An Isoform of the Oncogenic Splice Variant AIMP2-DX2 Detected by a Novel Monoclonal Antibody.

AIMP2-DX2, an exon 2-deleted splice variant of AIMP2 (aminoacyl-tRNA synthetase-interacting multifunctional protein 2), is highly expressed in lung cancer and involved in tumor progression in vivo. Oncogenic function of AIMP2-DX2 and its correlation with poor prognosis of cancer patients have been well established; however, the application of this potentially important biomarker to cancer research and diagnosis has been hampered by a lack of antibodies specific for the splice variant, possibly due to the poor immunogenicity and/or stability of AIMP2-DX2. In this study a monoclonal antibody, H5, that specifically recognizes AIMP2-DX2 and its isoforms was generated via rabbit immunization and phage display techniques, using a short peptide corresponding to the exon 1/3 junction sequence as an antigen. Furthermore, based on mutagenesis, limited cleavage, and mass spectrometry studies, it is also suggested that the endogenous isoform of AIMP2-DX2 recognized by H5 is produced by proteolytic cleavage of 33 amino acids from N-terminus and is capable of inducing cell proliferation similarly to the uncleaved protein. H5 monoclonal antibody is applicable to enzyme-linked immunosorbent assay, immunoblot, immunofluorescence, and immunohistochemistry, and expected to be a valuable tool for detecting AIMP2-DX2 with high sensitivity and specificity for research and diagnostic purposes.

Insulin-Inducible SMILE Inhibits Hepatic Gluconeogenesis.

The role of a glucagon/cAMP-dependent protein kinase-inducible coactivator PGC-1α signaling pathway is well characterized in hepatic gluconeogenesis. However, an opposing protein kinase B (PKB)/Akt-inducible corepressor signaling pathway is unknown. A previous report has demonstrated that small heterodimer partner-interacting leucine zipper protein (SMILE) regulates the nuclear receptors and transcriptional factors that control hepatic gluconeogenesis. Here, we show that hepatic SMILE expression was induced by feeding in normal mice but not in db/db and high-fat diet (HFD)-fed mice. Interestingly, SMILE expression was induced by insulin in mouse primary hepatocyte and liver. Hepatic SMILE expression was not altered by refeeding in liver-specific insulin receptor knockout (LIRKO) or PKB ß-deficient (PKBß(-/-)) mice. At the molecular level, SMILE inhibited hepatocyte nuclear factor 4-mediated transcriptional activity via direct competition with PGC-1α. Moreover, ablation of SMILE augmented gluconeogenesis and increased blood glucose levels in mice. Conversely, overexpression of SMILE reduced hepatic gluconeogenic gene expression and ameliorated hyperglycemia and glucose intolerance in db/db and HFD-fed mice. Therefore, SMILE is an insulin-inducible corepressor that suppresses hepatic gluconeogenesis. Small molecules that enhance SMILE expression would have potential for treating hyperglycemia in diabetes.

CCAR1 promotes chromatin loading of androgen receptor (AR) transcription complex by stabilizing the association between AR and GATA2.

Androgen receptor (AR), a ligand-dependent transcription factor, plays a critical role in prostate cancer onset and progression, and its transcriptional function is mediated largely by distinct nuclear receptor co-regulators. Here, we show that cell cycle and apoptosis regulator 1 (CCAR1) functions as an AR co-activator. CCAR1 interacted with and enhanced the transcriptional activity of AR. Depletion of CCAR1 caused reduction in androgen-dependent expression of a subset of AR target genes. We further showed that CCAR1 is required for recruitment of AR, MED1 and RNA polymerase II to the enhancers of AR target genes and for androgen-induced long-range prostate specific antigen enhancer-promoter interaction. The molecular mechanism underlying CCAR1 function in AR-mediated transcription involves CCAR1-mediated enhanced recruitment of GATA2, a pioneer factor for AR, to AR-binding sites. CCAR1 stabilized the interaction between AR and GATA2 by interacting directly with both proteins, thereby facilitating AR and GATA2 occupancy on the enhancers. Furthermore, CCAR1 depletion inhibited the growth, migration, invasion of prostate cancer cells and reduced the tumorigenicity of prostate cancer cells in vivo. Our results firmly established CCAR1 as an AR co-activator that plays a key role in AR transcription complex assembly and has an important physiological role in androgen signaling and prostate tumorigenesis.

