Sesamin is a lignan with a structure as shown (). The aim of the present study is to determine the effect of this lignan on NF-κB-mediated cellular responses, NF-κB-regulated gene products and on the signaling pathway leading to NF-κB activation. Most of our studies were done on KBM -5 cells as these cells express both types of TNF receptors. TNF was used for most studies because of a variety of reasons; first, TNF is a primary mediator of inflammation; second, TNF activates both apoptosis and NF-κB; third, TNF induced NF-κB activation cascade is well characterized; and fourth, TNF-induced NF-κB can downregulate apoptosis. Under the conditions we examined the effect of sesamin on NF-κB pathway, it had no effect on cell viability; thus downregulation of NF-κB was not due to decrease in viability.
FIGURE 1 (A) Chemical structure of sesamin. (B) Sesamin suppresses tumor cell proliferation. Cells were incubated with 0, 25, 50, and 100 μM sesamin for different days. Cell proliferation was then analyzed by the MTT method as described under “Materials (more ...)
Sesamin suppresses cell proliferation in various tumor cells
We examined whether sesamin can modulate the proliferationof various tumor cell types. As shown in , sesamin by itself suppressed the proliferation of human leukemic cells (such as KBM-5 and K562), and multiple myeloma cells (U266). The suppression was both dose- and time-dependent. Besides these cells, sesamin also inhibited the proliferation of solid tumor cells, such as human pancreatic cancer MiaPaCa-2 cells, human colon cancer HCT-116 cells, human prostate cancer DU145, human lung adenocarcinoma H1299 and human breast cancer MDA-MB-231 cells (). The fifty percent inhibitory sesamin dose was found to be 42.7, 48.3, 51.7, 60.2, 57.2, 58.3, 40.1, and 51.1 μM for KBM-5, K562, U266, DU145, HCT116, MiaPaCa-2, H1299 and MDA-MB-231 cells, respectively.
Sesamin potentiated TNF-induced apoptosis
We sought to determine whether sesamin affects TNF-induced apoptosis in human chronic myeloid leukemia KBM-5 cells. Using a Live/Dead assay, which measures intracellular esterase activity and assesses plasma membrane integrity, we found that sesamin increased the TNF-induced apoptosis from 9% to 65% in KBM-5 cells (, top panel).
To determine whether the effect is cell type specific, we also examine the effect of sesamin on TNF-induced apoptosis in human multiple myeloma U266 cells. Like KBM-5 cells, sesamin also increased the TNF-induced apoptosis from 10 % to 47% in U266 cells (, bottom panel).
Sesamin inhibited TNF-induced cell survival gene products
How sesamin potentiates the apoptotic effects of TNF was investigated. One of the possible mechanisms is through downregulation of cell survival gene products, such as Bcl-2 and survivin. We found that TNF-induced the expression of Bcl-2 and survivin and sesamin inhibited this expression ().
FIGURE 2 (A) Sesamin suppresses the expression of TNF-induced tumor cell survival proteins. KBM-5 cells were incubated with 100 μM sesamin for 12 h and then treated with 1 nM TNF for the indicated times. Whole cell extracts were prepared and analyzed by (more ...)
Sesamin inhibited the TNF-induced expression of cell-proliferative gene products
Both cyclin D1 and COX-2 have been linked with the proliferation of different types of tumor cells. Thus, we investigated the effect of sesamin on the expression of cyclin D1 and COX-2 induced by TNF treatment. We found that TNF induced the expression of these gene products and that pre-treatment with sesamin inhibited this expression ().
Sesamin inhibited the expression of gene products involved in invasion and angiogenesis
TNF also induces the expression of genes involved in adhesion (e.g. ICAM-1) and invasion (MMP-9). Whether sesamin affects the expression of ICAM-1 and MMP-9, was examined. As shown in , TNF-induced the expression ICAM-1 and MMP-9; and sesamin suppressed the expression.
VEGF plays a major role in the process of tumor angiogenesis (25
). We found that TNF-induced expression of VEGF and sesamin inhibited the expression ().
Sesamin inhibited the NF-κB activation induced by TNF in a dose- and time-dependent manner
All the cellular response and gene products modulated by sesamin as described above are regulated by NF-κB activation. Whether sesamin can suppress NF-κB activation, was examined directly. For this we exposed KBM-5 cells to sesamin at different concentrations and then exposed them to TNF and examined for the activation of NF-κB. We found that sesamin by itself had no effect on the activation of NF-κB. However, it suppressed the TNF-induced NF-κB activation in a dose-dependent manner, with maximum inhibition occurring at a concentration of 100 μM (, left panel). Moreover when we accessed the cells viability under these conditions it was greater than 90%.
