In pigmented
basal cell epithelioma (BCE), there seems to be an abnormal transfer of melanized melanosomes from proliferating melanocytes to basaloid
tumor cells. In this study, the interruption of that melanosome transfer was studied with special respect to the altered function of a phagocytic
receptor, protease-activated receptor (PAR)-2 in the basaloid
tumor cells. We used electron microscopy to clarify the disrupted transfer at the ultrastructural level and then performed immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to examine the regulation of a phagocytic
receptor, PAR-2, expressed on basaloid
tumor cells. Electron microscopic analysis revealed that basaloid
tumor cells of pigmented BCE have a significantly lower population of melanosomes ( approximately 16.4%) than do normal keratinocytes located in the perilesional normal epidermis ( approximately 91.0%). In contrast, in pigmented
seborrheic keratosis (SK), a similarly pigmented epidermal
tumor, the distribution of
melanin granules does not differ between the lesional ( approximately 93.9%) and the perilesional normal epidermis ( approximately 92.2 %), indicating that interrupted melanosome transfer occurs in BCE but not in all pigmented epithelial
tumors. RT-PCR analysis demonstrated that the expression of PAR-2
mRNA transcripts in basaloid cells is significantly decreased in pigmented BCE compared with the perilesional normal epidermis. In contrast, in pigmented SK, where melanosome transfer to basaloid
tumor cells is not interrupted, the expression of PAR-2
mRNA transcripts is comparable between the basaloid
tumor cells and the perilesional normal epidermis. Immunohistochemistry demonstrated that basaloid cells in pigmented BCE have less immunostaining for PAR-2 than do keratinocytes in the perilesional normal epidermis whereas in pigmented SK, there is no difference in immunostaining for PAR-2 between the basaloid
tumor and the perilesional normal epidermis. These findings suggest that the decreased expression of PAR-2 in the basaloid cells is associated in part with the observed interruption of melanosome transfer in pigmented BCE.