Current cancer therapies generally assume cancers to progress linearly from primary to metastatic sites. The model postulates a stepwise accumulation of genetic and epigenetic alterations within the primary tumor, and therefore, primary tumors may be used as surrogate markers for disseminated cancer cells. However, this model is challenged by (i) the insufficient therapy success and (ii) the genomic disparity between disseminated cancer cells (DCCs) and their matched primary tumors when isolated at surgery. The genomic disparity suggests that metastatic dissemination may occur early during tumor formation, however the mechanisms of early metastatic spread are unknown so far.

To uncover mechanisms of early metastatic dissemination we microdissected mammary tissue from a Her2-driven mouse model (Balb-NeuT) of breast cancer before and after microscopic invasion and performed gene expression analysis. Identified pathways were tested in a series of in vitro and in vivo experiments. Finally, the findings were validated in a panel of human breast cancer cell lines and investigation of more than 2000 patient samples.

Gene expression profiling identified a distinct signature at the time point of maximal seeding when only pre-invasive mammary lesions could be identified. The morphology of pre-invasive lesions coincided with a transient time window of progesterone receptor (PgR) and Her2 co-expression. Upon functional testing progesterone-induced signaling triggered migration of stem-like cells from early lesions shortly after Her2 activation, but promoted proliferation in advanced primary tumor cells. The switch from migration to proliferation was regulated by elevated Her2 expression and increased cell density involving miRNA-regulated PgR down-regulation. The combined result was a differential response of cancer cells to progesterone and its paracrine-signal mediators Wnt4 and Rankl imposing stemness and migration at low and proliferation at high cellular density. Exploring human breast cancer cell lines we confirmed the observed phenotypes and mechanisms. Molecular-genetic and transcriptomic analysis of DCCs isolated from the bone marrow of breast cancer patients provided strong support for early metastatic dissemination. Finally, we identified a subtype of breast cancer mimicking the Balb-NeuT model closest.

We identify microenvironmental signaling (PgR signaling), oncogenic signaling (Her2) and cellular density as central regulators of early dissemination and metastasis. In light of the patient-derived data we suggest that (i) dissemination is mostly early and (ii) clones that evolved over time at the primary site and became pre-dominant are less able to disseminate. We predict the findings not only to be relevant for Her2-driven cancers but for many other cancers as well.

Etwa 90% der krebsbedingten Todesfälle werden durch Metastasen verursacht. Ein tieferes Verständnis der Streuung und dem Auswachsen von Krebszellen in Zielorganen ist wichtig, um zukünftig bessere Behandlungsmethoden entwickeln zu können.

Wir konnten nachweisen, dass die Streuung von Krebszellen in einem Brustkrebsmodell (dem Her2 getriebenen Mausmodell BALB-NeuT) schon bei sehr frühen Läsionen beginnt, die morphologisch noch atypischen Hyperplasien ähneln. Bei der Analyse des molekularen Mechanismus fanden wir eine Verbindung zwischen den Her2- und Progesteron(Pgr)-Signalwegen. Dabei stimulierten Pgr-induzierte parakrine Signale sowohl Migration und Stammzell-ähnliche Eigenschaften in Zellen früher Läsionen, aber nicht in Primärtumorzellen. Weiterhin fanden wir, dass die Pgr-Expression durch einen Zelldichte-regulierten miRNA Mechanismus gesteuert wird und dass die Zelldichte auch Migration und Stammzellpotenzial reguliert. Die Ergebnisse legen nahe, dass durch das Tumorwachstums die Zelldichte steigt und die Zellen von einem Migrations- in ein Proliferationsprogramm wechseln, die metastatische Streuung also präferentiell früh stattfindet. Weitere Analysen an humanen Proben, einschließlich primärer Tumore und disseminierter Tumorzellen, legen die hohe Relevanz der Befunde aus dem Mausmodell für Brustkrebs nahe.