Signaling pathways involved in regulating nuclear-cytoplasmic distribution of BRCA1 have not been previously reported. Here, we provide evidence that
heregulin beta1-induced activation of the Akt pathway increases the nuclear content of BRCA1. First, treatment of T47D
breast cancer cells with
heregulin beta1 results in a two-fold increase in nuclear BRCA1 as assessed by FACS analysis, immunoblotting and immunofluorescence. This
heregulin-induced increase in nuclear BRCA1 is blocked by
siRNA-mediated down-regulation of Akt. Second, mutation of
threonine 509 in BRCA1, the site of Akt phosphorylation, to an
alanine, attenuates the ability of
heregulin to induce BRCA1 nuclear accumulation. These data suggest that Akt-catalyzed phosphorylation of BRCA1 is required for the
heregulin-regulated nuclear concentration of BRCA1. Because most functions ascribed to BRCA1 occur within the nucleus, we postulated that phosphorylation-dependent nuclear accumulation of BRCA1 would result in enhanced nuclear activity, specifically transcriptional activity, of BRCA1. This postulate is affirmed by our observation that the ability of BRCA1 to transactivate GADD45 promoter constructs was enhanced in T47D cells treated with
heregulin beta1. Furthermore, the heterologous expression of BRCA1 in HCC1937 human
breast cancer cells, which have constitutively active Akt, also induces GADD45 promoter activity, whereas the expression of BRCA1 in which
threonine 509 has been mutated to an
alanine is able to only minimally induce promoter activity. These findings implicate Akt in upstream events leading to BRCA1 nuclear localization and function.