CXXC5 is widely expressed in various human tissues and is pivotal in the cell signaling network, facilitating signal integration and information transfer [29, 51]. This broad expression and its pivotal role make CXXC5 closely linked to multiple cellular manners, including metabolism, proliferation, differentiation, and apoptosis [53, 70, 71]. Correspondingly, the appropriate regulation of CXXC5 and these cellular manners are intimately associated with various physiological processes, including bone formation and immune system regulation; conversely, abnormalities in CXXC5 expression or function have been linked to a wide range of pathological processes and diseases, including bone loss, infection, and especially cancers [25, 49, 55, 72, 73].

CXXC5 in cell proliferation, differentiation, and tissue generation

CXXC5 is a crucial regulator of cell proliferation, differentiation, and tissue generation. Given the intricate role of CXXC5 in the cell signaling network, its regulatory pathway in these processes is similarly complex. However, it can be divided into two main categories: the Wnt-associated pathways and the non-Wnt-associated pathways. CXXC5 plays a pivotal role in the growth and development of tissues, and it is also involved in pathological processes such as P. gingivalis infection and fibrosis [33, 44, 56].

Wnt associated pathways

The regulation of Wnt signaling by CXXC5 profoundly impacts proliferation and differentiation behaviors in many tissues and cells, including oligodendrocytes differentiation, as well as the development of telencephalon and kidney. During neurological development, CXXC5 has been demonstrated to promote the differentiation of oligodendrocytes and the production of myelin; Cxxc5−/− mice exhibit severely reduced expression of myelin basic protein and abnormal myelin structure [8]. Furthermore, CXXC5 expression is induced by BMP4 and inhibits Wnt3a signaling by interacting with Dvl2 during mouse telencephalon development; this regulation contributes to delineating BMP4 and Wnt3a signaling regions and maintaining normal developmental behaviors during mouse telencephalon development [3]. Similarly, CXXC5 and its upstream activators are co-expressed in podocytes of maturing nephrons during zebrafish kidney development. The ablation of CXXC5 can result in abnormal kidney development and the formation of large cysts in the glomerular-tubular regions of zebrafish embryos [20]. Additionally, it was demonstrated that activated Wnt signaling also results in the formation of cystic kidneys in zebrafish and mice, consistent with the discoveries that CXXC5 can inhibit Wnt signaling [74, 75].

Furthermore, negative feedback regulation of the Wnt signaling pathway by CXXC5 plays an essential role in bone formation, angiogenesis, skin repair, hair regeneration, and adipocyte differentiation. During osteoblast and bone differentiation, Cxxc5−/− mice exhibited high bone mass phenotypes as well as increased osteocyte dendrite formation and bone formation rate, and osteoblast differentiation and ex vivo calvaria growth can be promoted by blocking the CXXC5-Dvl interaction as well as reducing bone loss in ovariectomized (OVX) mouse models [57, 72]. In addition, CXXC5 expression is progressively elevated during the senescence process in rodent growth plates, and elevated CXXC5 expression inhibits Wnt signaling by interacting with Dvl, suppressing Wnt-regulated molecules related to chondrocyte maturation transcription. Cxxc5−/− mice exhibit delayed growth plate senescence and tibial elongation [25].

CXXC5 participates in the regulation of diabetic wound healing and angiogenesis, and blocking the Dvl-CXXC5 interaction to activate the Wnt signaling pathway can enhance angiogenesis and skin repair in diabetic mice [76, 77]. Moreover, a reduction in CXXC5 protein expression is evident in epidermal keratinocytes and skin fibroblasts from acute wounds in humans, and the blockade of the CXXC5-Dvl interaction was observed to promote hair healing, with Cxxc5−/− mice exhibiting faster skin healing [58, 59, 78, 79]. Similarly, CXXC5 is expressed in keratin-forming cells of hair follicles, hair follicle dermal papilla cells, and other hair production-related cells. CXXC5 inhibits the Wnt signaling pathway and suppresses the process of follicle regeneration by interacting with Dvl, and accelerated hair regeneration can be observed in Cxxc5−/− mice [26, 80,81,82]. The mRNA level of CXXC5 was found to be higher in differentiated adipocytes in comparison to preadipocytes from the human omentum and subcutaneous adipose tissue, and up-regulated CXXC5 has been observed to promote adipocyte differentiation by inhibiting Wnt signaling [83].

Non-Wnt-associated pathways

In addition to the CXXC5-Dvl interaction, CXXC5 exerts regulatory effects on cardiogenesis, angiogenesis, skeletal myogenesis, myelopoiesis, fibrosis, and proliferation or differentiation of embryonic, cementoblasts, and myeloid cells through other pathways.

