Tribbles 2 pseudokinase confers enzalutamide resistance in prostate cancer by promoting lineage plasticity.

Enzalutamide, a second-generation antiandrogen, is commonly prescribed for the therapy of advanced prostate cancer, but enzalutamide-resistant, lethal, or incurable disease invariably develops. To understand the molecular mechanism(s) behind enzalutamide resistance, here, we comprehensively analyzed a range of prostate tumors and clinically relevant models by gene expression array, immunohistochemistry, and Western blot, which revealed that enzalutamide-resistant prostate cancer cells and tumors overexpress the pseudokinase, Tribbles 2 (TRIB2). Inhibition of TRIB2 decreases the viability of enzalutamide-resistant prostate cancer cells, suggesting a critical role of TRIB2 in these cells. Moreover, the overexpression of TRIB2 confers resistance in prostate cancer cells to clinically relevant doses of enzalutamide, and this resistance is lost upon inhibition of TRIB2. Interestingly, we found that TRIB2 downregulates the luminal markers androgen receptor and cytokeratin 8 in prostate cancer cells but upregulates the neuronal transcription factor BRN2 (Brain-2) and the stemness factor SOX2 (SRY-box 2) to induce neuroendocrine characteristics. Finally, we show that inhibition of either TRIB2 or its downstream targets, BRN2 or SOX2, resensitizes resistant prostate cancer cells to enzalutamide. Thus, TRIB2 emerges as a potential new regulator of transdifferentiation that confers enzalutamide resistance in prostate cancer cells via a mechanism involving increased cellular plasticity and lineage switching.

Inhibition of 5-lipoxygenase selectively triggers disruption of c-Myc signaling in prostate cancer cells.

Myc is up-regulated in almost all cancer types and is the subject of intense investigation because of its pleiotropic effects controlling a broad spectrum of cell functions. However, despite its recognition as a stand-alone molecular target, development of suitable strategies to block its function is hindered because of its nonenzymatic nature. We reported earlier that arachidonate 5-lipoxygenase (5-Lox) plays an important role in the survival and growth of prostate cancer cells, although details of the underlying mechanisms have yet to be characterized. By whole genome gene expression array, we observed that inhibition of 5-Lox severely down-regulates the expression of c-Myc oncogene in prostate cancer cells. Moreover, inhibition of 5-Lox dramatically decreases the protein level, nuclear accumulation, DNA binding, and transcriptional activities of c-Myc. Both the 5-Lox inhibition-induced down-regulation of c-Myc and induction of apoptosis are mitigated when the cells are treated with 5-oxoeicosatetraenoic acid, a metabolite of 5-Lox, confirming a role of 5-Lox in these processes. c-Myc is a transforming oncogene widely expressed in prostate cancer cells and maintains their transformed phenotype. Interestingly, MK591, a specific 5-Lox inhibitor, strongly affects the viability of Myc-overactivated prostate cancer cells and completely blocks their invasive and soft agar colony-forming abilities, but it spares nontransformed cells where expression of 5-Lox is undetectable. These findings indicate that the oncogenic function of c-Myc in prostate cancer cells is regulated by 5-Lox activity, revealing a novel mechanism of 5-Lox action and suggesting that the oncogenic function of c-Myc can be suppressed by suitable inhibitors of 5-Lox.

MK591 (Quiflapon), a 5-lipoxygenase inhibitor, kills pancreatic cancer cells via downregulation of protein kinase C-epsilon.

