No abstract

Genetically engineered mouse models (GEMMs) have dramatically improved our understanding of tumor evolution and therapeutic resistance. However, sequential genetic manipulation of gene expression and targeting of the host is almost impossible using conventional Cre-loxP–based models. We have developed an inducible dual-recombinase system by combining flippase-FRT (Flp-FRT) and Cre-loxP recombination technologies to improve GEMMs of pancreatic cancer. This enables investigation of multistep carcinogenesis, genetic manipulation of tumor subpopulations (such as cancer stem cells), selective targeting of the tumor microenvironment and genetic validation of therapeutic targets in autochthonous tumors on a genome-wide scale. As a proof of concept, we performed tumor cell–autonomous and nonautonomous targeting, recapitulated hallmarks of human multistep carcinogenesis, validated genetic therapy by 3-phosphoinositide-dependent protein kinase inactivation as well as cancer cell depletion and show that mast cells in the tumor microenvironment, which had been thought to be key oncogenic players, are dispensable for tumor formation.

Background & AimsInhibitors of the epidermal growth factor receptor (EGFR) are the first-line therapy for patients with metastatic colorectal tumors without RAS mutations. However, EGFR inhibitors are ineffective in these patients, and tumor level of EGFR does not associate with response to therapy. We screened human colorectal tumors for EGFR-positive myeloid cells and investigated their association with patient outcome. We also performed studies in mice to evaluate how EGFR expression in tumor cells and myeloid cells contributes to development of colitis-associated cancer and ApcMin-dependent intestinal tumorigenesis.MethodsWe performed immunohistochemical and immunofluorescent analyses of 116 colorectal tumor biopsies to determine levels of EGFR in tumor and stroma; we also collected information on tumor stage and patient features and outcomes. We used the Mann-Whitney U and Kruskal-Wallis tests to correlate tumor levels of EGFR with tumor stage, and the Kaplan-Meier method to estimate patients’ median survival time. We performed experiments in mice lacking EGFR in intestinal epithelial cells (Villin-Cre; Egfrf/f and Villin-CreERT2; Egfrf/f mice) or myeloid cells (LysM-Cre; Egfrf/f mice) on a mixed background. These mice were bred with ApcMin/+ mice; colitis-associated cancer and colitis were induced by administration of dextran sodium sulfate (DSS), with or without azoxymethane (AOM), respectively. Villin-CreERT2 was activated in developed tumors by administration of tamoxifen to mice. Littermates that expressed full-length EGFR were used as controls. Intestinal tissues were collected; severity of colitis, numbers and size of tumors, and intestinal barrier integrity were assessed by histologic, immunohistochemical, quantitative reverse transcription polymerase chain reaction, and flow cytometry analyses.ResultsWe detected EGFR in myeloid cells in the stroma of human colorectal tumors; myeloid cell expression of EGFR associated with tumor metastasis and shorter patient survival time. Mice with deletion of EGFR from myeloid cells formed significantly fewer and smaller tumors than the respective EGFR-expressing controls in an ApcMin/+ background as well as after administration of AOM and DSS. Deletion of EGFR from intestinal epithelial cells did not affect tumor growth. Furthermore, tamoxifen-induced deletion of EGFR from epithelial cells of established intestinal tumors in mice given AOM and DSS did not reduce tumor size. EGFR signaling in myeloid cells promoted activation of STAT3 and expression of survivin in intestinal tumor cells. Mice with deletion of EGFR from myeloid cells developed more severe colitis after DSS administration, characterized by increased intestinal inflammation and intestinal barrier disruption, than control mice or mice with deletion of EGFR from intestinal epithelial cells. EGFR-deficient myeloid cells in the colon of DSS-treated LysM-Cre; Egfrf/f mice had reduced expression of interleukin 6 (IL6), and epithelial STAT3 activation was reduced compared with controls. Administration of re...