Endoplasmic reticulum stress promotes LIPIN2-dependent hepatic insulin resistance.

OBJECTIVE: Diet-induced obesity (DIO) is linked to peripheral insulin resistance-a major predicament in type 2 diabetes. This study aims to identify the molecular mechanism by which DIO-triggered endoplasmic reticulum (ER) stress promotes hepatic insulin resistance in mouse models. RESEARCH DESIGN AND METHODS: C57BL/6 mice and primary hepatocytes were used to evaluate the role of LIPIN2 in ER stress-induced hepatic insulin resistance. Tunicamycin, thapsigargin, and lipopolysaccharide were used to invoke acute ER stress conditions. To promote chronic ER stress, mice were fed with a high-fat diet for 8-12 weeks. To verify the role of LIPIN2 in hepatic insulin signaling, adenoviruses expressing wild-type or mutant LIPIN2, and shRNA for LIPIN2 were used in animal studies. Plasma glucose, insulin levels as well as hepatic free fatty acids, diacylglycerol (DAG), and triacylglycerol were assessed. Additionally, glucose tolerance, insulin tolerance, and pyruvate tolerance tests were performed to evaluate the metabolic phenotype of these mice. RESULTS: LIPIN2 expression was enhanced in mouse livers by acute ER stress-inducers or by high-fat feeding. Transcriptional activation of LIPIN2 by ER stress is mediated by activating transcription factor 4, as demonstrated by LIPIN2 promoter assays, Western blot analyses, and chromatin immunoprecipitation assays. Knockdown of hepatic LIPIN2 in DIO mice reduced fasting hyperglycemia and improved hepatic insulin signaling. Conversely, overexpression of LIPIN2 impaired hepatic insulin signaling in a phosphatidic acid phosphatase activity-dependent manner. CONCLUSIONS: These results demonstrate that ER stress-induced LIPIN2 would contribute to the perturbation of hepatic insulin signaling via a DAG-protein kinase C ε-dependent manner in DIO mice.

AMPK-dependent repression of hepatic gluconeogenesis via disruption of CREB.CRTC2 complex by orphan nuclear receptor small heterodimer partner.

Orphan nuclear receptor small heterodimer partner (SHP) plays a key role in transcriptional repression of gluconeogenic enzyme gene expression. Here, we show that SHP inhibited protein kinase A-mediated transcriptional activity of cAMP-response element-binding protein (CREB), a major regulator of glucose metabolism, to modulate hepatic gluconeogenic gene expression. Deletion analysis of phosphoenolpyruvate carboxykinase (PEPCK) promoter demonstrated that SHP inhibited forskolin-mediated induction of PEPCK gene transcription via inhibition of CREB transcriptional activity. In vivo imaging demonstrated that SHP inhibited CREB-regulated transcription coactivator 2 (CRTC2)-mediated cAMP-response element-driven promoter activity. Furthermore, overexpression of SHP using adenovirus SHP decreased CRTC2-dependent elevations in blood glucose levels and PEPCK or glucose-6-phosphatase (G6Pase) expression in mice. SHP and CREB physically interacted and were co-localized in vivo. Importantly, SHP inhibited both wild type CRTC2 and S171A (constitutively active form of CRTC2) coactivator activity and disrupted CRTC2 recruitment on the PEPCK gene promoter. In addition, metformin or overexpression of a constitutively active form of AMPK (Ad-CA-AMPK) inhibited S171A-mediated PEPCK and G6Pase gene expression, and hepatic glucose production and knockdown of SHP partially relieved the metformin- and Ad-CA-AMPK-mediated repression of hepatic gluconeogenic enzyme gene expression in primary rat hepatocytes. In conclusion, our results suggest that a delayed effect of metformin-mediated induction of SHP gene expression inhibits CREB-dependent hepatic gluconeogenesis.