CXXC5 is consistently expressed during cardiogenesis and regulates cardiac development and circulation through the TGF-β signaling pathway; knockdown of CXXC5 during zebrafish cardiac development has been observed to result in looping defects, cardiac dysplasia, pericardial edema, and other cardiac developmental abnormalities [18]. Furthermore, in mouse embryonic stem cells, CXXC5 forms a complex with Tet1, Tet2, Nanog, and Oct4, which positively regulates the transcription of pluripotency genes and Tet enzymes and ensures the normal differentiation of embryonic stem cells [50]. CXXC5 binds directly to the Flk-1 promoter region and promotes Flk-1 transcriptional activity; correspondingly, CXXC5 is required for BMP signaling-induced Flk-1 expression, motility, and tube formation in endothelial cells; conversely, aberrant expression of CXXC5 results in impaired venous angiogenesis during zebrafish development and the down-regulation of CXXC5 was also observed in vein wall tissue from patients with varicose veins (VV) [4, 84]. In the context of skeletal muscle cell differentiation, it has been found that CXXC5 can significantly increase the activity of the promoters of genes related to skeletal muscle differentiation in C2C12 myoblasts and promote skeletal muscle differentiation [85]. CXXC5 regulates the differentiation process of cementoblasts through multiple pathways, including p38, PI3K-Akt, Erk-1/2, and PGC-1α. P. gingivalis infection inhibits CXXC5 expression, thereby inhibiting cementoblasts differentiation [33, 44].

In the hematopoietic system, CXXC5 plays a vital role in normal bone marrow hematopoiesis, and during myeloid differentiation of normal CD34 + progenitor cells, the expression of CXXC5 is changed dynamically with the stage of cell maturation, and knockdown of CXXC5 has been discovered to lead to a significant accumulation of immature cells [29]. Furthermore, CXXC5 plays an essential role in the development of monocytes; during myeloid differentiation, the down-regulation of CXXC5 was observed to result in a reduction in monocyte differentiation and an increase in granulocyte differentiation; knockdown of CXXC5 has been shown to enhance the expression of genes involved in cell cycle regulation, and an increased proportion of S-phase cells [86]. The loss, mutation, or functional defects in CXXC5 are associated with a range of hematopoietic disorders, including abnormalities in proliferation and differentiation, and some studies have identified CXXC5 as an essential effector to promote terminal differentiation of cancer cells in the treatment of APL with ATRA [29].

Moreover, CXXC5 is also involved in the inhibition of cell proliferation and differentiation behaviors. CXXC5 is expressed in immature erythrocytes and inhibits cellular sensitivity to TGF-β signaling via Smad7, and CXXC5 knockdown has been observed to accelerate erythropoietin-driven maturation without affecting cell viability [52]. Furthermore, CXXC5 impedes mouse lung fibroblast (MLF) proliferation and transformation to myofibroblasts by inhibiting activation of the CD40/CD40L pathway and promotes apoptosis of MLFs, and CXXC5 overexpression has been discovered to inhibit the progression of bleomycin-induced lung fibrosis in mice [56].

CXXC5 in apoptosis

CXXC5 regulates apoptosis through multiple pathways, including TNF-α, ATM/p53, FOXL2, and CD40/CD40L. CXXC5 is essential for the regulation of DNA damage-induced apoptosis, and in an in vitro experiment using wild-type MCF7 and HeLa cells, knockdown of CXXC5 influenced ATM phosphorylation and downstream p53 signaling [19]. Intriguingly, overexpression of CXXC5 in HEK293T cells did not result in enhanced p53 signaling, indicating that the regulation of ATM/p53 signaling by CXXC5 may be context-dependent or a necessary but not sufficient condition for it [53]. Similarly, CXXC5 expression was also found to correlate with TP53 mutation status in a population of BC patients, with high CXXC5 expression correlating with wild-type TP-53 and low CXXC5 expression correlating with TP53 mutation status, suggesting that high expression of CXXC5 may inhibit apoptosis in BC cells through other pathways [87].

In addition, CXXC5, as a protein partner of FXOL2, can promote apoptotic activity in wild-type FOXL2 KGN cells [60]. CXXC5 could induce apoptosis in TNF-α induced HEK293 cells by affecting the function of Smad3/4 proteins, and the regulation is dependent on the mitochondria-mediated apoptosis pathway; inhibition of the mitochondria-mediated apoptosis pathway by co-transfection of Bcl-2 was observed to inhibit CXXC5-induced apoptosis in primary rat cortical neurons [53]. Moreover, evidence indicates that CXXC5 can facilitate the TGF-β signaling pathway in hepatocellular carcinoma (HCC) by interacting with HDAC1, and its overexpression was discovered to result in apoptosis and cell cycle arrest in HCC [5]. In human malignant peripheral nerve sheath tumors (MPNSTs), CXXC5 is one of the downstream apoptosis mediators of KANK1 [40]. Furthermore, CXXC5 can induce apoptosis of MLFs by suppressing the CD40/CD40L pathway [56].

