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,1
Departments of
* Clinical Cancer Prevention,
Gastrointestinal Medical Oncology,
Pathology, and
Biostatistics and Applied Mathematics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas, USA
1Correspondence: Department of Clinical Cancer Prevention, Unit 1360, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030-4009, USA. E-mail: ishureiqi{at}mdanderson.org
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: transcriptional regulation • apoptosis
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. GATA-4, –5, and –6 are expressed in overlapping patterns during the development of endoderm-derived organs such as the intestines (2)
.
15-LOX-1 (15-LOX-1) is an enzyme that is involved in oxidative metabolism of linoleic acid, which is expressed during terminal differentiation of normal cells (3
4
5)
but transcriptionally silenced in cancer cells (6)
. Expression of 15-LOX-1 is down-regulated in colonic tumorigenesis (7
8
9)
; restoring 15-LOX-1 expression induces apoptosis and inhibits tumorigenesis in colon cancer cells (8
,10
11
12
13)
. GATA-6 expression is inversely related to the induction of 15-LOX-1 expression during induced terminal differentiation in Caco-2 colon cancer cells by sodium butyrate (NaBT) treatment (14)
, and GATA-6 inhibits the ability of nonsteroidal antiinflammatory drugs (NSAIDs) to induce 15-LOX-1 transcription and thus apoptosis in colon cancer cells (6)
. These data suggested that GATA-6 promotes tumorigenesis by acting as a transcriptional suppressor of 15-LOX-1 to inhibit apoptosis in cancer cells. Additionally, the following lines of evidence suggest that GATA-6 may promote tumorigenesis, especially in the intestines. First, GATA-6 expression is higher in the proliferating region of the intestinal crypts than in the tips of the intestinal villi, where cells undergo differentiation and apoptosis (15)
. Second, GATA-6 is highly expressed in gastric, colonic, pancreatic, pulmonary, and prostatic cancer cell lines (16
17
18)
. Third, in contrast with the GATA-4 and –5 promoters, which are methylated in human gastric, colorectal (17)
, and lung cancers (18)
, the GATA-6 promoter remains unmethylated, so GATA-6 expression may contribute to tumorigenesis in these sites.
The relative mechanistic contribution of GATA-6 overexpression to colonic tumorigenesis and 15-LOX-1 transcriptional suppression has, however, remained unknown: specifically, whether GATA-6 overexpression occurs in vivo in human cancers and the biological effects of reversing GATA-6 overexpression on cancer cells. Therefore, we examined whether GATA-6 is overexpressed in human colorectal cancer in vivo and the effects of GATA-6 overexpression reversal on important molecular events in colorectal cancer such as 15-LOX-1 expression, cell proliferation, and apoptosis.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Caffeic acid (CAF) was purchased from Cayman Chemical Co. (Ann Arbor, MI). Dibutyryl cAMP (dbcAMP) was purchased from Sigma-Aldrich (St. Louis, MO). Other reagents, molecular-grade solvents, and chemicals were obtained from commercial manufacturers or as specified.
Acquisition of clinical samples
We obtained surgically resected specimens of normal and malignant colorectal tissues from patients at The University of Texas M. D. Anderson Cancer Center through the Tissue Procurement and Banking Facility. For each patient, samples were procured from both the tumor area and the normal-appearing mucosa. Fresh-frozen paired colorectal normal and malignant mucosa samples were obtained from each of 33 patients. Tissue blocks were kept frozen at –70°C until processed. M. D. Anderson Cancer Center’s Institutional Review Board approved this study.
Cell cultures
Caco-2 cells were grown in eagle’s minimal essential medium containing 15% FBS, and HCT-116 cells were grown in RPMI 1640 medium containing 10% FBS in a humidified atmosphere containing 5% CO2 at 37°C. Cell culture media were supplemented with 1% penicillin/streptomycin.
