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Clinical Cancer Research Vol. 12, 6444-6451, November 1, 2006
© 2006 American Association for Cancer Research

Imaging, Diagnosis, Prognosis

Peroxisome Proliferator-Activated Receptor {gamma} Is Highly Expressed in Pancreatic Cancer and Is Associated With Shorter Overall Survival Times

Glen Kristiansen1, Juliane Jacob1, Ann-Christin Buckendahl1, Robert Grützmann3, Ingo Alldinger3, Bence Sipos4, Günter Klöppel4, Marcus Bahra2, Jan M. Langrehr2, Peter Neuhaus2, Manfred Dietel1 and Christian Pilarsky3

Authors' Affiliations: 1 Institute of Pathology and 2 Department of Surgery, Charité University Hospital, Berlin, Germany; 3 Department of Visceral, Thoracic, and Vascular Surgery, University Hospital Dresden, Dresden, Germany; and 4 Institute of Pathology, University of Schleswig-Holstein Campus Kiel, Kiel, Germany

Requests for reprints: Glen Kristiansen, Institute of Pathology, Charité University Hospital, Schumannstrasse 20/21, 10117 Berlin, Germany. Phone: 49-30-450-53-6145; Fax: 49-30-450-53-6945; E-mail: glen.kristiansen{at}charite.de.


   Abstract
Top
 Abstract
Materials and Methods
 Results
 Discussion
 References
 
Purpose: Peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}) is a ligand-activated transcription factor that has been implicated in carcinogenesis and progression of various solid tumors, including pancreatic carcinoma. We aimed to clarify the expression patterns of PPAR{gamma} in pancreatic ductal carcinomas and to correlate these to clinicopathologic variables, including patient survival.

Experimental Design: Array-based expression profiling of 19 microdissected carcinomas and 14 normal ductal epithelia was conducted. Additionally, Western blots of pancreatic cancer cell lines and paraffinized tissue of 129 pancreatic carcinomas were immunostained for PPAR{gamma}. For statistical analysis, Fisher's exact test, {chi}2 test for trends, correlation analysis, Kaplan-Meier analysis, and Cox's regression were applied.

Results: Expression profiles showed a strong overexpression of PPAR{gamma} mRNA (change fold, 6.9; P = 0.04). Immunohistochemically, PPAR{gamma} expression was seen in 71.3% of pancreatic cancer cases. PPAR{gamma} expression correlated positively to higher pT stages and higher tumor grade. Survival analysis showed a significant prognostic value for PPAR{gamma}, which was found to be independent in the clinically important subgroup of node-negative tumors.

Conclusions: PPAR{gamma} is commonly up-regulated in pancreatic ductal adenocarcinoma and might be a prognostic marker in this disease. Both findings corroborate the importance of PPAR{gamma} in tumor progression of pancreatic cancer.

Pancreatic ductal adenocarcinoma (PDAC) confers one of the highest mortality rates in malignant human tumors. In 2005, the estimated incidence for this disease in the United States alone reached 32,180 cases, with an equally high rate of deaths related to this disease (1). The high disease-specific mortality is due to the fact that pancreatic carcinomas lack early symptoms. Consequently PDACs are often detected when the local tumor extent is high and metastatic spread to regional lymph nodes or to distant organ sites has already occurred. Therefore, only ~20% of patients are eligible for radical surgery. To date, no satisfactory progress in the treatment of pancreatic adenocarcinomas has been made and gemcitabine is the only substance that results in a short gain of patient survival time (2, 3). Most chemotherapeutic regimens target the proliferative potential of tumor cells, which is initially effective in many instances because uncontrolled proliferation is a hallmark of these cells. To specifically target other properties of tumor cells, a variety of candidate genes are being screened for their importance to tumor biology and eligibility as target genes for therapy. One of these candidates is peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}). PPAR{gamma} is a member of the nuclear receptor family of ligand-activated transcription factors of which three subtypes ({alpha}, ß, and {gamma}) have been identified thus far, which show different tissue distributions and ligand specificities (4). PPAR{gamma} was initially recognized as a key regulator of adipogenic differentiation and glucose homeostasis. Recent data supported evidence for participation of PPAR{gamma} in the biological mechanisms underlying the carcinogenic evolution, by affecting proliferation and differentiation of cancer cells in vitro and in vivo (5, 6). PPAR{gamma} expression has been described in a variety of human malignancies, including, among others, breast cancer, prostate cancer, glioblastoma cell lines, non–small cell lung carcinoma, ovarian cancer, and pancreatic carcinoma (7–13). To our knowledge, a detailed description of PPAR{gamma} expression in a larger cohort of clinically characterized pancreatic cancer cases is still missing. Therefore, we aimed to investigate the expression of PPAR{gamma} in a collection of pancreatic carcinomas and to correlate the data with clinicopathologic variables, including patient survival.


