Sildenafil potentiates bone morphogenetic protein signaling in pulmonary arterial smooth muscle cells and in experimental pulmonary hypertension

FK506 activates BMPR2, rescues endothelial dysfunction, and reverses pulmonary hypertension

Dysfunctional bone morphogenetic protein receptor-2 (BMPR2) signaling is implicated in the pathogenesis of pulmonary arterial hypertension (PAH). We used a transcriptional high-throughput luciferase reporter assay to screen 3,756 FDA-approved drugs and bioactive compounds for induction of BMPR2 signaling. The best response was achieved with FK506 (tacrolimus), via a dual mechanism of action as a calcineurin inhibitor that also binds FK-binding protein-12 (FKBP12), a repressor of BMP signaling. FK506 released FKBP12 from type I receptors activin receptor-like kinase 1 (ALK1), ALK2, and ALK3 and activated downstream SMAD1/5 and MAPK signaling and ID1 gene regulation in a manner superior to the calcineurin inhibitor cyclosporine and the FKBP12 ligand rapamycin. In pulmonary artery endothelial cells (ECs) from patients with idiopathic PAH, low-dose FK506 reversed dysfunctional BMPR2 signaling. In mice with conditional Bmpr2 deletion in ECs, low-dose FK506 prevented exaggerated chronic hypoxic PAH associated with induction of EC targets of BMP signaling, such as apelin. Low-dose FK506 also reversed severe PAH in rats with medial hypertrophy following monocrotaline and in rats with neointima formation following VEGF receptor blockade and chronic hypoxia. Our studies indicate that low-dose FK506 could be useful in the treatment of PAH.

Id proteins are critical downstream effectors of BMP signaling in human pulmonary arterial smooth muscle cells

Bone morphogenetic protein type II receptor (BMPR-II) mutations are responsible for over 70% of cases of heritable pulmonary arterial hypertension (PAH). Loss of BMP signaling promotes pulmonary vascular remodeling via modulation of pulmonary artery smooth muscle cell (PASMC) proliferation. Id proteins (Id1-4) are major downstream transcriptional targets of BMP signaling. However, the impact of BMPR-II mutation on the expression of the range of Id proteins and the contribution of individual Id proteins to abnormal PASMC function remain unclear. Human PASMCs were used to determine the expression of Id proteins (Id1-4) by real-time PCR and immunoblotting. The BMP responses in control cells were compared with PASMCs harboring BMPR-II mutations and cells in which BMPR-II was knocked down by siRNA transfection. Id3 expression in pulmonary vessels was also investigated in BMPR-II mutant mice and in patients with heritable PAH. BMP4 and BMP6, but not BMP9, induced mRNA expression of Id1, Id2, and Id3. The BMP-stimulated induction of Id1 and Id3 was markedly reduced in BMPR-II mutant PASMCs and in control PASMCs following siRNA silencing of BMPR-II. Pulmonary arteries in BMPR-II mutant mice and patients with heritable PAH demonstrated reduced levels of Id3 compared with control subjects. Lentiviral overexpression of Id3 reduced cell cycle progression and inhibited proliferation of PASMCs. Lipopolysaccharide further reduced Id3 expression in mutant PASMCs. In conclusion, Id proteins, and particularly Id1 and Id3, are critical downstream effectors of BMP signaling in PASMCs. Loss of BMPR-II function reduces the induction of Id genes in PASMCs, Id1, and Id3 regulate the proliferation of PASMCs via cell cycle inhibition, an effect that may be exacerbated by inflammatory stimuli.

Vascular remodeling in pulmonary hypertension

Pulmonary hypertension is a complex, progressive condition arising from a variety of genetic and pathogenic causes. Patients present with a spectrum of histologic and pathophysiological features, likely reflecting the diversity in underlying pathogenesis. It is widely recognized that structural alterations in the vascular wall contribute to all forms of pulmonary hypertension. Features characteristic of the remodeled vasculature in patients with pulmonary hypertension include increased stiffening of the elastic proximal pulmonary arteries, thickening of the intimal and/or medial layer of muscular arteries, development of vaso-occlusive lesions, and the appearance of cells expressing smooth muscle-specific markers in normally non-muscular small diameter vessels, resulting from proliferation and migration of pulmonary arterial smooth muscle cells and cellular transdifferentiation. The development of several animal models of pulmonary hypertension has provided the means to explore the mechanistic underpinnings of pulmonary vascular remodeling, although none of the experimental models currently used entirely replicates the pulmonary arterial hypertension observed in patients. Herein, we provide an overview of the histological abnormalities observed in humans with pulmonary hypertension and in preclinical models and discuss insights gained regarding several key signaling pathways contributing to the remodeling process. In particular, we will focus on the roles of ion homeostasis, endothelin-1, serotonin, bone morphogenetic proteins, Rho kinase, and hypoxia-inducible factor 1 in pulmonary arterial smooth muscle and endothelial cells, highlighting areas of cross-talk between these pathways and potentials for therapeutic targeting.