Published online before print October 14, 2004, doi: 10.1161/01.RES.0000147961.09840.fb
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© 2004 American Heart Association, Inc.
Molecular Medicine |
A G/C Element Mediates Repression of the SM22
Promoter Within Phenotypically Modulated Smooth Muscle Cells in Experimental Atherosclerosis
From the Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville.
Correspondence to Gary K. Owens, PhD, Department of Molecular Physiology and Biological Physics, University of Virginia, PO Box 800736, Charlottesville, VA 22908-0736. E-mail gko{at}virginia.edu
A hallmark of smooth muscle cell (SMC) phenotypic switching in atherosclerotic lesions is suppression of SMC differentiation marker gene expression. Yet little is known regarding the molecular mechanisms that control this process. Here we show that transcription of the SMC differentiation marker gene SM22
is reduced in atherosclerotic lesions and identify a cis regulatory element in the SM22 promoter required for this process. Transgenic mice carrying the SM22 promoter–ß-galactosidase (ß-gal) reporter transgene were crossed to apolipoprotein E (ApoE)–/– mice. Cells of the fibrous cap, intima, and underlying media showed complete loss of ß-gal activity in advanced atherosclerotic lesions. Of major significance, mutation of a G/C-rich cis element in the SM22 promoter prevented the decrease in SM22 promoter–ß-gal reporter transgene expression, including in cells that compose the fibrous cap of the lesion and in medial cells in proximity to the lesion. To begin to assess mechanisms whereby the G/C repressor element mediates suppression of SM22 in atherosclerosis, we tested the hypothesis that effects may be mediated by platelet-derived growth factor (PDGF)-BB–induced increases in the G/C binding transcription factor Sp1. Consistent with this hypothesis, results of studies in cultured SMCs showed that: (1) PDGF-BB increased expression of Sp1; (2) PDGF-BB and Sp1 profoundly suppressed SM22 promoter activity as well as smooth muscle myosin heavy chain promoter activity through mechanisms that were at least partially dependent on the G/C cis element; and (3) a short interfering RNA to Sp1 increased basal expression and attenuated PDGF-BB induced suppression of SM22. Together, these results support a model whereby a G/C repressor element within the SM22 promoter mediates transcriptional repression of this gene within phenotypically modulated SMCs in experimental atherosclerosis and provide indirect evidence implicating PDGF-BB and Sp1 as possible mediators of these effects.
Key Words: atherosclerosis • proliferation • smooth muscle differentiation • transcriptional regulation • vascular smooth muscle cell proliferation
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