A hallmark of cultured smooth muscle cells (SMCs) is the rapid down-regulation of several lineage-restricted genes that define their in vivo differentiated phenotype. Identifying factors that maintain an SMC differentiated phenotype has important implications in understanding the molecular underpinnings governing SMC differentiation and their subversion to an altered phenotype in various disease settings. Here, we show that several G-protein coupled receptors [α-thrombin, lysophosphatidic acid and angiotensin II (AII)] increase the expression of smooth muscle calponin (SM-Calp) in rat and human SMC. The increase in SM-Calp protein appears to be selective for G-protein-coupled receptors as epidermal growth factor was without effect. Studies using AII showed a 30-fold increase in SM-Calp protein, which was dose- and time-dependent and mediated by the angiotensin receptor-1 (AT1 receptor). The increase in SM-Calp protein with AII was attributable to transcriptional activation of SM-Calp based on increases in steady-state SM-Calp mRNA, increases in SM-Calp promoter activity and complete abrogation of protein induction with actinomycin D. To examine the potential role of extracellular signal-regulated kinase (Erk1/2), protein kinase B, p38 mitogen-activated protein kinase and protein kinase C in AII-induced SM-Calp, inhibitors to each of the signalling pathways were used. None of these signalling molecules appears to be crucial for AII-induced SM-Calp expression, although Erk1/2 may be partially involved. These results identify SM-Calp as a target of AII-mediated signalling, and suggest that the SMC response to AII may incorporate a novel activity of SM-Calp.
Abbreviations used: AII, angiotensin II; AT1, angiotensin receptor-1; AT2 receptor, angiotensin receptor-2; EGF, epidermal growth factor; Erk1/2, extracellular signal-regulated kinase; FBS, fetal bovine serum; GPCR, G-protein-coupled receptor; LPA, lysophosphatidic acid; MAP kinase, mitogen-activated protein kinase; PI 3-kinase, phosphoinositide 3-kinase; PKB/Akt, protein kinase B; PKC, protein kinase C; SM-Calp, smooth muscle calponin; SMC, smooth muscle cell; RASMC-WKY, Primary-derived aortic SMCs from Wistar–Kyoto rats; −549, 5′-proximal promoter construct containing 549nt upstream of initiation site; −549Int, −594 promoter construct plus the first intron of SM-Calp; −549uInt, −549Int construct with a mutated CarG box.