The vasculature is comprised of endothelial cells, vascular smooth muscle cells (VSMCs), fibroblasts and immune cells with closely integrated functions. It is exposed to a variable environment of pulsatile flow and changing blood pressure, plus fluctuations in nutrient, oxidative and cytokine stress. Adaptation to this changing environment is critical for sustained vascular health. VSMC undergo a phenotype change in response to vascular injury including migration, proliferation and matrix production also known as VSMC activation or phenotypic modulation. VSMC activation contributes to the progression of atherosclerosis and post-angioplasty restenosis. Understanding the endogenous regulators of vascular plasticity and adaptation has revealed potential targets for pharmacological intervention to prevent vascular stenosis. For example, Ross and colleagues established cyclic AMP as a critical signaling pathway for the maintenance of VSMC quiescence1. Since that time, agents which increase cAMP, such as phosphodiesterase inhibitors, have demonstrated usefulness as vasodilators with clinical implications for pulmonary hypertension 2, 3. Intracellular accumulation of cAMP signals to protein kinase A and cAMP-responsive Rap1 guanine nucleotide exchange factor, Epac; simultaneous activation of both of these pathways by cAMP is essential for the anti-proliferative impact on VSMCs 3, 4.
The article by Chen et. al. in this issue demonstrates that the phosphodiesterase inhibitor cilostazol increases VSMC differentiation via cAMP signaling to the transcription factor CREB, cAMP Response Element Binding Protein(). This result is consistent with reports that active CREB prevents mitogen stimulated migration and proliferation in VSMCs and that dominant negative CREB or CREB silencing have the converse effect 5, 6. Contradictions to the VSMC differentiating effect of CREB were suggested by an earlier report that balloon angioplasty increases CREB phosphorylation transiently and adenoviral delivery of dominant negative CREB (DN CREB) at the time of balloon angioplasty decreases neointimal plaque formation 7. Consistent with Tokunou et al, Chen et al. demonstrate that balloon injury leads to CREB phosphorylation. Chen’s data offer some new insights into how to interpret these apparently conflicting data. Chen et. al. demonstrate that cilostazol leads to CREB phosphorylation and nuclear localization, whereas balloon injury leads to CREB phosphorylation with exclusion from the nucleus(). In vivo proliferation with CREB nuclear export is consistent with our original report demonstrating loss of pulmonary artery CREB overlapping with increased in proliferation detected by Ki67 in the hypoxic neonatal calf model 6. We have previously reported nuclear exclusion of phosphorylated CREB in response to platelet derived growth factor (PDGF), oxidized LDL and cytokine mixtures 5, 8, 9. Cellular localization, nuclear environment (Redox state, activity of other transcription factors and co-repressor or co-activator proteins) and DNA context (acetylation, methylation) all contribute to the nuclear interpretation of CREB phosphorylation 10. Acute nuclear exclusion of CREB in response to balloon injury would be expected to acutely decrease the cAMP dependent CREB gene expression that maintains VSMC differentiation. It remains enigmatic that adenoviral DN CREB also decreased neointimal formation suggesting a difference between the CREB regulation by cilostazol and the transcriptional response to the DN CREB adenovirus.
VSMC plasticity is critical for vascular health and acutely regulated by nuclear localization of transcription factors and cofactors. In response to injury, changes in VSMC proliferation, migration and matrix production are essential for adaptive remodeling such as wound healing. In restenosis or the neointimal phase of atherosclerosis progression, this remodeling response is exaggerated. Nuclear export of CREB is a robust response to VSMC mitogen or oxidant challenge and now balloon injury5, 9(). Reports indicate that neuronal CREB is excluded from the nucleus in response to stress and can be targeted to the mitochondrial to enhance mitochondrial oxidative gene expression11, 12. The report by Chen et al. suggests that CREB nuclear export in a regulatory mechanism that occurs in vivo and correlates with VSMC phenotype. Dynamic CREB nuclear export in the vascular remodeling is similar to other transcriptional responses leading to phenotypic modulation. For example, leupaxin is a LIM protein family member and cofactor for Serum Response Factor (SRF). In response to FAK signaling (such a can be stimulated by fibronectin) leupaxin is sequestered in the cytosol leading to decreased expression of the SRF dependent contractile proteins α-smooth muscle actin and smooth muscle-myosin heavy chain 13. Myocardin and myocardin family members are critical SRF cofactors and determinants of VSMC differentiation. Myocardin is constitutively located in the nucleus, but other family members are translocated to the nucleus in response to activation of Rho kinase on Bone Morphogenic Protein 4 signaling leading to increased VSMC contractile protein expression 14, 15. Thus control of VSMC phenotype is tightly and rapidly regulated by nuclear trafficking.
The new data presented by Chen et al. adds support to the following model: In response to vascular injury VSMC undergo a rapid and reversible phenotype switch to permit wound healing. Nuclear exclusion of CREB and other cofactors is likely a permissive transcriptional modulator of this phenotype switch. Agents or local factors that stimulate nuclear CREB or myocardin nuclear localization, such as nitric oxide or cilostazol, will attenuate the proliferative response at the completion of adaptive remodeling. In disease states with diminished VSMC CREB content, the remodeling response may go unabated thereby contributing to the progression of vascular disease.