Recently, we demonstrated that the GTPase Rac1 is involved in the control of arterial pressure by modulating vascular smooth muscle cells (vSMC) contraction. Rac1 promotes vSMC relaxation by positively regulating cyclic GMP-
dependent signaling. In constrat, we observed a contractile action of Rac1 in non-vascular SMC. The objective of this study was thus to decipher the molecular mechanism explaining the different Rac1 effects depending on the type of SMC.
2">Methods
and results EHT1864, a selective Rac1 inhibitor, had no effect on vasoconstriction but dramatically decreased carbachol (Cch)-induced contraction of bronchial rings. In primary cell culture, measurements of intracellular free [Ca2+] by a fluorescent Ca2+ probe showed that Cchinduced rise in cytosolic [Ca2+] was prevented by EHT1864 in airways SMC (aSMC) but not in vSMC. In addition, Rac1 inhibition prevented IP3 formation in Cch-stimulated aSMC, indicating an essential and specific role of Rac1 in phospholipase C/IP3/Ca2+ signaling in aSMC. By RT-PCR and western blot analysis, we observed that PLCβ2 is the most expressed isoform in aSMC whereas PLCg is the predominant isoform in vSMC. Coimmunoprecipitation experiments indicated that muscarinic receptor stimulation triggers the formation of a protein complex associating Rac1 with PLCβ2 in aSMC. In contrast, no interaction Rac1/PLCγ€ was detected in vSMC stimulated with vasoconstrictors.
Conclusion
Our results demonstrated for the first time that stimulation of Rac1 activity lead to a rise of Ca2+ concentration, through a mechanism specific to SMC expressing PLCβ2 isoform. We unveiled that PLCβ2 requires association with Rac1 to initiate its enzymatic activity and promote SMC contraction.
The author hereby declares no conflict of interest
