Thyroid hormones (triiodothyronine, T3) practically influence every biological process. T3 is crucial for the development of numerous adult organs and the maintenance of appropriate physiological functions in these organs. This is especially true during the maturation period of embryonic development, when plasma T3 levels are at their highest, which corresponds to the weeks and months before and after birth in mice and humans, respectively. Extreme developmental issues, such as intestinal maturation deficiencies, are driven by T3 deficits during embryonic development [24]. The hypothalamic–pituitary–thyroid axis controls the production and release of thyroid hormones through thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH), which are both produced by the thyroid gland [25]. Nuclear thyroid hormone receptors (TRs), which are found in all vertebrates, mediate the action of thyroid hormones on the genome. TRs are nuclear hormone receptor superfamily transcription factors. T3 response elements (TREs) control gene transcription by recruiting various cofactors in a T3-dependent manner, and TRs primarily bind to 9-cis-retinoic acid receptors (RXRs) to form heterodimers with TREs [26]. TRs undergo conformational changes both before and after T3 engagement, thus activating or repressing the transcriptional apparatus. When bound to ligands, TRs recruit coactivator complexes to target genes to activate transcription, whereas unbound TRs recruit corepressor complexes to target genes to repress their expression. Chromatin remodelling and histone modifications play a significant role in the regulation of genes by TRs [26]. All vertebrates have two TR genes, namely, TRα and TRβ [27]. Thyroid hormone has been the subject of most studies reporting on hormones involved in controlling ISCs. Mice, anurans and in vitro intestine organoid cultures are among the experimental models used to explore thyroid hormones.

Several mouse models with single TR or T3 or double TR/T3 knockouts have been developed to examine the critical role of TR in controlling T3 signalling during mammalian development and identify the intriguing properties and significant molecular pathways of adult ISCs. To date, adult mammalian ISCs have been extensively studied. Recent studies have reported that the development of adult ISCs in mice occurs during the neonatal period when plasma T3 levels are highest. T3 can increase the crypt size, enhance the proliferation of crypt cells and increase the pool of ISCs and Paneth cells in mouse models stimulated with T3 [28, 29]. Additionally, intestinal abnormalities and altered ISC proliferation are driven by mutations or deficits in mice T3 receptors. In addition to having shorter intestinal villi and more differentiated Paneth and cup cells in the crypt, with decreased crypt cell proliferation and fewer ISCs, adult mice with TR knockout can develop constipation owing to poor intestinal motility. Therefore, delayed replacement of intestinal epithelial cells and impaired proliferation/regeneration of ISCs may be caused by TR mutations [28].

Owing to the dependence of a mammalian neonate on its mother and the uterine closure of the embryo, it is challenging to understand the significance of T3 in postembryonic development. Development of the anurans Xenopus laevis and Xenopus tropicalis is similar to the postembryonic development of mammals. Importantly, metamorphosis in anurans can be easily controlled by controlling the supply of T3 to tadpoles; therefore, anurans are very valuable models for studying organ maturation and ISC formation under the action of T3 during postembryonic development in vertebrates [30].

In the anuran X. laevis or X. tropicalis, the intestine undergoes significant modification during metamorphosis and resembles a basic tubular structure with a single epithelial fold in tadpoles. Thyroid hormone (TH) induces apoptosis in a majority of larval epithelial cells during differentiation. When TH is continuously expressed, the remaining epithelial cells (SC precursors) differentiate into ISCs, which de novo produce a multi-fold adult epithelium that resembles that of mammals and is encircled by thick layers of connective tissue and muscle [31]. Similar to mammals, anurans have two TR isoforms, namely, TRα and TRβ. TRα expression is only found in proliferating adult epithelial primordial cells produced by ISCs after the initiation of TH-dependent remodelling. As intestinal folds develop, TRα expression concentrates in the valleys of the folds, where ISCs are found [32, 33]. In contrast, as endogenous TH levels increase, TRβ expression is momentarily increased throughout the gut, with the adult epithelium primordium exhibiting the highest increase. TRβ is expressed on both ISCs and larval epithelial cells, whereas TRα is expressed on SC precursors but not on larval cells proper that are ready to undergo apoptosis [34]. Different organs express TRα and TRβ mRNAs differently. The expression of TRα mRNA is widespread during the larval and metamorphic stages and is primarily seen in certain adult organs, such as the hindlimbs [35]. However, TRβ mRNA is momentarily elevated during the peak of metamorphosis along with an increase in endogenous TH levels, and it is mostly expressed in organs unique to larvae, such as the tail and gills [36]. These results suggest that TRα and TRβ play different roles in intestinal larval-to-adult remodelling [36].

