Complex expression of the genes coding for plasminogen activators and their inhibitors in HeLa-smooth muscle cell hybrids.

As cells progress through the multistep process of neoplastic transformation, they eventually acquire the property of invasive behavior. Although both plasminogen activators (PA) and their inhibitors (PAI) contribute to this process, their regulation in normal and transformed cells remains poorly defined. Because somatic cell hybrids provide useful tools for examining the transformation pathway, tumorigenic and invasive HeLa cells were fused with human normal vascular smooth muscle cells and tested for invasion-related parameters, including the expression of PA and PAI genes, and matrix degradation. Both parental cell lines produced large amounts of PAI activities with no detectable PA in either cellular or secreted form. Opposite findings were obtained with the hybrid cell lines, which demonstrated the presence of receptor-bound and secreted PA but absence of enzymatically measurable PAI activities. Both urokinase-type and tissue-type PA were found in cell-associated and secreted form in the hybrid cells. In addition, expression of the urokinase-type PA receptor gene was found in the three hybrid cells and the vascular smooth muscle cells but not in the HeLa cells. Expression of active, receptor-bound and secreted PA provided the nontumorigenic hybrid cells with the enzymatic tools to degrade extracellular proteins in a plasminogen-dependent manner. Thus, the hybrid cells lost tumorigenicity while retaining the tissue-degrading capability of HeLa cells. These hybrid cell lines should prove to be important reagents for investigating the complex regulatory control of PA and PAI gene expression.