Honokiol induces apoptosis in human lymphoid leukemia Molt 4B cells

Current Perspectives of the Applications of Polyphenols and Flavonoids in Cancer Therapy

The development of anticancer therapies that involve natural drugs has undergone exponential growth in recent years. Among the natural compounds that produce beneficial effects on human health, polyphenols have shown potential therapeutic applications in cancer due to their protective functions in plants, their use as food additives, and their excellent antioxidant properties. The possibility of combining conventional drugs-which are usually more aggressive than natural compounds-with polyphenols offers very valuable advantages such as the building of more efficient anticancer therapies with less side effects on human health. This review shows a wide range of trials in which polyphenolic compounds play a crucial role as anticancer medicines alone or in combination with other drugs at different stages of cancer: cancer initiation, promotion, and growth or progression. Moreover, the future directions in applications of various polyphenols in cancer therapy are emphasized.

Phyto-polyphenols as potential inhibitors of breast cancer metastasis

Breast cancer is the most common cancer among women as metastasis is currently the main cause of mortality. Breast cancer cells undergoing metastasis acquire resistance to death signals and increase of cellular motility and invasiveness.Plants are rich in polyphenolic compounds, many of them with known medicinal effects. Various phyto-polyphenols have also been demonstrated to suppress cancer growth. Their mechanism of action is usually pleiotropic as they target multiple signaling pathways regulating key cellular processes such as proliferation, apoptosis and differentiation. Importantly, some phyto- polyphenols show low level of toxicity to untransformed cells, but selective suppressing effects on cancer cells proliferation and differentiation.In this review, we summarize the current information about the mechanism of action of some phyto-polyphenols that have demonstrated anti-carcinogenic activities in vitro and in vivo. Gained knowledge of how these natural polyphenolic compounds work can give us a clue for the development of novel anti-metastatic agents.

Honokiol induces apoptosis and autophagy via the ROS/ERK1/2 signaling pathway in human osteosarcoma cells in vitro and in vivo

Osteosarcoma is the most common primary malignant tumor of bone, the long-term survival of which has stagnated in the past several decades. In the present study, we investigated the anticancer effect of honokiol (HNK), an active component isolated and purified from the magnolia officinalis on human osteosarcoma cells. Our results showed that honokiol caused dose-dependent and time-dependent cell death in human osteosarcoma cells. The types of cell death induced by honokiol were primarily autophagy and apoptosis. Furthermore, honokiol induced G0/G1 phase arrest, elevated the levels of glucose-regulated protein (GRP)-78, an endoplasmic reticular stress (ERS)-associated protein, and increased the production of intracellular reactive oxygen species (ROS). In contrast, reducing production of intracellular ROS using N-acetylcysteine, a scavenger of ROS, concurrently suppressed honokiol-induced cellular apoptosis, autophagy, and cell cycle arrest. Consequently, honokiol stimulated phosphorylation of extracellular signal-regulated kinase (ERK)1/2. Furthermore, pretreatment of osteosarcoma cells with PD98059, an inhibitor of ERK1/2, inhibited honokiol-induced apoptosis and autophagy. Finally, honokiol suppressed tumor growth in the mouse xenograft model. Taken together, our results revealed that honokiol caused G0/G1 phase arrest, induced apoptosis, and autophagy via the ROS/ERK1/2 signaling pathway in human osteosarcoma cells. Honokiol is therefore a promising candidate for development of antitumor drugs targeting osteosarcoma.

Magnolol and honokiol exert a synergistic anti-tumor effect through autophagy and apoptosis in human glioblastomas

Glioblastoma (GBM) is a malignant brain tumor associated with a high mortality rate. The aim of this study is to investigate the synergistic effects of honokiol (Hono) and magnolol (Mag), extracted from Magnolia officinalis, on cytotoxicity and inhibition of human GBM tumor progression in cellular and animal models. In comparison with Hono or Mag alone, co-treatment with Hono and Mag (Hono-Mag) decreased cyclin A, D1 and cyclin-dependent kinase 2, 4, 6 significantly, leading to cell cycle arrest in U87MG and LN229 human glioma cells. In addition, phosphorylated phosphoinositide 3-kinase (p-PI3K), p-Akt, and Ki67 were decreased after Hono-Mag treatment, showing proliferation inhibition. Hono-Mag treatment also reduced p-p38 and p-JNK but elevated p-ERK expression. Besides, Hono-Mag treatment induced autophagy and intrinsic and extrinsic apoptosis. Both ERK and autophagy inhibitors enhanced Hono-Mag-induced apoptosis in LN229 cells, indicating a rescuer role of ERK. In human GBM orthotopic xenograft model, the Hono-Mag treatment inhibited the tumor progression and induced apoptosis more efficiently than Temozolomide, Hono, or Mag group. In conclusion, the Hono-Mag exerts a synergistic anti-tumor effect by inhibiting cell proliferation and inducing autophagy and apoptosis in human GBM cells. The Hono-Mag may be applied as an adjuvant therapy to improve the therapeutic efficacy of GBM treatment.

