Abstract
EML4–ALK+ lung cancer cells depend on the RAS–MAPK effector pathway for survival.
Major finding: EML4–ALK+ lung cancer cells depend on the RAS–MAPK effector pathway for survival.
Clinical relevance: Reactivation of MAPK signaling drives ALK inhibitor resistance in EML4–ALK+ lung adenocarcinomas.
Impact: Up-front co-targeting of ALK and MEK suppresses resistance and enhances the response to ALK inhibition.
Targeted inhibition of anaplastic lymphoma kinase (ALK) fusion–positive lung adenocarcinoma using crizotinib or ceritinib frequently results in acquired resistance that prevents durable responses. Fusion of the ALK kinase domain with echinoderm microtubule-associated protein like 4 (EML4) results in the activation of PI3K–AKT, RAS–MAPK, and JAK–STAT effector pathways; however, the effector pathway that mediates survival in ALK inhibitor–sensitive or resistant lung tumors is unknown. Hrustanovic and colleagues modulated these downstream pathways in EML4–ALK-positive lung adenocarcinoma cell lines and found that ALK fusion–positive cells were specifically dependent on the MAPK pathway for growth and survival. Immunoprecipitation and immunofluorescence experiments revealed that all three isoforms of RAS (HRAS, NRAS, and KRAS) are activated by EML4–ALK via the EML4 HELP domain, which was required for EML4–ALK subcellular localization and RAS-mediated activation of MAPK signaling downstream of EML4–ALK. Focal copy-number gains and gene duplication events of wild-type KRAS occurred in lung cancer cells lines and primary human tumor samples with acquired resistance to ALK inhibitors and resulted in reactivation of MAPK signaling. In addition, MAPK reactivation in ALK inhibitor–resistant EML4–ALK-positive lung cancer cell lines and tumor samples was also mediated by downregulation of dual-specificity phosphatase 6 (DUSP6), a negative regulator of MEK–ERK signaling. Treatment of EML4–ALK-positive lung cancer cells with inhibitors of MEK or ERK sensitized cells to ALK inhibition both in vitro and in vivo. Furthermore, in a preclinical mouse model, co-administration of the ALK inhibitor crizotinib and a sub-maximal dose of the MEK inhibitor trametinib forestalled the development of acquired ALK inhibitor resistance and improved the initial tumor response. These data identify the RAS–MAPK pathway as a critical mediator of ALK oncogene dependence and ALK inhibitor resistance and suggest a polytherapy approach to prevent resistance and promote enhanced clinical outcomes in EML4–ALK-positive lung cancer.
