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The ACTN3 gene encodes α-actinin-3 protein, which stabilizes the contractile apparatus at the Z-line in skeletal muscle cell fast fibers. A nonsense mutation of the arginine (R) at the codon for amino acid 577 of the ACTN3 gene generates a premature termination codon (PTC) and produces the R577X polymorphism in humans (X specifies translational termination). The ACTN3 577X genotype abolishes α-actinin-3 protein production due to targeted degradation of the mutant transcript by the cellular nonsense-mediated mRNA decay (NMD) system, which requires mRNA splicing. In humans, α-actinin-3 deficiency can decrease sprinting and power performance as well as skeletal muscle mass and strength. Here we investigated whether suppression of the in-frame PTC induced by treatment with the aminoglycosides gentamicin and G418 that promote termination codon readthrough could allow production of full-length α-actinin-3 protein from ACTN3 577X. We constructed expression plasmids encoding mature mRNA that lacks introns or pre-mRNA, which carries introns for the ACTN3 577X gene (X and X pre , respectively) and transfected the constructs into HEK293 cells. Similar constructs for the ACTN3 577R gene were used as controls. HEK293 cells carrying the X gene, but not the X pre gene, expressed exogenous truncated α-actinin-3 protein, indicating NMD-mediated suppression of exogenous X pre expression. Cells treated with aminoglycosides produced exogenous full-length α-actinin-3 protein in X-transfected cells, but not in X pre -transfected cells. The NMD inhibitor caffeine prevented suppression of X pre expression and thereby induced production of full-length α-actinin-3 protein in the presence of aminoglycoside. Together these results indicate that the ACTN3 R577X polymorphism could be a novel target for readthrough therapy, which may affect athletic and muscle performance in humans. [ABSTRACT FROM AUTHOR]

Abstract: Glycerol-3-phosphate acyltransferase 1 (GPAT1) acts as a rate limiting enzyme in triacylglycerol and phospholipid synthesis in mammals. GPAT1 regulates hepatic lipid accumulation associated with metabolic disorders. Here we have identified two transcriptional initiation sites and two promoters (promoter I and II) required for expression of the human GPAT1 (hGPAT1) gene. Promoter I regulates transcription of three alternative hGPAT1 mRNA variants, hGPAT1-V1, V2, and V3, while promoter II induces expression of a fourth variant, hGPAT1-V4. RT-PCR analysis and luciferase reporter assays revealed that promoter II acts in lipogenic tissues like the liver (and liver-derived HepG2 cells), whereas promoter I is differentially regulated and also acts in non-liver HeLa cells. Among liver-enriched transcription factors, HNF4α and C/EBPα slightly activated hGPAT1 promoter I, while factors including HNF1α altered promoter II activity. The lipogenic transcription factor SREBP1c greatly increased promoter II activity in HepG2 cells. The use of various truncated or mutated fragments of promoter II revealed that one sterol regulatory element-like motif and one inverted CCAAT box on promoter II contributed to the SREBP1c response. These cis-acting elements and trans-acting factors can be potential targets for manipulation of hepatic GPAT1 levels in humans. [Copyright &y& Elsevier]

Abstract: Glycerol-3-phosphate acyltransferase (GPAT) is a rate-limiting enzyme in mammalian triacylglycerol biosynthesis. GPAT is a target for the treatment of metabolic disorders associated with high lipid accumulation. Although the molecular basis for GPAT1 activation has been investigated extensively, the activation of other isoforms, such as GPAT2, is less well understood. Here the membrane topology of the GPAT2 protein was examined using an epitope-tag-based method. Exogenously expressed GPAT2 protein was present in the membrane fraction of transformed HEK293 cells even in the presence of Na2CO3 (100mM), indicating that GPAT2 is a membrane-bound protein. Trypsin treatment of the membrane fraction degraded the N-terminal (FLAG) and C-terminal (myc-epitope) protein tags of the GPAT2 protein. Bioinformatic analysis of the GPAT2 protein sequence indicated four hydrophobic sequences as potential membrane-spanning regions (TM1–TM4). Immunoblotting of the myc-epitope tag, which was inserted between each TM region of the GPAT2 protein, showed that the amino acid sequence between TM3 and TM4 was protected from trypsin digestion. These results suggest that the GPAT2 protein has two transmembrane segments and that the N-terminal and C-terminal regions of this protein face the cytoplasm. These results also suggest that the enzymatically active motifs I–III of the GPAT2 protein face the cytosol, while motif IV is within the membrane. It is expected that the use of this topological model of GPAT2 will be essential in efforts to elucidate the molecular mechanisms of GPAT2 activity in mammalian cells. [Copyright &y& Elsevier]