   Materials and Methods
Top
 Abstract
 Materials and Methods
Results
 Discussion
 References
 
Patients. The study was carried out with institutional review board consent using surgically resected tissue of 129 patients with PDAC. Of these, 95 cases had surgery in Berlin between the years 1991 and 2000, whereas 34 cases underwent surgery in Dresden. Additionally, frozen tissue from 33 patients undergoing pancreatic surgery was included for expression profiling. The study group for immunohistochemistry consisted of 70 male and 59 female patients whose age ranged from 34 to 80 years at the time of diagnosis (median, 64). Complete follow-up data were available for all cases. The overall survival time ranged from 2 to 492 weeks (median, 72). One hundred two patients died during follow-up after a median overall survival time of 55 weeks.

Tumor histology was determined according to the criteria of the WHO. All cases were ductal adenocarcinomas, which were predominantly localized in the head of the pancreas (n = 118; 91.5%), four tumors were in corpus/cauda region, and in seven cases, the papilla duodeni major was involved. The stage of tumors was assessed according to International Union Against Cancer. The clinicopathologic characteristics of the tumor cohort are described in Table 1 .


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  		Table 1. Clinicopathologic variables and PPAR{gamma} expression of the tumor cohort

 
Cell lines. Pancreatic cancer cell lines were obtained from American Type Culture Collection: (Wesel, Germany); AsPC-1 was originally isolated from ascites of a patient with a G2 PDAC. Mia Paca-2, PANC1, and PancTuI stemmed from weakly differentiated (G3) primary PDACs. Capan-1 was isolated from a lymph node metastasis of a PDAC patients, and Colo357 and Panc89 were from liver metastases.

Expression profiling. We analyzed the expression of microdissected PDACs from 19 patients and microdissected normal duct cells from 14 patients undergoing pancreatic surgery for nonmalignant diseases, as described before. Briefly, freshly frozen tissue samples of PDAC were obtained from surgical specimens of patients who were operated at the Department of Visceral, Thoracic, and Vascular Surgery, Carl Gustav Carus, Technical University of Dresden (Dresden, Germany), and the Department of General Surgery, University of Kiel (Kiel, Germany) between 1996 and 2003. Normal pancreatic tissue was obtained from 14 patients who underwent pancreatic resection for other pancreatic diseases. These tissues were histologically normal tissues without dysplastic changes in the ducts and were taken from the distal parts of the resected pancreas. PDAC cells and normal ductal cells were dissected manually using a sterile injection needle. The estimated cellularity was 10,000 to 11,000 cells per microdissected sample. The cellularity of the dissections was ~95%. The RNA was extracted, amplified, and hybridized to the U133 GeneChip set as described previously (14). The obtained data were normalized and condensed using the dChip software,5 which also was used to generate the heatmap. Identification of differential expression was done using the SAM software.6 Genes were assigned as differential expressed if the fold change was larger than two and the q value was below 5% resulting in a set of differentially expressed genes for PDAC. Pathway analysis of genes interacting with PPAR{gamma} was done using PathwayAssist 3.0 (Stratagene, Amsterdam, The Netherlands). Such genes were used to identify PPAR{gamma}-related genes within the set of differentially expressed genes.

