Increased intracellular Ca²⁺ signaling caused by the antitumor agent helenalin and its analogues

The antitumor sesquiterpene lactone helenalin, which is found in species of the plant genus Helenium, cause a marked potentiation of the increases in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) produced by mitogens such as vasopressin, bradykinin, and platelet-derived growth factor in Swiss mouse 3T3 fibroblasts. Removing external Ca²⁺ partly attenuated the increased [Ca²⁺]_i response. caused by helenalin. The increased [Ca²⁺]_i responses occurred at concentrations of helenalin that inhibited cell proliferation. At higher concentrations, helenalin inhibited the [Ca²⁺]_i responses. No change in resting [Ca²⁺]_i was caused by helenalin even at high concentrations. Other helenalin analogues also increased the [Ca²⁺]_i response. Helenalin did not inhibit protein kinase C (PKC) and PKC appeared to play a minor role in the effects of helenalin on [Ca²⁺]_i responses in intact cells. Studies with saponin-permeabilized HT-29 human colon carcinosarcoma cells indicated that helenalin caused an increased accumulation of Ca²⁺ into nonmitochondrial stores and that the potentiating effect of helenalin on mitogen-stimulated [Ca²⁺]_i responses was due in part to an increase in the inositol-(1,4,5)-trisphosphate-mediated release of Ca²⁺ from these stores.