Factors Determining the Specificity of Signal Transduction by Guanine Nucleotide-binding Protein-coupled Receptors

Abstract

To define the integration of multiple signals by different types of adenylyl cyclase (AC) within the cell, we altered the population of enzymes expressed in the cell and determined the subsequent processing of stimulatory and inhibitory input. DDT1-MF2 cells expressed AC VI-IX and were stably transfected with AC II, III, or IV. Enzyme expression was confirmed by RNA blot analysis and functional assays. Basal enzyme activity was only increased in AC II transfectants (6-fold). Maximum stimulation of enzyme activity was increased in each of the AC transfectants to varying extents. alpha2A/D-AR activation elicited enzyme type-specific responses. alpha2-AR activation inhibited the effect of isoproterenol in control transfectants, and this action was magnified in AC III transfectants. In AC II and AC IV transfectants, alpha2-AR activation initiated both positive (Gbetagamma) and negative signals (Gialpha) to the Gsalpha-stimulated enzyme, and both types of signals were blocked by cell pretreatment with pertussis toxin. The negative input to AC II from the alpha2-AR was blocked by protein kinase C activation in AC II transfectants, but it was the positive input to AC IV that was compromised by protein kinase C activation. These data indicate that the integration of multiple signals by adenylyl cyclases is a dynamic process depending upon the enzyme type and phosphorylation status.