Objective-Protease-activated receptor-1 (PAR1) mediates the thrombin-induced proliferation and hypertrophy of vascular smooth muscle cells. A role of Rac1 in the regulation of PAR1 expression was investigated. Methods and Results-Treatment with simvastatin, a hydroxy-3-methyl-glutaryl coenzyme A reductase inhibitor, for 24 hours attenuated the transient [Ca 2ϩ ] i elevation induced by thrombin. Immunofluorescence staining revealed that simvastatin decreased the surface expression of PAR1 in a manner dependent on protein geranylgeranylation. Introduction of a Rac1/Cdc42 inhibitory fragment but not a RhoA inhibitory fragment using a cell-penetrating peptide also attenuated the response to thrombin and decreased the surface expression of PAR1. Finally, downregulation of Rac1, but not RhoA, using an RNA interference technique attenuated the thrombin-induced [Ca 2ϩ ] i elevation. However, the level of PAR1 mRNA and the total amount of PAR1 protein remained unchanged. Conclusions-Here, we provide for the first time 3 lines of evidence that Rac1 plays a critical role in maintaining the surface expression of PAR1 and the responsiveness to thrombin in vascular smooth muscle cells. Rac1 is suggested to regulate the constitutive trafficking of PAR1 and thereby regulate the surface expression of PAR1. Key Words: expression Ⅲ protease-activated receptor Ⅲ Rac1 Ⅲ smooth muscle Ⅲ thrombin P rotease-activated receptor-1 (PAR1) belongs to a family of G-protein-coupled receptors (GPCRs), and it is known to mediate the cellular effects of thrombin in various types of cells. [1][2][3][4][5] In vascular smooth muscle cells, PAR1 regulates the contraction, proliferation, and hypertrophy. 1,4,6 The expression of PARs has been reported to either increase or decrease under various pathological conditions, including atherosclerosis, balloon injury of arteries, cerebral ischemia, and neuroinflammatory and neurodegenerative disorders. [7][8][9][10][11][12] Therefore, elucidating the mechanism regulating the expression of PARs is important to understand the pathophysiology of such diseases and also to establish new therapeutic strategies. The nascent PAR1 is considered to be targeted first to the plasma membrane, and then subjected to constitutive internalization, thus resulting in the formation of an intracellular pool. 13,14 PAR1 then cycles between the plasma membrane and the intracellular pool under resting conditions. The steady-state level of PAR1 on the cell surface is thus determined by a balance among the rate of de novo synthesis, the receptor internalization, and the recruitment from the intracellular pool. 14,15 However, the molecular mechanism regulating the expression of PAR1 remains to be elucidated.The small G-protein is known to regulate the intracellular vesicle trafficking. 16 The Rab family is the most studied small G-protein that regulates GPCR trafficking. 17 Rab5a has been shown to be required for the agonist-triggered internalization of PAR2, whereas Rab11a has been shown to contribute to the transport of PAR2 ...

Abstract. The level of receptors expressed on the cell surface determines the cellular responsiveness to agonists. Proteinase-activated receptors (PARs) have been reported to be either upregulated or down-regulated in response to various types of stimulation and pathological situations. In addition, the transcriptional regulation plays a major role in the alteration of the expression of PARs under pathological conditions, while post-translational mechanisms such as phosphorylation, arresting-binding, internalization, and lysosomal degradation, which desensitize activated PARs and terminate intracellular signaling, also play an important role in regulating the expression of PARs and the cellular responsiveness to the agonists. Elucidating the mechanisms related to the expression of PARs is a critical step to understand the pathophysiology of various diseases and establish new therapeutic strategies. However, the molecular mechanism regulating the expression of PARs still remains to be elucidated. This minireview discusses our current understanding of the mechanisms regulating the expression of PARs. The transcription factors and the regulatory elements in the promoter regions, and the proteins that interact with the receptors and thereby regulate their trafficking and desensitization are the main problems that need to be elucidated.