12/15 lipoxygenase regulation of colorectal tumorigenesis is determined by the relative tumor levels of its metabolite 12-HETE and 13-HODE in animal models

Colorectal cancer (CRC) continues to be a major cause of morbidity and mortality. The arachidonic acid (AA) pathway and linoleic acid (LA) pathway have been implicated as important contributors to CRC development and growth. Human 15-lipoxygenase 1 (15-LOX-1) converts LA to anti-tumor 13-S-hydroxyoctadecadienoic acid (13-HODE)and 15-LOX-2 converts AA to 15-hydroxyeicosatetraenoic acid (15-HETE). In addition, human 12-LOX metabolizes AA to pro-tumor 12-HETE. In rodents, the function of 12-LOX and 15-LOX-1 and 15-LOX-2 is carried out by a single enzyme, 12/15-LOX. As a result, conflicting conclusions concerning the role of 12-LOX and 15-LOX have been obtained in animal studies. In the present studies, we determined that PD146176, a selective 15-LOX-1 inhibitor, markedly suppressed 13-HODE generation in human colon cancer HCA-7 cells and HCA-7 tumors, in association with increased tumor growth. In contrast, PD146176 treatment led to decreases in 12-HETE generation in mouse colon cancer MC38 cells and MC38 tumors, in association with tumor inhibition. Surprisingly, deletion of host 12/15-LOX alone led to increased MC38 tumor growth, in association with decreased tumor 13-HODE levels, possibly due to inhibition of 12/15-LOX activity in stroma. Therefore, the effect of 12/15-LOX on colorectal tumorigenesis in mouse models could be affected by tumor cell type (human or mouse), relative 12/15 LOX activity in tumor cells and stroma as well as the relative tumor 13-HODE and 12-HETE levels.     

Alterations in lipoxygenase and cyclooxygenase-2 catalytic activity and mRNA expression in prostate carcinoma

Recent studies in prostate tissues and especially cell lines have suggested roles for arachidonic acid (AA) metabolizing enzymes in prostate adenocarcinoma (Pca) development or progression. The goal of this study was to more fully characterize lipoxygenase (LOX) and cyclooxygenase-2 (COX-2) gene expression and AA metabolism in benign and malignant prostate using snap-frozen tissues obtained intraoperatively and mRNA analyses and enzyme assays. Formation of 15-hydroxyeicosatetraenoic acid (15-HETE) was detected in 23/29 benign samples and 15-LOX-2 mRNA was detected in 21/25 benign samples. In pairs of pure benign and Pca from the same patients, 15-HETE production and 15-LOX-2 mRNA were reduced in Pca versus benign in 9/14 (P=.04) and 14/17 (P=.002), respectively. Under the same conditions, neither 5-HETE nor 12-HETE formation was detectable in 29 benign and 24 tumor samples; with a more sensitive assay, traces were detected in some samples, but there was no clear association with tumor tissue. COX-2 mRNA was detected by nuclease protection assay in 7/16 benign samples and 5/16 tumors. In benign and tumor pairs from 10 patients, COX-2 was higher in tumor versus benign in only 2, with similar results by in situ hybridization. Paraffin immunoperoxidase for COX-2 was performed in whole mount sections from 87 additional radical prostatectomy specimens, with strong expression in ejaculatory duct as a positive control and corroboration with in situ hybridization. No immunostaining was detected in benign prostate or tumor in 45% of cases. Greater immunostaining in tumor versus benign was present in only 17% of cases, and correlated with high tumor grade (Gleason score 8 and 9 vs. 5 to 7). In conclusion, reduced 15-LOX-2 expression and 15-HETE formation is the most characteristic alteration of AA metabolism in Pca. Increased 12-HETE and 5-HETE formation in Pca were not discernible. Increased COX-2 expression is not a typical abnormality in Pca in general, but occurs in high-grade tumors.     

