Krüppel-like Factor 4 Modulates Development of BMI1(+) Intestinal Stem Cell-Derived Lineage Following γ-Radiation-Induced Gut Injury in Mice

Enhanced infection efficiency and cytotoxicity mediated by vpx-containing lentivirus in chimeric antigen receptor macrophage (CAR-M)

Genetically modified macrophage infusion has been proven to be a novel treatment for cancer. One of the most important processes in macrophage-based therapy is the efficient transfer of genes. HIV-1-derived lentiviruses were widely used as delivery vectors in chimeric antigen receptor T and NK cell construction. While macrophages are relatively refractory to this lentiviral vector transduction as a result of the myeloid-specific restriction factor SAMHD1, which inhibited the virion cycle through exhausting the dNTPs pool and degradating RNAs. An efficient macrophage transduction strategy has been developed via packaging the HIV-2 accessory protein Vpx into the virion. Vpx counteracts SAMHD1 through CRL4 (DCAF1) E3 ubiquitin ligase mediated SAMHD1 degradation, yet the influence by the introduction of Vpx on macrophage has not been fully evaluated. Here, we constructed the chimeric lentiviral vector HIV-1-Vpx and systematically analyzed the infection efficiency of this vector in time-dependent manner. Our results showed that the simplified chimeric virus exhibited dramatically enhanced infection in human macrophages compared to normal lentivirus. Moreover, transcriptome sequencing was performed to evaluate the cellular status after chimeric virus infection. The sequencing results indicated that Vpx introduction promoted macrophage remodeling towards a proinflammatory phenotype, without affecting classic M1/M2 cell surface markers. Our results suggest that the Vpx-containing lentivirus could be used as an ideal tool for the generation of genetically engineered macrophages with high gene transfer efficiency and poised proinflammatory gene sets, especially for solid tumor treatment.

Overexpression of KLF4 Suppresses Pulmonary Fibrosis through the HIF-1α/Endoplasmic Reticulum Stress Signaling Pathway

The hypoxia-inducible factor-1α/endoplasmic reticulum stress signaling pathway (HIF-1α/ERS) has a crucial role in the pathogenetic mechanism of pulmonary fibrosis (PF). However, the upstream regulatory mediators of this pathway remain unclear. In the present study, by conducting bioinformatics analysis, we found that Krüppel-like factor 4 (KLF4) expression was decreased in the lung tissues of patients with idiopathic pulmonary fibrosis (IPF) as compared to that in patients with non-IPF. Furthermore, KLF4 expression was significantly reduced (p = 0.0331) in bleomycin-induced fibrotic HFL-1 cells. Moreover, in mice with bleomycin-induced PF, the degree of fibrosis was significantly reduced in mice overexpressing KLF4 as compared to that in wild-type mice. In mice and HFL-1 cells, KLF4 overexpression significantly reduced bleomycin-induced protein expression of HIF-1α (p = 0.0027) and ERS markers, particularly p-IRE1α (p = 0.0255) and ATF6 (p = 0.0002). By using the JASPAR database, we predicted that KLF4 has five binding sites for the HIF-1α promoter. The results of in vitro and in vivo studies suggest that KLF4 may inhibit PF through the HIF-1α/ERS pathway. This finding could guide the development of future therapies for PF and facilitate the identification of appropriate biomarkers for routine clinical diagnosis of PF.

Sonic Hedgehog and WNT Signaling Regulate a Positive Feedback Loop Between Intestinal Epithelial and Stromal Cells to Promote Epithelial Regeneration

BACKGROUND AND AIMS

Active intestinal stem cells are prone to injury by ionizing radiation. We previously showed that upon radiation-induced injury, normally quiescent reserve intestinal stem cells (rISCs) (marked by BMI1) are activated by Musashi-1 (MSI1) and exit from the quiescent state to regenerate the intestinal epithelium. This study aims to further establish the mechanism that regulates activation of Bmi1-CreER;Rosa26eYFP (Bmi1-CreER) rISCs following γ radiation-induced injury.

METHODS

Bmi1-CreER mice were treated with tamoxifen to initiate lineage tracing of BMI1 (eYFP+) cells and exposed to 12 Gy of total body γ irradiation or sham. Intestinal tissues were collected and analyzed by immunofluorescence, Western blot, reverse-transcription quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and chromatin immunoprecipitation real-time polymerase chain reaction.

