Adipocyte differentiation selectively represses the serum inducibility of c-jun and junB by reversible transcription-dependent mechanisms

H3K27ac modification and transcription characteristics of adipose and muscle tissues in Chuxiang Black pig

The establishment of high-quality pork breeds for improving meat quality in the pig industry is needed. The Chuxiang Black (CX) pig is a new breed developed from Chinese local pigs and Western lean pigs that has a high proportion of lean meat and excellent meat quality. However, the characteristics of cis-regulatory elements in CX pigs are still unknown. In this study, cis-regulatory elements of muscle and adipose tissues in CX pigs were investigated using ChIP-seq and RNA sequencing. Compared with the reported cis-regulatory elements of muscle and adipose tissues, 1768 and 1012 highly activated enhancers and 433 and 275 highly activated promoters in CX muscle and adipose tissues were identified, respectively. Motif analysis showed that transcription factors, such as MEF2A and MEF2C, were core regulators of highly activated enhancers and promoters in muscle. Similarly, the transcription factors JUNB and CUX1 were identified as essential for highly activated enhancers and promoters in CX adipose tissue. These results enrich the resources for the analysis of cis-regulatory elements in the pig genome and provide new basic data for further meat quality improvement through breeding in pigs.

Identification of SCARA3 with potential roles in metabolic disorders

Obesity is characterized by the expansion of adipose tissue which is partially modulated by adipogenesis. In the present study, we identified five differentially expressed genes by incorporating two adipogenesis-related datasets from the GEO database and their correlation with adipogenic markers. However, the role of scavenger receptor class A member 3 (SCARA3) in obesity-related disorders has been rarely reported. We found that Scara3 expression in old adipose tissue-derived mesenchymal stem cells (Ad-MSCs) was lower than it in young Ad-MSCs. Obese mice caused by deletion of the leptin receptor gene (db/db) or by a high-fat diet both showed reduced Scara3 expression in inguinal white adipose tissue. Moreover, hypermethylation of SCARA3 was observed in patients with type 2 diabetes and atherosclerosis. Data from the CTD database indicated that SCARA3 is a potential target for metabolic diseases. Mechanistically, JUN was predicted as a transcriptional factor of SCARA3 in different databases which is consistent with our further bioinformatics analysis. Collectively, our study suggested that SCARA3 is potentially associated with age-related metabolic dysfunction, which provided new insights into the pathogenesis and treatment of obesity as well as other obesity-associated metabolic complications.

A fine-mapping study of central obesity loci incorporating functional annotation and imputation

A recent genome-wide association study (GWAS) of central obesity identified 27 loci, from sex-combined analysis, associated with waist-to-hip ratio adjusted for body-mass index (WHRadjBMI) in European-ancestry individuals. Nevertheless, the identified variants may not be the biological causal ones due to the presence of linkage disequilibrium (LD). To better understand the mechanisms underlying the identified loci from the GWAS meta-analysis, we first imputed summary statistics at GWAS loci to increase genetic resolution, and then we applied a Bayesian statistical fine-mapping method through PAINTOR, incorporating LD structure and functional annotations to select and prioritize the most plausible causal variants across WHRadjBMI-associated regions. Using adipose tissue- and cell-specific annotations that showed significant associations with WHRadjBMI, we identified 33 single-nucleotide polymorphisms (SNPs) from 27 sex-combined fine-mapping loci with posterior probability of causality greater than 0.9. Six of the selected 33 SNPs belong to at least one of the top five identified annotations. SNPs rs1440372 (SMAD6) and rs12608504 (JUND) are particularly important since they not only have associated functional annotations but are also GWA hits in the original study. Incorporation of functional annotations helps identify additional plausible causal variants, such as rs2213731 (DNM3-PIGC) and rs4531856 (JUND), that did not reach genome-wide significance in GWAS. Our results provide promising candidates for future functional validation experiments.

