cGMP and PKGI are required for vascular BMP signaling

Background

Maintenance of vascular homeostasis depends on phenotypic switching of vascular smooth muscle cells (VSMCs) during development, vascular injury repair, and disease. In healthy blood vessels, VSMCs exhibit a differentiated, ‘contractile’ phenotype, but in diseased vascular tissue or after vascular injury, they de-differentiate into a ‘synthetic’ state, characterized by decreased smooth muscle (SM)-specific gene expression and increased proliferation and motility [1].

Results

Although still controversial, a large number of studies indicate that the NO/cGMP/PKGI pathway inhibits proliferation and de-differentiation of VSMCs. Subcultured primary VSMCs undergo de-differentiation to a ‘synthetic’ phenotype with both reduced SM-specific gene expression and loss of PKGI expression. A more ‘contractile’ phenotype can be regained by restoring PKGI [2]. PKGI stimulates SM-specific gene expression through regulation of the cystein-rich LIM only protein CRP4 that cooperates with SRF-containing transcription complexes [3]. But the mechanism how cGMP/PKGI regulates SM-specific phenotype is incompletely understood. In C2C12 myoblasts, PKGI phosphorylates the BMP type II receptor (BMPRII); in response to BMP-2, PKGI dissociates from the receptor, associates with the activated Smad1/4 complex, translocates to the nucleus, and forms a complex with Smads and the general transcription factor TFII-I to collaboratively activate transcription [4]. In the vascular system, BMP signaling inhibits proliferation and migration of VSMCs, and upregulates SM-specific genes, through Smad-dependent [5–8] and/or Smad-independent pathways [9–11]. Interestingly, heterozygous germline mutations within BMPRII can cause pulmonary arterial hypertension (PAH), a disease characterized by thickening of pulmonary arteries due to abnormal proliferation, migration, and/or apoptosis of VSMCs and endothelial cells [12]. Considering these data we hypothesize that PKGI promotes the differentiated ‘contractile’ phenotype of VSMCs at least in part through enhancing vascular BMP/Smad signaling. Indeed we found that cGMP/PKGI promoted Smad1/5 activation and BMP target gene expression in VSMCs and SM precursor cells. Pharmacological or siRNA-mediated inhibition of the NO/cGMP/PKGI pathway not only suppressed BMP-induced upregulation of SM-specific gene transcription, but also abrogated the anti-proliferative and anti-migratory effects of BMP on VSMCs. Furthermore preliminary data suggest that Smad crosstalk with other transcriptional regulators is involved.

Conclusion

Our data imply that within the vasculature, PKGI is a critical regulator of the VSMC differentiation-promoting effects of BMP. The integration of cGMP/PKGI pathway into BMP/Smad signaling in the vascular system might provide new insight into the mechanisms of vascular remodeling in diseases such as atherosclerosis, vascular restenosis and pulmonary hypertension.