Western blot of PPAR{gamma}. Pancreatic cell lines were cultivated in RPMI 1640 with 10% FCS (Invitrogen, Karlsruhe, Germany), lysed using Novex LDS sample buffer, and subjected to gel electrophoresis using the Novex Nupage system (Invitrogen, Karlsruhe, Germany). After completion, the proteins were transferred to a nitrocellulose membrane (Amersham Pharmacia Biosciences, Freiburg, Germany). PPAR{gamma} was detected by using the PPAR{gamma}-specific antibody (E8, Santa Cruz Biotechnology, Santa Cruz, CA) diluted 1:100 and the enhanced chemiluminescence plus Western blot detection system (Amersham Pharmacia Biosciences). After detection, the blots were stripped and ß-actin was detected by using a mouse monoclonal antibody (Abcam, Heidelberg, Germany) diluted 1:5,000. The secondary antibody was diluted 1:25,000 in both cases.

Quantitative reverse transcription-PCR of PPAR{gamma} in cell lines. For the determination of PPAR{gamma} expression in PDAC cell lines, total RNA was prepared and total RNA (5 µg) reverse transcribed was done using SuperScript II reverse transcriptase and random hexamer primers (Invitrogen, Heidelberg, Germany) according to the manufacturer's suggestion. The genes were amplified with the SYBR Green Universal PCR Master Mix according to the manufacturer's instructions, with the ABI Prism 5700 Sequence Detection System (Weitenstadt, Germany) using the following primers: PPAR{gamma}, 5'-atgacagcgacttggcaata-3' (forward) and 5'-gaatgtcttcaatgggcttca-3' (reverse); ß-actin, 5'-aatgctatcacctcccctgtgt-3' (forward) and 5'-aagccaccccacttctctctaa-3' (reverse).

Immunohistochemistry. Formalin-fixed paraffin-embedded tissue was freshly cut into slices of 4 µm. The sections were mounted on Superfrost slides (Menzel Gläser, Braunschweig, Germany), dewaxed with xylene, and gradually hydrated. Antigen retrieval was achieved by pressure cooking in 0.01 mol/L citrate buffer for 5 minutes. The primary PPAR{gamma} antibody (E8) was diluted 1:75 using a background reducing dilution buffer (DAKO, Hamburg, Germany). No other blocking agents were used. The primary antibody was incubated at room temperature for 1 hour. As a negative control, two slides were processed without primary antibody. Detection took place by the conventional labeled streptavidin-biotin (DAKO) method with alkaline phosphatase as the reporting enzyme according to the manufacturer's instructions. Fast Red (Sigma-Aldrich, Munich, Germany) served as chromogen. Afterwards, the slides were briefly counterstained with hematoxylin and aqueously mounted.

Evaluation of the immunohistochemical stainings. The slides were evaluated by two clinical pathologists. Classification of nuclear immunoreactivity was based on staining intensity and the amount of positive tumor and graded as 0, no staining; 1, weak; 2, moderate; and 3, strong. The additional cytoplasmic staining seen in a few cases was not evaluated.

Statistical analysis. The data were compiled with the software package SPSS, version 12.0 (Chicago, IL). Fisher's exact and {chi}2 tests were used to assess the statistical significance of the association between expression of PPAR{gamma} and clinicopathologic variables. Correlation analysis comprised Pearson's analysis for the array data and Spearman's analysis for the immunohistochemistry data. Univariate survival analysis was done according to Kaplan-Meier, and differences in survival curves were assessed with the log-rank test. Ps < 0.05 were considered significant. All statistics were accredited by the head biostatistician of the Tumor Centre, Charité University Hospital (Berlin, Germany).


