Potential role of mast cells in hamster cheek pouch carcinogenesis

During the process of activation, mast cells release products stored in their granules. Tryptase, a protease released from mast cell granules after activation, induces tumor cell proliferation through the activation of PAR-2 (protease activated receptor 2) on the plasma membrane of carcinoma cells. Chemical cancerization (DMBA) of the hamster cheek pouch is the most accepted model of oral cancer. However, there are no reports on the activation of mast cells during experimental carcinogenesis or on the correlation between mast cell activation and cell proliferation. The aim of the present study was to evaluate the potential effect of mast cells on the proliferation of epithelial cells at different times during the cancerization process. Paraffin serial sections of cancerized, tumor-bearing pouches were stained with Alcian Blue-Safranin to identify the different degrees of mast cell activation. Immunohistochemistry was performed to identify BrdU-positive cells to study tumor cell proliferation. Mast cells were counted and grouped into two categories: inactive mast cells AB-S+++ (red) and active mast cells AB+++S- (blue). Mast cell counts were performed in tumor stroma, base of the tumor (connective tissue immediately below the exophytic tumor), connective and muscle tissue underlying the cancerized epithelium (pouch wall) and adventitious tissue underlying the pouch wall. There was a significant increase in the number of mast cells at the base of tumors (p<0.001) compared to the number of mast cells in the wall of the pouch and in tumor stroma. In normal non-cancerized pouches, inactive mast cells were prevalent both in the wall (AB:S=1:2.15; p<0.001) and in the adventitious tissue (AB:S=1:1.6; p<0.004) of the hamster cheek pouch. At most of the experimental times examined, the ratio of active/inactive mast cells (AB/S) in the wall approximated unity and even reverted. The ratio of mast cells was AB:S 1:1.05 at the base of the tumor and 1:0.24 in tumor stroma (p<0.001). The evaluation of epithelial nuclei labeled for BrdU revealed a statistically significant increase in cells undergoing DNA synthesis in the epithelium of the wall of the cancerized pouch compared to control (p<0.017). Tumor parenchyma exhibited a highly statistically significant increase in DNA synthesis compared to control (p<0.001) and compared to the epithelium of the wall of the cancerized pouch (p<0.036). We conclude that mast cell activation in this model is associated to the increase in tumor cell proliferation, conceivably mediated by the release of tryptase.