Introduction: Pancreatic tumors and cell lines derived from them exhibit elevated expression of 5-lipoxygenase (5-Lox), whereas non-tumor glands or normal cells do not exhibit this overexpression. Arachidonic acid stimulates pancreatic cancer cell growth via metabolic conversion through the 5-Lox pathway, and inhibition of 5-Lox activity decreases the viability of pancreatic cancer cells. However, the downstream signaling mechanisms through which 5-Lox exerts its effects on the survival of pancreatic cancer cells remain to be elucidated. Methods: The effects of 5-Lox inhibition on cell proliferation, apoptosis, and invasive potential were investigated in pancreatic cancer cells. The protein expression was analyzed by Western blot. Apoptosis was analyzed by Annexin-V binding assay and by detecting the degradation of chromatin-DNA to nucleosomal fragments. The protein kinase C-epsilon (PKCε) activity was measured by an immunoprecipitation-kinase assay. The in vivo effects of MK591 were evaluated in pancreatic tumor xenograft model. Results: MK591, a specific inhibitor of 5-Lox activity, killed pancreatic cancer cells via induction of apoptosis, involving externalization of phosphatidylserine, cleavage of PARP (poly-ADP ribose polymerase) and degradation of chromatin DNA to nucleosomes. MK591 effectively blocked in vitro invasion and soft-agar colony formation by pancreatic cancer cells and decreased pancreatic tumor growth in nude mice xenografts. Furthermore, inhibition of 5-Lox downregulated K-Ras and inhibited phosphorylation of c-Raf and ERKs. Interestingly, 5-Lox inhibition induced apoptosis in pancreatic cancer cells without the inhibition of Akt but the protein level of PKCε was dramatically downregulated. Furthermore, inhibition of 5-Lox decreased the phosphorylation of Stat3 at Serine-727. Pre-treatment of pancreatic cancer cells with peptide activators of PKCε prevented apoptosis induced by 5-Lox inhibition, suggesting that the mechanism by which 5-Lox inhibition causes cell death in pancreatic cancer involves downregulation of PKCε. The combination of low doses of MK591 and gemcitabine synergistically reduced the oncogenic phenotype and killed pancreatic cancer cells by inducing apoptosis. Discussion: These findings indicate that inhibition of 5-Lox interrupts an Akt-independent, PKCε-dependent survival mechanism in pancreatic cancer cells and suggest that metabolism of arachidonic acid through the 5-Lox pathway plays an integral part in the survival of pancreatic cancer cells via signaling through PKCε, an oncogenic, pro-survival serine/threonine kinase.

Daclatasvir, an Antiviral Drug, Downregulates Tribbles 2 Pseudokinase and Resensitizes Enzalutamide-Resistant Prostate Cancer Cells.

FDA-approved enzalutamide is commonly prescribed to reduce the growth of advanced prostate cancer by blocking androgen receptor function. However, enzalutamide-resistant prostate cancer (ERPC) invariably develops and progresses to metastatic, lethal disease. Management of ERPC poses a special problem not only because available therapeutic regimens cannot effectively kill ERPC cells but also due to their propensity to invade large bones. Moreover, molecular mechanism(s) behind enzalutamide resistance is not properly understood, which is delaying development of newer agents. We found that the pseudokinase, Tribbles 2 (TRIB2), is overexpressed in ERPC cells and plays a critical role in their survival. Forced overexpression of TRIB2 enhances prostate cancer cell growth and confers resistance to physiologic doses of enzalutamide, suggesting that TRIB2 plays an important role in the development and progression of ERPC. Though TRIB2 has emerged as an excellent molecular target for ERPC, suitable inhibitors are not commercially available for effective targeting. By designing a luciferase-tagged TRIB2 fusion protein-based assay system, we screened a library of about 1,600 compounds and found that daclatasvir (DCV), an antiviral drug, effectively inhibits TRIB2-luciferase. We also found that DCV degrades TRIB2 proteins by direct binding and resensitizes ERPC cells to enzalutamide treatment. Moreover, DCV at lower, sublethal doses synergizes with enzalutamide to decrease the viability and induce apoptosis in prostate cancer cells. Because DCV is already approved by the FDA and well tolerated in humans, based on our findings, it appears that DCV is a promising new agent for development of an effective therapy for advanced, enzalutamide-resistant, lethal prostate cancer.

Inhibition of 5-lipoxygenase triggers apoptosis in prostate cancer cells via down-regulation of protein kinase C-epsilon.

Previous studies have shown that human prostate cancer cells constitutively generate 5-lipoxygenase (5-LOX) metabolites from arachidonic acid, and inhibition of 5-LOX blocks production of 5-LOX metabolites and triggers apoptosis in prostate cancer cells. This apoptosis is prevented by exogenous metabolites of 5-LOX, suggesting an essential role of 5-LOX metabolites in the survival of prostate cancer cells. However, downstream signaling mechanisms which mediate the survival-promoting effects of 5-LOX metabolites in prostate cancer cells are still unknown. Recently, we reported that MK591, a specific inhibitor of 5-LOX activity, induces apoptosis in prostate cancer cells without inhibition of Akt, or ERK, two well-characterized regulators of pro-survival mechanisms, suggesting the existence of an Akt and ERK-independent survival mechanism in prostate cancer cells regulated by 5-LOX. Here, we report that 5-LOX inhibition-induced apoptosis in prostate cancer cells occurs via rapid inactivation of protein kinase C-epsilon (PKCε), and that exogenous 5-LOX metabolites prevent both 5-LOX inhibition-induced down-regulation of PKCε and induction of apoptosis. Interestingly, pre-treatment of prostate cancer cells with diazoxide (a chemical activator of PKCε), or KAE1-1 (a cell-permeable, octa-peptide specific activator of PKCε) prevents 5-LOX inhibition-induced apoptosis, which indicates that inhibition of 5-LOX triggers apoptosis in prostate cancer cells via down-regulation of PKCε. Altogether, these findings suggest that metabolism of arachidonic acid by 5-LOX activity promotes survival of prostate cancer cells via signaling through PKCε, a pro-survival serine/threonine kinase.

