Abstract 523: Bone Morphogenetic Protein (BMP) Type II Receptor, but Not Activin Type II Receptor, Mediates the Anti-Proliferative Effects of BMPs in Pulmonary Artery Smooth Muscle Cells
Mutations in the BMP type II receptor (BMPR-II) are implicated in familial primary pulmonary hypertension. BMPs reportedly antagonize pulmonary artery smooth muscle cell (PASMC) proliferation. We previously found PASMCs lacking BMPR-II (KO) to have decreased BMP4 and greatly increased BMP7 signaling, consistent with substitution of Activin type II and type I receptors (ActRIIa, ALK2) in KO cells for canonical BMP receptors (BMPR-II, ALK3). We hypothesized that disrupting BMPR-II modulates the anti-proliferative effects of BMP in PASMC. PASMC isolated from mice homozygous for BMPR-II alleles with loxP sites flanking exons 4 and 5 were infected with adenovirus expressing Cre recombinase (Ad.Cre) or GFP (Ad.GFP) to yield KO and wild type (WT) cells. WT and KO cells exhibited similar proliferative responses to PDGF-BB, bFGF, and serum stimulation, as assessed using [3H]thymidine incorporation, while BMPs alone had minimal impact. BMP4 potently inhibited PDGF-mediated proliferation of WT PASMCs up to 65±2% in a dose-dependent manner (IC50=1ng/ml, P<0.01), whereas BMP7 had no effect. In KO cells, BMP4 inhibited PDGF-mediated proliferation only up to 20±4% (IC50=25ng/ml, P<0.05 vs WT cells). BMP7 elicited 5-fold greater SMAD signaling and Id transcription in KO versus WT cells and elicited greater responses than did BMP4 in WT cells, yet BMP7 inhibited PDGF-mediated proliferation in KO cells much less effectively than BMP4 in WT cells (10±3%, IC50=15ng/ml, P<0.05). No cross-talk between PDGF and BMP signaling was observed in SMAD1/5/8, p38, ERK, or Akt activation. Specific inhibition of p38 (SB239063) did not affect BMP inhibition of PDGF-mediated growth.
Conclusions: Disrupting BMPR-II attenuated the anti-proliferative effect of BMP4 in PDGF-stimulated PASMCs. Disrupting BMPR-II greatly enhanced BMP7 signaling but did not lead to commensurate gains in BMP7’s anti-mitogenic activity. BMP anti-mitogenic activity appears restricted to classical BMP ligands (BMP4) via canonical receptors (BMPR-II, ALK3), whereas alternate ligands (BMP7) or receptors (ActRIIa, ALK2) do not antagonize mitogenic activity effectively. BMPR-II/ALK3 may exert antiproliferative effects by targeting unique effectors or SMAD subsets as compared to ActRIIa/ALK2.