Bortezomib induces protective autophagy through AMP-activated protein kinase activation in cultured pancreatic and colorectal cancer cells

Background

Bortezomib, a selective and potent inhibitor of the proteasome, has demonstrated broad anti-tumor activities in many malignancies. In the current study, we aimed to understand the potential resistance factor of bortezomib in cultured pancreatic and colorectal cancer cells.

Results

We observed that bortezomib-induced protective autophagy in cultured PANC-1 pancreatic cancer cells and HT-29 colorectal cancer cells. Inhibition of autophagy by 3-methyladenine (3-MA) and chloroquine enhanced bortezomib-induced apoptosis and cytotoxicity in both PANC-1 and HT-29 cells. Activation of AMP-activated protein kinase (AMPK) was required for bortezomib-induced autophagy induction in PANC-1 and HT-29 cells, and AMPK inhibition by its inhibitor compound C (CC) or RNAi-depletion suppressed bortezomib-induced autophagy, while dramatically enhancing cancer cell apoptosis/cytotoxicity. Meanwhile, significant AMPK activation and autophagy induction were observed after bortezomib stimulation in primary cultured pancreatic cancer cells derived from a patient’s tumor tissue. Both CC and 3-MA facilitated bortezomib-induced cytotoxicity in primary cultured pancreatic cancer cells.

Conclusions

In conclusion, our data here suggest that bortezomib induces protective autophagy in pancreatic and colorectal cancer cells through activating AMPK-Ulk1 signalings. AMPK or autophagy inhibitors could be developed as an adjunct or chemo-sensitizer for bortezomib.     

Resveratrol induces apoptosis via ROS-triggered autophagy in human colon cancer cells

Resveratrol (Res; 3,4',5-trihydroxy-trans-stilbene), which is a polyphenol found in grapes, can block cell proliferation and induce growth arrest and/or cell death in several types of cancer cells. However, the precise mechanisms by which Res exerts anticancer effects remain poorly understood. Res blocked both anchorage-dependent and -independent growth of HT-29 and COLO 201 human colon cancer cells in a dose- and time-dependent manner. Annexin V staining and Western blot analysis revealed that Res induced apoptosis accompanied by an increase in Caspase-8 and Caspase-3 cleavage. In HT-29 cells, Res caused autophagy as characterized by the appearance of autophagic vacuoles by electron microscopy and elevation of microtubule-associated protein 1 light chain 3 (LC3)-II by immunoblotting, which was associated with the punctuate pattern of LC3 detected by fluorescein microscopy. Inhibition of Res-induced autophagy by the autophagy inhibitor 3-methyladenine caused a significant decrease in apoptosis accompanied by decreased cleavage of Casapse-8 and Caspase-3, indicating that Res-induced autophagy was cytotoxic. However, inhibition of Res-induced apoptosis by the pan-caspase inhibitor Z-VAD(OMe)-FMK did not decrease autophagy but elevated LC3-II levels. Interestingly, Res increased the intracellular reactive oxygen species (ROS) level, which correlated to the induction of Casapse-8 and Caspase-3 cleavage and the elevation of LC3-II; treatment with ROS scavenger N-acetyl cysteine diminished this effect. Therefore, the effect of Res on the induction of apoptosis via autophagy is mediated through ROS in human colon cancer cells.     

Selenium regulates cyclooxygenase-2 and extracellular signal-regulated kinase signaling pathways by activating AMP-activated protein kinase in colon cancer cells

Epidemiologic and experimental evidences indicate that selenium, an essential trace element, can reduce the risk of a variety of cancers. Protection against certain types of cancers, particularly colorectal cancers, is closely associated with pathways involving cyclooxygenase-2 (COX-2). We found that AMP-activated protein kinase (AMPK), which functions as a cellular energy sensor, mediates critical anticancer effects of selenium via a COX-2/prostaglandin E(2) signaling pathway. Selenium activated AMPK in tumor xenografts as well as in colon cancer cell lines, and this activation seemed to be essential to the decrease in COX-2 expressions. Transduction with dominant-negative AMPK into colon cancer cells or application of cox-2(-/-)-negative cells supported the evidence that AMPK is an upstream signal of COX-2 and inhibits cell proliferation. In HT-29 colon cancer cells, carcinogenic agent 12-O-tetradecanoylphorbol-13-acetate (TPA) activated extracellular signal-regulated kinase (ERK) that led to COX-2 expression and selenium blocked the TPA-induced ERK and COX-2 activation via AMPK. We also showed the role of a reactive oxygen species as an AMPK activation signal in selenium-treated cells. We propose that AMPK is a novel and critical regulatory component in selenium-induced cancer cell death, further implying AMPK as a prime target of tumorigenesis.     

