Calotropin: A Cardenolide from Calotropis gigantea that Inhibits Wnt Signaling by Increasing Casein Kinase 1α in Colon Cancer Cells

Wnt signaling plays key roles in embryonic development and various human diseases. Activity-guided testing to isolate Wnt signaling inhibitors from the methanol extract of Calotropis gigantea (Asclepiadaceae) exudutes identified six Wnt inhibitory cardenolides (1-6), of which 1, 3, 5, and 6 exhibited potent TCF/-catenin inhibitory activities (IC50 0.7-3.6 nM). Calotropin (1) inhibited Wnt signaling by decreasing both nuclear and cytosolic -catenin in a dose-dependent manner, and promoted degradation of -catenin by increasing the phosphorylation of -catenin at Ser45 through casein kinase 1 (CK1). Moreover, 1 significantly increased CK1 protein and mRNA levels. The results suggest that 1 inhibits the Wnt signaling pathway by increasing CK1 protein levels. To the best of our knowledge, calotropin is the first small molecule to increase CK1 levels.