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Molecular Oncology |
Second Department of Surgery, Shimane Medical University, Izumo 693-8501, Japan
| ABSTRACT |
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| INTRODUCTION |
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COX, constitutively expressed COX-1, and inducible COX-2, are the rate-limiting enzymes for production of prostanoids (prostaglandins and thromboxanes) from arachidonic acid (6) . COX-2 has recently been categorized as an immediate-early gene associated with inflammation (7) , cellular growth (8) , differentiation (9) , prevention of apoptosis (9) , and tumorigenesis (3) . It has also been reported that COX-2 induces angiogenesis, which is essential for tumor growth (10) . Increased prostaglandin production and enhanced release of angiogenic growth factor by COX-2 may induce neovascularization (3 , 8) .
Evidence suggests that NO produced by iNOS enhances the activity of COX-2 (11) . Expression of both iNOS and COX-2 has been reported to be increased in Barretts esophagus and esophageal adenocarcinomas and also in Helicobacter pylori-induced gastritis (12 , 13) . NO and COX-2 have carcinogenic effects achieved either directly or by producing mediators that regulate cellular growth (10 , 14) . These effects may be mediated by the formation of peroxinitrite and prostanoids and involve "cross-talk" between the two enzyme systems (15) . To determine the roles of iNOS and COX-2 in HCC, we compared their expressions by the immunohistochemical method.
| MATERIALS AND METHODS |
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Immunohistochemical Staining of iNOS, COX-2, and CD34.
Tumor samples free of necrosis or hemorrhage on macroscopic inspection were selected for histology. Samples were routinely fixed in 10% formalin and subsequently embedded in paraffin. Serial sections 4-µm thick were prepared from paraffin blocks. Sections were deparaffinized and hydrated by sequential immersion in xylene and graded alcohol solutions. The sections were then incubated in 3% H2O2 for 30 min to block the endogenous peroxidase activity. For iNOS staining, slides were incubated with 2% skim milk for 10 min. Slides were treated with normal serum obtained from the same species in which the secondary antibody was developed for 30 min to block nonspecific staining. Subsequently, slides were incubated with primary antibodies [anti-iNOS (WAKO Pharmaceuticals Ltd., Osaka, Japan) at 1:500 dilution for 60 min at room temperature; anti-COX-2 (Cayman Chemical, Ann Arbor, MI) at 1:300 dilution overnight at 4°C; and anti-CD34 (Dako, Glostrup, Denmark) at 1:50 dilution for 1 h at room temperature], as appropriate. Immunostaining was performed according to the avidin-biotin peroxidase complex method by a commercially available kit [Histofine, SAB-PO(R); Nichirei Corporation, Tokyo, Japan]. Slides were treated with a biotin-conjugated secondary antibody for 30 min followed by incubation with peroxidase-conjugated streptavidin for 30 min at room temperature. All steps were followed by washing in PBS. Peroxidase activity was detected by incubating the samples with 3-amino-9-ethylcarbazol (Histofine, Tokyo, Japan). Finally, the sections were counterstained with hematoxylin. For CD34 staining, counter staining was not done.
Evaluation of iNOS and COX-2 Expression.
The degree of staining was categorized by the extent and intensity of the staining. Two independent observers screened all sections as a semiquantitative evaluation of iNOS and COX-2 immunostaining. The immunoreactive score was determined by the sum of extension and intensity as reported previously (18)
The intensity of staining was scored on a scale of 0 to 3, in which 0 = negative staining, 1 = weakly positive staining, 2 = moderately positive staining, and 3 = strongly positive staining. The extent of positivity ("extent of distribution" of positive cells) was estimated on a scale of 0 to 4, in which 0 = negative, 1 = positive staining in 125% of cells, 2 = positive staining in 2650%; 3 = positive staining in 5175%; and 4 = positive staining in 76100%. The combined staining score (extension + intensity)
3 was considered as positive staining.
Quantitation of MVD.
Because of the technical difficulty in counting linearly immunostained sinusoids, we used an image analysis system to assess the MVD as a percentage of the endothelial area. Because the immunoreactivity of CD34 showed slight heterogeneity within the same tumor, the five most highly vascularized areas (hot spots) were selected in x200 magnification fields. The images were scanned with a Olympus CCD camera and stored on a computer for subsequent analysis. On the computer screen image, the background color was subtracted and the 3,3'-diaminobenzidine-stained area was evaluated using the NIH (Bethesda, MD) image analysis system (19)
. A MVD value
3 (median value of the MVD) was considered high MVD.
Statistical Analysis.
