Lowering glucose depletes a thapsigargin-sensitive calcium pool and inhibits transcription of the S14 gene.

Abstract

S14 is a nuclear protein that is rapidly and synergistically induced by glucose and thyroid hormone, and the level of it's messenger RNA correlates with hepatocyte and adipocyte lipogenesis. We previously reported that the calcium ionophore A23187 markedly inhibits the carbohydrate response of the S14 gene without inhibiting glucose metabolism. Because the calcium ionophore not only increased intracellular cytosolic free calcium but also depletes intracellular calcium stores, we examined which of these two possibilities accounts for the regulation of S14 gene transcription. We found that increasing cytosolic calcium with arginine vasopressin is insufficient to inhibit S14 gene transcription. Furthermore, reduction of intracellular calcium by addition of EGTA to medium containing A23187 leads to further inhibition of S14 transcription. Measurement of intracellular free calcium in indo-1-loaded hepatocytes showed no significant changes induced by high glucose. These results suggested that depletion of an intracellular pool of calcium by A23187 causes the inhibition of S14 transcription. Addition of thapsigargin, which depletes intracellular calcium pools by inhibition of endoplasmic reticulum Ca2+-ATPase, led to significant inhibition of glucose-regulated S14 transcription. Lastly, continuous incubation in 5.5 mM glucose depletes the thapsigargin-sensitive calcium pool. These studies imply that the ability of glucose to induce S14 transcription is related to a thapsigargin-sensitive calcium pool, and depletion of this pool by lowering glucose inhibits S14 transcription.