HETEROGENOUS EXPRESSION OF PLASMINOGEN ACTIVATOR (PA) GENES IN THE HUMAN SARCOMA CELL LINE HT1080

Tumor cell derived PA activities are of crucial importance for tissue invasion and destruction by tumor cells. Therefore, we studied the expression of the PA genes in HT1080 cells using immunoenzymatic methods and specific PA gene probes.Immunenzymatic methods allowed only for the detection of urokinase like PA (u-PA) activities in HT1080 cells which was suppressed by treatment of the cells with dexamethasone (10-7 m). Despite the lack of u-PA activities, the cells still degraded extracellular tissue glycoproteins. Northern blot analysis with specific PA gene probe showed that HT1080 cells express both tissue type PA (t-PA) and u-PA. The enzymatic activities of t-PA were most likely masked by the simultaneous production of inhibitors of PA (PAI). Treatment of HT1080 cells with dexamethasone resulted in increased transcription of t-PA and decreased expression of the u-PA gene, explaining the unchanged tissue destruction by dexamethasone treated HT1080 cells.Cell clones secreting either large or small amounts of enzymatic PA activities were isolated from the parental HT1080 cell line using a fibrin agarose overlay technique.The expression of the u-PA gene was enhanced in high secreting PA clones compared to low secreting PA clones when analyzed on Northern blots. This heterogenous expression of the u-PA gene within the HT1080 cell line was confirmed by in situ hybridization with a specific u-PA gene probe.These findings demonstrate that PA gene expression can be missed with immunenzymatic methods due to simultaneous production of inhibitors of PA. In addition our results show that the expression of a given PA gene may be heterogenous on the cellular level within an established tumor cell line. These findings, therefore, suggest cellular variation of PA gene expression in tumor which may be of fundamental importance for tissue invasion and metastasis by cancer cells.