Peroxisome proliferator-activated receptor gamma depletion stimulates Nox4 expression and human pulmonary artery smooth muscle cell proliferation

Hypoxia stimulates pulmonary hypertension (PH) in part by increasing the proliferation of pulmonary vascular wall cells. Recent evidence suggests that signaling events involved in hypoxia-induced cell proliferation include sustained nuclear factor-kappaB (NF-kappa B) activation, increased NADPH oxidase 4 (Nox4) expression, and downregulation of peroxisome proliferator-activated receptor gamma (PPAR gamma) levels. To further understand the role of reduced PPAR gamma levels associated with PH pathobiology, siRNA was employed to reduce PPAR gamma levels in human pulmonary artery smooth muscle cells (HPASMC) in vitro under normoxic conditions. PPAR gamma protein levels were reduced to levels comparable to those observed under hypoxic conditions. Depletion of PPAR gamma for 24-72 h activated mitogen-activated protein kinase, ERIC 1/2, and NF-kappa B. Inhibition of ERIC 1/2 prevented NF-kappa B activation caused by PPAR gamma depletion, indicating that ERIC 1/2 lies upstream of NF-kappa B activation. Depletion of PPAR gamma for 72 h increased NF-kappa B-dependent Nox4 expression and H2O2 production. Inhibition of NF-kappa B or Nox4 attenuated PPAR gamma depletion-induced HPASMC proliferation. Degradation of PPAR gamma depletion-induced H2O2 by PEG-catalase prevented HPASMC proliferation and also ERIC 1/2 and NF-kappa B activation and Nox4 expression, indicating that H2O2 participates in feed-forward activation of the above signaling events. Contrary to the effects of PPAR gamma depletion, HPASMC PPAR gamma overexpression reduced ERIC 1/2 and NF-kappa B activation, Nox4 expression, and cell proliferation. Taken together these findings provide novel evidence that PPAR gamma plays a central role in the regulation of the ERK1/2-NF-kappa B-Nox4-H2O2 signaling axis in HPASMC. These results indicate that reductions in PPAR gamma caused by pathophysiological stimuli such as prolonged hypoxia exposure are sufficient to promote the proliferation of pulmonary vascular smooth muscle cells observed in PH pathobiology. Published by Elsevier Inc.