A spatially resolved single-cell genomic atlas of the adult human breast.

The adult human breast is comprised of an intricate network of epithelial ducts and lobules that are embedded in connective and adipose tissue1-3. Although most previous studies have focused on the breast epithelial system4-6, many of the non-epithelial cell types remain understudied. Here we constructed the comprehensive Human Breast Cell Atlas (HBCA) at single-cell and spatial resolution. Our single-cell transcriptomics study profiled 714,331 cells from 126 women, and 117,346 nuclei from 20 women, identifying 12 major cell types and 58 biological cell states. These data reveal abundant perivascular, endothelial and immune cell populations, and highly diverse luminal epithelial cell states. Spatial mapping using four different technologies revealed an unexpectedly rich ecosystem of tissue-resident immune cells, as well as distinct molecular differences between ductal and lobular regions. Collectively, these data provide a reference of the adult normal breast tissue for studying mammary biology and diseases such as breast cancer.

Microenvironment in metastasis: roadblocks and supportive niches.

In many cancers, malignant cells can spread from the primary tumor through blood circulation and initiate metastasis in secondary organs. Metastatic colonization may depend not only on inherent properties of cancer cells, but also on suitable microenvironments in distant sites. Increasing evidence suggests that the nature of the microenvironment may determine the fate of disseminated cancer cells, providing either hindrance or support for cancer cell propagation. This can result in strong selective pressure where the vast majority of cancer cells, invading a secondary organ, are either eliminated or maintained in a dormant state. The ability of cancer cells to fend off or circumvent anti-metastatic signals from the stroma and the capacity to manipulate the local microenvironment towards a supporting environment, a metastatic niche, may be essential for metastatic growth. The molecular interactions between cancer cells and the stroma are still enigmatic, but recent studies are beginning to reveal their nature. Here, we discuss the interactive relationship between metastatic cancer cells and host stroma, involving selection and adaptation of metastasis-initiating cells and host tissue remodeling. Understanding the dynamic and continuously evolving cross talk between metastatic cancer cells and the stroma may be crucial when developing cancer treatments.

Preneoplastic stromal cells promote BRCA1-mediated breast tumorigenesis.

Women with germline BRCA1 mutations (BRCA1+/mut) have increased risk for hereditary breast cancer. Cancer initiation in BRCA1+/mut is associated with premalignant changes in breast epithelium; however, the role of the epithelium-associated stromal niche during BRCA1-driven tumor initiation remains unclear. Here we show that the premalignant stromal niche promotes epithelial proliferation and mutant BRCA1-driven tumorigenesis in trans. Using single-cell RNA sequencing analysis of human preneoplastic BRCA1+/mut and noncarrier breast tissues, we show distinct changes in epithelial homeostasis including increased proliferation and expansion of basal-luminal intermediate progenitor cells. Additionally, BRCA1+/mut stromal cells show increased expression of pro-proliferative paracrine signals. In particular, we identify pre-cancer-associated fibroblasts (pre-CAFs) that produce protumorigenic factors including matrix metalloproteinase 3 (MMP3), which promotes BRCA1-driven tumorigenesis in vivo. Together, our findings demonstrate that precancerous stroma in BRCA1+/mut may elevate breast cancer risk through the promotion of epithelial proliferation and an accumulation of luminal progenitor cells with altered differentiation.

A spatially resolved single cell genomic atlas of the adult human breast.

The adult human breast comprises an intricate network of epithelial ducts and lobules that are embedded in connective and adipose tissue. While previous studies have mainly focused on the breast epithelial system, many of the non-epithelial cell types remain understudied. Here, we constructed a comprehensive Human Breast Cell Atlas (HBCA) at single-cell and spatial resolution. Our single-cell transcriptomics data profiled 535,941 cells from 62 women, and 120,024 nuclei from 20 women, identifying 11 major cell types and 53 cell states. These data revealed abundant pericyte, endothelial and immune cell populations, and highly diverse luminal epithelial cell states. Our spatial mapping using three technologies revealed an unexpectedly rich ecosystem of tissue-resident immune cells in the ducts and lobules, as well as distinct molecular differences between ductal and lobular regions. Collectively, these data provide an unprecedented reference of adult normal breast tissue for studying mammary biology and disease states such as breast cancer.

Perivascular tenascin C triggers sequential activation of macrophages and endothelial cells to generate a pro-metastatic vascular niche in the lungs.

Disseminated cancer cells frequently lodge near vasculature in secondary organs. However, our understanding of the cellular crosstalk invoked at perivascular sites is still rudimentary. Here, we identify intercellular machinery governing formation of a pro-metastatic vascular niche during breast cancer colonization in the lung. We show that specific secreted factors, induced in metastasis-associated endothelial cells (ECs), promote metastasis in mice by enhancing stem cell properties and the viability of cancer cells. Perivascular macrophages, activated via tenascin C (TNC) stimulation of Toll-like receptor 4 (TLR4), were shown to be crucial in niche activation by secreting nitric oxide (NO) and tumor necrosis factor (TNF) to induce EC-mediated production of niche components. Notably, this mechanism was independent of vascular endothelial growth factor (VEGF), a key regulator of EC behavior and angiogenesis. However, targeting both macrophage-mediated vascular niche activation and VEGF-regulated angiogenesis resulted in added potency to curb lung metastasis in mice. Together, our findings provide mechanistic insights into the formation of vascular niches in metastasis.

