Cell-type plasticity within a tumor has recently been suggested to cause a bidirectional conversion between tumor-initiating stem cells and nonstem cells triggered by an inflammatory stroma. NF-κB represents a key transcription factor within the inflammatory tumor microenvironment. However, NF-κB's function in tumor-initiating cells has not been examined yet. Using a genetic model of intestinal epithelial cell (IEC)-restricted constitutive Wnt-activation, which comprises the most common event in the initiation of colon cancer, we demonstrate that NF-κB modulates Wnt signaling and show that IEC-specific ablation of RelA/p65 retards crypt stem cell expansion. In contrast, elevated NF-κB signaling enhances Wnt activation and induces dedifferentiation of nonstem cells that acquire tumor-initiating capacity. Thus, our data support the concept of bidirectional conversion and highlight the importance of inflammatory signaling for dedifferentiation and generation of tumor-initiating cells in vivo.

Inactivation of APC is a strongly predisposing event in the development of colorectal cancer1,2, prompting us to search for vulnerabilities specific to cells that have lost APC function. Signalling through the mTOR pathway is known to be required for epithelial cell proliferation and tumour growth3-5 and the current paradigm suggests that a critical function of mTOR activity is to upregulate translational initiation through phosphorylation of 4EBP16,7. This model predicts that the mTOR inhibitor rapamycin, which does not efficiently inhibit 4EBP18, would be ineffective in limiting cancer progression in APC deficient lesions. Here we show that mTORC1 activity is absolutely required for the proliferation of APC deficient (but not wild type) enterocytes, revealing an unexpected opportunity for therapeutic intervention. Although APC deficient cells show the expected increases in protein synthesis, our studies reveals that it is translation elongation, and not initiation, which is the rate-limiting component. Mechanistically, mTORC1 mediated inhibition of eEF2 kinase is required for the proliferation of APC deficient cells. Importantly, treatment of established APC deficient adenomas with rapamycin (which can target eEF2 through the mTORC1 – S6K – eEF2K axis) causes tumour cells to undergo growth arrest and differentiation. Taken together our data suggest that inhibition of translation elongation using existing, clinically approved drugs such as the Rapalogs, would provide clear therapeutic benefit for patients at high-risk of developing colorectal cancer.

The chemokine receptor CXCR2 is a key mediator of neutrophil migration that also plays a role in tumor development. However, CXCR2 influences tumors through multiple mechanisms and might promote or inhibit tumor development depending on context. Here, we used several mouse models of spontaneous and inflammation-driven neoplasia to define indispensable roles for CXCR2 in benign and malignant tumors. CXCR2-activating chemokines were part of the secretome of cultured primary benign intestinal adenomas (Apc Min/+ ) and highly expressed by all tumors in all models. CXCR2 deficiency profoundly suppressed inflammation-driven tumorigenesis in skin and intestine as well as spontaneous adenocarcinoma formation in a model of invasive intestinal adenocarcinoma (AhCreER;Apc fl/+ ;Pten fl/fl mice). Pepducin-mediated CXCR2 inhibition reduced tumorigenesis in Apc Min/+ mice. Ly6G + neutrophils were the dominant source of CXCR2 in blood, and CXCR2 deficiency attenuated neutrophil recruitment. Moreover, systemic Ly6G + cell depletion purged CXCR2-dependent tumor-associated leukocytes, suppressed established skin tumor growth and colitis-associated tumorigenesis, and reduced Apc Min/+ adenoma formation. CXCR2 is thus a potent protumorigenic chemokine receptor that directs recruitment of tumor-promoting leukocytes into tissues during tumor-inducing and tumor-driven inflammation. Similar leukocyte populations were also found in human intestinal adenomas, which suggests that CXCR2 antagonists may have therapeutic and prophylactic potential in the treatment of cancer.

SummaryThe Adenomatous Polyposis Coli (APC) gene is mutated in the majority of colorectal cancers (CRCs). Loss of APC leads to constitutively active WNT signaling, hyperproliferation, and tumorigenesis. Identification of pathways that facilitate tumorigenesis after APC loss is important for therapeutic development. Here, we show that RAC1 is a critical mediator of tumorigenesis after APC loss. We find that RAC1 is required for expansion of the LGR5 intestinal stem cell (ISC) signature, progenitor hyperproliferation, and transformation. Mechanistically, RAC1-driven ROS and NF-κB signaling mediate these processes. Together, these data highlight that ROS production and NF-κB activation triggered by RAC1 are critical events in CRC initiation.

SummaryTaxol and other antimitotic agents are frontline chemotherapy agents but the mechanisms responsible for patient benefit remain unclear. Following a genome-wide siRNA screen, we identified the oncogenic transcription factor Myc as a taxol sensitizer. Using time-lapse imaging to correlate mitotic behavior with cell fate, we show that Myc sensitizes cells to mitotic blockers and agents that accelerate mitotic progression. Myc achieves this by upregulating a cluster of redundant pro-apoptotic BH3-only proteins and suppressing pro-survival Bcl-xL. Gene expression analysis of breast cancers indicates that taxane responses correlate positively with Myc and negatively with Bcl-xL. Accordingly, pharmacological inhibition of Bcl-xL restores apoptosis in Myc-deficient cells. These results open up opportunities for biomarkers and combination therapies that could enhance traditional and second-generation antimitotic agents.