   Results
Top
 Abstract
 Materials and Methods
 Results
Discussion
 References
 
Expression profiling
To analyze the expression of PPAR{gamma} in PDAC, we evaluated the obtained gene expression profiles from microdissected samples of normal ductal epithelia and adenocarcinomas. PPAR{gamma} was overexpressed in ductal adenocarcinomas by a factor of (median) 6.92 (P = 0.04; Fig. 1 ). We were then interested if target genes of PPAR{gamma} or genes regulating PPAR{gamma} function were also differentially expressed. These genes were identified by the PathwayAssist program. Of the eight identified target genes, (CEACAM5, MCM7, AGR2, JUND, AGTR1, LOR, NR5A2, and RORC) three are also overexpressed in pancreatic cancer (CEACAM5, MCM7, and AGR2), whereas the others are underexpressed. Of seven genes known to regulate PPAR{gamma} function (STRA13, EGR1, CEBPD, BMPR1A, BMP2, EGF, and FGFR1), the activator BMPR1A and the inhibitor STRA13 were up-regulated in PDAC, whereas the others were down-regulated (Table 2 ). In a stratified correlation analysis of PPAR{gamma} with these 15 genes in normal and tumor tissue, we found quite divergent results: in normal tissue, PPAR{gamma} was significantly correlated with CEACAM5 [correlation coefficient (cc), 0.997; P < 0.001], MCM7 (cc, 0.740; P = 0.002), JUND (cc, –0.725; P = 0.002), STRA13 (cc, 0.784; P = 0.001), EGR1 (cc, –0.570; P = 0.026), CEBPD (cc, –0.556; P = 0.032), BMPR1A (cc, 1; P < 0.001), and NR0B2 (cc, –0.526; P = 0.044). In tumor tissue, PPAR{gamma} correlated only to CEACAM (cc, 0.478; P = 0.033), JUND (cc, 0.554; P = 0.011), and FGFR1 (cc, 0.449; P = 0.047). This discrepancy of different correlations in normal and malignant tissue suggests an impaired function of PPAR{gamma} in tumor tissue. The coactivator of PPAR{gamma} retinoid X receptor {alpha} was not differentially expressed in PDAC (Fig. 2A and B ; Table 2).


Figure 1
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  		Fig. 1. Box plot of arbitrary fluorescent units of PPAR{gamma} expression in microdissected pancreatic tissue. T, expression in microdissected PDAC; N, expression in microdissected normal duct epithelia. To confirm the differential expression, t test was applied (P = 0.04).

 

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  		Table 2. PPAR{gamma}-associated genes differentially expressed in PDAC identified using gene expression analysis of microdissected samples

 

Figure 2
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  		Fig. 2. Expression of PPAR{gamma} and related genes in PDAC. A, PPAR{gamma} pathway containing PPAR{gamma} target genes in the lower part and genes that regulate PPAR{gamma} function in the upper part. Left of PPAR{gamma}, the coactivators of PPAR{gamma}. CCND1 is depicted on the right of PPAR{gamma} because it appears that it regulates and is regulated by PPAR{gamma} (green, underexpressed; red, overexpressed; blue, not differentially expressed; +, positive regulation; –, negative regulation). B, heatmap of PPAR{gamma}-related genes (red, high expression; blue, low expression).

 
Western blot and quantitative reverse transcription-pCR
Western blot of PPAR{gamma} with the E8 antibody revealed a specific band between 50 and 60 kDa, as expected (Fig. 3A ). Expression of PPAR{gamma} protein corresponded well with the expression levels of PPAR{gamma} mRNA as seen on the microarrays (Fig. 2) and CT values of the reverse transcription-PCR (Fig. 3B). This indicates that the differences in expression seen in the primary tissue is represented in pancreatic cancer cell lines and also shows that the RNA and protein level of PPAR{gamma} are correlated.


Figure 3
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  		Fig. 3. Detection of PPAR{gamma} by Western blot and quantitative reverse transcription-PCR in pancreatic cancer cell lines. A, top lane, PPAR{gamma}; bottom lane, ß-actin, which served as a loading control. Expression of PPAR{gamma} protein corresponded well with the expression levels of PPAR{gamma} mRNA as seen on the microarrays (Fig. 2). B, expression of PPAR{gamma} RNA in PDAC cell lines normalized to Capan-1 expression in percent. For PPAR{gamma}, quantitative reverse transcription-PCR was done and the obtained CT values were first normalized to ß-actin as housekeeping gene. The obtained {Delta}CT values were then normalized to the {Delta}CT value from the Capan-1 cell line, which was arbitrarily set to 100%.

