Shikonin (SHK), a natural small agent (MW 288.3), reportedly induces cell death in various tumor cells. We have found that SHK also exerts potent cytocidal effects on human multiple myeloma (MM) cells, but its anticancer mechanism in MM cells remains to be elucidated. SHK at 2.5–5 μM induced apoptosis in seven MM cell lines, including the bortezomib-resistant cell line KMS11/BTZ. The IC50 value of SHK against KMS11/BTZ was comparable to that of a parental cell line KMS11 (1.1 and 1.56 μM, respectively). SHK induces accumulation of ubiquitinated proteins and activates XBP-1 in MM cells, suggesting that SHK functions as a proteasome inhibitor, eventually inducing ER stress-associated apoptosis. SHK increases levels of HSP70/72, which protects cells from apoptosis, and exerts greater cytocidal effects in combination with the HSP70/72 inhibitor VER-155008. At higher concentrations (10–20 μM), SHK induced cell death, which was completely inhibited by a necroptosis inhibitor, necrostatin-1 (Nec-1), while the cytocidal activity was unaffected by Z-VAD-FMK, strongly suggesting that cell death is induced by SHK at high concentrations through necroptosis. The present data show for the first time that SHK induces cell death in MM cells. SHK efficiently induces apoptosis and combination of heat shock protein inhibitor with low dose SHK enhances apoptosis, while high dose SHK induces necroptosis in MM cells. These findings together support the use of SHK as a potential therapeutic agent for MM.
Despite recent advances in developing therapeutic strategy for multiple myeloma (MM), MM still remains incurable and thus a novel therapeutic approach is urgently needed (
SHK has been reported to induce cell death in various tumor cell lines, such as breast cancer, leukemia and prostate cancer (
However, there are no reports showing the efficacy of SHK in MM cells. Although mechanisms by which SHK regulates cell death have not been fully analyzed, SHK has been shown to induce activation of caspases (
Previous studies have implicated a function for HSP70 in MM cells. Inhibition of HSP70 reversed drug resistance and induced apoptosis in MM cells (
Interestingly, SHK is also an inducer of necroptosis in osteosarcoma and glioma cells (
In this study, we investigated the mechanisms underlying SHK regulation of cell death in MM cells, including apoptosis and necroptosis.
Human myeloma cell lines KMS-12-PE (
Shikonin, Necrostatin-1 and thapsigargin were purchased from Sigma-Aldrich (St. Louis, MO, USA). VER-155008, an ATP-derivative inhibitor of HSP70, was purchased from Enzo Life Sciences (Farmingdale, NY, USA). These reagents were dissolved in phosphate-buffered saline. Bortezomib was purchased from Janssen Pharmaceutical (Tokyo, Japan).
Cell viability was determined by WST-8 assay using the Cell Counting Kit-8 (Dojindo, Kumamoto, Japan). Briefly, cells were seeded in 96-well plates at a concentration of 2×104/100 μl and incubated with reagents SHK, bortezomib or VER-155008, for 24 h. Following treatment, cells were incubated with WST-8 reagent for a subsequent 3 h. The light absorbance of each well was measured at 450 nm using a VMax absorbance microplate reader (Molecular Devices, Sunnyvale, CA, USA). Data were obtained from three independent experiments.
Cells were incubated at a concentration of 5×105/ml in the presence of SHK for 7 h, or with bortezomib or VER-155008 for 24 h. Cell death was evaluated using the trypan blue exclusion assay (Gibco, Carlsbad, CA, USA). Inhibitors of pan-caspase, Z-VAD-FMK (MBL, Nagoya, Japan) at a concentration of 50 μM, and necroptosis, Nec-1 (necrostatin-1) at a concentration of 60 μM, were employed to distinguish apoptosis and necroptosis, respectively. In some experiments, MM cells were pretreated for 20 min with Z-VAD-FMK before analysis of cell death. Morphological examinations of cells were performed with May-Giemsa staining and transmission electron microscopy. For transmission electron microscopy, cells were centrifuged at 2,000 g for 10 min, fixed in 2.5% glutaraldehyde buffer (pH 7.4, 4°C), and then post-fixed in 1% osmium tetraoxide and embedded in Epon. Ultrathin sections were cut with an ultramicrotome, stained with uranyl acetate and lead citrate, and examined in a Hitachi H-7500 (Tokyo, Japan).
