Suppression of a human colon cancer cell line by introduction of an exogenous NF1 gene.

Human colon carcinoma cell line HCT116 harbors an oncogenic Ki-ras gene. Introduction of an exogenous full-length NF1 gene or its GTPase-activating protein (GAP)-related domain suppressed the tumor-forming ability of this cell line in nude mice. A GAP-related domain peptide carrying a K1423E mutation, which shows greatly diminished GAP activity but a normal binding affinity for p2lras-GTP, was also tested. This construct was able to suppress tumor formation by the HCT116 cell line, thus ruling out the possibility that the observed tumor suppression is due to the GAP activity of NF1. Reduced Raf-1 kinase activity in cells which expressed these NF1 constructs suggested that neurofibromin may interfere with the interaction between Ras and Raf. Introduction of a mutationally activated Raf-1 kinase domain reversed tumor suppression by neurofibromin, implicating Raf-1 as the primary downstream transducer of the oncogenic Ras signal. An increase in apoptotic cell death, which could be delayed by activated Raf-1 kinase, was also seen in cells carrying the exogenous NF1 gene.