Received for publication May 22, 2006.
Revised July 9, 2006.
Accepted for publication July 12, 2006.

BASILIOLIDES, A CLASS OF TETRACYCLIC C-19 DILACTONES FROM THAPSIA GARGANICA, RELEASE CA2⁺ FROM THE ENDOPLASMIC RETICULUM AND REGULATE THE ACTIVITY OF THE TRANSCRIPTION FACTORS NF-AT, NF-B AND AP-1 IN T LYMPHOCYTES

Abstract

Calcium concentration within the endoplasmic reticulum (ER) plays an essential role in cell physiology. We have investigated the effects of basiliolides, a novel class of C-19 dilactones isolated from Thapsia garganica, on Ca²⁺ mobilization in T cells. Basiliolide A1 induced a biphasic elevation of intracellular Ca²⁺ in the leukemia T-cell line, Jurkat. First, a rapid calcium peak was observed and inhibited by BAPTA-AM. This initial calcium mobilization was followed by a sustained elevation, mediated by the entry of extracellular calcium through store-operated calcium release-activated Ca²⁺ (CRAC) channels and sensitive to inhibition by EGTA and by the CRAC channel inhibitor BTP-2. Basiliolide A1 mobilized Ca2⁺ from ER stores but in contrast to thapsigargin did not induce apoptosis. Basiliolide A1 induced NF-AT1 dephosphorylation and activation that was inhibited by BTP-2 and CsA. In addition, we found that basiliolide A1 alone did not mediate IB degradation nor RelA phosphorylation (ser536) but it synergized with PMA to induce a complete degradation of the NF-B inhibitory protein and to activate the c-Jun N-terminal Kinase (JNK). Moroever, basiliolide A1 regulated both IL- 2 and TNF- gene expression at the transcriptional levels. In basiliolide B, oxidation of one of the two geminal methyls to a carboxymethyl group retained most of the activity of basiliolide A1. In contrast, basiliolide C, where the 15-carbon is oxidized to an acetoxymethine, was much less active. These findings qualify these compounds as new probes to investigate intracellular calcium homeostasis.

Key words: Calcium homeostasis, Cytokines, NFAT, Signaling, T cells, Transcription