RESULTS

After irradiation, increased expression of Msi1 in eYFP+ cells was accompanied by increased expression of Axin2, a WNT marker. Promoter studies of the Msi1 gene indicated that Msi1 is a WNT target gene. Coculture of stromal cells isolated from irradiated mice stimulated Bmi1-CreER-derived organoid regeneration more effectively than those from sham mice. Expression of WNT ligands, including Wnt2b, Wnt4, Wnt5a, and Rspo3, was increased in irradiated stromal cells compared with sham-treated stromal cells. Moreover, expression of the Sonic hedgehog (SHH) effector Gli1 was increased in stromal cells from irradiated mice. This was correlated with an increased expression of SHH in epithelial cells postirradiation, indicating epithelial-stromal interaction. Finally, preinjury treatment with SHH inhibitor cyclopamine significantly reduced intestinal epithelial regeneration and Msi1 expression postirradiation.

CONCLUSIONS

Upon ionizing radiation-induced injury, intestinal epithelial cells increase SHH secretion, stimulating stromal cells to secrete WNT ligands. WNT activators induce Msi1 expression in the Bmi1-CreER cells. This stromal-epithelial interaction leads to Bmi1-CreER rISCs induction and epithelial regeneration.

New insights into KLFs and SOXs in cancer pathogenesis, stemness, and therapy

Despite the development of cancer therapies, the success of most treatments has been impeded by drug resistance. The crucial role of tumor cell plasticity has emerged recently in cancer progression, cancer stemness and eventually drug resistance. Cell plasticity drives tumor cells to reversibly convert their cell identity, analogous to differentiation and dedifferentiation, to adapt to drug treatment. This phenotypical switch is driven by alteration of the transcriptome. Several pluripotent factors from the KLF and SOX families are closely associated with cancer pathogenesis and have been revealed to regulate tumor cell plasticity. In this review, we particularly summarize recent studies about KLF4, KLF5 and SOX factors in cancer development and evolution, focusing on their roles in cancer initiation, invasion, tumor hierarchy and heterogeneity, and lineage plasticity. In addition, we discuss the various regulation of these transcription factors and related cutting-edge drug development approaches that could be used to drug "undruggable" transcription factors, such as PROTAC and PPI targeting, for targeted cancer therapy. Advanced knowledge could pave the way for the development of novel drugs that target transcriptional regulation and could improve the outcome of cancer therapy.

Exposure to MoS2 nanosheets or bulk activated Kruppel-like factor 4 in 3D Caco-2 spheroids in vitro and mouse intestines in vivo

MoS₂ nanosheets (NSs) are novel 2D nanomaterials (NMs) being used in many important fields. Recently, we proposed the need to evaluate the influences of NMs on Kruppel-like factors (KLFs) even if these materials are relatively biocompatible. In this study, we investigated the influences of MoS₂ NSs or bulk on KLF4 signaling pathway in 3D Caco-2 spheroids in vitro and mouse intestines in vivo. Through the analysis of our previous RNA-sequencing data, we found that exposure to MoS₂ NSs or bulk activated KLF4 expression in 3D Caco-2 spheroids. Consistently, these materials also activated KLF4-related gene ontology (GO) terms and down-regulated a panel of KLF4-downstream genes. To verify these findings, we repeatedly exposed mice to MoS₂ NSs or bulk materials via intragastrical administration (1 mg/kg bodyweight, once a day, for 4 days). It was shown that oral exposure to these materials decreased bodyweight, leading to relatively higher organ coefficients. As expected, exposure to both types of materials increased Mo elements as well as other trace elements, such as Zn, Fe, and Mn in mouse intestines. The exposure also induced morphological changes of intestines, such as shortening of intestinal villi and decreased crypt depth, which may result in decreased intestinal lipid staining. Consistent with RNA-sequencing data, we found that material exposure increased KLF4 protein staining in mouse intestines and decreased two KLF4 downstream proteins, namely extracellular signal-regulated kinase (ERK) and serine/threonine kinase (AKT). We concluded that MoS₂ materials were capable to activate KLF4-signaling pathway in intestines both in vivo and in vitro.

