Calotropin: a cardenolide from calotropis gigantea that inhibits Wnt signaling by increasing casein kinase 1α in colon cancer cells.

ABSTRACT

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/ß-catenin inhibitory activities (IC50 0.7-3.6 nM). Calotropin (1) inhibited Wnt signaling by decreasing both nuclear and cytosolic ß-catenin in a dose-dependent manner, and promoted degradation of ß-catenin by increasing the phosphorylation of ß-catenin at Ser45 through casein kinase 1α (CK1α). Moreover, 1 significantly increased CK1α protein and mRNA levels. The results suggest that 1 inhibits the Wnt signaling pathway by increasing CK1α protein levels. To the best of our knowledge, calotropin is the first small molecule to increase CK1α levels.