Antibodies against caspase-3, HSP70, HSP90, ubiquitinated proteins and actin were purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Antibodies against caspase-8 and RIP-1 were purchased from Cell Signaling Technology (Beverly, MA, USA). Anti-HSP70 (HSP72) antibody was purchased from Enzo Life Sciences. Cell lysates were prepared using the M-PER mammalian protein extraction reagent (Thermo Scientific Inc., Rockford, IL, USA) after addition of Halt EDTA-free phosphatase inhibitor cocktail and Halt protease inhibitor cocktail (both from Thermo Scientific Inc.). The cell lysates were separated in NuPAGE Bis-Tris precast gels (Invitrogen, Carlsbad, CA, USA) and transferred to PVDF membranes using an iBlot Dry Blotting system (Invitrogen). The membranes were blocked with 5% non-fat dry milk dissolved in Tris-buffered saline (TBS) containing 0.5% Tween-20 (TBS-T) for 1 h at room temperature, followed by incubation with the primary antibodies at 4°C overnight. After washing with TBS-T, the membranes were incubated with a horseradish peroxidase-conjugated secondary antibody (Amersham Biosciences, Oxford, UK) diluted in TBS-T for 2 h at room temperature. The antibody-bound proteins were visualized using an ECL plus kit (Amersham Bioscience).
Proteasome activity was analyzed by 20S proteasome activity assay kit (Chemicon USA & Canada, cat. no. APT280) according to the manufacturer’s protocol using Corona multi-microplate reader MTP-800AFC (Ibaragi, Japan). Data were obtained from three independent experiments.
RNA was extracted using TRIzol reagent (Invitrogen). cDNA synthesis was performed using the SuperScript III First-Strand Synthesis system for RT-PCR (Invitrogen) according to the manufacturer’s protocol. Thapsigargin, a sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitor, was used as an ER stress inducer. ER stress was assessed by detecting activated
Primers for
Statistical analyses were examined using Student’s t-test. P-values <0.05 were considered statistically significant. The interactions between SHK and VER-155008 was analyzed by Chou’s combination index (CI) using CalcuSyn software Version 2.1 (Biosoft, Cambridge, UK) to determine whether the combination was additive or synergistic (
The human MM cell lines were cultured for 24 h in the presence of various concentrations of SHK and cell viability was analyzed by WST-8 assay. As shown in
To further investigate the mechanisms of SHK in regulating cell death, we utilized the pan-caspase inhibitor Z-VAD-FMK. After pretreatment of three MM cell lines, KMS-12-PE, RPMI-8226 and U266, with Z-VAD-FMK for 20 min, cells were incubated with SHK at 2.5 or 5 μM for 7 h and then analyzed by trypan blue assay. As shown in
Western blot analysis revealed that caspase-3 was activated by SHK (
In combination with bortezomib, SHK increased cytotoxic effects for KMS-12-PE cells at a concentration of 0.5 μM (
We further investigated whether SHK can overcome resistance to bortezomib. We used the bortezomib resistant MM cell line KMS-11/BTZ, which has a 9.9-fold higher IC50 value to bortezomib than that of its parental cell line KMS11 (
We further investigated whether SHK could induce cell death in primary MM cells from a patient clinically refractory to both bortezomib and lenalidomide. We observed marked cell death in response to treatment with SHK, and the proportion of dead cells was clearly inhibited by Z-VAD-FMK (
We further analyzed in detail the mechanisms underlying SHK-induced apoptosis. A previous report (
As shown in
SHK increases HSP 70/72 and exerts cytotoxic effect in combination with HSP70/72-inhibitor, VER-155008. Our preliminary observations using mass-spectrometry analysis suggested induction of heat shock proteins (HSPs) by SHK (data not shown), which was consistent with previous reports in monocytes and leukemia cells (
Because HSPs are known to support cell survival under various stresses, we utilized the HSP70/72 inhibitor VER-155008, to analyze the influence of HSPs in cytotoxic effects delivered by SHK. VER-155008 alone increased cytotoxicity of MM cells in a dose-dependent manner (
Because SHK has been reported to induce necroptosis in various tumor cell lines, we examined whether SHK could induce necroptosis in MM cells. As shown in
Our above results demonstrate that SHK exerted antitumor effects in MM cells. No previously examination of the potential cytotoxic effects of SHK in MM cells, has been reported and to the best of our knowledge, ours is the first report providing possible therapeutic efficacy of SHK in MM. We also showed that SHK exerted cytotoxicity to both the bortezomib-resistant cell line and freshly isolated MM cells from a patient clinically refractory in both bortezomib and lenalidomide. The IC50 value of SHK to a bortezomib-resistant cell line was even lower than that of the parental cell line. The mechanisms regulating cell death in bortezomib resistant cell line is unknown. Western blot analysis of KMS11/BTZ treated with SHK revealed slight accumulation of ubiquitinated proteins but the amount was rather less than what found in parental cells (data not shown), suggesting SHK should possess other mechanisms than proteasome inhibition. Collectively, these findings suggested that monotherapy using SHK may overcome refractoriness to bortezomib. Alternatively, since treatment of the bortezomib resistant MM cell line KMS-11/BTZ, with very low concentration of SHK (0.5 μM) sensitized cells to bortezomib; this suggests that SHK may be utilized as a combinational reagent with bortezomib. These results indicate that SHK may overcome refractoriness of MM cells to bortezomib by either monotherapy or in combination with bortezomib. Future studies in other refractory cases should be performed to examine this possibility.
