Paradoxical regulation of Sp1 transcription factor by glucagon

Identification of Metabolic Syndrome-Related miRNA-mRNA Regulatory Networks and Key Genes Based on Bioinformatics Analysis

Metabolic syndrome, which affects approximately one-quarter of the world's population, is a combination of multiple traits and is associated with high all-cause mortality, increased cancer risk, and other hazards. It has been shown that the epigenetic functions of miRNAs are closely related to metabolic syndrome, but epigenetic studies have not yet fully elucidated the regulatory network and key genes associated with metabolic syndrome. To perform data analysis and screening of potential differentially expressed target miRNAs, mRNAs and genes based on a bioinformatics approach using a metabolic syndrome mRNA and miRNA gene microarray, leading to further analysis and identification of metabolic syndrome-related miRNA-mRNA regulatory networks and key genes. The miRNA gene set (GSE98896) and mRNA gene set (GSE98895) of peripheral blood samples from patients with metabolic syndrome from the GEO database were screened, and set|logFC|> 1 and adjusted P < 0.05 were used to identify the differentially expressed miRNAs and mRNAs. Differentially expressed miRNA transcription factors were predicted using FunRich software and subjected to GO and KEGG enrichment analysis. Next, biological process enrichment analysis of differentially expressed mRNAs was performed with Metascape. Differentially expressed miRNAs and mRNAs were identified and visualized as miRNA-mRNA regulatory networks based on the complementary pairing principle. Data analysis of genome-wide metabolic syndrome-related mRNAs was performed using the gene set enrichment analysis (GSEA) database. Finally, further WGCNA of the set of genes most closely associated with metabolic syndrome was performed to validate the findings. A total of 217 differentially expressed mRNAs and 158 differentially expressed miRNAs were identified by screening the metabolic syndrome miRNA and mRNA gene sets, and these molecules mainly included transcription factors, such as SP1, SP4, and EGR1, that function in the IL-17 signalling pathway; cytokine-cytokine receptor interaction; proteoglycan syndecan-mediated signalling events; and the glypican pathway, which is involved in the inflammatory response and glucose and lipid metabolism. miR-34C-5P, which was identified by constructing a miRNA-mRNA regulatory network, could regulate DPYSL4 expression to influence insulin β-cells, the inflammatory response and glucose oxidative catabolism. Based on GSEA, metabolic syndrome is known to be closely related to oxidative phosphorylation, DNA repair, neuronal damage, and glycolysis. Finally, RStudio and DAVID were used to perform WGCNA of the gene sets most closely associated with metabolic syndrome, and the results further validated the conclusions. Metabolic syndrome is a common metabolic disease worldwide, and its mechanism of action is closely related to the inflammatory response, glycolipid metabolism, and impaired mitochondrial function. miR-34C-5P can regulate DPYSL4 expression and can be a potential research target. In addition, UQCRQ and NDUFA8 are core genes of oxidative phosphorylation and have also been identified as potential targets for the future treatment of metabolic syndrome.

Transforming growth factor-β-inducible early response gene 1 is a novel substrate for atypical protein kinase Cs

The protein kinase C (PKC) family of serine/threonine kinases consists of ten different isoforms grouped into three subfamilies, denoted classical, novel and atypical PKCs (aPKCs). The aPKCs, PKCι/λ and PKCζ serve important roles during development and in processes subverted in cancer such as cell and tissue polarity, cell proliferation, differentiation and apoptosis. In an effort to identify novel interaction partners for aPKCs, we performed a yeast two-hybrid screen with the regulatory domain of PKCι/λ as bait and identified the Krüppel-like factors family protein TIEG1 as a putative interaction partner for PKCι/λ. We confirmed the interaction of both aPKCs with TIEG1 in vitro and in cells, and found that both aPKCs phosphorylate the DNA-binding domain of TIEG1 on two critical residues. Interestingly, the aPKC-mediated phosphorylation of TIEG1 affected its DNA-binding activity, subnuclear localization and transactivation potential.

