Microenvironmental Metabolites in the Intestine: Messengers between Health and Disease

The intestinal mucosa is a highly absorptive organ and simultaneously constitutes the physical barrier between the host and a complex outer ecosystem. Intestinal epithelial cells (IECs) represent a special node that receives signals from the host and the environment and translates them into corresponding responses. Specific molecular communication systems such as metabolites are known to transmit information across the intestinal boundary. The gut microbiota or food-derived metabolites are extrinsic factors that influence the homeostasis of the intestinal epithelium, while mitochondrial and host-derived cellular metabolites determine the identity, fitness, and regenerative capacity of IECs. Little is known, however, about the role of intrinsic and extrinsic metabolites of IECs in the initiation and progression of pathological processes such as inflammatory bowel disease and colorectal cancer as well as about their impact on intestinal immunity. In this review, we will highlight the most recent contributions on the modulatory effects of intestinal metabolites in gut pathophysiology, with a particular focus on metabolites in promoting intestinal inflammation or colorectal tumorigenesis. In addition, we will provide a perspective on the role of newly identified oncometabolites from the commensal and opportunistic microbiota in shaping response and resistance to antitumor therapy.

Rpl24Bst mutation suppresses colorectal cancer by promoting eEF2 phosphorylation via eEF2K

Increased protein synthesis supports the rapid cell proliferation associated with cancer. The Rpl24Bst mutant mouse reduces the expression of the ribosomal protein RPL24 and has been used to suppress translation and limit tumorigenesis in multiple mouse models of cancer. Here, we show that Rpl24Bst also suppresses tumorigenesis and proliferation in a model of colorectal cancer (CRC) with two common patient mutations, Apc and Kras. In contrast to previous reports, Rpl24Bst mutation has no effect on ribosomal subunit abundance but suppresses translation elongation through phosphorylation of eEF2, reducing protein synthesis by 40% in tumour cells. Ablating eEF2 phosphorylation in Rpl24Bst mutant mice by inactivating its kinase, eEF2K, completely restores the rates of elongation and protein synthesis. Furthermore, eEF2K activity is required for the Rpl24Bst mutant to suppress tumorigenesis. This work demonstrates that elevation of eEF2 phosphorylation is an effective means to suppress colorectal tumorigenesis with two driver mutations. This positions translation elongation as a therapeutic target in CRC, as well as in other cancers where the Rpl24Bst mutation has a tumour suppressive effect in mouse models.

Oxidized Linoleic Acid Interacts With Gut Microbiota to Promote Colitis and Colorectal Cancer

Background: Emerging human studies support that a high intake of linoleic acid (LA), which is an essential fatty acid and the most abundant polyunsaturated fatty acid (PUFA) in human diet, is associated with increased risks of developing inflammatory bowel disease (IBD). As a PUFA, LA is highly prone to oxidation; to date, it remains unknown whether the observed IBD-enhancing effect of dietary LA is caused by LA itself (un-oxidized LA) or oxidized LA. Answering this question will help us to identify the exact risk factor of IBD and to develop targeted strategies to reduce the risks of IBD; in addition, the obtained information could have important implications for optimizing dietary recommendations. Results: Here we show that oxidized LA, rather than LA itself, exacerbates colitis and colorectal tumorigenesis, via gut microbiota- and microbial receptor Toll-like receptor 4 (TLR4)-dependent mechanisms. Administration of a diet containing oxidized LA, at low human-consumption levels, increases the severity of colitis and exaggerates the development of colorectal tumorigenesis in mice. In addition, oxidized LA alters gut microbiota and fails to promote colitis in antibiotic-treated mice. Finally, oxidized LA activates TLR4 signaling in vivo and fails to promote colitis in Tlr4-/- mice. Conclusions: Overall, these results support that oxidized LA could be a risk factor of IBD and associated diseases, highlighting the need to develop novel strategies to further control oxidation of dietary LA and potentially update policies regulating the levels of oxidized LA in food products.

Atypical chemokine receptor 3 (ACKR3) induces the perturbation of rRNA biogenesis in colonic cells: a novel mechanism of colorectal tumorigenesis

Background Atypical chemokine receptor 3 (ACKR3) has emerged as a key player in several biologic processes. Its atypical “intercepting receptor” signaling properties have established ACKR3 as the main regulator in many pathophysiological processes. In this study, we investigated the mechanisms of ACKR3 in promoting Colitis and colorectal tumorigenesis. Methods ACKR3 and clinically relevant was evaluated in human colonic cancer specimens. The mechanism of ACKR3-induced perturbation of rRNA biogenesis was performed in Villin-ACKR3-IREF mice specifically expressed ACKR3 in intestines. Nuclear β-arr1 and the interaction of NOLC1 to Fibrillarin were analyzed in vitro and in vivo assays. Results Activation of ACKR3 promotes Colitis and colorectal tumorigenesis, in human and animal model, through NOLC1-induced perturbations of rRNA biogenesis. Human colonic cancer tissues demonstrated higher expression of ACKR3, and high ACKR3 expression was associated with the increased severity of Colitis and colorectal tumorigenesis. Villin-ACKR3 transgenic mice demonstrated the characteristics of ACKR3-induced colorectal cancer, showing the nuclear β-arrestin-1-activated perturbation of rRNA biogenesis. Activation of ACKR3 induced nuclear translocation of β-arrestin-1 (β-arr1), leading to the interaction of β-arr1 with nucleolar and coiled-body phosphoprotein 1 (NOLC1). As the highly phosphorylated protein in the nucleolus, NOLC1 further interacted with Fibrillarin, a conserved nucleolar methyltransferase responsible for ribosomal RNA methylation, leading to the increase of methylation in Histone H2A, resulting in the promotion of rRNA transcription of ribosome biogenesis. Conclusion ACKR3 promotes Colitis and colorectal tumorigenesis through the perturbation of rRNA biogenesis by nuclear β-arr1-induced interaction of NOLC1 with Fibrillarin.

ZNF545 loss promotes ribosome biogenesis and protein translation to initiate colorectal tumorigenesis in mice

Ribosome biogenesis plays a pivotal role in tumorigenesis by supporting robust protein translation. We investigate the functional and molecular mechanism of Zinc finger protein 545 (ZNF545), a transcriptional repressor for ribosomal RNA (rRNA), in colorectal cancer (CRC). ZNF545 was silenced in CRC compared to adjacent normal tissues (P < 0.0001), implying a tumor-suppressive role. Colon-specific Znf545 knockout in mice accelerated CRC in ApcMin/+ and azoxymethane/dextran sulfate sodium-induced CRC. Mechanistically, we demonstrated that ZNF545 uses its two zinc finger clusters to bind to minimal rDNA promoter, where it assembled transcriptional repressor complex by interacting with KAP1. Znf545 deletion in mouse embryonic fibroblasts not only increased rRNA transcription rate and the nucleolar size and number but also altered the nucleolar composition and architecture with an increased number of fibrillar centers surrounded by net-like dense fibrillar components. Consequently, Znf545 deletion promoted the gene expression of translation machinery, protein translation, and cell growth. Consistent with its tumor-suppressive role, ZNF545 overexpression in CRC cells induced growth arrest and apoptosis. Finally, administration of rRNA synthesis inhibitor, CX-5461, inhibited CRC development in Znf545Δ/ΔApcMin/+ mice. In conclusion, ZNF545 suppresses CRC through repressing rRNA transcription and protein translation. Targeting rRNA biosynthesis in ZNF545-silenced tumors is a potential therapeutic strategy for CRC.