Overexpression of 12/15-lipoxygenase, an ortholog of human 15-lipoxygenase-1, in the prostate tumors of TRAMP mice

Changes in the expression and activity of lipid-metabolizing enzymes, including the linoleic acid (LA)-metabolizing enzyme 15-lipoxygenase-1 (15-LO-1), may play a role in the development and progression of human prostate carcinoma (PCa). We reported that human 15-LO-1 (designated as leukocyte type 12-LO or 12/15-LO in mouse) is expressed in human prostate and increased in PCa, particularly high-grade PCa. Genetically engineered mouse (GEM) models of PCa could facilitate the study of this gene and its regulation and function in PCa progression. In this study, we examine the protein expression and enzyme activity levels of 12/15-LO associated with PCa progression in the TRansgenic Adenocarcinoma of Mouse Prostate (TRAMP) model of PCa. This GEM model develops prostatic intraepithelial neoplasia (PIN), followed by invasive gland-forming PCa and invasive and metastatic less differentiated PCa, with neuroendocrine (NE) differentiation (NE Ca). In the wild-type and TRAMP prostates, the most prominent LA metabolite was 13-hydroxyoctadecadienoic acid (13-HODE). Lesser amounts of 12-hydroxyeicosatetraenoic acid and 15-hydroxyeicosatetraenoic acid (HETE) were made from arachidonic acid (AA). In TRAMP prostates, 12/15-LO activity was increased compared to wild type at 20, 29, 39, and 49 weeks, as assessed by LA conversion to 13-HODE, and by AA conversion to 12/15-HETE, respectively. Immunostaining demonstrated that the increased capacity to generate 13-HODE was paralleled by an increase in neoplastic epithelial expression of 12/15-LO in PIN and invasive carcinomas. In conclusion, although there is a basal 12/15-LO activity in the wild-type mouse prostate, there is a marked increase in the expression of 12/15-LO with TRAMP PCa progression, paralleling our previously reported increased expression of the ortholog 15-LO-1 in high-grade human PCa. Thus, 12/15-LO and LA metabolism in the TRAMP model shares similarities to human PCa, and may allow to confirm a role for LA metabolism and other biologic functions of 15-LO-1 in human PCa. In addition, the TRAMP model will serve as a tool for testing the suitability of 12/15-LO-and ultimately human 15-LO--as a therapeutic target during PCa progression.     

Eicosanoid metabolism in squamous cell carcinoma cell lines derived from primary and metastatic head and neck cancer and its modulation by celecoxib

Eicosanoid metabolism through cyclooxygenases (COXs) and lipoxygenases (LOXs) generates various lipids that play a role in squamous cell carcinogenesis. We used pairs of head and neck squamous cell carcinoma (HNSCC) cell lines derived from primary and metastatic tumors of the same patient to analyze eicosanoid metabolites by ESI-LC/MS/MS and COX/LOX expression by western immunoblotting. The effects of celecoxib on eicosanoid synthesis and HNSCC cell growth were examined. Prostaglandin E2 (PGE2) was the major metabolite in three of six cell lines. COX-2 was detected in three cell lines, which produced PGE2 (two from metastases). We found low expression of COX-1 at similar intensities for each pair of cell lines. 5-LOX was detected in all cells. Some expressed 12-LOX, 15-LOX-1, and 15-LOX-2, but there was no correlation between enzyme expression and endogenous product content. Exogenous arachidonic acid did not change the profile of eicosanoid biosynthesis. Low doses of celecoxib inhibited formation of PGE2 in UMSCC-14A cells by 84% as early as 6 hours. In contrast, 5-HETE, 12-HETE, and 15-HETE levels were increased by approximately 40-, 5- and 3-fold, respectively, with a decline to baseline levels within 24 hours. High dose celecoxib increased the 12-HETE level 2.3-fold after 3 days of incubation. Celecoxib inhibited growth of all HNSCC cell lines in a concentration-dependent manner regardless of their COX expression (IC50 values after 3 days; 33 to 62 microM). Our findings provide new informations about individual eicosanoids produced by HNSCC cells and their differential regulation by the selective COX-2 inhibitor celecoxib.     

