CALCINEURIN FEEDBACK INHIBITION OF AGONIST-EVOKED CAMP FORMATION

The effects of immunosuppressant blockers of calcineurin (protein phos phatase 2B) on cAMP formation and hormone release were investigated in mouse pituitary tumor (AtT20) cells, Immunosuppressants enhanced cort icotropin-releasing factor- and isoproterenol-evoked cAMP production i n proportion with their potency to block calcineurin. Further analysis of cAMP production revealed that intracellular Ca2+ derived through v oltage-regulated calcium channels reduces cAMP formation induced by co rticotropin releasing-factor or beta(2)-adrenergic stimulation and tha t this effect of Ca2+ is inhibited by blockers of calcineurin. AtT20 c ells were found to express at least three species of adenylyl cyclase mRNA-encoding types 1 and 6 as well as a novel isotype, which appeared to be the predominant species. In two cell lines expressing very low or undetectable levels of the novel cyclase mRNA (NCB20 and HEK293 cel ls respectively), corticotropin-releasing factor-induced cAMP formatio n was not altered upon blockage of calcineurin activity, These data id entify calcineurin as a Ca2+ sensor that mediates the negative feedbac k effect of intracellular Ca2+ on receptor-stimulated cAMP production. Furthermore, the effect of calcineurin on cAMP synthesis appears to b e associated with the expression of a novel adenylyl cyclase isotype, which is highly abundant in AtT20 cells.