Metastasis-initiating cells induce and exploit a fibroblast niche to fuel malignant colonization of the lungs.

Metastatic colonization relies on interactions between disseminated cancer cells and the microenvironment in secondary organs. Here, we show that disseminated breast cancer cells evoke phenotypic changes in lung fibroblasts, forming a supportive metastatic niche. Colonization of the lungs confers an inflammatory phenotype in metastasis-associated fibroblasts. Specifically, IL-1α and IL-1ß secreted by breast cancer cells induce CXCL9 and CXCL10 production in lung fibroblasts via NF-κB signaling, fueling the growth of lung metastases. Notably, we find that the chemokine receptor CXCR3, that binds CXCL9/10, is specifically expressed in a small subset of breast cancer cells, which exhibits tumor-initiating ability when co-transplanted with fibroblasts and has high JNK signaling that drives IL-1α/ß expression. Importantly, disruption of the intercellular JNK-IL-1-CXCL9/10-CXCR3 axis reduces metastatic colonization in xenograft and syngeneic mouse models. These data mechanistically demonstrate an essential role for the molecular crosstalk between breast cancer cells and their fibroblast niche in the progression of metastasis.

LNK suppresses interferon signaling in melanoma.

LNK (SH2B3) is a key negative regulator of JAK-STAT signaling which has been extensively studied in malignant hematopoietic diseases. We found that LNK is significantly elevated in cutaneous melanoma; this elevation is correlated with hyperactive signaling of the RAS-RAF-MEK pathway. Elevated LNK enhances cell growth and survival in adverse conditions. Forced expression of LNK inhibits signaling by interferon-STAT1 and suppresses interferon (IFN) induced cell cycle arrest and cell apoptosis. In contrast, silencing LNK expression by either shRNA or CRISPR-Cas9 potentiates the killing effect of IFN. The IFN-LNK signaling is tightly regulated by a negative feedback mechanism; melanoma cells exposed to IFN upregulate expression of LNK to prevent overactivation of this signaling pathway. Our study reveals an unappreciated function of LNK in melanoma and highlights the critical role of the IFN-STAT1-LNK signaling axis in this potentially devastating disease. LNK may be further explored as a potential therapeutic target for melanoma immunotherapy.

Stress signaling in breast cancer cells induces matrix components that promote chemoresistant metastasis.

Metastatic progression remains a major burden for cancer patients and is associated with eventual resistance to prevailing therapies such as chemotherapy. Here, we reveal how chemotherapy induces an extracellular matrix (ECM), wound healing, and stem cell network in cancer cells via the c-Jun N-terminal kinase (JNK) pathway, leading to reduced therapeutic efficacy. We find that elevated JNK activity in cancer cells is linked to poor clinical outcome in breast cancer patients and is critical for tumor initiation and metastasis in xenograft mouse models of breast cancer. We show that JNK signaling enhances expression of the ECM and stem cell niche components osteopontin, also called secreted phosphoprotein 1 (SPP1), and tenascin C (TNC), that promote lung metastasis. We demonstrate that both SPP1 and TNC are direct targets of the c-Jun transcription factor. Exposure to multiple chemotherapies further exploits this JNK-mediated axis to confer treatment resistance. Importantly, JNK inhibition or disruption of SPP1 or TNC expression sensitizes experimental mammary tumors and metastases to chemotherapy, thus providing insights to consider for future treatment strategies against metastatic breast cancer.

The hepatic microenvironment essentially determines tumor cell dormancy and metastatic outgrowth of pancreatic ductal adenocarcinoma.

Pancreatic ductal adenocarcinoma (PDAC) is often diagnosed when liver metastases already emerged. This study elucidated the impact of hepatic stromal cells on growth behavior of premalignant and malignant pancreatic ductal epithelial cells (PDECs). Liver sections of tumor-bearing KPC mice comprised micrometastases displaying low proliferation located in an unobtrusive hepatic microenvironment whereas macrometastases containing more proliferating cells were surrounded by hepatic myofibroblasts (HMFs). In an age-related syngeneic PDAC mouse model livers with signs of age-related inflammation exhibited significantly more proliferating disseminated tumor cells (DTCs) and micrometastases despite comparable primary tumor growth and DTC numbers. Hepatic stellate cells (HSC), representing a physiologic liver stroma, promoted an IL-8 mediated quiescence-associated phenotype (QAP) of PDECs in coculture. QAP included flattened cell morphology, Ki67-negativity and reduced proliferation, elevated senescence-associated ß galactosidase activity and diminished p-Erk/p-p38-ratio. In contrast, proliferation of PDECs was enhanced by VEGF in the presence of HMF. Switching the micromilieu from HSC to HMF or blocking VEGF reversed QAP in PDECs. This study demonstrates how HSCs induce and maintain a reversible QAP in disseminated PDAC cells, while inflammatory HMFs foster QAP reversal and metastatic outgrowth. Overall, the importance of the hepatic microenvironment in induction and reversal of dormancy during PDAC metastasis is emphasized.