Cross-talk between the receptor tyrosine kinases AXL and ERBB3 regulates invadopodia formation in melanoma cells
The invasive phenotype of metastatic cancer cells is accompanied by the formation of actin-rich invadopodia, which adhere to the extracellular matrix, and degrade it. In this study, we explored the role of the tyrosine kinome in the formation of invadopodia in metastatic melanoma cells. Using a microscopy-based siRNA screen, we identified novel invadopodia regulators, the knock-down of which either suppresses (e.g., TYK2, IGFR1, ERBB3, TYRO3, FES, ALK, PTK7) or enhances invadopodia formation and function (e.g., ABL2, AXL, CSK). Particularly intriguing was the discovery that the receptor tyrosine kinase AXL displays a dual regulatory function, manifested by enhancement of invadopodia function upon knock-down or long-term inhibition, as well as following its over-expression. We show here that this apparent contradiction may be attributed to the capacity of AXL to directly stimulate invadopodia; yet its suppression up-regulates the ERBB3 signaling pathway, which consequently activates core invadopodia regulators, and greatly enhances invadopodia function. Bioinformatic analysis of multiple melanoma cells points to an inverse expression pattern of AXL and ERBB3, with the apparent association of high-AXL melanomas, with high expression of invadopodia components and an invasive phenotype. The relevance of these results to melanoma metastasis in vivo, and to potential anti-invasion therapy, is discussed.