Bone morphogenetic protein-7 modulates genes that maintain the vascular smooth muscle cell phenotype in culture

Background: The vasculature is an important component in the musculoskeleta l system, and vascularization is a key event in the development of normal c artilage and bone formation. Blood vessels deliver nutrients, oxygen, and p recursor cells to maintain the structural and functional integrity of joint s and soft and hard tissues. Therefore, agents that help to inhibit prolife ration and retain the phenotype of vascular smooth muscle cells (SMCs) are of critical importance. In this study, we examined the capacity of bone mor phogenetic protein-7 (BMP-7) to inhibit the proliferation of SMCs and maint ain their phenotype. Methods: A thymidine-incorporation assay was used to monitor the proliferat ive activity of SMCs on stimulation with platelet-derived growth factor (PD GF) and transforming growth factor-beta (TGF-beta), agents known to be stim ulatory for these cells. Reverse transcriptase-polymerase chain reaction (R T-PCR), Northern blot analysis, and enzyme-linked immunosorbent assay (ELIS A) were used to monitor the modulation of various genes and gene products. Immunolocalization of SMC specific markers was also performed. Results: BMP-7 inhibited both serum-stimulated and growth factor-induced (P DGF-BB and TGF-beta1) SMC growth, as measured by H-3-thymidine uptake and c ell number, in primary human aortic smooth muscle (HASM) cell cultures. The addition of BMP-7 stimulated the expression of developmentally regulated a s well as SMC-specific markers, namely, id-1 and 1d-2, alpha -actin, and SM C-specific heavy-chain myosin, as examined by semiquantitative and quantita tive RT-PCR and by Northern blot analysis. Additionally, BMP-7 exhibited an ti-inflammatory activity by downregulating intercellular adhesion molecule- 1 (ICAM-1) expression. The collagen type III/I ratio that becomes lower wit h the transdifferentiation of SMCs into myofibroblasts is maintained in BMP -7-treated cultures compared with untreated controls. Studies on the mechan ism of action indicate that BMP-7 treatment induces cyclin-dependent kinase -2 inhibitor, p21, which was inhibited during PDGF-BB-induced proliferation of SMCs. Finally, BMP-7 upregulates the expression of the inhibitory Smads , Smad6 and Smad7, which are known to inhibit TGF-beta superfamily signalin g. Conclusions: These results suggest that BMP-7 maintains the expression of t he vascular SMC phenotype. Thus, BMP-7 may prevent vascular proliferative d isorders and potentially could act as a palliative agent following damage t o the vasculature. Clinical Relevance: In musculoskeletal disorders in which the vasculature p lays an important role, BMP-7 may be of benefit as an anti-inflammatory and anti-proliferative agent for vascular endothelium and help maintain vascul ar integrity.