Comprehensive review and annotation of susceptibility SNPs associated with obesity-related traits

We aimed to summarize the results of genetic association studies for obesity and provide a comprehensive annotation of all susceptibility single nucleotide polymorphisms (SNPs). A total of 72 studies were summarized, resulting in 90,361 susceptibility SNPs (738 index SNPs and 89,623 linkage disequilibrium SNPs). Over 90% of the susceptibility SNPs are located in non-coding regions, and it is challenging to understand their functional significance. Therefore, we annotated these SNPs by using various functional databases. We identified 24,623 functional SNPs, including 4 nonsense SNPs, 479 missense SNPs, 399 untranslated region SNPs which might affect microRNA binding, 262 promoter and 5,492 enhancer SNPs which might affect transcription factor binding, 7 splicing sites, 76 SNPs which might affect gene methylation levels, 1,839 SNPs under natural selection and 17,351 SNPs which might modify histone binding. Expression quantitative trait loci analyses for functional SNPs identified 98 target genes, including 69 protein coding genes, 27 long non-coding RNAs and 3 processed transcripts. The percentage of protein coding genes that could be correlated with obesity-related pathways directly or through gene-gene interaction is 75.36 (52/69). Our results may serve as an encyclopaedia of obesity susceptibility SNPs and offer guide for functional experiments.

c-Jun regulates adipocyte differentiation via the KLF15-mediated mode

Abnormal adipocyte differentiation is implicated in the development of metabolic disorders such as obesity and type II diabetes. Thus, an in-depth understanding of the molecular mechanisms associated with adipocyte differentiation is the first step in overcoming obesity and its related metabolic diseases. Here, we examined the role of c-Jun as a transcription factor in adipocyte differentiation. c-Jun overexpression in murine 3T3-L1 preadipocytes significantly inhibited adipocyte differentiation. In addition, the expression level of KLF15, an upstream effector of the key adipogenic factors C/EBPα and PPARγ, was decreased upon the ectopic expression of c-Jun. We found that c-Jun inhibited basal and glucocorticoid receptor (GR)-induced promoter activities of KLF15. c-Jun directly bound near the glucocorticoid response element (GRE) sites in the KLF15 promoter and inhibited adjacent promoter occupancies of GR. Furthermore, the restoration of KLF15 expression in 3T3-L1 cells with the stable ectopic expression of c-Jun partially rescued adipocyte differentiation. Our results demonstrate that c-Jun can suppress adipocyte differentiation through the down-regulation of KLF15 at the transcriptional level. This study proposes a novel mechanism by which c-Jun regulates adipocyte differentiation.

Therapeutic Implications of PPARgamma in Human Osteosarcoma

Osteosarcoma (OS) is the most common nonhematologic malignancy of bone in children and adults. Although dysregulation of tumor suppressor genes and oncogenes, such as Rb, p53, and the genes critical to cell cycle control, genetic stability, and apoptosis have been identified in OS, consensus genetic changes that lead to OS development are poorly understood. Disruption of the osteogenic differentiation pathway may be at least in part responsible for OS tumorigenesis. Current OS management involves chemotherapy and surgery. Peroxisome proliferator-activated receptor (PPAR) agonists and/or retinoids can inhibit OS proliferation and induce apoptosis and may inhibit OS growth by promoting osteoblastic terminal differentiation. Thus, safe and effective PPAR agonists and/or retinoid derivatives can be then used as adjuvant therapeutic drugs for OS therapy. Furthermore, these agents have the potential to be used as chemopreventive agents for the OS patients who undergo the resection of the primary bone tumors in order to prevent local recurrence and/or distal pulmonary metastasis.

Green tea polyphenol stimulates a Ras, MEKK1, MEK3, and p38 cascade to increase activator protein 1 factor-dependent involucrin gene expression in normal human keratinocytes