Colored potato extracts induce superoxide dismutase-2 mRNA via ERK1/2 pathway in HepG2 cells

Rats fed a diet containing Shadow Queen (SQ), an anthocyanin-rich potato cultivar, previously showed an increase in the hepatic superoxide dismutase (SOD)-2 mRNA level. We investigated whether an extract of SQ would directly increase the hepatic SOD-2 mRNA level in HepG2 cells. Furthermore, we estimated the intracellular signaling pathway for the induction of SOD-2 mRNA expression. HepG2 cells were stimulated using extracts of four crops, including SQ, for 12 h; only extracts of colored potatoes induced SOD-2 mRNA expression significantly. This induction of SOD-2 mRNA expression was blocked by an inhibitor of the extracellular signal-related kinase (ERK) 1/2 pathway. Furthermore, an extract of SQ increased the phosphorylation of ERK1/2 after 15 or 30 min of stimulation. These data indicate that an extract of SQ directly induces hepatic SOD-2 mRNA expression via activation of ERK1/2 pathway in HepG2 cells.

O-linked N-acetylglucosaminylation of Sp1 inhibits the human immunodeficiency virus type 1 promoter

Human immunodeficiency virus type 1 (HIV-1) gene expression and replication are regulated by the promoter/enhancer located in the U3 region of the proviral 5' long terminal repeat (LTR). The binding of cellular transcription factors to specific regulatory sites in the 5' LTR is a key event in the replication cycle of HIV-1. Since transcriptional activity is regulated by the posttranslational modification of transcription factors with the monosaccharide O-linked N-acetyl-D-glucosamine (O-GlcNAc), we evaluated whether increased O-GlcNAcylation affects HIV-1 transcription. In the present study we demonstrate that treatment of HIV-1-infected lymphocytes with the O-GlcNAcylation-enhancing agent glucosamine (GlcN) repressed viral transcription in a dose-dependent manner. Overexpression of O-GlcNAc transferase (OGT), the sole known enzyme catalyzing the addition of O-GlcNAc to proteins, specifically inhibited the activity of the HIV-1 LTR promoter in different T-cell lines and in primary CD4(+) T lymphocytes. Inhibition of HIV-1 LTR activity in infected T cells was most efficient (>95%) when OGT was recombinantly overexpressed prior to infection. O-GlcNAcylation of the transcription factor Sp1 and the presence of Sp1-binding sites in the LTR were found to be crucial for this inhibitory effect. From this study, we conclude that O-GlcNAcylation of Sp1 inhibits the activity of the HIV-1 LTR promoter. Modulation of Sp1 O-GlcNAcylation may play a role in the regulation of HIV-1 latency and activation and links viral replication to the glucose metabolism of the host cell. Hence, the establishment of a metabolic treatment might supplement the repertoire of antiretroviral therapies against AIDS.

Tertiary structural rearrangements upon oxidation of Methionine145 in calmodulin promotes targeted proteasomal degradation

The selectivity underlying the recognition of oxidized calmodulin (CaM) by the 20S proteasome in complex with Hsp90 was identified using mass spectrometry. We find that degradation of oxidized CaM (CaMox) occurs in a multistep process, which involves an initial cleavage that releases a large N-terminal fragment (A1-F92) as well as multiple smaller carboxyl-terminus peptides ranging from 17 to 26 amino acids in length. These latter small peptides are enriched in methionine sulfoxides (MetO), suggesting a preferential degradation around MetO within the carboxyl-terminal domain. To confirm the specificity of CaMox degradation and to identify the structural signals underlying the preferential recognition and degradation by the proteasome/Hsp90, we have investigated how the oxidation of individual methionines affect the degradation of CaM using mutants in which all but selected methionines in CaM were substituted with leucines. Substitution of all methionines with leucines except Met144 and Met145 has no detectable effect on the structure of CaM, permitting a determination of how site-specific substitutions and the oxidation of Met144 and Met145 affects the recognition and degradation of CaM by the proteasome/Hsp90. Comparable rates of degradation are observed upon the selective oxidation of Met144 and Met145 in CaM-L7 relative to that observed upon oxidation of all nine methionines in wild-type CaM. Substitution of leucines for either Met144 or Met145 promotes a limited recognition and degradation by the proteasome that correlates with decreases in the helical content of CaM. The specific oxidation of Met144 has little effect on rates of proteolytic degradation by the proteasome/Hsp90 or the structure of CaM. In contrast, the specific oxidation of Met145 results in both large increases in the rate of degradation by the proteasome/Hsp90 and significant circular dichroic spectral shape changes that are indicative of changes in tertiary rather than secondary structure. Thus, tertiary structural changes resulting from the site-specific oxidation of a single methionine (i.e., Met145) promote the degradation of CaM by the proteasome/Hsp90, suggesting a mechanism to regulate cellular metabolism through the targeted modulation of CaM abundance in response to oxidative stress.

