Grandy, Rodrigo A.

ABSTRACTThe chromatin remodeling complex SWI/SNF and the transcription factor C/EBPβ play critical roles in osteoblastic cells as they jointly control transcription of a number of bone‐related target genes. The largest C/EBPβ isoform, LAP*, possesses a short additional N‐terminal domain that has been proposed to mediate the interaction of this factor with SWI/SNF in myeloid cells. Here we examine the requirement of a functional N‐terminus in C/EBPβ‐LAP* for binding SWI/SNF and for recruiting this complex to the Ric‐8B gene to mediate transcriptional repression. We find that both C/EBPβ‐LAP* and SWI/SNF simultaneously bind to the Ric‐8B promoter in differentiating osteoblasts that repress Ric‐8B expression. This decreased expression of Ric‐8B is not accompanied by significant changes in histone acetylation at the Ric‐8B gene promoter sequence. A single aminoacid change at the C/EBPβ‐LAP* N‐terminus (R3L) that inhibits C/EBPβ‐LAP*‐SWI/SNF interaction, also prevents SWI/SNF recruitment to the Ric‐8B promoter as well as C/EBPβ‐LAP*‐dependent repression of the Ric‐8B gene. Inducible expression of the C/EBPβ‐LAP*R3L protein in stably transfected osteoblastic cells demonstrates that this mutant protein binds to C/EBPβ‐LAP*‐target promoters and competes with the endogenous C/EBPβ factor. Together our results indicate that a functional N‐terminus in C/EBPβ‐LAP* is required for interacting with SWI/SNF and for Ric‐8B gene repression in osteoblasts. J. Cell. Physiol. 229: 1521–1528, 2014. © 2014 Wiley Periodicals, Inc.

Abstract The regulatory information for phenotype, proliferation, and growth of normal and tumor cells must be maintained through genome replication in the S phase and cell division during mitosis. Epigenetic mechanisms that include DNA methylation, posttranslational modifications of histones, selective utilization of histone variants, and inheritable RNA molecules play pivotal roles in maintaining cellular identity through mitotic divisions. Recent studies demonstrate that mitotic occupancy of genes, which are determinants of cell fate, growth, and proliferation, by lineage-restricted transcription factors is a key epigenetic mechanism for retention and transmission of cellular expression memory. Evidence is emerging for the presence of distinct transcriptional regulatory microenvironments in mitotic chromosomes in which the genes bookmarked for reactivation postmitotically reside. Importantly, some oncoproteins are present in mitotic microenvironments where they occupy target genes during mitosis and may contribute to perpetuating the transformed phenotype. We discuss emerging regulatory implications of epigenetically bookmarking genes during mitosis for physiologic control as well as for the onset and progression of cancer. Cancer Res; 74(2); 420–5. ©2014 AACR.