CCAAT/enhancer-binding protein homologous protein (CHOP) regulates osteoblast differentiation

Distinct cytoplasmic and nuclear functions of the stress induced protein DDIT3/CHOP/GADD153

DDIT3, also known as GADD153 or CHOP, encodes a basic leucine zipper transcription factor of the dimer forming C/EBP family. DDIT3 is known as a key regulator of cellular stress response, but its target genes and functions are not well characterized. Here, we applied a genome wide microarray based expression analysis to identify DDIT3 target genes and functions. By analyzing cells carrying tamoxifen inducible DDIT3 expression constructs we show distinct gene expression profiles for cells with cytoplasmic and nuclear localized DDIT3. Of 175 target genes identified only 3 were regulated by DDIT3 in both cellular localizations. More than two thirds of the genes were downregulated, supporting a role for DDIT3 as a dominant negative factor that could act by either cytoplasmic or nuclear sequestration of dimer forming transcription factor partners. Functional annotation of target genes showed cell migration, proliferation and apoptosis/survival as the most affected categories. Cytoplasmic DDIT3 affected more migration associated genes, while nuclear DDIT3 regulated more cell cycle controlling genes. Cell culture experiments confirmed that cytoplasmic DDIT3 inhibited migration, while nuclear DDIT3 caused a G1 cell cycle arrest. Promoters of target genes showed no common sequence motifs, reflecting that DDIT3 forms heterodimers with several alternative transcription factors that bind to different motifs. We conclude that expression of cytoplasmic DDIT3 regulated 94 genes. Nuclear translocation of DDIT3 regulated 81 additional genes linked to functions already affected by cytoplasmic DDIT3. Characterization of DDIT3 regulated functions helps understanding its role in stress response and involvement in cancer and degenerative disorders.

Regulation of bone and cartilage development by network between BMP signalling and transcription factors

Bone morphogenetic protein(s) (BMP) are very powerful cytokines that induce bone and cartilage formation. BMP also stimulate osteoblast and chondrocyte differentiation. During bone and cartilage development, BMP regulates the expression and/or the function of several transcription factors through activation of Smad signalling. Genetic studies revealed that Runx2, Osterix and Sox9, all of which function downstream of BMP, play essential roles in bone and/or cartilage development. In addition, two other transcription factors, Msx2 and Dlx5, which interact with BMP signalling, are involved in bone and cartilage development. The importance of these transcription factors in bone and cartilage development has been supported by biochemical and cell biological studies. Interestingly, BMP is regulated by several negative feedback systems that appear necessary for fine-tuning of bone and cartilage development induced by BMP. Thus, BMP harmoniously regulates bone and cartilage development by forming network with several transcription factors.

C/EBPβ and RUNX2 cooperate to degrade cartilage with MMP-13 as the target and HIF-2α as the inducer in chondrocytes

To elucidate the molecular mechanism underlying the endochondral ossification process during the skeletal growth and osteoarthritis (OA) development, we examined the signal network around CCAAT/enhancer-binding protein-β (C/EBPβ, encoded by CEBPB), a potent regulator of this process. Computational predictions and a C/EBP motif-reporter assay identified RUNX2 as the most potent transcriptional partner of C/EBPβ in chondrocytes. C/EBPβ and RUNX2 were induced and co-localized in highly differentiated chondrocytes during the skeletal growth and OA development of mice and humans. The compound knockout of Cebpb and Runx2 in mice caused growth retardation and resistance to OA with decreases in cartilage degradation and matrix metalloproteinase-13 (Mmp-13) expression. C/EBPβ and RUNX2 cooperatively enhanced promoter activity of MMP13 through specific binding to a C/EBP-binding motif and an osteoblast-specific cis-acting element 2 motif as a protein complex. Human genetic studies failed to show the association of human CEBPB gene polymorphisms with knee OA, nor was there a genetic variation around the identified responsive region in the human MMP13 promoter. However, hypoxia-inducible factor-2α (HIF-2α), a functional and genetic regulator of knee OA through promoting endochondral ossification, was identified as a potent and functional inducer of C/EBPβ expression in chondrocytes by the CEBPB promoter assay. Hence, C/EBPβ and RUNX2, with MMP-13 as the target and HIF-2α as the inducer, control cartilage degradation. This molecular network in chondrocytes may represent a therapeutic target for OA.

