Lucidin 3-methyl ether from Rubia philippinensis suppresses the proliferation of multiple myeloma cells through the promotion of β-catenin degradation

Background: Constitutive accumulation of β-catenin has been frequently observed in multiple myeloma. Extracts from genus Rubia plants exhibit cytotoxic activity against several types of cancer cells; however, little is known about their chemopreventive mechanisms and bioactive metabolites.

Purpose: Purpose: The study aimed to identify the underlying antiproliferative mechanisms of Rubia philippinensis extract in multiple myeloma cells and the major active metabolites responsible for cytotoxic activity of R. philippinensis.

Methods: The effects of R. philippinensis extracts and lucidin 3-methyl ether on the Wnt/β-catenin pathway were determined by cell-based reporter assay, Western blot analysis, and RT-PCR. The antiproliferative activity was evaluated by cell viability assay and apoptosis analysis in RPMI8226 and MM.1S multiple myeloma cells.

Results: R. philippinensis extracts inhibited Wnt/β-catenin signaling and lucidin 3-methyl ether, an anthraquinone derivative, was identified as the major active metabolite responsible for the inhibition of Wnt/β-catenin signaling. Lucidin 3-methyl ether induced β-catenin phosphorylation at Ser33/Ser37/Thr41 residues and promoted proteasomal degradation of β-catenin via a GSK-3β-independent mechanism, thereby downregulating Wnt3a-induced β-catenin response transcription (CRT). Moreover, lucidin 3-methyl ether repressed the expression of β-catenin/T-cell factor (TCF)-dependent genes, such as cyclin D1, c-myc, and axin-2, thus inhibiting MM cell proliferation. Apoptosis was also elicited by lucidin 3-methyl ether, as indicated by the increase in the population of annexin V-FITC positive cells and caspase-3/7 activity in MM cells.

Conclusion: These findings indicate that R. philippinensis and its active metabolite lucidin 3-methyl ether prevent cell proliferation through the suppression of the Wnt/β-catenin pathway and exhibit potential as chemopreventive agents for the treatment of MM.