Analysis of GATA-6 expression by immunohistochemical staining
Frozen (–20°C) 5-µm-thick sections from both the tumor area and the normal-appearing mucosa were cut, air dried, and fixed in acetone for 30 s. At the time of staining, the sections were incubated with 3% hydrogen peroxidase in ethanol for 30 min to inactivate endogenous peroxides. Nonspecific antibody (Ab) binding sites were blocked using 20% goat serum. Tissue sections were incubated overnight in 1:50 rabbit anti-human GATA-6 polyclonal antibody (pAb) (H92) (Santa Cruz Biotechnology) at 4°C. The next day, the sections were washed and then incubated with biotinylated anti-rabbit Ab solution followed by avidin-biotinylated horseradish peroxidase complex (Vectastain Elite avidin-biotin complex (ABC); Vector Laboratories, Burlingame, CA). Slides were washed, reincubated in a solution of 0.1 M 3,3'-diaminobenzidine (DAB) in 0.05 M tris-buffered saline with 0.5 ml of 3% hydrogen peroxide DAB solution enhanced with nickel cobalt (DAB-Ni kit, Zymed Laboratories, San Francisco, CA), and then counterstained with 1% methyl green for 2 min. For negative-control experiments, rabbit serum was substituted for the primary GATA-6 Ab solution.
GATA-6 expression analyses by cDNA arrays
The Cancer Profiling Array I (BD Biosciences, Clontech, Palo Alto, CA) consists of 241 paired cDNA samples representing 13 different tissue types. Each pair consists of a tumor sample and a corresponding normal tissue sample obtained from the same individual. Colon tissues represented on this array included 38 normal and tumor pairs from 34 patients. Clinical characteristics of samples are available on the Web site http://bioinfo.clontech.com/dparray.
We used polymerase chain reaction (PCR) to amplify a human GATA-6 cDNA fragment (486 bp) from the template of GATA-6 expression vector (pCMV6-XL6-GATA6 vector, OriGene Technologies, Rockville, MD) with the primers 5'-GAG GGA ATT CAA ACC AGG AA-3' (forward) and 5'-CAA GCC TCT TGG GAA AAA CA-3' (reverse). The cDNA probe was labeled with 
-32P-deoxycytidine triphosphate by random primer labeling and hybridized to the Cancer Profiling Array I membrane, according to the manufacturer’s protocol. Equal loading was confirmed by probing randomly selected arrays from the same lot of printed array with ubiquitin as part of the printing quality control. Data were acquired and quantified using the Storm PhosphoImager and ImageQuant version 5.2 software (Molecular Dynamics, Sunnyvale, CA).
siRNA transfection and treatment with NS-398 and NaBT
Cells were cultured to 40–50% confluence and then transfected with 100 nmol of a pooled mixture of four SMART–selected siRNA duplexes (SMARTpool; Dharmacon) for GATA-6 or a nonspecific control siRNA (siGLO RISC-Free siRNA; Dharmacon) using Lipofectamine 2000 (Invitrogen, Carlsbad, CA). In some experiments, NS-398 (30 µM), or NaBT (0.25 mM), or an equal amount of solvent (control) was added 48 h after the siRNA transfection.
Quantitative real-time reverse-transcriptase PCR
Total RNA was extracted from cells using TRI reagent (Molecular Research Center, Cincinnati, OH). The isolated RNA was size fractionated by electrophoresis on a 1% agarose-formaldehyde gel, stained with ethidium bromide, and confirmed to be of adequate quality (clear RNA bands for 18S, 28S, and 5S; 28S:18S of 2:1). Extracted RNA was quantified using an RNA quantitation kit (RiboGreen; Molecular Probes, Eugene, OR). A 500-ng aliquot of each RNA sample was reverse transcribed in a 20-µl reaction volume using the iScript cDNA synthesis kit (Bio-Rad Laboratories, Hercules, CA). Real-time reverse-transcriptase (RT)-PCR was carried out in 25 µl of a reaction mixture containing 1 µl of cDNA (25 ng/µl), 12.5 µl of 2x TaqMan Universal PCR Master Mix (Applied Biosystems, Foster City, CA), 10.25 µl double-distilled water, and 1.25 µl of primer and probe mixture (Applied Biosystems). Real-time PCR assays were performed in triplicate using a 7300 real-time PCR system (Applied Biosystems) with the following conditions: 50°C for 2 min, 95°C for 10 min, 40 cycles at 95°C for 15 s, and 60°C for 1 min. A sequence-detection program calculated a threshold cycle number at which the probe cleavage-generated fluorescence exceeded the background signal (19)
.
