The gastrointestinal tract is supported by myofibroblasts (MF) that regulate epithelial homeostasis. In cancer the physiological stroma is replaced by activated cells including bone marrow (BM)-derived cancer-associated fibroblasts (CAFs) that drive carcinogenesis. MFs and a significant proportion of CAFs express αSMA. The origin of these cells and their relation to the stem cell niche in cancer, however, is unclear. We examined gene expression to determine markers of the MSC niche, from which MFs and CAFs develop, and then generated three transgenic mouse models to trace and examine the kinetics of the physiological and cancer stroma. MSC cultures from whole BM of αSMA-RFP transgenic reporter mice consist of a heterogeneous population of αSMA-RFP (+) and (-) cells (FACS). The αSMA-RFP (-) population contained the true MSC (CFU-F and differentiation). The BMP-antagonist, Grem1, was specifically upregulated in the combined RFP+/− population but not RFP+ cells suggesting that Grem1 may mark the MSC within its niche. Additionally, we confirmed the expansion of BM-derived Grem1 cells in the gastric peritumoral stroma supporting our hypothesis that the mesenchymal niche is recruited to the cancer microenvironment. In order to confirm that Grem1 labels MSCs, we developed a Grem1-BAC-CreER transgenic mouse to lineage trace this cell. 24 hours after tamoxifen induction single recombined cells were evident throughout the gastrointestinal stroma adjacent to the epithelial stem cell zone. By 3 months post-induction the recombined cells had expanded giving rise to αSMA+ MFs. Importantly, Grem1(+) cells were identified within BM-derived MSC cultures, where they also gave rise to αSMA(+) MFs and adipocytes, and are currently being differentiated into osteocytes and chondrocytes. Furthermore, in a syngeneic tumor model, subcutaneous injection of MC38 cells into Grem1-BAC-CreER mice, yielded infiltrating CAFs derived from Grem1 cells, suggesting that Grem1 marks the cellular origin of both physiological and pathological mesenchyme. Gastrointestinal carcinogenesis models (AOM/DSS, MNU/H. felis), metastasis models (splenic injection MC38) and longer term lineage tracing are proceeding. We also developed αSMA-BAC-CreER and Vimentin-BAC-CreER transgenic mice to characterize the cellular kinetics of the stroma and further refine markers co-expressed by MSCs. These studies have confirmed the turnover of intestinal MFs. In summary, Grem1 expressing stromal cells co-localize with the epithelial stem cell niche within the gastrointestinal tract and specifically label a mesenchymal cell that gives rise to αSMA+ MFs. Furthermore, syngeneic tumor models suggest that Grem1 cells also give rise to peritumoral mesenchyme. Grem1 expression identifies a likely progenitor to gastrointestinal MFs/CAFs in health and cancer. Grem1 is a promising marker of the MSC in health in cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2969. doi:1538-7445.AM2012-2969