Calcium Receptor-Induced Serotonin Secretion by Parafollicular Cells: Role of Phosphatidylinositol 3-Kinase-Dependent Signal Transduction Pathways

Elevation of extracellular Ca 2+ (↑[Ca 2+ ] e ) stimulates the Ca 2+ receptor (CaR) to induce secretion of 5-hydroxytryptamine (5-HT) from the calcium-sensing parafollicular (PF) cells. The CaR has been reported to couple to Gαq with subsequent activation of protein kinase C-γ (PKCγ). We have identified a parallel transduction pathway in primary cultures of sheep PF cells by using a combinatorial approach in which we expressed adenoviral-encoded dominant-negative signaling proteins and performed in vitro kinase assays. The role of the CaR was established by expression of a dominant-negative CaR that eliminated calcium-induced 5-HT secretion but not secretion in response to KCl or phorbol esters. The calcium-induced secretion was inhibited by a dominant-negative p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-K). PI3-K activity was also assayed using isoform-specific antibodies. The activity of p85/p110β (PI3-Kβ) immunocomplexes was elevated by ↑[Ca 2+ ] e and activated by Gβγ subunits. In addition, secretion of 5-HT was antagonized by the expression of a minigene encoding a peptide scavenger of Gβγ subunits (C-terminal fragment peptide of bovine β-adrenergic receptor kinase). One target of PI3-K activity is phosphoinositide-dependent kinase-1 (PDK1), which in turn activated PKCζ. Expression of a dominant-negative PKCζ in PF cells reduced 5-HT secretion. Together, these observations establish that ↑[Ca 2+ ] e evokes 5-HT secretion from PF cells by stimulating both Gαq- and Gβγ-signaling pathways downstream of the CaR. The βγ cascade subsequently activates PI3-Kβ-dependent signaling that is coupled to PDK1 and the downstream effector PKCζ, and results in an increase in 5-HT release.