C/EBPβ expression in ALK-positive anaplastic large cell lymphomas (ALCL) is required for cell proliferation and is induced by the STAT3 signaling pathway.
Background: ALK+ALCL is characterized by the t(2;5) chromosomal translocation, resulting in the expression of the fusion protein NPM-ALK. Recently, we reported the abnormal expression of the transcription factor C/EBPβ in ALK+ALCL, and demonstrated its dependence on NPM-ALK kinase activity. Design and Methods: In this study, the role of C/EBPβ in proliferation and survival of ALK+ALCL was investigated, as well as, the mechanism of its expression and activity. Highly effective shRNA sequences and/or pharmacological inhibitors were used to abrogate C/EBPβ, STAT3, AKT, ERK1/2 and mTOR expression or activity.. Results: Interference with C/EBPβ expression resulted in a dramatic decrease in cell proliferation in ALK+ALCLs, with a mild induction of apoptosis after 6 days. Downregulation of STAT3 resulted in a marked decrease in C/EBPβ 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/EBPβ Thr235 dephosphorylation and moderate growth retardation. The AKT/mTOR signaling pathway did not have any influence on C/EBPβ expression or C/EBPβ phosphorylation. Conclusions: These findings reveal the convergence of STAT3 and ERK1/2 signaling pathways activated by NPM-ALK, in mediating the regulation of C/EBPβ expression, a transcription factor central to NPM-ALK transformation.