FIGURE 3 (A) Dose- and time-dependent effect of sesamin on TNF-induced NF-κB activation. (Left) KBM-5 cells were incubated with the indicated concentrations of sesamin for 12 h and treated with 0.1 nM TNF for 30 min. (Right) KBM-5 cells were pre-incubated (more ...)
Whether suppression of NF-κB by sesamin was time-dependent, was also examined. For this KBM-5 cells were treated with sesamin for different time intervals followed by 30 minute exposure to TNF. We observed that sesamin inhibited the activation of NF-κB triggered by TNF in a time-dependent manner, with optimum inhibition occurring at 12 h (, right panel).
Sesamin inhibited NF-κB activation induced by carcinogens and other inflammatory stimuli
Earlier studies reported from our laboratory and by others clearly showed that a wide variety of agents which include cigarette smoke condensate (CSC), tumor promoters such as okadaic acid (OA), phorbol myristate acetate (PMA), inflammatory agents such as hydrogen peroxide and lipopolysaccharide (LPS), can activate NF-κB but the mechanisms by which these agents induce activation of NF-κB vary significantly (22
). Whether sesamin affects NF-κB activation induced by all these agents, was examined. As shown in , sesamin suppressed the activation NF-κB induced by all these agents. So it can be concluded that this lignan acts at a step in the NF-κB activation pathway that is common to all of these agents.
Sesamin does not have any effect on the binding of NF-κB p65 subunit to the DNA
We also investigated whether this lignan can directly interact with the p65 subunit of NF-κB and inhibit its binding to DNA. Nuclear extracts isolated from TNF-treated KBM-5 cells were exposed to sesamin at different concentrations and then examined for binding to DNA. As given in we found that sesamin did not modulate the p65 binding to DNA even at the highest dose.
Sesamin inhibited constitutive NF-κB expression
A wide variety of tumor cell types are known to harbor constitutively active form NF-κB which often results in chemoresistance and treatment failure (19
). Multiple myeloma cell lines (e.g., RPMI-8226) are known to express constitutively active NF-κB (27
). Whether sesamin affects NF-κB expression in these cells, was examined. For this we exposed cells to sesamin at different concentrations for 12 hr and then analyzed them for DNA binding by EMSA. We found that this lignan completely suppressed constitutive NF-κB activation in RPMI-8226 cells (, left panel), indicating that this sesamin can suppress both inducible as well as constitutive NF-κB activation.
Inhibition of NF-κB activation by sesamin is not cell type-specific
Our next aim was to examine whether the inhibition of activation of NF-κB by sesamin is specific to a particular cell type. We treated human lung adenocarcinoma H1299 cell and human colon cancer cell line HCT116 to sesamin and then exposed to TNF. EMSA showed that this lignan inhibited the activation of NF-κB indicating that the effect is not cell type specific (, right panel).
Sesamin inhibits TNF-dependent phosphorylation and degradation of IκBα
Translocation of NF-κB to the nucleus is accompanied by phosphorylation, ubiquitination, and degradation of IκBα, the inhibitory subunit associated with the NF-κB complex (28
). To determine whether inhibition of TNF-induced NF-κB activation is associated with degradation of IκBα, we pretreated KBM-5 cells with sesamin and then exposed them to TNF at various time intervals. We analyzed nuclear extracts for NF-κB activation using EMSA and cytoplasmic extracts for IκBα degradation using Western blotting. TNF activated NF-κB in a time-dependent manner; however, in sesamin-pretreated cells we found that there is no activation of NF-κB ().
FIGURE 4 (A) Sesamin inhibits TNF-induced activation of NF-κB. KBM-5 cells were incubated with 100 μM sesamin for 12 hr, treated with 0.1 nM TNF for the indicated time intervals, and then analyzed by EMSA for NF-κB activation. (B) Effect (more ...)
Western blot analysis showed that TNF-induced IκBα degradation started at 5 min after TNF treatment and reached maximum level at 10 min. The resynthesis of IκBα started at 60 min after TNF exposure (; left panel). However, we noticed that no degradation of IκBα in sesamin-pretreated cells (; right panel). These results indicated that sesamin mediates its effect via suppression of TNF-induced IκBα degradation, which in turn leads to suppression of the activation of NF-κB.
