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Osamu Araki

Publications and source records attributed to Osamu Araki.

8 recordsLinked to original sources
PubMed2007

Impaired adipogenesis caused by a mutated thyroid hormone alpha1 receptor.

Thyroid hormone (T3) is critical for growth, differentiation, and maintenance of metabolic homeostasis. Mice with a knock-in mutation in the thyroid hormone receptor alpha gene (TRalpha1PV) were created previously to explore the roles of mutated TRalpha1 in vivo. TRalpha1PV is a dominant negative mutant with a frameshift mutation in the carboxyl-terminal 14 amino acids that results in the loss of T3 binding and transcription capacity. Homozygous knock-in TRalpha1(PV/PV) mice are embryonic lethal, and heterozygous TRalpha1(PV/+) mice display the striking phenotype of dwarfism. These mutant mice provide a valuable tool for identifying the defects that contribute to dwarfism. Here we show that white adipose tissue (WAT) mass was markedly reduced in TRalpha1(PV/+) mice. The expression of peroxisome proliferator-activated receptor gamma (PPARgamma), the key regulator of adipogenesis, was repressed at both mRNA and protein levels in WAT of TRalpha1(PV/+) mice. Moreover, TRalpha1PV acted to inhibit the transcription activity of PPARgamma by competition with PPARgamma for binding to PPARgamma response elements and for heterodimerization with the retinoid X receptors. The expression of TRalpha1PV blocked the T3-dependent adipogenesis of 3T3-L1 cells and repressed the expression of PPARgamma. Thus, mutations of TRalpha1 severely affect adipogenesis via cross talk with PPARgamma signaling. The present study suggests that defects in adipogenesis could contribute to the phenotypic manifestation of reduced body weight in TRalpha1(PV/+) mice.

Adipocytes, White↗
PubMed2006

Aberrant accumulation of PTTG1 induced by a mutated thyroid hormone beta receptor inhibits mitotic progression.

Overexpression of pituitary tumor-transforming 1 (PTTG1) is associated with thyroid cancer. We found elevated PTTG1 levels in the thyroid tumors of a mouse model of follicular thyroid carcinoma (TRbeta(PV/PV) mice). Here we examined the molecular mechanisms underlying elevated PTTG1 levels and the contribution of increased PTTG1 to thyroid carcinogenesis. We showed that PTTG1 was physically associated with thyroid hormone beta receptor (TRbeta) as well as its mutant, designated PV. Concomitant with thyroid hormone-induced (T3-induced) degradation of TRbeta, PTTG1 proteins were degraded by the proteasomal machinery, but no such degradation occurred when PTTG1 was associated with PV. The degradation of PTTG1/TRbeta was activated by the direct interaction of the liganded TRbeta with steroid receptor coactivator 3 (SRC-3), which recruits proteasome activator PA28gamma. PV, which does not bind T3, could not interact directly with SRC-3/PA28gamma to activate proteasome degradation, resulting in elevated PTTG1 levels. The accumulated PTTG1 impeded mitotic progression in cells expressing PV. Our results unveil what we believe to be a novel mechanism by which PTTG1, an oncogene, is regulated by the liganded TRbeta. The loss of this regulatory function in PV led to an aberrant accumulation of PTTG1 disrupting mitotic progression that could contribute to thyroid carcinogenesis.

Animals↗
PubMed2005

Thyroid hormone receptor beta mutants: Dominant negative regulators of peroxisome proliferator-activated receptor gamma action.

Thyroid hormone (T3) and peroxisome proliferators have overlapping metabolic effects in the maintenance of lipid homeostasis. Their actions are mediated by their respective receptors: thyroid hormone receptors (TR) and peroxisome proliferator-activated receptors (PPAR). We recently found that a dominantly negative TRbeta mutant (PV) that causes a genetic disease, resistance to thyroid hormone, acts to repress the ligand (troglitazone)-mediated transcriptional activity of PPARgamma in cultured thyroid cells. This finding suggests that TRbeta mutants could crosstalk with PPARgamma-signaling pathways. The present study explored the molecular mechanisms by which PV represses the PPARgamma transcriptional activity. Gel-shift assays show that the PV, similar to wild-type TRbeta, bound to the peroxisome proliferator response element (PPRE) as homodimers and heterodimers with PPARgamma or the retinoid X receptor (RXR), thereby competing with PPARgamma for binding to PPRE and for sequestering RXR. Association of PPRE-bound PV with corepressors [e.g., nuclear receptor corepressor (NCoR)] that led to transcriptional repression was independent of T3 and troglitazone. Chromatin immunoprecipitation assay further demonstrated that, despite the presence of ligands, NCoR was recruited to PPRE-bound PV on a PPARgamma-target gene, the lipoprotein lipase, in vivo, suggesting the dominant action of PV on PPARgamma-mediated transcriptional activity. Thus, the dominant negative action of PV is not limited on the wild-type TRs. The findings that TRbeta mutants affect PPARgamma functions through dominant negative action provide insights into the molecular mechanisms by which TR regulates the PPARgamma-target genes involved in metabolic pathways, lipid homeostasis, and carcinogenesis.

