Michelina Plateroti

During amphibian metamorphosis, a postembryonic developmental process, the thyroid hormone T3 induces multiple responses across organs and tissues, including intestinal remodeling. Indeed, the intestine reaches full maturity under the influence of this hormone, which, by acting through the nuclear receptors TRs, also induces the neogenesis of epithelial stem cells. A symmetric function of T3/TRα1 has been established in the developing mammalian intestine, as well as in the control of epithelial homeostasis and stem cell biology. While most studies have focused on the mammalian small intestine, very few have investigated T3 function in the colon, and none have done so at the single-cell level. We investigated here how T3 affects the physiological renewal of colon epithelium using a single-cell (sc) RNA-seq approach. Our results reveal a T3-dependent alteration of differentiation trajectories and immunolabeling experiments show an accumulation of committed goblet progenitors at the bottom of colonic crypts. Altogether, these results describe, for the first time, the action of T3 on distinct colon epithelial cell populations in the mouse. In addition, they provide further evidence on the mechanisms underlying T3’s role in controlling cell fate during physiological regeneration.

Thyroid hormones (THs, namely T3 and T4) regulate intestinal development and homeostasis via thyroid hormone nuclear receptors (TRs), which are T3-modulated transcription factors. Previous work has highlighted the importance of THs and the TRα1 receptor in intestinal stem cell biology and tumor formation, through actions on WNT, NOTCH, and BMP signaling pathways, which mediate epithelial-stromal cell interactions. Recent findings underscore the critical role of stromal cells in maintaining homeostasis and interacting with colonic stem cells. Stromal cells, especially cancer-associated fibroblasts (CAFs), are also essential in colorectal cancer (CRC). While the TH/TR signaling on gut epithelia-stromal interactions is well characterized in amphibians during the TH-dependent metamorphosis process, its function in the normal mammalian colon is still poorly defined, and in CRCs, it remains underexplored. In addition, it is worth underlining that TRα1 mutations in patients are responsible for Resistance to Thyroid Hormone-α (RTH-α) syndrome. This syndrome is a complex pathology that recapitulates typical traits of hypothyroidism, including gut malfunction. Up to now, very little is known about the cellular alterations in the gut of RTH-α patients. This review summarizes recent studies on the roles of T3 and TRα1 in colon physiopathology, with an emphasis on epithelial/stromal or tumor/stromal interactions via cell-cell signaling.

The urochordate Ciona robusta exhibits numerous functional and morphogenetic traits that are shared with vertebrate models. While prior investigations have identified several analogies between the gastrointestinal tract (i.e., gut) of Ciona and mice, the molecular mechanisms responsible for these similarities remain poorly understood. This study seeks to address this knowledge gap by investigating the transcriptional landscape of the adult stage gut. Through comparative genomics analyses, we identified several evolutionarily conserved components of signaling pathways of pivotal importance for gut development (such as WNT, Notch, and TGFβ-BMP) and further evaluated their expression in three distinct sections of the gastrointestinal tract by RNA-seq. Despite the presence of lineage-specific gene gains, losses, and often unclear orthology relationships, the investigated pathways were characterized by well-conserved molecular machinery, with most components being expressed at significant levels throughout the entire intestinal tract of C. robusta. We also showed significant differences in the transcriptional landscape of the stomach and intestinal tract, which were much less pronounced between the proximal and distal portions of the intestine. This study confirms that C. robusta is a reliable model system for comparative studies, supporting the use of ascidians as a model to study gut physiology.

While the toxicity of pollutants has been rather well explored in simple laboratory conditions, there is little knowledge on their real toxicity in natural environments and living organisms because pollutants are often interacting and trapped into organic matter. Because of these interactions, their real concentrations can also be underestimated. Here we studied the nature, intensity, and strength of the interactions between bisphenol A and nano-sized natural organic matter. The bioavailability and toxicity of the complexed bisphenol A were tested with human colon adenocarcinoma cell lines. Results show that that interaction of bisphenol A with organic matter reduces bisphenol A cytotoxicity. Moreover, the bisphenol A-organic matter interaction is weak in the first hour then very stable after 24 h. Once formed, the bisphenol A-organic matter complex escapes detection and, as a consequence, the levels of pollutants in organic-rich media is most probably underestimated. The mechanism of interaction involves hydrophobic and π-stacking forces inside the core of nano-sized organic matter.

