Cyclosporin A Inhibits Inositol 1,4,5-Trisphosphate-dependent Ca 2+ Signals by Enhancing Ca2+ Uptake into the Endoplasmic Reticulum and Mitochondria

Abstract

Cytosolic Ca2+ ([Ca2+]i) oscillations may be generated by the inositol 1,4,5-trisphosphate receptor (IP3R) driven through cycles of activation/inactivation by local Ca2+ feedback. Consequently, modulation of the local Ca2+ gradients influences IP3R excitability as well as the duration and amplitude of the [CA2+]i oscillations. In the present work, we demonstrate that the immunosuppressant cyclosporin A (CSA) reduces the frequency of IP3-dependent [Ca2+]i oscillations in intact hepatocytes, apparently by altering the local Ca 2+ gradients. Permeabilized cell experiments demonstrated that CSA lowers the apparent IP3 sensitivity for Ca2+ release from intracellular stores. These effects on IP3-dependent [Ca 2+]i signals could not be attributed to changes in calcineurin activity, altered ryanodine receptor function, or impaired Ca 2+ fluxes across the plasma membrane. However, CSA enhanced the removal of cytosolic Ca2+ by sarco-endoplasmic reticulum Ca 2+-ATPase (SERCA), lowering basal and interspike [Ca 2+]i. In addition, CSA stimulated a stable rise in the mitochondrial membrane potential (ΔΨm), presumably by inhibiting the mitochondrial permeability transition pore, and this was associated with increased Ca2+ uptake and retention by the mitochondria during a rise in [Ca2+]i. We suggest that CSA suppresses local Ca2+ feedback by enhancing mitochondrial and endoplasmic reticulum Ca2+ uptake, these actions of CSA underlie the lower IP3 sensitivity found in permeabilized cells and the impaired IP3-dependent [Ca2+]i signals in intact cells. Thus, CSA binding proteins (cyclophilins) appear to fine tune agonist-induced [Ca2+]i signals, which, in turn, may adjust the output of downstream Ca2+-sensitive pathways.