Modulation of intracellular calcium concentrations and T cell activation by prickly pear polyphenols

Opuntia ficus indica(prickly pear) polyphenolic compounds (OFPC) triggered an increase in Ca²⁺]_i in human Jurkat T-cell lines. Furthermore, OFPC-induced rise in Ca²⁺]_i was significantly curtailed in calcium-free buffer (0% Ca²⁺) as compared to that in 100% Ca²⁺ medium. Preincubation of cells with tyrphostin A9, an inhibitor of Ca²⁺ release-activated Ca²⁺(CRAC) channels, significantly diminished the OFPC-induced sustained response on the increases in Ca²⁺]_i. Lanthanum and nifedipine, the respective inhibitors of voltage-dependent and L-type calcium channels, failed to curtail significantly the OFPC-induced calcium response. As OFPC still stimulated increases in Ca²⁺]_i in 0% Ca²⁺ medium, the role of intracellular calcium was investigated. Hence, addition of thapsigargin (TG), an inhibitor of Ca²⁺-ATPase of the endoplasmic reticulum (ER), during the OFPC-induced peak response exerted an additive effect, indicating that the mechanism of action of these two agents are different. Furthermore, U73122, an inhibitor of IP₃ production, completely abolished increases in Ca²⁺]_i, induced by OFPC, suggesting that these polyphenols induce the production of IP₃ that recruits calcium from ER pool. Polyphenolic compounds do act extracellularly as addition of fatty acid-free bovine serum albumin (BSA) significantly diminished the rise in Ca²⁺]_i evoked by the formers. OFPC also induced plasma membrane hyperpolarisation which was reversed by addition of BSA. OFPC were found to curtail the expression of IL-2 mRNA and T-cell blastogenesis. Together these results suggest that OFPC induce increases in Ca²⁺]_i via ER pool and opening of CRAC channels, and exert immunosuppressive effects in Jurkat T-cells.