In particular, IL-6 has been put forward as the molecular

component released by non-stem cancer cells to allow their conversion to cancer stem cells, and thereby maintain a dynamic equilibrium between these two tumor intrinsic cell types.82 Stat3 upregulates proteins of the Bcl-2 pro-survival family. In epithelial cells, it also induces other proteins that indirectly suppress apoptosis, such as the chaperone protein Hsp70, the C-type lectin-type RegIIIβ, and survivin,83 which are all overexpressed in CRC and IBD. The latter proteins not only suppress apoptosis, but might also promote cell cycle progression through binding to Cdc2. Stat3 also promotes the G1/S phase transition of the cell cycle more directly through the transcriptional Apoptosis inhibitor induction of cyclinB1, cdc2, c-myc, and cyclinD1, Alpelisib order and repression of the cell cycle inhibitor p21.83 As a third tumor-intrinsic property, Stat3 induces expression of the angiogenic factors, VEGF and HIF1α.83 Thus, excessive activation of Stat3 correlates with tumor invasion and metastasis in a variety of cancers. In the absence of epithelial Stat3 expression, the CAC model yields reduced tumor formation. Conversely, excessive Stat3 activation, through epithelial-specific Socs3 ablation or introduction of the Socs3-binding deficient gp130Y757F mutation, results in increased multiplicity and size of these tubular adenomas.84,85 Administration of

hyper-IL-6 (a fusion protein between IL-6 and soluble IL-6Rα), but not of enough IL-6, also increased tumor burden in CAC-challenged mice,85 suggesting that the extent of membrane-bound IL-6Rα, rather than gp130, limit the tumor-promoting

response. Consistent with these observations, we found functional redundancy between IL-6 and IL-11, and that both cytokines conferred Stat3-dependent, epithelial resistance to apoptosis and colitis.84 Genetic deficiency for the ligand binding IL-11Rα subunit in the CAC model significantly abrogates colonic tumor formation in gp130Y757F mice, while systemic reduction of Stat3 expression in gp130Y757FStat3+/− mice also reduced their susceptibility to colon tumorigenesis in the CAC model (Ernst et al., unpubl. observ., 2011). Furthermore, intestinal tumor burden is reduced in ApcMin mice lacking IL-6, and in ApcMin mice that are also haplo-insufficient for IL-11Rα or Stat3. However, IL-11 administration protected against radiation-induced mucositis, suggesting that IL-11 signaling might play a physiological role in the maintenance of intestinal epithelial integrity. Notwithstanding the central role played by excessive epithelial Stat3 signaling for the promotion of intestinal tumorigenesis, it has been recently suggested that this might also be part of an epigenetic switch mechanism that initiates tumor formation from non-transformed cells, rather than solely-expanding neoplastic cells that have arisen after exposure to mutagens.