Non-toxic dose of liposomal honokiol suppresses metastasis of hepatocellular carcinoma through destabilizing EGFR and inhibiting the downstream pathways

At present, there is no specific anti-metastasis drug in HCC treatment. Drugs used for primary HCC tumors and tumor metastasis are very similar, among which cytotoxic drugs are prevalent, such as cisplatin, doxorubicin and 5-FU. The EGFR pathway plays an important role in promoting hepatocellular carcinoma (HCC) metastasis. Hence, development of non-toxic anti-metastasis drugs, such as EGFR or downstream pathways inhibitors, is of great importance. In our present study, we found non-toxic dose of liposomal honokiol (LH) could inhibit the HCC metastasis by destabilizing EGFR and inhibiting the downstream pathways. Non-toxic dose of LH significantly inhibited the motility, migration and lamellipodia formation of HepG2 cells in vitro and decreased extravasation of HepG2 cells in a novel metastasis model of transgenic zebrafish. In two lung metastasis models (HepG2 and B16F10) and a spontaneous metastasis model of HepG2 cells, LH remarkably inhibited pulmonary metastasis and regional lymph nodes metastasis without obvious toxicity. Further study showed that destabilizing EGFR and inhibiting the downstream pathways were the main mechanisms of non-toxic dose of LH on metastasis inhibition. Our results provide the preclinical rationale and the underlying mechanisms of LH to suppress HCC metastasis, implicating LH as a potential therapeutic agent to block HCC metastasis without severe side effects.

Pro-Apoptotic Activity of New Honokiol/Triphenylmethane Analogues in B-Cell Lymphoid Malignancies

Honokiol and triphenylmethanes are small molecules with anti-tumor properties. Recently, we synthesized new honokiol analogues (HAs) that possess common features of both groups. We assessed the anti-tumor effectiveness of HAs in B-cell leukemia/lymphoma cells, namely in chronic lymphocytic leukemia (CLL) cells ex vivo and in pre-B-cell acute lymphoblastic leukemia (Nalm-6), Burkitt lymphoma (BL; Raji), diffuse large B-cell lymphoma (DLBCL; Toledo) and multiple myeloma (MM; RPMI 8226) cell lines. Four of these compounds appeared to be significantly active against the majority of cells examined, with no significant impact on healthy lymphocytes. These active HAs induced caspase-dependent apoptosis, causing significant deregulation of several apoptosis-regulating proteins. Overall, these compounds downregulated Bcl-2 and XIAP and upregulated Bax, Bak and survivin proteins. In conclusion, some of the HAs are potent tumor-selective inducers of apoptosis in ex vivo CLL and in BL, DLBCL and MM cells in vitro. Further preclinical studies of these agents are recommended.

Honokiol activates reactive oxygen species-mediated cytoprotective autophagy in human prostate cancer cells

BACKGROUND

Honokiol (HNK), derived from the bark of an oriental medicinal plant (Magnolia officinalis), is a promising anticancer agent with preclinical in vitro (PC-3 and LNCaP cells) and in vivo (PC-3 xenografts) efficacy against prostate cancer. However, the mechanisms affecting anticancer response to HNK are not fully understood.

METHODS

Human (androgen-independent PC-3 and androgen-responsive LNCaP) and murine (Myc-CaP) prostate cancer cells, and PC-3 tumor xenografts were used for various assays. Autophagy was assessed by transmission electron microscopy, immunofluorescence (LC3 puncta), and immunoblotting (LC3BII detection). Cell viability was determined by trypan blue assay. Apoptosis was quantitated by DNA fragmentation detection and Annexin V/propidium iodide assay. Reactive oxygen species (ROS) were detected by electron paramagnetic resonance spectrometry and flow cytometric/microscopic analysis of MitoSOX red fluorescence.

