Loss of PPARγ Activates ERK 1/2‐NF‐κB‐Nox4‐H2O2 Signaling Axis to Promote Human Pulmonary Artery Smooth Muscle Cell Proliferation

Abstract

Hypoxia stimulates pulmonary hypertension (PH) in part by increasing the proliferation of pulmonary vascular wall cells. Recent evidence suggests that hypoxia stimulates sustained nuclear factor‐kappaB (NF‐κB) activation, increased NADPH oxidase4 (Nox4) expression, and downregulation of peroxisome proliferator‐activated receptor‐gamma (PPARγ) to cause human pulmonary artery smooth muscle cell (HPASMC) proliferation. To further understand the role of reduced PPARγ in PH pathobiology, siRNA was employed to reduce PPARγ levels in HPASMC in vitro under normoxic conditions. PPARγ protein levels were reduced to levels comparable to those observed under hypoxic conditions. Knockdown of PPARγ for 24‐72 hours activated mitogen‐activated protein kinase, ERK 1/2, and NF‐κB. Inhibition of ERK 1/2 prevented NF‐κB activation caused by PPARγ knockdown indicating that ERK 1/2 lies upstream of NF‐κB activation. Knockdown of PPARγ for 72 hours increased NF‐κB‐dependent Nox4 protein expression and H2O2 production. Degradation of H2O2 using PEG‐catalase or inhibition of NF‐κB or Nox4 attenuated HPASMC proliferation caused by PPARγ knockdown. In contrast, PPARγ overexpression reduced ERK 1/2 and NF‐κB activation, Nox4 expression, and proliferation in HPASMC. Collectively, these findings provide novel evidence that PPARγ plays a central role in the regulation of the ERK 1/2‐NF‐κB‐Nox4‐H2O2 signaling axis in HPASMC and that reductions in PPARγ 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.