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.