Background: Liver kinase 1 (LKB1) is emerging as a multifunctional protein, acting as a key metabolic enzyme, regulator of cell polarity, and transcription factor. Altered LKB1 expression has been linked with various cancers and may be a potential prognostic marker. While the functional role of LKB1 continues to undergo intensive investigation, the molecular mechanisms that regulate its expression remain to be defined more clearly. Recent reports have established a possible link between estrogen receptor alpha (ER alpha) signaling and LKB1 in MCF-7 human breast cancer cells. The current study aimed to investigate whether LKB1 is transcriptionally regulated by ER alpha. in MCF-7 cells. Methods: siRNA transfections were used to transiently knock down LKB1 and ER alpha. LKB1 and ER alpha mRNA and protein levels were evaluated by real-time PCR and Western blotting, respectively. An approximately 3 kilobase pair human LKB1 promoter construct and various truncations were generated, transfected into MCF-7 cells, and luciferase reporter assays were performed. Cells were also treated with various doses of 17-beta-estradiol (E2) to evaluate the effect on LKB1 and ER alpha mRNA levels. Results: LKB1 mRNA and protein levels were significantly lower in ER alpha-positive MCF-7 compared to ER alpha-negative MDA-MB-231 breast cancer cells, suggesting that ER alpha may act as a repressor. siRNA-mediated knock-down of ER alpha in MCF-7 cells significantly increased LKB1 promoter activity and expression at the mRNA and protein levels, and computational analysis revealed the presence of several putative estrogen response element (ERE) DNA binding sites in the LKB1 promoter region. In addition, treatment with E2 led to an increase in LKB1 expression, concomitant with decreased expression of ER alpha in MCF-7 cells. The E2-mediated increase was abrogated by pretreatment with actinomycin D, supporting that the observed changes in LKB1 levels were transcriptionally regulated. Conclusions: ER alpha repressively modulates the expression of LKB1 at the transcriptional level. Targeting the expression of LKB1 by modulating ERa signaling may provide a potential approach to further evaluate its function in ERcx-positive breast cancers. (C) 2011 Elsevier Inc. All rights reserved.