Abstract 1670: Over-expression of SOCS3 Negatively Regulates STAT3 Activation and Attenuates IGF-1 Mediated Human Coronary Artery Smooth Muscle Cell Proliferation
Stability of a plaque which depends on release of extracellular matrix by human coronary artery smooth muscle cells (hCASMCs) is a major cause of morbidity and mortality. IGF-1 is one of the most potent inducers of proliferation in hCASMCs, mainly through the activation of PI3-K and MAPK pathways. This study was conducted to understand the mechanism of suppression of IGF-1 induced proliferation of hCASMCs in presence of atheromatous cytokines. We hypothesized that IGF-1 at least, in part, induces proliferation through STAT3 activation, and this pathway is blocked with over-expression of SOCS3. For the first time, our study has shown activation of STAT3 in response to IGF-1 (100 ng/ml; 30min) in hCASMCs (n=3). AG490, an inhibitor of STAT3, inhibited IGF-1 induced proliferation of hCASMCs as detected by BrdU cell proliferation assay (n=3; p<0.05). We treated hCASMCs with IGF-1 and TNF-α (100 ng/ml each) and examined the expression of SOCS3, a novel protein primarily associated with negative regulation of STAT and cytokine signaling. We used SOCS3 over-expression and SOCS3 siRNA approaches to confirm the role of SOCS3 in hCASMCs proliferation. Following IGF-1 stimulation, cells over-expressing SOCS3 were susceptible to a higher degree of apoptosis and became resistant to IGF-1 induced proliferation (n=3; p<0.05). SOCS3 siRNA on the other hand, increased IGF-1 induced hCASMCs proliferation (n=3; p<0.05). Western blot analysis revealed a reduction in IGF-1 induced phosphorylation of Akt and Insulin Receptor Substrate-1 in cells overexpressing SOCS3. IGF-1 and not TNF-α induced STAT3 phosphorylation was blocked upon over-expression of SOCS3 in hCASMCs. Immunoprecipitation studies revealed that SOCS3 associated with IGF-1R upon IGF-1 stimulation, indicating its inhibitory role on IGF-1 pathway upon association with the receptor. These novel findings suggest that SOCS3 expression in hCASMCs regulates the expression and activity of IGF-1 signaling. This leads to decreased cell proliferation which might result in instability of atherosclerotic plaque. Thus, SOCS3 may serve as an important target in stabilizing unstable plaques.