15-Lipoxygenase-1 mediates cyclooxygenase-2 inhibitor-induced apoptosis in gastric cancer

It has been found that expression of 15-lipoxygenase-1 (15-LOX-1) and its main product, 13-S-hydroxyoctadecadienoic acid (13-S-HODE), are decreased in human colorectal and esophageal cancers and that non-steroidal anti-inflammatory drugs (NSAIDs) can therapeutically induce 15-LOX-1 expression to trigger apoptosis in those cancer cells. We found that a specific cyclooxygenase-2 (COX-2) inhibitor SC-236 similarly induced apoptosis in gastric cancer cells. In the present study, we tested whether SC-236 induced apoptosis through up-regulation of 15-LOX-1 in gastric cancer. We found that: (i) SC-236 inhibited growth of gastric cancer cells mainly by inducing apoptosis; (ii) SC-236 induced 15-LOX-1 expression and increased endogenous 13-S-HODE product, instead of 15-S-HETE during apoptosis; (iii) SC-236 did not affect expression of COX-1, COX-2, 5-LOX and 12-LOX; and (iv) 15-LOX-1 inhibition suppressed SC-236 induced apoptosis. These findings demonstrated that SC-236 induced apoptosis in gastric cancer cells via up-regulation of 15-LOX-1, and 13-S-HODE. These are potential and new targets for prevention and treatment of gastric cancer.     

Zyflamend-mediated inhibition of human prostate cancer PC3 cell proliferation: effects on 12-LOX and Rb protein phosphorylation

The multiherb anti-inflammatory product Zyflamend was investigated for its antiproliferative effects on PC3 human prostate cancer cells and eicosanoid metabolism in this prostate cancer cell line. Zyflamend produced a concentration-dependent inhibition of cloned COX-1, COX-2, and 5-LOX enzyme activities, with inhibition of 5-HETE production being greater than that of PGE(2) formation. Applied to intact PC3 cells, Zyflamend was found to be most potent against 12-LOX, followed by 5-LOX and then COX activities. The concentration-dependent inhibition of PC3 cell proliferation was associated with a selective G(2)/M arrest of the cell cycle and induction of apoptosis, as evidenced by flow cytometric staining of PC3 cells with annexin V. Zyflamend also produced a concentration-dependent down-regulation of 5-LOX and 12-LOX expression. Determination of cell signal transduction proteins demonstrated that Zyflamend produced an increase in p21 phosphorylation but down-regulated phosphorylation of retinoblastoma (Rb) protein. The decrease in pRb protein was shown to be due to 12-LOX inhibition and a decline in 12-HETE levels in the cells. Replenishing 12-HETE in Zyflamend-treated cells overcame the ability of this multiple herb product to inhibit cell proliferation, and concordantly, 12-HETE blocked Zyflamend's ability to down-regulate phosphorylation of Rb protein. We conclude that the effective control of human prostate cancer cell proliferation with Zyflamend is multi-mechanistic but, in part, involves regulation of aberrant tumor cell eicosanoid metabolism, especially on 5- and 12-LOX, as well as restoration of Rb tumor suppressor protein function through regulation of its phosphorylation status.     

The prevalence of high-grade prostatic intraepithelial neoplasia in surgical resection specimens: an Indian experience

BACKGROUND: To the authors' knowledge there is a paucity of literature regarding the prevalence of high-grade prostatic intraepithelial neoplasia (HGPIN) in the Indian subcontinent. The objective of the current study was to document the prevalence of HGPIN in a low-risk Indian population. METHOD: SA total of 110 prostate specimens (61 taken from the test group and 49 taken from the control group) were studied to document the prevalence of HGPIN in a low-risk Indian population. RESULTS: None of the benign prostate samples were found to harbor HGPIN, whereas 85.24% of the samples from malignant prostates did so. CONCLUSION: SA strong correlation between HGPIN and invasive carcinoma was observed, a finding that reinforces the view that HGPIN is a strong indicator of concurrent invasive prostate carcinoma.     

脂氧合酶在胃癌中的表达及其意义

 目的 检测5-脂氧合酶（5-lipoxygenase,5-LOX）和15-脂氧合酶-1（15-lipoxygenase-1,15-LOX-1）mRNA及蛋白在胃癌及癌旁正常组织中的表达,并研究两者间的关系。方法 采用RT-PCR和western blot检测胃癌组织及相匹配的癌旁正常组织中5-LOX、15-LOX-1mRNA和蛋白的表达。结果 5-LOX mRNA及蛋白在胃癌组织中的表达均显著高于癌旁正常组织（P〈0.05）,而15-LOX-1mRNA及蛋白在胃癌组织中的表达均显著低于癌旁正常组织（P〈0.05）。5-LOX与15-LOX-1在胃癌组织中的表达无明显相关（P〉0.05）。结论 胃癌组织中存在5-LOX和15-LOX-1的表达失衡,且可能与胃癌的发生发展有关,但二者之间无相关性。     

