Claudin 18 and annexin A8 are frequently highly overexpressed in infiltrating ductal adenocarcinomas when compared with normal reactive ducts, suggesting a biologic role for these molecules in pancreatic ductal adenocarcinomas. Additionally, claudin 18 overexpression is detected in PanIN precursor lesions. Furthermore, claudin 18 and annexin A8 may serve as diagnostic markers, as screening tests and as therapeutic targets.
The claudins are a large family of tight junction proteins with 4 transmembrane domains.41
Over 20 claudins have been described and are expressed in epithelial cells from a variety of tissues. Claudins interact with one another to form a branching network that helps create the tight junctions of epithelial cells.55
Claudins function to produce a seal between cells, and they play a critical role in maintaining cell polarity.41,52,55
Although the biologic role of claudins in cancer is not entirely understood, the abnormal expression of a number of claudins has been reported in a variety of cancer types.41,51
Early reports identified the down-regulation of selected claudin members in some cancer types. For example, claudin 7 levels are decreased in some breast carcinomas,35
and in squamous cell carcinomas of the head and neck.2
In these cases, the loss of cell polarity normally maintained by the claudins likely contributed to tumorigenesis. Other claudins are increased in cancers, including claudin 10 in hepatocellular and papillary thyroid carcinomas, claudins 3 and 4 in ovarian carcinoma, and claudin 4 in pancreatic cancer.41
Here we demonstrate that claudin 18 protein is also overexpressed in pancreatic cancer, and PanIN lesions, a finding supported by recent reports from Iacobuzio-Donahue et al32
and Hewitt et al24
that claudin 18 mRNA is overexpressed in pancreatic cancer as measured by oligonucleotide arrays and real time reverse transcriptionpolymerase chain reaction, respectively.
It has been suggested that the overexpression of claudins in cancers may be related to a tight junction independent function.24
Recent studies, using human ovarian epithelial cells, have shown that claudin 3 and claudin 4 overexpression may enhance tumorigenesis and metastasis.24
The overexpression of selected claudins by pancreatic cancer may have therapeutic implications. Our group has previously demonstrated that most of pancreatic cancers overexpress claudin 4.42
Claudin 4 is also a receptor for the Clostridium perfringens
cytolytic enterotoxin, and recent studies in pancreatic cancer xenografts have demonstrated that the C. perfringens
enterotoxin causes tumor cell necrosis in vivo.39,42
Similarly, the demonstration here that claudin 18 is also strongly and diffusely overexpressed in the majority of pancreatic cancers and their precursors suggests that claudin 18 may also be a useful therapeutic target.45
Additionally, the detection of PanINs, a precursor to invasive carcinoma, offers a window of opportunity for early surgical resection and treatment of neoplasia before the development of an incurable invasive adenocarcinoma.
It is interesting to note that we found that claudin 18 protein expression was most pronounced in well-differentiated cancers, because it is the well-differentiated carcinomas that present the greatest challenge diagnostically. Immunolabeling for claudin 18 may be useful in distinguishing the reactive glands of chronic pancreatitis from well-differentiated adenocarcinoma.
Claudin 18 expression levels may also have prognostic implications for patients with pancreatic carcinoma. We found that patients whose carcinomas diffusely and strongly labeled with antibodies to the claudin 18 protein had a significantly better survival when compared with those cancers that had weak or no claudin 18 protein expression (). The multivariable Cox proportional hazards model suggests a 48% reduction in mortality associated with this labeling pattern, independent of other known prognostic factors including tumor grade, tumor size, positive lymph nodes, positive margins, and older age.
The annexins are a family of calcium-regulated proteins that bind the cell membrane and provide a link between calcium signaling and cell membrane functions. 15,46
The overexpression of annexin A8 in pancreatic cancer is biologically interesting for several reasons. Annexins have calcium regulatory functions and a number of other calcium-regulated proteins, particularly the S100 family of proteins, are also overexpressed in pancreatic cancers suggesting a critical role for calcium homeostasis in pancreatic cancer.11,37,48,56
In addition, one of the other annexins, annexin A2, binds S100 A10, and S100 A10 is also overexpressed in pancreatic cancer.11,33,37,48,50,56
Our finding that annexin A8 is overexpressed in pancreatic cancers is supported by other reports that have also identified annexin A8 overexpression by cDNA representational difference analysis and oligonucleotide-directed gene array experiments.16,36
The strong and diffuse overexpression of annexin A8 and claudin 18 in the majority of pancreatic cancers suggests that annexin A8 and claudin 18 may be useful markers for pancreatic cancer. For example, we have previously identified a number of proteins that are overexpressed in pancreatic cancer and subsequently some of these proteins have been used to create new diagnostic markers, imaging and therapeutic targets for pancreatic cancer.1,13,42
Using serial analysis of gene expression, we discovered that mesothelin is overexpressed in most of pancreatic cancers, and several clinical trials are now underway treating pancreatic cancer by targeting mesothelin.4,20,22,30,51,53
Similarly, mesothelin has also proven useful in evaluating difficult pancreas biopsies and cytology samples, and mesothelin may be useful as an early detection marker for pancreatic neoplasia.8,21,23,25,34,38,43,58
In summary, we report the overexpression of 2 new proteins in pancreatic cancer. Claudin 18 and annexin A8 may serve as diagnostic markers, as screening tests and as therapeutic targets.