5-ALA mediated photodynamic therapy induces autophagic cell death via AMP-activated protein kinase

Photodynamic therapy (PDT) has been developed as an anticancer treatment, which is based on the tumor-specific accumulation of a photosensitizer that induces cell death after irradiation of light with a specific wavelength. Depending on the subcellular localization of the photosensitizer, PDT could trigger various signal transduction cascades and induce cell death such as apoptosis, autophagy, and necrosis. In this study, we report that both AMP-activated protein kinase (AMPK) and mitogen-activated protein kinase (MAPK) signaling cascades are activated following 5-aminolevulinic acid (ALA)-mediated PDT in both PC12 and CL1-0 cells. Although the activities of caspase-9 and -3 are elevated, the caspase inhibitor zVAD-fmk did not protect cells against ALA-PDT-induced cell death. Instead, autophagic cell death was found in PC12 and CL1-0 cells treated with ALA-PDT. Most importantly, we report here for the first time that it is the activation of AMPK, but not MAPKs that plays a crucial role in mediating autophagic cell death induced by ALA-PDT. This novel observation indicates that the AMPK pathway play an important role in ALA-PDT-induced autophagy.     

Apigenin, a chemopreventive bioflavonoid, induces AMP-activated protein kinase activation in human keratinocytes

AMP-activated protein kinase (AMPK) is a cellular energy sensor that is conserved in eukaryotes. Although AMPK is traditionally thought to play a major role in the regulation of cellular lipid and protein metabolism, recent discoveries reveal that AMPK inhibits mammalian target of rapamycin (mTOR) signaling and connects with several tumor suppressors such as liver kinase B1 (LKB1), p53, and tuberous sclerosis complex 2 (TSC2), indicating that AMPK may be a potential target for cancer prevention and treatment. For the first time, we demonstrated that apigenin, a naturally occurring nonmutagenic flavonoid, induced AMPK activation in human keratinocytes (both cultured HaCaT cell line and primary normal human epidermal keratinocytes). Through experiments with over-expression of constitutively active Akt and knockdown of LKB1 expression by siRNAs, we further found that the activation of AMPK by apigenin was not dependent on its inhibition of Akt, and was independent of the activation of upstream kinase LKB1. Instead, another upstream kinase of AMPK, calcium/calmodulin-dependent protein kinase kinase-β (CaMKKβ), was required for apigenin-induced AMPK activation. We have demonstrated that knockdown of CaMKKβ expression by siRNA or inhibition of CaMKKβ activity by either CaMKK inhibitor STO-609 or BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester; a chelator of intracellular Ca(2+)) prevented apigenin-induced AMPK activation. Apigenin-induced AMPK activation inhibited mTOR signaling and further induced autophagy in human keratinocytes. These results suggest that one of the mechanisms by which apigenin exerts its chemopreventive action may be through activation of AMPK and induction of autophagy in human keratinocytes.     

Hedyotis Diffusa Willd extract induces apoptosis via activation of the mitochondrion-dependent pathway in human colon carcinoma cells

Hedyotis Diffusa Willd has been used as a major component in several Chinese medicine formulations for the clinical treatment of colorectal cancer. However, the molecular mechanism of the anti-cancer activity of Hedyotis Diffusa Willd remains unclear. In the present study, we investigated the cellular effects of the ethanol extract of Hedyotis Diffusa Willd (EEHDW) in the HT-29 human colon carcinoma cell line. We found that EEHDW inhibited the growth of HT-29 cells demonstrating EEHDW-induced cell morphological changes and reduced cell viability in a dose- and time-dependent manner. Furthermore, we observed that EEHDW treatment resulted in DNA fragmentation, loss of plasma membrane asymmetry, collapse of mitochondrial membrane potential, activation of caspase-9 and caspase-3, and increase of the ratio of pro-apoptotic Bax to anti-apoptotic Bcl-2, suggesting that the HT-29 cell growth inhibitory activity of EEHDW was due to mitochondrion-mediated apoptosis, which may partly explain the anti-cancer activity of Hedyotis Diffusa Willd.     

