Resistance to IL-1 anti-proliferative effect, accompanied by characteristics of advanced melanoma, permits invasion of human melanoma cells in vitro, but not metastasis in the nude mouse.

We reported earlier that IL-1 inhibits the growth of human melanoma cells (A375-6), and that these cells become resistant to IL-1 after prolonged periods of culture. The resistant cells constitutively produce IL-alpha and IL-6 with IL-6 production was induced by endogenous IL-1 in an autocrine manner. The cells are also resistant to IL-6 anti-proliferative effects. In the present study, we show that the resistant clones exhibited up-regulated expression of intercellular-adhesion molecule 1 (ICAM-1) and vitronectin receptor (integrin alpha(v)beta3) when compared with the IL-1-sensitive clone, A375-6. Moreover, these IL-1-resistant clones exhibited many other metastatic characteristics, such as expression of IL-8 mRNA, production of matrix metalloproteinases (MMP-2 and MMP-9), and augmented invasion activity. However, contrary to our expectations, the IL-1-resistant cells did not exhibit experimental metastasis in a nude-mouse model, similarly to the IL-1-sensitive parental A375-6 cell line. In contrast, the highly metastatic clone A375-SM exhibited alpha(v)beta3 expression at a level comparable to that of the IL-1-resistant cells, but expressed low or no ICAM-1, metalloproteinase and displayed little in vitro invasion activity. These results show that the metastatic characteristics of IL-1-resistant cells are not sufficient to produce metastasis in vivo and suggest that these resistant clones may provide a good model system for characterizing the molecular mechanisms of metastasis.