The potential role of YAP in Axl-mediated resistance to EGFR tyrosine kinase inhibitors.

Yes-associated protein (YAP) is a transcription co-regulator downstream of the Hippo pathway, and plays a critical role in cancer. Although YAP regulation in the canonical Hippo pathway is well established, the Hippo-independent regulation of YAP is not well explored. Here, we showed the possible new mechanism of YAP regulation by the receptor tyrosine kinase Axl. Co-immunoprecipitation and Western blot analysis demonstrated the interaction between YAP and Axl, which was enhanced by Axl ligand Growth Arrest Specific 6 (GAS6) stimulation. Furthermore, we found that YAP is phosphorylated at tyrosine residues by GAS6 stimulation in vivo and Axl directly phosphorylates YAP in vitro. Axl overexpression or GAS6 stimulation increased YAP-mediated transcriptional activity, and YAP-mediated colony forming activity in soft agar was enhanced by co-expression of Axl. In EGFR tyrosine kinase inhibitor (TKI)-sensitive lung cancer cells, YAP protein was downregulated in response to TKI treatment, while overexpression of YAP attenuated TKI sensitivity, suggesting that YAP is a key determinant of TKI response. Moreover Axl overexpression reversed TKI-induced YAP downregulation and induced TKI-resistance, which was reversed by YAP knockdown, further supporting the notion that YAP functions downstream of Axl. Together, these findings suggest a novel role of YAP in Axl-mediated TKI resistance. AJCR