RESULTS

Exposure of PC-3, LNCaP, and Myc-CaP cells to pharmacologic doses of HNK resulted in autophagy induction. The PC-3 tumor xenografts from HNK-treated mice contained higher levels of LC3BII protein compared with control tumors. Cell viability inhibition and apoptosis induction resulting from HNK exposure were significantly augmented by pharmacological inhibition of autophagy using 3-methyladenine as well as RNA interference of autophagy regulator ATG5. HNK-mediated increase in levels of LC3BII protein was partially but markedly diminished in the presence of antioxidants, including N-acetylcysteine, polyethylene glycol-conjugated (PEG)-superoxide dismutase, and PEG-catalase. On the other hand, antioxidants had no impact on HNK-induced apoptosis.

CONCLUSIONS

In conclusion, the present study demonstrates, for the first time, that HNK induces ROS-mediated cytoprotective autophagy in prostate cancer cells.

Synthesis of tetrahydrohonokiol derivates and their evaluation for cytotoxic activity against CCRF-CEM leukemia, U251 glioblastoma and HCT-116 colon cancer cells

Biphenyl neolignans such as honokiol and magnolol, which are the major active constituents of the Asian medicinal plant Magnolia officinalis, are known to exert a multitude of pharmacological and biological activities. Among these, cytotoxic and tumor growth inhibitory activity against various tumour cell lines are well-documented. To further elucidate the cytotoxic effects of honokiol derivatives, derivatizations were performed using tetrahydrohonokiol as a scaffold. The derivatizations comprised the introduction of functional groups, e.g., nitro and amino groups, as well as alkylation. This way, 18 derivatives, of which 13 were previously undescribed compounds, were evaluated against CCRF-CEM leukemia cells, U251 glioblastoma and HCT-116 colon cancer cells. The results revealed no significant cytotoxic effects in any of the three tested cell lines at a test concentration of 10 µM.

Honokiol inhibits epithelial-mesenchymal transition in breast cancer cells by targeting signal transducer and activator of transcription 3/Zeb1/E-cadherin axis

Epithelial-mesenchymal transition (EMT), a critical step in the acquisition of metastatic state, is an attractive target for therapeutic interventions directed against tumor metastasis. Honokiol (HNK) is a natural phenolic compound isolated from an extract of seed cones from Magnolia grandiflora. Recent studies from our lab show that HNK impedes breast carcinogenesis. Here, we provide molecular evidence that HNK inhibits EMT in breast cancer cells resulting in significant downregulation of mesenchymal marker proteins and concurrent upregulation of epithelial markers. Experimental EMT induced by exposure to TGFβ and TNFα in spontaneously immortalized nontumorigenic human mammary epithelial cells is also completely reversed by HNK as evidenced by morphological as well as molecular changes. Investigating the downstream mediator(s) that may direct EMT inhibition by HNK, we found functional interactions between HNK, Stat3, and EMT-signaling components. In vitro and in vivo analyses show that HNK inhibits Stat3 activation in breast cancer cells and tumors. Constitutive activation of Stat3 abrogates HNK-mediated activation of epithelial markers whereas inhibition of Stat3 using small molecule inhibitor, Stattic, potentiates HNK-mediated inhibition of EMT markers, invasion and migration of breast cancer cells. Mechanistically, HNK inhibits recruitment of Stat3 on mesenchymal transcription factor Zeb1 promoter resulting in decreased Zeb1 expression and nuclear translocation. We also discover that HNK increases E-cadherin expression via Stat3-mediated release of Zeb1 from E-cadherin promoter. Collectively, this study reports that HNK effectively inhibits EMT in breast cancer cells and provide evidence for a previously unrecognized cross-talk between HNK and Stat3/Zeb1/E-cadherin axis.

The natural product honokiol inhibits calcineurin inhibitor-induced and Ras-mediated tumor promoting pathways

Although calcineurin inhibitors (CNIs) are very useful in preventing allograft rejection, they can mediate a rapid progression of post-transplantation malignancies. The CNI cyclosporine A (CsA) can promote renal tumor growth through activation of the proto-oncogene ras and over-expression of the angiogenic cytokine VEGF; the ras activation also induces over-expression of the cytoprotective enzyme HO-1, which promotes survival of renal cancer cells. Here, we show that the natural product honokiol significantly inhibited CsA-induced and Ras-mediated survival of renal cancer cells through the down-regulations of VEGF and HO-1. Thus, honokiol treatment may help to prevent tumor-promoting effects of CsA in transplant patients.

