Pertussis Toxin Can Distinguish the Augmentary Effect Elicited by Epidermal Growth Factor from That of Phorbol Ester on Luteal Adenylate Cyclase Activity

The preincubation of luteal cells with epidermal growth factor (EGF) increases LH/GTP-stimulated adenylate cyclase activity measured subsequently in luteal cell membrane preparations. This reflects an EGF-stimulated increase in the maximum velocity, with no distinct change in the Km value of the enzyme. The augmentary effect of EGF was rapid (maximum after 5-15 min of preincubation and declining thereafter) and was inhibited by preincubation of luteal cells with pertussis toxin. The treatment with this toxin had no effect on [125I] EGF binding to luteal cells. Radiolabeling experiments carried out under ADP-ribosylating conditions revealed diminished radiolabeling of a 40/41-kilodalton pertussis toxin substrate in membranes from pertussis toxin-pretreated cells. In contrast, although the preincubation of luteal cells with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA) led to a similar increase in LH/GTP-stimulated adenylate cyclase, pretreatment with pertussis toxin did not inhibit the augmentary effects of PMA. In fact, prior exposure of PMA-treated cells to the toxin resulted in a further increase in the enzyme activity. We report here that pertussis toxin can be used to discriminate between the potentiating effects of EGF and PMA on luteal adenylate cyclase. The data reinforce the concept that "cross-talk" with other signal-transducing pathways may modulate hormone-stimulated cAMP production in luteal cells and reveal that although EGF and PMA both amplify this response, they appear to do so by distinct mechanisms that can be distinguished by their sensitivity to pertussis toxin.