Elevated 12- and 20-hydroxyeicosatetraenoic acid in urine of patients with prostatic diseases

The role of eicosanoids (metabolites of arachidonic acid) in prostate diseases is receiving increased attention. We investigated the relationship between the concentrations of urinary free acids of 12- and 20-hydroxyeicosatetraenoic acids (12- and 20-HETE) and the benign prostatic hypertrophy (BPH) and prostate cancer (Pca). Urinary concentrations of 12-HETE and 20-HETE of BPH and Pca patients were significantly higher than normal subjects. After removal of the prostate gland, the urinary concentrations of these eicosanoids decreased to concentrations similar to the normal subjects. These results suggest that urinary free acids of 12-HETE and 20-HETE indicate an abnormality of the prostate gland.     

A probasin-large T antigen transgenic mouse line develops prostate adenocarcinoma and neuroendocrine carcinoma with metastatic potential

Neuroendocrine (NE) cells may be involved not only in growth and differentiation of the normal prostate but also in carcinogenesis and progression of prostate adenocarcinoma (Pca), including development of androgen resistance. However, the exact pathophysiology of NE cells in Pca remains poorly understood. Here we describe a transgenic model of Pca with progressive NE differentiation. Seven lines of transgenic mice with the rat prostate-specific large probasin promoter linked to the SV40-large T antigen (Tag) that develop prostatic neoplasia have been established. In this study, one of the seven lines (12T-10) was characterized by examination of 52 mice aged from 2-12 months. With advancing age, low-grade prostatic intraepithelial neoplasia, high-grade prostatic intraepithelial neoplasia, microinvasion, invasive carcinoma, and poorly or undifferentiated carcinoma with NE differentiation appeared in the prostates in sequential order. Whereas Tag is expressed uniformly in prostate epithelium, only an increasing subset of cells in prostatic intraepithelial neoplasia showed NE differentiation by chromogranin immunostaining. Frankly invasive carcinoma developing subsequently showed occasional definitive glandular differentiation (adenocarcinoma) and particularly undifferentiated carcinoma with NE histological features similar to those observed in NE carcinomas in humans. The NE carcinomas occurred in the dorsolateral and ventral lobes and were generally androgen receptor negative. Twenty-one of 32 (66%) mice aged > or = 6 months and 15 of 17 (88%) mice aged > or = 9 months developed metastatic tumors, as confirmed by histology and/or Tag immunohistochemistry. Metastases occurred at the later time points, with metastasis to regional lymph nodes, liver, and lung being particularly common. Metastases showed histological features of NE differentiation, as confirmed by chromogranin immunostaining and electron microscopy. An athymic nude mouse that received a s.c. implant of a primary NE tumor developed Tag-positive metastatic tumors with similar NE differentiation. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry identified identical protein profiles between the primary NE tumor and lesions in the extraprostatic organs. Hence, in the 12T-10 large probasin promoter-Tag mouse, high-grade prostatic intraepithelial neoplasia develops progressively greater NE differentiation and progresses to invasive adenocarcinoma and NE carcinoma, with a high percentage of metastases. The predictable progression through these stages will allow testing of therapeutic interventions as well as possible further delineation of the role of NE cells in Pca progression.     

脂氧合酶在胃癌中的表达及其临床意义

目的:探讨脂氧合酶（lipoxygenases,LOXs）,包括5-LOX、15-LOX-1/2在胃癌组织中的表达及其与临床病理特征的关系。方法:收集42例胃癌患者的临床资料和胃癌组织及相应的癌旁组织的石蜡包块做病理切片。免疫组化法分析5-LOX、15-LOX-1/2在胃癌组织及癌旁组织中的表达,统计学分析两者表达的差异,及其与胃癌临床病理特征的关系。结果:5-LOX表达的阳性率在胃癌组织高于相应的癌旁组织（P＜0.05）;相反的,15-LOX-1和15-LOX-2的阳性率在胃癌组织表达低于相应的癌旁组织（P＜0.01;P＜0.01）;5-LOX与胃癌的TNM分期、淋巴结转移相关(P＜0.05);15-LOX-2与胃癌肿瘤大小、TNM分期、淋巴结转移相关(P＜0.05);15-LOX-1与胃癌临床特征未见明显相关。结论:5-LOX在胃癌组织中高表达;15-LOX-1和15-LOX-2在胃癌组织中低表达;提示LOXs在胃癌的发生发展中起作用。     