We also decided to determine whether inhibition of TNF-induced degradation of IκBα was caused by inhibition of phosphorylation of IκBα. For this purpose we used the proteasome inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) to block this degradation. We performed a western blot analysis using an antibody that specifically recognized the IκBα phosphorylated at serine 32. The results of this analysis showed that TNF induced IκBα phosphorylation at serine 32 and that sesamin suppressed this phosphorylation ().
Sesamin inhibits IκBα kinase (IKK) activation induced by TNF
TNF-induced activation of the enzyme IKK which triggers the phosphorylation of IκBα (28
). We sought to determine whether in KBM-5 cells sesamin inhibits TNF-induced activation of IKK using immune complex assays. As given in , TNF activated IKK in a time-dependent manner and sesamin suppressed TNF-induced activation of IKK. Neither TNF nor sesamin affected the expression of IKK-α (, left panel).
We then decided to examine whether sesamin suppresses IKK activity directly by binding to IKK or indirectly by suppressing its activation. For this we incubated the immune complexes with various concentrations of sesamin and then examined the IKK activity in vitro. The results showed that sesamin did not directly inhibit activity of IKK (, right panel).
TNF-induced translocation of NF-κB p65 subunit to nucleus is inhibited by sesamin
Once the degradation of IκBα started, it triggers the nuclear translocation of p65 subunit. We decided to determine whether sesamin has any effect on TNF-induced nuclear translocation of p65 subunit. Immunocytochemical analysis showed that sesamin suppressed the TNF-induced translocation of p65 subunit to the nucleus in KBM-5 cells (). However in both untreated cells and cells pre-treated with sesamin, p65 was localized in the cytoplasm, whereas in cells treated with TNF alone, p65 subunit was translocated to the nucleus. These results support the conclusion that sesamin inhibits nuclear translocation of p65.
FIGURE 5 (A) Sesamin inhibits TNF-induced nuclear translocation of p65 assayed by immunocytochemical analysis. KBM-5 cells were initially treated with 100 μM sesamin for 12 hr and then exposed to 0.1 nM TNF for 15 min. After cytospin, immunocytochemical (more ...)
Sesamin suppressed TNF-induced phosphorylation of NF-κB p65 subunit
It is now established that transcriptional activation of p65 requires phosphorylation at the serine 536 residues (29
). Thus, we also sought to investigate the effect of sesamin on TNF-induced phosphorylation of p65. We found that in a time-dependent manner phosphorylation of p65 occurred in TNF-treated KBM-5 cells. We also observed that in sesamin pre-treated cells there was no phosphorylation of p65 ().
Sesamin suppressed TNF-induced NF-κB-dependent reporter gene expression
Although we showed using EMSA, that sesamin inhibited TNF-induced NF-κB expression, DNA binding alone does not always correlate with NF-κB-dependent gene transcription, suggesting that additional regulatory steps are involved. Thus, we decided to examine whether sesamin affects TNF-induced reporter gene transcription. For this, cells were transiently transfected with an NF-κB-regulated SEAP reporter construct, followed by treatment with sesamin, and then exposed to TNF. We found that TNF induced NF-κB reporter gene activity, and in a dose-dependent manner sesamin inhibited the TNF-induced NF-κB reporter activity ().
FIGURE 6 (A) Sesamin inhibits TNF-induced NF-κB-dependent reporter gene (SEAP) expression. A293 cells treated were transiently transfected with a NF-κB-containing plasmid linked to the SEAP gene. After 24 hr of transfection cells were treated with (more ...)
Sesamin inhibits NF-κB activation induced by TNFR1, TRADD, NIK, TAK1/TAB1 and IKK
TNF-induced NF-κB activation requires a sequential recruitment of TNFR1, TRADD, TAK1, and IKK (30
). To determine where in the pathway sesamin blocks the TNF-induced NF-κB activation, we decided to examine the effect of sesamin on TNFR1, TRADD, NIK, TAK1/TAB1 and IKK-induced NF-κB–dependent reporter gene transcription. We observed that all of these plasmids induced NF-κB reporter activity and sesamin was found to inhibit the activation ().