(-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive constituent of green tea that efficiently reduces epidermal cancer cell proliferation. This inhibition is associated with a reduction in activator protein 1 (AP1) transcription factor level and activity. However, its effects on AP1 function in normal epidermal cells have not been extensively explored. Our present studies show that EGCG regulates normal keratinocyte function. To understand the mechanism of action, we examined the effects of EGCG on AP1 factor activity, MAPK signal transduction, and expression of the AP1 factor-regulated human involucrin (hINV) gene. EGCG increases hINV promoter activity in a concentration-dependent manner that requires the presence of an intact hINV promoter AP1 factor binding site. This response appears to be physiologic, as endogenous hINV gene expression is also increased. Fra-1, Fra-2, FosB, JunB, JunD, c-Jun, and c-Fos levels are increased by EGCG treatment, as is AP1 factor binding to hINV promoter AP1 site. Gel mobility shift studies show that this complex contains Fra-1 and JunD. Signal transduction analysis indicates that the EGCG response requires Ras, MEKK1, MEK3, and p38 kinases. Kinase assays and inhibitor studies suggest that p38delta is the p38 isoform responsible for the regulation. These changes are also associated with a cessation of cell proliferation and enhanced cornified envelope formation. These studies show that in normal human keratinocytes EGCG markedly increases, via a MAPK signaling mechanism, AP1 factor-associated responses.

Senescence represses the nuclear localization of the serum response factor and differentiation regulates its nuclear localization with lineage specificity

The differentiation of cultured 3T3T mesenchymal stem cells into adipocytes represses growth factor responsiveness by limiting the nuclear localization of the serum response factor (SRF) that binds to and activates the promoters of growth control genes that contain the serum response elements (SRE), such as junB and c-fos. The regulation of SRF nuclear localization by adipocyte differentiation is specific, because we show that adipocyte differentiation does not repress the nuclear localization of six other transacting factors. To determine if repression of growth factor responsiveness that occurs during senescence also represses the nuclear localization of SRF, we studied normal human WI-38 fibroblasts at low versus high population doublings. The results show that SRF localizes to the nucleus of proliferative cells whereas in senescent cells SRF can not be detected in the nucleus. This result is apparent in both immunofluorescence assays and in western blot analysis. We next evaluated the cellular distribution of SRF in selected human tissues to determine whether the loss of proliferative potential in vivo could have a different effect on SRF nuclear localization. We found that in cells of the small bowel mucosa, differentiation modulates SRF nuclear localization in an opposite manner. Minimal SRF expression and nuclear localization is evident in undifferentiated cells at the base of crypts whereas increased SRF expression and nuclear localization is evident in differentiated cells at the surface tip of the villus. These results together establish that regulation of SRF expression and nuclear localization is important in senescence and differentiation in a lineage specific manner.

Down-regulation of AP1 activities after polarization of vas deferens epithelial cells correlates with androgen-induced gene expression

Vas deferens epithelial cell subcultures were used to study the sequential regulation of jun/fos proto-oncogene expression and AP1 activities during cell proliferation, polarization and androgen-induced expression of a terminal differentiation marker, i. e. the mvdp gene. Proliferation of epithelial cells is associated with a high expression in the nucleus of most Jun and Fos oncoproteins. After cell seeding on an extracellular matrix which allows polarization and expression of the mvdp gene in response to androgens, AP1 protein accumulation is greatly altered and consists in a loss of JunB, Fra1, FosB and a decrease in c-Fos, c-Jun and Fra2, while JunD remained at the same level. This was correlated with a drop in AP1 binding activity as evaluated by gel shift assay using either AP1 consensus sequence or AP1 binding sites of the mvdp gene promoter region, and in AP1 transactivating activity, as estimated by stable transfection experiments using an AP1 responsive promoter (TRE-TK-luc). Androgens did not significantly influence AP1 activities. On the contrary, stimulation of AP1 proteins by the tumor-promoting phorbol ester caused a decrease in androgen-induced mvdp mRNA accumulation, and this effect was reversed by staurosporine, a potent inhibitor of PKC. Our data suggest that a down-regulation of AP1 activities induced by epithelial cell differentiation is a prerequisite to androgen-induced mvdp gene expression. The high AP1 activities observed during proliferative state or induced in TPA-treated polarized cells, exert a repressive effect on androgen action.

