The transactivated epidermal growth factor receptor recruits Pyk2 to regulate Src kinase activity.

G protein-coupled receptors such as proteinase-activated receptor 1 induce phosphorylation of mitogen-activated protein kinases through multiple pathways including transactivation of receptor tyrosine kinases. In vascular smooth muscle cells, both matrix-metalloproteinase-dependent extracellular shedding of membrane-bound epidermal growth factor (EGF) receptor ligands and activation of the nonreceptor tyrosine kinases Pyk2 and Src contributed to the thrombin-induced ERK1/2 phosphorylation. Surprisingly, disruption of the HB-EGF-mediated extracellular mode of EGF receptor transactivation also prevented the phosphorylation of the nonreceptor tyrosine kinases Pyk2 and Src, locating these kinases downstream of the transactivated EGF receptor. The ionomycin-induced Pyk2 phosphorylation was partially sensitive to AG1478, heparin, or the matrix-metalloproteinase inhibitor BB2116, and the ionomycin-induced EGF receptor phosphorylation was almost completely blocked by these inhibitors of extracellular transactivation. Coimmunoprecipitation experiments revealed that, upon thrombin stimulation, a signaling complex consisting of Pyk2 and Src assembles at the EGF receptor. Reconstitution of the signaling molecules in HEK293 or vascular smooth muscle cells and subsequent determination of the EGF-induced Src kinase activity applying fluorescent sensor proteins demonstrated that a Ca(2+)-independent mode of Pyk2 activation is critical for the activation of Src downstream of the EGF receptor.

ERK1/2-dependent contractile protein expression in vascular smooth muscle cells.

In vivo, vascular smooth muscle (VSM) cells change their contractile phenotype toward a more proliferative phenotype during the pathogenesis of vascular diseases. Because these dedifferentiated VSM cells may gradually regain contractile functions, we aimed to identify signaling pathways that result in an increased expression of contractile proteins in VSM cells. In vitro, serum and thrombin induced a reversible upregulation of smooth muscle myosin heavy-chain (SM-MHC) in cultured neonatal rat VSM cells. Cotransfection of a SM-MHC-promoter chloramphenicol acetyltransferase-construct with dominant-negative N17Ras or N17Raf or treatment with the mitogen-activated/ERK-activating kinase (MEK) inhibitor PD 98059 concentration dependently decreased the serum- or thrombin-induced SM-MHC promoter activity. Consistently, the serum- or thrombin-induced phosphorylation of MEK and extracellular signal-regulated kinase 1/2 (ERK1/2) coincided with a MEK-dependent nuclear accumulation of phosphorylated ERK1/2 and subsequent nuclear phosphorylation of the transcription factors c-myc and Elk-1. A 5'-deletion analysis of cis-elements within the SM-MHC promoter demonstrated that a conserved region (nucleotide -1346 to -1102) was required for both cell type-specific expression and serum- or thrombin-induced upregulation of the SM-MHC promoter in VSM cells. Within this region, 2 CArG-boxes, a GC-rich element, and a CTF/NF-1 site are critical positively acting cis-elements for the serum- or thrombin-induced upregulation of SM-MHC. We conclude that the serum- or thrombin-induced differentiation requires an intact Ras/Raf/MEK/ERK signaling cascade, nuclear translocation of activated ERK1/2, phosphorylation of transcription factors, and several cis-elements within the SM-MHC promoter.