C/EBPβ binds the P1 promoter of the Runx2 gene and up-regulates Runx2 transcription in osteoblastic cells

The Runx2 factor is an essential component of the regulatory mechanisms that control transcription during skeletogenesis. Runx2/p57 expression in osteoblastic cells is controlled by the P1 promoter, which is recognized by key regulators of osteoblast differentiation including homeodomain factors and Wnt- and BMP-signaling mediators. Here, we report that the transcription factor C/EBPβ up-regulates Runx2/p57 expression by directly binding to the Runx2 P1 promoter in mesenchymal, pre-osteoblastic, and osteoblastic cells. This C/EBPβ-mediated up-regulation is principally dependent on C/EBP site II that is located within the first 180 bp of the proximal P1 promoter region and is highly conserved among mouse, rat, and human Runx2 genes. Our studies reveal how the C/EBPβ factor, known to have a key role during osteogenesis, contributes to regulating the expression of Runx2, the master regulator of osteoblast differentiation.

CHOP-independent apoptosis and pathway-selective induction of the UPR in developing plasma cells

Upon antigen stimulation, B lymphocytes differentiate into antibody secreting cells (ASC), most of which undergo apoptosis after a few days of intense Ig production. Differentiation entails expansion of the endoplasmic reticulum (ER) and requires XBP1 but not other elements of the unfolded protein response, like PERK. Moreover, normal and malignant ASC are exquisitely sensitive to proteasome inhibitors, but the underlying mechanisms are poorly understood. Here we analyze the role of C/EBP homologous protein (CHOP), a transcription factor mediating apoptosis in many cell types that experience high levels of ER stress. CHOP is transiently induced early upon B cell stimulation: covalent IgM aggregates form more readily and IgM secretion is slower in chop(-/-) cells. Despite these subtle changes, ASC differentiation and lifespan are normal in chop(-/-) mice. Unlike fibroblasts and other cell types, chop(-/-) ASC are equally or slightly more sensitive to proteasome inhibitors and ER stressors, implying tissue-specific roles for CHOP in differentiation and stress.

The flavoheme reductase Ncb5or protects cells against endoplasmic reticulum stress-induced lipotoxicity

NCB5OR is a novel flavoheme reductase with a cytochrome b5-like domain at the N-terminus and a cytochrome b5 reductase-like domain at the C terminus. Ncb5or knock-out mice develop insulin deficient diabetes and loss of white adipose tissue. Ncb5or(-/-) mice have impairment of Delta9 fatty acid desaturation with elevated ratios of palmitate to palmitoleate and stearate to oleate. In this study we assess the role of the endoplasmic reticulum (ER) stress response in mediating lipotoxicity in Ncb5or(-/-) mice. The ER stress response was assessed by induction of BiP, ATF3, ATF6, XBP-1, and C/EBP homologous protein (CHOP). Exposure to palmitate, but not oleate or mixtures of oleate and palmitate induced these markers of ER stress to a much greater extent in Ncb5or(-/-) hepatocytes than in wild-type cells. In contrast, Ncb5or(-/-) and Ncb5or(+/+) hepatocytes were equally sensitive to ER stress imposed by increasing concentrations of tunicamycin. In order to assess the role of ER stress in vivo, we prepared mice that lack both NCB5OR and CHOP, a proapoptotic transcription factor important in the ER stress response. Onset of hyperglycemia in the Chop(-/-);Ncb5or(-/-) mice was delayed two weeks beyond that observed in Chop(+/+);Ncb5or(-/-) mice. Taken together these results suggest that ER stress plays a critical role in palmitate-induced lipotoxicity both in vitro and in vivo.

