PGC1α-mediated Mitofusin-2 Deficiency in Female Rats and Humans with Pulmonary Arterial Hypertension

Rationale: Pulmonary arterial hypertension (PAH) is a lethal, female-predominant, vascular disease. Pathologic changes in PA smooth muscle cells (PASMC) include excessive proliferation, apoptosis-resistance, and mitochondria! fragmentation. Activation of dynamin-related protein increases mitotic fission and promotes this proliferation-apoptosis imbalance. The contribution of decreased fusion and reduced mitofusin-2 (MFN2) expression to PAH is unknown. Objectives: We hypothesize that decreased MFN2 expression promotes mitochondrial fragmentation, increases proliferation, and impairs apoptosis. The role of MFN2's transcriptional coactivator, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1 alpha), was assessed. MFN2 therapy was tested in PAH PASMC and in models of PAH. Methods: Fusion and fission mediators were measured in lungs and PASMC from patients with PAH and female rats with monocrotaline or chronic hypoxia + Sugen-5416 (CH + SU) PAN. The effects of adenoviral mitofusin-2 (Ad-MFN2) overexpression were measured in vitro and in vivo. Measurements and Main Results: In normal PASMC, siMFN2 reduced expression of MFN2 and PGC1 alpha; conversely, siPGC1 alpha reduced PGC1 alpha and MFN2 expression. Both interventions caused mitochondrial fragmentation. siMFN2 increased proliferation. In rodent and human PAH PASMC, MFN2 and PGC1 alpha were decreased and mitochondria were fragmented. Ad-MFN2 increased fusion, reduced proliferation, and increased apoptosis in human PAH and CH + SU. In CH + SU, Ad-MFN2 improved walking distance (381 +/- 35 vs. 245 +/- 39 m; P < 0.05); decreased pulmonary vascular resistance (0.18 +/- 0.02 vs. 0.38 +/- 0.14 mm Hg/ml/min; P < 0.05); and decreased PA medial thickness (14.5 +/- 0.8 vs. 19 +/- 1.7%; P < 0.05). Lung vascularity was increased by MFN2. Conclusions: Decreased expression of MFN2 and PGC1 alpha contribute to mitochondrial fragmentation and a proliferation-apoptosis imbalance in human and experimental PAH. Augmenting MFN2 has therapeutic benefit inhuman and experimental PAH.