Abstract: We identified a novel alternative splicing event that constitutively produces a truncated active form of human sterol regulatory element-binding protein 1 (SREBP-1). A cDNA of this splicing variant (named SREBP-1Δ) contains a translational stop codon-encoding exon sequence between exons 7 and 8. It produces SREBP-1aΔ (470 a.a.) and SREBP-1cΔ (446 a.a.) proteins that lack transmembrane and C-terminal regulatory sequences necessary for localization of SREBP-1 to the endoplasmic reticulum. A luciferase reporter assay showed that SREBP-1aΔ and SREBP-1cΔ transactivated lipogenic gene promoters to the same extent as that induced by N-terminal active fragments of SREBP-1a and SREBP-1c, respectively. SREBP-1Δ mRNA is expressed in human cell lines as well as adipose and liver tissues. Expression levels ranged from 5% to 16% of total SREBP-1 expression. The ratio of SREBP-1Δ expression to total SREBP-1 expression in HepG2 cells was not affected by either insulin or high glucose treatment. [Copyright &y& Elsevier]

Abstract: In adipocytes and myocytes, insulin stimulation translocates glucose transporter 4 (Glut4) storage vesicles (GSVs) from their intracellular storage sites to the plasma membrane (PM) where they dock with the PM. Then, Glut4 is inserted into the PM and initiates glucose uptake into these cells. Previous studies using chemical inhibitors demonstrated that myosin II participates in fusion of GSVs and the PM and increase in the intrinsic activity of Glut4. In this study, the effect of myosin IIA on GSV trafficking was examined by knocking down myosin IIA expression. Myosin IIA knockdown decreased both glucose uptake and exposures of myc-tagged Glut4 to the cell surface in insulin-stimulated cells, but did not affect insulin signal transduction. Interestingly, myosin IIA knockdown failed to decrease insulin-dependent trafficking of Glut4 to the PM. Moreover, in myosin IIA knockdown cells, insulin-stimulated binding of GSV SNARE protein, vesicle-associated membrane protein 2 (VAMP2) to PM SNARE protein, syntaxin 4 was inhibited. These data suggest that myosin IIA plays a role in insulin-stimulated docking of GSVs to the PM in 3T3-L1 adipocytes through SNARE complex formation. [Copyright &y& Elsevier]

Researchers frequently use 3T3-L1 adipocytes as a fat cell line, but the capacity of this line for insulin-mediated glucose transport is lower than that of primary isolated fat cells. In this study, we found that 5-azacytidine (5-aza-C), DNA methyltransferase 1 inhibitor, enhanced insulin-stimulated 2-deoxyglucose (2-DG) transport in 3T3-L1 cells after adipogenic differentiation. We next examined the expression of the genes related to glucose transport and insulin signal transduction. The insulin independent glucose transporter, glucose transporter 1 (GLUT1), showed lower expression in 5-aza-C pre-treated 3T3-L1 adipocytes, than in un-treated control adipocytes, while the expression of insulin dependent transporter GLUT4 remained unchanged. In addition, insulin receptor substrate-1 (IRS-1) was highly expressed in 5-aza-C pre-treated adipocytes. Based on DNA microarray and functional annotation analysis, we noticed that 5-aza-C pretreatment activated the tumor suppressor p53 pathway. We confirmed that in 5-aza-C adipocytes, p53 expression was markedly higher, and the methylation level of CpGs in its promoter region was lower than in un-treated control adipocytes. Moreover, pharmacological inhibition of p53 restored GLUT1 and IRS-1 expression to the same level as in un-treated 3T3-L1 adipocytes, and significantly decreased insulin-mediated 2-DG uptake. These results suggest that glucose transport capacity in adipocytes is influenced by DNA methylation status, and demethylation induced by 5-aza-C increased it possibly through the p53 signaling pathway. • The treatment of 5-azacytidine enhanced 2-deoxyglucose uptake in 3T3-L1 adipocytes. • 5-azacytidine treatment suppressed GLUT1, and increased IRS-1 mRNA expression. • The p53 signaling pathway was activated in 5-azacytidine treated adipocytes. • p53 inhibition restored the effect of 5-azacytidine on 2-deoxyglucose uptake. [ABSTRACT FROM AUTHOR]

