G-Protein-Coupled Receptor 5 (LGR5) Overexpression Activates β-Catenin Signaling in Breast Cancer Cells via Protein Kinase

Background: Targeting cancer stem cells (CSCs) in breast cancer (BrCa) may improve treatment outcome and patient prognosis. Leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5) is a well-recognized adult stem cell and CRC marker, and previous reports have suggested the cancer-promoting role of LGR5 in breast cancer, but the mechanism remains unclear. Material/Methods: Potential LGR5-associating genes were explored using STRING database, and LGR5 overexpression and knockdown was constructed in MCF-7 and MDA-MB-453 human BrCa cells, respectively. PKA catalytic subunit activation and PKA kinase activity in human BrCa cells was examined by Western blot and PKA kinase activity assay, respectively. Protein expression level or activation of beta-catenin and GSK-3b in human BrCa cells was investigated by Western blot. Cell proliferation, colony formation, Transwell migration, cisplatin sensitivity, and in vivo tumor formation of human BrCa cells were examined. Results: LGR5 overexpression increased PKA activation and its kinase activity in human BrCa cells, which was decreased by LGR5 knockdown. LGR5 expression level or PKA kinase activity were correlated with beta-catenin Ser 552 phosphorylation but inversely correlated with GSK-3b Ser9 phosphorylation in human BrCa cells in vitro. LGR5/PKA increased cell proliferation, colony formation, Transwell migration, and cisplatin resistance in vitro, as well as tumor formation in vivo, of human BrCa cells. Conclusions: LGR5 activates the Wnt/beta-catenin signaling pathway in human BrCa cells in vitro via PKA.