DAX-1 acts as a novel corepressor of orphan nuclear receptor HNF4alpha and negatively regulates gluconeogenic enzyme gene expression.

DAX-1 (dosage-sensitive sex reversal adrenal hypoplasia congenital critical region on X chromosome, gene 1) is an atypical member of the nuclear receptor family and acts as a corepressor of a number of nuclear receptors. HNF4alpha (hepatocyte nuclear factor 4alpha) is a liver-enriched transcription factor that controls the expression of a variety of genes involved in cholesterol, fatty acid, and glucose metabolism. Here we show that DAX-1 inhibits transcriptional activity of HNF4alpha and modulates hepatic gluconeogenic gene expression. Hepatic DAX-1 expression is increased by insulin and SIK1 (salt-inducible kinase 1), whereas it is decreased in high fat diet-fed and diabetic mice. Coimmunoprecipitation assay from mouse liver samples depicts that endogenous DAX-1 interacts with HNF4alpha in vivo. In vivo chromatin immunoprecipitation assay affirms that the recruitment of DAX-1 on the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is inversely correlated with the recruitment of PGC-1alpha and HNF4alpha under fasting and refeeding, showing that DAX-1 could compete with the coactivator PGC-1alpha for binding to HNF4alpha. Adenovirus-mediated expression of DAX-1 decreased both HNF4alpha- and forskolin-mediated gluconeogenic gene expressions. In addition, knockdown of DAX-1 partially reverses the insulin-mediated inhibition of gluconeogenic gene expression in primary hepatocytes. Finally, DAX-1 inhibits PEPCK and glucose-6-phosphatase gene expression and significantly lowers fasting blood glucose level in high fat diet-fed mice, suggesting that DAX-1 can modulate hepatic gluconeogenesis in vivo. Overall, this study demonstrates that DAX-1 acts as a corepressor of HNF4alpha to negatively regulate hepatic gluconeogenic gene expression in liver.

Salt-inducible kinase regulates hepatic lipogenesis by controlling SREBP-1c phosphorylation.

Liver plays a major role in regulating energy homeostasis in mammals. During feeding conditions, excessive glucose is converted into a preferred storage form of energy sources as triacylglycerol in liver via a collective metabolic pathway termed lipogenesis. Sterol regulatory element-binding protein 1c is a master regulator for this process by activating number of enzyme genes, such as Fasn or Acaca, that are involved in this pathway at the transcriptional level. Here we show that the salt-inducible kinase (SIK) family of proteins regulates the hepatic lipogenesis by modulating SREBP-1c activity. Overexpression of SIK1 inhibits hepatic expression of lipogenic genes, such as Fasn, whereas knockdown of SIK1 in liver greatly enhances their expression. Regulation of the Fasn gene by SIK kinases is mediated at the level of transcription via phosphorylation and inactivation of nuclear SREBP-1c. Among candidate sites for SIK-dependent regulation of SREBP-1c, the serine 329 residue is shown to be a critical regulatory site for SIK-mediated repression of SREBP-1c activity by in vitro kinase assay and reverse transcription-PCR analysis in primary hepatocytes. Finally, reduced hepatic triacylglycerol levels and lipogenic gene expression by adenoviral SIK1 transgenic expression are restored to normal levels by co-infection of mutant SREBP-1c, suggesting that SIK-dependent regulation of hepatic lipogenesis is indeed mediated through the phosphorylation of SREBP-1c in vivo. The process for the development of nonalcoholic fatty liver involves de novo lipogenesis via the activation of SREBP-1c. Modulation of SREBP-1c activity by SIK proteins would provide an attractive means for the regulation of such diseases.