1,25-Dihydroxyvitamin D(3) stimulates activator protein-1-dependent Caco-2 cell differentiation

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is a potential chemopreventive agent for human colon cancer. We have reported that 1,25(OH)(2)D(3) specifically activated protein kinase C-alpha (PKC-alpha) and also caused a reduction in proliferation while increasing apoptosis and differentiation in CaCo-2 cells, a cell line derived from a human colon cancer. The mechanisms by which this secosteroid influences these important cellular processes, however, remain unclear. The transcription factor, activator protein-1 (AP-1), regulates many genes involved in these processes. Therefore, we asked whether 1,25(OH)(2)D(3) activated AP-1 in CaCo-2 cells and, if so, by what mechanisms? 1,25(OH)(2)D(3) caused a time-dependent increase in AP-1 DNA binding activity and significantly enhanced the protein and mRNA abundance of c-Jun, a component of AP-1. 1, 25(OH)(2)D(3) also induced a rapid and transient activation of ERK2 (where ERK is extracellular signal-regulated kinase) and a more persistent activation of JNK1 (where JNK Jun N-terminal kinase). Transfection experiments revealed that 1,25(OH)(2)D(3) also increased AP-1 gene-transactivating activity. This AP-1 activation was completely blocked by PD 098059, a specific mitogen-activated protein kinase/ERK kinase inhibitor, as well as by a dominant negative JNK or a dominant negative Jun, indicating that the AP-1 activation induced by 1,25(OH)(2)D(3) was mediated by ERK and JNK. Using a specific inhibitor of the Ca(2+)-dependent PKC isoforms, Gö6976, and CaCo-2 cells stably transfected with antisense PKC-alpha cDNA, demonstrated that PKC-alpha mediated the AP-1 activation induced by this secosteroid. Inhibition of JNK activation or c-Jun protein expression significantly reduced 1, 25(OH)(2)D(3)-induced alkaline phosphatase activity, a marker of CaCo-2 cell differentiation, in secosteroid-treated cells. Taken together, the present study demonstrated that 1,25(OH)(2)D(3) stimulated AP-1 activation in CaCo-2 cells by a PKC-alpha- and JNK-dependent mechanism leading to increases in cellular differentiation.

Increased tyrosine phosphorylation of the insulin receptor, the insulin receptor substrate-1 and a 73 kDa protein associated with insulin-induced mitogenesis in SV40-transformed 3T3T cells

Insulin selectively induces mitogenesis in quiescent SV40 large T antigen-transformed murine 3T3T (CSV3-1) cells but not in quiescent nontransformed 3T3T cells. This mitogenic effect induced by insulin in CSV3-1 cells requires an induction of AP-1 activity associated with c-Jun and JunB. To further investigate the mechanisms that are involved in insulin-induced mitogenesis in CSV3-1 cells, the current experiments were performed. The results show that following insulin stimulation, the insulin receptor beta-subunit and the insulin receptor substrate-1 undergo a much more significant tyrosine phosphorylation in CSV3-1 cells than in 3T3T cells. Insulin also induces tyrosine phosphorylation of a 73 kDa protein that is coprecipitated with the tyrosine-phosphorylated insulin receptor in CSV3-1 cells but not in 3T3T cells. The increased tyrosine phosphorylation in response to insulin stimulation in CSV3-1 cells does not appear to be due to an increase in the level of expression of the insulin receptor and does not appear to result from a significant change in tyrosine phosphatase activity compared to nontransformed cells. The results also show that the insulin effect in CSV3-1 cells is not mediated by insulin-like growth factor 1 receptor because insulin at the concentrations that induce mitogenesis does not increase the tyrosine phosphorylation of the insulin-like growth factor 1 receptor and the expression level of the receptor is not significantly changed in CSV3-1 cells compared to nontransformed cells. These data together indicate that the selective mitogenic effect of insulin on CSV3-1 cells involves increased tyrosine phosphorylation of the insulin receptor, the insulin receptor substrate-1 and the 73 kDa protein, although the underlying mechanisms need to be further elucidated.

