Abstract Abstract #3050 Expression of the GATA-3 binding protein correlates with estrogen receptor (ER) alpha expression in primary and cultured breast cancer cells. While the significance of this correlation and role of GATA-3 in breast cancer remains unclear, GATA-3 may serve as a good prognostic indicator in breast tumors. Recently, a reciprocal interaction between GATA-3 and ER has been demonstrated. GATA-3 may regulate ER levels through binding to an enhancer region upstream of the ER gene while ER can associate with and regulate GATA-3 expression. Meta-analyses of several cancer microarrays indicate that GATA-3 is co-expressed with ER and other ER responsive genes, further suggesting a role of GATA-3 in the ER signaling pathway. Our laboratory has been investigating the role of the RNA binding protein HuR/ELAV1 in control of mRNA stability in breast cancer cells. We have previously reported that ER mRNA stability is increased through binding of HuR to the 3'UTR. Microarray analysis of those transcripts associated with HuR in MCF7 cells showed that GATA-3 mRNA associates with HuR compared to an IgG control. Biotin pulldown assays using a biotinylated GATA-3 RNA probe confirmed that HuR binds to the GATA-3 3'untranslated region (UTR). Inhibition of HuR using siRNA probes decreased GATA-3 mRNA and protein expression, further indicating a role for HuR in regulation of GATA-3 mRNA stability. siRNA mediated inhibition of GATA-3 expression significantly reduced ER mRNA and protein levels. Surprisingly, we found that GATA-3 bound to the ER mRNA by binding to two out of four regions within the ER 3'UTR, suggesting that while GATA-3 may play a role in upregulation of ER mRNA transcription through the enhancer interaction, GATA-3 may stabilize ER mRNA once transcribed by binding to the ER 3'UTR. Studies are ongoing to fully characterize this role of GATA-3 in control of ER mRNA stability. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3050.