熊果酸诱导结肠癌HT-29细胞凋亡的实验研究

目的探讨熊果酸（UA）诱导结肠癌HT-29细胞凋亡的作用及机制。方法用不同浓度的UA处理HT-29细胞,采用四甲基偶氮唑蓝（MTT）比色法,检测UA对HT-29细胞的增殖抑制效应;采用形态学、TUNEL法和流式细胞术,检测细胞凋亡的发生;应用免疫组织化学S-P法,检测凋亡相关基因caspase-9和bcl-2的表达,并用病理图像分析软件进行半定量分析。结果UA在体外对HT-29细胞有中度增殖抑制效应,在UA作用下,HT-29细胞出现显著的细胞凋亡征象。TUNEL法显示细胞固缩,核染色质聚集或断裂,形成凋亡小体。流式细胞术检测结果显示,在G1期之前出现Sub—G1峰,凋亡率最高为11．63％,UA的作用具有浓度和时间依赖性。在HT-29细胞凋亡过程中,凋亡相关基因caspase-9的表达增强,bcl-2的表达减弱。结论凋亡是UA杀伤肿瘤细胞的机制之一;UA诱导结肠癌HT-29细胞凋亡主要与促进caspase-9的活化、下调bcl-2的表达有关。     

FTY720 induces apoptosis in hepatocellular carcinoma cells through activation of protein kinase C delta signaling

This study was aimed at elucidating the mechanism by which FTY720, a synthetic sphingosine immunosuppressant, mediated antitumor effects in hepatocellular carcinoma (HCC) cells. The three HCC cell lines examined, Hep3B, Huh7, and PLC5, exhibited differential susceptibility to FTY720-mediated suppression of cell viability, with IC(50) values of 4.5, 6.3, and 11 mumol/L, respectively. Although FTY720 altered the phosphorylation state of protein kinase B and p38, our data refuted the role of these two signaling kinases in FTY720-mediated apoptosis. Evidence indicates that the antitumor effect of FTY720 was attributable to its ability to stimulate reactive oxygen species (ROS) production, which culminated in protein kinase C (PKC)delta activation and subsequent caspase-3-dependent apoptosis. We showed that FTY720 activated PKC delta through two distinct mechanisms: phosphorylation and caspase-3-dependent cleavage. Cotreatment with the caspase-3 inhibitor Z-VAD-FMK abrogated the effect of FTY720 on facilitating PKC delta proteolysis. Equally important, pharmacologic inhibition or shRNA-mediated knockdown of PKC delta protected FTY720-treated Huh7 cells from caspase-3 activation. Moreover, FTY720 induced ROS production to different extents among the three cell lines, in the order of Hep3B > Huh7 > PLC5, which inversely correlated with the respective glutathione S-transferase pi expression levels. The low level of ROS generation might underlie the resistant phenotype of PLC5 cells to the apoptotic effects of FTY720. Blockade of ROS production by an NADPH oxidase inhibitor protected Huh7 cells from FTY720-induced PKC delta activation and caspase-3-dependent apoptosis. Together, this study provides a rationale to use FTY720 as a scaffold to develop potent PKC delta-activating agents for HCC therapy.     

gamma-Tocopheryl quinone induces apoptosis in cancer cells via caspase-9 activation and cytochrome c release

Recently, it was suggested the potential role of gamma-tocopheryl quinone (gamma-TQ), an oxidative metabolite of gamma-tocopherol, as a powerful chemotherapeutic agent, since it was shown that this molecule exerts powerful cytotoxic effects, induces apoptosis and escapes drug resistance in human acute lymphoblastic leukemia and promyelocytic leukemia cells. We have studied the apoptogenic potential of gamma-TQ in cultured human leukemia HL-60 and colon adenocarcinoma WiDr cells, and in murine thymoma cells growing in vivo in ascites form. The cells were treated with gamma-TQ and apoptosis was evaluated morphologically by acridine-orange staining and cytofluorimetrically by Annexin V binding assay. gamma-TQ-induced apoptosis in a dose- and time-dependent manner in all the cell types tested, although HL-60 and thymoma cells were much more sensitive than WiDr cells. In HL-60 cells apoptosis was mediated by the activation of the caspase-3 cascade. In particular, we observed a time- and dose-dependent increase in the activities of the upstream caspase-9 and caspase-8 and of the downstream caspase-3. The activation of caspase-9 preceded that of caspase-8 and its specific inhibition completely prevented apoptosis. These findings and data showing the precocious release of cytochrome c from mitochondria, a decrease in Bcl-2, and a change in mitochondrial transmembrane potential (Delta psi(m)), all suggest that the intrinsic mitochondrial pathway is primarily involved in the development of gamma-TQ-induced apoptosis. The late activation of caspase-8 and data showing the partial cleavage of pro-apoptotic protein BID suggest that the initial activation of caspase-9 may be potentiated by a feedback amplification loop involving the caspase-8/BID pathway.     