Krüppel-like Factors 4 and 5 in Colorectal Tumorigenesis

Krüppel-like factors (KLFs) are transcription factors regulating various biological processes such as proliferation, differentiation, migration, invasion, and homeostasis. Importantly, they participate in disease development and progression. KLFs are expressed in multiple tissues, and their role is tissue- and context-dependent. KLF4 and KLF5 are two fascinating members of this family that regulate crucial stages of cellular identity from embryogenesis through differentiation and, finally, during tumorigenesis. They maintain homeostasis of various tissues and regulate inflammation, response to injury, regeneration, and development and progression of multiple cancers such as colorectal, breast, ovarian, pancreatic, lung, and prostate, to name a few. Recent studies broaden our understanding of their function and demonstrate their opposing roles in regulating gene expression, cellular function, and tumorigenesis. This review will focus on the roles KLF4 and KLF5 play in colorectal cancer. Understanding the context-dependent functions of KLF4 and KLF5 and the mechanisms through which they exert their effects will be extremely helpful in developing targeted cancer therapy.

Klf4 protects thymus integrity during late pregnancy

Pregnancy causes abrupt thymic atrophy. This atrophy is characterized by a severe decrease in the number of all thymocyte subsets and qualitative (but not quantitative) changes in thymic epithelial cells (TECs). Pregnancy-related thymic involution is triggered by progesterone-induced functional changes affecting mainly cortical TECs (cTECs). Remarkably, this severe involution is rapidly corrected following parturition. We postulated that understanding the mechanisms of pregnancy-related thymic changes could provide novel insights into signaling pathways regulating TEC function. When we analyzed genes whose expression in TECs was modified during late pregnancy, we found a strong enrichment in genes bearing KLF4 transcription factor binding motifs. We, therefore, engineered a Psmb11-iCre : Klf4^lox/lox mouse model to study the impact of TEC-specific Klf4 deletion in steady-state conditions and during late pregnancy. Under steady-state conditions, Klf4 deletion had a minimal effect on TEC subsets and did not affect thymic architecture. However, pregnancy-induced thymic involution was much more pronounced in pregnant females lacking Klf4 expression in TECs. These mice displayed a substantial ablation of TECs with a more pronounced loss of thymocytes. Transcriptomic and phenotypic analyses of Klf4 ^-/- TECs revealed that Klf4 maintains cTEC numbers by supporting cell survival and preventing epithelial-to-mesenchymal plasticity during late pregnancy. We conclude that Klf4 is essential for preserving TEC's integrity and mitigating thymic involution during late pregnancy.

Intestinal Epithelial Regeneration in Response to Ionizing Irradiation

The intestinal epithelium consists of a single layer of cells yet contains multiple types of terminally differentiated cells, which are generated by the active proliferation of intestinal stem cells located at the bottom of intestinal crypts. However, during events of acute intestinal injury, these active intestinal stem cells undergo cell death. Gamma irradiation is a widely used colorectal cancer treatment, which, while therapeutically efficacious, has the side effect of depleting the active stem cell pool. Indeed, patients frequently experience gastrointestinal radiation syndrome while undergoing radiotherapy, in part due to active stem cell depletion. The loss of active intestinal stem cells in intestinal crypts activates a pool of typically quiescent reserve intestinal stem cells and induces dedifferentiation of secretory and enterocyte precursor cells. If not for these cells, the intestinal epithelium would lack the ability to recover from radiotherapy and other such major tissue insults. New advances in lineage-tracing technologies allow tracking of the activation, differentiation, and migration of cells during regeneration and have been successfully employed for studying this in the gut. This study aims to depict a method for the analysis of cells within the mouse intestinal epithelium following radiation injury.

KLF4 Regulates Goblet Cell Differentiation in BMI1＋ Reserve Intestinal Stem Cell Lineage during Homeostasis

Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor, expressed in villus cells of the intestinal epithelium, that promotes cellular differentiation and tissue homeostasis. Previous studies suggest that BMI1⁺ cells represent secretory progenitors with reserve intestinal stem cell (rISC) activity. However, it has not been elucidated how KLF4 contributes to crypt regeneration originated from BMI1⁺ rISC lineage during homeostasis. In this study, Bmi1-Cre^ER;Rosa26^eYFP (Bmi1^Ctrl) and Bmi1-Cre^ER;Rosa26^eYFP;Klf4^fl/fl (Bmi1^ΔKlf4) mice were injected with tamoxifen to label BMI1⁺ cells and their lineage and to delete Klf4. During homeostasis, MUC2⁺ goblet cells appeared in the BMI1⁺ cell lineage 2, 3 and 7 days after tamoxifen administration. After Klf4 deletion in BMI1⁺ cells, the number of KLF4⁺ and MUC2⁺ cells in eYFP⁺ cells decreased in Bmi1^ΔKlf4 mice compared with Bmi1^Ctrl mice. Thus, KLF4 was positively correlated with goblet cell differentiation in BMI1⁺ cell derived lineage. In ex-vivo analysis, organoids derived from single eYFP⁺ cells of Bmi1^Ctrl mice contained MUC2-expressing cells that co-expressed KLF4. On the other hand, organoids derived from Klf4-deleted eYFP⁺ cells from Bmi1^ΔKlf4 mice showed reduced number of MUC2-expressing cells. In conclusion, these results suggest that KLF4 regulates goblet cell differentiation in BMI1⁺ ISC-derived lineage during homeostasis.

Interplay among p21Waf1/Cip1, MUSASHI-1 and Krüppel-like factor 4 in activation of Bmi1-CreER reserve intestinal stem cells after gamma radiation-induced injury

Gamma radiation is a commonly used adjuvant treatment for abdominally localized cancer. Since its therapeutic potential is limited due to gastrointestinal (GI) syndrome, elucidation of the regenerative response following radiation-induced gut injury is needed to develop a preventive treatment. Previously, we showed that Krüppel-like factor 4 (KLF4) activates certain quiescent intestinal stem cells (ISCs) marked by Bmi1-Cre^ER to give rise to regenerating crypts following γ irradiation. In the current study, we showed that γ radiation-induced expression of p21^Waf1/Cip1 in Bmi1-Cre^ER cells is likely mitigated by MUSASHI-1 (MSI1) acting as a negative regulator of p21^Waf1/Cip1 mRNA translation, which promotes exit of the Bmi1-Cre^ER cells from a quiescent state. Additionally, Bmi1-specific Klf4 deletion resulted in decreased numbers of MSI1⁺ cells in regenerating crypts compared to those of control mice. We showed that KLF4 binds to the Msi1 promoter and activates its expression in vitro. Since MSI1 has been shown to be crucial for crypt regeneration, this finding elucidates a pro-proliferative role of KLF4 during the postirradiation regenerative response. Taken together, our data suggest that the interplay among p21^Waf1/Cip1, MSI1 and KLF4 regulates Bmi1-Cre^ER cell survival, exit from quiescence and regenerative potential upon γ radiation-induced injury.

Epithelial WNT2B and Desert Hedgehog Are Necessary for Human Colonoid Regeneration after Bacterial Cytotoxin Injury

Intestinal regeneration and crypt hyperplasia after radiation or pathogen injury relies on Wnt signaling to stimulate stem cell proliferation. Mesenchymal Wnts are essential for homeostasis and regeneration in mice, but the role of epithelial Wnts remains largely uncharacterized. Using the enterohemorrhagic E. coli-secreted cytotoxin EspP to induce injury to human colonoids, we evaluated a simplified, epithelial regeneration model that lacks mesenchymal Wnts. Here, we demonstrate that epithelial-produced WNT2B is upregulated following injury and essential for regeneration. Hedgehog signaling, specifically activation via the ligand Desert Hedgehog (DHH), but not Indian or Sonic Hedgehog, is another driver of regeneration and modulates WNT2B expression. These findings highlight the importance of epithelial WNT2B and DHH in regulating human colonic regeneration after injury.

Successful Introduction of Human Renovascular Units into the Mammalian Kidney

BACKGROUND

Cell-based therapies aimed at replenishing renal parenchyma have been proposed as an approach for treating CKD. However, pathogenic mechanisms involved in CKD such as renal hypoxia result in loss of kidney function and limit engraftment and therapeutic effects of renal epithelial progenitors. Jointly administering vessel-forming cells (human mesenchymal stromal cells [MSCs] and endothelial colony-forming cells [ECFCs]) may potentially result in in vivo formation of vascular networks.

METHODS

We administered renal tubule-forming cells derived from human adult and fetal kidneys (previously shown to exert a functional effect in CKD mice) into mice, alongside MSCs and ECFCs. We then assessed whether this would result in generation of "renovascular units" comprising both vessels and tubules with potential interaction.

