Honokiol is an active component isolated and purified from Magnolia, a plant used in traditional Chinese medicine. It is an anti-oxidant, and inhibits both xanthine oxidase and angiogenesis. In this study, we first examined the direct toxicity of honokiol against human multiple myeloma (MM) cell lines in vitro. Honokiol significantly inhibited growth of MM cell lines (RPMI8226, U266 and MM.1S) via induction of G1 growth arrest, followed by apoptosis, with IC50 values at 48h of 5 to 10 μg/ml. Moreover, honokiol similarly inhibited growth of doxorubicin (Dox)-resistant (RPMI-Dox40), melphalan resistant (RPMI-LR5), and dexamethasone (Dex)-resistant (MM.1R) cell lines. Furthermore, flow cytometric analysis demonstrated that honokiol (6–10 μg/ml, 48h) induced death of CD38+CD138+ tumor cells isolated from 5 patients with relapsed refractory MM. In contrast, no toxicity was observed in normal peripheral blood mononuclear cells or long term-cultured bone marrow stromal cells (BMSCs) treated with honokiol (≤20 mg/ml). Neither culture of MM cells with BMSCs nor interleukin-6 (IL-6) and insulin like growth factor-1 (IGF-1) protected against honokiol-induced cytotoxicity in MM.1S cells. We next delineated the mechanism of honokiol-triggered cytotoxicity. Honokiol triggered increased expression of Bax and Bad; down regulated Mcl-1 protein expression, followed by caspase-8/9/3 cleavage. Importantly, the pan-caspase inhibitor z-VAD-fmk only partially inhibited honokiol-induced apoptosis in MM.1S cells. Furthermore, honokiol induced apoptosis even in SU-DHL4 cells, which express low level of caspase-8 and -3 and are resistant to both conventional (doxorubicin, melphalan, dexamethason) and novel (bortezomib, revimid) drugs. These results suggest that honokiol may induce apoptosis via both caspase-dependent and -independent pathways. Finally, honokiol inhibited IL-6-induced phosphorylation of ERK1/2, STAT3, and Akt, known to mediate growth, survival, and drug resistance, respectively. Taken together, our results suggest that providing the rationale for clinical evaluation of honokiol to improve patient outcome in MM.

Author notes

Corresponding author