Background: Recent data suggest that anti-tumor activities of interleukin-12 (IL-12) involve the induction of apoptosis. Fas (APO-1/CD95) is a type I membrane protein that is capable of initiating an apoptosis signaling pathway when bound to its ligand (FasL). We undertook this study to test the hypothesis that Fas-FasL-mediated apoptosis plays a role in IL-12-induced tumor regression.
Methods: An mIL-12 expression vector driven by cytomegalovirus promoter was used to express murine IL-12 cDNA in the RM-9 murine prostate carcinoma cell line. Control RM-9 cells and RM-9 cells stably transfected with IL-12 gene (RM-9-IL12) were inoculated subcutaneously in 4- to 6-week-old male C57BL/J6 mice. Tumor size was measured every 3 days. Western blot and immunohistochemical assays were used to evaluate Fas and FasL protein expression. In situ fluorescent end labeling was used to label apoptotic cells.
Results: IL-12-expressing RM-9 prostate carcinoma cells transplanted into C57BL/J6 mice grew more slowly than control RM-9 cells and vector control RM-9-Luc cells. The average survival time of the RM-9-IL12 mice was longer than 53 days, whereas the mean survival for mice transplanted with control RM-9 cells was only 16 days. Apoptotic cells were more numerous in RM-9-IL12 tumors: 10.3% vs. 1.5% in control (P = 0.001). Fas and FasL proteins were increased approximately twofold in the RM-9-IL12 tumors compared with the RM-9 control tumors as determined by Western blot and immunohistochemical analyses (P < 0.05).
Conclusion: The Fas-FasL-mediated apoptosis pathway may contribute to the IL-12-induced rejection of prostate carcinoma.
Copyright 2002 Wiley-Liss, Inc.