GPCR responses in vascular smooth muscle can occur predominantly through dual transactivation of kinase receptors and not classical G?q protein signalling pathways

GPCR signalling is well known to proceed through several linear pathways involving activation of G proteins and their downstream signalling pathways such as activation of phospholipase C. In addition, GPCRs signal via transactivation of Protein Tyrosine Kinase receptors such as that for Epidermal Growth Factor (EGF) and Platelet-Derived Growth Factor (PDGF) where GPCR agonists mediate increase levels of phosphorylated Erk (pErk) the immediate downstream product of the activation of EGF receptor. It has recently been shown that this paradigm can be extended to include the GPCR transactivation of a Protein Serine/Threonine Kinase receptor, specifically the Transforming Growth Factor β Type I receptor (also known as Alk V) (TβRI) in which case GPCR activation leads to the formation of carboxy terminal polyphosphorylated Smad2 (phosphoSmad2) being the immediate downstream product of the activation of TβRI. Growth factor and hormone regulation of proteoglycan synthesis in vascular smooth muscle cells represent one component of an in vitro model of atherosclerosis because modified proteoglycans show enhanced binding to lipoproteins as the initiating step in atherosclerosis. In the example of proteoglycan synthesis stimulated by GPCR agonists such as thrombin and endothelin-1, the transactivation pathways for the EGF receptor and TβRI are both active and together account for essentially all of the response to the GPCRs. In contrast, signalling downstream of GPCRs such as increased inositol 1,4,5 trisphosphate (IP3) and intracellular calcium do not have any effect on GPCR stimulated proteoglycan synthesis. These data lead to the conclusion that dual transactivation pathways for protein tyrosine and serine/threonine kinase receptors may play a far greater role in GPCR signalling than currently recognised.