 
PPAR{gamma} immunostaining in pancreatic tissue
To validate the observed overexpression of PPAR{gamma} in PDAC, we did immunohistochemistry using a mouse monoclonal antibody. Within normal pancreatic acinar epithelium, staining for PPAR{gamma} was not detected. Normal ductal epithelium was mostly PPAR{gamma} negative or showed a very weak nuclear or cytoplasmic staining (Fig. 4A ). Intraductal proliferations with atypia diagnostic of pancreatic intraepithelial neoplasia (PanIN) occasionally exhibited a nuclear pronounced PPAR{gamma} expression (Fig. 4B). In invasive carcinomas, a heterogenous nuclear immunoreactivity was noted, which reached from weak to strong (Fig. 4C and D). In total, 71.3% of carcinomas were PPAR{gamma} positive, 28.7% of cases were PPAR{gamma} negative. The positive carcinomas were further classified into weak (32.6%), moderate (25.6%), and strong (13.2 %) expression of PPAR{gamma}. For statistical analysis, we grouped carcinomas, which had either no or a weak PPAR{gamma} expression together (low level group, 61.3% of cases) and opposed them to cases with a moderate or strong expression of PPAR{gamma} (high level group, 38.7% of cases).


Figure 4
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  		Fig. 4. PPAR{gamma} immunohistochemistry. A, normal pancreatic parenchyma. No significant PPAR{gamma} expression is seen in normal ductal epithelium (central), whereas acinar epithelia exhibit a very weak cytoplasmic staining. B, moderate nuclear PPAR{gamma} expression in atypical epithelia of a PanIN lesion. C and D, moderate/strong nuclear PPAR{gamma} expression in pancreatic adenocarcinoma.

 
In cross-tables, we did not find any significant association of PPAR{gamma} staining intensity with patient age, nodal status, lymphangiosis or hemangiosis carcinomatosa, and histologic grade. However, there is a significant association of higher PPAR{gamma} expression levels with higher pT stages (Table 1). However, Spearman's rank correlations showed a significant correlation of PPAR{gamma} immunoreactivity with tumor grade and pT stage (Table 3 ).


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  		Table 3. Correlation of PPAR{gamma} protein expression in pancreatic cancer with conventional clinicopathologic variables

 
PPAR{gamma} expression and patient survival
Univariate survival analysis. In univariate survival analyses, cumulative survival curves were calculated according to the Kaplan-Meier method. Differences in survival were assessed with the log-rank test. The conventional prognostic markers histologic tumor grade, nodal status, and patient age, but also PPAR{gamma} immunoreactivity reached significance for different overall survival times (Fig. 5A ; Table 4 ). No significant differences in survival time became apparent for patient gender or pT stage.


Figure 5
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  		Fig. 5. Survival analysis. Kaplan-Meier analysis of overall survival times stratified according to PPAR{gamma} expression in patients with pancreatic adenocarcinomas (dotted line, low level; continuous line, high levels). A, all cases (n = 129). B, subgroup of pT3/pT4 cases (n = 85). C, subgroup of G3 cases (n = 53). D, subgroup of node-negative cases (n = 46).

 

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  		Table 4. Kaplan-Meier survival analysis of PPAR{gamma} expression and selected tumor variables

 
Survival analysis in patient subgroups
To assess if PPAR{gamma} might bear a pronounced prognostic effect in patient subgroups, we conducted an extensive Kaplan-Meier analysis of PPAR{gamma} expression stratified for pT stage, tumor grading, and nodal status. In the subgroup of pT3/pT4 tumors, high levels of PPAR{gamma} expression were associated with shorter overall survival times (median survival time, 97 versus 56 weeks; P = 0.001; Fig. 5B), which was not found in the group of pT1/pT2 tumors (P = 0.243). In the subgroup of G1/G2 tumors, high levels of PPAR{gamma} expression were associated with shorter overall survival times (median survival time, 128 versus 77 weeks; P = 0.011; Fig. 5C), which was not found in the group of G3 tumors (P = 0.726). Stratifying for nodal status, in node-negative cases, survival time was significantly shorter in the group of tumors with high levels of PPAR{gamma} expression (median survival time, 148 versus 52 weeks; P = 0.012; Fig. 5D), which again was not seen in node positive cases (P = 0.964).

Multivariate survival analysis
In the Cox regression model, we included pT stage (pT1/pT2 versus pT3/pT4), tumor grade (G1/G2 versus G3), patient age, nodal status (pN0 versus pN1), and PPAR{gamma} expression (0/1+ versus 2/3+). In the analysis of all cases, overall survival time was significantly dependent on nodal status, patient age, and tumor grade (Table 5 ). Analogous to the stratified univariate analysis, the Cox regression analysis in the subgroup of node-negative patients (n = 46) revealed that PPAR{gamma} expression was a significant indicator for shortened patient survival (relative risk, 2.46; P = 0.039) as shown in Table 6 .