Pharmacological and genetic targeting of 5-lipoxygenase interrupts c-Myc oncogenic signaling and kills enzalutamide-resistant prostate cancer cells via apoptosis.

Much of the morbidity and mortality due to prostate cancer happen because of castration-resistant prostate cancer (CRPC) which invariably develops after anti-androgenic therapy. FDA-approved enzalutamide is commonly prescribed for CRPC which works by blocking androgen receptor function. However, even after initial good response, enzalutamide-resistant prostate cancer (ERPC) develops which eventually leads to widespread metastasis. Management of ERPC is extremely difficult because available therapeutic regimen cannot effectively kill and eliminate ERPC cells. Though the mechanism behind enzalutamide-resistance is not properly understood, over-activation of c-Myc has been found to be a common event which plays an important role in the maintenance and progression of ERPC phenotype. However, direct-targeting of c-Myc poses special problem because of its non-enzymatic nature and certain amount of c-Myc activity is needed by non-cancer cells as well. Thus, c-Myc has emerged as an elusive target which needs to be managed by novel agents and strategies in a cancer-specific way. We investigated the effects of pharmacological and genetic inhibition of 5-lipoxygenase (5-Lox) on cell proliferation, apoptosis and invasive potential of enzalutamide-resistant prostate cancer cells. Transcriptional activity of c-Myc was analyzed by DNA-binding, luciferase-assays, and expression of c-Myc-target genes. We found that 5-Lox regulates c-Myc signaling in enzalutamide-resistant prostate cancer cells and inhibition of 5-Lox by Quiflapon/MK591 or shRNA interrupts oncogenic c-Myc signaling and kills ERPC cells by triggering caspase-mediated apoptosis. Interestingly, MK591 does not affect normal, non-cancer cells in the same experimental conditions. Our findings indicate that inhibition of 5-Lox may emerge as a promising new approach to effectively kill ERPC cells sparing normal cells and suggest that development of a long-term curative therapy of prostate cancer may be possible by killing and eliminating ERPC cells with suitable 5-Lox-inhibitors.

Inhibition of 5-lipoxygenase downregulates stemness and kills prostate cancer stem cells by triggering apoptosis via activation of c-Jun N-terminal kinase.

The cancer stem cell (CSC) concept suggests that neoplastic clones are maintained exclusively by a rare group of cells possessed with stem cell properties. CSCs are characterized by features that include self-renewal, pluripotency and tumorigenicity, and are thought to be solely responsible for tumor recurrence and metastasis. A hierarchically organized CSC model is becoming increasingly evident for various types of cancer, including prostate cancer. The CD44 (+), CD133 (+) cell subpopulations were isolated from human prostate tumors which exhibit stem-like properties showing therapeutic-resistance, capacity of self-renewal, and exact recapitulation of the original tumor in vivo. Thus, an important challenge is to find measures to eliminate these cancer stem cells, which will stop tumor growth and prevent disease-recurrence. However, knowledge about molecular features critical for the survival of prostate cancer stem cells (PCSC) is meager. Here we report that inhibition of 5-lipoxygenase (5-Lox) by shRNA or MK591 dramatically kills PCSC by inducing apoptosis, suggesting that 5-Lox plays an essential role in the survival of PCSC. Interestingly, MK591 treatment decreases protein levels and inhibits transcriptional activities of Nanog and c-Myc. Since Nanog and c-Myc play important roles as stemness factors, our findings indicate that the 5-Lox activity plays a causal role in maintaining prostate cancer stemness via regulation of Nanog and c-Myc, and suggest that further exploration of 5-Lox-mediated signaling in PCSC may lead to development of novel, target-based, durable strategies to effectively block development and growth of prostate tumors, and prevent prostate cancer recurrence.

Wedelolactone, an Anti-inflammatory Botanical, Interrupts c-Myc Oncogenic Signaling and Synergizes with Enzalutamide to Induce Apoptosis in Prostate Cancer Cells.