The critical role of 15-lipoxygenase-1 in colorectal epithelial cell terminal differentiation and tumorigenesis

Terminal differentiation is an important event for maintaining normal homeostasis in the colorectal epithelium, and the loss of apoptosis is an important mechanism underlying colorectal tumorigenesis. The very limited current data on the role of lipoxygenase (LOX) metabolism in tumorigenesis suggests that the oxidative metabolism of linoleic and arachidonic acid possibly shifts from producing antitumorigenic 15-LOX-1 and 15-LOX-2 products to producing protumorigenic 5-LOX and 12-LOX products. We examined whether this shift occurs in vitro in the human colon cancer cell line Caco-2 in association with the loss of terminal differentiation and apoptosis, or in vivo during the formation of colorectal adenomas in patients with familial adenomatous polyposis (FAP). Restoring terminal differentiation and apoptosis of Caco-2 cells increased the mRNA levels of 5-LOX, 15-LOX-2, and 15-LOX-1, but the only significant increases in protein expression and enzymatic activity were of 15-LOX-1. In FAP patients, 15-LOX-1 expression and activity were significantly down-regulated in adenomas (compared with paired nonneoplastic epithelial mucosa), whereas 5-LOX and 15-LOX-2 protein expressions and enzymatic activities were not. We conducted a validation study with immunohistochemical testing in a second group of FAP patients; 15-LOX-1 expression was down-regulated in colorectal adenomas (compared with nonneoplastic epithelial mucosa) in 87% (13 of 15) of this group. We confirmed the mechanistic relevance of these findings by demonstrating that ectopically restoring 15-LOX-1 expression reestablished apoptosis in Caco-2 cells. Therefore, 15-LOX-1 down-regulation rather than a shift in the balance of LOXs is likely the dominant alteration in LOX metabolism which contributes to colorectal tumorigenesis by repressing apoptosis.     

Diverse roles of 2-arachidonoylglycerol in invasion of prostate carcinoma cells: Location, hydrolysis and 12-lipoxygenase metabolism

Endogenous 2-arachidonoylglycerol (2-AG) is antiinvasive in androgen-independent prostate carcinoma (PC-3) cells. Invasion of PC-3 cells is also inhibited by exogenously added noladin ether, a non-hydrolyzable analog of 2-AG. In contrast, exogenous 2-AG has the opposite effect. Cell invasion significantly increased with high concentrations of exogenous 2-AG. In PC-3 cells, arachidonic acid (AA) and 12-hydroxyeicosatetraenoic acid (12-HETE) concentrations increased along with exogenously added 2-AG, and 12-HETE concentrations increased with exogenously added AA. Invasion of PC-3 cells also increased with exogenously added AA and 12(S)-HETE but not 12(R)-HETE. The exogenous 2-AG-induced invasion of PC-3 cells was inhibited by 3-octylthio-1,1,1-trifluoropropan-2-one (OTFP, an inhibitor of 2-AG hydrolysis) and baicalein (a 12-LO inhibitor). Western blot and RT-PCR analyses indicated expression of 12-HETE producing lipoxygenases (LOs), platelet-type 12-LO (P-12-LO) and leukocyte-type 12-LO (L-12-LO), in PC-3 cells. These results suggest that exogenous 2-AG induced, rather inhibited, cell invasion because of its rapid hydrolysis to free AA, and further metabolism by 12-LO of AA to 12(S)-HETE, a promoter of PC cell invasion. The results also suggest that PC-3 cells and human prostate stromal (WPMY-1) cells released free AA, 2-AG, and 12-HETE. In the microenvironment of the PC cells, this may contribute to the cell invasion. The 2-AG hydrolysis and concentration of 2-AG in microenvironment are critical for PC cell's fate. Therefore, inhibitors of 2-AG hydrolysis could potentially serve as therapeutic agents for the treatment of prostate cancer. (c) 2007 Wiley-Liss, Inc.     