AMPK活性对HeLa细胞内质网功能稳态的影响

目的:探讨腺苷酸活化蛋白激酶(AMPK)对HeLa细胞内质网应激的影响。方法:试验分为对照组(1‰ DMSO)、内质网应激诱导组(2 μg/mL衣霉素或500 nmol/L MG132内质网应激诱导剂诱导)和内质网应激干预组[衣霉素或MG132+2 mmol/L AMPK激动剂5-氨基咪唑-4-甲酰胺-1-B-呋喃核糖苷(AICAR)干预组、衣霉素或MG132+2 mmol/L AMPK激动剂二甲双胍干预组以及衣霉素或MG132+0.5 mmol/L AMPK抑制剂Compound C干预组],以探究各处理组中AMPK活性状态对内质网应激的影响。收集对数生长期的HeLa细胞,分组诱导孵育后,用Western blot检测HeLa细胞AMPK/T-172磷酸化水平及内质网应激标志蛋白(p-elf2α、Grp78和XBP-1s)的表达,并采用细胞内钙集群检测分析(Indo-1 ratiometric Ca²⁺ analysis)检测各组HeLa细胞内Ca²⁺浓度的变化。结果:衣霉素和MG132可诱导内质网应激标志蛋白p-elf2α、Grp78和XBP-1s的表达。AMPK激动剂AICAR和二甲双胍干预可降低上述3种内质网应激标志蛋白的表达,并降低细胞胞浆内因内质网应激导致的Ca²⁺浓度升高,与对照组比较,差异均有统计学意义(P均<0.05)。AMPK抑制剂Compound C的干预对上述内质网应激标志蛋白的表达无明显影响(P均>0.05)。结论:AMPK激活能缓解HeLa细胞中的内质网应激,维持细胞内质网的功能稳态,可能对提高肿瘤细胞的应激耐受能力起到主要作用。     

Tumor necrosis factor-alpha induces interleukin-6 production via extracellular-regulated kinase 1 activation in breast cancer cells

Interleukin-6 (IL-6) and interleukin-11 (IL-11) are frequently produced by breast cancer cells. These interleukins promote osteoclast formation and may mediate osteolysis at the site of breast cancer bone metastases. Transforming growth factor- (TGF-), tumor necrosis factor- (TNF-) and interleukin-1 (IL-1) up-regulate IL-6 and IL-11 production in a cytokine-dependent fashion in breast cancer cells, but very little is known about their intracellular signaling pathways in breast cancer cells. To study TGF-, TNF- and IL-1 regulation of IL-6 and IL-11 production in human MDA-MB-231 breast cancer cells, we established single cell clones stably expressing dominant negative (DN) forms of the mitogen-activated protein kinases p38 (p38/AF) or ERK1 (ERK1K71R). We show here, that while basal, TGF- and IL-1 induced IL-6 production was similar in parental cells and in pcDNA3 control, ERK1K71R and p38/AF clones, TNF- induced IL-6 production was blunted in the ERK1K71R clones. TGF- and IL-1, but not TNF-, induced IL-11 production in parental MDA-MB-231 cells. Similar findings were detected in clones stably expressing p38/AF and ERK1K71R, which did not change basal IL-11 production either. In conclusion, TNF- induced IL-6 production is mediated via ERK1 activation in MDA-MB-231 cells. These observations may be helpful in designing new anti-osteolytic therapies.     

CLP induces apoptosis in human leukemia K562 cells through Ca regulating extracellular-related protein kinase ERK activation

The cyclic lipopeptide (CLP) has been known to inhibit proliferation and induce apoptosis in cancer cells. However, the molecular mechanisms involved in CLP-induced apoptosis are still uncharacterized in human leukemic K562 cells. The current study investigated the molecular mechanism of action of CLP, purified from Bacillus natto T-2. CLP-induced a sustained increase in concentration of intracellular Ca²⁺. This increase in [Ca²⁺]i was associated with CLP-induced cell apoptosis and ERK phosphorylation. CLP-induced cell apoptosis was reversed by PD98059 (an inhibitor of ERK), but not by SB203580 (an inhibitor of p38) and SP200125 (an inhibitor of JNK), suggesting that the action of CLP on K562 cells was via ERK, but not via p38 and JNK. On the other hand, pretreatment with Bapta-AM, a well-known calcium chelator, partially blocked CLP-induced apoptosis, indicating that the elevation of [Ca²⁺]i may play an important role in the apoptosis. Collectively, in K562 cells, CLP-induced an increase in [Ca²⁺]i which evoked ERK phosphorylation. This ERK phosphorylation subsequently activated Bax, cytochrome c and caspase-3 leading to apoptosis.     