RESULTS

Directly injecting vessel-forming cells and renal tubule-forming cells into the subcutaneous and subrenal capsular space resulted in self-organization of donor-derived vascular networks that connected to host vasculature, alongside renal tubules comprising tubular epithelia of different nephron segments. Vessels derived from MSCs and ECFCs augmented in vivo tubulogenesis by the renal tubule-forming cells. In vitro coculture experiments showed that MSCs and ECFCs induced self-renewal and genes associated with mesenchymal-epithelial transition in renal tubule-forming cells, indicating paracrine effects. Notably, after renal injury, renal tubule-forming cells and vessel-forming cells infused into the renal artery did not penetrate the renal vascular network to generate vessels; only administering them into the kidney parenchyma resulted in similar generation of human renovascular units in vivo.

CONCLUSIONS

Combined cell therapy of vessel-forming cells and renal tubule-forming cells aimed at alleviating renal hypoxia and enhancing tubulogenesis holds promise as the basis for new renal regenerative therapies.

BMI1-KLF4 axis deficiency improves responses to neoadjuvant concurrent chemoradiotherapy in patients with rectal cancer

PURPOSE

Neoadjuvant concurrent chemoradiotherapy (CCRT) is a gold standard treatment for patients with stage II/III rectal cancer. B-cell-specific Moloney murine leukemia virus insertion site 1 (BMI1) is a member of the polycomb group of proteins that are involved in regulating gene expression. High levels of BMI1 have been demonstrated to contribute to the malignant phenotypes of several cancers; however, its relevance in rectal cancer treated with CCRT is largely unknown.

METHODS AND MATERIALS

We used two patient cohorts to address the clinical relevance of BMI1 in human cancers. In addition, HT-29 and HCT-116 cells were chosen as our in vitro models to verify the role of BMI1 in cell response to ionizing radiation. Stemness-related proteins were analyzed by western blotting and cell survival was determined using clonogenic assays.

RESULTS

BMI1 overexpression was found to significantly correlate with advanced pre-treatment nodal status (N1-N2; p < 0.001), post-treatment tumor stage (T1-T2; p = 0.015), inferior tumor regression grade (p = 0.001), and also an independent prognosis factor in 172 rectal cancer patients receiving CCRT. Serial cell-based functional examination indicated that BMI1 deficiency sensitized cells to radiation treatment by modulating the gene expression of Kruppel-like factor 4 (KLF4) and enhanced radiosensitivity in microsatellite stable (MSS) colorectal cancers. Overexpression of KLF4 partially overcame BMI1-deficiency-mediated γ-H2AX expression after ionizing radiation exposure. Consistent with in vitro data, an analysis of an additional 30 rectal cancer tissue specimens revealed a positive correlation between BMI1 and KLF4 (p = 0.02).

CONCLUSION

Higher levels of BMI1 are associated with poor therapeutic response and adverse outcomes in rectal cancer patients receiving CCRT.

Overexpression of Krüppel-Like Factor 4 Suppresses Migration and Invasion of Non-Small Cell Lung Cancer Through c-Jun-NH2-Terminal Kinase/Epithelial-Mesenchymal Transition Signaling Pathway

Krüppel-like factor 4 (KLF4) is a transcription factor and plays a vital role in cancer initiation and development. However, the role of Krüppel-like factor 4 in the metastasis of non-small cell lung cancer (NSCLC) is not clear. Here, we demonstrated that the expression of Krüppel-like factor 4 was significantly decreased in human non-small cell lung cancer tissues compared with that in normal tissues using Western blot. We performed immunohistochemical staining and observed the decreased expression of Krüppel-like factor 4 in human lung cancer tissues, and metastatic tumor tissues located in the trachea and main bronchus. We also found that the E-cadherin expression was decreased, while vimentin expression was increased in human NSCLC tissues and metastatic tumor tissues located in the trachea and main bronchus. Additionally, enforced expression of Krüppel-like factor 4 in mouse lungs significantly inhibited the metastasis of circulating Lewis lung carcinoma cells to the lungs by attenuating mesenchymal-epithelial transition (MET). Furthermore, cell scratch assays and Matrigel invasion assays revealed that overexpression of Krüppel-like factor 4 inhibited the migration and invasion of non-small cell lung cancer cell lines A549, H1299, H226, and H1650 cells. Moreover, overexpression of Krüppel-like factor 4 attenuated TGF-β1-induced epithelial-mesenchymal transition (EMT) in A549, and inhibited the phosphorylation of c-Jun-NH2-terminal kinase (JNK), an important pathway in metastasis in non-small cell lung cancer. Our in vivo and in vitro findings illustrate that Krüppel-like factor 4 inhibited metastasis and migration of non-small cell lung cancer, and indicate that Krüppel-like factor 4 could be a potential therapeutic target for the treatment of non-small cell lung cancer.

