NPM1 mutant maintains ULK1 protein stability via TRAF6-dependent ubiquitination to promote autophagic cell survival in leukemia.

Abstract

Nucleophosmin (NPM1) mutations are the most frequent genetic alteration in acute myeloid leukemia (AML) and aberrant cytoplasm-dislocated NPM1 mutant is a distinct biological characterization of this disease. Our group previously reported that NPM1 mutant elevated autophagy activity and autophagy activation contributed to leukemic cell survival. However, the molecular mechanisms by which cytoplasmic NPM1 mutant involving in the autophagy pathway has not been fully elucidated. Here, we showed that Unc-51-like kinase 1 (ULK1) as a core autophagy protein was highly expressed in NPM1-mA positive OCI-AML3 cells and primary NPM1-mutated AML blasts. Meanwhile, we found that NPM1-mA could interact with ULK1 protein and positively regulated ULK1 protein levels. Mechanically, NPM1-mA promoted TRAF6-dependent K63 ubiquitination and further maintained ULK1 stability and kinase activity via miR-146a. In addition, ULK1 high expression-mediated autophagy activation and facilitated to leukemic cell proliferation. Finally, we demonstrated that restoring ULK1 expression, ULK1 inhibitor SBI-0206965 treatment and using shULK1 partially rescued the effect of NPM1-mA on autophagy and cell survival. In conclusion, our findings suggest that NPM1 mutant interacts with ULK1, and thus, maintains its protein stability, which is required for NPM1 mutant-mediated autophagic cell survival. These data extend our understanding of the functions of NPM1 mutant in the regulation of autophagy activation in NPM1-mutated AML.