Inducible activation of CEBPB, a gene negatively regulated by BCR/ABL, inhibits proliferation and promotes differentiation of BCR/ABL-expressing cells

Abstract

Translational regulation by oncogenic proteins may be a rapid and efficient mechanism to modulate gene expression. We report here the identification of the CEBPB gene as a target of translational regulation in myeloid precursor cells transformed by the BCR/ABL oncogene. Expression of CEBPB was repressed in 32D-BCR/ABL cells and reinduced by imatinib (STI571) via a mechanism that appears to depend on expression of the CUG-repeat RNA-binding protein CUGBP1 and the integrity of the CUG-rich intercistronic region of c/ebp␤ mRNA. Constitutive expression or conditional activation of wildtype CEBPB induced differentiation and inhibited proliferation of 32D-BCR/ABL cells in vitro and in mice, but a DNA binding-deficient CEBPB mutant had no effect. The proliferation-inhibitory effect of CEBPB was, in part, mediated by the CEBPB-induced GADD45A gene. Because IntroductionChronic myelogenous leukemia (CML) is a clonal myeloproliferative disorder arising from neoplastic transformation of the hematopoietic stem cell. 1,2 Its clinical course involves progression from a protracted chronic phase (CML-CP), characterized by accumulation of apparently normal neutrophils, to a rapidly fatal blast crisis (CML-BC), characterized by clonal expansion of differentiationarrested myeloid or lymphoid blasts. CML is consistently associated with a reciprocal translocation of the long arms of chromosomes 9 and 22, 3,4 which generates the BCR/ABL fusion gene, in turn translated in the p210 BCR/ABL oncoprotein in almost all patients. 5 Expression activity of p210 BCR/ABL is necessary and sufficient for hematopoietic cell transformation and disease maintenance as demonstrated by in vitro assays, leukemogenesis in mice, and the antileukemia effects of imatinib, a specific BCR/ABL tyrosine kinase inhibitor. [6][7][8][9] BCR/ABL-dependent transformation of hematopoietic cells involves the assembly of multiprotein complexes and the phosphorylation of various substrates, which is essential to generate proliferative and antiapoptotic signals [10][11][12] and is often accompanied by transcriptional and posttranscriptional changes in gene expression. In regard to the latter mechanism, BCR/ABL can regulate both positively and negatively mRNA translation and protein stability. [13][14][15] We recently identified translation-regulatory mechanisms involving increased expression of RNA-binding proteins that modulate MDM2 and CEBPA levels in BCR/ABL-expressing cells. 16,17 Enhanced expression of MDM2 by increased expression of the RNA-binding protein La reduces the susceptibility of BCR/ABLexpressing cells to apoptosis induced by DNA-damaging agents. 16 Suppression of CEBPA expression by increased expression of the RNA-binding protein hnRNPE2 is important for the differentiation arrest of BCR/ABL-expressing leukemic cells as indicated by the rapid induction of differentiation on reactivation of CEBPA expression or activity. 17,18 Because translational regulation by BCR/ABL might not be limited to MDM2 and C/EBPA, we sought to identify other translation-regula...

Src homology 2 domain–containing inositol-5-phosphatase and CCAAT enhancer-binding protein β are targeted by miR-155 in B cells of Eμ-MiR-155 transgenic mice

“…Western blotting was performed according to previously published protocols. 20,21 Antibody-reactive proteins were detected with horseradish peroxidase-labeled sheep anti-rabbit, -mouse, and/or -goat Ig sera and enhanced chemiluminescence (Amersham). Proteins were analyzed in 4% to 15% SDS-PAGE (Bio-Rad) using reducing conditions.…”

ReSETting PP2A tumour suppressor activity in blast crisis and imatinib-resistant chronic myelogenous leukaemia

“…Bcl-X L , GCSFR, C/EBPa and b, MDM2 and MYC) are aberrantly regulated by increased BCR/ABL expression through the activity of specific mRNA binding proteins (e.g. TLS/FUS, hnRNP A1, hnRNP E2, La, hnRNP K and CUGBP1) (Perrotti et al, 1998;Iervolino et al, 2002;Perrotti et al, 2002;Trotta et al, 2003;Guerzoni et al, 2006;Notari et al, 2006). Specifically, the mRNA export activity of hnRNP A1, an RNAbinding protein overexpressed in CML-BC, is required for cytokine-independent proliferation, survival and tumorigenesis of acute phase CML blasts and BCR/ABL-expressing myeloid progenitor cell lines .…”

Airway epithelial cell differentiation during lung organogenesis requires C/EBPα and C/EBPβ

“…In support of this, the majority of mice with a lung-specific inactivation of C/ EBPa die from respiratory failure at birth, and lungs analyzed just before birth display pulmonary immaturity including disrupted surfactant production (Martis et al, 2006). C/EBPb plays a pivotal role in differentiation of myeloid cells, skin, and adipose tissue (Natsuka et al, 1992;Tanaka et al, 1997;Sterneck et al, 2005;Guerzoni et al, 2006) and in contrast to C/EBPa is thought to promote proliferation (Ramji and Foka, 2002). In addition, C/EBPb is important for the respiratory system by regulating the expression of both inflammatory and lung-specific genes (Cassel et al, 2000a(Cassel et al, , 2002Didon et al, 2011).…”