C/EBPalpha and beta couple interfollicular keratinocyte proliferation arrest to commitment and terminal differentiation.

The transcriptional regulators that couple interfollicular basal keratinocyte proliferation arrest to commitment and differentiation are yet to be identified. Here we report that the basic region leucine zipper transcription factors C/EBPalpha and C/EBPbeta are co-expressed in basal keratinocytes, and are coordinately upregulated as keratinocytes exit the basal layer and undergo terminal differentiation. Mice lacking both C/EBPalpha and beta in the epidermis showed increased proliferation of basal keratinocytes and impaired commitment to differentiation. This led to ectopic expression of keratin 14 (K14) and DeltaNp63 in suprabasal cells, decreased expression of spinous and granular layer proteins, parakeratosis and defective epidermal water barrier function. Knock-in mutagenesis revealed that C/EBP-E2F interaction was required for control of interfollicular epidermis (IFE) keratinocyte proliferation, but not for induction of spinous and granular layer markers, whereas C/EBP DNA binding was required for DeltaNp63 downregulation and K1/K10 induction. Finally, loss of C/EBPalpha/beta induced stem cell gene expression signatures in the epidermis. C/EBPs, therefore, couple basal keratinocyte cell cycle exit to commitment to differentiation through E2F repression and DNA binding, respectively, and may act to restrict the epidermal stem cell compartment.

Modeling of C/EBPalpha mutant acute myeloid leukemia reveals a common expression signature of committed myeloid leukemia-initiating cells.

Mutations in the CEBPA gene are present in 7%-10% of human patients with acute myeloid leukemia (AML). However, no genetic models exist that demonstrate their etiological relevance. To mimic the most common mutations affecting CEBPA-that is, those leading to loss of the 42 kDa C/EBPalpha isoform (p42) while retaining the 30kDa isoform (p30)-we modified the mouse Cebpa locus to express only p30. p30 supported the formation of granulocyte-macrophage progenitors. However, p42 was required for control of myeloid progenitor proliferation, and p42-deficient mice developed AML with complete penetrance. p42-deficient leukemia could be transferred by a Mac1+c-Kit+ population that gave rise only to myeloid cells in recipient mice. Expression profiling of this population against normal Mac1+c-Kit+ progenitors revealed a signature shared with MLL-AF9-transformed AML.

Neurotrophin/Trk receptor signaling mediates C/EBPalpha, -beta and NeuroD recruitment to immediate-early gene promoters in neuronal cells and requires C/EBPs to induce immediate-early gene transcription.

BACKGROUND: Extracellular signaling through receptors for neurotrophins mediates diverse neuronal functions, including survival, migration and differentiation in the central nervous system, but the transcriptional targets and regulators that mediate these diverse neurotrophin functions are not well understood. RESULTS: We have identified the immediate-early (IE) genes Fos, Egr1 and Egr2 as transcriptional targets of brain derived neurotrophic factor (BDNF)/TrkB signaling in primary cortical neurons, and show that the Fos serum response element area responds to BDNF/TrkB in a manner dependent on a combined C/EBP-Ebox element. The Egr1 and Egr2 promoters contain homologous regulatory elements. We found that C/EBPalpha/beta and NeuroD formed complexes in vitro and in vivo, and were recruited to all three homologous promoter regions. C/EBPalpha and NeuroD co-operatively activated the Fos promoter in transfection assays. Genetic depletion of Trk receptors led to impaired recruitment of C/EBPs and NeuroD in vivo, and elimination of Cebpa and Cebpb alleles reduced BDNF induction of Fos, Egr1 and Egr2 in primary neurons. Finally, defective differentiation of cortical dendrites, as measured by MAP2 staining, was observed in both compound Cebp and Ntrk knockout mice. CONCLUSION: We here identify three IE genes as targets for BDNF/TrkB signaling, show that C/EBPalpha and -beta are recruited along with NeuroD to target promoters, and that C/EBPs are essential mediators of Trk signaling in cortical neurons. We show also that C/EBPs and Trks are required for cortical dendrite differentiation, consistent with Trks regulating dendritic differentiation via a C/EBP-dependent mechanism. Finally, this study indicates that BDNF induction of IE genes important for neuronal function depends on transcription factors (C/EBP, NeuroD) up-regulated during neuronal development, thereby coupling the functional competence of the neuronal cells to their differentiation.

Acetylation inactivates the transcriptional repressor BCL6.

The proto-oncogene BCL6 encodes a BTB/POZ-zinc finger transcriptional repressor that is necessary for germinal-center formation and has been implicated in the pathogenesis of B-cell lymphomas. Here we show that the co-activator p300 binds and acetylates BCL6 in vivo and inhibits its function. Acetylation disrupts the ability of BCL6 to recruit histone deacetylases (HDACs), thereby hindering its capacity to repress transcription and to induce cell transformation. BCL6 is acetylated under physiologic conditions in normal germinal-center B cells and in germinal center-derived B-cell tumors. Treatment with specific inhibitors shows that levels of acetylation of BCL6 are controlled by both HDAC-dependent and SIR2-dependent pathways. Pharmacological inhibition of these pathways leads to the accumulation of the inactive acetylated BCL6 and to cell-cycle arrest and apoptosis in B-cell lymphoma cells. These results identify a new mechanism of regulation of the proto-oncogene BCL6 with potential for therapeutic exploitation. Furthermore, these findings provide a new mechanism by which acetylation can promote transcription not only by modifying histones and activating transcriptional activators, but also by inhibiting transcriptional repressors.