Synergistic antitumor effects of liposomal honokiol combined with cisplatin in colon cancer models

Honokiol, a novel antitumor agent, may induce apoptosis and inhibit the growth of vascular endothelium in a number of tumor cell lines and xenograft models. It has been proposed that the antitumor effects of chemotherapy may be increased in combination with an antiangiogenesis agent as an anticancer strategy. In the present study, we examined the potential of honokiol to increase the antitumor effect of cisplatin (DDP) when the agent and drug were combined in murine CT26 colon cancer models, and investigated the underlying mechanism. Liposomal honokiol (LH) was prepared, and female BALB/c mice were administered LH at various doses to determine the optimum doses for honokial. Evaluation of cell apoptosis was analyzed using flow cytometry. Honokiol was encapsulated with liposome to improve its water insolubility. In vitro, LH inhibited the proliferation of CT26 cells via apoptosis and significantly enhanced the DPP-induced apoptosis of CT26 cells. In vivo, the systemic administration of LH plus DDP resulted in the inhibition of subcutaneous tumor growth beyond the effects observed with either LH or DDP alone. This growth reduction was associated with elevated levels of apoptosis (TUNEL staining) and reduced endothelial cell density (CD31 staining) compared with either treatment alone. Collectively, these findings indicate that LH may augment the induction of apoptosis in CT26 cells in vitro and in vivo, and this combined treatment has exhibited synergistic suppression in tumor progression according to the synergistic analysis. The present study may be significant to future exploration of the potential application of the combined approach in the treatment of colon cancer.

Honokiol crosses BBB and BCSFB, and inhibits brain tumor growth in rat 9L intracerebral gliosarcoma model and human U251 xenograft glioma model

Background

Gliosarcoma is one of the most common malignant brain tumors, and anti-angiogenesis is a promising approach for the treatment of gliosarcoma. However, chemotherapy is obstructed by the physical obstacle formed by the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB). Honokiol has been known to possess potent activities in the central nervous system diseases, and anti-angiogenic and anti-tumor properties. Here, we hypothesized that honokiol could cross the BBB and BCSFB for the treatment of gliosarcoma.

Methodologies

We first evaluated the abilities of honokiol to cross the BBB and BCSFB by measuring the penetration of honokiol into brain and blood-cerebrospinal fluid, and compared the honokiol amount taken up by brain with that by other tissues. Then we investigated the effect of honokiol on the growth inhibition of rat 9L gliosarcoma cells and human U251 glioma cells in vitro. Finally we established rat 9L intracerebral gliosarcoma model in Fisher 344 rats and human U251 xenograft glioma model in nude mice to investigate the anti-tumor activity.

Principal Findings

We showed for the first time that honokiol could effectively cross BBB and BCSFB. The ratios of brain/plasma concentration were respectively 1.29, 2.54, 2.56 and 2.72 at 5, 30, 60 and 120 min. And about 10% of honokiol in plasma crossed BCSFB into cerebrospinal fluid (CSF). In vitro, honokiol produced dose-dependent inhibition of the growth of rat 9L gliosarcoma cells and human U251 glioma cells with IC₅₀ of 15.61 µg/mL and 16.38 µg/mL, respectively. In vivo, treatment with 20 mg/kg body weight of honokiol (honokiol was given twice per week for 3 weeks by intravenous injection) resulted in significant reduction of tumor volume (112.70±10.16 mm³) compared with vehicle group (238.63±19.69 mm³, P = 0.000), with 52.77% inhibiting rate in rat 9L intracerebral gliosarcoma model, and (1450.83±348.36 mm³) compared with vehicle group (2914.17±780.52 mm³, P = 0.002), with 50.21% inhibiting rate in human U251 xenograft glioma model. Honokiol also significantly improved the survival over vehicle group in the two models (P<0.05).

Conclusions/Significance

This study provided the first evidence that honokiol could effectively cross BBB and BCSFB and inhibit brain tumor growth in rat 9L intracerebral gliosarcoma model and human U251 xenograft glioma model. It suggested a significant strategy for offering a potential new therapy for the treatment of gliosarcoma.