CXXC5 in metabolism

CXXC5 can inhibit Cytochrome c oxidase Subunit 4 isoform 2 (COX4I2) promoter activity through an RBPJ-dependent pathway or independent pathway and interact with MNRR1 and RBPJ to affect COX4I2 expression and thus regulate cellular energy demand under different oxygen concentrations, which is a non-HIF1A-dependent ORE pathway [61]. In addition, CXXC5 has been shown to interact with CHCHD10, enhance its inhibition of oxygen-responsive element (ORE) transcriptional activity, and indirectly affect the composition and function of the mitochondrial respiratory chain [62]. CXXC5 is highly expressed inadipose tissues from obese type 2 diabetes (T2DM) patients. Blocking the CXXC5-Dvl interaction can improve the disease metabolic status of mice, reduce insulin resistance, inhibit adipocyte differentiation, facilitate the regeneration of pancreatic beta-cells, and enhance glucose homeostasis [71, 83]. Furthermore, a study has demonstrated that CXXC5 is associated with genes involved in insulin endocytosis, suggesting a potential role in the regulation of insulin resistance [88].

CXXC5 in immunity and inflammation

CXXC5 appears to be repressed in non-immune tissues during inflammatory states. The expression of CXXC5 is downregulated in muscle tissues in COVID-19 induced cytokine storms and correlates with infection-induced muscle loss [45]. In the pathological state of periapical periodontitis due to P. gingivalis infection, CXXC5 expression is downregulated in c-cells and is associated with inhibition of differentiation of cementoblasts [33, 44, 46].

CXXC5 expression appears crucial for the immune response of immune cells. CXXC5 is highly expressed in mouse plasmacytoid dendritic cells (pDCs) and plays an important role in TLR7/9- and virus-induced IFN responses by recruiting Tet2 to maintain hypomethylation of specific CpG islands and stabilizing IRF7 expression. CXXC5-deficient mice are impaired in their early IFN responses and are susceptible to infection by herpes simplex virus and vesicular stomatitis virus [49]. In addition, the expression of CXXC5 is elevated in E. granulosus-infected mouse T follicular helper 2 (Tfh2) cells, which may be involved in the immune response they regulate [73]. CXXC5 interacts with SUV39H1, inducing trimethylation of H3K9. This process is involved in ThPOK-mediated inhibition of CD40L expression in CD8+ cytotoxic T cells [22]. Furthermore, the expression of CXXC5 was found to correlate with the infiltration of CD8+ T cells, resting memory CD4+ T cells, resting NK cells, activated dendritic cells, and memory B cells in chronic myeloid leukemia (CML), indicating that CXXC5 may be involved in anti-tumor immunity in CML [89].The identification of a heterozygous variant of CXXC5 in a patient with primary immunodeficiency disorders (PIDs) with decreased antibody production, progressive loss of B cells, and infections of the lungs and gastrointestinal tract also suggests an important role for CXXC5 in the immune system [90]. However, high expression of CXXC5 in immune cells is not always beneficial to the immune response of the organism. CXXC5 can enhance the activity of the HIV-1 promoter, and its expression is upregulated in CD4+ T cells from patients with low-level viremia (LLV), which may be related to the activation of signaling pathways associated with viral replication and the low-level viremia state in these patients [55].

CXXC5: from physiology to pathology

As mentioned above, CXXC5 regulates various cellular behaviors, including proliferation, differentiation, and apoptosis, as well as physiological processes, such as growth and development, hair regeneration, and the immune response. Correspondingly, the abnormal expression or function of CXXC5 is closely associated with pathological conditions, including uncontrolled proliferation, restricted differentiation, immune dysregulation, and other abnormalities (shown in Fig. 3).

