmoM2 expressing cells were stimulating proliferation via a paracrine or juxtacrine mechanism. In contrast, Smo expression altered luminal cell differentiation in a cell-autonomous manner. SmoM2 expressing cells, purified by fluorescence activated cell sorting (FACS) via the genetic fluorescent tag, expressed high levels of Ptch2, Gli1, Gli2, Jag2 and Dll-1, and lower levels of Notch4 and Hes6, in comparison to wildtype cells. These studies provide insight into the mechanism of Smo activation in the mammary gland and its possible roles in breast tumorigenesis. In addition, these results also have potential implications for the interpretation of proliferative phenotypes commonly observed in other organs as a consequence of hedgehog signaling activation.

In breast cancer (BrCa), overexpression of the nuclear co-activator NCOA1 (SRC-1) is associated with disease recurrence and resistance to endocrine therapy. To examine the impact of NCOA1 overexpression on morphogenesis and carcinogenesis in the mammary gland (MG), we generated MMTV-hNCOA1 transgenic [Tg(NCOA1)] mice. In the context of two distinct transgenic models of breast cancer, NCOA1 overexpression did not affect the morphology or tumor forming capability of MG epithelial cells. However, NCOA1 overexpression increased the number of circulating BrCa cells and the efficiency of lung metastasis. Mechanistic investigations showed that NCOA1 and c-Fos were recruited to a functional AP-1 site in the macrophage attractant CSF1 promoter, directly upregulating CSF1 expression to enhance macrophage recruitment and metastasis. Conversely, silencing NCOA1 reduced CSF1 expression and decreased macrophage recruitment and BrCa cell metastasis. In a cohort of 453 human breast tumors, NCOA1 and CSF1 levels correlated positively with disease recurrence, higher tumor grade and poor prognosis. Together, our results define an NCOA1/AP-1/CSF1 regulatory axis that promotes BrCa metastasis, offering a novel therapeutic target for impeding this process.