Pancreatic Ductal Adenocarcinoma in a Wistar Hannover GALAS Rat.

There are no reported spontaneous cases of pancreatic ductal adenocarcinoma (PDAC), and there are few reports about chemically-induced PDAC in rats. We encountered a PDAC in a Wistar Hannover GALAS rat that had been subjected to a medium-term multiorgan carcinogenicity bioassay. This article describes the histological and histochemical findings of the tumor. The tumor was located in the pancreatic tissue and had not invaded the liver parenchyma or the mucosal layer of the alimentary tract. The tumor cells were atypical and were mainly arranged in small tubules. In addition, abundant stroma and mucus production were observed in the tumor. In an immunohistochemical examination, the tumor cells were positive for cytokeratin, Sox9 and pancreas duodenum homeobox 1 and negative for amylase 2A and insulin. Therefore, the tumor was diagnosed as a PDAC based on its histological and histochemical findings. We considered that the tumor was caused by the carcinogens administered during the abovementioned bioassay.

Pancreatic acinar cell carcinomas and pancreatic ductal adenocarcinomas (PDAC) are pancreatic tumors that originate from exocrine glands. Pancreatic acinar cell carcinoma rarely develops spontaneously in rats, but it can be induced by azaserine or 4-hydroxyaminoquinoline-1-oxide[1],[2],[3]. On the other hand, as far as we know, no spontaneous cases of PDAC in rats have been reported, and there are few reports about chemically-induced cases[1], [4],[5],[6]. We encountered a chemically-induced PDAC in a Wistar Hannover GALAS rat. This article describes the histological and histochemical findings of this tumor.

The animal was one of 114 rats in the treated group of a study in which the toxicity of copper gluconate and the preventive effects of green tea catechins were evaluated using a medium-term multiorgan carcinogenicity bioassay[7]. In this group, 6-week-old male Wistar Hannover GALAS rats (BrlHan:WIST@Jcl, GALAS) (CLEA Japan, Inc., Tokyo, Japan) were subjected to combined treatment with 5 carcinogens targeting different organs: a single intraperitoneal injection of 100 mg/kg body weight of N-nitrosodiethylamine at commencement, 4 intraperitoneal injections of 20 mg/kg body weight of N-methylnitrosourea (MNU) during the first 2 weeks, 4 subcutaneous injections of 40 mg/kg body weight of 1, 2-dimethylhydrazine during the second 2 weeks and continuous administrations by admixing into drinking water with N-butyl-N-(4-hydroxybutyl)nitrosamine at a concentration of 0.05% for the first 2 weeks and 2,2’-dihydroxy-di-n-propylnitrosamine at a concentration of 0.1% for the second 2 weeks (DMBDD treatment). Subsequently, all rats were given a diet containing copper gluconate and green tea catechins for 25 weeks before being sacrificed under light ether anesthesia. Their major organs/tissues were obtained[7], fixed in 10% neutrally buffered formalin solution, embedded in paraffin and sectioned into 4-μm-thick specimens. All experiments were conducted according to the Guidelines for Animal Experimentation developed by the Japanese Association for Laboratory Animal Science (1987).

The specimens of the tumor were stained with hematoxylin and eosin (HE) and also subjected to periodic acid-Schiff (PAS), Alcian blue and Masson’s trichrome staining. In immunohistochemical examination, the specimens were stained for cytokeratin (CK) (monoclonal; MNF116; Dako, Kyoto, Japan), SRY-related high-mobility group box 9 (Sox9) (polyclonal; Millipore, Temecula, CA, USA), amylase 2A (AMY2A) (polyclonal; Proteintech, Chicago, IL, USA), insulin (polyclonal, Dako), pancreas duodenum homeobox 1 (PDX1) (polyclonal; TransGenic, Kumamoto, Japan), vimentin (monoclonal, VIM 3B4, Millipore), desmin (monoclonal, D33, Dako), S-100 (polyclonal, Dako) and proliferating cell nuclear antigen (PCNA) (monoclonal, PC10, Dako) using the avidin-biotin complex method (Vectastain ABC Kit; Vector Laboratories, Burlingame, CA, USA) and for C-ERC/mesothelin (polyclonal; Immuno-Biological Laboratories, Fujioka, Japan) using the streptavidin-biotin method (Histofine Simple Stain MAX-PO; Nichirei, Tokyo, Japan).

At necropsy, a whitish solid tumor (gross size: approximately 3 cm × 2 cm × 2 cm) was found in the proximity of the greater curvature of the stomach. The tumor involved the pancreas and adhered to the liver, stomach and duodenum.

Histologically, the tumor had replaced some of the pancreatic tissue (Figs. 1 and 2A). Although the tumor cells had partly invaded the hepatic capsules and the muscular layer of the stomach, they had not invaded the liver parenchyma or the mucosal layer of the alimentary tract including the stomach (Figs. 2B and 2C). The tumor cells were mainly arranged in small tubules within the abundant stroma (Fig. 2D), and some of them showed a clumpy and solitary arrangement. The tubules were usually irregular in shape and sometimes contained debris (Fig. 2E). The tumor cells were atypical and had eosinophilic cytoplasm, polymorphic nuclei and indistinct borders. Mitotic figures were often observed in the tumor cells, and some of them contained intracytoplasmic vacuoles (Fig. 2F), which were positively stained by PAS (Fig. 2G) and Alcian blue. The stroma was characterized by proliferation of fibroblasts and abundant collagenous fibrous tissue (Fig. 2H) with infiltration of lymphocytes, neutrophils and macrophages. Myxoid regions were scattered throughout the stroma (Fig. 2F), and the spaces in these regions were positively stained by PAS (Fig. 2G) and Alcian blue.

