Multiple features of advanced melanoma recapitulated in tumorigenic variants of early stage (radial growth phase) human melanoma cell lines: evidence for a dominant phenotype.

The vast majority of primary human cutaneous melanomas undergo a slow and gradual progression from a clinically indolent, curable radial growth phase (RGP) to a malignant vertical growth phase. We sought to develop a way of isolating genetically related malignant variants from a benign RGP human melanoma, called WM35. The parent and variants were then used as a model system to examine to what extent the expression of clinically and biologically relevant phenotypic features characteristic of advanced melanomas are associated with (and thus perhaps causative of) such a malignant conversion. Such a model system could also be used as a means of eventually identifying genetic alterations and cellular changes involved in the malignant switch in melanoma progression. To develop such a model, we subjected WM35 cells to retroviral insertional mutagenesis, which was followed by selection for progressive growth of solid tumors in nude mice. Highly aggressive and phenotypically stable tumorigenic variants were derived which contained at least four integrated proviruses. In contrast to the parental WM35 cells, these cell lines expressed several phenotypic features characteristic of naturally derived, advanced-stage malignant melanoma cells. Thus, in addition to tumor-forming ability in nude mice, the variants were growth factor and anchorage independent, overexpressed the MUC18 adhesion molecule, and lost responsiveness to the growth-inhibitory effect of several cytokines, including interleukin 6, transforming growth factor beta, interleukin 1beta, and tumor necrosis factor-alpha. Tumorigenicity and "multicytokine resistance" were dominant traits since in somatic cell hybrids between the parental cells and a tumorigenic subline no suppressive effect of the former cell population was observed. These findings suggest that one or more dominantly acting genetic alterations might be involved in this progression of RGP melanoma cells. The identity of such alterations remains to be determined.