Troglitazone inhibits mitogenic signaling by insulin in vascular smooth muscle cells

Troglitazone (TRO) is an oral insulin-sensitizer that has direct effects on the vasculature to inhibit cell growth and migration. In vascular smooth muscle cells (VSMCs), insulin transduces a mitogenic signal that is dependent on the ERK1/2 MAP kinases, We examined the effects of TRO on this pathway and found that it inhibits mitogenic signaling. In quiescent VSMCs, insulin (1 mu M) induced a 3.2-fold increase in DNA synthesis. TRO (1-20 mu M) inhibited insulin-stimulated DNA synthesis by 72.8% at the maximal concentration. TRO at and 10 mu M had no significant effect on insulin-stimulated ERK1/2 activity. At 20 mu M, however, TRO modestly enhanced insulin-stimulated ERK1/2 activity by 1.5-fold. ERKs transduce a mitogenic signal by phosphorylating transcription factors such as Elk-1, which regulate critical growth-response genes. We used GAL-Elk-1 expression plasmids to detect ERK-dependent activation of Elk-l. TRO at 1-20 mu M potently inhibited insulin-stimulated. ERK1/2-dependent Elk-l transcription factor activity. Neither early steps in insulin signaling nor the phosphatidylinositol 3-kinase (PI3K) branch of this pathway were affected by TRO, because it had no effect on IRS-I phosphorylation, PI3K/IRS-1 association, or Akt phosphorylation. Because TRO is a known ligand for the nuclear transcription factor peroxisome proliferator-activated receptor gamma (PPAR-gamma), we tested two other ligands for this receptor, rosiglitazone (RSG) and 15-deoxy-Delta(12,14) prostaglandin J2 (15d-PGJ(2)). Both also inhibited insulin-induced DNA synthesis. In summary, these data show that TRO inhibits mitogenic signaling by insulin at a point distal of ERK1/2 activation, potentially by a PPAR gamma-mediated inhibition of ERK-dependent phosphorylation and activation of nuclear transcription factors that regulate cell growth.