BACKGROUNDLiver kinase B1 (LKB1) is a serine/threonine kinase that functions as a tumor suppressor and regulates cell polarity, proliferation, and metabolism. Mutations in LKB1 are associated with Peutz‐Jeghers syndrome as well as sporadic cervical and lung cancers. Although LKB1‐null mice develop invasive endometrial cancers, the role and regulation of LKB1 in the pathogenesis of human endometrial cancer are not well defined and are the focus of these studies.METHODSLKB1 protein and messenger RNA (mRNA) expression levels were evaluated in high‐grade and low‐grade endometrioid endometrial cancer (EEC) and cell lines by reverse transcriptase‐polymerase chain reaction analysis, Western blot analysis, and immunohistochemistry. Mutational and promoter analyses of the LKB1 gene (serine/threonine kinase 11 [STK11]) were performed to identify the mechanisms that contribute to the loss of LKB1 in high‐grade EEC.RESULTSAnalysis of the LKB1 gene in low‐grade and high‐grade EECs revealed no genetic mutations, suggesting that alterations in LKB1 transcription may be responsible for LKB1 protein loss in high‐grade EEC. Analysis of the LKB1 promoter revealed 4 putative tumor protein 53 (p53) binding sites. Quantitative chromatin immunoprecipitation demonstrated that p53 bound directly to 1 of these sites and increased LKB1 promoter activity 140‐fold. LKB1 promoter activity, mRNA, and protein levels were suppressed after silencing of p53 with small interfering RNA and were elevated in cells that overexpressed p53. Levels of p53 mRNA and protein expression were decreased in high‐grade EEC and were positively correlated with LKB1 protein levels (Spearman correlation, r=0.601; P<.001).CONCLUSIONSLKB1 is a direct transcriptional target of p53. The loss of wild‐type p53 in high‐grade EEC may contribute to the LKB1 loss observed in these more aggressive tumors. Cancer 2014;120:3457–3468. © 2014 American Cancer Society.