Metastatic dissemination is a complex, multi-step process in which cells leave the primary tumor, move into the blood vessels and colonize distant organs in the body. The underlying mechanisms of metastatic dissemination are yet to be fully defined.
Main Finding. In a recent paper published in Nature, authors show that tumor cells induce an increased expression of SLIT2 (a protein guiding developing neurons along their appropriate path) in endothelial cells (cells lining the blood vessels), which in turn promotes metastatic dissemination.
Experimental details. Authors observed higher expression of SLIT2 in endothelial cells of highly metastatic melanoma and breast cancer models compared to low metastatic tumors. Moreover, deletion of SLIT2 in endothelial cells of breast cancer murine models reduced metastasis formation, without affecting primary tumor growth or angiogenesis (cancer-induced formation of new blood vessels): mice showed increased survival after primary tumor removal.
So, higher expression of SLIT2 in endothelial cells was associated with metastases, whereas its deletion reduced metastasis formation, demonstrating a role of SLIT2 in the process of metastatic dissemination.
What is the underlying mechanism of endothelial SLIT2-mediated promotion of metastases? Highly metastatic tumor cells release double-stranded RNA (dsRNA). dsRNA binds TLR3 receptor expressed on endothelial cells. dsRNA binding to TLR3 triggers an increase of SLIT2 in endothelial cells. Then, SLIT2 produced by endothelial cells binds Robo1 receptor expressed on tumor cells, thus promoting migration of cancer cells into the blood vessels.
Indeed, in vitro experiments showed that Robo1 deletion reduced SLIT2-induced migration of cancer cells. Moreover, Robo1 was more expressed in highly metastatic cancer cells compared to non-metastatic cancer cells.
Analysis of gene expression databases revealed 1) higher endothelial expression of SLIT2 in high-stage primary breast cancer compared to low-stage primary breast cancer and 2) higher SLIT2 expression in the endothelium of metastatic samples compared to primary tumors.
Additionally, pharmacological inhibition or genetic inactivation of TLR3 suppress SLIT2 increase in endothelial cells. Furthermore, endocytosis is a known mechanism of TLR3 signalling and the use of specific drugs targeting this process also suppressed the increased SLIT2 expression.
Interestingly, the authors observed that the same protein, SLIT2, can act both as a driver or a suppressor of tumor progression, depending on its expression in endothelial or tumor cells: metastatic dissemination is associated with enhanced SLIT2 expression in endothelial cells or reduced SLIT2 expression in breast cancer cells.
Indeed, deletion of SLIT2 in tumor cells promotes metastasis formation (while SLIT2 inactivation in endothelial cells reduced metastasis formation) without affecting primary tumor growth or angiogenesis. Consistently, analysis of public databases showed that reduced SLIT2 expression in tumors was associated with a worse patient prognosis.
Conclusions. The authors show a double role of SLIT2 in tumor progression, promoting metastasis formation when highly expressed in endothelial cells while suppressing metastasis formation when highly expressed in tumor cells. The endothelial SLIT2-induced metastasis formation is mediated by dsRNA released from tumor cells, which binds TLR3, induces SLIT2 expression, ultimately increasing cancer cell migration.
Interestingly, TLR3 agonists (drugs activating TLR3) are currently in clinical trials to activate the immune system in order to control tumor growth. However, the side effect linked to the formation of metastases, via the SLIT2/Robo1 axis, should be considered as well, and perhaps a combined treatment with TLR3 agonists (to activate the immune system) and drugs aimed at the specific inhibition of the SLIT2/Robo1 axis in endothelial cells may enhance the beneficial effect of TLR3 treatment in a clinical setting.
Reference. Tumoral activation of TLR3-SLIT2 axis in endothelium drives metastasis. Tavora, Mederer, Wessel, […], Tavazoie. Nature 2020.