MMP-2 expression is associated with, but not sufficient for, malignant conversion of murine LTA cells

We previously showed that tumorigenic, non-metastatic LTA cells can be converted to a metastatic phenotype either by cell fusion with non-malignant NIH 3T3 cells, or by transfection with genomic DNA from metastatic murine B16F1 or human 1GR37 melanoma cells. In order to identify a gene present in NIH 3T3 cells that is responsible for this conversion, we transferred DNA from an NIH 3T3 genomic library into LTA cells and tested for changes in metastatic properties, assessed in the chick embryo. We found that 3 of 4 pools of transfectant clones showed significantly increased metastatic ability over the vector-only control transfectants. All three metastatic transfectant pools showed significantly increased RNA levels of the 72 kDa type IV gelatinase (MMP-2). To test whether increased expression of MMP-2 was sufficient to convert LTA cells to metastatic ability, we transfected full length MMP-2 cDNA, in a CMV-promoter expression construct, into LTA cells. Stable transfectants with elevated MMP-2 RNA and enzymatic activity were obtained. The highest MMP-2 expressing clone was assayed for experimental metastatic ability in the chick embryo, and found to be no more metastatic than LTA parental cells. We conclude that increased MMP-2 expression accompanies the malignant conversion of LTA cells, but MMP-2 expression alone is not sufficient to bring about this change. The inability of LTA cells to metastasize thus appears to be due to a more complex defect than insufficient MMP-2. This study supports the idea that malignant conversion may require the concerted activation of multiple genes, which are in turn controlled by regulatory genes, whose identification will be important in understanding and controlling metastasis.