Nuclear export of BRCA1 occurs during early S phase and is calcium-dependent

B108

Although the breast cancer susceptibility gene 1 (BRCA1) protein is predominantly nuclear, its localization can vary during the cell cycle in response to cellular insults. For example, in S-phase cells, BRCA1 forms subnuclear foci and localizes to the perinuclear region in response to DNA damage. The present study provides evidence that BRCA1 is transiently excluded from the nucleus during the early part of S phase in the absence of DNA damage. The percentage of MCF-7 cells expressing nonnuclear BRCA1 significantly correlates with the percentage of cells within early S phase. Redistribution of BRCA1, observed as a decrease in nuclear content with a concomitant increase in nonnuclear content, is sensitive to leptomycin B, indicating that CRM-1-mediated nuclear export is involved. The abilities of BAPTA-AM, an intracellular calcium chelator, to inhibit the changes in BRCA1 localization, and of A23187, a calcium ionophore, to mimic nuclear exclusion of BRCA1, provide evidence for the involvement of calcium in this process. When nuclear export-defective BRCA1 is expressed in HCC1937 cells, which express nonfunctional endogenous BRCA1, decreased levels of DNA synthesis occur relative to that of wildtype BRCA1. These data suggest that cell cycle-regulated, calcium-dependent nuclear export of BRCA1 may be important for cell cycle progression.