Although numerous studies have indicated that sesamin exhibits activity against hypertension, hyperlipidimia, septic shock and carcinogenesis, its precise mechanism of action is not understood. Antihypertensive effects have been documented even in human clinical trials (32
). Since inflammation has been linked with most chronic diseases including cancer, it is possible that modulation of inflammatory pathway is one of the major sites of action of sesamin. Over the last decade NF-κB pathway has emerged as a major mediator of inflammation (19
). The major aim of current study was to determine the effects of sesamin on NF-κB mediated cellular responses linked to prevention of cancer. We found that sesamin inhibited the NF-κB pathway induced by various carcinogens, inflammatory stimuli and cytokines. It also inhibited constitutive expression of NF-κB activation.
We found sesamin suppressed the proliferation of wide variety of tumor cells including leukemia and solid tumor cells of the prostate, colon, pancreas, lung and breast. Suppression of proliferation of these cells is most likely linked to inhibition of gene products linked with survival and proliferation of cells such as Bcl-2, survivin, cyclin D1 and COX-2.
By using DNA binding assay, we showed NF-κB activated by highly diverse stimuli was blocked by sesamin suggesting that sesamin acts at a step common to all. Our results are in agreement with a recent report about suppression of LPS-induced NF-κB monitored by nuclear pool of p65 in microglia cells (17
). How sesamin inhibits LPS-induced NF-κB activation was not examined by these investigators. We found that sesamin inhibited the activation of IKK thereby prevents the phosphorylation as well as degradation of IκBα. When we examined the effects of sesamin on IKK in details we found that this lignan did not directly modulate the activity of IKK. However it blocked the activation of the kinase. Numerous kinases have been linked with activation of IKK. TAK1 is one of the kinase that has been shown to mediate TNF-induced NF-κB (31
). We found that sesamin blocked TNF-induced TAK1 mediated NF-κB activation. IKK has also been shown to mediate the phosphorylation of p65, the DNA binding subunit (28
). We found that sesamin also inhibited the phosphorylation of p65. NF-κB reporter gene expression induced by TNF and TNF signaling components was also suppressed by this lignan.
When examined for the expression of antiapoptotic gene products survivin and Bcl-2, both regulated by NF-κB, were suppressed by sesamin. In addition, sesamin inhibited the expression of protein COX-2 closely linked with inflammation. Although sesamin has been shown to exhibit anti-inflammatory activity and downregulate prostaglandin production (7
); ours is the first report to show that this agent can downregulate COX-2 expression. Reports about the anti-inflammatory activity due to downregulation of IL-1, IL-6 (10
), NO (16
) and thromboxane B2 could also be due to its ability to downregulate NF-κB as described here, as inducible NO synthase and lipooxygenase are also regulated by NF-κB. Lee et al., (33
) showed that sesamin inhibited the expression of phospholipase C (PLC)-γ1 but the mechanism was not shown. Because PLC-γ1 expression is also regulated by NF-κB (34
), downregulation of NF-κB signaling sesamin may decrease the expression of PLC-γ.
Our results also indicate that sesamin inhibition of NF-κB led to the downregulation of the expression cyclin D1 closely associated with proliferation of cells. This is in agreement with observations of Yokota et al (18
). It is possible that reports which indicated the suppression of proliferation of various tumor cells including leukemia (12
), breast cancer (18
) and gastric cancer (14
), is due to the suppression of cyclin D1 expression.
We also found for the first time that the expression of protein linked with adhesion (ICAM-1), invasion (MMP-9) and angiogenesis (VEGF), was also abrogated by sesamin. Although sesamin has been shown to inhibit DMBA-induced breast carcinogenesis in rats, it is possible that these effects of sesamin involve suppression of NF-κB pathway. DMBA indeed has been shown to activate NF-κB (35
Additionally we also found that downregulation of expression of antiapoptotic gene products led to the enhancements of apoptosis induced by cytokines as well as by chemotherapeutic agents. That NF-κB activation can inhibit apoptosis and induce chemoresistance is well established (19
). These studies suggest that sesamin can be used not only alone but also in combination with existing therapies both to potentiate their effect and to overcome chemoresistance.
One of the earliest activity reported of sesamin is it ability to lower cholesterol (3
). Like sesamin, well known cholesterol-lowering drugs such as statin, have also been shown to suppress NF-κB pathway and sensitize the cells to chemotherapeutic agents (37
). Besides numerous animal studies (39
), several clinical trials have been done with sesamin, demonstrating its safety and bioavailability (1
). In conclusion, our results demonstrate the mechanism by which sesamin could mediate antiinflamatory, antiproliferative, and antiangiogenic effects against cancer through the inhibition of NF-κB and NF-κB-regulated gene products. A further investigation in animals and humans with sesamin is needed to fully realize its potential.