TRα controls the subsequent development and/or maintenance of ISCs and the dedifferentiation of SC precursors into ISCs. The lack of TRα delays these processes as observed in TRα-knockout tadpoles [37]. TRα deficiency prevents intestinal remodelling in X. tropicalis during T3-stimulated development, indicating that TRα is necessary to initiate gut remodelling. After T3 treatment, cell proliferation gradually increases in the gut of wild-type tadpoles but not in TRα-KO tadpoles [38]. Additionally, the gut of wild-type tadpoles has enhanced expression of the stem cell marker gene Lgr5, whereas this upregulation is repressed or delayed in TRα-KO tadpoles [39]. Therefore, TRα may play a role in regulating the proliferation of ISCs. Furthermore, high levels of apoptotic signals are observed in the intestinal epithelial cells of wild-type tadpoles but not in those of TRα-KO tadpoles after 2 days of T3 treatment, indicating that TRα is necessary for T3-induced apoptosis. A majority of larval epithelial cells undergo apoptosis with increased levels of T3 during intestinal metamorphosis [40]. Activation and promotion of cell cycle-related genes by TRα are crucial during intestinal metamorphosis. In addition, TRα may play an important role in remodelling the extracellular matrix by metalloproteinases (MMPs), which is induced by T3 in larval epithelial cells [39]. TRα mediates T3 during intestinal metamorphosis and ISC maintenance. However, TRα knockdown inhibits T3-induced intestinal remodelling and significantly reduces the apoptosis and proliferation of adult stem cells after prolonged T3 treatment.

TRβ may play a role in the formation of ISCs after their emergence and the death of larval cells. Although TRβ expression is briefly and significantly upregulated during gut metamorphosis, there are only very minor variations in gut remodelling between thyroid hormone receptor β knockout (TRβ-KO) and wild-type tadpoles [41]. Contrary to TRα, which is expressed at high levels from the pre-metamorphosis stage to the end of metamorphosis, TRβ is expressed at very low levels before metamorphosis but is triggered as a direct target gene of T3, peaking at the end of metamorphosis. According to external morphology, TRβ KO does not affect tadpole development before metamorphosis but delays tail regression during metamorphosis. In X. tropicalis, TRβ KO has relatively minimal effects on hindlimb development and gut remodelling. However, T3-induced intestinal remodelling, including reduced length, adult stem cell development and proliferation, and larval epithelial cell death were inhibited by TRβ knockout. These facts suggest that TRβ has an effect on the intestinal remodelling process [42].

Reconstituted organoid cultures and 3D primary intestinal epithelial organoid cultures are excellent in vitro models to study ISCs. Studies employing these models have validated the involvement of T3 and TR in the development of adult stem cells during intestinal metaphase [43]. T3 can control cell proliferation in organoid cultures. Organoids treated with T3 have longer and larger buds and faster cell cycles, resulting in the earlier emergence of buds during in vitro growth. The loss of ISCs and their differentiation to intestinal secretory cells can increase the turnover [44]. Intestinal Lgr5 + crypt cells exclusively express TRα, which directly activates the Wnt and Notch pathways and promotes cell growth in these cells. TRβ, which is encoded by the TRβ gene, is only expressed on differentiated epithelial cells of the villi; however, its function in the gut remains unknown. In vitro organoid dysplasia and reduced stem cell activity result from the loss of a specific TRα function, and TRβ-KO organoids do not exhibit any similar symptoms. In addition, it has been demonstrated that the absence of TRα but not TRβ reduces ISCs activity [45].

Identifying pathways involved in TR control in the gut and the related target genes is essential for understanding the regulatory role of T3 in ISCs. During intestinal remodelling, T3 can stimulate numerous signalling pathways crucial for the proliferation and function of stem cells. These pathways include the Notch, WNT, hedgehog and BMP signalling pathways [46,47,48,49,50]. However, further research is required to determine the effects of these signalling pathways on the growth of adult ISCs.

