The tryptophan-metabolizing enzyme indoleamine 2,3 dioxygenase 1 (IDO1) is frequently overexpressed in epithelial-derived malignancies, where it plays a recognized role in promoting tumor immune tolerance. We previously demonstrated that the IDO1–kynurenine pathway (KP) also directly supports colorectal cancer growth by promoting activation of β-catenin and driving neoplastic growth in mice lacking intact adaptive immunity. In this study, we sought to delineate the specific role of epithelial IDO1 in colon tumorigenesis and define how IDO1 and KP metabolites interact with pivotal neoplastic signaling pathways of the colon epithelium. We generated a novel intestinal epithelial–specific IDO1 knockout mouse and utilized established colorectal cancer cell lines containing β-catenin–stabilizing mutations, human colorectal cancer samples, and human-derived epithelial organoids (colonoids and tumoroids). Mice with intestinal epithelial–specific knockout of IDO1 developed fewer and smaller tumors than wild-type littermates in a model of inflammation-driven colon tumorigenesis. Moreover, their tumors exhibited reduced nuclear β-catenin and neoplastic proliferation but increased apoptosis. Mechanistically, KP metabolites (except kynurenic acid) rapidly activated PI3K-Akt signaling in the neoplastic epithelium to promote nuclear translocation of β-catenin, cellular proliferation, and resistance to apoptosis. Together, these data define a novel cell-autonomous function and mechanism by which IDO1 activity promotes colorectal cancer progression. These findings may have implications for the rational design of new clinical trials that exploit a synergy of IDO1 inhibitors with conventional cancer therapies for which Akt activation provides resistance such as radiation. Significance: This study identifies a new mechanistic link between IDO1 activity and PI3K/AKT signaling, both of which are important pathways involved in cancer growth and resistance to cancer therapy.

Regenerating Gene 4 (Reg4) is highly upregulated in gastrointestinal (GI) malignancies including colorectal and pancreatic cancers. Numerous studies demonstrated an association between higher Reg4 expression and tumor aggressiveness, intrinsic resistance to apoptotic death, and poor outcomes from GI malignancies. However, the precise receptor and underlying signaling mechanism have remained unknown. Although we previously reported a Reg4-mediated induction of EGFR activity in colorectal cancer cells, a direct interaction between Reg4 and EGFR was not observed. This study is focused on identifying the cell surface binding partner of Reg4 and dissecting its role in colorectal cancer and pancreatic cancer growth and stem cell survival. In vitro models of human colorectal cancer and pancreatic cancer were used to evaluate the results. Results of this study find: (i) Reg4 interacts with CD44, a transmembrane protein expressed by a population of colorectal cancer and pancreatic cancer cells; (ii) Reg4 activates regulated intramembrane proteolysis of CD44 resulting in γ-secretase–mediated cleavage and release of the CD44 intracytoplasmic domain (CD44ICD) that functions as a transcriptional activator of D-type cyclins involved in the regulation of cancer cell proliferation and Klf4 and Sox2 expression involved in regulating pluripotency of cancer stem cells; and (iii) Reg4 significantly increases colorectal cancer and pancreatic cancer cell proliferation and their clonogenic potential in stem cell assays. Implications: These results suggest that pro-proliferative and pro-stemness effects of Reg4 are mediated through γ-secretase–mediated CD44/CD44ICD signaling, hence strategies to disrupt Reg4-CD44-γ-secretase-CD44ICD signaling axis may increase cancer cell susceptibility to chemo- and radiotherapeutics.

Expression of anti-apoptotic genes is frequently elevated in tumors, where they increase resistance to chemotherapeutic agents and predict poor patient outcomes. However, key cellular factors regul...

Editorial: Regulators of radiosensitivity in colorectal cancer

Ferroptosis has recently been described as an iron-dependent subroutine of programmed cell death. Cancers driven by oncogenic Ras mutations, such as pancreatic ductal adenocarcinoma, are particularly vulnerable to ferroptosis and are thus promising candidates for antineoplastic drugs targeting this unique form of programmed cell death. Our group has developed a cancer-specific drug conjugate (ACXT-3102), consisting of a proapoptotic sigma-2 ligand as a delivery moiety (SV119), linked to an inhibitor of the cystine antiporter xCT (dm-erastin), an established inducer of ferroptosis. We hypothesized that ACXT-3102 would trigger apoptosis and ferroptosis via its discrete chemical components, representing a new approach to clinical therapy for pancreatic ductal adenocarcinoma. In vitro cell viability assays corroborated our earlier findings that ACXT-3102 is a potent inducer of cancer cell death. The sigma-2 delivery component of ACXT-3102 induced canonical markers of apoptosis, including cleaved caspase-3/7 and PARP, whereas the dm-erastin cargo component induced canonical markers of ferroptosis, including lipid peroxidation and consumption of glutathione peroxidase 4. These changes resulted in the accumulation of reactive oxygen species. Subsequently, we found that ACXT-3102–mediated cell death was accompanied by the activation of MAPK/ERK signaling, presumably via the reactive oxygen species–dependent degradation of dual-specificity phosphatase 6, a negative regulator of MAPK/ERK phosphorylation. We suspected that this was a compensatory reaction and that ACXT-3102–induced cancer cell death would be augmented by inhibition of MAPK/ERK signaling. We successfully combined ACXT-3102 with trametinib (MEK inhibitor) to enhance the overall efficacy of treatment in vitro and in vivo, presumably by targeting ACXT-3102–induced upregulation of MAPK/ERK.

Trametinib potentiates ACXT-3102 cytotoxicity in vitro.