Measurement of relative RNA expression level
We calculated the relative RNA expression level using a comparative threshold cycle method (19)
. The sets of gene primer and probe for the target genes (e.g., GATA-6 and 15-LOX-1) were confirmed to have amplification efficiency equal to that of the internal reference gene (HPRT1). The relative expression level of an individual target gene was normalized to the reference gene and to a calibrator sample that was run on the same plate. We calculated the normalized relative expression level of a target gene in an individual sample using the following formula:
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. Thus, the relative RNA expression level of a gene is a unitless number relative to that of the calibrator sample (20)
. We calculated relative RNA expression with commercial software (SDS V1.2; Applied Biosystems).
Western blot analyses
Cells were homogenized in lysis buffer [0.5% Nonidet P-40, 20 mM MOPS (pH 7.0), 2 mM EGTA, 5 mM EDTA, 30 mM sodium fluoride, 40 mM ß-glycerophosphate, 2 mM sodium orthovanadate, 1 mM phenylmethysulfonylfluoride, and 1x complete protease inhibitor cocktail (Roche Applied Science, Indianapolis, IN)]. Equivalent amounts of protein (60 µg crude protein (CP)/sample) were subjected to sodium dodecyl sulfate–PAGE and transferred to a nitrocellulose membrane. Blots were incubated with a rabbit Ab to human GATA-6 (1:500, Santa Cruz Biotechnology) overnight at 4°C and then analyzed by the enhanced chemiluminescence (ECL) method. HCT-116 cells transfected with GATA-6 full-length cDNA expression vector (pCMV6-XL6-GATA6 vector) were used as a positive control, and histone H1 was used as an internal control. Cells were processed for 15-LOX-1 Western blot analyses, as has been described previously (21)
.
Chromatin immunoprecipitation assays
Caco-2 and HCT-116 cells were transfected with GATA-6 SMARTpool siRNA and nonspecific control siRNA (siGLO RISC-Free siRNA), as described previously. Cross-linking was performed 48 h after transfection by adding formaldehyde to the culture medium to a final concentration of 1% and incubating for 10 min at 37°C. We performed chromatin immunoprecipitation (ChIP) assays using a commercial assay kit, according to the manufacturer’s protocol (Upstate Cell Signaling Solutions, Waltham, MA). Chromatin was immunoprecipitated using an affinity purified goat anti-human GATA-6 pAb (C-20) (Santa Cruz Biotechnology). To amplify a 355-bp fragment of the human 15-LOX-1 promoter, we used the primers 5'-TAC ACA CGT GCA TAA CTC C-3' (sense) and 5'-CCC ATC TTG CTC AAA GAT G-3' (antisense) using the following conditions: 94°C for 3 min and then 94°C for 20 s; 60°C for 30 s; and 71.5°C for 70 s for 28 cycles. We optimized the PCR conditions for the primer set to ensure that the product yield was within the linear range (data not shown).
In vitro cell-survival assays
We measured cell survival for Caco-2 and HCT-116 cells in vitro using the sulforhodamine B (SRB) technique (22)
. Cells were plated in 96-well plates at equal cell densities, and cells were transfected the next day with GATA-6 siRNA, nonspecific siRNA, or transfection medium (Lipofectamine) alone. At 72 and 96 h after transfection, the cells were fixed with trichloroacetic acid and stained with SRB. Optical densities were measured at 490 nm with a microplate reader (Emax; Molecular Devices, Sunnyvale, CA).
Enzyme immunoassay measurements
13-S-HODE levels were measured in cell lysates and cell culture media by use of a commercially available enzyme immunoassay (EIA) kit (Assay Designs, Ann Arbor, MI), as described previously (21)
.
Assessment of apoptosis
We measured apoptosis by caspase-3 activity and DNA fragmentation assays. Cells were plated to 50% confluence in 100 cm2 dishes and were then transfected with GATA-6 siRNA, nonspecific siRNA, or transfection medium alone. Cells were harvested at 72 and 96 h after transfection. Caspase-3 activity and DNA fragmentation assays were performed as described previously (13)
.