Numerous T3-regulated targets play a role in intestinal remodelling and the generation and proliferation of ISCs (Fig. 2). In the ISCs and surrounding connective tissue cells of adult X. laevis, TH can upregulate hyaluronate synthase (HAS), which is implicated in the development of ISCs through newly synthesised hyaluronic acid (HA). During the late stage of vertebrate embryonic development and maturation, signal transduction is important for controlling ISCs [51]. LGR5, a marker for ISCs, is inhibited via inhibition of HA production [52]. HAL2 is selectively produced during metaplasia of developing/proliferating adult ISCs in X. laevis, and T3 can induce HAL2 during the early stage of adult stem cell development. The histidine gene in HAL2 is the first enzyme in the histidine catabolic pathway, which breaks down histidine into ammonia and uridine. Therefore, histidine catabolism may play a role in the formation of adult stem cells [53]. The Ctnnb1 gene, which produces a β-catenin protein, is directly regulated by TRα through transcription. The cell cycle proteins D1 and D2 and c-Myc are some targets that are activated as a result of increased expression of β-catenin protein [54]. Additionally, secreted frizzled-related protein (sFRP2) is a direct target of TRs and positively controls the common WNT pathway in intestinal progenitor cells in vitro [55]. In X. tropicalis, the Myc/Mad/Max axis is involved in the development and/or proliferation of adult stem cells and the death of larval epithelial cells. Both Mad1 and c-Myc heterodimerise with MAX and bind to the same target genes with contrasting consequences. Mad suppresses the expression of c-Myc target genes and is associated with quiescence or cell differentiation, whereas c-Myc is a well-known oncogene that increases target gene transcription and promotes cell proliferation. During intestinal metamorphosis, Mad1 and c-Myc are expressed on various epithelial cells, with Mad1 expression being high on apoptotic larval epithelial cells and c-Myc expression being high on proliferative adult stem cells [31]. During metamorphosis, TRs can directly activate Mettl1 at the transcriptional level through the TRE of the promoter region in the gut. During intestinal remodelling, Mettl1 modulates target tRNAs to affect translation, thus promoting the generation and/or proliferation of stem cells [56]. The Mtfp1 gene is directly activated by T3 through the TRE of introns, and this activation influences mitochondrial fission, which in turn stimulates the growth and/or proliferation of adult ISCs [57]. Forkhead box l1 (Foxl1) expression is indirectly upregulated by TH via Shh signalling, and organ-autonomous induction of Foxl1-expressing cells by TH occurs simultaneously with the emergence of stem cells in the tadpole gut in vitro. Recent studies have reported that Foxl1-expressing mesenchymal cells are an important component of the intestinal niche. Intestinal niche cells that express Foxl1 are evolutionarily conserved in terrestrial vertebrates and can stimulate the formation of stem cells during amphibian metamorphosis by activating TH/Shh signalling [58]. PRMT1 is a well-known TR co-activator and a histone H4R3 methyltransferase. It is induced in response to T3 during intestinal metamorphosis in X. laevis. In developing stem cells, T3 appears to activate the transcription factor cMyc, and cMyc, in turn, appears to activate the PRMT1 promoter. Therefore, T3 can indirectly increase PRMT1 expression, and hat PRMT1 can act as a TR co-activator to further enhance T3 signalling and support the formation of ISCs [59]. Dot1L, a histone methyltransferase, is the only enzyme that methylates histone H3K79 in vitro and is directly regulated by T3 at the transcriptional level through the binding of TR to the TRE of its promoter. It enhances transcriptional activation of TR and functions as a TR coactivator through a positive feedback mechanism during intestinal remodelling and adult stem cell development [60]. In a study, RNA-sequencing analysis revealed that the clusterin (Clu) gene, which is associated with the suppression of the cycling/quiescent/revitalised population of ISCs, was an important upregulated differentially expressed gene (DEG) associated with SC characteristics in T3-treated organoids [45]. It is noteworthy that TH has a different mechanism of action on colon cancer stem cells than on small intestine stem cells. Thyroid hormones control the balance between proliferation and differentiation of colon cancer stem cells (CSC), specifically, TH activates the Wnt/β-catenin pathway, while deiodinase 3 (D3) is a direct target of β-catenin, which binds to the D3 promoter and activates D3 transcription, ultimately promoting the proliferation of colon cancer stem cells [61].

