THAPSIGARGIN-INDUCED GENE-EXPRESSION IN NONEXCITABLE CELLS IS DEPENDENT ON CALCIUM INFLUX

Agents such as thapsigargin and endothelin elevate intracellular calci um levels by a combination of calcium release from intracellular store s and calcium influx across the plasma membrane; however, the relative contribution of influx vs. release in modulating calcium-dependent ge ne expression is not as well understood in nonexcitable cells as in ex citable cells, In this report we have been able to separate thapsigarg in-induced elevation of intracellular calcium into release and influx components, using carboxyamido-triazole (CAI), a known inhibitor of ca lcium influx with antiproliferative activity against a number of human carcinomas, to selectively inhibit influx without affecting release, The results of these experiments indicate that the ability of thapsiga rgin to induce calcium-dependent gene expression in nonexcitable cells is dependent on the induction of calcium influx, presumably through s tore-operated calcium channels. CAI treatment specifically inhibited t hapsigargin- or endothelin-stimulated expression from the c-fos promot er in Rat-1 cells and in epithelial cell lines derived from ovary and breast, Use of the VL30 model system confirmed the ability of CAI to i nhibit calcium-dependent gene expression and further demonstrated that the ability of elevated calcium to synergize with other signaling pat hways required close temporal coupling, In addition to inhibiting endo thelin-induced calcium influx, CAI treatment also resulted in a partia l inhibition of IP3 production and calcium release, CAI treatment also blocked the increase in ERK1 kinase activity observed in response to either endothelin or thapsigargin, suggesting a role for calcium influ x in the activation of mitogen-activated protein kinase pathways.