Statistical analyses
We used the sign test for nonparametric data analyses for GATA-6 expression (immunohistochemistry and cDNA arrays). We used one-way ANOVA to compare various quantifiable outcome measures (e.g., 15-LOX-1 and GATA-6 expression levels) in different experimental conditions (e.g., GATA-6 siRNA, nonspecific siRNA, or transfection medium alone) after log transformation of data. Data were analyzed with SAS software (SAS Institute, Cary, NC). Reported P values were two-sided, were considered statistically significant at the 0.05 level, and reflected the Bonferroni adjustment for multiple comparisons.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A). GATA-6 immunohistochemical staining (regardless of intensity) was detected in 94% of cancer mucosa compared with only 23% of normal mucosa (P<0.01) (Fig. 1B
). The median intensity of staining was + 2 in the tumors and 0 in the normal tissues. GATA-6 mRNA expression in the colon cancer part of the Cancer Profiling Array I that consisted of cDNA prepared from paired normal and cancerous colon tissues showed that GATA-6 mRNA was overexpressed in the majority of tumors (29 of 38 pairs, 76%; P<0.01) (Fig. 1C
) with a median ratio of normal-to-tumor expression of 0.69 (Fig. 1D
). The normal-to-tumor GATA-6 expression ratio was higher in patients with stages I–III disease [0.92±0.15 (means±SE)] than in patients with stage IV disease (0.466±0.12) (P=0.058).
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GATA-6 knockdown effects on 15-LOX-1 expression
GATA-6 siRNA decreased GATA-6 mRNA expression by 
72% at 24 h and by 78% at 48 h in Caco-2 cells (P<0.0001), as measured by real-time RT-PCR assays (Fig. 2
A). Similarly, GATA-6 siRNA reduced GATA-6 mRNA expression by 75% in HCT-116 cells at 24 and 48 h (P<0.0001) (Fig. 2B
). Transfection with nonspecific siRNA failed to reduce GATA-6 expression [compared with transfection with medium alone (mock)] at either 24 or 48 h in either Caco-2 or HCT-116 cells (Fig. 2A and B
). GATA-6 siRNA transfection also down-regulated GATA-6 protein expression in Caco-2 and HCT-116 cells (Fig. 2C and D
). GATA-6 binding to the 15-LOX-1 promoter was detected in both Caco-2 and HCT-116 cells with ChIP assays, using GATA-6 antibodies in cells transfected with the control (nonspecific) siRNA but not in cells transfected with GATA-6 siRNA (Fig. 2E
). Expression levels for 15-LOX-1 mRNA were similar in cells transfected with nonspecific siRNA or GATA-6 siRNA in either the Caco-2 or HCT-116 cell lines at 24 h (P=0.12 for Caco-2 and P=0.29 for HCT-116) or 48 h (P = 0.13 for Caco-2 and P = 0.54 for HCT-116) (Fig. 2F and G
). As in Caco-2 and HCT-116, GATA-6 down-regulation by GATA-6 siRNA failed to increase 15-LOX-1 mRNA expression levels in RKO or DLD-1 colorectal cancer cells (data not shown). GATA-6 siRNA transfections significantly reduced the expression of human hepatocyte NF 4-alpha (HNF4A), a GATA-6 target gene, compared with this expression in mock- or nonspecific siRNA-transfected cells (data not shown) in Caco-2 cells (P<0.0001) and HCT-116 cells (P<0.0001).
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GATA-6 knockdown effects on colon cancer cell proliferation and apoptosis
Survival rates after GATA-6 siRNA transfection were identical to those of Caco-2 and HCT-116 cells transfected with nonspecific siRNA or mock transfection at 72 h (P=0.2 for Caco-2 and P=0.09 for HCT-116) and 96 h (P=0.23 for Caco-2 and P=0.55 for HCT-116) (Fig. 3
A and B). Apoptosis induction rates as measured by caspase-3 activity at 72 and 96 h were similar in Caco-2 and HCT-116 cells transfected with GATA-6 siRNA, nonspecific siRNA or mock (P=0.95 for Caco-2 and P=0.1 for HCT-116) (Fig. 3C and D
). Treatment of both Caco-2 and HCT-116 cells with NaBT at 5 mM (as positive control) significantly increased caspase-3 activity (P<0.0001 for both). Apoptosis measurement by DNA laddering at 72 and 96 h also showed no differences between Caco-2 or HCT-116 cells transfected with either GATA-6, nonspecific siRNA or mock, whereas treatment with NaBT(5 mM) induced the typical DNA laddering indicative of apoptosis (Fig. 3E and F
).