Animals↗
PubMed2005

Simultaneous cranial and spinal subdural hematoma.

A 59-year-old male presented with spinal subdural hematoma (SDH) with concomitant cranial chronic SDH manifesting as mild paraparesis and numbness in both lower extremities. Magnetic resonance (MR) imaging showed simultaneous occurrence of cranial and spinal SDHs. The patient was treated conservatively because of poor medical condition and mild neurological symptoms, and recovered well within 1 month. Serial follow-up MR imaging revealed spontaneous resolution of both lesions, with signal intensity changes suggesting the degenerative process of subacute hematoma. The spinal hematoma may have migrated from the cranial lesion. Spinal SDH is a potential sequela of chronic SDH in the cranium.

Hematoma, Subdural, Intracranial↗
PubMed2004

Site-directed mutagenesis establishes aspartic acids-227 and -342 as essential for enzyme activity in an isomalto-dextranase from Arthrobacter globiformis.

Isomalto-dextranase, from Arthrobacter globiformis T6, is a member of the glycoside hydrolase family 27. However, the alignments of the whole amino acid sequence are distinct from other members of this family. The enzymes cleave the glycosidic bond of the substrate in two different manners: either retaining or inverting the anomeric configuration. We believe that a retaining enzyme is involved in a two-step, double-displacement mechanism utilizing active site carboxylic acids as the nucleophile and general acid/base catalysts in the hydrolytic reaction. The critical amino acid residues at the isomalto-dextranase active site that catalyzes the hydrolysis reaction of dextran have been identified and the roles of nine amino acid residues (D107, D163, D227, D295, D340, D342, D373, D396, and E420) in the isomalto-dextranase from A. globiformis analyzed by site-directed mutagenesis. Of 15 mutant enzymes that were prepared, eight had reduced activities for dextran hydrolysis. Aspartic acids-227 and -342, which are part of the apparent catalytic dyad, were essential for hydrolase activity toward dextran.

Animals↗
PubMed2003

Expression of type 2 iodothyronine deiodinase in corticotropin-secreting mouse pituitary tumor cells is stimulated by glucocorticoid and corticotropin-releasing hormone.

We identified the presence of iodothyronine deiodinase in AtT-20 mouse pituitary tumor cells that secrete corticotropin. Iodothyronine deiodinating activity in AtT-20 cells fulfills all the characteristics of type 2 iodothyronine deiodinase (D2), including the inhibition by thyroid hormones, the insensitivity to inhibition by 6-propyl-2-thiouracil, and the low Michaelis-Menten constant value for T4. Northern analysis using mouse D2 cRNA probe demonstrated the hybridization signal of approximately 7.0 kb in size in AtT-20 cells. D2 activity and D2 mRNA were stimulated by glucocorticoid in a dose-dependent manner but were not stimulated by testosterone or beta-estradiol. D2 expression was stimulated by (Bu)2cAMP, and CRH in a dose-dependent manner in the presence of dexamethasone. These results suggest the previously unrecognized role of local thyroid hormone activation by D2 in the regulation of pituitary corticotrophs.

Adrenocorticotropic Hormone↗
PubMed2003

Relationship between gastric disease and deletion of cag pathogenicity island genes of Helicobacter pylori in gastric juice.

The cag pathogenicity island genes of Helicobacter pylori (ie, cag1, cag5, cagT, cagE, and cagA) were detected by PCR in DNA extracted from endoscopically collected gastric juice, and the relationship between these genes and gastric disease was studied in 25 patients with early gastric cancer, 9 patients with gastric ulcer, and 15 patients with chronic active gastritis. In three patients with early gastric cancer and one patient with gastric ulcer, cag pathogenicity island genes were amplified although H. pylori was not detected by conventional methods. Compared with conventional methods, the sensitivity of detection of cag genes was 92.3% (36/39) and the specificity was 60% (6/10). Among the patients with cagA amplification, only cagE was not amplified in one case each with early cancer and chronic active gastritis. In addition, none of cag1, cag5, cagT, and cagE were amplified in spite of cagA amplification in one patient with gastric ulcer. This method is a simple procedure, has a high sensitivity, and appears to be useful for accurate assessment of infection with cagA-positive strains. Because deletion of cag PAI genes was found in the patients with all three gastric diseases that we studied, it was suggested that the pathogenicity of H. pylori might not be determined by cag PAI genes in those cases.

Aged↗