Peroxisome proliferator-activated receptor γ inhibits expression of minichromosome maintenance proteins in vascular smooth muscle cells

Abstract

Using a cDNA array consisting only of cell cycle genes, we found that a novel nonthiazolidinedione partial peroxisome proliferator-activated receptor γ (PPARγ) agonist (nTZDpa) inhibited expression of minichromosome maintenance (MCM) proteins 6 and 7 in vascular smooth muscle cells. MCM proteins are required for the initiation and elongation stages of DNA replication and are regulated by the transcription factor E2F. Mitogen-induced MCM6 and MCM7 mRNA expression was potently inhibited by nTZDpa and to a lesser degree by the full PPARγ agonist, rosiglitazone. Inhibition of MCM6 and MCM7 expression by nTZDpa and rosiglita-zone paralleled their effect to inhibit phosphorylation of the retinoblastoma protein and cell proliferation. Transient transfection experiments revealed that the nTZDpa inhibited mitogen-induced MCM6 and MCM7 promoter activity, implicating a transcriptional mechanism. Adenoviral-mediated E2F overexpression reversed the suppressive effect of nTZDpa on MCM6 and MCM7 expression. Furthermore, activity of a luciferase reporter plasmid driven by multiple E2F elements was inhibited by nTZDpa, indicating that their down-regulation by nTZDpa involves an E2F-dependent mechanism. Overexpression of dominant-negative PPARγ or addition of a PPARγ antagonist, GW 9662, blocked nTZDpa inhibition of MCM7 transcription. Adenovirus-mediated overexpression of constitutively active PPARγ inhibited MCM7 expression in a similar manner as the nTZDpa. These findings provide strong evidence that activation of PPARγ attenuates MCM7 transcription and support the important role of this nuclear receptor in regulating vascular smooth muscle cell proliferation.