Next, we attempted to elucidate the biological mechanisms of SHK in inducing cell death. Because previous reports showed induction of HSP70 (
That HSP70 induction is detected by SHK treatment may seem conflicting, as SHK alone causes cytotoxic effects and HSP70 is considered to support cell survival. HSP70 is overexpressed in MM cells and HSP70 inhibition is reported to reverse drug resistance (
To evaluate if antitumor effects delivered by SHK could be enhanced by HSP70 inhibition, a combination of SHK and the HSP70 inhibitor VER-155008 was used. As expected, VER-155008 alone exerted cytotoxicity to MM cells in a dose-dependent manner. This was not surprising because HSP inhibitors are reported to be therapeutic candidates for MM (
Furthermore, we showed that SHK at higher concentrations induced necroptosis in MM cells. Necroptosis (programmed necrosis) is regulated by RIP1 and RIP3 through complex formation and activation (
Taken together, our results show that SHK at low concentrations may have a potential role as an inducer of apoptosis in MM cells, including drug resistant clones, through affecting caspases. SHK shows potential to be a new therapeutic agent for treating MM in combination with HSP70 inhibitors. Moreover, SHK at high concentrations may have a potential role as an inducer of necroptosis in MM cells. Since necroptosis has not been considered as a therapeutic strategy in treating MM, this approach might serve as a new modality leading to better control of MM cells. Elucidating the mechanisms underlying the multiple effects of SHK such as reversal of bortezomib resistance and induction of necroptosis by determination of molecules targeted by SHK is currently underway.
We are grateful to the Institute of Natural Medicine, Toyama University, Japan for providing natural herbal compounds. This study was supported in part by a grant from the Amyloidosis Research Committee from the Ministry of Health, Labour, and Welfare, Japan. We are also grateful to Ms. Y. Otake for technical assistance. We thank Gene Technology Center, Institute of Resource Development and Analysis, Kumamoto University for technical assistances regarding proteasome activity analysis.
Induction of apoptosis in MM cells by low concentrations of SHK. (A) Five human MM cell lines (U266, KMS-12-PE, KMS-12-BM, RPMI-8226 and KMM1) were cultured for 24 h in the presence of various concentrations of SHK and analyzed by WST-8 assay. All MM cells tested showed dose-dependent cytotoxic effects of SHK. (B) Three representative MM cell lines (KMS-12-PE, RPMI-8226 and U266) were incubated with SHK at 2.5 or 5 μM for 7 h with or without 20 min pre-treatment with Z-VAD-FMK (indicated as Z) and then analyzed by trypan blue dye exclusion assay. SHK-induced cell death was partly inhibited by Z-VAD-FMK. *P<0.01. (C) Western blot analyses of caspase-3. SHK activated caspase-3 at concentrations of 2.5 and 5 μM for 7 h. (D) Morphological changes of MM cells after incubation with SHK. KMS-12-PE cells were treated with 2.5 μM SHK for 5 h either with or without pre-treatment of Z-VAD-FMK and evaluated by cytospin analysis. Cells were stained with May-Giemsa staining solution. SHK clearly induced apoptotic morphological changes, such as fragmented nucleus (arrows), and apoptosis was inhibited by Z-VAD-FMK. (E) Enhanced cytotoxic effects of bortezomib by SHK. KMS-12-PE cells were incubated with various concentrations of bortezomib either with (dotted line) or without 0.5 μM SHK (solid line). Cell viability was analyzed by WST-8 assay. Marked sensitization of MM cells to bortezomib by SHK was observed.