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GATA-6 knockdown effects on 15-LOX-1 expression and apoptosis induction by NS-398 and NaBT
NaBT at 0.25 mM [markedly lower than the previously reported concentration (5 mM) used to down-regulate GATA-6 and induce apoptosis and 15-LOX-1 expression (14
, 23)
] failed to down-regulate GATA-6 in Caco-2 cells (P=0.079) (Fig. 4
A). GATA-6 siRNA significantly reduced GATA-6 expression in Caco-2 cells with and without NaBT treatment [compared with cells transfected with either nonspecific siRNA or transfection media alone (mock)] (P<0.0001) (Fig. 4A
). GATA-6 siRNA transfection alone failed to increase 15-LOX-1 expression, when compared with mock or nonspecific siRNA transfection (P=0.96) (Fig. 4B
). Caco-2 cells transfected with GATA-6 siRNA and treated with NaBT, however, had significantly higher 15-LOX-1 expression than did cells either transfected with nonspecific siRNA and treated with NaBT or transfected with GATA-6 siRNA but not treated with NaBT (P<0.0001) (Fig. 4B
).
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NS-398 at 30 µM [below the concentration (120 µM) previously reported to down-regulate GATA-6, induce 15-LOX-1 expression, and induce apoptosis (6)
] did not reduce GATA-6 expression in Caco-2 cells (Fig. 4C
). GATA-6 siRNA reduced GATA-6 to similar levels in cells whether treated or not with NS-398 (Fig. 4C
); the reduction was significantly different, however, from mock- or nonspecific siRNA–transfected cells (P<0.0001). Although GATA-6 siRNA transfection alone failed to increase 15-LOX-1 expression (P=0.15) (Fig. 4D
), GATA-6 siRNA transfection plus NS-398 (30 µM) increased 15-LOX-1 expression significantly, compared with cells transfected with nonspecific siRNA plus NS-398 treatment and compared with cells transfected with GATA-6 siRNA but not treated with NS-398 (P<0.0001) (Fig. 4D
). GATA-6 down-regulation alone or combined with either NS-398 or NaBT did not significantly change 15-LOX-2 or 5-LOX RNA expression; COX-2 expression increased modestly in association with GATA-6 siRNA transfection alone or combined with NS-398 (<one-fold) (P<0.0001) but not with NaBT (data not shown). As with GATA-6 siRNA, dbcAMP (2 mM) reduced GATA-6 expression in Caco-2 cells (data not shown) but failed (by itself) to induce 15-LOX-1 expression (P=0.09). dbcAMP (2 mM) combined with NaBT (0.25 mM) increased 15-LOX-1 mRNA expression 3.5-fold compared with control or dbcAMP alone (P=0.0008) (data not shown). GATA-6 siRNA alone failed to induce 15-LOX-1 protein expression, but GATA-6 siRNA combined with a low concentration of either NS-398 or NaBT did induce 15-LOX-1 protein expression (Fig. 4E
) and significantly increased 13-S-HODE, the main product of 15-LOX-1 (vs. GATA-6 siRNA, NS-398, or NaBT alone) (P<0.0001) (Fig. 4F
). The increased 13-S-HODE level was reversed by CAF at 2.2 µM, a concentration that specifically inhibits 15-LOX-1 enzymatic activity (10)
(P<0.0001) (Fig. 4F
). GATA-6 siRNA transfection combined with either NS-398 or NaBT also significantly reduced cell survival (as measured by the number of attached cells and SRB assay) compared with effects of GATA-6 siRNA transfection alone or either agent alone (P<0.0001) (Fig. 4G
for SRB assays; data not shown for cell count), and CAF (2.2 µM) reversed this inhibition of cell survival (P<0.0001) (Fig. 4G
for SRB assays; data not shown for cell count). GATA-6 siRNA transfection without treatment (either NS-398 or NaBT) failed to induce apoptosis, as measured by caspase-3 activation levels (Fig. 4H
