GREB1 isoforms regulate proliferation independent of ERα co-regulator activities in breast cancer

Activation of the transcription factor estrogen receptor a (ER alpha) and the subsequent regulation of estrogen-responsive genes play a crucial role in the development and progression of the majority of breast cancers. One gene target of ER alpha, growth regulation by estrogen in breast cancer 1 (GREB1), is associated with proliferation and regulation of ER alpha activity in estrogen-responsive breast cancer cells. The GREB1 gene encodes three distinct isoforms: GREB1a, GREB1b and GREB1c, whose molecular functions are largely unknown. Here, we investigate the role of these isoforms in regulation of ER alpha activity and proliferation. Interaction between GREB1 and ER alpha was mapped to the amino terminus shared by all GREB1 variants. Analysis of isoform-specific regulation of ER alpha activity suggests none of the GREB1 isoforms possess potent co-regulator activity. Exogenous expression of GREB1a resulted in elevated expression of some ER-target genes, independent of ER alpha activity. Despite this slight specificity of GREB1a for gene regulation, exogenous expression of either GREB1a or GREB1b resulted in decreased proliferation in both ER-positive and ER-negative breast carcinoma cell lines, demonstrating an ER-independent function of GREB1. Interestingly, we show an increase in the expression of GREB1b and GREB1c mRNA in malignant breast tissue compared to normal patient samples, suggesting a selective preference for these isoforms during malignant transformation. Together, these data suggest GREB1a has an isoform-specific function as a transcriptional regulator while all isoforms share an ER-independent activity that regulates proliferation.