Ha-il Kim

Regulation of glucose transporter type 4 isoform gene expression in muscle and adipocytes

The gene expression of glucose transporter type 4 isoform (GLUT4) is known to be controlled by metabolic, nutritional, or hormonal status. Understanding the molecular mechanisms governing GLUT4 gene expression is critical, because glucose disposal in the body depends on the activities of GLUT4 in the muscle and adipocytes. The GLUT4 activities are regulated by a variety of mechanisms. One of them is transcriptional regulation. GLUT4 gene expression is regulated by a variety of transcriptional factors in muscle and adipose tissue. These data are accumulating regarding the transcriptional factors regulating GLUT4 gene expression. These include MyoD, MEF2A, GEF, TNF‐α, TR‐1α, KLF15, SREBP‐1c, C/EBP‐α, O/E‐1, free fatty acids, PAPRγ, LXRα, NF‐1, etc. These factors are involved in the positive or negative regulation of GLUT4 gene expression. In addition, there is a complex interplay between these factors in transactivating GLUT4 promoter activity. Understanding the mechanisms controlling GLUT4 gene transcription in these tissues will greatly promote the potential therapeutic drug development for obesity and T2DM. IUBMB Life, 59: 134‐145, 2007

Regulation of GLUT4 gene expression by SREBP-1c in adipocytes

Expression of the GLUT4 (glucose transporter type 4 isoform) gene in adipocytes is subject to hormonal or metabolic control. In the present study, we have characterized an adipose tissue transcription factor that is influenced by fasting/refeeding regimens and insulin. Northern blotting showed that refeeding increased GLUT4 mRNA levels for 24 h in adipose tissue. Consistent with an increased GLUT4 gene expression, the mRNA levels of SREBP (sterol-regulatory-element-binding protein)-1c in adipose tissue were also increased by refeeding. In streptozotocin-induced diabetic rats, insulin treatment increased the mRNA levels of GLUT4 in adipose tissue. Serial deletion, luciferase reporter assays and electrophoretic mobility-shift assay studies indicated that the putative sterol response element is located in the region between bases -109 and -100 of the human GLUT4 promoter. Transduction of the SREBP-1c dominant negative form to differentiated 3T3-L1 adipocytes caused a reduction in the mRNA levels of GLUT4, suggesting that SREBP-1c mediates the transcription of GLUT4. In vivo chromatin immunoprecipitation revealed that refeeding increased the binding of SREBP-1 to the putative sterol-response element in the GLUT4. Furthermore, treating streptozotocin-induced diabetic rats with insulin restored SREBP-1 binding. In addition, we have identified an Sp1 binding site adjacent to the functional sterol-response element in the GLUT4 promoter. The Sp1 site appears to play an additive role in SREBP-1c mediated GLUT4 gene upregulation. These results suggest that upregulation of GLUT4 gene transcription might be directly mediated by SREBP-1c in adipose tissue.

Transcriptional Regulation of Glucose Sensors in Pancreatic β Cells and Liver

Derangement of glucose metabolism is a key feature of T2DM, with the liver and pancreatic β-cells playing a key role in glucose homeostasis. In the postprandial state, glucose is transported into hepatocytes and either metabolized to fatty acids or CO2, or stored as glycogen. Glucose also acts as a key signal in pancreatic β-cells for regulating insulin secretion. Because GLUT2 and GK expressed in liver and β-cells are responsible for sensing glucose levels in the blood, studies on the regulation of these biomolecules are important in understanding glucose homeostasis in vivo. These molecules are known to be regulated either transcriptionally or post-transcriptionally, and recent studies on the structure and function of promoters of these genes have revealed the involvement of various transcriptional factors in their regulation. Here, we review recent progress in elucidating the transcriptional regulation of glucose sensors in the liver and pancreatic β-cells and the relevance to T2DM.

Troglitazone activates p21Cip/WAF1 through the ERK pathway in HCT15 human colorectal cancer cells

In this study, we identified a new mechanism for the anti-proliferation of HCT15 colorectal cancer cells by troglitazone (TRO). Treating HCT15 cells with 20 microM of TRO transiently increased extracellular signal regulated kinase (ERK) activity within 15 min, and this subsequently induced p21Cip/WAF1 cell cycle regulator and localized in the nucleus. Raf-1 modification and MEK activation also occurred after TRO treatment, and Elk-1-dependent trans-reporter gene expression was concomitantly induced. The induction and nuclear localization of p21Cip/WAF1 by TRO were blocked by PD98059 pre-treatment, which suggested a role for the ERK pathway in p21Cip/WAF1 activation. TRO inhibited BrdU incorporation and no BrdU incorporation was observed in most p21Cip/WAF1-activated cells. Therefore, TRO regulates the proliferation of HCT15 cells at least partly by a mechanism involving the activation of p21Cip/WAF1.

Hepatitis B virus X protein modulates peroxisome proliferator-activated receptor γ through protein–protein interaction

Ligand activation of peroxisome proliferator‐activated receptor γ (PPARγ) has been reported to induce growth inhibition and apoptosis in various cancers including hepatocellular carcinoma (HCC). However, the effect of hepatitis B virus X protein (HBx) on PPARγ activation has not been characterized in hepatitis B virus (HBV)‐associated HCC. Herein, we demonstrated that HBx counteracted growth inhibition caused by PPARγ ligand in HBx‐associated HCC cells. We found that HBx bound to DNA binding domain of PPARγ and HBx/PPARγ interaction blocked nuclear localization and binding to recognition site of PPARγ. HBx significantly suppressed a PPARγ‐mediated transactivation. These results suggest that HBx modulates PPARγ function through protein–protein interaction.

Anti-Histone Acetyltransferase Activity from Allspice Extracts Inhibits Androgen Receptor-Dependent Prostate Cancer Cell Growth

Histone acetylation depends on the activity of two enzyme families, histone acetyltransferase (HAT) and deacetylase (HDAC). In this study, we screened various plant extracts to find potent HAT inhibitors. Hot water extracts of allspice inhibited HAT activity, especially p300 and CBP (40% at 100 μg/ml). The mRNA levels of two androgen receptor (AR) regulated genes, PSA and TSC22, decreased with allspice treatment (100 μg/ml). Importantly, in IP western analysis, AR acetylation was dramatically decreased by allspice treatment. Furthermore, chromatin immunoprecipitation indicated that the acetylation of histone H3 in the PSA and B2M promoter regions was also repressed. Finally, allspice treatment reduced the growth of human prostate cancer cells, LNCaP (50% growth inhibition at 200 μg/ml). Taken together, our data indicate that the potent HAT inhibitory activity of allspice reduced AR and histone acetylation and led to decreased transcription of AR target genes, resulting in inhibition of prostate cancer cell growth.

The functional relationship between co-repressor N-CoR and SMRT in mediating transcriptional repression by thyroid hormone receptor α

in BCL3, Spot14, FAS and ADRB2 genes. Our data clearly show that SMRT and N-CoR are independently recruited to various TR target genes. We also present evidence that overexpression of N-CoR can restore repression of endogenous genes after knocking down SMRT. Finally, unliganded, co-repressor-free TR is defective in repression and interacts with a co-activator, p300. Collectively, theseresults suggestthatbothSMRTandN-CoRare limited in cells and that knocking down either of them results in co-repressor-free TRandconsequently de-repressionofTRtarget genes.