) (P = 0.52). NS-398 (30 µM) plus GATA-6 siRNA transfection significantly increased caspase-3 activation levels, compared with NS-398 plus mock or nonspecific siRNA transfections (P<0.0001). Similarly, NaBT (0.25 mM) plus GATA-6 siRNA transfection significantly increased caspase-3 activation levels, compared with NaBT plus mock or nonspecific siRNA transfection (P<0.0001) (Fig. 4H
). CAF, at 2.2 µM concentration, reduced caspase-3 activation levels by GATA-6 siRNA combined with either NS-398 or NaBT to levels that were similar to those produced by GATA-6 siRNA transfection, NS-398, or NaBT alone or by other controls (nonspecific siRNA or mock transfection) (P=0.52) (Fig. 4H
).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We confirmed our findings of GATA-6 overexpression by assessing two different patient groups and using two different methods of measuring expression (RNA and protein). To our knowledge, this is the first report showing differential GATA-6 expression between malignant and normal clinical colorectal mucosa. The ratios of GATA-6 RNA expression in tumors to that in paired normal tissues were approximately twofold higher in patients with distant metastases than in patients with earlier stage disease. The statistical significance of this finding was only borderline, possibly because the sample size was small. If confirmed in larger studies, this finding would indicate an association between the degree of GATA-6 overexpression and the potential for distant metastases in human colorectal tumors.
Although not studied directly, a protumorigenic role (possibly via inhibiting the expression of 15-LOX-1) for GATA-6 has been suggested by prior data. For example, restoring terminal cell differentiation in Caco-2 colon cancer cells is associated with down-regulation of GATA-6 expression (14)
. GATA-6 overexpression inhibits NSAIDs from inducing both 15-LOX-1 expression and apoptosis (6)
. Since the direct role of GATA-6 in tumorigenesis has not been examined previously, we evaluated the effects of reversing GATA-6 overexpression on 15-LOX-1 expression, cell proliferation, and apoptosis in Caco-2 and HCT-116 colon cancer cells. We selected Caco-2 because it has higher GATA-6 expression than do various other colorectal cancer cell lines (data not shown) and because it has the special ability to undergo terminal differentiation with NaBT treatment, thus resembling normal colonic cells (24)
. Caco-2 terminal differentiation has been associated with GATA-6 down-regulation (14)
and mechanistically linked to 15-LOX-1 expression (21)
. For comparison with effects in Caco-2, we used HCT-116 cells, which have one of the lowest levels of GATA-6 expression (data not shown). The siRNA approach allowed us to adequately reduce GATA-6 expression and to block GATA-6 binding to the 15-LOX-1 promoter, where GATA-6 acts as a transcriptional suppressor (14)
. We also measured the expression of HNF4, which is a target gene for the transcriptional activity of GATA-6 (25)
, to further confirm that GATA-6 down-regulation modulated GATA-6 transcriptional activity. GATA-6 knockdown by GATA-6 siRNA significantly reduced HNF4, when compared with mock or nonspecific siRNA transfection, thus confirming that knocking down GATA-6 effectively modulated its transcriptional activity. Contrary to our expectations, however, down-regulating GATA-6 failed to induce 15-LOX-1 expression or induce apoptosis in HCT-116 or Caco-2 colon cancer cells.