Fig. 3

CXXC5 related phthological and physiological processes

CXXC5 in cancers

CXXC5 affects the activity of multiple signaling pathways, including TGF-β/BMP, Wnt/β-catenin, ATM/p53, and others [5, 19, 51]. Furthermore, CXXC5 influences the expression of its downstream target genes through epigenetic modification or transcriptional regulation. The complex signaling pathways and target genes associated with CXXC5 have different regulatory effects on tumors, which has resulted in the intricate and multifaceted role of CXXC5 in regulating tumorigenesis and cancer progression. CXXC5 is expressed at low levels in hematopoietic system tumors, gastric cancer (GC), and DIPG, indicating a potential tumor-suppressive role [6, 39, 47]. Conversely, CXXC5 is highly expressed in metastatic melanoma, thyroid carcinoma (THCA), BC, endometrial cancer (EC), PCa, and ovarian cancer (OC), suggesting a potential tumor-promoting function (shown in Fig. 4) [48, 87, 91, 92].

Fig. 4

CXXC5 in cancer: turn on or off?

Hematopoietic tumors

CXXC5 is intimately associated with the development of hematopoietic tumors. The expression of CXXC5 varies considerably between individuals in AML and acute lymphoblastic leukemia patient populations [93]. However, downregulated expression of CXXC5 is associated with low-risk abnormalities and higher overall survival in patients with newly diagnosed AML receiving intensive chemotherapy, indicating that CXXC5 may act as a tumor suppressor involved in tumor development [47, 89, 93].

Multiple factors play a role in the regulation of the expression or function of CXXC5 in hematopoietic tumors. The chromosomal location of the CXXC5 gene (5q31.2) is a frequently deleted region in AML/MDS patients, which implicates that haploinsufficiency or functional defects in CXXC5 may be involved in the development of AML/MDS [12, 13]. In addition, in patients with AML, MLL rearrangements, t(8;21), and GATA2 mutations have also been identified to downregulate the expression of CXXC5 [47]. In one study that included 22 AML and higher-risk MDS patients, all exhibiting a deletion of 5q, including CXXC5, 13 patients demonstrated CXXC5 expression levels below 50% of those observed in the standard control group. This finding suggests that factors other than haploinsufficiency may influence CXXC5 expression. Additionally, this study found that methylation and somatic mutations of CXXC5 were rare in the AML/MDS patient population, with only 0/182 and 1/175 [94]. However, in a separate study that included 46 AML and 6 CD34+ samples, the promoter region of CXXC5 was found to be highly methylated and associated with decreased expression in AML. Treatment with DAC inhibited methylation of the CXXC5 promoter and promoted its expression in highly methylated cell lines U937 and NB4 [47]. Further data are required to elucidate whether methylation is involved in the regulation of CXXC5 in hematopoietic tumors. In myeloid malignancies, EZH2 regulates CXXC5 expression and influences disease development, and the mutation status of EZH2 affects CXXC5 expression [37].

CXXC5 exerts regulatory effects on the development of hematopoietic system tumors through several different mechanisms. One early study demonstrated that CXXC5, an essential effector in the treatment of APL with all-trans retinoic acid, could promote terminal differentiation of cancer cells [29]. In acute myeloid leukemia (AML), CXXC5 can inhibit leukemia cell proliferation and Wnt signaling and affect p53-dependent DNA damage response. Furthermore, the downregulation of CXXC5 was found to increase the susceptibility of AML cell lines to chemotherapy-induced apoptosis, and there were differences in the activity of apoptosis between primary human AML cells with high expression of CXXC5 and low expression, suggesting that CXXC5 may influence tumor cell development by regulating cellular drug resistance and apoptosis [47, 93]. The expression of CXXC5 in CML was related to the P53 pathway, DNA repair, MYC targets, and apoptosis, which may be involved in the regulation of CML cell proliferation. Furthermore, the expression of CXXC5 was associated with immune cell infiltration, suggesting a potential involvement of CXXC5 in anti-tumor immunity in CML [89].

HCC

The function of CXXC5 in HCC development remains controversial. One study revealed that CXXC5 expression was reduced in most HCC tissue samples compared with normal tissues, and the competitive binding of CXXC5 to HDAC1 was found to upregulate TGF-β signaling and thus induce cell cycle arrest and apoptosis in HCC cells under in vitro test conditions [5]. However, it has also been found that over-regulation of CXXC5 expression in Hep3B cells promotes the growth, migration, and invasion of HCC cells [35]. Hepatic fibrosis represents a critical step in the transformation of normal liver cells to HCC [95], and CXXC5 expression is down-regulated during the activation of hepatic stellate cells, which inhibits the expression of the proto-oncogene MYCL1 by binding to the promoter region of the MYCL1 gene. Deletion of CXXC5 leads to hepatic stellate cells activation and promotion of hepatic fiber progression [9].

BC

CXXC5 plays a role as a tumor-promoting factor in BC. The expression of CXXC5 is significantly upregulated in patients with advanced BC, and its high expression correlates with higher tumor grade and poorer prognosis in BC patients [31,