The c-Myc gene encodes an oncoprotein transcription factor that is frequently upregulated in almost all cancer types and is the subject of intense investigation for management of cancer because of its pleiotropic effects controlling a spectrum of cellular functions. However, due of its nonenzymatic nature, development of suitable strategies to block its protein-protein or protein-DNA interaction is challenging. Thus, c-Myc has been recognized as an elusive molecular target for cancer control, and various approaches are in development to inhibit c-Myc transcriptional activity. We observed that wedelolactone (WDL), an anti-inflammatory botanical compound, severely downregulates the expression of c-Myc mRNA in prostate cancer cells. Moreover, WDL dramatically decreases the protein level, nuclear accumulation, DNA-binding, and transcriptional activities of c-Myc. c-Myc is a transforming oncogene widely expressed in prostate cancer cells and is critical for maintaining their transformed phenotype. Interestingly, WDL was found to strongly affect the viability of Myc-activated prostate cancer cells and completely block their invasion as well as soft agar colony formation in vitro WDL was also found to downregulate c-Myc in vivo in nude mice xenografts. Moreover, WDL synergizes with enzalutamide to decrease the viability of androgen-sensitive prostate cancer cells via induction of apoptosis. These findings reveal a novel anticancer mechanism of the natural compound WDL, and suggest that the oncogenic function of c-Myc in prostate cancer cells can be effectively downregulated by WDL for the development of a new therapeutic strategy against Myc-driven prostate cancer. Mol Cancer Ther; 15(11); 2791-801. ©2016 AACR.

MK591, a second generation leukotriene biosynthesis inhibitor, prevents invasion and induces apoptosis in the bone-invading C4-2B human prostate cancer cells: implications for the treatment of castration-resistant, bone-metastatic prostate cancer.

Castration-resistant prostate cancer (CRPC) is a major clinical challenge for which no cure is currently available primarily because of the lack of proper understanding about appropriate molecular target(s). Previously we observed that inhibition of 5-lipoxygenase (5-Lox) activity induces apoptosis in some types of prostate cancer cells, suggesting an important role of 5-Lox in the viability of prostate cancer cells. However, nothing is known about the role of 5-Lox in the survival of castration-resistant, metastatic prostate cancer cells. Thus, we tested the effects of MK591, a second-generation, specific inhibitor of 5-Lox activity, on the viability and metastatic characteristics of CRPC cells. We observed that MK591 effectively kills the bone-invading C4-2B human prostate cancer cells (which bear characteristics of CRPC), but does not affect normal, non-cancer fibroblasts (which do not express 5-Lox) in the same experimental conditions. We also observed that MK591 dramatically inhibits the in vitro invasion and soft-agar colony formation of C4-2B cells. Interestingly, we found that treatment with MK591 dramatically down-regulates the expression of c-Myc and its targets at sub-lethal doses. In light of frequent over-activation of c-Myc in a spectrum of aggressive cancers (including CRPC), and the challenges associated with inhibition of c-Myc (because of its non-enzymatic nature), our novel findings of selective killing, and blockade of invasive and soft-agar colony-forming abilities of the castration-resistant, bone-metastatic C4-2B prostate cancer cells by MK591, open up a new avenue to attack CRPC cells for better management of advanced prostate cancer while sparing normal, non-cancer body cells.

OXER1, a G protein-coupled oxoeicosatetraenoid receptor, mediates the survival-promoting effects of arachidonate 5-lipoxygenase in prostate cancer cells.

Inhibition of 5-Lox induces apoptosis in prostate cancer cells by inactivating PKCε which is prevented by 5-oxoETE, and activators of PKCε prevent 5-Lox inhibition-induced apoptosis, suggesting that 5-Lox metabolites exert survival signaling via PKCε. However, mechanisms by which 5-Lox metabolites activate PKCε are not understood yet. We found that prostate cancer cells express high levels of OXER1, a G protein-coupled 5-oxoETE receptor, which delivers signal by generating diacyl-glycerol through phospholipase C-beta. Interestingly, we found that U73122, an inhibitor of PLC-beta, interrupts the apoptosis-preventing effect of 5-oxoETE, and exogenous diacyl-glycerol effectively prevents 5-Lox inhibition-induced apoptosis, suggesting that 5-oxoETE signals via OXER1 to promote prostate cancer cell survival.

Wedelolactone, a medicinal plant-derived coumestan, induces caspase-dependent apoptosis in prostate cancer cells via downregulation of PKCε without inhibiting Akt.