We previously reported that GATA-6 ectopic expression in RKO and DLD-1 colorectal cancer cells inhibits the induction of 15-LOX-1 expression by NSAIDs (6)
. As in Caco-2 and HCT-116, however, our present study showed that GATA-6 knockdown failed to reverse 15-LOX-1 transcriptional suppression in RKO or DLD-1 cells. These findings suggested that although GATA-6 is sufficient to initiate 15-LOX-1 transcriptional suppression (6)
, GATA-6 is not the sole transcriptional factor maintaining 15-LOX-1 silencing in cancer cells. Therefore, we examined the relative contribution of GATA-6 toward maintaining 15-LOX-1 transcriptional silencing by evaluating whether specific GATA-6 knockdown by GATA-6 siRNA acts with other transcriptional modulatory events to reverse 15-LOX-1 transcriptional suppression. The HDAC inhibitor NaBT and the NSAID NS-398 (which is a selective cyclooxygenase-2 inhibitor) down-regulate GATA-6 and induce 15-LOX-1 expression and apoptosis at high concentrations (5 mM for NaBT and 120 µM for NS-398) (6
, 14)
. We found that a much lower concentration of NaBT (0.25 mM) or NS-398 (30 µM) failed to down-regulate GATA-6. We examined the effects of combining each drug at the low concentration (that failed to induce GATA-6 down-regulation) with specific GATA-6 down-regulation (via GATA-6 siRNA). These combinations induced 15-LOX-1 RNA and protein expression, 13-S-HODE production, and apoptosis and inhibited cell survival. Down-regulation of GATA-6 was not increased by the combinations over that by siRNA alone, indicating that modulation of transcriptional events other than GATA-6 down-regulation was required to restore 15-LOX-1 transcription. We also used the experimental approach of cAMP treatment to test the effects of down-regulating GATA-6 on 15-LOX-1 transcription. cAMP alone at a concentration reported to induce GATA-6 proteolysis (26)
failed to induce 15-LOX-1 expression. The same concentration of cAMP combined with NaBT (0.25 mM), however, increased 15-LOX-1 expression by 3.5-fold (compared with cAMP treatment alone). These results indicate 1) that GATA-6 down-regulation alone is insufficient to induce 15-LOX-1 expression and apoptosis but significantly enhances the reversal of 15-LOX-1 suppression by an HDAC inhibitor or NSAID, 2) that 15-LOX-1 transcriptional silencing in cancer cells is multifactorial because reversal of 15-LOX-1 silencing required GATA-6 down-regulation combined with the modulation of other transcriptional and/or epigenetic factors by NS-398 or NaBT, and 3) that induction of apoptosis was related to the induction of 15-LOX-1 expression since the reversal of 15-LOX-1 enzymatic activity by CAF [at a concentration that specifically inhibits 15-LOX-1 enzymatic activity (10)
] blocked apoptosis induction. This last conclusion is consistent with our prior findings that 15-LOX-1 expression significantly contributes to apoptosis induction by either NSAIDs or HDAC inhibitors in colorectal cancer cells (11
, 21)
. The specific link between apoptosis and 15-LOX-1 expression induction also was supported by our findings that GATA-6 siRNA transfection combined with either NS-398 or NaBT induced 15-LOX-1 expression but failed to alter the expression of 5-LOX or 15-LOX-2 or to down-regulate COX-2 expression. HDAC inhibitors such as NaBT and NSAIDs such as NS-398 are known to influence various transcriptional/epigenetic factors such as histone acetylation and protein kinase G (12
, 26)
. These and other factors, such as signal transducers and activators of transcription-6 (27
, 28)
and methylation (29)
, have been reported to individually influence 15-LOX-1 transcriptional regulation in cancer cells.
Our current findings demonstrate that the transcriptional regulation of 15-LOX-1 is combinatorial and that the modulation of more than one individual factor is required to reverse 15-LOX-1 suppression in cancer cells. These results provide a rationale for evaluating therapeutic strategies using GATA-6-targeting interventions combined with NSAIDs and HDAC inhibitors. Both classes of agents have shown promising antitumorigenic activity in the clinic. They also appear to have limited efficacy, however, and certainly have important potential adverse effects (e.g., cardiovascular toxicity of NSAIDs) that would limit their use at high doses (30
31
32)
. Both limitations may be improved by adding GATA-6-inhibiting approaches. Future mechanistic studies of the other transcriptional and/or epigenetic factors that significantly contribute to maintaining 15-LOX-1 transcriptional silencing will further elucidate the mechanisms of this silencing and thus may lead to improved approaches for treating colonic tumorigenesis.
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ACKNOWLEDGMENTS |
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Received for publication July 12, 2006. Accepted for publication September 29, 2006.
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