Enhancement role of host 12/15-lipoxygenase in melanoma progression

12/15-Lipoxygenase (12/15-LOX) is a non-haeme iron-containing dioxygenase that forms 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) or 15(S)-HETE. Several biological mediators including cytokines, growth factors and lipid metabolites released during tumour cell-endothelial cell adhesion are associated with malignant tumour progression. Here we found that HETEs released from the host organ played a critical role in tumour metastasis. Intravenous injection of B16F10 melanoma cells caused lung nodule formation, which was markedly attenuated in 12/15-LOX null mice. Co-injection of melanoma cells with 12(S)-HETE increased the lung homing activity of B16F10 melanoma cells. In vitro studies showed that 12(S)-HETE and 15(S)-HETE treatment resulted in a concentration-dependent increase of adhesion of B16F10 cells on collagen or fibronectin. The melanoma cell adhesion was then evaluated in pulmonary primary cell culture isolated from wild-type (WT) and 12/15-LOX knockout (KO) mice. It was found that the adhesion of melanoma cells on the epithelial cells isolated from 12/15-LOX null mice was reduced in comparison with those isolated from WT mice. Treatment of 12(S)-HETE increased the pFAK in melanoma cells adhering on collagen-coated slide. The enhancement of adherence elicited by 12(S)-HETE in B16F10 cells could be antagonised by focal adhesion kinase (FAK) inhibitor 14 (FAK inhibitor) or PD98059 (extracellular signal-regulated kinase (ERK) inhibitor). 12(S)-HETE increased the phosphorylation of FAK and ERK in adhering melanoma cells. The FAK phosphorylation induced by 12(S)-HETE was further inhibited by PD98059, indicating that FAK is the downstream target of ERK. The adhesion and lung metastasis of human melanoma cells of C32 in NOD/SCID mice were also potentiated by co-treatment with 12(S)-HETE. These results demonstrate that 12(S)-HETE/15(S)-HETE activates ERK and FAK signalling pathways, thereby upregulates the adhesion and metastatic potential of melanoma cells. The endogenous release of 12(S)-HETE/15(S)-HETE in the host organ may affect the metastatic potential of melanoma.     

15-LOX-1: a novel molecular target of nonsteroidal anti-inflammatory drug-induced apoptosis in colorectal cancer cells

BACKGROUND: Nonsteroidal anti-inflammatory drugs (NSAIDs) appear to act via induction of apoptosis-programmed cell death-as potential colorectal cancer chemopreventive agents. NSAIDs can alter the production of different metabolites of polyunsaturated fatty acids (linoleic and arachidonic acids) through effects on lipoxygenases (LOXs) and cyclooxygenases. 15-LOX-1 is the main enzyme for metabolizing colonic linoleic acid to 13-S-hydroxyoctadecadienoic acid (13-S-HODE), which induces apoptosis. In human colorectal cancers, the expression of this enzyme is reduced. NSAIDs can increase 15-LOX enzymatic activity in normal leukocytes, but their effects on 15-LOX in neoplastic cells have been unknown. We tested the hypothesis that NSAIDs induce apoptosis in colorectal cancer cells by increasing the protein expression and enzymatic activity of 15-LOX-1. METHODS: We assessed 15-LOX-1 protein expression and enzymatic activity, 13-S-HODE levels, and 15-LOX-1 inhibition in association with cellular growth inhibition and apoptosis induced by NSAIDs (primarily sulindac and NS-398) in two colorectal cancer cell lines (RKO and HT-29). All P values are two-sided. RESULTS: Sulindac and NS-398 progressively increased 15-LOX-1 protein expression in RKO cells (at 24, 48, and 72 hours) in association with subsequent growth inhibition and apoptosis. Increased 13-S-HODE levels and the formation of 15-hydroxyeicosatetraenoic acid on incubation of the cells with the substrate arachidonic acid confirmed the enzymatic activity of 15-LOX-1. Inhibition of 15-LOX-1 in RKO cells by treatment with caffeic acid blocked NS-398-induced 13-S-HODE production, cellular growth inhibition, and apoptosis (P =. 007, P<.0001, and P<.0001, respectively); growth inhibition and apoptosis were restored by adding exogenous 13-S-HODE (P<.0001 for each) but not its parent compound, linoleic acid (P = 1.0 for each). Similar results occurred with other NSAIDs and in HT-29 cells. CONCLUSIONS: These data identify 15-LOX-1 as a novel molecular target of NSAIDs for inducing apoptosis in colorectal carcinogenesis.