Correlation between factors was evaluated using the Spearman rank correlation coefficient test. Survival rates were obtained by the Kaplan-Meier method, and the differences were compared by the log-rank and Wilcoxon tests. Significance differences between categorical variables were compared by the
2 test or Fishers exact probability test when appropriate. Continuous variables were compared by the Mann-Whitney U test. Multivariate analyses was performed by the Cox proportional hazards regression analysis using StatView 4.11 (Abacus Concepts, Inc., Berkeley, CA) software on a Macintosh computer. P < 0.05 was considered significant.
| RESULTS |
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4 cm in diameter (P = 0.042) and also in tumors with positive iNOS staining scores (P = 0.034; Table 2
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60 years), combined iNOS and COX-2 expression (positive group versus negative group), liver cirrhosis, serum
-fetoprotein (<100 ng/ml versus
100 ng/ml), and differentiation (well, moderate versus others). Four factors also had prognostic impact on recurrence-free survival: age (<60 versus
60 years), combined iNOS and COX-2 expression (positive group versus negative group), liver cirrhosis, and
-fetoprotein (<100 ng/ml versus
100 ng/ml). Combined iNOS and COX-2 expression in HCV-positive HCC cases remained an independent prognostic factor both in the recurrence-free and overall survival analyses (Table 3)
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| DISCUSSION |
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4 cm in diameter in HCV-positive HCCs. COX-2 expression is also significantly higher in larger tumors in colorectal cancer (25)
. Prostanoid levels in adenomas from patients with familial adenomatous polyposis are significantly elevated with increased tumor size (26)
. Perhaps a similar mechanism is responsible for higher COX-2 expression in larger HCCs.
We have found positive correlations between COX-2 and iNOS expression and MVD in HCV-positive HCCs. This suggests that both iNOS and COX-2 might be important factors in the pathogenesis of HCV-positive HCCs. Higher expressions of the iNOS and COX-2 genes in HCV-positive HCCs might be caused by a secondary effect of cytokines produced in response to HCV infection (27
, 28)
or by the direct activation of the HCV core protein (29)
. In particular, both genes have nuclear factor
B binding sites in their promoter regions (29)
, and HCV is known to stimulate the activation of nuclear factor
ß (30)
. Evidence suggests that iNOS is capable of inducing COX-2 expression, and there is a possibility of cross-talk between the iNOS and COX-2 systems, producing a synergistic effects in tumorigenesis (15)
.
It has been reported that HCV infection causes elevated iNOS transcription (31)
. Kane et al. (32)
showed a significant up-regulation of iNOS in HCV-positive hepatitis and suggested that enhanced iNOS expression might be responsible for carcinogenesis in HCV-positive chronic hepatitis. We also found higher positive iNOS expression in HCV-positive HCC cases. In our study, iNOS expression was significantly higher in the surrounding liver in cirrhotic patients. In addition, it has been reported that iNOS expression becomes higher in cirrhotic liver in animal models (33)
. It has been suggested that in cirrhosis, the endotoxemia and/or the increased circulating levels of cytokines may induce iNOS expression. Aberrant iNOS expression may be one of the phenotypical changes associated with the carcinogenic process in the cirrhotic liver. It might also be possible that the increased iNOS expression in the surrounding cirrhotic liver was induced by some factors released by the tumor itself. Another interesting finding of this study was a negative correlation, although not statistically significant, between iNOS expression and the serum AFP level. It was reported that the transgenic mice that expressed and produced human AFP had significantly reduced production of IFN-
in their livers and sera. (34)
. IFN-
induces iNOS; it therefore might be possible that higher AFP-secreting tumors down-regulate iNOS production through IFN-
. In addition, reduced plasma nitrite/nitrate levels were reported in AFP-positive patients compared with AFP-negative patients (35)
.
The biological significance of high iNOS and COX-2 expression remains controversial and poorly understood in cancers. Most of the studies indicate that high concentrations of NO and prostanoids can be either mutagenic or tumorigenic and that these effects depend on their concentrations (36 , 37) . iNOS and COX-2 expression is up-regulated in different types of premalignant and malignant lesions (3, 4, 5 , 8) . Both iNOS and COX-2 are abundantly expressed in premalignant Barretts esophagus (12) and in H. pylori-related gastritis (13) . We could not find any impact of iNOS and COX-2 expression on disease-free survival in all patients, but in HCV-positive cases, patients with negative expression of both iNOS and COX-2 had a significant survival advantage over the rest of the patients with variable expression of iNOS and/or COX-2. Recently, Kondo et al. (38) reported that overexpression of COX-2 in the surrounding liver was associated with a shorter disease-free survival in patients with HCV-positive HCCs.
HCC is the leading cancer in men in Taiwan and is one of the most common causes of malignancy-related death in Africa and Asia (39) . Unfortunately, to date, there is no effective preventive measure for this highly malignant disease. Many reports have indicated that COX-2 is up-regulated in most human tumors (20) . The clinical data of this study indicate that COX-2 expression may play a role in tumor angiogenesis because a direct correlation between the COX-2 expression score and MVD was observed in HCV-positive HCC cases. Recently, we have shown that COX-2 inhibitors induce growth arrest and apoptosis in several HCC-derived cell lines (40) . All of these findings indicate that COX-2 inhibitors might be effective in prevention of both cancer development and disease progression of HCC. Further research in this direction may elucidate the roles of iNOS and COX-2 in the development of HCC, which may form the basis for future novel therapeutic and/or preventive strategies to inhibit these genes.
| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at Second Department of Surgery, Shimane Medical University, Izumo 693-8501, Japan. Phone: (81) 853-202235; Fax: (81) 853-202229; E-mail: rahman{at}shimane-med.ac.jp ![]()
2 The abbreviations used are: NOS, nitric oxide synthase; iNOS, inducible NOS; COX, cyclooxygenase; HCC, hepatocellular carcinoma; HCV, hepatitis C virus; AFP,
-fetoprotein; MVD, microvessel density. ![]()
Received 10/18/00; revised 1/30/01; accepted 2/ 2/01.
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