Transcriptional regulation of methionine adenosyltransferase 2A by peroxisome proliferator-activated receptors in rat hepatic stellate cells

Authors

  • Komal Ramani,

    Corresponding author
    1. Division of Gastroenterology and Liver Diseases, USC Research Center for Liver Diseases, Keck School of Medicine, University of Southern California, Los Angeles, CA
    • Division of Gastrointestinal and Liver Diseases, HMR Building, 413, Department of Medicine, Keck School of Medicine, University of Southern California, 2011 Zonal Ave., Los Angeles, CA 90033
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    • fax: 323-442-3234

  • Maria Lauda Tomasi

    1. Division of Gastroenterology and Liver Diseases, USC Research Center for Liver Diseases, Keck School of Medicine, University of Southern California, Los Angeles, CA
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  • Potential conflict of interest: Nothing to report.

  • Supported by National Institutes of Health grant 5K99AA017774-02 (to K. R.). Maria Lauda Tomasi is a recipient of the F32 training grant from the National Institutes of Health (F32AA201502). The National Institute on Alcohol Abuse and Alcoholism's Non-Parenchymal Liver Cell Core (R24 AA012885) provided isolation and culture of hepatic stellate cells.

Abstract

Methionine adenosyltransferases (MATs) are critical enzymes that catalyze the formation of the methyl donor S-adenosyl methionine (SAM). The MAT2A gene, which encodes the catalytic subunit α2, is induced in dedifferentiated liver. We previously demonstrated that MAT2A expression is enhanced in activated hepatic stellate cells (HSCs) and that silencing this gene reduces HSC activation. In this study, we examined the molecular mechanisms responsible for the transcriptional regulation of the MAT2A gene in HSCs. We identified peroxisome proliferator-activated receptor (PPAR) response elements (PPREs) in the rat MAT2A promoter. The PPARγ agonist rosiglitazone (RSG) promoted quiescence in the activated rat HSC cell line (BSC) or culture-activated primary rat HSCs, decreased MAT2A expression and promoter activity, and enhanced PPARγ binding to MAT2A PPREs. In vivo HSC activation in bile duct–ligated rats lowered PPARγ interaction with MAT2A PPREs. Silencing PPARγ increased MAT2A transcription, whereas overexpressing it had the opposite effect, demonstrating that PPARγ negatively controls this gene. Site-directed mutagenesis of PPREs abolished PPARγ recruitment to the MAT2A promoter and its inhibitory effect on MAT2A transcription in quiescent HSCs. PPRE mutations decreased the basal promoter activity of MAT2A in activated HSCs independent of PPARγ, indicating that other factors might be involved in PPRE interaction. We identified PPARβ binding to wild-type but not to mutated PPREs in activated cells. Furthermore, silencing PPARβ inhibited MAT2A expression and promoter activity. Forced expression of MAT2A in RSG-treated HSCs lowered PPARγ and enhanced PPARβ expression, thereby promoting an activated phenotype. Conclusion: We identified PPARγ as a negative regulator of MAT2A in quiescent HSCs. A switch from quiescence to activation abolishes this control and allows PPARβ to up-regulate MAT2A transcription. (HEPATOLOGY 2012;55:1942–1953)

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