Satori Tokudome

Intramolecular control of protein stability, subnuclear compartmentalization, and coactivator function of peroxisome proliferator-activated receptor γ coactivator 1α

Peroxisome proliferator-activated receptor γ coactivator (PGC)-1 is a critical transcriptional regulator of energy metabolism. Here we found that PGC-1α is a short lived and aggregation-prone protein. PGC-1α localized throughout the nucleoplasm and was rapidly destroyed via the ubiquitin-proteasome pathway. Upon proteasome inhibition, PGC-1α formed insoluble polyubiquitinated aggregates. Ubiquitination of PGC-1α depended on the integrity of the C terminus-containing arginine-serine-rich domains and an RNA recognition motif. Interestingly, ectopically expressed C-terminal fragment of PGC-1α was autonomously ubiquitinated and aggregated with promyelocytic leukemia protein. Cooperation of the N-terminal region containing two PEST-like motifs was required for prevention of aggregation and targeting of the polyubiquitinated PGC-1α for degradation. This region thereby negatively controlled the aggregation properties of the C-terminal region to regulate protein turnover and intranuclear compartmentalization of PGC-1α. Exogenous expression of the PGC-1α C-terminal fragment interfered with degradation of full-length PGC-1α and enhanced its coactivation properties. We concluded that PGC-1α function is critically regulated at multiple steps via intramolecular cooperation among several distinct structural domains of the protein.

Ligand-based gene expression profiling reveals novel roles of glucocorticoid receptor in cardiac metabolism

Recent studies have documented various roles of adrenal corticosteroid signaling in cardiac physiology and pathophysiology. It is known that glucocorticoids and aldosterone are able to bind glucocorticoid receptor (GR) and mineralocorticoid receptor, and these ligand-receptor interactions are redundant. It, therefore, has been impossible to delineate how these nuclear receptors couple with corticosteroid ligands and differentially regulate gene expression for operation of their distinct functions in the heart. Here, to particularly define the role of GR in cardiac muscle cells, we applied a ligand-based approach involving the GR-specific agonist cortivazol (CVZ) and the GR antagonist RU-486 and performed microarray analysis using rat neonatal cardiomyocytes. We indicated that glucocorticoids appear to be a major determinant of GR-mediated gene expression when compared with aldosterone. Moreover, expression profiles of these genes highlighted numerous roles of glucocorticoids in various aspects of cardiac physiology. At first, we identified that glucocorticoids, via GR, induce mRNA and protein expression of a transcription factor Kruppel-like factor 15 and its downstream target genes, including branched-chain aminotransferase 2, a key enzyme for amino acid catabolism in the muscle. CVZ treatment or overexpression of KLF15 decreased cellular branched-chain amino acid concentrations and introduction of small-interfering RNA against KLF15 cancelled these CVZ actions in cardiomyocytes. Second, glucocorticoid-GR signaling promoted gene expression of the enzymes involved in the prostaglandin biosynthesis, including cyclooxygenase-2 and phospholipase A2 in cardiomyocytes. Together, we may conclude that GR signaling should have distinct roles for maintenance of cardiac function, for example, in amino acid catabolism and prostaglandin biosynthesis in the heart.