Cellular compartmentation of calcium in mouse fibroblasts in vitro depends on cell density and transformation

Altered calcium regulation in SV40-transformed Swiss 3T3 fibroblasts

Calcium homeostasis has long been thought to be altered in transformed cells but mechanisms have not been established. In this study, the photoprotein, aequorin, was used to examine calcium regulation in 3T3 and SV40-transformed 3T3 cells. It was found that calcium transients induced by bradykinin or serum in serum-starved cells are lower and delayed in the transformed cells and decay kinetics are altered. These changes are not related to differences in cell cycle distribution. Though the serum transient is insensitive to nifedipine, verapamil, or lanthanum, removal of extracellular calcium accelerates transient decay in both cell types. Treatment of unstimulated cells with the ER Ca(2+)-ATPase inhibitor, thapsigargin, causes a 4-5-fold greater increase in [Ca2+]i in the transformed than in the nontransformed cells. Following serum stimulation, transformed cells still exhibit a large thapsigargin-induced increase in [Ca2+]i whereas the response in nontransformed cells is nearly abolished. When the 3T3 or SV3T3 cells are exposed to serum or thapsigargin in the absence of extracellular calcium and subsequently exposed to 11.8 mM Ca2+, a much greater influx of calcium again occurs in the SV3T3 cells. The observed changes in SV3T3 cells are most likely due to an alteration in a capacitative mechanism which regulates influx of calcium through the plasma membrane.