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  		Table 5. Cox regression model, including conventional variables and PPAR{gamma} expression in all cases (n = 129)

 

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  		Table 6. Cox regression model, including conventional variables and PPAR{gamma} expression in the subgroup of node-negative cases (n = 46)

 

   Discussion
Top
 Abstract
 Materials and Methods
 Results
 Discussion
References
 
PPAR{gamma} belongs to the nuclear hormone receptor superfamily that includes the receptors for steroid, thyroid hormone, vitamin D, and retinoid acids (15). Activated by binding of specific ligands, PPAR{gamma} forms a complex with retinoid X receptors (16). This complex binds to so-called peroxisome proliferator response elements in target genes, which activates transcription. PPAR{gamma} is expressed at high levels in fat tissue but has also been described in other tissues (e.g., muscle, adrenal gland, and liver; refs. 17–20). PPAR{gamma} functions in many processes related to cellular development proliferation and differentiation. PPAR{gamma} is expressed in the majority of malignant human tumors, ranging from hematologic malignancies (21, 22) to solid tumors [e.g., gastric and colon cancer (23, 24), prostate, breast and ovarian cancer (8, 11, 25), non–small cell lung cancer (10), urothelial carcinoma (26), renal cell carcinoma (27), thyroid cancer (28, 29), esophageal squamous cell carcinoma (30), osteosarcoma (31), liposarcoma (32), glioma (33), and neuroblastoma (34)]. Administration of synthetic PPAR{gamma} ligands (thiazolidindiones) has shown marked antineoplastic effects on various cell lines and tumors. PPAR{gamma} agonists are antiproliferative, at least in part by up-regulating cyclin-dependent kinase inhibitors and they induce apoptosis in some tumor cell lines (35–37) or differentiation of cells in others (25, 38). This might be explained by the stimulation of overexpressed but inactive PPAR{gamma} in tumors and might also be the case in PDAC because we observed overexpression of only three typical PPAR{gamma} target genes in PDAC and also an underexpression of five PPAR{gamma} target genes. This is also mirrored in the loss of correlations of PPAR{gamma} with PPAR{gamma}-associated genes we found in tumor tissue. These data indicate that other signal transduction pathways might be involved in regulating PPAR{gamma} target gene expression. Interestingly, STRA13, a gene reported to negatively regulate PPAR{gamma}, is overexpressed in PDAC. This overexpression might result in an inactivation of PPAR{gamma} and might not be compensated by the overexpression of the PPAR{gamma} activator BMPR1A resulting in PPAR{gamma} inactivation.

The stock of literature on PPAR{gamma} in pancreatic cancer thus far focused on cell lines and small tumor cohorts. Eibl et al. (35) found PPAR{gamma} expressed in six pancreatic cancer cell lines (AsPC-1, BxPC-3, Capan-2, HPAF-II, MIA Paca-2, and PANC1) and could show a decreased cell viability by treatment with PPAR{gamma} agonists, which was time and dose dependent. Farrow et al. describe PPAR{gamma} expression in the cell lines AsPC-1, SUIT-2, BxPC-3, and MIA Paca-2 and conducted invasion assays under PPAR{gamma} agonist treatment. They found a reduced invasiveness and reduced levels of proinvasive factors as tissue plasminogen activator, matrix metalloproteinase 3, matrix metalloproteinase 7, and urokinase-type plasminogen activator (39, 40). Sasaki et al. (12) found PPAR{gamma} mRNA in five of seven pancreatic carcinoma cell lines and also in five of seven cancer samples, whereas all four adjacent normal tissues showed no expression. Functionally, they could show an inhibition of anchorage-independent growth of cell lines. Motomura et al. described PPAR{gamma} expression on RNA and protein level in 4 of 4 cell lines and in 9 of 10 pancreatic cancer samples. No expression was found in normal ductal epithelium. Their immunohistologic analysis clearly showed the nuclear pattern that is to be expected of a transcription factor and which we found as well (41). In addition, a dose-dependent inhibition of cell growth by troglitazone accompanied by an up-regulation of p27 was seen. Because this could be blocked by p27 antisense oligonucleotides, the authors conclude that the PPAR{gamma} effect on cell growth is p27 mediated. Itami et al. found PPAR{gamma} expressed in 5 of 5 cell lines and conducted immunohistochemistry on 47 primary pancreas cancer samples and 15 liver metastases. Seventy-five percent of primary tumors and 80% of metastases showed a high rate of PPAR{gamma} expression. Unfortunately, no further correlation of PPAR{gamma} with clinicopathologic variables of the primary tumors was investigated (42).

