title: Dual effect of Kras(D12) knockdown on tumorigenesis: increased immune-mediated tumor clearance and abrogation of tumor malignancy.
abstract: Activating mutations in the human KRAS proto-oncogene are acquired during the earliest stages of colorectal cancer development. If mutant KRAS is to be used as a target for therapy in colorectal cancer, tumor growth should depend on its continued presence. Here, we report that stable knockdown of Kras(D12) in murine C26 colorectal cancer cells by RNA interference resulted in loss of transformed properties in vitro. The incidence of subcutaneous tumor formation was reduced by 60% and the lag time was increased sevenfold. Kras(D12)-knockdown tumors grew noninvasively and did not cause morbidity. Remarkably, some of the Kras(D12)-knockdown tumors regressed spontaneously, which rendered these mice resistant to parental C26 tumor growth. In immune-deficient hosts, the incidence of tumor formation by Kras(D12)-knockdown cells was 100%. None of these tumors regressed spontaneously. We conclude that the reduced incidence of tumor formation by Kras(D12)-knockdown cells is due to tumor cell clearance by the host immune system, but not to an intrinsic inability of these cells to grow out as tumors. Interestingly, Kras(D12) knockdown resulted in increased production of interleukin 18 (Il-18), an immune-stimulatory cytokine that has been implicated in limiting colorectal tumor formation. Thus, mutant Kras(D12) suppresses Il-18 production in colorectal tumor cells, which may contribute to evasion of the local immune system during tumor development.
authors.affiliation: Department of Surgery, University Medical Center Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands.
authors.forename: Niels
authors.initials: N
authors.lastname: Smakman
authors.forename: Liesbeth M
authors.initials: LM
authors.lastname: Veenendaal
authors.forename: Paul
authors.initials: P
authors.lastname: van Diest
authors.forename: Rinke
authors.initials: R
authors.lastname: Bos
authors.forename: Rienk
authors.initials: R
authors.lastname: Offringa
authors.forename: Inne H M
authors.initials: IH
authors.lastname: Borel Rinkes
authors.forename: Onno
authors.initials: O
authors.lastname: Kranenburg
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic / Q000235:genetics* / Q000276:immunology* / Q000378:metabolism / Q000473:pathology; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000276:immunology / Q000473:pathology / Q000517:prevention & control*; D017353:Gene Deletion; D006801:Humans; D020382:Interleukin-18 / Q000037:antagonists & inhibitors / Q000096:biosynthesis; D051379:Mice; D008807:Mice, Inbred BALB C; D009368:Neoplasm Transplantation; D000090063:Proto-Oncogene Mas; D016283:Proto-Oncogene Proteins p21(ras) / Q000235:genetics* / Q000502:physiology; D034622:RNA Interference
chemical_list: D020382:Interleukin-18; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D016283:Proto-Oncogene Proteins p21(ras)