Bipterostilbene, a Novel Compound From Rhubarb, Specifically Inhibits the Proliferation of Malignant B Cells Via Inactivation of AKT/mTOR Signaling Pathway

Abstract 1423

Patients with B cell malignaces initially respond to current treatment modalities, however, such malignances remain incurable. Many new therapeutic options have become available during the past several years but nearly all patients develop resistance to currently available therapeutic options. Ideally, a new treatment should inhibit tumor growth, improve the efficacy of other anti-tumor agents, and improve both the overal survial and the quality of life for patients. Pterostilbene is predominantly found in Rhubarb. We synthesized bipterostilbene (5-(4-(4-(3,5-dihydroxylstyryl)phenoxy)styryl)-benzene-1,3-diol) (C₂₈H₂₂O₅) of a molecular weight of 438.48 Kda. In this study, we first examined whether bipterostilbene affects tumor cells proliferation using breast cancer, ovarian cancer, lymphoma and multiple myeloma (MM) cell lines. The results of the MTS assay demonstrated that bipterostilbene significantly inhibited tumor cell proliferation of the lymphoma cell line (Raji) and the MM cell lines (RPMI1640 and MM1s) at 48 hours (IC50: 5μM for Raji, 4μM for RPMI8226, and 2 μM for MM1s). The induction of tumor cell apoptosis was most prominent at 72 hours. The extent of the inhibition of tumor cell proliferation and the induction of apoptosis was concentration-dependent. Bipterostilbene had minimal effects on breast and ovarian cancer cell lines. Noteworthy, bipterostilbene had no detectable cytotoxic effects on normal human peripheral blood mononuclear cells (PBMCs). The molecular mechanism by which bipterostilbene mediates its effects was examined. Both the AKT and the NF-κB signaling transduction pathways have been reported to play key roles in B cell metabolism, proliferation and survival. Using RT-PCR, bipterostilbene specifically inhibited AKT1 and mTOR gene expression when Raji or RPMI8226 tumor cells were treated with the IC50 concentration of bipterostilbene for 24 hours. Analysis of downstream gene products of the AKT pathway revealed that Cyclin D1 expression was slightly reduced and P21Cip and P27 kip expressions were not changed. Bipterostilbene did not alter AKT2 or AKT3 gene expression, demonstrating that this compound is specifically targeting AKT1. We further determined whether bipterostilbene interfered with IGF1-induced AKT/mTOR activation or IL-1β –mediated NF-κB phosphorylation by Western blot. The results showed that bipterostilbene markedly inhibited IGF1-induced phosphorylation of AKT but did not interfere with IL-1β-induced NF-κB activity and IκB phosphorylation. Overall, the results of our in vitro studies demonstrate that bipterostilbene inhibits tumor cell proliferation and enhances apoptosis of B-cell malignancies via inhibition of the AKT/mTOR signaling pathway with no detectable effect on the NF-κB signaling pathway. Importantly, bipterostilbene is not cytotoxic on normal hematopoietic cells at concentrations that were highly toxic to B-cell malignancies. We propose that bipterostilbene may be better tolerated than other anti- cancer drugs that are currently being used for the treatment of B-cell malignancies.