The intestine is very susceptible and is affected by the harsh conditions caused by DNA-altering agents, such as radiation and chemotherapy, during cancer treatment. For example, many patients with tumors in the gastrointestinal cavity receive radiotherapy, a treatment that often also damages the healthy intestine and affects its regenerative capacity.
It is therefore very important to understand how intestinal epithelial regeneration occurs. The cellular and molecular mechanisms involved in this key process are not yet fully understood.
Researchers at the Spanish National Cancer Research Centre (CNIO) have now discovered one of the cellular and molecular mechanisms essential for the regeneration of the intestinal epithelium. This finding lays the foundations for stimulating this process if it fails, and for protecting it against damage caused by radiotherapy and chemotherapy.
According to the study, what prompts intestinal stem cells to regenerate the mucosa depends on the communication between different cell types in the epithelial tissue. The researchers have also found a way to intervene in this communication, and thereby, boost intestinal regeneration.
The paper is published this week in the Journal of Experimental Medicine. The research was led by the head of the CNIO’s Growth Factors, Nutrients and Cancer Group, Nabil Djouder, and Almudena Chaves-Pérez and Karla Santos-de-Frutos are first authors.
The group has spent years researching how to improve the regeneration of various organs—particularly the liver and intestinal mucosa—and thus mitigate the effects of radiotherapy. Their findings during this period have been published in high-impact journals.
‘Fascinating’ four-way cellular communication
“Regeneration of the intestinal epithelium is very important in the proper functioning of the intestine,” explains Djouder. “Until now, we knew that it was driven by powerful mitogenic factors—proteins—that stimulate the proliferation of intestinal stem cells, but we didn’t know how these factors were regulated.”
This new study suggests that—unexpectedly for the researchers—it is the progenitor cells involved in regenerating the epithelial mucosa that modulate the production of mitogenic factors. The process is as follows: when severe damage occurs, injury to the progenitor cells leads to tissue inflammation; this in turn slows down the production of mitogenic factors and thus the proliferation of stem cells and the subsequent regeneration of the mucous membrane.
“For us, this communication between at least four different cell types is new: progenitor cells, which differentiate to form the epithelial mucosa; cells that secrete mitogenic factors; inflammatory cells; and intestinal stem cells themselves,” says Djouder. “This communication must be very well controlled, so that the tissue responds appropriately to aggressions.”
“That progenitor cells communicate with inflammatory cells and coordinate the proliferation rate of intestinal stem cells is fascinating,” he adds.
Djouder places particular emphasis on the new role that progenitor cells have been found to play: “Our study suggests that progenitor cells are not mere bystanders in the process of epithelial regeneration, but play an active and important role in the decisions that intestinal stem cells make in regeneration. Progenitor cells tell intestinal stem cells when and how to divide, and thus control their self-regeneration.”
Reducing the side effects of radiotherapy
“This study has allowed us to better understand cell cooperation in order to find new ways to reduce adverse effects in traditional cancer treatments,” say Chaves-Pérez and Santos-de-Frutos, lead authors of the paper.
The group has also confirmed findings observed in previous work, namely that c-MYC oncogene plays a key role in regeneration. Due to radiation damage and the increase of c-MYC in progenitor cells, inflammation in the intestine increases and mitogenic protein levels are reduced; however, by removing or inhibiting c-MYC, the process is reversed: inflammation is reduced, mitogenic factors increase and intestinal regeneration during severe damage improves.
“Our data show an unexpected role for progenitor cells in the control of inflammatory signaling and mitogenic factor production, essential for maintaining intestinal stem cell proliferation and tissue regeneration,” the authors write.
The bone marrow and the small intestine are radiosensitive tissues, and a high dose of radiation results in hematopoietic and intestinal injury (McBride and Schaue, 2020). Radiation-induced intestinal injury (RIII) manifests following whole-body or abdominal irradiation at a dose of more than 6 Gy in humans.