Molecular mechanisms of the LPS-induced non-apoptotic ER stress-CHOP pathway

The expression of C/EBP homologous protein (CHOP), which is an endoplasmic reticulum (ER) stress-induced transcription factor, induces apoptosis. Our previous study demonstrated that lipopolysaccharide (LPS)-induced CHOP expression does not induce apoptosis, but activates a pro-IL-1beta activation process. However, the mechanism by which CHOP activates different pathways, depending on the difference in the inducing stimuli, remains to be clarified. The present study shows that LPS rapidly activates the ER function-protective pathway, but not the PERK pathway in macrophages. PERK plays a major role in CHOP induction, and other ER stress sensors-mediated pathways play minor roles. The induction of CHOP by LPS was delayed and weak, in comparison with CHOP induction by ER stress-inducer thapsigargin. In addition, LPS-pre-treatment or overexpression of ER chaperone, IgH chain binding protein (BiP), prevented ER stress-mediated apoptosis. LPS plus IFN-gamma-treated macrophages produce a larger amount of nitric oxide (NO) in comparison with LPS-treated cells. Treatment with the NO donor, SNAP (S-nitro-N-acetyl-dl-penicillamine), induces CHOP at an earlier period than LPS treatment. The depletion of NO retards CHOP induction and prevents apoptosis in LPS plus IFN-gamma-treated cells. We concluded that apoptosis is prevented in LPS-treated macrophages, because the ER function-protective mechanisms are induced before CHOP expression, and induction level of CHOP is low.

Transcription factor C/EBPbeta isoform ratio regulates osteoclastogenesis through MafB

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancer-binding protein β (C/EBPβ) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP^*, LAP) and a short repressive (LIP) isoforms from a single C/EBPβ transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBPβ null (c/ebpβ^−/−) or in LIP knock-in (L/L) osteoclast precursors. C/EBPβ mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBPβ isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression.

Misexpression of CCAAT/enhancer binding protein beta causes osteopenia

CCAAT/enhancer binding proteins (C/EBPs) are expressed by osteoblasts and adipocytes during differentiation. C/EBP beta is critical for adipogenesis; however, its role in osteoblastogenesis is unclear, and its function in the postnatal skeleton is not known. To study C/EBP beta in osteoblasts in vivo, we created transgenic mice expressing full length C/EBP beta under the control of a 3.8 kb fragment of the human osteocalcin promoter. Two transgenic lines were established in a friend leukemia virus strain B genetic background, and compared with wild type littermate controls. Both C/EBP beta transgenic lines exhibited osteopenia, with a 30% decrease in bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. Bone marrow stromal cells from C/EBP beta transgenics showed reduced mineralization, and reduced alkaline phosphatase mRNA levels. Calvarial osteoblasts from C/EBP beta transgenics displayed reduced alkaline phosphatase activity. To determine the consequences of the Cebpb deletion in vivo, the phenotype of Cebpb null mice was compared with that of wild type controls of identical genetic composition. Cebpb null mice exhibited reduced weight, body fat, and bone mineral density, and decreased bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. C/EBP beta downregulation by RNA interference in calvarial osteoblasts had no effect on osteoblast differentiation/function. The phenotype of the Cebpb inactivation may be secondary to systemic indirect effects, and to direct effects of C/EBP beta in osteoblasts. In conclusion, C/EBP beta plays a role in mesenchymal cell differentiation and its misexpression in vivo causes osteopenia.

Expression of osterix inhibits bone morphogenetic protein-induced chondrogenic differentiation of mesenchymal progenitor cells

Osteoblasts and chondrocytes arise from common bipotential mesenchymal progenitor cells. Although the differentiation of these two cell lineages can be induced by treatment with bone morphogenetic proteins (BMPs), the responses of mesenchymal progenitors to BMP differ from cell line to cell line. Here we demonstrate that C3H/10T1/2 cells preferred chondrogenic differentiation, primary bone marrow stroma cells (MSCs) tended to convert to osteoblasts, and ST-2 cells differentiated into both the osteoblastic and chondrocytic lineages simultaneously, suggesting that a molecular switch functions to select cell fate. Osterix, the secondary master regulator of osteoblastogenesis, was induced by BMP at high and low levels in MSCs and ST-2 cells, respectively; in contrast, C3H/10T1/2 cells demonstrated only faint expression. As osterix has been suggested as a negative regulator of chondrogenesis, we hypothesized that the intense chondrocyte differentiation of C3H/10T1/2 cells may have resulted from an absence of osterix. We therefore restored osterix gene expression in C3H/10T1/2 cells using an adenovirus vector. Following BMP treatment, infection with an osterix-encoding virus dramatically inhibited the chondrocytic differentiation of C3H/10T1/2 cells, resulting instead in prominent osteoblast differentiation. These results indicate the chondrogenic potential of C3H/10T1/2 cells was abrogated by osterix expression. Chondrocyte differentiation of MSCs, however, was not enhanced by silencing the osterix gene using lentivirus-mediated shRNA, despite successful suppression of osteoblast differentiation. These results suggest that the low levels of osterix expression remaining after knockdown are sufficient to block chondrogenesis, whereas higher expression may be required to promote osteoblastic differentiation.

