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Evaluation of malignant human tumors in a xenobiotic nude mouse system has demonstrated that not all cells in tumors exhibit the capacity to form progressively growing tumors. However, nontumorigenic cells isolated from human tumors can be converted to a tumorigenic phenotype in nude mice by treatment with chemical carcinogens or by transfection with antisense to tumor suppressor genes. A newly discovered gene, designated ML-1, appears to be associated with tumorigenesis, because an ML-1 antisense cDNA construct, transfected into nontumorigenic, anchorage-independent growth (AIG) cells, was sufficient to convert these cells into a tumorigenic phenotype. The AIG cells transfected with ML-1 antisense cDNA constructs and converted to tumorigenic cells did not exhibit expression of normal ML-1 mRNA transcripts in the converted cells when evaluated by Northern analysis, whereas premalignant and normal cells expressed ML-1 transcripts at a high level. The converted cells exhibited a loss of growth control and produced tumors in a surrogate nude mouse that were greater than 2.0 cm in less than 2 months. The ML-1 gene has a DNA sequence that is 2177 bp in size and is located on chromosome number 13 on the q arm at site 12-14. Sequence analysis and investigation of GenBank sequences indicate that this is a newly described human gene.

A linear model for human cell metastasis has been developed in vitro from chemically transformed normal human cells. The chemically transformed cells are nontumorigenic in nude mice, but can be converted to a tumorigenic phenotype by transfection with a nondirectional cDNA library or antisense cDNA to the ML-1 gene. The primary transfected cell line (TR1T) forms localized, progressively growing tumors in nude mice that do not invade into the surrounding tissue. This tumorigenic TR1T cell line could be advanced into a metastatic stage following an additional transfection (TR2M cell line) with the cDNA expression library or antisense cDNA to the ML-1 gene. Metastatic cells, selected from tumors that were attached to internal organs, exhibited an increase in invasiveness as measured in vitro using an invasion chamber. The metastatic cells also exhibited an increased expression of matrix metalloproteinase-1 (MMP-1), although MMP-1 was not part of the cDNA that was transfected into either the TR1T cells or the doubly transfected metastatic TR2M cells. These data suggest that the increase in MMP-1 expression was a secondary downstream event responding to an upstream genetic change that initiated the conversion of cells from a tumorigenic to a metastatic stage. In summary, human cell lines representing premalignant, malignant, and metastatic phenotypes have been established in culture that can be used to identify gene changes that occur as normal human cells progress to a metastatic stage during tumor development. One gene, ML-1, that is found in the expression library appears to be involved in malignant progression, because ML-1 antisense cDNA will convert chemically transformed cells to both tumorigenic and metastatic stages, and cells from both local and metastatic tumors have a reduced or complete loss of expression of the ML-1 gene.