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Activation of the GTPase RhoA increases actinomyosin contractility and a population of highly invasive amoeboid-like cells in tumours. Here we identified that the pro-metastatic oncogene RASSF1C is an activator of RhoA/ROCK signaling and amoeboid motility in cells with stem cell-like attributes. RhoA-driven amoeboid cell movement also induces the release of extracellular vesicles that promote non-cell autonomous migration and metastasis in recipient cells. Mechanistically, RASSF1C localises in the uropod like structure of fast A2-amoeboid motile cells where it activates pMLCII and supports contractility required for forward movement. In addition, amoeboid cells secret integrin-rich extracellular vesicles that support uropod attachment which are released in to the milieu upon initiation of movement. The higher level of amoeboid cells in RASSF1C+/RhoA+ tumours results in elevated A2 motility, an associated increase in vesicle release and promotion of metastasis in benign tumour cells in vivo, unravelling a common mechanism behind diverse phenotypic attributes of cancer cells.

RASSF1A is a tumour suppressor, the silencing of which occurs through promoter methylation in a variety of human cancers. Loss of RASSF1A is associated with decreased sensitivity to DNA damaging agents and worse prognosis in breast, colon and lung cancers amongst others. RASSF1A functions in a number of cellular processes, promoting apoptosis in response to DNA damage or death receptor signalling, or cell cycle arrest at both G1/S and pro-metaphase checkpoints. As a scaffold protein, RASSF1A imparts these functions through direct interaction with target proteins. We have identified a novel interaction between RASSF1A and the SRC activator, OSSA. Further studies identify a role for RASSF1 in SRC signalling. We find that a second isoform of RASSF1, RASSF1C, the expression of which is maintained in cancers, is able to activate SRC. We also identify a novel tumour suppressor role for RASSF1A inhibiting SRC activation through binding of RASSF1C. SRC activation by RASSF1C expression promotes internalisation of adherens junctions leading to subsequent loss of tight junctions and cell polarity markers from sites of cell-cell contact. -catenin is also found to be re-localised throughout the cells from where it is hypothesised to be able to upregulate pro-proliferative genes. In addition, we find that RASSF1C expression promotes cell motility in both scratch wound and transwell assays. Finally, we show that RASSF1C expression enhances tumour cell aggressiveness using a mammosphere growth assay. We conclude that RASSF1C is an oncogene that can promote EMT through the activation of SRC family kinases. This function is inhibited by the tumour suppressor RASSF1A. This work highlights why RASSF1A is lost through epigenetic mechanisms and not mutation and why loss of RASSF1A is associated with more aggressive, metastatic cancers.