Abstract 664: Disruption of the BMP Type II Receptor Alters the Dynamics of BMP Pathway Activation, Abrogating BMP-mediated Growth Inhibition and Differentiation in Pulmonary Artery Smooth Muscle Cells
Bone morphogenetic protein (BMP) signals regulate growth and differentiation of vascular cells and may contribute to vascular calcific disease. The association of BMP type II receptor (BMPRII) gene mutations with idiopathic pulmonary arterial hypertension suggests important roles in vascular remodeling. Pulmonary artery smooth muscle cells (PASMCs) lacking BMPRII transduce BMP signals via the Activin type II receptor (ActRIIa), but it is unclear whether BMP signaling via the two receptors leads to different phenotypes. In order to test the functional role of BMPRII, BMPRII-deficient (BMPRII−/ −) and control (WT) PASMCs were generated from conditional BMPRII-knockout mice. BMP2 and BMP4, but not BMP7, (1–25 ng/ml) inhibited PDGF-activated proliferation of WT PASMC (72 ± 4%, mean ± SD). None of the BMP ligands tested inhibited the proliferation of BMPRII−/ − PASMC. Adenoviral gene transfer of short or long isoforms of BMPRII restored the ability of BMP4 (10 ng/ml) to inhibit proliferation in BMPRII−/ −PASMC (80 ± 7%), and enabled BMP7 to inhibit proliferation (60 ± 8%), suggesting BMPRII extracellular and kinase domains are necessary and sufficient for this function, while the long C-terminal domain is not required. BMP4 induced osteogenic differentiation in WT PASMC, measured by induction of RANKL (10-fold), osteoprotegerin (3-fold), and alkaline phosphatase (15-fold) expression. Disruption of BMPRII abrogated BMP-induced osteogenic differentiation, while adenoviruses expressing short or long BMPRII isoforms restored BMP-induced differentiation. BMP-induced effects on cell proliferation and differentiation were not dependent on activation of specific SMADs, nor dependent upon MAPK p38 activation or notch-mediated signaling. However, only BMPRII-mediated signals led to persistent activation of SMAD1/5/8 and sustained increases in Id1 mRNA and protein levels for 48 hours, whereas ActRIIa-mediated signals caused only transient pathway activation. Taken together, these findings show that BMPRII disruption alters the dynamics of BMP pathway activation, leading to loss-of-function, consistent with an essential role of BMPRII in regulating vascular smooth muscle proliferation and differentiation.