A molecular model of the action of thyroid hormone (TH) on the intestinal stem cell pathway was proposed. In intestinal stem cells, TH enters the nucleus and binds to TR to directly regulate the expression of HAS, HAL2, Mettl1, Mtfp1, DotiL and CLU genes. In addition, TH regulates the expression of multiple targets involved in the Wnt, Notch and hedgehog pathways. In the Wnt signalling pathway, the binding of TH to TR positively regulates the expression of Ctnnb1 and sFRP2 genes. In turn, Ctnnb1 and sFRP2 proteins allow β-catenin translocation to the nucleus by interacting with Wnt/β-catenin, where β-catenin forms a complex with transcription factors TCF/LEF, leading to increased expression of Wnt target genes, such as Cell cycle proteins D1/D2, Deiodinase 3 and c-Myc. In hedgehog signalling pathways, TH can act directly on Shh and promote enhanced transcription and expression of BMP4 and Foxl1

Glucagon-like peptide 2 (GLP-2) is produced by L cells in the gastrointestinal tract and maintains the dynamic homeostasis of intestinal epithelial cells. It is an intestinal hormone comprising 33 amino acids derived from proglucagon cleaved by protein hydrolase 1/3 [62]. It can exert direct and indirect effects on responding cells. Cells that respond to GLP-2 include enterocytes, cup cells, neurons, subepithelial myofibroblasts, endothelial cells and certain enteroendocrine cells [63]. The effects of GLP-2 on cells located near the intestine are mediated by signalling molecules released from cells with surface expression of the GLP-2 receptor (GLP-2R). When GLP-2 binds to its receptor (GLP-2R), it initiates signals that promote the proliferation of intestinal crypt cells, thereby promoting intestinal growth and improving intestinal digestion, absorption and barrier function. GLP-2R is a G protein-coupled receptor localised in cells within the lamina propria and scattered neurons of the enteric nervous system. Its activation on subepithelial myofibroblasts leads to the release of growth factors, mainly insulin-like growth factors 1 and 2, keratinocyte growth factor, epidermal growth factor and transforming growth factor-beta. Through this paracrine effect, GLP-2 can expand different cells in the intestine [64]. To overcome the short half-life of the natural GLP-2, a degradable GLP-2 analogue (h[Gly2]-GLP-2) named teduglutide has been developed. Teduglutide has been approved for the chronic treatment of short bowel syndrome in the United States of America and Europe based on a placebo-controlled phase 3 study and is used to reduce the requirement of parenteral nutrition while increasing the size and absorption of the intestinal epithelium [65].

In mice with acute graft-versus-host disease (GVHD), Norona et al. used the GLP-2 analogue teduglutide [66] and discovered that GLP-2 treatment overcame the deficiency in ISCs caused by graft-versus-host disease and encouraged ISC regeneration. In addition, injection of 500-nM teduglutide significantly increased the surface area of intestinal organoids [66]. In another study, immunofluorescent-labelled mice with GVHD were examined using confocal microscopy to evaluate whether GLP-2 treatment offers protection against GVHD by increasing the number of ISCs. The results demonstrated that teduglutide treatment increased the number of Lgr5 + ISCs, which counteracted the effects of GVHD on the number of Lgr5 + ISCs. Furthermore, qPCR was used to examine the expression of the stem cell marker Olfm4. Compared with mice treated with a vehicle, those treated with teduglutide had high Olfm4 expression. Additionally, mice treated with teduglutide and developing GVHD had increased expression of the stem cell marker prominin-1 (Prom1). These results indicate that GLP-2 possesses anti-apoptotic, histo-protective and regenerative properties, and the GLP-2 analogue teduglutide increases the number of ISCs and the expression of ISC markers during GVHD. In addition, reduced expression of the apoptosis execution factor caspase 3, downregulated transcription of several genes associated with cell death and increased transcription of the KGF gene, a gene associated with growth factors, are some processes that were demonstrated in the study [66].

In a study by Chen et al. [67], acute GLP-2 administration significantly enhanced the progression of ISCs from the G1 phase to the S phase, and long-term GLP-2 treatment increased ISC expansion, which may be dependent on IGF-1. Lgr5-enhanced green-fluorescent protein-internal ribosome entry site-Cre recombinase-oestrogen receptor T2 (eGFP-IRES-creERT2) mice were administered teduglutide or a vehicle 6 and 3 h before the mice were sacrificed. S-phase cells were labelled with EDU 1 h before the mice were sacrificed. Acute treatment with teduglutide increased the proportion of S-phase ISCs, whereas treatment with a GLP-2 antagonist decreased the proportion of S-phase ISCs. After GLP-2 treatment, transcriptional analysis of the entire jejunal and colonic sections using gene microarrays revealed that MCM3 expression was elevated in both the jejunum and colon. This information was used to identify potential targets that may mediate the proliferative effects of GLP-2 on the intestine. The stimulatory effects of GLP-2 on the number of ISCs were diminished in intestinal epithelial (IE)-IGF-1R-KO mice. Therefore, GLP-2 substantially stimulates the advancement of S-phase Lgr5 + /Olfm4 + ISCs via GLP-2R, and MCM3 may be a potential target for its proliferative effects. In addition, GLP-2 persistently increases the number of ISCs in an IGF-1R-dependent manner [67]. In a study by Bradley et al. [68], GLP-2 increased the length of the functional crypt-villus axis by promoting the proliferation of crypt cells and inhibiting the apoptosis of epithelial cells. The pro-intestinal effects of GLP-2 may depend on the expression of the BMI-1 gene, and the role of BMI-1 in the proliferation of TA cells and/or preventing their differentiation may be affected by GLP-2.