The physiological activity of the steroid derived hormone vitamin D is regulated by several enzymatic steps. Both 25-hydroxy vitamin D 3 1α-hydroxylase (CYP27B1) and 25-hydroxyvitamin D 3 24-hydroxylase (CYP24A1) modulate blood levels of 1,25-dihydroxyvitamin D 3 , an activated form of vitamin D. We previously demonstrated that CYP27B1 expression was trans-activated by sterol regulatory element binding protein 1 (SREBP1), although whether SREBP1 also regulates CYP24A1 transcription was unclear. Here we investigated the ability of SREBP1 to affect CYP24A1 transcription. In a luciferase reporter assay, mouse and human CYP24A1 promoter activity was strongly activated by SREBP1 in opossum kidney proximal tubular cells (OK-P). Three putative SREs (pSREs) were found in the mouse Cyp24a1 gene promoter and the SREBP1 protein showed specific binding to the pSRE1 element in EMSAs. Site-directed mutagenesis of the pSRE1 element strongly decreased SREBP1-mediated Cyp24a1 gene transcription. Moreover, siRNA-mediated SREBP1 knock-down repressed CYP24A1 expression in human renal proximal tubular epithelial cells (HKC-8). In animal studies, mice given various doses of thyroid hormone (T 3 ) showed dose-dependent reductions in renal Srebp1c and Cyp24a1 mRNA levels. Taken together, our results suggest that SREBP1 trans-activates CYP24A1 expression through SREBP binding elements present in the promoter. [ABSTRACT FROM AUTHOR]

Abstract: Insulin-responsive aminopeptidase (IRAP) and GLUT4 are two major cargo proteins of GLUT4 storage vesicles (GSVs) that are translocated from a postendosomal storage compartment to the plasma membrane (PM) in response to insulin. The cytoplasmic region of IRAP is reportedly involved in retention of GSVs. In this study, vimentin was identified using the cytoplasmic domain of IRAP as bait. The validity of this interaction was confirmed by pull-down assays and immunoprecipitation in 3T3-L1 adipocytes. In addition, it was shown that GLUT4 translocation to the PM by insulin was decreased in vimentin-depleted adipocytes, presumably due to dispersing GSVs away from the cytoskeleton. These findings suggest that the IRAP binding protein, vimentin, plays an important role in retention of GSVs. [Copyright &y& Elsevier]

Abstract: Exendin-4 (Ex-4) is a glucagon-like peptide-1 receptor (GLP-1R) agonist that has been used as a drug injected subcutaneously for treatment of type 2 diabetes. Many studies have revealed molecular targets of Ex-4, but its influence on adipokines has not been determined. Our study showed that Ex-4 induced secretion of adiponectin into the culture medium of 3T3-L1 adipocytes. This effect of Ex-4 is due to increased adiponectin mRNA level through the GLP-1R. Both forskolin and 3-isobutyl-1-methylxanthine (IBMX), which may finally elevate cyclic adenosine monophosphate (cAMP) concentration, prevented the induction of adiponectin expression by Ex-4. Moreover, H89, a protein kinase A inhibitor, blocked the effect of Ex-4 on adiponectin. On the other hand, Ex-4 decreased the mRNA levels of inflammatory adipokines. The results indicate that Ex-4 directly promotes adiponectin secretion via the protein kinase A pathway in 3T3-L1 adipocytes and may ameliorate insulin resistance. [Copyright &y& Elsevier]

Abstract: Bacteria produce many types of hemolysin that induce diarrhea by mechanisms that are not completely understood. Aeromonas sobria hemolysin (ASH) is a major virulence factor produced by A. sobria, a human pathogen that causes diarrhea. Since epithelial cells in the intestine are the primary targets of hemolysin, we investigated the effects of ASH on ion transport in human colonic epithelial (Caco-2) cells. ASH increased short-circuit currents (Isc) in a dose-dependent manner, and it also activated a 125I efflux from Caco-2 cells. ASH-induced Isc increases and 125I efflux activations were both suppressed by low Ca2+ levels in the extracellular solution or by pretreatment with the Ca2+ chlelator BAPTA-AM. Intracellular Ca2+ levels were increased by ASH in a biphasic fashion characterized by a rapid sharp increase (peak 1) followed by a sustained low plateau (peak 2). ASH-induced peak 1 was inhibited by pretreatment with pertussis toxin, indicating that Ca2+ was mobilized from intracellular stores, and peak 2 was induced by an influx of extracellular Ca2+. Peak 2 but not peak 1 was related to Cl− secretion. These results indicate that ASH activates Ca2+-dependent Cl− secretion. [Copyright &y& Elsevier]