It has been hypothesized that carcinoma metastasis is initiated by a subpopulation of circulating tumor cells (CTCs) found in the blood of patients. However, although the presence of CTCs is an indicator of poor prognosis in several carcinoma entities, the existence and phenotype of metastasis-initiating cells (MICs) among CTCs has not been experimentally demonstrated. Here we developed a xenograft assay and used it to show that primary human luminal breast cancer CTCs contain MICs that give rise to bone, lung and liver metastases in mice. These MIC-containing CTC populations expressed EPCAM, CD44, CD47 and MET. In a small cohort of patients with metastases, the number of EPCAM(+)CD44(+)CD47(+)MET(+) CTCs, but not of bulk EPCAM(+) CTCs, correlated with lower overall survival and increased number of metastasic sites. These data describe functional circulating MICs and associated markers, which may aid the design of better tools to diagnose and treat metastatic breast cancer.

SummaryLifelong blood cell production is governed through the poorly understood integration of cell-intrinsic and -extrinsic control of hematopoietic stem cell (HSC) quiescence and activation. MicroRNAs (miRNAs) coordinately regulate multiple targets within signaling networks, making them attractive candidate HSC regulators. We report that miR-126, a miRNA expressed in HSC and early progenitors, plays a pivotal role in restraining cell-cycle progression of HSC in vitro and in vivo. miR-126 knockdown by using lentiviral sponges increased HSC proliferation without inducing exhaustion, resulting in expansion of mouse and human long-term repopulating HSC. Conversely, enforced miR-126 expression impaired cell-cycle entry, leading to progressively reduced hematopoietic contribution. In HSC/early progenitors, miR-126 regulates multiple targets within the PI3K/AKT/GSK3β pathway, attenuating signal transduction in response to extrinsic signals. These data establish that miR-126 sets a threshold for HSC activation and thus governs HSC pool size, demonstrating the importance of miRNA in the control of HSC function.

The metastatic spread of cancer is achieved by the hematogenous dissemination of circulating tumor cells (CTCs). Generally, however, the temporal dynamics that dictate the generation of metastasis-competent CTCs are largely uncharacterized, often assuming that CTCs are constantly shed from growing tumors or shed as a consequence of mechanical insults 1 . Here, we observe a striking and unexpected pattern of CTC generation dynamics in both patients with breast cancer and mouse models, highlighting that the vast majority of spontaneous CTC intravasation events occur during the rest phase. Further, we demonstrate that rest-phase CTCs are highly metastasis-prone, while CTCs generated during active phase are devoid of metastatic ability. Mechanistically, single cell-resolution RNA sequencing analysis of CTCs reveals a dramatic upregulation of mitotic genes exclusively during the rest phase in both patients and mouse models, enabling metastasis proficiency. Systemically, we find that key circadian rhythm hormones such as melatonin, testosterone and glucocorticoids dictate CTC generation dynamics, and as a consequence, that insulin directly promotes tumor cell proliferation in vivo, yet in a time-dependent manner. Thus, the spontaneous generation of CTCs with a high proclivity to metastasize does not occur continuously but it is concentrated within the rest phase of the host, providing a new rationale for time-controlled interrogation and treatment of metastasis-prone cancers.3/24 Main Circulating tumor cells (CTCs) are pioneers of the metastatic cascade in several cancer types, including breast cancer 1 . The factors that regulate spontaneous CTC intravasation in physiological settings are poorly understood, and the general assumption is that CTCs are constantly generated from invasive cancerous tissues 2 , or generated upon mechanical cues such as surgery 3 or physical activity 4 . In patients and in mouse cancer models, the exact timing of the events that characterize metastatic cancer progression, as well as the principles that dictate CTC intravasation and their proclivity to metastasize are unclear. A better understanding of these processes may result in new approaches for cancer investigation and treatment. Circadian rhythm and CTC intravasationWe first sought to determine CTC abundance and composition in hospitalized women with progressive breast cancer that had no treatment or were temporarily off-treatment and that consented to donate blood during the active (10:00am) and rest (4:00am) phase of the same day, including a total of 30 patients (Fig. 1a). Of these, 21 patients were diagnosed with early breast cancer (no metastasis) and 9 were diagnosed with stage IV metastatic disease at the time of blood sampling (Supplementary Table 1). Strikingly, upon antigen-agnostic microfluidic capture of CTCs and confirmation via immunofluorescence staining 5 , we found the vast majority of CTCs (78.3%) in samples obtained at nighttime during rest phase, including single CTCs, CTC clusters and CTC-white blood cell (WBC) cl...

Key Points miR-155 knockdown in myeloid cells accelerates spontaneous breast cancer development. miR-155 is required by TAMs for deploying antitumoral activity.