652

BMP4

BMP2B|BMP2B1|MCOPS6|OFC11|ZYME;bone morphogenetic protein 4;BMP4;bone morphogenetic protein 4

BMP-2B|BMP-4|bone morphogenetic protein 2B

14q22-q23

protein-coding

Count: Bmp4; 12159

The bone morphogenetic protein (BMP) type 2 receptor ligand, Bmp2, is upregulated in the peripheral pulmonary vasculature during hypoxia-induced pulmonary hypertension (PH). This contrasts with the expression of Bmp4, which is expressed in respiratory epithelia throughout the lung. Unlike heterozygous null Bmp4 mice (Bmp4(LacZ/+)), which are protected from the development of hypoxic PH, mice that are heterozygous null for Bmp2 (Bmp2(+/-)) develop more severe hypoxic PH than their wild-type littermates. This is associated with reduced endothelial nitric oxide synthase (eNOS) expression and activity in the pulmonary vasculature of hypoxic Bmp2(+/-) but not Bmp4(LacZ/+) mutant mice. Furthermore, exogenous BMP2 upregulates eNOS expression and activity in intrapulmonary artery and pulmonary endothelial cell preparations, indicating that eNOS is a target of Bmp2 signaling in the pulmonary vasculature. Together, these data demonstrate that Bmp2 and Bmp4 exert opposing roles in hypoxic PH and suggest that the protective effects of Bmp2 are mediated by increasing eNOS expression and activity in the hypoxic pulmonary vasculature.

Fibroblast proliferation, differentiation, and migration contribute to the characteristic pulmonary vascular remodeling seen in primary pulmonary hypertension (PPH). The identification of mutations in the bone morphogenetic protein type II receptor (BMPRII) in PPH have led us to question what role BMPRII and its ligands play in pulmonary vascular remodeling. Thus, to further understand the functional significance of BMPRII in the pulmonary vasculature, we examined the expression of TGF-beta superfamily receptors in human fetal lung fibroblasts (HFL) and investigated the role of BMP4 on cell cycle regulation, fibroblast proliferation, and differentiation. Furthermore, signaling pathways involved in these processes were examined. HFL expressed BMPRI and BMPRII mRNA and demonstrated specific I(125)-BMP4 binding sites. BMP4 inhibited [(3)H]thymidine incorporation and proliferation of HFL; protein expression was increased for the cell cycle inhibitor p21 and reduced for the positive regulators cyclin D and cdk2 by BMP4. BMP4 induced differentiation of HFL into a smooth muscle cell phenotype since protein expression of alpha-smooth muscle actin and smooth muscle myosin was increased. Furthermore, p38(MAPK), ERK1/2, JNK, and Smad1 were phosphorylated by BMP4. Using specific MAPK inhibitors, a dominant negative Smad1 construct, and Smad1 siRNA, we found that the antiproliferative and prodifferentiation effects of BMP4 were Smad1 dependent with JNK also contributing to differentiation. Because failure of Smad phosphorylation is a major feature of BMPRII mutations, these results imply that BMPRII mutations may promote the expansion of fibroblasts resistant to the antiproliferative, prodifferentiation effects of BMPs and suggest a mechanism for the vascular obliteration seen in familial PPH.

mutations in the bone morphogenetic protein (BMP) type II receptor (BMPR2) gene cause familial pulmonary arterial hypertension (FPAH), a disease characterized by excessive smooth muscle and endothelial cell proliferation. However, the specific receptors mediating responses to BMPs in human vascular cells are not known. We show that human pulmonary artery smooth muscle cells (HPASMCs) express high specific (125)I-BMP4 binding, whereas human microvascular endothelial cells (HMEC-1) and human pulmonary artery endothelial cells (HPAECs) exhibit low binding. BMP4 competes for both high- and low-affinity (125)I-BMP4 binding sites on HPASMCs, yet BMP2 competes only at the low-affinity binding sites. In addition, BMP4, but not BMP2, induced Smad1/5 phosphorylation at low concentrations in HPASMCs. Conversely, HMEC-1 cells exhibited a single binding site population with equal affinity for BMP2 and BMP4. In both cell types, growth differentiation factor-5 (GDF5), BMP6, and BMP7 stimulated Smad1/5 phosphorylation and competed for (125)I-BMP4 less efficiently than BMP2 or BMP4. HPAECs exhibited weak Smad responses to BMPs. expression analysis suggested the low binding in endothelial cells corresponded to lower ALK3 and ALK6 expression. Although transfection of small interfering RNAs (siRNAs) for ALK3 and BMPR-II abrogated Smad1/5 phosphorylation to BMP4, BMP2, and GDF5 in HMEC-1 and HPASMCs, they had little effect on (125)I-BMP4 binding. ALK6 siRNA did not alter binding or Smad1/5 responses, even to GDF5, a reported ALK6 selective ligand. Therefore, ALK3/BMPR-II is the BMP4/BMP2/GDF5-responsive receptor in human vascular cells, but these studies suggest that a BMP4/GDF5 selective binding protein exists in HPASMCs. These cell-specific differences in BMP responses are important for understanding the pathogenesis of FPAH.