Recently, it was hypothesised that tumours comprise heterogeneous populations of cells that differ in their abilities to proliferate and exhibit self-renewal. Only a small number of cells, so-called CSCs, can proliferate and exhibit extensive self-renewal; most tumour cells have a limited ability to do so and tend to differentiate into cells that form the tumour mass (Reya et al, 2001
; Presnell et al, 2002
; Pardal et al, 2003
). Cancer stem cells appear to be able to initiate and drive tumour growth in different haematological and solid tumours. CD133, a recently reported prospective marker for CSC, is expressed in a variety of tumours. However, to our knowledge, no attempt has been made to detect CD133 expression in pancreatic cancer specimens.
In this study, we used immunohistochemical staining to detect positive expression of CD133 in 60% of 80 pancreatic head carcinoma specimens, and found that the percentage of CD133-positive cells per specimen was less than 15%. These data are in agreement with the finding that CSCs represent only a very small portion of the total tumour cell population. For example, CD133-positive cells were detected in 0.7–6.1% (Ricci-Vitiani et al, 2007
), 1.8–24.5% (O'Brien et al, 2007
), 0.3–3% (Todaro et al, 2007
) of primary colon cancer cells, and in 0.7–3.2% of primary pancreatic cancer cells using flow cytometric analysis (Hermann et al, 2007
). Immunohistochemical staining revealed CD133 expression in 1–3% of hepatocellular carcinoma specimens (Ma et al, 2007
). Notably, we observed no staining for CD133 in normal pancreatic ductal epithelium in the present study (). The present immunohistochemical data appear to support the hypothesis that CD133-positive pancreatic carcinoma cells are involved in the tumorigenic process.
We found that CD133 expression was significantly associated with histological type, lymphatic invasion, and lymph node metastasis (). It is noteworthy that lymph node metastasis and lymphatic invasion are correlated with CD133 expression, as a recent study demonstrated that a melanoma cell line highly enriched with CD133-expressing cells concomitantly expresses lymphoangiogenic markers, vascular endothelial growth factor receptor (VEGFR)-3 and lymphatic vessel endothelial hyaluronan receptor (LYVE)-1 (Monzani et al, 2007
). As possible correlation between VEGF-C expression and lymphangiogenesis has previously been described (Kurahara et al, 2004
; Mohammed et al, 2007
), we examined VEGF-C expression in pancreatic cancer specimens and found it to be significantly associated with CD133 expression (). This suggests that CD133-positive pancreatic carcinoma cells promote lymphatic metastasis not only by inducing lymphangiogenesis, but also by facilitating self-entry into the lymphatic system. In view of the potential link to lymphangiogenesis, further studies are needed to address the possible role of CD133 in lymph node metastasis of pancreatic cancers.
Recently, the niche microenvironments of CSCs in brain tumours have been shown to be important for maintaining and self-renewing CSCs through close interaction with endothelial cells (Calabrese et al, 2007
). In this study, we could not reveal the histological site of such niches, although we observed that CD133-positive cells were localised in the peripheral site (facing interstitial space) of adenocarcinoma glandular structures, and did not express the epithelial differentiation marker CK (Hermann et al, 2007
). Considering the relationship between CD133-positive cells and lymph node metastasis, lymphangiogenesis and/or lymphatic vessels might be involved in forming the niche microenvironments of CSCs for pancreatic cancer. An understanding of niche histological sites would therefore be of use in preventing progression of pancreatic cancer cells.
High-grade MVD was more frequently found in CD133-positive tumours (), reflecting a possible effect of CD133 on tumour vascularity in pancreatic cancer. Bao et al (2006b)
made similar observations that CD133-positive tumour cells were more vascular than CD133-negative ones. In this context, investigations are currently underway to examine whether CD133-positive cells established from primary pancreatic carcinomas express VEGF more strongly than CD133-negative cells. There is a clear difference between CD133-positive and CD133-negative cells derived from pancreatic cancer regarding angiogenic potential.
A specific subpopulation of CD133+
CSCs was recently identified as being responsible for tumour metastasis (Hermann et al, 2007
). The CXCR4 protein has multiple essential functions, including homing of stem cells and metastasis of cancer cells (Miki et al, 2007
). Several cancers express CXCR4, and a relationship between CXCR4 expression and malignant potentiality has been suggested (Muller et al, 2001
; Kaifi et al, 2005
). In the present study, 85% of specimens exhibited positive expression of CXCR4. Notably, positive expression of CXCR4 was observed in all epithelial cells of adenocarcinoma, which is consistent with previous observations (Gockel et al, 2006
). Furthermore, there was no significant association between CXCR4 expression and lymph node metastasis, lymphatic invasion, together with prognosis in the CD133-positive group. Taking these findings into consideration, it appears that CXCR4 expression is not associated with CD133 expression, nor with other cancer-related events in the CD133-positive group.
It remains to be determined whether CD133-positive cancer cells exhibit higher degrees of proliferative activity than CD133-negative ones. As a preliminary test, we assessed this possibility using Ki-67 antibody staining and observed the presence of both highly proliferative activity and less proliferative activity of CD133-positive cells. To confirm these findings, we are currently examining the MIB-1 labelling index in CD133-positive and CD133-negative cells.
In conclusion, our immunohistochemical results indicate that CD133 expression in pancreatic head cancer is associated with histological type, lymphatic invasion, lymph node metastasis, VEGF-C expression, and prognosis. Multivariate survival analysis revealed that CD133 expression is an independent prognostic factor. These results suggest that CD133 expression is a useful marker for predicting the outcome of patients with pancreatic cancer. This is the first report of an association between CD133 expression and pancreatic cancer. Understanding the biological function of CD133 expression as a CSC marker in pancreatic cancer will be helpful in elucidating its role in the pathogenesis of pancreatic cancer and developing more effective therapeutic approaches.