Polyamine depletion is associated with an increase in JunD/AP-1 activity in small intestinal crypt cells

Activator protein 1 (AP-1) is a group of dimeric transcription factors composed of protooncogene (Jun and Fos) subunits that bind to a common DNA site, the AP-1 binding site. The proteins of c-Jun, JunB, and Fos are essential for initiation of the cell cycle. Conversely, the activation of the junD gene slows cell growth in some cell types. The current study tests the hypothesis that polyamines influence cell growth by altering the balance of positive and negative Jun/AP-1 activities in intestinal epithelial cells. Studies were conducted in the IEC-6 cell line derived from rat small intestinal crypt cells. Administration of alpha-difluoromethylornithine (DFMO), a specific inhibitor for polyamine synthesis, for 4 and 6 days completely depleted cellular polyamine levels, while AP-1 binding activity was significantly increased. Spermidine, when given together with DFMO, restored AP-1 binding activity toward normal. The increased AP-1 complexes in polyamine-deficient cells were dramatically supershifted by the anti-JunD antibody but not by antibodies against c-Jun, JunB, or Fos proteins. There were significant increases in JunD mRNA and protein in DFMO-treated cells, although expression of the c-fos, c-jun, and junB genes decreased. The increase in JunD/AP-1 activity in DFMO-treated cells was associated with a significant decrease in cell division. Exposure of control quiescent cells to 5% dialyzed serum increased c-Jun/AP-1 but not JunD/AP-1 activities. DFMO prevented the stimulation of c-Jun/AP-1 activity induced by 5% dialyzed serum. These results indicate that 1) polyamine depletion is associated with an increase in AP-1 binding activity and 2) the increase in AP-1 activity in the DFMO-treated cells was primarily contributed by an increase in the JunD/AP-1. These findings suggest that polyamines regulate cell growth at least partially by modulating the balance of positive and negative Jun/AP-1 activities in the intestinal mucosa.

Induction of AP-1 activity associated with c-Jun and JunB is required for mitogenesis induced by insulin and vanadate in SV40-transformed 3T3T cells

Insulin and vanadate function as complete mitogens for SV40-transformed murine 3T3T (CSV3-1) cells but not for nontransformed 3T3T cells. Mitogenesis induced by insulin and vanadate in CSV3-1 cells is associated with the induction of the expression of protooncogenes c-jun and junB, two major AP-1 transcription factor components. We now report that both insulin and vanadate induce a significant increase in AP-1 DNA binding activity in CSV3-1 cells but not in 3T3T cells. Gel supershift assays and Western blot analysis using specific antibodies demonstrate that the increased AP-1 binding activity induced by insulin and vanadate in CSV3-1 cells is primarily contributed by an increase in the expression of c-Jun and JunB protein levels. Furthermore, treatment of CSV3-1 cells with antisense oligodeoxyribonucleotides to c-jun or to junB blocks insulin- and vanadate-induced mitogenesis whereas antisense junD oligomers have no inhibitory effects. These results therefore demonstrate that the induction of AP-1 binding activity associated with c-Jun and JunB is required for insulin- and vandate-induced mitogenesis in SV40-transformed murine 3T3T cells. Additional data presented in this paper show that JunD/AP-1 binding activity, which is thought to play a negative role in regulating cell proliferation, is also slightly induced following insulin and vanadate stimulation in CSV3-1 cells. Nevertheless, the ratio of proliferation promoting c-Jun/AP-1 and JunB/AP-1 binding activities to proliferation inhibiting JunD/AP-1 binding activity is significantly increased following insulin and vanadate stimulation. These results therefore support the concept that modulation of the balance of positive Jun/AP-1 and negative Jun/AP-1 activities is important in regulating cell proliferation.