高表达GRIM-19诱导结肠癌SW480细胞凋亡

目的：研究干扰素/维甲酸诱导死亡基因（retinoidinterferoninduced mortality,GRIM19）对结肠癌SW480细胞凋亡的影响。方法：构建GRIM19真核表达载体（pCMVFlagGRIM19）,转染入SW480细胞中,Western blotting检测GRIM19及凋亡相关蛋白Balxl的表达,采用AnnexinV/PI和线粒体膜电位JC-1染色检测SW480细胞的凋亡。结果：成功构建pCMVFlagGRIM19真核表达载体。pCMV-Flag-GRIM-19质粒转染SW480细胞后,GRIM19的表达上调,凋亡抑制蛋白Bclxl的表达则下调。转染空质粒pCMVFlag组SW480细胞凋亡率为（7.7±1.39）%,转染pCMV-Flag-GRIM-19质粒后SW480细胞凋亡率为（15.0±2.52）%（P<0.05）。线粒体膜电位检测显示转染空质粒pCMVFlag组SW480细胞膜电位降低细胞为（7.5±2.09）%,而转染pCMVFlagGRIM19后细胞线粒体膜电位降低细胞为（17.5±3.07）%（P<0.05）。结论：GRIM-19体外转染可有效诱导结肠癌SW480细胞凋亡。     

Widdrol induces cell cycle arrest,associated with MCM down-regulation,in human colon adenocarcinoma cells

Widdrol, an odorous compound extracted from Juniperus chinensis, has been shown to inhibit the in vitro growth of in human cancer cells. This study was conducted on cultured human colon adenocarcinoma HT29 cells to elucidate the possible mechanisms by which widdrol exerts its anti-proliferative activity, which until now has remained poorly understood. It was found that widdrol induces accumulation of sub-G1 phase and arrests in the G1 phase of the cell cycle. Induction of G1 arrest by widdrol was correlated with induction of Chk2, p53 phosphorylation and CDK inhibitor p21 expression as well as inhibition of cyclin E, cyclin-dependent kinase (CDK2) and retinoblastoma protein (pRB). Moreover, mini-chromosome maintenance (MCM) proteins were markedly down-regulated in HT29 cells treated with widdrol. Altogether, these results show widdrol possesses potential anti-cancer activity against colon adenocarcinoma cells by inhibiting their proliferation and inducing cell cycle G1 arrest.     

Tetrandrine induces apoptosis and growth suppression of colon cancer cells in mice

Tetrandrine, a bisbenzylisoquinoline alkaloid, exerts antitumor effects against some cancers. We explored tetrandrine’s effects on colon cancer with cultured mouse CT-26 cells and with subcutaneous tumors. Tetrandrine induced apoptosis in concentration- and time-dependent manner. Tetrandrine increased expression of ERK 1/2 and p38 MAPK; inhibition of p38 MAPK reduced tetrandrine-induced apoptosis; inhibition of ERK1/2 did not. Tetrandrine had significant effects on tumors including slower growth and longer animal survival time and higher survival rate. Higher dose and earlier treatment were more effective than lower dose and delayed treatment. TUNEL staining showed prominent tetrandrine-induced apoptosis of tumors. These data suggest that tetrandrine induced significant apoptosis of cultured and subcutaneous CT-26 cells. Tetrandrine-induced apoptosis might be at least partially related to activation of the p38 MAPK signaling pathway.     

Chitosan induces apoptosis via caspase-3 activation in bladder tumor cells.

Recently, because of its low toxicity and biological effects, chitosan has been widely used in the medical and pharmaceutical fields, e.g., for nasal or oral delivery of peptide or polar drug delivery. Here, we report a growth-inhibitory effect of chitosan on tumor cells. The growth inhibition was examined by WST-1 colorimetric assay and cell counting. We also observed DNA fragmentation, which is characteristic of apoptosis, and elevated caspase-3-like activity in chitosan-treated cancer cells. The findings suggest that chitosan may have potential value in cancer therapy.     

