Tumor progression to the stage of metastasis may result in part from the selection of certain primary tumor cell clones which are phenotypically competent for survival, invasion, and growth at secondary sites. Selection for traits such as loss of growth inhibitory responses, acquisition of increased adhesiveness, increased local immunosuppression, and enhanced motility and collagenase activities likely contribute to cancer progression and may be regulated through the action of growth factors. The transforming growth factor β (TGF-β) family of growth factors has often been associated with these traits and tumor progression; therefore, elimination or subversion of TGF-β-responsive pathways should be considered as a mechanistic framework for metastatic events.
In this report, we have compared growth and extracellular matrix responses to TGF-β in six metastatic and six primary tumor-derived cell lines in a mouse model of prostate cancer. We have found that tumor cell lines derived from focal pulmonary metastasis secreted relatively greater quantities of total TGF-βs, lost most or all TGF-β1 growth inhibition, but responded to TGF-β1 through induction of the type IV collagenase matrix metalloproteinase-9, whereas cell lines derived from tumors which proliferated at the primary site retained the growth inhibition but lacked collagenase activity. Synthesis of another extracellular matrix protein, plasminogen activator inhibitor 1, was stimulated by TGF-β1 in both primary as well as metastatic tumors. These results suggest that acquisition of differential responses to the TGF-β family could results in phenotypic traits which facilitate tumor metastasis from certain primary site clones.
This study was supported by NIH Grants CA50588 and CA68814 (to T.C.T.) and a Postdoctoral Fellowship F32 CA66323 (to I.S.).