Transcription factor CCAAT enhancer binding protein α (C/EBPα) is the master regulator of myeloid differentiation programme. Tumor suppressor function of C/EBPα is shown by the deregulation of C/EBPα in AML by various mechanisms. Mutations in the CEBPA gene are reported in around 9% of patients with acute myeloid leukemia (AML). The CEBPA mutations reported are frame shift mutation at N-terminal domain, which results in C/EBPα-p30 and point mutations at the basic region Leucine zipper. The mutant form of C/EBPα (C/EBPα-p30) displays dominant negative function over the wild type protein and induces AML in mouse model. The mechanism for the dominant negative action of C/EBPα-p30 in AML is poorly understood. Peptidyl-prolyl cis/trans isomerase, PIN1 binds to and isomerizes specific pSer/Thr-Pro motifs. Recent studies show that PIN1 is overexpressed in many types of cancers and it acts as a crucial catalyst for oncogenesis. Here we investigated the role of PIN1 in AML with CEBPA mutations. We report C/EBPα-p30 could induce PIN1 mRNA and protein levels as revealed by quantitative Real-Time RT-PCR analysis and western blot analysis, respectively. Affymetrix mRNA expression analysis shows that PIN1 mRNA is upregulated in patients with AML. Interestingly, the wild type C/EBPα down regulates PIN1 mRNA and protein levels. Chromatin immunoprecipitation assays demonstrate that induction of C/EBPα-p30 results in recruitment of E2F1 in the PIN1 promoter. Silencing PIN1 with inhibitor against PIN1 (PiB) in AML blast cells with CEBPA mutation and in Kasumi-6 cells leads to myeloid differentiation as assessed by myeloid markers by FACS analysis and quantitative Real-Time RT-PCR analysis. Furthermore, PIN1 inhibition by PiB leads to upregulation of wild type C/EBPα protein level. Next, we investigated the mechanism through which the C/EBPα-p30 blocks the wild type protein. c-Jun expression has been shown to be high in AML patients with CEBPA mutation and c-Jun blocks C/EBPα function by proteinprotein interaction. In vivo ubiquitination assay demonstrates that PIN1 increases the stability of the c-Jun protein by inhibiting c-Jun ubiquitination. We also show that c-Jun blocks granulocytic differentiation mediated by C/EBPα as assessed by myeloid specific markers. In summary, C/EBPα-p30 induces PIN1 expression and increases the stability of c-Jun and c-Jun blocks the wild type C/EBPα. Our data suggest the inhibition of PIN1 could be a potential strategy in the treatment of AML patients with CEBPA mutation.

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