Emerging studies indicate that metabolism of arachidonic acid through the 5-lipoxygenase (5-Lox) pathway plays a critical role in the survival of prostate cancer cells raising the possibility that 5-Lox can be targeted for an effective therapy of prostate cancer. Wedelolactone (WDL), a medicinal plant-derived natural compound, is known to inhibit 5-Lox activity in neutrophils. However, its effect on apoptosis in prostate cancer cells has not been addressed. Thus, we tested the effects of WDL on human prostate cancer cells in vitro. We observed that WDL kills both androgen-sensitive as well as androgen-independent prostate cancer cells in a dose-dependent manner by dramatically inducing apoptosis. We also found that WDL-induced apoptosis in prostate cancer cells is dependent on c-Jun N-terminal Kinase (c-JNK) and caspase-3. Interestingly, WDL triggers apoptosis in prostate cancer cells via downregulation of protein kinase Cε (PKCε), but without inhibition of Akt. WDL does not affect the viability of normal prostate epithelial cells (PrEC) at doses that kill prostate cancer cells, and WDL-induced apoptosis is effectively prevented by 5-oxoETE, a metabolite of 5-Lox (but not by 15-oxoETE, a metabolite of 15-Lox), suggesting that the apoptosis-inducing effect of WDL in prostate cancer cells is mediated via inhibition of 5-Lox activity. These findings indicate that WDL selectivity induces caspase-dependent apoptosis in prostate cancer cells via a novel mechanism involving inhibition of PKCε without affecting Akt and suggest that WDL may emerge as a novel therapeutic agent against clinical prostate cancer in human.

MK591, a leukotriene biosynthesis inhibitor, induces apoptosis in prostate cancer cells: synergistic action with LY294002, an inhibitor of phosphatidylinositol 3'-kinase.

MK591 is a synthetic compound which specifically inhibits the activity of 5-Lox and is currently under development for the treatment of asthma. We observed that human prostate cancer cells treated with MK591 undergo apoptosis within hours of treatment. Apoptosis involves severe morphological alteration, externalization of phosphatidyl-serine, cleavage of PARP, and degradation of chromatin-DNA. MK591 also induced rapid activation of the stress kinase, c-Jun N-terminal kinase (JNK), which plays an important role in the apoptosis process. The phosphatidylinositol 3'-kinase-Akt/protein kinase B (PI3K/Akt) axis is a well-known pro-survival pathway which prevents apoptosis through defined anti-apoptotic mechanisms in a variety of cancer cells. Interestingly, we observed that MK591 triggers apoptosis in prostate cancer cells without inhibition of PI3K-Akt, or ERK. Moreover, it was observed that MK591 and LY294002 (an inhibitor of PI3K) exert synergistic effect in inducing apoptosis in prostate cancer cells. Altogether, these findings indicate that 5-Lox inhibition-induced apoptosis in prostate cancer cells occurs without inhibition of PI3K-Akt, or ERK, and suggest for the existence of an Akt- and ERK-independent survival mechanism(s) in these cancer cells maintained via signals generated by metabolites of 5-Lox.

Selenite triggers rapid transcriptional activation of p53, and p53-mediated apoptosis in prostate cancer cells: Implication for the treatment of early-stage prostate cancer.

Supra-nutritional selenium supplementation has emerged as an attractive new approach to intervene in a range of human cancers, in particular prostate cancer. However, scanty information is currently available on molecular mechanisms underlying selenium's anticancer action. The tumor suppressor p53 plays an important role in preventing transformation by transcriptional regulation of a range of genes that are involved in vital cell functions such as DNA repair, cell cycle arrest, and induction of apoptosis. Here we report that incubation of LNCaP human prostate cancer cells (p53 +/+) with a natural form of selenium triggers rapid transcriptional activation of p53, and up-regulation of the expression of p53-target genes as well as induction of miR-34 class of microRNAs. Moreover, blocking p53 function by transfection of cells with a dominant-negative, mutated p53 gene, or by siRNA, significantly reduced selenium-induced expression of p53-target genes and induction of apoptosis. Since majority of the early-stage human prostate cancers bear functional p53 gene (p53+/+), our findings indicate that the anticancer action of selenium may involve transactivation of p53 as a potential mechanism, and suggest that selenite may be useful not only for prevention but also for treatment of human prostate cancer.

Expression of 5-oxoETE receptor in prostate cancer cells: critical role in survival.