Control of mitochondrial biogenesis during myogenesis

We used expression and reporter gene analysis to understand how changes in transcription factors impinge on mitochondrial gene expression during myogenesis of cultured murine myoblasts (C2C12 and Sol8). The mRNA levels for nuclear respiratory factor-1 (NRF-1) and NRF-2α increased 60% by the third day of myogenesis, whereas NRF-1 and NRF-2 reporter gene activity increased by fivefold over the same period. Although peroxisome proliferator activated receptor (PPARα) mRNA levels increased almost 10-fold, the activity of a PPAR reporter was unchanged during myogenesis. The PPAR coactivator PPAR-γ coactivator-1α (PGC1α), a master controller of mitochondrial biogenesis, was not expressed at detectable levels. However, the mRNA for both PGC1α-related coactivator and PGC1β was abundant, with the latter increasing by 50% over 3 days of differentiation. We also conducted promoter analysis of the gene for citrate synthase (CS), a common mitochondrial marker enzyme. The proximal promoter (∼2,100 bp) of the human CS lacks binding sites for PPAR, NRF-1, or NRF-2. Deletion mutants, a targeted mutation, and an Sp1 site-containing reporter construct suggest that changes in Sp1 regulation also participate in mitochondrial biogenesis during myogenesis. Because most mitochondrial genes are regulated by PPARs, NRF-1, and/or NRF-2, we conducted inhibitor studies to further support the existence of a distinct pathway for CS gene regulation in myogenesis. Although both LY-294002 (a phosphatidylinositol 3-kinase inhibitor) and SB-203580 (a p38-MAPK inhibitor) blocked myogenesis (as indicated by creatine phosphokinase activity), only SB-203580 prevented the myogenic increase in cytochrome oxidase activity, whereas only LY-294002 blocked the increase in CS (enzyme and reporter gene activities). Collectively, these studies help delineate the roles of some transcriptional regulators involved in mitochondrial biogenesis associated with myogenesis and underscore an import role for posttranscriptional regulation of transcription factor activity.

Insulin Dynamically Regulates Calmodulin Gene Expression by Sequential O-Glycosylation and Phosphorylation of Sp1 and Its Subcellular Compartmentalization in Liver Cells

