Human malignant melanoma cells release a factor that inhibits the expression of smooth muscle alpha-actin

Smooth muscle alpha-actin (SMA) is a cytoskeletal protein expressed in vascular smooth muscle cells, pericytes, hair follicle dermal sheaths and myofibroblasts which appear in the process of wound healing and tumor growth. To examine the effect of malignant melanoma on the expression of SMA in these non-neoplastic cells, we carried out immunohistochemical staining and a cell culture study. Conditioned medium prepared from a melanoma cell line M14 was incubated with a rat fibroblastic cell line 3Y1, which had been shown to express SMA. Human cells that had migrated from nevus tissue were also cultured either with or without M14 conditioned medium. Immuno-histochemical staining of human melanoma tissues suggested that the expression of SMA was low in the vicinity of the tumor as well as within the tumor nodules. The conditioned medium from melanoma, but not the medium from control non-neoplastic cells, suppressed the expression of SMA both in the 3Y1 cells and human cells that migrated from the nevus. Preincubation of the medium with anti-platelet-derived growth factor allowed 76% recovery of SMA expression. These data thus imply that melanoma cells release a platelet-derived growth factor-like substance which has a suppressive effect on the contractile elements in non-neoplastic cells.