Inhibition of Wnt signaling by cucurbitacin B in breast cancer cells: Reduction of Wnt-associated proteins and reduced translocation of galectin-3-mediated β-catenin to the nucleus

The cucurbitacins are tetracyclic triterpenes found in plants of the family Cucurbitaceae. Cucurbitacins have been shown to have anti‐cancer and anti‐inflamatory activities. We investigated the anti‐cancer activity of cucurbitacin B extracted from Thai medicinal plant Trichosanthes cucumerina Linn. Cell viability was assessed by MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide) assay. Results indicated that cucurbitacin B from T. cucumerina Linn. has a cytotoxic effect on breast cancer cell lines SKBR‐3 and MCF‐7 with an IC50 of 4.60 and 88.75 µg/ml, respectively. Growth inhibition was attributed to G2/M phase arrest and apoptosis. Cyclin D1, c‐Myc, and β‐catenin expression levels were reduced. Western blot analysis showed increased PARP cleavage and decreased Wnt‐associated signaling molecules β‐catenin, galectin‐3, cyclin D1 and c‐Myc, and corresponding changes in phosphorylated GSK‐3β levels. Cucurbitacin B treatment inhibited translocation to the nucleus of β‐catenin and galectin‐3. The depletion of β‐catenin and galectin‐3 in the nucleus was confirmed by cellular protein fractionation. T‐cell factor (TCF)/lymphoid enhancer factor (LEF)‐dependent transcriptional activity was disrupted in cucurbitacin B treated cells as tested by a TCF reporter assay. The relative luciferase activity was reduced when we treated cells with cucurbitacin B compound for 24 h. Our data suggest that cucurbitacin B may in part induce apoptosis and exert growth inhibitory effect via interruption the Wnt signaling. J. Cell. Biochem. 113: 49–60, 2012. © 2011 Wiley Periodicals, Inc.