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 NPM1mutated 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.

Autophagy and beyond: Unraveling the complexity of UNC-51-like kinase 1 (ULK1) from biological functions to therapeutic implications

“…GTPases RAB2 can also interact with the ULK1 complex, thereby promoting the recruitment of the ULK1 complex to form autophagosomes 189 . Interestingly, NPM1 mutant NPM1-mA could interact with ULK1 protein and positively regulate ULK1 protein levels and contribute to leukemic cell survival 190 . IRGM , encoded by a human gene for inflammatory disease risk, plays anti-inflammatory roles by promoting the interaction and co-assembly of ULK1 and Beclin 1 and the formation of the ULK1 complex 191 .…”

Autophagy a Close Relative of AML Biology

“…Another group demonstrated that in NPM1-mutated AML, the glycolytic enzyme PKM2 (pyruvate kinase M2) activates autophagy by increasing the phosphorylation of key autophagy protein BECLIN1 and contributes to cell survival [ 36 ]. Finally, NPM1 mutant can also interact with ULK1, a core autophagy protein that promotes TRAF6-dependent ULK1 ubiquitination via miR-146, maintaining ULK1 stability and activity and promoting autophagic cell survival [ 38 ].…”

Autophagy Regulation by Crosstalk between miRNAs and Ubiquitination System

“…K63-linked ubiquitination mediated by the E3 ubiquitin ligase TRAF6 is a crucial posttranslational modification for ULK1 protein. Unlike K48 linkage, K63-linked ubiquitination has a positive effect on protein stability of ULK1 and its participation in autophagosome formation [ 63 ].…”