Abstract 468: The Role of Ca2+/PKA Signaling Enhancer Protein Phosphatase Inhibitor 1 in the Control of Ca2+ Cycling and Signaling in VSMCs

Vascular remodeling is associated with trans-differentiation of contractile vascular smooth muscle cells (VSMC) towards a proliferating/synthetic phenotype. We have recently demonstrated (Bobe et al., 2011) that the Ca2+ cycling in “contractile” VSMC requires the expression of the fast isoform of sarco/endo plasmic ATPase (SERCA2a), whereas the Ca2+ cycling in “proliferating” ones is associated with the expression of the ubiquitous isoform SERCA2b only. Phospholamban (PLB), a negative regulator of SERCA2 activity, is inhibited by PKA phosphorylation and activated by protein phosphatase 1 (PP1) dependent dephosphorylation. Inhibitor-1 (I-1), a highly specific inhibitor of PP1, enhances both PKA-dependent PLB phosphorylation and SERCA activity. The goal of this study was to elucidate the role of PKA signaling enhancer I-1 in the control of VSMC Ca2+ cycling. In humans and rodents, the expression of I-1 was found to be specific for contractile VSMC, whereas its target PP1 was highly expressed in synthetic VSMC. Consequently, PLB phosphorylation was decreased in synthetic VSMC whereas the expression of total PLB remained unchanged. Genetic deletion of I-1 in mice model (I-1 KO) resulted in lack of PLB phosphorylation in VSMC of adult animals. Despite the fact that SERCA2a was expressed, VSMCs from I-1 deficient mice were locked in the synthetic state. Consistent with this, the adult I-1 deficient mice developed a vascular proliferative disorder and excessive neointimal proliferation after vascular injury. Adenovirus-directed gene transfer of constitutively active I-1 (I-1c) significantly increased PLB phosphorylation and Ca2+ uptake in both synthetic and contractile VSMCs, however, there was no effect on the type of Ca2+ transient, which appears to be a SERCA2 isoform-dependent characteristic. Therefore, I-1c prevented proliferation and remodeling of contractile VSMC, but had no effect on synthetic VSMC, which predominantly express SERCA2b. In conclusion, synergistic action of I-1 and SERCA2a is necessary for acquisition of the VSMC contractile phenotype. Gene transfer of I-1c may be considered as a promising therapeutic strategy for preventing vascular proliferative diseases.