O-glycosylation and phosphorylation of Sp1 are thought to modulate the expression of a number of genes in normal and diabetic state. Sp1 is an obligatory transcription factor for constitutive and insulin-responsive expression of the calmodulin gene (Majumdar, G., Harmon, A., Candelaria, R., Martinez-Hernandez, A., Raghow, R., and Solomon, S. S. (2003) Am. J. Physiol. 285, E584-E591). Here we report the temporal dynamics of accumulation of total, O-GlcNAc-modified, and phosphorylated Sp1 in H-411E hepatoma cells by immunohistochemistry with monospecific antibodies, confocal microscopy, and matrix-assisted laser desorption and ionization-time of flight mass spectrometry. Insulin elicited sequential and reciprocal post-translational modifications of Sp1. The O-glycosylation of Sp1 and its nuclear accumulation induced by insulin peaked early (approximately 30 min), followed by a steady decline of O-GlcNAc-modified Sp1 to negligible levels by 240 min. The accumulation of phosphorylated Sp1 in the nuclei of insulin-treated cells showed an opposite pattern, increasing steadily until reaching a maximum around 240 min after treatment. Analyses of the total, O-GlcNAc-modified, or phosphorylated Sp1 by Western blot and mass spectrometry corroborated the sequential and reciprocal control of post-translational modifications of Sp1 in response to insulin. Treatment of cells with streptozotocin (a potent inhibitor of O-GlcNAcase) led to hyperglycosylation of Sp1 that failed to be significantly phosphorylated. The mass spectrometry data indicated that a number of common serine residues of Sp1 undergo time-dependent, reciprocal O-glycosylation and phosphorylation, paralleling its rapid translocation from cytoplasm to the nucleus. Later, changes in the steady state levels of phosphorylated Sp1 mimicked the enhanced steady state levels of calmodulin mRNA seen after insulin treatment. Thus, O-glycosylation of Sp1 appears to be critical for its localization into the nucleus, where it undergoes obligatory phosphorylation that is needed for Sp1 to activate calmodulin gene expression.

Role of receptor for advanced glycation end-products and signalling events in advanced glycation end-product-induced monocyte chemoattractant protein-1 expression in differentiated mouse podocytes

BACKGROUND

Upregulation of local monocyte chemoattractant protein-1 (MCP-1) production is involved in glomerular damage through macrophage recruitment and activation in diabetic nephropathy. Treatment of db/db mice with soluble receptor for advanced glycation end-products (RAGE) prevented recruitment of macrophages to the glomeruli and reduced albuminuria, suggesting that binding of ligands and RAGE may be involved in MCP-1 expression. Therefore, we investigated the role of advanced glycation end-products (AGEs) in MCP-1 production by podocytes and signalling events after RAGE activation.

METHODS

MCP-1 gene and protein expression were examined by using reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay in differentiated mouse podocytes. Dichlorofluorescein-sensitive intracellular reactive oxygen species (ROS) generation was measured by confocal microscopy. RAGE, phosphorylation of mitogen-activated protein kinases, nuclear factor (NF)-kappaB, c-Jun and Sp1 were studied using western blotting and immunocytochemistry.

RESULTS

Both differentiated and undifferentiated podocytes expressed RAGE. MCP-1 was induced by AGEs and carboxymethyllysine (CML) in a time-dependent and dose-dependent manner in differentiated podocytes. Neutralizing antibody for RAGE suppressed AGE- and CML-induced MCP-1 production. AGEs and CML rapidly generated intracellular ROS in podocytes. Blocking of ROS by using N-acetyl-l-cysteine abolished CML and H(2)O(2)-induced MCP-1 expression. Phosphorylated extracellular signal-regulated kinase (ERK) was found in podocytes incubated with CML and was prevented by N-acetyl-l-cysteine or 7'-amino 4 [trifluoromethyl]. PD98059, an inhibitor of ERK, partially prevented CML-induced MCP-1 gene expression. NF-kappaB and Sp1 were translocated into the nucleus after podocytes were incubated with CML for 60 min. Parthenolide and mithramycin A, inhibitors of NF-kappaB and Sp1, respectively, abolished CML-induced MCP-1 gene expression in a dose-dependent manner.

CONCLUSIONS

These results suggest that AGEs and CML induce MCP-1 expression in podocytes through activation of RAGE and generation of intracellular ROS. NF-kappaB and Sp1 regulate MCP-1 gene transcription.

Synergistic role of specificity proteins and upstream stimulatory factor 1 in transactivation of the mouse carboxylesterase 2/microsomal acylcarnitine hydrolase gene promoter

