Calcineurin Feedback Inhibition of Agonist-evoked cAMP Formation

Abstract

The effects of immunosuppressant blockers of calcineurin (protein phosphatase 2B) on cAMP formation and hormone release were investigated in mouse pituitary tumor (AtT20) cells. Immunosuppressants enhanced corticotropin-releasing factor- and isoproterenol-evoked cAMP production in proportion with their potency to block calcineurin. Further analysis of cAMP production revealed that intracellular Ca2+ derived through voltage-regulated calcium channels reduces cAMP formation induced by corticotropin releasing-factor or beta 2-adrenergic stimulation and that this effect of Ca2+ is inhibited by blockers of calcineurin. AtT20 cells 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 cells respectively), corticotropin-releasing factor-induced cAMP formation was not altered upon blockage of calcineurin activity. These data identify calcineurin as a Ca2+ sensor that mediates the negative feedback effect of intracellular Ca2+ on receptor-stimulated cAMP production. Furthermore, the effect of calcineurin on cAMP synthesis appears to be associated with the expression of a novel adenylyl cyclase isotype, which is highly abundant in AtT20 cells.