Arkadia represses the expression of myoblast differentiation markers through degradation of Ski and the Ski-bound Smad complex in C2C12 myoblasts

The differentiation of myoblasts is regulated by multiple extracellular and intracellular factors. Of the extracellular regulators, members of transforming growth factor-beta (TGF-beta) family play critical roles in the regulation of osteoblasts and myoblast differentiation. Little is known, however, about the regulation of Myostatin/TGF-beta signaling during myoblast differentiation. In this study, we examined the roles of Arkadia, an E3 ubiquitin ligase, in Myostatin/TGF-beta signaling and the regulation of myoblast differentiation. Knockdown of Arkadia reduced Myostatin/TGF-beta signaling and enhanced the differentiation of C2C12 myoblasts. In addition, exogenous overexpression of Arkadia enhanced Myostatin/TGF-beta signaling, preventing myoblast differentiation. In the absence of the activation of Myostatin/TGF-beta signaling, knockdown of Arkadia enhanced myoblast differentiation via upregulation of Ski protein, an intracellular enhancer of myoblast differentiation. Arkadia likely affected the differentiation of myoblasts in a Smad-independent fashion by inducing Ski degradation. Knockdown of Arkadia increased the Myostatin-induced phosphorylation of Smad2/3 in C2C12 cells. Arkadia bound Smad2/3 via Ski to induce the ubiquitination of Smad2/3. These results suggest that Arkadia targets Ski-bound, inactive phospho-Smad2/3 to regulate positively Myostatin/TGF-beta signaling. Taken together, this study indicates that Arkadia regulates myoblast differentiation through both Smad-dependent and Smad-independent pathways.

Cardiotrophin-1 is an osteoclast-derived stimulus of bone formation required for normal bone remodeling

Cardiotrophin (CT-1) signals through gp130 and the LIF receptor (LIFR) and plays a major role in cardiac, neurological, and liver biology. We report here that CT-1 is also expressed within bone in osteoclasts and that CT-1 is capable of increasing osteoblast activity and mineralization both in vitro and in vivo. Furthermore, CT-1 stimulated CAAT/enhancer-binding protein-delta (C/EBP delta) expression and runt-related transcription factor 2 (runx2) activation. In neonate CT-1(-/-) mice, we detected low bone mass associated with reduced osteoblasts and many large osteoclasts, but increased cartilage remnants within the bone, suggesting impaired resorption. Cultured bone marrow (BM) from CT-1(-/-) mice generated many oversized osteoclasts and mineralized poorly compared with wildtype BM. As the CT-1(-/-) mice aged, the reduced osteoblast surface (ObS/BS) was no longer detected, but impaired bone resorption continued resulting in an osteopetrotic phenotype in adult bone. CT-1 may now be classed as an essential osteoclast-derived stimulus of both bone formation and resorption.

Zhangfei, a novel regulator of the human nerve growth factor receptor, trkA

The replication of herpes simplex virus (HSV) in epithelial cells, and during reactivation from latency in sensory neurons, depends on a ubiquitous cellular protein called host cell factor (HCF). The HSV transactivator, VP16, which initiates the viral replicative cycle, binds HCF as do some other cellular proteins. Of these, the neuronal transcription factor Zhangfei suppresses the ability of VP16 to initiate the replicative cycle. It also suppresses Luman, another cellular transcription factor that binds HCF. Interactions of nerve growth factor (NGF) and its receptor tropomyosin-related kinase (trkA) appear to be critical for maintaining HSV latency. Because the neuronal transcription factor Brn3a, which regulates trkA expression, has a motif for binding HCF, we investigated if Zhangfei had an effect on its activity. We found that Brn3a required HCF for activating the trkA promoter and Zhangfei suppressed its activity in non-neuronal cells. However, in neuron-like NGF-differentiated PC12 cells, both Brn3a and Zhangfei activated the trkA promoter and induced the expression of endogenous trkA. In addition, capsaicin, a stressor, which activates HSV in in vitro models of latency, decreased levels of Zhangfei and trkA transcripts in NGF-differentiated PC12 cells.

