Autophagy mediates proteolysis of NPM1 and HEXIM1 and sensitivity to BET inhibition in AML cells

// Min Huang 1 , Jacqueline S. Garcia 2 , Daniel Thomas 2, 3 , Li Zhu 4 , Le Xuan Truong Nguyen 1 , Steven M. Chan 2, 3 , Ravindra Majeti 1, 2, 3 , Bruno C. Medeiros 1, 2 , Beverly S. Mitchell 1, 2 1 Stanford Cancer Institute, Stanford University, Stanford, California, USA 2 Division of Hematology, Department of Medicine, Stanford University, Stanford, California, USA 3 Institute for Stem Cell Biology and Regenerative Medicine, Stanford University, Stanford, California, USA 4 Department of Pathology, Stanford University School of Medicine, Stanford, California, USA Correspondence to: Beverly S. Mitchell, email: bmitchell@stanford.edu Keywords: autophagy, NPM1, HEXIM1, BET inhibitors, AML Received: March 14, 2016      Accepted: September 26, 2016      Published: October 06, 2016 ABSTRACT The mechanisms underlying activation of the BET pathway in AML cells remain poorly understood. We have discovered that autophagy is activated in acute leukemia cells expressing mutant nucleophosmin 1 (NPMc+) or MLL-fusion proteins. Autophagy activation results in the degradation of NPM1 and HEXIM1, two negative regulators of BET pathway activation. Inhibition of autophagy with pharmacologic inhibitors or through knocking down autophagy-related gene 5 (Atg5) expression increases the expression of both NPM1 and HEXIM1. The Brd4 inhibitors JQ1 and I-BET-151 also inhibit autophagy and increase NPM1 and HEXIM1 expression. We conclude that the degradation of NPM1 and HEXIM1 through autophagy in certain AML subsets contributes to the activation of the BET pathway in these cells.