In conclusion, GLP-2 promotes ISC regeneration, inhibits apoptosis, exerts histo-protective and regenerative effects on the intestine, dilates intestinal organs, lengthens the functional crypt-villus axis and stimulates ISC proliferation. Teduglutide, an anti-degradative GLP-2 analogue, is used in clinical practice and improves enteral nutrition and intestinal growth in patients with short bowel syndrome [69]. Related mechanisms include downregulation of the transcription of several cell death-related genes by GLP-2, including decreased expression of caspase 3; promotion of proliferation as evidenced by the promotion of upregulation of the growth factor-related gene KGF and BMI-1; and induction of the progression of S-phase ISCs through the action of GLP-2R on the MCM3 gene, which in turn stimulates ISC expansion. To date, a few studies have examined the role of GLP-2 in Lgr5 + ISCs. Therefore, further investigation should be performed using organoid models that resemble intestinal exosomes.

Androgens (from the Greek root word ‘andro,’ meaning male or man) are typical male hormones that maintain masculine characteristics by activating the androgen receptor (AR), a ligand-induced nuclear receptor that, when activated, serves as a transcription factor [70]. The primary androgen in men is testosterone (T, androst-4-ene-17-ol-3-one), which is produced by mesenchymal cells in the testis and delivered into the bloodstream. When T is converted to the most potent natural androgen, 5-dihydrotestosterone (DHT, 5-androst-17-ol-3-one), the androgen signal is further enhanced in a few selected target tissues [71].

DHT activates AR in primary stromal cells in vitro, resulting in the promotion of organoid development. In a study on male mice, the number of enterocytes and intestinal secretory epithelial-like cells was increased and that of S-phase cells was decreased after 2 weeks of treatment with AR antagonists, which prevented the proliferation of ISCs. However, AR agonists prevented ISC differentiation and enhanced ISC proliferation in mice [71]. Therefore, androgens can simultaneously decrease the number of enterocytes and intestinal secretory lineage cells and increase the proliferation of ISCs, thereby promoting the growth of intestinal organs.

Mechanistically, the BMP pathway prevents and the Wnt pathway promotes ISC proliferation. Androgens can promote the growth of ISCs by activating the Wnt pathway and suppressing the BMP pathway. In particular, they can downregulate the Wnt-related antagonists Dkk2, Dkk3 and Sfrp1 and the BMP signalling-related proteins Bmp4 and Tgfb1 and upregulate b-catenin, a key component of the Wnt pathway, and the BMP signalling antagonists angptl2 and chrd. Additionally, AR expression is higher on intestinal mesenchymal cells than in the crypt foci, and androgens can influence stromal cells to promote crypt cell proliferation. Therefore, androgens simultaneously promote the proliferation of ISCs and suppress the development of intestinal epithelial cells by upregulating Wnt signalling and negatively regulating BMP signalling in stromal cells [72]. The increased incidence of colon cancer among men may be explained by these findings.

Insulin is a crucial hormone that controls glucose levels and is involved in the control of energy metabolism throughout the body [73]. In addition, it controls several signalling pathways. The IIS pathway controls various processes in multicellular organisms, including ageing, reproduction, nutrition, metabolism, stress resistance and growth [74]. High insulin levels in obesity-induced stem cell niches influence ISCs, thereby regulating organ size. Insulin can promote the proliferation of crypt cells, including ISCs, and several detrimental effects on tissue function are associated with these proliferative effects. Increased proliferation increases the likelihood of developing mutations and restricts the development of DNA repair mechanisms, which may promote the formation of tumours and cancer and contribute to the increased risk of bowel cancer associated with obesity [75].

