Etk interaction with PFKFB4 modulates chemoresistance of small cell lung cancer by regulating autophagy

Purpose: Epithelial and endothelial tyrosine kinase (Etk), also known as bone marrow X kinase (Bmx), was found to be critical in modulating the chemoresistance of small cell lung cancer (SCLC) in our preliminary study. However, the molecular mechanisms of Etk in SCLC chemoresistance remain poorly understood. The present study investigated the downstream factor and pathway involved. Experimental Design: We demonstrated first that knockdown of Etk by siRNAs suppressed autophagy in chemoresistant SCLC cells, and that direct inhibition of autophagy sensitized cells to chemotherapy. Our subsequent microarray, co-immunoprecipitation (co-IP) and GST-pull down experiments identified 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 4 (PFKFB4) as a downstream target of Etk and an Etk-interacting protein. We demonstrated knockdown of PFKFB4 suppressed autophagy in SCLC cells. Our in vitro and in vivo gain or loss-of-function analyses of PFKFB4 revealed PFKFB4 plays a critical role in SCLC chemoresistance. Our analysis of PFKFB4 expression in SCLC specimens demonstrated that high levels of PFKFB4 are associated with poor therapeutic response and prognosis. Furthermore, as Etk shares conserved domains with Btk (Bruton's tyrosine kinase) family, we explored the potential of Ibrutinib, a Btk inhibitor clinically effective in treating leukemia, in targeting Etk and found that Ibrutinib exhibited a synergistic anti-tumor effect with chemotherapy in SCLC preclinical models including a PDX model. Results: Described above Conclusions:Our results demonstrated for the first time that Etk interacts with PFKFB4 to promote SCLC chemoresistance through regulation of autophagy. Aberrant Etk and PFKFB4 can be predictive factors for the chemotherapy response as well as potential therapeutic targets in SCLC.

http://ift.tt/2AyF7Uz