The rapid activation of cPKCβII by progesterone results in the negative regulation of Ca2+ influx in human resting T cells

Abstract

Background: Progesterone-stimulated rapid suppression of phytohemagglutinin (PHA)-activated sustained membrane Ca2+ influx is revealed by Mn2+ quenching fura-2 fluorescence. Ca2+ influx suppression results in immunosuppression of T-cell proliferation. Downregulation of protein kinase C (PKC) activity by phorbol 12-myristate 13-acetate (PMA) enhances the PHA-activated increase in sustained intracellular Ca2+ concentration ([Ca2+]i) via Ca2+ influx in T cells. Conventional PKC (cPKC) inhibitors also enhance the [Ca2+]i increase in resting T cells caused by progesterone. This study explores whether cPKC activation by progesterone results in suppression of Ca2+ influx in resting T cells. Methods: Progesterone, its analogs (R5020/Org OD 02-0), and plasma membrane-impermeable progesterone-bovine serum albumin conjugate were used to stimulate human resting T cells. Inhibitors and PKC downregulation by PMA were used to investigate whether cPKC affects Ca2+ influx. Results: Progesterone and analogs dose-dependently suppressed Ca2+ influx in T cells. One cPKC inhibitor, Ro318220, attenuated Ca2+ influx suppression, and enhanced the increase in [Ca2+]i caused by progesterone and analogs. U73122 did not affect Ca2+ influx suppression but did decrease the [Ca2+]i increase. Ca2+ influx suppression was not attenuated by the cPKCα/βI isoform-selective inhibitor, Go6976, nevertheless, a cPKCβI/βII isoform-selective inhibitor, LY333531 did. Ca2+ influx suppression was attenuated by the cPKCβII-specific inhibitor CGP53353. After PKC downregulated by PMA, Ca2+ influx suppression by progesterone and analogs was almost abolished in parallel with a massive reduction in cPKCβII expression. This suggests cPKCβII activation by progesterone and analogs mediate Ca2+ influx suppression in resting T cells. Conclusion: Nongenomic membrane activation of cPKCβII by progesterone causes immunosuppression via negative regulation of Ca2+ influx into human resting T cells. This prevents resting T-cell activation and proliferation, which protects the fetus from maternal immune attack while decreasing maternal autoimmune disease flare-ups during pregnancy. Thus, cPKCβII modulators might provide a new therapeutic approach to balancing T-cell tolerance and immunity.