SLC22A5/OCTN2 expression in breast cancer is induced by estrogen via a novel intronic estrogen-response element (ERE)

Estrogen signaling is a critical pathway that plays a key role in the pathogenesis of breast cancer. In a previous transcriptional profiling study, we identified a novel panel of estrogen-induced genes in breast cancer. One of these genes is solute carrier family 22 member 5 (SLC22A5), which encodes a polyspecific organic cation transporter (also called OCTN2). In this study, we found that estrogen stimulates SLC22A5 expression robustly in an estrogen receptor (ER)-dependent manner and that SLC22A5 expression is associated with ER status in breast cancer cell lines and tissue specimens. Although the SLC22A5 proximal promoter is not responsive to estrogen, a downstream intronic enhancer confers estrogen inducibility. This intronic enhancer contains a newly identified estrogen-responsive element (ERE) (GGTCA-CTG-TGACT) and other transcription factor binding sites, such as a half ERE and a nuclear receptor related 1 (NR4A2/Nurr1) site. Estrogen induction of the luciferase reporter was dependent upon both the ERE and the NR4A2 site within the intronic enhancer. Small interfering RNA against either ER or Nurr1 inhibited estrogen induction of SLC22A5 expression, and chromatin immunoprecipitation assays confirmed the recruitment of both ER and Nurr1 to this enhancer. In functional assays, knockdown of SLC22A5 inhibited l-carnitine intake, resulted in lipid droplet accumulation, and suppressed the proliferation of breast cancer cells. These results demonstrate that SLC22A5 is an estrogen-dependent gene regulated via a newly identified intronic ERE. Since SLC22A5 is a critical regulator of carnitine homeostasis, lipid metabolism, and cell proliferation, SLC22A5 may serve as a potential therapeutic target for breast cancer in the future.