Alpha-2 Adrenergic Receptors and Signal Transduction Effector Output in Relation to G-Protein Coupling and Signalling Cross-Talk
Sammanfattning: The alpha-2 adrenergic receptor (?2-AR) subfamily includes three different subtypes, ?2A-, ?2B- and ?2C-AR, all believed to exert their function through heterotrimeric Gi/o-proteins. The present study was undertaken in order to investigate subtype differences in terms of cellular response and to explore other potential signalling pathways of ?2-ARs.Evidence is provided for a strong Gs-protein coupling capability of the ?2B-AR, leading to stimulation of adenylyl cyclase (AC). The difference between the ?2A- and ?2B-AR subtypes, in this respect, was shown to be due to differences in the second intracellular loops of the receptor proteins. Substitution of the second loop in ?2A-AR with the corresponding domain of ?2B-AR enrolled the chimeric ?2A/?2B receptor with functional ?2B-AR properties. Dual Gi and Gs coupling can have different consequences for AC output. Using coexpression of receptors and G-proteins, it was shown that the ultimate cellular response of ?2B-AR activation is largely dependent on the ratio of Gi- to Gs-protein amounts in the cell. Also Gi- and Go-proteins appear to have different regulatory influences on AC. Heterologous expression of AC2 together with Gi or Go and the ?2A-AR resulted in receptor-mediated inhibition of protein kinase C-stimulated AC2 activity through Go, whereas activation of Gi potentiated the activity. ?2-ARs mobilize Ca2+ in response to agonists in some cell types. This response was shown to depend on tonic purinergic receptor activity in transfected CHO cells. Elimination of the tonic receptor activity almost completely inhibited the Ca2+ response of ?2-ARs.In conclusion, ?2-ARs can couple to multiple G-proteins, including Gi, Go and Gs. The cellular response to ?2-AR activation depends on which receptor subtype is expressed, which cellular signalling constituents are engaged (G-proteins and effectors), and the signalling status of the effectors (dormant or primed).
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