Peroxisome proliferator-activated receptor-γ and retinoic acid X receptor α represses the TGFβ1 gene via PTEN-mediated p70 ribosomal S6 kinase-1 inhibition:: Role for Zf9 dephosphorylation

Peroxisome proliferator-activated receptor (PPAR)-gamma and retinoic acid X receptor (RXR) heterodimer regulates cell growth and differentiation. Zinc finger transcription factor-9 (Zf9), whose phosphorylation promotes target genes, is a transcription factor essential for transactivation of the transforming growth factor (TGF)-beta 1 gene. This study investigated whether activation of PPAR gamma-RXR heterodimer inhibits TGF beta 1 gene transcription and Zf9 phosphorylation and, if so, what signaling pathway regulates it. Either 15-deoxy- delta(12,14)-prostaglandin J(2) (PGJ(2)) or 9-cis-retinoic acid ( RA) treatment decreased the TGF beta 1 mRNA level in L929 fibroblasts. PGJ(2) + RA, compared with individual treatment alone, synergistically inhibited the TGF beta 1 gene expression, which was abrogated by PPAR gamma antagonists. Likewise, PGJ2 + RA decreased luciferase expression from the TGF beta 1 gene promoter. Promoter deletion analysis of the TGF beta 1 gene revealed that pGL3-323 making up to -323-base pair region, but lacking PPAR-responsive elements, responded to PGJ(2) + RA. PGJ(2) + RA treatment inhibited the activity of p70 ribosomal S6 kinase-1 (S6K1), abolishing Zf9 phosphorylation at serine as did rapamycin [ a mammalian target of rapamycin ( mTOR) inhibitor]. Zf9 dephosphorylation by PGJ(2) + RA was reversed by transfection of cells with the plasmid encoding constitutively active S6K1 (CA-S6K1). Transfection with dominant negative S6K1 inhibited the TGF beta 1 gene. TGF beta 1 gene repression by PGJ(2) + RA was consistently antagonized by CA-S6K1. Ectopic expression of PPAR gamma 1 and RXR alpha repressed pGL3-323 transactivation with S6K1 inhibition, which was abrogated by CA-S6K1 transfection. PGJ(2) + RA induced phosphatase and tensin homolog deleted on chromosome 10 ( PTEN), whose overexpression repressed the TGF beta 1 gene through S6K1 inhibition, decreasing extracellular signal-regulated kinase 1/2-90-kDa ribosomal S6 kinase 1 and Akt-mTOR phosphorylations. Data indicate that activation of PPAR gamma-RXR heterodimer represses the TGF beta 1 gene and induces Zf9 dephosphorylation via PTEN-mediated S6K1 inhibition, providing insight into pharmacological manipulation of the TGF beta 1 gene regulation.