Summary
Treatment of rat C6 glioma with high doses of 13 cis-retinoic acid (cRA) was responsible
for death related to haemorrhagic necrosis localized to the tumor. Our aim was to
explore this adverse effect of retinoid treatment. We show that cRA-treated C6 glioma
at 25 mg/kg/day for 18 days exhibits in vivo an increased t-PA activity, which is
responsible for a localized tumor fibrinolytic activity. Production of t-PA is supported
by specific enhancement of gene expression, as was shown by the increase in t-PA mRNA
(X 2.3). This production is a direct effect of cRA when treating the tumor, since
tumor cells themself do not produce enough t-PA and treatment of control rats does
not increase the t-PA level. t-PA production by rat C6 glioma is in vivo related to
the specific synthesis of t-PA by the C6 cell-line. The stimulation of C6 cell-line
by cRA in vitro is dose-dependent and reached a maximum for 3 and 30 |iM at the 72nd
h. So cRA-treated C6 glioma cells produce t-PA which appears to be the major species
associated with the fibrinolytic activity-induced intra-tumoral haemorrhage after
exposure to retinoid treatment.