Comparison of Proton and Photon Beam Irradiation in Radiation-Induced Intestinal Injury Using a Mouse Model

When radiotherapy is applied to the abdomen or pelvis, normal tissue toxicity in the gastrointestinal (GI) tract is considered a major dose-limiting factor. Proton beam therapy has a specific advantage in terms of reduced doses to normal tissues. This study investigated the fundamental differences between proton- and X-ray-induced intestinal injuries in mouse models. C57BL/6J mice were irradiated with 6-MV X-rays or 230-MeV protons and were sacrificed after 84 h. The number of surviving crypts per circumference of the jejunum was identified using Hematoxylin and Eosin staining. Diverse intestinal stem cell (ISC) populations and apoptotic cells were analyzed using immunohistochemistry (IHC) and a terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) assay, respectively. The crypt microcolony assay revealed a radiation-dose-dependent decrease in the number of regenerative crypts in the mouse jejunum; proton irradiation was more effective than X-ray irradiation with a relative biological effectiveness of 1.14. The jejunum is the most sensitive to radiations, followed by the ileum and the colon. Both types of radiation therapy decreased the number of radiosensitive, active cycling ISC populations. However, a higher number of radioresistant, reserve ISC populations and Paneth cells were eradicated by proton irradiation than X-ray irradiation, as shown in the IHC analyses. The TUNEL assay revealed that proton irradiation was more effective in enhancing apoptotic cell death than X-ray irradiation. This study conducted a detailed analysis on the effects of proton irradiation versus X-ray irradiation on intestinal crypt regeneration in mouse models. Our findings revealed that proton irradiation has a direct effect on ISC populations, which may result in an increase in the risk of GI toxicity during proton beam therapy.

The Krüppel-Like Factors and Control of Energy Homeostasis

Nutrient handling by higher organisms is a complex process that is regulated at the transcriptional level. Studies over the past 15 years have highlighted the critical importance of a family of transcriptional regulators termed the Krüppel-like factors (KLFs) in metabolism. Within an organ, distinct KLFs direct networks of metabolic gene targets to achieve specialized functions. This regulation is often orchestrated in concert with recruitment of tissue-specific transcriptional regulators, particularly members of the nuclear receptor family. Upon nutrient entry into the intestine, gut, and liver, KLFs control a range of functions from bile synthesis to intestinal stem cell maintenance to effect nutrient acquisition. Subsequently, coordinated KLF activity across multiple organs distributes nutrients to sites of storage or liberates them for use in response to changes in nutrient status. Finally, in energy-consuming organs like cardiac and skeletal muscle, KLFs tune local metabolic programs to precisely match substrate uptake, flux, and use, particularly via mitochondrial function, with energetic demand; this is achieved in part via circulating mediators, including glucocorticoids and insulin. Here, we summarize current understanding of KLFs in regulation of nutrient absorption, interorgan circulation, and tissue-specific use.