At these doses of radiation, massive intestinal stem cells (ISCs) are depleted, which compromises epithelial integrity and impairs epithelial renewal. The breakdown of the mucosal barrier further leads to fluid loss, electrolyte imbalance, sepsis and even death (Macià I Garau et al., 2011). In contrast to hematopoietic failure that can be rescued by bone marrow transplantation and supportive care, there are no approved agents to prevent or mitigate RIII (Singh and Seed, 2017).
Importantly, despite advances in treatment delivery techniques, abdominal and pelvic radiotherapy inevitably evoke intestinal toxicity that restricts maximum doses of irradiation, which prevents patients from competing treatment and causes acute and chronic gastrointestinal tract complications, thus limiting the efficiency of therapy and reducing the patient’s quality of life (Harb et al., 2014). Therefore, highly effective radioprotectors with fewer side effects are urgently needed for RIII.
ISCs reside at the bottom of the crypts and differentiate into highly proliferative transit amplifying cells that give rise to all differentiated cell types, including enterocytes, goblet cells, tuft cells, enteroendocrine cells and Paneth cells. Thus, ISCs are fundamental to the maintenance of intestinal homeostasis and epithelia regeneration following radiation exposure (Barker et al., 2012).
Ionizing radiation (IR) induces DNA damage, and DNA double-strand break (DSB) is the most serious type of DNA damage. DSBs in ISCs and intestinal progenitors initiate a coordinated signaling network that recognizes exposed DNA lesions, induces cell cycle arrest, activates DSB repair or signals cell death (Lund, 2012).
Various modes of radiation-induced intestinal cell death, including apoptosis, mitotic catastrophe and pyroptosis have been implicated (Kirsch et al., 2010; Leibowitz et al., 2011; Hu et al., 2016). Accumulating evidence shows that IR-induced gastrointestinal epithelium apoptosis occurs in two distinct phases. Radiation exposure rapidly activates p53 in crypt cells, which initiates p53-upregulated modulator of apoptosis-dependent (PUMA-dependent) apoptosis via the intrinsic pathway within 3–6 h following radiation.
A delayed wave of p53-independent apoptosis occurs at 24 h after irradiation when ISCs and crypt progenitors are released from G2 arrest (Kirsch et al., 2010). Unsolved DSBs in proliferative crypt cells render aberrant mitosis or mitotic catastrophe that compromises epithelium regeneration. Researches indicate that the delayed apoptosis probably arise as the consequence of mitotic catastrophe (Vitale et al., 2011). Thus, the resolution of accumulated DNA damage in ISCs and progenitors is critical in their capabilities for epithelial regeneration.
Generally, new drug development, including radioprotectors, is a costly and time-consuming process. Therefore, an alternative strategy to repurpose approved drugs with radioprotective effects could be promising. Disulfiram (DSF), also known as Antabuse, has been used as a treatment for alcoholism since it was approved by the FDA in 1948 (Johansson, 1992).
The radioprotective properties of DSF have been extensively observed. Several groups have reported that DSF alleviates radiation-induced oxidative stress and DNA damage in vitro and in vivo (Gandhi et al., 2003; Nemavarkar et al., 2004). Moreover, the thiol compound diethyldithiocarbamate (DTC) is an obligatory intermediate in the metabolism of disulfiram and DTC has been reported to protect mice against a lethal dose of X-ray irradiation (Stromme and Eldjarn, 1966).
In the present study, we explored whether DSF protects against intestinal injury induced by high single-dose irradiation in a mouse model. Our results revealed that DSF decreased DNA damage accumulation in intestinal epithelium crypt cells and enhanced ISCs survival and crypt regeneration, thus ameliorating intestinal injuries and improving the survival of mice after lethal irradiation.
Interestingly, DSF treatment did not inhibit radiation-induced apoptosis of crypt cells at the early phase but decreased delayed apoptosis in crypt cells. Our data suggested that DSF may be a promising protectant in the prevention of radiation-induced intestinal injury.
reference link :https://www.frontiersin.org/articles/10.3389/fphar.2022.852669/full
reference : More information: Almudena Chaves-Pérez et al, Transit-amplifying cells control R-spondins in the mouse crypt to modulate intestinal stem cell proliferation, Journal of Experimental Medicine (2022). DOI: 10.1084/jem.20212405