Signaling of the Human Calcium-Sensing Receptor Expressed in HEK293-Cells is Modulated by Protein Kinases A and C

Abstract

In this study, the human calcium-sensing receptor (CaR) stably expressed in HEK293 cells was investigated with regard to the phosphorylation-induced desensitization of its signaling pathway. The receptor is known to activate the phospholipase C/inositol-1,4,5-trisphosphate (IP₃) signaling cascade, thus stimulating protein kinase C (PKC). In contrast, the adenylylcyclase/cAMP signaling pathway that activates protein kinase A (PKA) is believed to be coupled to the receptor via an inhibitory G-protein. We elucidated the roles of PKC and PKA by measuring Ca²⁺ _o-stimulated accumulation of total inositol phosphates and by individually and simultaneously inhibiting the two kinases pharmacologically in HEK293 cells, which stably expressed the human CaR. Pharmacological inhibition of PKC resulted in a 5-fold enhancement of IP₃ signaling, whereas blocking PKA had almost no effect. IP₃ signaling activity increased even more (10-fold) however, when the two kinases were inhibited simultaneously. Apart from validating the role of PKC as a potent down-regulator of signaling of the human CaR in this cell system, this study suggests that both kinases synergize in inhibiting Ca²⁺ _o-stimulated IP₃ signaling in CaR-transfected HEK293 cells.

Ca²⁺-sensing receptor - G protein-coupled receptor - Desensitization - Protein kinase A - Protein kinase C

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