The Role of Nrf2 in the Response to Normal Tissue Radiation Injury

The transcription factor Nrf2 is an important modulator of antioxidant and drug metabolism, carbohydrate and lipid metabolism, as well as heme and iron metabolism. Regulation of Nrf2 expression occurs transcriptionally and post-transcriptionally. Post-transcriptional regulation entails ubiquitination followed by proteasome-dependent degradation. Additionally, Nrf2-mediated gene expression is subject to negative regulation by ATF3, Bach1 and cMyc. Nrf2-mediated gene expression is an important regulator of a cell's response to radiation. Although a majority of studies have shown that Nrf2 deficient cells are radiosensitized and Nrf2 over expression confers radioresistance, Nrf2's role in mediating the radiation response of crypt cells is controversial. The Nrf2 activator CDDO attenuates radiation-mediated crypt injury, whereas intestinal crypts in Nrf2 null mice are radiation resistant. Further investigation is needed in order to define the relationship between Nrf2 and radiation sensitivity in Lgr5+ and Bmi1+ cells that regulate regeneration of crypt stem cells. In hematopoietic compartments Nrf2 promotes the survival of irradiated osteoblasts that support long-term hematopoietic stem cell (LT-HSC) niches. Loss of Nrf2 in LT-HSCs increases stem cell intrinsic radiosensitivity, with the consequence of lowering the LD50₃₀. An Nrf2 deficiency drives LT-HSCs from a quiescent to a proliferative state. This results in hematopoietic exhaustion and reduced engraftment after myoablative irradiation. The question of whether induction of Nrf2 in LT-HSC enhances hematopoietic reconstitution after bone marrow transplantation is not yet resolved. Irradiation of the lung induces pulmonary pneumonitis and fibrosis. Loss of Nrf2 promotes TGF-β/Smad signaling that induces ATF3 suppression of Nrf2-mediated target gene expression. This, in turn, results in elevated reactive oxygen species (ROS) and isolevuglandin adduction of protein that impairs collagen degradation, and may contribute to radiation-induced chronic cell injury. Loss of Nrf2 impairs ΔNp63 stem/progenitor cell mobilization after irradiation, while promoting alveolar type 2 cell epithelial-mesenchymal transitions into myofibroblasts. These studies identify Nrf2 as an important factor in the radiation response of normal tissue.

The Role of Intestinal Stem Cells in Epithelial Regeneration Following Radiation-Induced Gut Injury

Purpose of Review

Intestinal epithelial cells show remarkable plasticity in regenerating the epithelium following radiation injury. In this review, we explore the regenerative capacity and mechanisms of various populations of intestinal stem cells (ISCs) in response to ionizing radiation.

Recent Findings

Ionizing radiation targets mitotic cells that include “active” ISCs and progenitor cells. Lineage-tracing experiments showed that several different cell types identified by a single or combination of markers are capable of regenerating the epithelium, confirming that ISCs exhibit a high degree of plasticity. However, the identities of the contributing cells marked by various markers require further validation.

Summary

Following radiation injury, quiescent and/or radioresistant cells become active stem cells to regenerate the epithelium. Looking forward, understanding the mechanisms by which ISCs govern tissue regeneration is crucial to determine therapeutic approaches to promote intestinal epithelial regeneration following injury.

Recent Advances in Intestinal Stem Cells

Purpose of review

The intestine is a dynamic organ with rapid stem cell division generating epithelial cells that mature and apoptose in 3-5 days. Rapid turnover maintains the epithelial barrier and homeostasis. Current insights on intestinal stem cells (ISCs) and their regulation are discussed here.

Recent findings

The Lgr5+ ISCs maintain intestinal homeostasis by dividing asymmetrically, but also divide symmetrically to extinguish or replace ISCs. Following radiation or mucosal injury, reserve BMI1+ ISCs as well as other crypt cells can de-differentiate into Lgr5+ ISCs. ISC niche cells, including Paneth, immune and myofibroblast cells secrete factors that regulate ISC proliferation. Finally, several studies indicate that the microbiome metabolites regulate ISC growth.

Summary

ISC cells can be plastic and integrate a complexity of environmental/niche cues to trigger or suppress proliferation as needed.

SP and KLF Transcription Factors in Digestive Physiology and Diseases

Specificity proteins (SPs) and Krüppel-like factors (KLFs) belong to the family of transcription factors that contain conserved zinc finger domains involved in binding to target DNA sequences. Many of these proteins are expressed in different tissues and have distinct tissue-specific activities and functions. Studies have shown that SPs and KLFs regulate not only physiological processes such as growth, development, differentiation, proliferation, and embryogenesis, but pathogenesis of many diseases, including cancer and inflammatory disorders. Consistently, these proteins have been shown to regulate normal functions and pathobiology in the digestive system. We review recent findings on the tissue- and organ-specific functions of SPs and KLFs in the digestive system including the oral cavity, esophagus, stomach, small and large intestines, pancreas, and liver. We provide a list of agents under development to target these proteins.

Krüppel-like factor 4 (KLF4): What we currently know

Krüppel-like factor 4 (KLF4) is an evolutionarily conserved zinc finger-containing transcription factor that regulates diverse cellular processes such as cell growth, proliferation, and differentiation. Since its discovery in 1996, KLF4 has been gaining a lot of attention, particularly after it was shown in 2006 as one of four factors involved in the induction of pluripotent stem cells (iPSCs). Here we review the current knowledge about the different functions and roles of KLF4 in various tissue and organ systems.