Colorectal cancers (CRCs) are highly heterogeneous and show a hierarchical organization, with cancer stem cells (CSCs) responsible for tumor development, maintenance, and drug resistance. Our previous studies showed the importance of thyroid hormone-dependent signaling on intestinal tumor development and progression through action on stem cells. These results have a translational value, given that the thyroid hormone nuclear receptor TRα1 is upregulated in human CRCs, including in the molecular subtypes associated with CSC features. We used an established spheroid model generated from the human colon adenocarcinoma cell line Caco2 to study the effects of T3 and TRα1 on spheroid formation, growth, and response to conventional chemotherapies. Our results show that T3 treatment and/or increased TRα1 expression in spheroids impaired the response to FOLFIRI and conferred a survival advantage. This was achieved by stimulating drug detoxification pathways and increasing ALDH1A1-expressing cells, including CSCs, within spheroids. These results suggest that clinical evaluation of the thyroid axis and assessing TRα1 levels in CRCs could help to select optimal therapeutic regimens for patients with CRC. Proposed mechanism of action of T3/TRα1 in colon cancer spheroids. In the control condition, TRα1 participates in maintaining homeostatic cell conditions. The presence of T3 in the culture medium activates TRα1 action on target genes, including the drug efflux pumps ABCG2 and ABCB1. In the case of chemotherapy FOLFIRI, the increased expression of ABC transcripts and proteins induced by T3 treatment is responsible for the augmented efflux of 5-FU and Irinotecan from the cancer cells. Taken together, these mechanisms contribute to the decreased efficacy of the chemotherapy and allow cells to escape the treatment. Created with BioRender.com.

Intestinal crypts are composed of heterogeneous and highly plastic cell populations. Lgr5high-stem cells (SC) are responsible for homeostatic renewal, but other cells can revert to an SC-like phenotype to maintain epithelial integrity. Despite their distinct roles in orchestrating homeostasis, both populations have been designated as the putative “cell-of-origin” of colorectal cancer. However, their respective involvement in the emergence of drug-resistant cancer SCs (CSC), responsible for tumor relapse and associated with poor outcome of colorectal cancer, remains elusive. In this context, the intestinal SC/progenitor-marker Musashi1 (MSI1) is interesting as it plays important functions in intestinal homeostasis and is frequently overexpressed in human colorectal cancer. Therefore, our aims were: (i) to study the impact of chemotherapy on Lgr5-expressing and MSI1-expressing cell populations, (ii) to explore the effect of increased MSI1 levels in response to treatment, and (iii) to evaluate the relevance in human colorectal cancer. Engineered mouse models treated with the therapeutic agent 5-fluorouracil showed that upon increased MSI1 levels, Lgr5high SCs remain sensitive while Lgr5low progenitors reprogram to a drug-resistant phenotype. This resulted in the expansion of an MSI1-expressing cell subpopulation with improved resistance to DNA damage and increased detoxification, typical properties of dormant-CSCs that can reactivate after chemotherapy. Analysis in patients with colorectal cancer revealed a correlation between MSI1 levels and tumor grading, CSC phenotype, and chemoresistance. Altogether, these results shed new light on the biology and plasticity of normal crypt and cancer cell populations and also open new perspectives to target MSI1 to improve chemotherapy outcome.Significance:This study unveils paradoxical roles for MSI1, underlining its importance in facilitating intestinal regeneration upon injury but also unraveling its new function in drug-resistant colorectal cancer stem cells.