Differentiation modulates the balance of positive and negative Jun/AP-1 DNA binding activities to regulate cellular proliferative potential: different effects in nontransformed and transformed cells

Differentiation of 3T3T cells into adipocytes results in the progressive repression of growth factor responsiveness. This is associated with the transcriptional repression of the inducibility of c-jun and junB expression by serum. In contrast, differentiation of SV-40 large T antigen-transformed 3T3T cells (CSV3-1) does not repress growth factor responsiveness nor c-jun or junB inducibility even though CSV3-1 cells can differentiate into adipocytes. To better explain these observations, we have studied compositional changes in AP-1 DNA binding activity attributed to c-Jun, JunB, and JunD during the differentiation process in 3T3T and CSV3-1 cells. The results show that in nontransformed 3T3T cells, differentiation represses AP-1 DNA binding activity via a proportionate downregulation of c-Jun, JunB, and JunD. In contrast, in CSV3-1 cells, AP-1 DNA binding activity increases twofold during differentiation, which is accounted for by an increase in JunD with no change in c-Jun and JunB. If c-Jun and JunB serve as positive regulators and JunD serves as a negative regulator for cell proliferation as suggested by previous studies, the repression of JunD expression in differentiating CSV3-1 cells should be mitogenic because decreasing JunD/AP-1 DNA binding activity would allow c-Jun/AP-1 and JunB/AP-1 DNA binding activities to be dominant. The results confirm this prediction showing that antisense junD oligodeoxyribonucleotides are mitogenic for differentiating CSV3-1 cells whereas antisense c-jun and junB inhibit mitogenesis. These data support the conclusion that differentiation can regulate cellular proliferative potential by modulating the balance of positive and negative Jun/AP-1 DNA binding activities in distinct ways in nontransformed and transformed cells.

Unique and selective mitogenic effects of vanadate on SV40-transformed cells

Vanadate and insulin both function as unique complete mitogens for SV40-transformed 3T3T cells, designated CSV3-1, but not for nontransformed 3T3T cells. The mitogenic effects induced by vanadate and insulin in CSV3-1 cells are mediated by different signaling mechanisms. For example, vanadate does not stimulate the tyrosine phosphorylation of the insulin receptor beta-subunit nor the 170 kDa insulin receptor substrate-1. Instead, vanadate induces a marked increase in tyrosine phosphorylation of 55 and 64 kDa proteins that is not observed in insulin-stimulated CSV3-1 cells. Perhaps most interestingly, vandate-induced mitogenesis is associated with the selective induction of c-jun and junB expression without significantly inducing c-fos or c-myc. Furthermore, treatment of CSV3-1 cells with genistein abolishes the effects of vanadate on protein tyrosine phosphorylation and c-jun induction. These and related data suggest that modulation of protein tyrosine phosphorylation and c-jun and junB expression may serve the critical roles in mediating vandate-induced mitogenesis in SV40-transformed cells.

Chemically and photochemically initiated DNA cleavage by an insulin-mimetic bisperoxovanadium complex

Chemically and photochemically induced cleavage of DNA by the insulin-mimetic compound NH4[VO(O2)2-(1,10-phenanthroline)], bpV(phen), have been studied. 51V NMR and absorption indicate that photoirradiation with low energy UV light of aqueous solutions containing bpV(phen) leads to the conversion of the compound to simple vanadates. Photoillumination of the compound in the presence of supercoiled pBR322 DNA results in cutting of the plasmid to produce nicked circular and linear DNA. Quantitative analysis of agarose gel data shows that bpV(phen) is a single strand nicking agent exhibiting sequence and/or base specificity.

In vivo protein-DNA interactions at the c-jun promoter in quiescent and serum-stimulated fibroblasts

c-Jun is an important component in the regulation of cell proliferation. As a member of the early response gene family, c-jun is induced within minutes in the presence of mitogenic agents such as serum growth factors. Using in vivo footprinting, we have analyzed protein-DNA interactions at the c-jun promoter in human fibroblasts subjected to growth arrest and serum stimulation. We located seven footprints upstream of the transcription initiation site. Protein-DNA interactions were detected at two AP-1-like sequences, A CCAAT box, an SP-1 sequence, an NF-jun sequence, a putative RSRF (related to serum response factor) binding site, and a sequence bound by an unknown factor. All of these binding sites were occupied in serum-starved cells, and no additional protein-DNA interactions were detected upon serum stimulation. Evidence from this study supports a model in which expression of the c-jun gene is mediated by phosphorylation events taking place on the transactivation domains of promoter-bound transcriptional activators.