The IIS pathway plays a survival-sustaining role in the ISCs of Drosophila melanogaster. The midgut of adult Drosophila is an appealing model for research into the regulation of stem cell maintenance and proliferation because it has several features similar to those of the mammalian gut. ISCs are found in the epithelial basement membrane in the midgut of Drosophila. They undergo symmetric division, with one daughter cell retaining its stemness and the other developing into an enteroblast (EB). The preservation of tissue homeostasis is significantly impacted by the control of self-renewal, proliferation and differentiation of ISCs. In a study, suppression of the IIS pathway via RNAi knockdown of the insulin receptor (InR) in ISCs reduced the lifespan of experimental flies compared with control flies and reduced their survival under starvation or malnutrition conditions. Additionally, the reproductive and eating capacities of these flies were diminished, and their bodies contained less glucose and glycogen. Therefore, InR knockdown can reduce the functional integrity of ISCs, which reduces the lifespan of flies and suggests that insulin is crucial for the normal function of stem cells via IIS [76].

The pancreas releases circulating insulin, which affects ISCs through mesenchymal cells below the crypt. ISCs express InR subtypes A and B. The activity of insulin-IGF-1 can be influenced by InR activity, and the IIS pathway positively controls the growth of ISCs. Mechanistically, the IIS pathway, when activated, regulates ISC proliferation by activating JAK–STAT and EGFR signalling in the gut [76]. However, the proliferation of ISCs induced by insulin is reduced when the PI3K/Akt pathway is inhibited [75].

Leptin was discovered as the first signalling protein produced by adipocytes (adipokines). It was first discovered in mouse adipocytes and is a peptide hormone produced by the OB gene. Leptin plays a critical role in maintaining body weight, hunger and energy balance and is primarily secreted into the bloodstream by adipocytes [77]. Recent studies have reported the association between leptin and control of the ISC milieu.

For the development and maintenance of intestinal homeostasis, pericrypt mesenchymal cells serve as the milieu for ISCs. The proliferation of ISCs near the base of the epithelial crypt is necessary for the regeneration of epithelial cells, and mesenchymal cells that surround these stem cells strictly control their maintenance and development. WNTs, gremlin (GREM) and BMP, which are crucial for intestinal renewal and stem cell maintenance, are secreted by several subpopulations of mesenchymal cells, such as intraepithelial myofibroblasts, GLI family zinc finger 1 + (Gli1+) fibroblasts and telocytes [78]. Leptin receptors are highly expressed on the surface of crypt MSCs, which suggests that endogenous leptin is an important factor in the control of MSC activity, especially in the crypt, based on the gene expression profile of crypt MSCs. A study showed that leptin stimulation increased the expression of the crypt ecological niche-associated factor Wnt2b in Lepr-deficient mice, indicating that leptin and WNT can stimulate stem cell proliferation. In addition, leptin was found to be involved in the proliferation of ISCs and the protection and repair of mucosal damage because the expression of Wnt2b was significantly lower in crypt mesenchymal cells than in controls. These results demonstrate the significance of Lepr signalling for the expression of Wnt2b, which is required for the survival and proliferation of ISCs [78].

In the colon, leptin can influence the biology of colorectal tumour stem cells. It was found that colorectal cancer stem cells express leptin receptor ObR and respond to leptin. Leptin activates extracellular signal-associated kinase (ERK) 1/2 and AKT signalling pathways and enhances cell proliferation. Blockade of ERK1/2 completely counteracts leptin-enhanced cell growth, suggesting that the ERK1/2 pathway is responsible for the increased proliferation of leptin-induced colorectal cancer stem cells [79].

Growth hormone (GH) is a pleiotropic hormone that is crucial for regulating several physiological processes. It promotes the expression of genes and intracellular signalling pathways by binding to the growth hormone receptor (GHR) [80]. GH can promote the proliferation of ISCs, formation of crypt-like organs, expression of ISC stemness markers in intestinal-like organs and differentiation of ISCs into Paneth cells and enterocytes. It primarily exerts anabolic effects by promoting the synthesis of IGF-1 [81].

In animal models, GH has been reported to influence intestinal tissue adaptability. Compared with control mice, transgenic mice overexpressing GH had increased body and intestinal weight [82]. Similarly, GH can affect the activity of ISCs or crypt cells. Intestinal shrinkage caused by GH deficiency after pituitary resection in rats is mostly attributed to decreased mitosis of epithelial ISCs. The size of intestinal epithelial cells, crypt volume and villi volume are lower in GH-deficient rats than in controls; and intraperitoneal administration of GH can restore these parameters to their normal levels [83]. Additionally, subcutaneous injection of bovine GH can increase the small intestine crypt depth/villi height ratio, thereby de