Mouse carboxylesterase 2 (mCES2), a microsomal acylcarnitine hydrolase, is thought to play some important roles in fatty acid (ester) metabolism, and it is therefore thought that the level of transcription of the mCES2 gene is under tight control. Examination of the tissue expression profiles revealed that mCES2 is expressed in the liver, kidney, small intestine, brain, thymus, lung, adipose tissue and testis. When the mCES2 promoter was cloned and characterized, it was revealed that Sp1 (specificity protein 1) and Sp3 could bind to a GC box, that USF (upstream stimulatory factor) 1 could bind to an E (enhancer) box, and that Sp1 could bind to an NFkappaB (nuclear factor kappaB) element in the mCES2 promoter. Co-transfection assays showed that all of these transcription factors contributed synergistically to transactivation of the mCES2 promoter. Taken together, our results indicate that Sp1, Sp3 and USF1 are indispensable factors for transactivation of the mCES2 gene promoter. To our knowledge, this is the first study in which transcription factors that interact with a CES2 family gene have been identified. The results of the present study have provided some clues for understanding the molecular mechanisms regulating mCES2 gene expression, and should be useful for studies aimed at elucidation of physiological functions of mCES2.

Insulin stimulates and diabetes inhibits O-linked N-acetylglucosamine transferase and O-glycosylation of Sp1

Insulin stimulates both the biosynthesis of transcription factor Sp1 and its O-linked N-acetylglucosaminylation (O-GlcNAcylation), which promotes nuclear localization of Sp1 and its ability to transactivate calmodulin (CaM) gene transcription. To investigate this further, we incubated H-411E liver cells with insulin (10,000 microU/ml) and quantified the subcellular distribution of O-GlcNAc transferase (OGT) and O-GlcNAc-modified Sp1. We also examined the phosphorylation of Sp1 using both Western blot and incorporation of 32P into Sp1. The results demonstrate that insulin, but not glucagon, stimulates OGT synthesis and enhances cytosolic staining of OGT (histochemical). Insulin increases O-GlcNAc-Sp1, which peaks at 30 min, followed by decline at 4 h. In contrast, insulin initiates phosphorylation of Sp1 early, followed by a continued increase in phosphorylated Sp1 (PO4-Sp1) at 4 h. A reciprocal relationship between O-GlcNAc-Sp1 and PO4-Sp1 was observed. To explore the pathophysiological relevance, we localized OGT in liver sections from streptozotocin (STZ)-induced diabetic rats. We observed that staining of OGT in STZ-induced diabetic rat liver is clearly diminished, but it was substantially restored after 6 days of insulin treatment. We conclude that insulin stimulates CaM gene transcription via a dynamic interplay between O-glycosylation and phosphorylation of Sp1 that modulates stability, mobility, subcellular compartmentalization, and activity.

O-glycosylation of Sp1 and transcriptional regulation of the calmodulin gene by insulin and glucagon

Both insulin and glucagon stimulate steady-state levels of Sp1 transcription factor, but only insulin stimulates transcription of the calmodulin (CaM) gene in liver. Because O-glycosylation of Sp1 by O-linked N-acetylglucosamine (O-GlcNAc) is thought to regulate its ability to activate transcription, we assayed the levels of Sp1 with anti-Sp1 and anti-O-GlcNAc antibodies in Western blots by use of extracts of H-411E liver cells treated with insulin (10,000 microU/ml) or glucagon (1.5 x 10(-5) M). We also assessed subcellular localization of the native and glycosylated Sp1 in H411E cells treated with either hormone in the presence of deoxynorleucine (DON, an indirect inhibitor of O-glycosylation) or streptozotocin (STZ, an indirect stimulator of O-glycosylation). Insulin stimulated both total and O-GlcNAc-modified Sp1 primarily in the nucleus and induced CaM gene transcription (P < 0.0001). In contrast, glucagon promoted accumulation of Sp1 in the cytoplasm but not the nucleus, without significantly stimulating (P = not significant) either its O-glycosylation or transcription of the CaM gene. DON inhibited O-glycosylation of Sp1 and its ability to migrate to the nucleus and transactivate CaM gene transcription. In contrast, cotreatment of cells with STZ and glucagon enhanced O-glycosylation of Sp1, promoting its migration to the nucleus and resulting in increased CaM gene transcription. Thus O-glycosylation of Sp1 by insulin, but not glucagon, apparently enhances its (Sp1) nuclear recruitment and results in activation of CaM gene transcription.