CCAAT/enhancer-binding protein beta promotes osteoblast differentiation by enhancing Runx2 activity with ATF4

Although CCAAT/enhancer-binding protein beta (C/EBPbeta) is involved in osteocalcin gene expression in osteoblast in vitro, the physiological importance of and molecular mechanisms governing C/EBPbeta in bone formation remain to be elucidated. In particular, it remains unclear whether C/EBPbeta acts as a homodimer or a heterodimer with other proteins during osteoblast differentiation. Here, deletion of the C/EBPbeta gene from mice resulted in delayed bone formation with concurrent suppression of chondrocyte maturation and osteoblast differentiation. The expression of type X collagen as well as chondrocyte hypertrophy were suppressed in mutant bone, providing new insight into the possible roles of C/EBPbeta in chondrocyte maturation. In osteoblasts, luciferase reporter, gel shift, DNAP, and ChIP assays demonstrated that C/EBPbeta heterodimerized with activating transcription factor 4 (ATF4), another basic leucine zipper transcription factor crucial for osteoblast maturation. This complex interacted and transactivated osteocalcin-specific element 1 (OSE1) of the osteocalcin promoter. C/EBPbeta also enhanced the synergistic effect of ATF4 and Runx2 on osteocalcin promoter transactivation by enhancing their interaction. Thus, our results provide evidence that C/EBPbeta is a crucial cofactor in the promotion of osteoblast maturation by Runx2 and ATF4.

Regulation of interleukin-8 gene at a distinct site of its promoter by CCAAT enhancer-binding protein homologous protein in prostaglandin E2-treated human T cells

For a long period of time, the transcription factor CCAAT/enhancer-binding protein homologous protein (CHOP) has been thought to inhibit transcriptional activity for its ability to interact with CCAAT enhancer-binding protein family factors, thus preventing their binding to DNA. We have previously shown that in human T lymphocytes the CHOP phosphorylation induced by prostaglandin E(2) (PGE(2))-increased interleukin-8 (IL-8) gene expression. Given the CHOP positive role in the regulation of transcription, here we have investigated the molecular mechanism(s) by which CHOP increases IL-8 gene activity under PGE(2) stimulus. Transfection experiments with mutants showed both that the CHOP transactivation domain is essential for IL-8 transcription and that the IL-8/activator protein 1 (AP-1) promoter mutated in NF-kappaB and NF-IL-6, but not in the AP-1 site, harbors essential CHOP-responsive elements. CHOP silencing confirmed its role in the IL-8 transcriptional regulation and protein production, whereas c-Jun small interfering RNA experiments showed that the PGE(2)-induced activation of IL-8 promoter is mainly c-Jun-independent. Moreover, PGE(2) induced CHOP-DNA complexes only when the entire IL-8/AP-1 promoter or the wild type sequences encompassing the AP-1 upstream region were employed. Mutations introduced in these sequences prevented the DNA-CHOP complex formation. The IL-8/AP-1 mutant promoter lacking the sequence immediately upstream the AP-1 site is PGE(2)-unresponsive. Finally, chromatin immunoprecipitation data confirmed in vivo that PGE(2) induces CHOP binding to the IL-8 promoter. Taken together, our results suggest that the increased expression of CHOP in response to PGE(2) exerts a positive transcriptional regulation of the IL-8 promoter mediated by direct binding to a novel consensus site.

Doubly truncated FosB isoform (Delta2DeltaFosB) induces osteosclerosis in transgenic mice and modulates expression and phosphorylation of Smads in osteoblasts independent of intrinsic AP-1 activity

INTRODUCTION

Activator protein (AP)-1 family members play important roles in the development and maintenance of the adult skeleton. Transgenic mice that overexpress the naturally occurring DeltaFosB splice variant of FosB develop severe osteosclerosis. Translation of Deltafosb mRNA produces both DeltaFosB and a further truncated isoform (Delta2DeltaFosB) that lacks known transactivation domains but, like DeltaFosB, induces increased expression of osteoblast marker genes.

