An increase in plasminogen activator mRNA occurs at an early stage in ethylnitrosourea-induced transformation of rat brain cells.

Brain tumor cells cultured after transplacental induction by the nitrosamide, N-ethyl-N-nitrosourea had a higher level of plasminogen activator activity than control cells from adult rat brain. Cultures (BE10) derived 2 days after exposure to the carcinogen showed a rise in this proteolytic activity at the 17th passage but were not able to form colonies in agar or tumours in syngeneic rats until passages 44/45. Cultures (BE11) derived 2 days after exposure to buffer did not show a rise in this enzyme activity nor were they able to grow in agar or animals at comparable passages. Zymography and inhibition studies showed that the enzyme produced by the tumour cells was related to human tissue-type plasminogen activator rather than to urokinase. Northern blot analysis showed a higher level of tissue plasminogen activator related mRNA in tumour cells than control cells. There was an increase in mRNA level during passaging of the carcinogen-exposed culture, BE10, which correlated with the increased enzyme activity. There was no rise in the barely detectable mRNA levels of the comparable buffer-exposed culture, BE11. The results suggest that alteration in the transcriptional control of this proteolytic enzyme occurs at an early stage in the transformation process.