Honokiol, a multifunctional antiangiogenic and antitumor agent

Honokiol is a small-molecule polyphenol isolated from the genus Magnolia. It is accompanied by other related polyphenols, including magnolol, with which it shares certain biologic properties. Recently, honokiol has been found to have antiangiogenic, antiinflammatory, and antitumor properties in preclinical models, without appreciable toxicity. These findings have increased interest in bringing honokiol to the clinic as a novel chemotherapeutic agent. In addition, mechanistic studies have tried to find the mechanism(s) of action of honokiol, for two major reasons. First, knowledge of the mechanisms of action may assist development of novel synthetic analogues. Second, mechanistic actions of honokiol may lead to rational combinations with conventional chemotherapy or radiation for enhanced response to systemic cancers. In this review, we describe the findings that honokiol has two major mechanisms of action. First, it blocks signaling in tumors with defective p53 function and activated ras by directly blocking the activation of phospholipase D by activated ras. Second, honokiol induces cyclophilin D, thus potentiating the mitochondrial permeability transition pore, and causing death in cells with wild-type p53. Knowledge of the dual activities of honokiol can assist with the development of honokiol derivatives and the design of clinical trials that will maximize the potential benefit of honokiol in the patient setting.

Liposomal honokiol, a potent anti-angiogenesis agent, in combination with radiotherapy produces a synergistic antitumor efficacy without increasing toxicity

Honokiol is an active compound purified from magnolia that has been shown to induce cell differentiation, apoptosis, and anti-angiogenesis effects, as well as an enhancement in tumor growth delay in combination with chemotherapeutic agents in several mouse xenograft models. Our goal was to investigate the radiosensitization effect of honokiol on lung carcinoma. The radiosensitization effect of liposomal honokiol in Lewis lung carcinoma cells (LL/2) was analyzed using an in vitro clonogenic survival assay. For an in vivo study, Lewis lung carcinoma-bearing C57BL/6 mice were treated with either liposomal honokiol at 25 mg/kg or 5 Gy of single tumor radiation, or a combination of both over 12 days of treatment. The tumor growth delay and the survival time were evaluated. In addition, histological analysis of tumor sections was performed to examine changes by detecting the microvessel density and apoptosis in tumor tissues. In the clonogenic survival assay, LL/2 cells treated with IC(50) Lipo-HNK for 24 h showed a radiation enhancement ratio of 1.9. After 12 days of combination treatment, the tumor volume decreased 78% and produced an anti-tumor activity 1.3-fold greater than a predicted additive effect of honokiol and radiation alone. This combination treatment also caused an 8.7 day delay in tumor growth. The cell cycle distribution and histological analysis demonstrated that liposomal honokiol has an anti-tumor effect via inducing apoptosis and inhibiting angiogenesis. Liposomal honokiol can enhance tumor cell radiosensitivity in vitro and in vivo, indicating that radiotherapy combined with liposomal honokiol can lead to greater anti-tumor efficacy.

The natural product honokiol preferentially inhibits cellular FLICE-inhibitory protein and augments death receptor-induced apoptosis

Targeting death receptor-mediated apoptosis has emerged as an effective strategy for cancer therapy. However, certain types of cancer cells are intrinsically resistant to death receptor-mediated apoptosis. In an effort to identify agents that can sensitize cancer cells to death receptor-induced apoptosis, we have identified honokiol, a natural product with anticancer activity, as shown in various preclinical studies, as an effective sensitizer of death receptor-mediated apoptosis. Honokiol alone moderately inhibited the growth of human lung cancer cells; however, when combined with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), greater effects on decreasing cell survival and inducing apoptosis than TRAIL alone were observed, indicating that honokiol cooperates with TRAIL to enhance apoptosis. This was also true to Fas-induced apoptosis when combined with Fas ligand or an agonistic anti-Fas antibody. Among several apoptosis-associated proteins tested, cellular FLICE-inhibitory protein (c-FLIP) was the only one that was rapidly down-regulated by honokiol in all of the tested cell lines. The down-regulation of c-FLIP by honokiol could be prevented by the proteasome inhibitor MG132. Moreover, honokiol increased c-FLIP ubiquitination. These results indicate that honokiol down-regulates c-FLIP by facilitating its degradation through a ubiquitin/proteasome-mediated mechanism. Enforced expression of ectopic c-FLIP abolished the ability of honokiol to enhance TRAIL-induced apoptosis. Several honokiol derivatives, which exhibited more potent effects on down-regulation of c-FLIP than honokiol, showed better efficacy than honokiol in inhibiting the growth and enhancing TRAIL-induced apoptosis as well. Collectively, we conclude that c-FLIP down-regulation is a key event for honokiol to modulate the death receptor-induced apoptosis.