Antitumor effect of SHK on bortezomib resistant cells, primary MM cells and peripheral blood mononuclear cells. The bortezomib resistant MM cell line KMS-11/BTZ, and the parental cell line KMS11, were cultured with various concentrations of bortezomib (A), SHK (B) or in combination (C) for 24 h and cell viabilities were analyzed by WST-8 assay. (A) The bortezomib resistant MM cell line KMS-11/BTZ (dotted line), and the parental cell line, KMS11 (solid line), were treated with bortezomib for 24 h and subsequently analyzed by WST8 assay. The IC50 values of the KMS11 and KMS11/BTZ cells were 9.9 and 98.5 nM, respectively. (B) The IC50 value of SHK for KMS11/BTZ cells (dotted line) was even lower than that of KMS11 cells (solid line) (1.1 vs. 1.56 μM, respectively). *P<0.05, ¶P<0.01. (C) Treatment of KMS11/BTZ with (dotted line) or without (solid line) low concentration of SHK (0.5 μM), which alone does not show cytotoxic effects, increased the sensitivity to bortezomib. *P<0.005, ¶P<0.0001. (D and E) MM cells from primary bone marrow sample were incubated with 0.5 μM SHK for 16 h and then either evaluated by cytospin analysis (D) or WST-8 assay (E). Both analyses revealed marked increase of dead cells in response to treatment with SHK and inhibition by Z-VAD-FMK. (F) Lack of cytotoxic effect of SHK in normal PBMCs. PBMCs were cultured with 0.5 μM SHK for 24 h and evaluated by trypan blue dye exclusion assay. There was no increase of dead cells by SHK.
Accumulation of ubiquitinated proteins and activation of XBP-1 by SHK. (A) Left panel, western blot analyses of ubiquitinated proteins. U266 and KMS-12-PE cells were incubated with 2.5 or 5 μM SHK for 7 h. SHK induced an accumulation of ubiquitinated proteins in a dose-dependent manner. Right panel, inhibition of 20S chymotrypsin-like activity by SHK. SHK decreased 20S chymotrypsin-like activity at a dose-dependent manner. (B) Activation of
Increase of HSP70/72 by SHK and synergistic cytotoxic effects of SHK in combination with HSP70/72 inhibitor. (A) Western blot analyses of HSP70. SHK at a concentration of 2.5 μM transiently increased HSP70 in KMS-12-PE cells in a time-dependent manner, while this was less evident at 5 μM. (B) Western blot analyses of HSP70, HSP72, and HSP90. U266, KMS-12-PE and KMM1 were treated with 2.5 and 5 μM SHK for 7 h. Induction of HSP70 and HSP72 by SHK was observed in all cell lines and maximized at 2.5 μM. There was no change in the expression of HSP90. (C) Cytotoxic effect of the HSP70/72 inhibitor VER-155008 in KMS-12-PE cells. Cells were cultured with various concentrations of VER-155008 for 24 h and evaluated by WST-8 analysis. VER-155008 alone induced cytotoxic effects in MM cells. Note that VER-155008 at ~3 μM showed 55% growth inhibition (dotted line). (D) Combination effects of VER-155008 and SHK. KMS-12-PE cells were treated with SHK at concentrations varying from 0.19 to 0.5 μM either with 3 μM VER-155008 (solid bars) or SHK alone (blank bars) for 24 h. Combinations of SHK and VER-155008 showed significant synergistic effects in induction of cytotoxicity (CI=0.72). (E) The populations of dead cells induced by the combination of SHK and VER-155008 (VER) were partly inhibited by Z-VAD-FMK (P<0.0001). (F) Combination of SHK and VER-155008 did not show toxic effects in normal PBMCs. PBMCs from a normal donor were cultured with SHK and VER-155008 (VER) at 0.5 and 3 μM, respectively, for 24 h and evaluated by trypan blue dye exclusion analysis. No cytotoxic effect was observed.
Induction of necroptosis in MM cells by SHK. (A) Electron microscopic examination of KMS-12-PE cells treated with 5 μM SHK for 4 h showed typical apoptotic changes, such as fragmented and condensed nuclei (middle panel). In contrast, treatment of 10 μM SHK for 2 h induced typical necrotic changes, such as translucent cytoplasm and swelling of cell membranes (right panel). Scale bar, 5 μm. (B) Morphological changes of KMS-12-PE cells after incubation with 10 or 20 μM SHK for 5 h. SHK induced ghost cells (left panel) and this was inhibited by treatment with Nec-1 (right panel). No apparent inhibition of cell death was found by Z-VAD-FMK treatment (middle panel). (C) Nec-1 inhibited cell death induced by SHK. KMS-12-PE, RPMI-8226 and U266 cells were incubated with 10 or 20 μM SHK in the presence or absence of Nec-1 (indicated as N) or Z-VAD-FMK (indicated as Z) for 7 h and then analyzed by trypan blue dye exclusion assay. SHK-induced cell death was significantly inhibited by Nec-1 (P<0.01) and not affected by Z-VAD-FMK (*P<0.01). (D) Western blot analyses of caspase-8, -3, and RIP-1. RPMI-8226 cells were treated with SHK at concentrations from 2.5 to 20 μM. SHK activated caspase-8 and -3 at concentrations <10 μM while no changes were detected at 20 μM SHK. RIP-1 was cleaved by SHK <10 μM and remained intact at 20 μM.