C/EBPβ expression in ALK-positive anaplastic large cell lymphomas is required for cell proliferation and is induced by the STAT3 signaling pathway

Background Anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma is characterized by the t(2;5) chromosomal translocation, resulting in the expression of a fusion protein formed of nucleophosmin (NPM) and ALK. Recently, we reported the abnormal expression of the transcription factor CCAAT/enhancer binding protein-beta (C/EBP beta) in ALK-positive anaplastic large cell lymphomas, and demonstrated its dependence on NPM-ALK activity. Design and Methods In this study, the role of C/EBP beta in proliferation and survival of ALK-positive anaplastic large cell lymphomas was investigated, as well as the mechanism of its expression and activity. Highly effective short hairpin RNA sequences and/or pharmacological inhibitors were used to abrogate the expression or activity of C/EBP beta, signal transducer and activator of transcription 3 (STAT3), AKT, extracellular signal-related kinase 1/2 (ERK1/2) and mammalian target of rapamycin (mTOR). Results Interference with C/EBP beta expression resulted in a dramatic decrease in cell proliferation in ALK-positive anaplastic large cell lymphomas, with a mild induction of apoptosis after 6 days. Down-regulation of STAT3 resulted in a marked decrease in C/EBP beta mRNA and protein levels with impairment in cell proliferation and viability, underscoring the important role of these two proteins in ALK-mediated oncogenesis. Additionally, we demonstrated that reduction of ERK1/2 activity led to C/EBP beta Thr(235) dephosphorylation and moderate growth retardation. The AKT/mTOR signaling pathway did not have any influence on C/EBP beta expression or C/EBP beta phosphorylation. Conclusions These findings reveal the convergence of STAT3 and ERK1/2 signaling pathways activated by NPM-ALK in mediating the regulation of C/EBP beta expression, a transcription factor central to NPM-ALK transformation.