Chromatin Remodeling and Transcriptional Activity of the Bone-specific Osteocalcin Gene Require CCAAT/Enhancer-binding Protein β-dependent Recruitment of SWI/SNF Activity

Abstract

Tissue-specific activation of the osteocalcin (OC) gene is associated with changes in chromatin structure at the promoter region. Two nuclease-hypersensitive sites span the key regulatory elements that control basal tissue-specific and vitamin D 3 -enhanced OC gene transcription. To gain understanding of the molecular mechanisms involved in chromatin remodeling of the OC gene, we have examined the requirement for SWI/SNF activity. We inducibly expressed an ATPase-defective BRG1 catalytic subunit that forms inactive SWI/SNF complexes that bind to the OC promoter. This interaction results in inhibition of both basal and vitamin D 3 -enhanced OC gene transcription and a marked decrease in nuclease hypersensitivity. We find that SWI/SNF is recruited to the OC promoter via the transcription factor CCAAT/enhancer-binding protein ␤, which together with Runx2 forms a stable complex to facilitate RNA polymerase II binding and activation of OC gene transcription. Together, our results indicate that the SWI/SNF complex is a key regulator of the chromatin-remodeling events that promote tissue-specific transcription in osteoblasts.Within the eukaryotic nucleus, the packaging of DNA into nucleosomes and higher order chromatin structures have been implicated in the regulation of key cellular events, such as replication and transcription. During the last decade, a large family of protein complexes that promote transcription by altering chromatin structure have been described (1-3). Among them is the SWI/SNF complex subfamily that remodels chromatin in an ATP-dependent manner (1-3). SWI/SNF complexes are composed of several subunits, which have been implicated in a wide range of cellular events, including gene regulation, cell cycle control, development, and differentiation (1, 3). The mammalian SWI/SNF complexes contain a catalytic subunit that can be either BRG1 or BRM, which includes ATPase activity. Mutations in the ATPase domain of BRG1 or BRM that abrogate the ability of these proteins to bind ATP result in the formation of inactive SWI/SNF complexes (4 -6). Furthermore, expression of mutant BRG1 or BRM proteins in NIH3T3 cells impairs the ability of these cells to activate endogenous stress response genes in the presence of arsenite (5) and to differentiate into muscle or adipocytic cells (4, 5, 7). In addition, we have recently shown that the presence of the mutant BRG1 protein in these NIH3T3 cell lines inhibits BMP2-induced differentiation into the osteoblast lineage (8).The rat osteocalcin (OC) 3 gene encodes a 10-kDa bone-specific protein that is expressed at late stages of osteoblast differentiation, concomitant with the mineralization of the extracellular matrix (9). Osteoblast-specific transcription of the OC gene is controlled by modularly organized basal and hormoneresponsive elements located within two DNase I-hypersensitive sites (distal site Ϫ605 to Ϫ400 and proximal site Ϫ170 to Ϫ70; see Fig. 1) that are present only in osteoblastic cells expressing this gene (10). Thus, chromatin remodeling of the OC gene p...

p38 Regulates Expression of Osteoblast-specific Genes by Phosphorylation of Osterix

“…Interestingly, BMP-2 has been shown to increase binding of Osx to Ibsp or osteocalcin promoters with a concomitant decrease of NO66 binding and activity (15). Similarly, BMPs increase the expression and function of the ATPdependent chromatin-remodeling complex SWI/SNF, which has been shown to be absolutely essential for osteogenesis, especially the complexes containing Brg1 as a catalytic core (53)(54)(55). Our study demonstrates that Osx is able to increase recruitment of p300 and Brg1 to the promoters of its target genes Fmod and Ibsp in vivo and that Osx directly associates to these cofactors through protein-protein interactions that are further enhanced by BMP signaling.…”

High glucose-induced increased expression of endothelin-1 in human endothelial cells is mediated by activated CCAAT/enhancer-binding proteins

“…It is noteworthy that in addition to the canonical interactions, several mechanisms of C/EBP-induced gene transcription have been demonstrated including interplays with other transcription factors such as (NF-kB, AP-1, Sp1, CREB) [37][38][39][40] or recruitment of various transcriptional co-regulators that affect chromatin conformation, a condition that untimely facilitates the trapping of basal transcription factors. [41,42] Based on that fact that NF-kB and AP-1 are direct regulators of ET-1 transcription, the existence of non-canonical associations among transcriptional complexes formed by heterodimers consisting of C/EBP and other transcription factors that might or might not contain the basicleucine zipper domain [18] should be considered as well.…”

Novel Mechanism of Negative Regulation of 1,25-Dihydroxyvitamin D3-induced 25-Hydroxyvitamin D3 24-Hydroxylase (Cyp24a1) Transcription

“…BRG1 has been shown to direct different cellular processes by recruitment to cis elements through divergent transcription factors including hormone receptors, the erythroid factors erythroid Krüppel-like factor and GATA-1 (which activate the ␤-globin gene) and C/EBP␤ (16,17,42,43). Similar to our findings related to the regulation of Cyp24a1 transcription, C/EBP␤ has been reported to recruit and cooperate with BRG1 to regulate the expression of myeloid genes (15), the osteocalcin gene in osteoblastic cells (16), and the mammary tissue-specific ␤-and ␥-casein genes (17).…”

“…In additional studies in COS-7 cells, suboptimal 1,25(OH) 2 Previous studies from our laboratory showed that C/EBP␤ is induced by 1,25(OH) 2 D 3 in kidney and osteoblastic cells and is a potent enhancer of VDR-mediated Cyp24a1 transcription (26). C/EBP␤ has also been reported to recruit the SWI/SNF complex to regulate cell type-specific genes (15)(16)(17). Hence, we next investigated the role of BRG1 in the C/EBP␤ enhancement of 1,25(OH) 2 D 3 -induced Cyp24a1 transcription.…”

“…BRG1-null mice are embryonic lethal, whereas Brm-null mice are viable (exhibiting a mild phenotype of increased body weight) (13,14). C/EBP␤ has been reported to recruit the SWI/SNF complex to specific gene promoters to promote tissue-specific transcription (15)(16)(17). In addition, it has been shown that ATP-dependent chromatin remodeling complexes can act together with histone-modifying enzymes to modulate transcription (18 -20).…”