MATERIALS AND METHODS

To test Delta2DeltaFosB's ability to induce bone formation in vivo, we generated transgenic mice that overexpress only Delta2DeltaFosB using the enolase 2 (ENO2) promoter-driven bitransgenic Tet-Off system.

RESULTS

Despite Delta2DeltaFosB's failure to induce transcription of an AP-1 reporter gene, the transgenic mice exhibited both the bone and the fat phenotypes seen in the ENO2-DeltaFosB mice. Both DeltaFosB and Delta2DeltaFosB activated the BMP-responsive Xvent-luc reporter gene and increased Smad1 expression. Delta2DeltaFosB enhanced BMP-induced Smad1 phosphorylation and the translocation of phospho-Smad1 (pSmad1) to the nucleus more efficiently than DeltaFosB and showed a reduced induction of inhibitory Smad6 expression.

CONCLUSIONS

DeltaFosB's AP-1 transactivating function is not needed to induce increased bone formation, and Delta2DeltaFosB may act, at least in part, by increasing Smad1 expression, phosphorylation, and translocation to the nucleus.

Pitx2 prevents osteoblastic transdifferentiation of myoblasts by bone morphogenetic proteins

Muscle cells are often exposed to bone morphogenetic proteins (BMPs) in pathological muscle and/or bone conditions. Because BMPs function as strong bone inducers as well as myogenesis inhibitors, certain molecules likely prevent muscle cells from converting into pathologic bone; without these molecules, de novo bone would form as observed in myositis ossificans traumatica. When C2C12 myoblasts are exposed to BMPs, they differentiate into osteoblastic cells but cannot mature into bone cells. As the Osterix gene, a transcription factor for osteoblast differentiation, is only transiently induced upon BMP stimulation in C2C12 cells, we hypothesized that unknown transcriptional repressor(s) inhibit Osterix expression and prevent complete osteoblastic differentiation. Gene microarray analyses were performed to identify putative inhibitors for osteoblastic differentiation, and the paired-like homeodomain transcription factor Pitx2 (also termed Rieg), which plays an important regulatory role in left-right asymmetry, was identified. Pitx2 was induced 2 days after BMP stimulation in C2C12 cells in concert with Osterix down-regulation. Overexpression of Pitx2 repressed Osterix expression and subsequent osteoblastic differentiation, whereas Runx2, the most upstream regulator of osteogenesis, was unaffected. Conversely, the induction of short hairpin RNA for Pitx2 in C2C12 cells enhanced Osterix expression and osteoblastic maturation upon BMP stimulation. Moreover, mouse embryonic fibroblasts containing myoblasts from Pitx2-null embryos showed enhanced Osterix expression upon BMP stimulation. These findings suggest that Pitx2 suppresses osteogenic signals induced by BMPs in myoblasts to prevent their osteoblastic conversion.

Modulator recognition factor-2 is required for adipogenesis in mouse embryo fibroblasts and 3T3-L1 cells

Previous study showed that mice lacking modulator recognition factor-2 (Mrf-2) were lean, with significant decreases in white adipose tissue. One postulated mechanism for the lean phenotype in Mrf-2 knockout mice is a defect in adipogenesis. In order to investigate this further, we examined the effects of Mrf-2 deficiency on adipogenesis in vitro. In mouse fibroblasts (MEFs) derived from Mrf-2(-/-) embryos, and in 3T3-L1 cells after knockdown of Mrf-2 by small interference RNA (siRNA) there was a potent inhibition of hormone-induced lipid accumulation, and significant decreases in the expression of the adipogenic transcription factors CCAAT/enhancer-binding protein (C/EBP) alpha and peroxisome proliferator-activated receptor-gamma and the mature adipocyte genes they control. Transduction of Mrf-2(-/-) MEFs with a retroviral vector expressing the longer Mrf-2 splice variant (Mrf-2B) stimulated both gene expression and lipid accumulation. Because 3T3-L1 cells are committed to the adipocyte lineage, we used this simpler model system to examine the effects of Mrf-2 deficiency on adipocyte maturation. Analyses of both mRNA and protein revealed that knockdown of Mrf-2 in 3T3-L1 cells prolonged the expression of C/EBP homologous protein-10, a dominant-negative form of C/EBP. Consistent with these findings, suppression of Mrf-2 also inhibited the DNA-binding activity of C/EBPbeta. These data suggest that Mrf-2 facilitates the induction of the two key adipogenic transcription factors C/EBPalpha and peroxisome proliferator-activated receptor-gamma indirectly by permitting hormone-mediated repression of the adipogenic repressor C/EBP homologous protein-10.