Intestinal crypts are composed of heterogeneous and highly plastic cell populations. Lgr5high-stem cells (SC) are responsible for homeostatic renewal, but other cells can revert to an SC-like phenotype to maintain epithelial integrity. Despite their distinct roles in orchestrating homeostasis, both populations have been designated as the putative “cell-of-origin” of colorectal cancer. However, their respective involvement in the emergence of drug-resistant cancer SCs (CSC), responsible for tumor relapse and associated with poor outcome of colorectal cancer, remains elusive. In this context, the intestinal SC/progenitor-marker Musashi1 (MSI1) is interesting as it plays important functions in intestinal homeostasis and is frequently overexpressed in human colorectal cancer. Therefore, our aims were: (i) to study the impact of chemotherapy on Lgr5-expressing and MSI1-expressing cell populations, (ii) to explore the effect of increased MSI1 levels in response to treatment, and (iii) to evaluate the relevance in human colorectal cancer. Engineered mouse models treated with the therapeutic agent 5-fluorouracil showed that upon increased MSI1 levels, Lgr5high SCs remain sensitive while Lgr5low progenitors reprogram to a drug-resistant phenotype. This resulted in the expansion of an MSI1-expressing cell subpopulation with improved resistance to DNA damage and increased detoxification, typical properties of dormant-CSCs that can reactivate after chemotherapy. Analysis in patients with colorectal cancer revealed a correlation between MSI1 levels and tumor grading, CSC phenotype, and chemoresistance. Altogether, these results shed new light on the biology and plasticity of normal crypt and cancer cell populations and also open new perspectives to target MSI1 to improve chemotherapy outcome.Significance:This study unveils paradoxical roles for MSI1, underlining its importance in facilitating intestinal regeneration upon injury but also unraveling its new function in drug-resistant colorectal cancer stem cells.

The THRA gene, encoding the thyroid hormone nuclear receptor TR alpha 1, is expressed in an increasing gradient at the bottom of intestinal crypts, overlapping with high Wnt and Notch activities. Importantly, THRA is upregulated in colorectal cancers, particularly in the high-Wnt molecular subtype. The basis of this specific and/or altered expression pattern has remained unknown. To define the mechanisms controlling THRA transcription and TR alpha 1 expression, we used multiple in vitro and ex vivo approaches. Promoter analysis demonstrated that transcription factors important for crypt homeostasis and altered in colorectal cancers, such as transcription factor 7-like 2 (TCF7L2; Wnt pathway), recombining binding protein suppressor of hairless (RBPJ; Notch pathway), and homeobox protein CDX2 (epithelial cell identity), modulate THRA activity. Specifically, although TCF7L2 and CDX2 stimulated THRA, RBPJ induced its repression. In-depth analysis of the Wnt-dependent increase showed direct regulation of the THRA promoter in cells and of TR alpha 1 expression in murine enteroids. Given our previous results on the control of the Wnt pathway by TR alpha 1, our new results unveil a complex regulatory loop and synergy between these endocrine and epithelial-cell-intrinsic signals. Our work describes, for the first time, the regulation of the THRA gene in specific cell and tumor contexts.

Several studies emphasized the function of the thyroid hormones in stem cell biology. These hormones act through the nuclear hormone receptor TRs, which are T3-modulated transcription factors. Pioneer work on T3-dependent amphibian metamorphosis showed that the crosstalk between the epithelium and the underlying mesenchyme is absolutely required for intestinal maturation and stem cell emergence. With the recent advances of powerful animal models and 3D-organoid cultures, similar findings have now begun to be described in mammals, where the action of T3 and TRα1 control physiological and cancer-related stem cell biology. In this review, we have summarized recent findings on the multiple functions of T3 and TRα1 in intestinal epithelium stem cells, cancer stem cells and their niche. In particular, we have highlighted the regulation of metabolic functions directly linked to normal and/or cancer stem cell biology. These findings help explain other possible mechanisms by which TRα1 controls stem cell biology, beyond the more classical Wnt and Notch signaling pathways.