Akt-Induced Phosphorylation of N-CoR at Serine 1450 Contributes to Its Misfolded Conformational Dependent Loss (MCDL) in Acute Myeloid Leukemia of the M5 Subtype

The nuclear receptor co-repressor (N-CoR) is a key component of the generic co-repressor complex that plays an important role in the control of cellular growth and differentiation. As shown by us recently, the growth suppressive function of N-CoR largely relies on its capacity to repress Flt3, a key regulator of cellular gorwth during normal and malignant hematopoesis. We further demonstrated how de-repression of Flt3 due to the misfolded conformation dependent loss (MCDL) of N-CoR contributed to malignant growth in acute myeloid leukemia (AML). However, the molecular mechanism underlying the MCDL of N-CoR and its implication in AML pathogenesis is not fully understood. Here, we report that Akt-induced phosphorylation of N-CoR at the consensus Akt motif is crucial for its misfolding and subsequent loss in AML (AML-M5). N-CoR displayed significantly higher level of serine specific phosphorylation in almost all AML-M5 derived cells and was subjected to processing by AML-M5 specific aberrant protease activity. To identify the kinase linked to N-CoR phosphorylation, a library of activated kinases was screened with the extracts of AML cells; leading to the identification of Akt as the putative kinase linked to N-CoR phosphorylation. Consistent with this finding, a constitutively active Akt consistently phosphorylated N-CoR leading to its misfolding; while the therapeutic and genetic ablation of Akt largely abrogated the MCDL of N-CoR in AML-M5 cells. Site directed mutagenic analysis of N-CoR identified serine 1450 as the crucial residue whose phosphorylation by Akt was essential for the misfolding and loss of N-CoR protein. Moreover, Akt-induced phosphorylation of N-CoR contributed to the de-repression of Flt3, suggesting a cross talk between Akt signaling and N-CoR misfolding pathway in the pathogenesis of AML-M5. The N-CoR misfolding pathway could be the common downstream thread of pleiotropic Akt signaling activated by various oncogenic insults in some subtypes of leukemia and solid tumors.