Radiation in Human Malignant Melanoma Cells Induction of Tissue-type Plasminogen Activator by Ionizing

Two differently timed extracellular and intracellular enzymatic and mRNA peaks of tissue-type plasrninogen activator (t-PA) were induced following ionizing radiation. The first peak appeared within 10 min following \-irradiation but rapidly declined. The appearance of early tPA mRNA transcripts and enzymatic activity were not prevented by actinomycin D treatment. In contrast, cycloheximide prevented the early, minor enzymatic induction peak of t-PA. Stabilization of t-PA mRNA transcripts appears to be an early initial response of human cells to ionizing radiation, since the synthesis of new mRNA transcripts within the first 30 min was not observed via nuclear run-on analyses. Nearly 12 h following \-irradiation, a second major enzymatic peak of t-PA was observed. Cycloheximide or actinomycin D treatments blocked the later t-PA response. t-PA mRNA levels were induced > 100fold in 4 h by ionizing radiation as assayed via Northern or nuclear runon analyses. During the major induction period, t-PA mRNA transcripts reached their maximum levels at 4-8 h, and intracellular enzyme levels accumulated 6-8 h after X-irradiation. Unirradiated Ul-Mel cells dem onstrated only low basal levels of t-PA mRNA and enzymatic activity. Similar induction responses were found following UV-irradiation or 12O-tetradecanoyl-phorboI-13-acetate (I'M A) treatments. Normal human fibroblast (i.e., GM 2936B, GM2907A, and IMR-90) cells also demonstrated the induction of t-PA, although only one later enzymatic peak was detected. The induction of t-PA mRNA levels and intracellular and extracellular enzymatic activities for these cells were 50-fold lower than with Ul-Mel cells given equitoxic doses of X-rays. Differential expression of t-PA in some tumor as compared to normal tissues may be utilized in future chemotherapeutic regimens.