Honokiol activates AMP-activated protein kinase in breast cancer cells via an LKB1-dependent pathway and inhibits breast carcinogenesis

Introduction

Honokiol, a small-molecule polyphenol isolated from magnolia species, is widely known for its therapeutic potential as an antiinflammatory, antithrombosis, and antioxidant agent, and more recently, for its protective function in the pathogenesis of carcinogenesis. In the present study, we sought to examine the effectiveness of honokiol in inhibiting migration and invasion of breast cancer cells and to elucidate the underlying molecular mechanisms.

Methods

Clonogenicity and three-dimensional colony-formation assays were used to examine breast cancer cell growth with honokiol treatment. The effect of honokiol on invasion and migration of breast cancer cells was evaluated by using Matrigel invasion, scratch-migration, spheroid-migration, and electric cell-substrate impedance sensing (ECIS)-based migration assays. Western blot and immunofluorescence analysis were used to examine activation of the liver kinase B1 (LKB1)-AMP-activated protein kinase (AMPK) axis. Isogenic LKB1-knockdown breast cancer cell line pairs were developed. Functional importance of AMPK activation and LKB1 overexpression in the biologic effects of honokiol was examined by using AMPK-null and AMPK-wild type (WT) immortalized mouse embryonic fibroblasts (MEFs) and isogenic LKB1-knockdown cell line pairs. Finally, mouse xenografts, immunohistochemical and Western blot analysis of tumors were used.

Results

Analysis of the underlying molecular mechanisms revealed that honokiol treatment increases AMP-activated protein kinase (AMPK) phosphorylation and activity, as evidenced by increased phosphorylation of the downstream target of AMPK, acetyl-coenzyme A carboxylase (ACC) and inhibition of phosphorylation of p70S6kinase (pS6K) and eukaryotic translation initiation factor 4E binding protein 1 (4EBP1). By using AMPK-null and AMPK-WT (MEFs), we found that AMPK is required for honokiol-mediated modulation of pACC-pS6K. Intriguingly, we discovered that honokiol treatment increased the expression and cytoplasmic translocation of tumor-suppressor LKB1 in breast cancer cells. LKB1 knockdown inhibited honokiol-mediated activation of AMPK and, more important, inhibition of migration and invasion of breast cancer cells. Furthermore, honokiol treatment resulted in inhibition of breast tumorigenesis in vivo. Analysis of tumors showed significant increases in the levels of cytoplasmic LKB1 and phospho-AMPK in honokiol-treated tumors.

Conclusions

Taken together, these data provide the first in vitro and in vivo evidence of the integral role of the LKB1-AMPK axis in honokiol-mediated inhibition of the invasion and migration of breast cancer cells. In conclusion, honokiol treatment could potentially be a rational therapeutic strategy for breast carcinoma.     

Unconjugated bilirubin induces apoptosis in colon cancer cells by triggering mitochondrial depolarization

Bilirubin is the principal end product of heme degradation. Prompted by epidemiologic analyses demonstrating an inverse correlation between serum bilirubin levels and cancer mortality, we examined the effect(s) of bilirubin on the growth and survival of colon adenocarcinoma cells. Adenocarcinoma cell monolayers were treated with bilirubin over a range of bilirubin:BSA molar ratios (0-0.6), and viability was assessed colorimetrically. Apoptosis was characterized by TUNEL assay, annexin V staining and caspase-3 activation. The mechanism(s) by which bilirubin induces apoptosis was investigated by Western blotting for cytochrome c release, assaying for caspase-8 and caspase-9 activation and for mitochondrial depolarization by JC-1 staining. The direct effect of bilirubin on the membrane potential of isolated mitochondria was evaluated using light-scattering and fluorescence techniques. Bilirubin decreased the viability of all colon cancer cell lines tested in a dose-dependent manner. Cells exhibited substantial apoptosis when exposed to bilirubin concentrations ranging 0-50 microM, as demonstrated by an 8- to 10-fold increase in TUNEL and annexin V staining and in caspase-3 activity. Bilirubin treatment evokes specific activation of caspase-9, enhances cytochrome c release into the cytoplasm and triggers the mitochondrial permeability transition in colon cancer monolayers. Additionally, bilirubin directly induces the depolarization of isolated rat liver mitochondria, an effect that is not inhibited by cyclosporin A. Bilirubin stimulates apoptosis of colon adenocarcinoma cells in vitro through activation of the mitochondrial pathway, apparently by directly dissipating mitochondrial membrane potential. As this effect is triggered at concentrations normally present in the intestinal lumen, we postulate a physiologic role for bilirubin in modulating colon tumorigenesis.