Previously, we reported that metabolism of arachidonic acid through the 5-lipoxygenase (5-LOX) pathway plays an important role in the survival and growth of human prostate cancer cells. Inhibition of 5-LOX by pharmacological inhibitors triggers apoptosis in prostate cancer cells within hours of treatment, which is prevented by the metabolites of arachidonate 5-lipoxygenase, 5(S)-hydroxyeicosatetraenoic acid (5(S)-HETE), and its dehydrogenated derivative, 5-oxoeicosatetraenoic acid (5-oxoETE). These findings suggested that 5-lipoxygenase metabolites are critical survival factors of prostate cancer cells. However, molecular mechanisms by which 5(S)-HETE and its derivative 5-oxoETE exert their effects on prostate cancer cell survival are yet to be understood. Here, we report that human prostate cancer cells differentially express a G-protein-coupled 5-oxoETE receptor (5-oxoER) in them. Blocking expression of 5-oxoER by short-interfering RNA (siRNA) significantly reduced the viability of prostate cancer cells, suggesting that 5-oxoER is critical for prostate cancer cell survival, and that the 5-LOX metabolite, 5-oxoETE, controls survival of prostate cancer cells through its own G-protein-coupled receptor, 5-oxoER.

Rapid induction of apoptosis in prostate cancer cells by selenium: reversal by metabolites of arachidonate 5-lipoxygenase.

Recent clinical trials have documented that selenium significantly reduces the incidence of clinical prostate cancer. However, nothing is clearly known about the underlying molecular mechanisms by which selenium exerts its anti-cancer effect. This report provides evidence that selenium at micro-molar concentrations induces rapid apoptotic death in human prostate cancer cells, but not in normal prostate epithelial cells. Apoptosis involves activation of caspase 3 which plays a critical role in the cell death process. Interestingly, the apoptosis-inducing effect of selenium in prostate cancer cells is substantially alleviated by the 5-lipoxygenase metabolites, 5(S)-HETE and its dehydrogenated derivative 5-oxoETE, but not by metabolites of 12-lipoxygenase (12(S)-HETE) or 15-lipoxygenase (15(S)-HETE). Apoptosis is also prevented by their precursor, arachidonic acid, an omega-6, polyunsaturated fatty acid, presumably by metabolic conversion through the 5-lipoxygenase pathway. These results indicate that selenium's anticancer effect may involve induction of apoptosis specifically in prostate cancer cells sparing normal prostate epithelial cells, and that 5-lipoxygenase may be a molecular target of selenium's anticancer action. The present report warrants that care should be taken about high intake of dietary fat containing arachidonic acid or its precursor fatty acids when selenium is used for the management of prostate cancer, and suggests that a combination of selenium and 5-lipoxygenase inhibitors may be a more effective regimen for prostate cancer control.

Inhibition of arachidonate 5-lipoxygenase triggers prostate cancer cell death through rapid activation of c-Jun N-terminal kinase.

Previously, we reported that inhibition of arachidonate 5-lipoxygenase triggers massive apoptosis in both androgen-sensitive (LNCaP) and androgen-refractory (PC3) human prostate cancer cells within hours of treatment [Proc. Natl. Acad. Sci. USA 95 (1998) 13182-13187]. Apoptosis was prevented by exogenous 5(S)-HETE, a product of 5-lipoxygenase, indicating a role of this eicosanoid as an essential survival/anti-apoptotic factor for prostate cancer cells. However, nothing was clearly known about details of the underlying molecular mechanisms or events mediating the induction of fulminating apoptosis in these cells. This report documents the fact that inhibition of arachidonate 5-lipoxygenase induces rapid activation of c-Jun N-terminal kinase (JNK) in human prostate cancer cells which is prevented by the 5-lipoxygenase metabolite, 5(S)-HETE. Activation of JNK is unaffected by the cell-permeable tetra-peptide inhibitors of caspase 8 or caspase 3 (IETD-FMK and DEVD-FMK), though these inhibitors effectively blocked apoptosis triggering, suggesting that activation of JNK is independent or upstream of caspase activation. Both 5-lipoxygenase inhibition-induced activation of JNK and induction of apoptosis are prevented by curcumin, an inhibitor of JNK-signaling pathway. Apoptosis is also blocked by SP600125, a specific inhibitor of JNK activity, indicating that JNK activity is required for the induction of apoptosis in these cells. These findings suggest that the metabolites of arachidonate 5-lipoxygenase promote survival of prostate cancer cells involving down-regulation of stress-activated protein kinase.