Selection for transformation and met protooncogene amplification in NIH 3T3 fibroblasts using tumor necrosis factor alpha.

Alterations in the structure and expression of protein tyrosine kinases are associated with cellular transformation and tumorigenicity. These enzymes may contribute to tumor progression by interfering with host mechanisms of antitumor surveillance. In the present work, we show that selection of NIH 3T3 fibroblasts for resistance to the cytotoxic effects of tumor necrosis factor alpha (TNF-alpha), a major mediator of macrophage-induced tumor cell cytotoxicity, leads to enrichment in the remaining cells for those with transformed morphology and is often associated with amplified copy number and expression of the met protooncogene. We further substantiate the relationship between met protooncogene amplification and resistance to TNF-alpha by showing that spontaneous (non-TNF-alpha-selected) NIH 3T3 cell transformants which have amplified met protooncogene copy number have increased resistance to the growth-inhibitory activity of this cytokine. These results provide evidence for one mechanism by which the activated macrophage may select for tumor cells within a developing focus with properties associated with tumor progression. In addition, our ability to select cells with such properties, as demonstrated using the met protooncogene as a model system, may also provide a unique means (i.e., selection with TNF-alpha) for identifying other gene products, including other tyrosine kinases, associated with aggressive tumor growth.