Proteomic identification of C/EBP-DBD multiprotein complex: JNK1 activates stem cell regulator C/EBPalpha by inhibiting its ubiquitination.

Functional inactivation of transcription factors in hematopoietic stem cell development is involved in the pathogenesis of acute myeloid leukemia (AML). Stem cell regulator C/enhancer binding protein (EBP)alpha is among such transcription factors known to be inactive in AML. This is either due to mutations or inhibition by protein-protein interactions. Here, we applied a mass spectrometry-based proteomic approach to systematically identify putative co-activator proteins interacting with the DNA-binding domain (DBD) of C/EBP transcription factors. In our proteomic screen, we identified c-Jun N-terminal kinase (JNK) 1 among others such as PAK6, MADP-1, calmodulin-like skin proteins and ZNF45 as proteins interacting with DBD of C/EBPs from nuclear extract of myelomonocytic U937 cells. We show that kinase JNK1 physically interacts with DBD of C/EBP alpha in vitro and in vivo. Furthermore, we show that active JNK1 inhibits ubiquitination of C/EBP alpha possibly by phosphorylating in its DBD. Consequently, JNK1 prolongs C/EBP alpha protein half-life leading to its enhanced transactivation and DNA-binding capacity. In certain AML patients, however, the JNK1 mRNA expression and its kinase activity is decreased which suggests a possible reason for C/EBP alpha inactivation in AML. Thus, we report the first proteomic screen of C/EBP-interacting proteins, which identifies JNK1 as positive regulator of C/EBP alpha

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