ADAM9, a member of the ADAM family that is involved in various biological processes (Moss et al, 2001
), was found to be overexpressed in prostate carcinoma cell lines, hepatocellular carcinoma and breast carcinoma (McCulloch et al, 2000
; Le Pabic et al, 2003
; O'Shea et al, 2003
). In PDAC, we and others observed ADAM9 overexpression by gene expression profiling. While Iacobuzio-Donahue et al (2003)
used cDNA microarray analysis, with subsequent validation of the ADAM9 overexpression by RT–PCR in PDAC cell lines, we based our examination on the use of Affymetrix GeneChips and validated the distinct immunohistochemical expression of ADAM9 in a small series of PDACs (Grutzmann et al, 2003
This study was designed to further validate the significance of ADAM9 overexpression in PDACs by comparing the results with those obtained in pancreatic tumours other than PDACs, and by correlating the immunohistochemical labelling of the individual PDACs with the survival of the patients.
ADAM9 was detected in 58 of 59 PDACs along the apical lumen-oriented membrane of the neoplastic glandular structures. In addition, 32 of the 59 PDACs showed staining of the basolateral cell membrane and 17 revealed cytoplasmic positivity. In the normal adjacent pancreatic tissue, ADAM9 staining, although weak, was also detected at the luminal surface of the interlobular ductal cells and centroacinar cells. It appears that ADAM9 is preferentially a luminal membrane-bound protein of duct-type pancreatic cells. As the apical membrane labelling, observed in the adjacent normal pancreatic tissue, is preserved in almost all PDACs, we might speculate that ADAM9 function could at least partly be maintained in the tumour cells. Among the pancreatic tumours, that is neoplasms of the acinar, endocrine and ductal phenotype, ADAM9 expression was selective for PDACs. Interestingly, endocrine neoplasms did not express ADAM9, although the islet cells in the normal pancreas displayed consistent granular cytoplasmic staining. These results indicate that ADAM9 might not play a role in the biology of nonduct-type neoplasms of the pancreas, but may be important for the biology of PDACs.
The distribution pattern of ADAM9 in PDACs was related to the differentiation of the individual tumours. More than two thirds of the well and moderately differentiated PDACs showed only apical membranous ADAM9 labelling, while poorly differentiated PDACs usually exhibited additional basolateral membranous and cytoplasmic staining. Whether this change bestows a progression advantage on the tumour cells is not yet known. However, because we found that cytoplasmic and basolateral ADAM9 staining correlates with poor survival in PDAC patients, it may be speculated that this over-expression pattern of ADAM9 promotes PDAC progression.
The relationship between the ADAM9 expression pattern in PDAC and the survival probability was tested in a series of curatively resected patients. This test, using a multivariate analysis, revealed cytoplasmic expression of ADAM9 to be an independent prognostic factor in patient survival. The second independent factor detected by this analysis was tumour grade, confirming earlier studies (Luttges et al, 2000
). As there was a relationship between differentiation and the ADAM9 expression pattern, the possibility has to be considered that the two factors might be interrelated.
So far the precise function of ADAM9 in the pancreas is unknown. The designation ADAM is derived from their two transmembrane domains, which possess A Disintegrin And a Metalloprotease function (Izumi et al, 1998
). The ADAMs are a multifunctional gene family, some members of which have been shown to play a role in diverse biological processes such as fertilization, myogenesis, neurogenesis and the activation of growth factors/immune regulators such as TNF-alpha. Moreover, ADAM9 is known to cleave heparin-binding EGF-like growth factor (Roghani et al, 1999
). The disintegrin function probably relates to cell-to-cell and cell-to-extracellular matrix (ECM) adhesive interactions and transduction of signals from the ECM to the cell interior and vice versa
. It may be involved in cell migration, invasion, intra- and extravasation and platelet interaction (Poggi et al, 1993
; Mizejewski, 1999
If ADAM9 overexpression is involved in PDAC progression, it may exert its action either via its disintegrin domain or its metalloproteinase domain or, most likely, via both. Various matrix metalloproteinases (MMP) like MMP2 and MMP9 have been described as being overexpressed in PDACs and seem to play an important role in the progression of PDAC (for a review cf. Bloomston et al, 2002
). These observations led to a clinical trial of the metalloproteinase inhibitor marimastat in PDAC, which provided evidence of a dose response (Bramhall et al, 2001
). Moreover, marimastat is potent not only against MMPs but also against ADAM9 (Moss et al, 2001
). It may therefore be speculated that the response to marimastat in patients with PDAC may be in part due to inhibition of ADAM9. If this proved true, ADAM9 might play a role in tumour progression, and might be used not only for prognostic and diagnostic purposes but also for novel therapeutic approaches. Misallocation of ADAM9 from the luminal membrane to the cytoplasm and the basolateral membrane might add to an activation of growth factors and the degradation of ECM by ADAM9.
In conclusion, we have demonstrated that ADAM9 is overexpressed in PDACs but not in endocrine tumours or acinar cell carcinomas. Furthermore, we found a significant association between cytoplasmic ADAM9 overexpression and survival in patients curatively resected for PDAC. This suggests that cytoplasmic ADAM9 overexpression may be a useful diagnostic marker and could also become a potential target in the treatment of PDAC.