ALPHA-SMOOTH MUSCLE ACTIN EXPRESSION IN TUMOR AND STROMAL CELLS OF BENIGN AND MALIGNANT HUMAN PIGMENT CELL TUMORS

We examined the altered expression of alpha-smooth muscle actin (alpha-Sm) in human benign, pre-malignant, and malignant pigment cell tumors by immunohistochemical as well as biochemical (Western blot) analysis using anti-alpha-Sm monoclonal antibody (anti-alpha-Sm MoAb). The expression of alpha-Sm has been revealed immunohistochemically to be associated with mesodermal cells rather than with pigment cells. Western blot analysis using anti-alpha-Sm MoAb detected alpha-Sm expression as a 43-kD band in the extracts from normal papillary dermis, nevus cell nevus, and metastatic melanoma with stromal tissues, but not from primary melanoma with stromal tissues examined. The above findings of alpha-Sm expression by Western blot analysis were further characterized immunohistochemically in terms of the localization at the cellular level as follows. 1) In normal papillary dermis, pericytes encircling capillary vessels showed only positive staining with anti-alpha-Sm MoAb. 2) In nevus tissues, nevus cells were not shown to be positively stained, despite similar positivity of pericytes in normal papillary dermis. 3) In melanoma tissues, alpha-Sm expression of metastatic melanoma detected by Western blot analysis was found to be derived from fibroblasts with smooth-muscle differentiation (myofibroblasts), but not from melanoma cells. Such myofibroblastic stromal changes could not be found on primary melanoma tissue sections, which showed no reactivity in Western blot analysis. We conclude that the major sources of alpha-Sm in benign and pre-malignant pigment cell tumors are capillary pericytes, whereas alpha-Sm found in malignant melanoma tissue is primarily from melanoma-surrounding stromal fibroblasts that were changed to myofibroblasts by some cytokine factor(s), presumably secreted from melanoma cells.