To our knowledge, our study is the first description of PPAR{gamma} expression in a larger cohort of clinically characterized primary pancreatic tumors by immunohistochemistry. We used a monoclonal PPAR{gamma} antibody, which is applicable to paraffinized tissue and enabled us to analyze clinically characterized archive material. This antibody is widely used in PPAR{gamma} research (23, 28, 36, 42) and yields clear immunohistologic stainings we found reproducible and easy to analyze. We saw PPAR{gamma} expression in 71.3% of pancreatic carcinoma tissues. High rates of PPAR{gamma} expression were noted in 38.7% of carcinomas with a clear predominance in larger (pT3) and less well-differentiated (G3) tumors, still no correlation to nodal status was apparent. On survival analysis, higher rates of PPAR{gamma} expression where associated with shorter overall survival times were seen, which even reached an independent prognostic value in the subgroup of node-negative cases. In the subgroups of patients with pT3 stage disease or less aggressive tumors (G1/G2), PPAR{gamma} could as well define significant risk groups. PPAR{gamma} might therefore be considered a new candidate marker of patient prognosis in patients with pancreatic cancer, which clearly warrants further study. It seems paradoxical that we found PPAR{gamma} overexpressed in more aggressive tumors, although PPAR{gamma} ligands are known to decrease tumor growth. As stated earlier, this could hint at a disrupted PPAR{gamma} downstream pathway, which would be endorsed by our array data that does not show a stringent correlation of PPAR{gamma} RNA with typical PPAR{gamma} target genes. It also parallels the findings of Zhang et al. (11) who reported higher PPAR{gamma} levels in more aggressive ovarian cancers. The data on PPAR{gamma} expression in primary tumors and patient prognosis is sparse and divergent. Jiang et al. (43) found lower mRNA levels in breast cancer patients with high-risk tumors. Papadaki et al. (44) investigated breast cancer tissues immunohistochemically and found low rates of PPAR{gamma} to be prognostic of shorter disease-free survival times. It is of interest to note, however, that although they used the same antibody we used in our study, they reported a predominantly cytoplasmic and even membranous immunostaining, which they subjected to statistical evaluation.

Given the wealth of experimental data on the growth-inhibitory effect of agonistic PPAR{gamma} ligands on pancreatic cancer cell lines and the high rate of PPAR{gamma} expression found in pancreatic cancer, we think that this strongly suggests to further investigate the potential of PPAR{gamma} ligands for treatment of pancreatic cancer. This idea is also supported in consideration of the convenient toxicity profile of these drugs and the very limited treatment options that are available for patients with this disastrous disease. Still, there are two caveats. First, previous studies with PPAR{gamma} ligands in liposarcoma as well as in prostate and breast cancer have shown diverse effects and were only partially successful (32, 45, 46). Also has troglitazone, the PPAR{gamma} ligand used in the breast cancer study, meanwhile been withdrawn by the FDA because of hepatic toxicity. Second, there is evidence that PPAR{gamma} ligands might even further induce tumor malignancy in some tissues (47–49). In conclusion, further studies are needed to clarify the possible role of PPAR{gamma} expression in pancreatic cancer and to elucidate its diagnostic or prognostic role and to clarify the therapeutic potential of PPAR{gamma} ligands.


   Acknowledgments
 
We thank Britta Beyer for the excellent technical assistance.


   Footnotes
 
Grant support: Deutsche Krebshilfe (70-2937-SaI).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

5 http://www.dchip.org. Back

6 http://www-stat.stanford.edu/~tibs/SAM/. Back

Received 4/ 4/06; revised 7/26/06; accepted 8/24/06.


   References
Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 

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