Prohibitin 2 represents a novel nuclear AKT substrate during all-trans retinoic acid-induced differentiation of acute promyelocytic leukemia cells.

The AKT/PKB kinase is essential for cell survival, proliferation, and differentiation; however, aberrant AKT activation leads to the aggressiveness and drug resistance of many human neoplasias. In the human acute promyelocytic leukemia cell line NB4, nuclear AKT activity increases during all-trans retinoic acid (ATRA)-mediated differentiation. As nuclear AKT activity is associated with differentiation, we sought to identify the nuclear substrates of AKT that were phosphorylated after ATRA treatment. A proteomics-based search for nuclear substrates of AKT in ATRA-treated NB4 cells was undertaken by using 2D-electrophoresis/mass spectrometry (MS) in combination with an anti-AKT phospho-substrate antibody. Western blot analysis, an in vitro kinase assay, and/or site-directed mutagenesis were performed to further characterize the MS findings. MS analysis revealed prohibitin (PHB)-2, a multifunctional protein involved in cell cycle progression and the suppression of oxidative stress, to be a putative nuclear substrate of AKT. Follow-up studies confirmed that AKT phosphorylates PHB2 on Ser-91 and that forced expression of the PHB2(S91A) mutant results in a rapid loss of viability and apoptotic cell death. Activation of nuclear AKT during ATRA-mediated differentiation results in the phosphorylation of several proteins, including PHB2, which may serve to coordinate nuclear-mitochondrial events during differentiation.