Differential regulation of nuclear and cytosolic Ca2+ in HeLa cells

Abstract

The results reported in this study address the controversial issue that nuclear free Ca2+ ([Ca2+](n)) may be regulated independently of cytosolic free Ca2+ ([Ca2+](c)). We have measured [Ca2+](n) and [Ca2+](c) with recombinant aequorin targeted to the nucleus and cytosol in HeLa cells. We found that histamine, ATP, and ionomycin increased [Ca2+](c) quantitatively more than [Ca2+](n), although the time course of these changes was similar. The difference between [Ca2+](c) and [Ca2+](n) depended on the stimulus, and the relative difference between [Ca2+](n) and [Ca2+](c) was less with ionomycin than with histamine or ATP. After depletion of the internal Ca2+ store, restoration of extracellular Ca2+ resulted in only increased [Ca2+](c) without a significant increase in [Ca2+](n). Treatment with cyclopiazonic acid resulted in a delayed increases in [Ca2+](n) compared to [Ca2+](c). These differences in both timing and magnitude of nuclear Ca2+ signals confirm that the cell can limit or delay increases in nuclear free Ca2+. Taken with the fact that an inositol phosphate signaling system resides in the nucleus and its envelope, our data support the hypothesis that [Ca2+](n) may be independently regulated.