Improved therapeutic effectiveness by combining liposomal honokiol with cisplatin in lung cancer model

Background

Honokiol is a major bioactive compound extracted from Magnolia. The present study was designed to determine whether liposomal honokiol has the antitumor activity against human lung cancer as well as potentiates the antitumor activity of cisplatin in A549 lung cancer xenograft model, if so, to examine the possible mechanism in the phenomenon.

Methods

human A549 lung cancer-bearing nude mice were treated with liposomal honokiol, liposomal honokiol plus DDP or with control groups. Apoptotic cells and vessels were evaluated by fluorescent in situ TUNEL assay and by immunohistochemistry with an antibody reactive to CD31 respectively.

Results

The present study showed that liposomal honokiol alone resulted in effective suppression of the tumor growth, and that the combined treatment with honokiol plus DDP had the enhanced inhibition of the tumor growth and resulted in a significant increase in life span. The more apparent apoptotic cells in the tumors treated with honokiol plus DDP was found in fluorescent in situ TUNEL assay, compared with the treatment with control groups. In addition, the combination of honokiol and DDP apparently reduced the number of vessels by immunolabeling of CD31 in the tissue sections, compared with control groups.

Conclusion

In summary, our data suggest that honokiol alone had the antitumor activity against human lung cancer in A549 lung cancer xenograft model, and that the combination of honokiol with DDP can enhance the antitumor activity, and that the enhanced antitumor efficacy in vivo may in part result from the increased induction of the apoptosis and the enhanced inhibition of angiogenesis in the combined treatment. The present findings may be of importance to the further exploration of the potential application of the honokiol alone or the combined approach in the treatment of lung carcinoma.

The natural product honokiol induces caspase-dependent apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells

B-cell chronic lymphocytic leukemia (B-CLL) remains an incurable disease that requires innovative new approaches to improve therapeutic outcome. Honokiol is a natural product known to possess potent antineoplastic and antiangiogenic properties. We examined whether honokiol can overcome apoptotic resistance in primary tumor cells derived from B-CLL patients. Honokiol induced caspase-dependent cell death in all of the B-CLL cells examined and was more toxic toward B-CLL cells than to normal mononuclear cells, suggesting greater susceptibility of the malignant cells. Honokiol-induced apoptosis was characterized by the activation of caspase-3, -8, and -9 and cleavage of poly(adenosine diphosphate-ribose) polymerase (PARP). Exposure of B-CLL cells to honokiol resulted in up-regulation of Bcl2-associated protein (Bax) and down-regulation of the expression of the key survival protein myeloid-cell leukemia sequence 1 (Mcl-1), which is associated with response to treatment in B-CLL patients. In addition, B-CLL cells pretreated with interleukin-4 (IL-4), a cytokine known to support B-CLL survival, underwent apoptosis when subsequently incubated with honokiol, indicating that honokiol could also overcome the prosurvival effects of IL-4. Furthermore, honokiol enhanced cytotoxicity induced by fludarabine, cladribine, or chlorambucil. These data indicate that honokiol is a potent inducer of apoptosis in B-CLL cells and should be examined for further clinical application either as a single agent or in combination with other anticancer agents. (Blood. 2005;106:690-697)

Honokiol overcomes conventional drug resistance in human multiple myeloma by induction of caspase-dependent and -independent apoptosis

Honokiol (HNK) is an active component purified from magnolia, a plant used in traditional Chinese and Japanese medicine. Here we show that HNK significantly induces cytotoxicity in human multiple myeloma (MM) cell lines and tumor cells from patients with relapsed refractory MM. Neither coculture with bone marrow stromal cells nor cytokines (interleukin-6 and insulin-like growth factor-1) protect against HNK-induced cytotoxicity. Although activation of caspases 3, 7, 8, and 9 is triggered by HNK, the pan-caspase inhibitor z-VAD-fmk does not abrogate HNK-induced apoptosis. Importantly, release of an executioner of caspase-independent apoptosis, apoptosis-inducing factor (AIF), from mitochondria is induced by HNK treatment. HNK induces apoptosis in the SU-DHL4 cell line, which has low levels of caspase 3 and 8 associated with resistance to both conventional and novel drugs. These results suggest that HNK induces apoptosis via both caspase-dependent and -independent pathways. Furthermore, HNK enhances MM cell cytotoxicity and apoptosis induced by bortezomib. In addition to its direct cytotoxicity to MM cells, HNK also represses tube formation by endothelial cells, suggesting that HNK inhibits neovascurization in the bone marrow microenvironment. Taken together, our results provide the preclinical rationale for clinical protocols of HNK to improve patient outcome in MM.