Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are second messengers that relay external signals to downstream effector proteins. In cardiomyocytes and smooth muscle cells, cyclic nucleotides control several key functions such as the relaxation tone or the proliferative / hypertrophic phenotype. The modulation of cyclic nucleotides levels thus appears as an appealing therapeutic objective. Signalling events triggered by extracellular stimuli arise from an ingenious regulation of intracellular cyclic nucleotide levels, which result from a balance between their production and their elimination. Classically, cyclic nucleotide elimination has been attributed to hydrolysis mediated by cyclic nucleotide phosphodiesterases (PDEs). However, PDEs may not be the sole regulator of cyclic nucleotide pathways. Our first results have indeed established that efflux of cyclic nucleotides out of cells represents an alternative or complementary mechanism, in addition to PDEs, ensuring intracellular cyclic nucleotide homeostasis. We have indeed identified the expression of an energy-dependent efflux pump called MRP4 (multidrug-resistance-associated protein MRP4, ABCC4) in smooth muscle cells as in cardiomyocytes and demonstrated that MRP4 is responsible for an active transmembrane efflux of cyclic nucleotides thereby limiting the activation of mediated signal transduction. Specific inhibition of MRP4 in cells modified the intracellular content of cyclic nucleotides and markedly enhanced their signalling effect in vitro and in vivo. We thus show that MRP4 is an important and independent regulator of endogenous cAMP and cGMP levels. The modulation of MRP4 activity or expression might therefore have therapeutic potential by modulating cAMP- and cGMP-mediated signal transduction. The following project will aim to further specify the function of MRP4 in the cardio-vascular system and certify the therapeutic impact of its inhibition. In addition to investigations on the prevention of coronary smooth muscle cells proliferation, we will focus on other cardio-vascular diseases where cyclic nucleotides may play a key role. Specifically, we will study the physiopathological and pharmacological role of MRP4 and MRP5 in the prevention of pulmonary artery hypertension where the inhibition of smooth muscle proliferation may represente a therapeutic challenge. Furthermore, we will study the role of MRP4 in the heart and more specifically on the hypertrophic remodelling and on the inotropic response, both phenomenon which are at least partly controlled by cyclic nucleotides-mediated signalling pathways. Our research strategy will be based on the use of RNA interference inserted in viral vectors (as already performed for our previous experiments), and on the investigations of knock-out mice (MRP4-/-). On top of cellular experiments in isolated cardiomyocytes and/or smooth muscle cells, several pathological models (aortic constriction and isoproterenol infusion for the heart; chronic hypoxy for pulmonary hypertension) will be performed. This scientific program will allow validating the function of MRP4 in the cardio-vascular system.