FoxO-dependent and -independent mechanisms mediate SirT1 effects on IGFBP-1 gene expression.
ABSTRACT Sirtuin 1 (SirT1), an NAD-dependent deacetylase that is important for promoting longevity during caloric restriction, can deacetylate and enhance the function of forkhead box transcription factors, O subfamily (FoxO). We examined the effect of SirT1 on the regulation of insulin-like growth factor-binding protein 1 (IGFBP-1), a known target of FoxO proteins that is increased in fasting. Co-transfection with a SirT1 expression vector dose-dependently stimulated IGFBP-1 promoter activity and a heterologous reporter gene construct containing three FoxO-binding sites linked to a minimal promoter. This effect is mimicked by 20muM resveratrol, a potent SirT1 activator, and immunoprecipitation and Western blotting confirm that SirT1 and FoxO1 interact in cells. Interestingly, mutation of FoxO-binding sites in the IGFBP-1 promoter reduces, but does not completely disrupt, the stimulatory effect of SirT1 on promoter activity. We found that overexpression of SirT1 is accompanied by enhanced mitogen-activated protein kinase (MAPK) activation. Treatment of SirT1-cotransfected cells with PD98059, which inhibits MAPK activation, decreased IGFBP-1 promoter activity by approximately 50%, in a FoxO-binding site-independent manner, and disrupts the residual effect of SirT1. These results indicate that SirT1 stimulates IGFBP-1 promoter activity through FoxO-dependent and -independent mechanisms, and provides the first evidence that activation of MAPK contributes to effects of SirT1 on gene expression.
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ABSTRACT: The transition from a chemotherapy-responsive cancer to a chemotherapy-resistant one is accompanied by increased expression of multidrug resistance 1 (MDR1, p-glycoprotein), which plays an important role in the efflux from the target cell of many anticancer agents. We recently showed that a Forkhead box-containing protein of the O subfamily 1 (FoxO1) is a key regulator of MDR1 gene transcription. Because nuclear localization of FoxO1 is regulated by silent information regulator two ortholog 1 (SIRT1) deacetylase, we wondered whether SIRT1 dominates MDR1 gene expression in breast cancer cells. Overexpression of SIRT1 enhanced both FoxO reporter activity and nuclear levels of FoxO1. Protein expression of MDR1 and gene transcriptional activity were also up-regulated by SIRT1 overexpression. In addition, SIRT1 inhibition reduced both nuclear FoxO1 levels and MDR1 expression in doxorubicin-resistant breast cancer cells (MCF-7/ADR) cells. A potent SIRT1 inhibitor, amurensin G (from Vitis amurensis), was identified by screening plant extracts and bioassay-guided fractionation. The compound suppressed FoxO1 activity and MDR1 expression in MCF-7/ADR cells. Moreover, pretreatment of MCF-7/ADR cells with 1 μg/ml amurensin G for 24 h increased cellular uptake of doxorubicin and restored the responsiveness of MCF-7/ADR cells to doxorubicin. In xenograft studies, injection of 10 mg/kg i.p. amurensin G substantially restored the ability of doxorubicin to inhibit MCF-7/ADR-induced tumor growth. These results suggest that SIRT1 is a potential therapeutic target of MDR1-mediated chemoresistance and that it may be possible to develop amurensin G as a useful agent for chemoresistance reversal.Molecular pharmacology 11/2010; 78(5):855-64. · 4.53 Impact Factor
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ABSTRACT: Insulin regulation of hepatic gene transcription is a vital component of glucose homeostasis. Understanding the molecular regulationof thisprocess aids the searchfor the defect(s) that promotesinsulin-resistant states, such asdiabetesmellitus. We havepreviously shownthat the insulin regulationof hepatic IGF-binding protein-1 (IGFBP1) expression requiresthe signalling proteins phosphatidylinositol 3-kinase (PI 3-kinase) and mammalian target of rapamycin (mTOR). In this report, we demonstrate that activation of the mTOR pathway, without activation of its upstream regulator PI 3-kinase, reduces IGFBP1 expression. Therefore, mTOR activation is sufficient to mimic insulin regulation of this gene. However, longer exposure (>3 h) of cells to insulin reduces the importance of this pathway in insulin regulation of the gene, suggesting a temporal switch in signalling mechanisms linking insulin action to the IGFBP1 gene promoter. In contrast, the activation of PI 3-kinase is required for insulin regulation of IGFBP1 under all conditions tested. Therefore, an mTOR-independent, PI 3-kinase-dependent pathway becomes more important in IGFBP1 regulation after long exposure to insulin. This is a novel concept in insulin regulation of gene expression and demonstrates the importance of temporal analysis of signalling processes.Journal of Molecular Endocrinology 11/2006; 37(2):227-37. · 3.58 Impact Factor
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ABSTRACT: The forkhead box O transcription factors convert a variety of external stimuli, including growth factors, nutrients, and oxidative stress, into diverse biological responses through modulation of specific gene expression. Forkhead box O regulation is principally achieved by two distinct mechanisms: post-translational modifications and protein-protein interactions. Among several modifications of forkhead box O factors, we focus on reversible acetylation, describing past research and current advances. In the latter part of this review, we also provide an overview of forkhead box O-binding partners that control the transcriptional activity of forkhead box O factors. These two layers of regulation mostly overlap and thereby enable a more precise fine-tuning of forkhead box O functions involved in metabolism, longevity, and tumor suppression. This article is part of a Special Issue entitled: PI3K-AKT-FoxO axis in cancer and aging.Biochimica et Biophysica Acta 03/2011; 1813(11):1954-60. · 4.66 Impact Factor