Bax gene expression alters Ca(2+) signal transduction without affecting apoptosis in an epithelial cell line.

Abstract

Bax is an oncogene that has proapoptotic properties but not all cells that express Bax undergo apoptosis. Bax may have a function unrelated to apoptosis. To elucidate the role of Bax in cell signaling, an epithelial cell line called SMG-C6 was transfected with the human bax gene. Stable transfectants were studied for their response to carbachol, a muscarinic receptor agonist, by measuring the increase in intracellular free Ca(2+) and Ca(2+) influx. Carbachol-mediated release of Ca(2+) from intracellular stores was significantly higher in Bax transfectants compared to control transfectants (empty vector). Ca(2+) influx was also increased in Bax transfectants. Bax had no affect on the storage operated channels. However, the concentration of Ca(2+) in the intracellular stores (i.e., mitochondria and granules) was 40-50% lower in the Bax transfectants. There was no significant difference in thapsigargin-mediated apoptosis in Bax transfectants compared to wild-type and control transfectants. Measurement of glutathione was reduced in the Bax transfectant. Restoration of glutathione levels with glutathione monoethyl ester partially normalized Ca(2+) mobilization and storage capacity in the mitochondria to control levels. This study shows that sub-apoptotic levels of Bax can reduce Ca(2+) content in intracellular stores and Ca(2+) homeostasis. Bax may mediate these effects by reducing the levels of antioxidants resulting in mild oxidative stress.