Fig. 1.

The tumor found in the abdominal cavity of the Wistar Hannover GALAS rat. HE stain. The tumor (Tt) is in contact with the pancreas (P), liver (L), stomach (S) and duodenum (D). Bar = 4 mm.

Fig. 2.

The tumor found in the abdominal cavity of the Wistar Hannover GALAS rat. HE stain (A–F), PAS stain (G) and Masson’s trichrome stain (H). The tumor replaced some of the pancreatic tissue (A). It did not invade the liver parenchyma (B) or the mucosal layer of the alimentary tract (C). The tumor cells are mainly arranged in small tubules within abundant stroma (D). The tubules are irregular in shape and sometimes contain debris (E). Some tumor cells with intracytoplasmic vacuoles (arrows) and myxoid regions are observed in the stroma (F). The vacuoles and spaces of the myxoid regions are positively stained with PAS (G). An abundant amount of collagenous fibrous tissue in the stroma is stained blue by Masson’s trichrome (H).

The results of the immunohistochemical examination are summarized in Table 1. The tumor cells were positive for CK, Sox9 and PDX1 and negative for AMY2A, insulin, vimentin, desmin, S-100 and C-ERC/mesothelin (Figs. 3A-E). PCNA-positive tumor cells were frequently detected. Among the normal pancreatic tissues in the tumor specimen, the ductal, acinar and islet cells were positive for Sox9, AMY2A and insulin/PDX1, respectively. In addition, the bile duct cells in the liver were positive for Sox9 and negative for PDX1. CK MNF116 antibody reacts with CK5, 6, 8, 17 and 19 and shows an especially broad pattern of reactivity with epithelial tissue[8], [9]. Sox9 is a transcription factor implicated in the control of diverse tissue development and carcinogenicity[10]. PDX1 is expressed in islet cells of the pancreas and posterior foregut in the fetus[11].

Table 1.

The Results of Immunohistochemical Examination of the Tumor, Pancreas and Liver

Fig. 3.

Immunohistochemical staining of the tumor for CK (A), Sox9 (B), PDX1 (C), AMY2A (D) and insulin (E). The tumor cells are positive for CK, Sox9 and PDX1 but negative for AMY2A and insulin.

As for other organs, the serous surfaces of the seminal vesicles contained masses (up to 4 mm in diameter) with histological findings similar to those of the abovementioned tumor. These masses were considered to be metastatic lesions.

The striking histological findings of the present tumor were that the tumor cells formed duct-like structures with abundant stroma in the pancreas and often produced mucus. These histological features are consistent with PDAC[1], [3]. The tumor cells were positive for CK, Sox9 and PDX1. Sox9 is specifically expressed in ductal cells and centriacinar cells in the normal pancreatic tissue of the adult rat. In addition, Sox9 and PDX1 have been detected in the human PDAC[12], [13] and in regenerative proliferating ductules after partial pancreatectomy in adult rats[14]. In contrast, the tumor cells did not contain zymogen granules, which are a feature of acinar cells, and were negative for AMY2A. Therefore, the present tumor was diagnosed as a PDAC.

Regarding the possible differential diagnoses for the present tumor, cholangiocarcinomas have similar histological features to PDAC[1], and in the present case, the bile duct cells in the normal liver were positive for Sox9, but the tumor had not invaded the liver parenchyma. Although it was reported that Sox9 and PDX1 were also expressed in pyloric glands and gastric carcinomas in humans[15], [16], gastric carcinomas were excluded for the same reasons as cholangiocarcinomas. Mesothelioma was also suspected because the tumor was located in the abdominal cavity. However, both the tumor and stromal cells were negative for C-ERC/mesothelin, and no continuity was detected between the tumor and the normal mesothelium (C-ERC/mesothelin-positive) around the invasive area.

It was reported that precursor lesions of PDAC are tubular complexes resulting from transdifferentiation of acinar cells in animal models of PDAC[17], [18]. Although acinar to ductal metaplasia was observed in some of the residual pancreatic tissues in the present tumor, the association in the above reports was unclear.

Regarding cases of chemically-induced PDAC in rodents, PDACs were induced in hamsters subcutaneously injected with N-nitrosobis(2-oxopropyl)amine[1], [3], [19], [20] or intraperitoneally injected with MNU[21]. In rats, it was reported that PDACs were induced by direct implantation of dimethylbenzanthracene into the pancreas[4],[5],[6]. Although to the best of our knowledge there are no reports about PDAC being induced in rats by the carcinogens used in DMBDD treatment, the histopathological features of the present case were quite similar to those of the PDACs that developed in the abovementioned studies. In conclusion, we considered that the present tumor was a PDAC that had been induced by the administration of the carcinogens used in DMBDD treatment in the Wistar Hannover GALAS rat.