Serotonin Mechanisms in Heart Valve Disease II : The 5-HT2 Receptor and Its Signaling Pathway in Aortic Valve Interstitial Cells

Serotonin [5-hydroxytryptamine (5-HT)]-mediated cardiac valvular disease has been commonly observed in patients with carcinoid tumors. Previous research by others using reverse transcriptase-polymerase chain reaction demonstrated that aortic valve cells expressed predominantly 5-HT 2A/2B receptors (5-HT 2A R). Related investigations by our group using sheep aortic valve interstitial cell (SAVIC) cultures demonstrated that 5-HT both up-regulates transforming growth factor (TGF)-β1 expression and activity, and also results in increased phospholipase C (PLC) activity. Thus, the present study investigated the hypothesis that the 5-HT signaling pathway in SAVICs involves 5-HT 2 Rs with associated G-protein signal transduction. The objectives were to functionally characterize in SAVIC cultures the native serotonin receptor subtypes using specific agonists and antagonists, and to delineate the serotonin-signaling pathway. 5-HT administration caused a marked stimulation of PLC activity. SAVIC studies of specific agents that target the 5-HT 2 R subtypes indicate that this response seemed to be mediated predominantly by 5-HT 2A Rs. Furthermore, the sheep 5-HT 2A R was identified by reverse transcriptase-polymerase chain reaction with sequence confirmation including comparisons to pig and human 5-HT 2A R. Extracellular signal-regulated kinase (Erk 1/2) is a signaling molecule downstream from the 5-HT 2A R. Both a protein kinase C inhibitor, GF109203X, and a Src inhibitor, PP1, attenuated 5-HT-stimulated Erk 1/2 activation. However, a 5-HT 2A R antagonist, MDL 100907, inhibited 5-HT up-regulation of PLC and TGF-β1, while having far less pronounced effects on Erk 1/2. In conclusion, these studies of the signal transduction activity of SAVICs in response to 5-HT have demonstrated that the 5-HT 2A Rs are the most functionally active of the 5-HT 2 Rs in this cell type. Furthermore, 5-HT 2A Rs are also involved in 5-HT up-regulation of active TGF-β. 5-HT also mediated strong Erk 1/2 signaling via the MAP-kinase pathway, which was only in part because of 5-HT 2A R activity. Thus, major 5-HT Erk 1/2 signaling beyond that controlled by 5-HT 2 Rs must involve other serotonin receptor types and/or secondary signaling events.