Target Proteins of C/EBPa-p30 in AML: A Critical Role of Ubiquitin-Conjugating Enzyme (Ubc9) in Modulating C/EBPa-p42 Functions.

The mutations in the transcription factor CCAAT/enhancer binding protein-a (C/EBPa) occur in 10% of patients with acute myeloid leukemia which often leads to the expression of an N-terminal truncated 30KDa isoform of C/EBPa. The mutated 30KDa isoform has been shown to be dominant negative over wild type isoform that affects most of its biological functions. In the present study, we applied a global proteomics approach to identify the target proteins of C/EBPap30. This analysis revealed that C/EBPap30 modulates the expression of 60 different proteins notably, ubiquitin-conjugating enzyme (Ubc9), Peptidylprolyl isomerase (pin1), hnRNP A2/B1, Cacyclin binding protein, calgranulin B, and Tubulin beta5. Further, we show that in AML patients with C/EBPap30 mutations Ubc9 was predominantly upregulated. We discovered that this ligase was responsible for the enhanced sumolyation of C/EBPap42 on a lysine residue (K161) which results in its reduced transactivation in a promoter assay. When lysine (K) residue at 161 of C/EBPa-p42 was mutated to argenine residue(R), transcriptional activity of C/EBPap42 was restored. This finding was further validated by silencing the expression of Ubc9 expression which completely abrogates the sumoylation at K161 residue and further restored the transactivation and differentiation capacity of C/EBPap42. Our results demonstrate that increasing expression of Ubc9 by C/EBPap30 enhances sumolyation of C/EBPap42 and thereby inhibits its transactivation and differentiation capacity.