Differential regulation of CHOP-10/GADD153 gene expression by MAPK signaling in pancreatic beta-cells

CHOP-10 (GADD153/DDIT-3) is a bZIP protein involved in differentiation and apoptosis. Its expression is induced in response to stresses such as nutrient deprivation, perturbation of the endoplasmic reticulum, redox imbalance, and UV exposure. Here we show that CHOP expression is induced in cultured pancreatic beta-cells maintained in a basal glucose concentration of 5.5 mM and repressed by stimulatory glucose (>or=11 mM). Both induction and repression of CHOP are dependent on the MAPKs ERK1 and ERK2. Two regulatory composite sites containing overlapping MafA response elements (MARE) and CAAT enhancer binding (CEB) elements regulate transcription in an ERK1/2-dependent manner. One site (MARE-CEB), from -320 to -300 bp in the promoter, represses transcription. The other site (CEB-MARE), from +2,628 to +2,641 bp in the first intron of the CHOP gene, activates it. MafA can influence transcription of both sites. The MARE-CEB is repressed by MafA, whereas the CEB-MARE site, which is homologous to the A2C1 component of the glucose-sensitive RIPE3b region of the insulin gene promoter, is activated by MafA. These results indicate that ERK1/2 have dual roles in regulating CHOP gene expression via both promoter and intronic regions, depending on environmental and metabolic stresses imposed on pancreatic beta-cells.

Stress to endoplasmic reticulum of mouse osteoblasts induces apoptosis and transcriptional activation for bone remodeling

ATF4 is an essential regulator in osteogenesis as well as in stress responses to the endoplasmic reticulum (ER). We addressed a question: Does ER stress to osteoblasts upregulate ATF4 expression? If so, do they exhibit ATF4-mediated bone remodeling or apoptosis? ER stress, induced by Thapsigargin and tunicamycin, elevated a phosphorylated form of eIF2alpha and ATF4, but the cellular fate depended on treatment duration. The treatment for 1h, for instance, activated Runx2, and type I collagen, while the treatment for 24h induced apoptosis. Our observations suggest that there is a threshold for ER stress and osteoblasts present a bi-phasic pattern of their fate.

C/EBPbeta regulates human immunodeficiency virus 1 gene expression through its association with cdk9

Transcriptional regulation of the human immunodeficiency virus type 1 (HIV-1) is a complex event that requires the cooperative action of both viral (e.g. Tat) and cellular (e.g. C/EBPbeta, NF-kappaB) factors. The HIV-1 Tat protein recruits the human positive transcription elongation factor P-TEFb, consisting of cdk9 and cyclin T1, to the HIV-1 transactivation response (TAR) region. In the absence of TAR, Tat activates the HIV-1 long terminal repeat (LTR) through its association with several cellular factors including C/EBPbeta. C/EBPbeta is a member of the CCAAT/enhancer-binding protein family of transcription factors and has been shown to be a critical transcriptional regulator of HIV-1 LTR. We examined whether Tat-C/EBPbeta association requires the presence of the P-TEFb complex. Using immunoprecipitation followed by Western blot, we demonstrated that C/EBPbeta-cyclin T1 association requires the presence of cdk9. Further, due to its instability, cdk9 was unable to physically interact with C/EBPbeta in the absence of cyclin T1 or Tat. Using kinase assays, we demonstrated that cdk9, but not a cdk9 dominant-negative mutant (cdk9-dn), phosphorylates C/EBPbeta. Our functional data show that co-transfection of C/EBPbeta and cdk9 leads to an increase in HIV-1 gene expression when compared to C/EBPbeta alone. Addition of C/EBP homologous protein (CHOP) inhibits C/EBPbeta transcriptional activity in the presence and absence of cdk9 and causes a delay in HIV-1 replication in T-cells. Together, our data suggest that Tat-C/EBPbeta association is mediated through cdk9, and that phosphorylated C/EBPbeta may influence AIDS progression by increasing expression of HIV-1 genes.