Inflammatory auxo-action in the stem cell division theory of cancer

Inflammatory response: the key to awakening quiescent stem cells

Historically, ASCs have been considered to exist in either a quiescent state, in which the cell is not actively cycling, or an activated state, in which the cell has entered the cell cycle (Cheung & Rando, 2013; Rossi et al., 2012). In the quiescent state, stem cells are in the reversible G0 phase and can re-enter the cell cycle in response to normal physiological stimuli. This is different from the G0 phase for terminally differentiated cells or senescent cells, which have left cell cycle and cannot re-enter (Cheung & Rando, 2013). The non-proliferative quiescent state of ASCs can protect the cell’s DNA from mutations acquired during continuous cell division (Walter et al., 2015), making them less likely to acquire cancer.

Many factors can trigger ASCs to exit the quiescent state. ASCs activation can be induced during tissue damage through mechanisms such as cell–cell contact and cell–extracellular matrix interaction (Cho et al., 2019). In addition, the mechanism by which inflammation awakens quiescent stem cells to promote the regeneration of damaged tissues has been widely reported (Schuettpelz & Link, 2013). Reactive oxygen species (ROS) generated during tissue damage and infection are important cues linking inflammation to intestinal stem cells (ISCs) proliferation through activating Jun N-terminal kinases (JNKs) and inhibiting nuclear factor erythroid2-related factor 2 (Nrf2) signal (Hochmuth et al., 2011; Biteau, Hochmuth & Jasper, 2008). Lipopolysaccharide (LPS) -induced transient mild inflammation stimulates the selective activation and proliferation of stationary stem cell populations, such as Clara-like cells (CLCs) (Verckist et al., 2018). Moreover, IFN-γ treatment increased the expression of bone marrow stromal cell antigen 2 (BST2), a cell surface protein essential for INF γ-dependent hematopoietic stem cell (HSC) activation, thereby disrupting HSC quiescence and promoting excessive terminal differentiation (Florez et al., 2020). Additionally, IL-6 and TNF directly act on epithelial cells by binding to the IL-6 receptor, gp130 heterotetramer, and TNF receptor 1 (TNFR1), thereby promoting the regeneration of damaged intestinal mucosa (Grivennikov et al., 2009; Böhm et al., 2010). IL-22 acts on epithelial cells and fibroblasts to stimulate proliferation, inhibit cell death, and delay terminal differentiation. The binding of IL-22 to its receptor leads to the activation of Janus kinases (JAK)-STAT3, MAPK, extracellular signal-regulated kinase (ERK), and JNK (Nikoopour, Bellemore & Singh, 2015). Therefore, inflammation is an important mediator that mobilizes quiescent stem cells to regenerate after tissue damage.

Inflammation mobilizes tissue stem cells: regeneration and tumorigenicity coexist

After tissue damage, inflammatory cells and cytokines are the main components of the tissue stem cell niche. Tissue stem cells, which are activated from a quiescent state owing to niche changes, repair damaged tissues through proliferation and differentiation and exhibit strong regenerative capabilities (Naik et al., 2018). For example, multiple subsets of innate lymphoid cells can play context-specific roles in stem cell regeneration and differentiation (Lindemans et al., 2015; von Moltke et al., 2016). However, mutations can occur with cell division. Although stem cells can self-renew for extended periods, but this also poses an inherent challenge. Being the longest-lived cells in an organism, they are at an increased risk of acquiring genomic damage, which undoubtedly increases the likelihood of accumulation of mutations in the stem cell genome. Gradual accumulation of genomic mutations throughout life may lead to carcinogenesis. Therefore, under the influence of inflammation, the regeneration of tissue stem cells coexists with tumorigenic ability. Although mutations and cancer incidence in some tissues depend primarily on exposure to external mutagens (Yoshida et al., 2020), intrinsic factors, such as the number of cell divisions in the tissue, appear to dominate other cancer types (Cheung & Rando, 2013). For example, a high-fat diet can drive a surge in intestinal stem cells and produce a range of other cells that behave very similarly to stem cells by multiplying indefinitely and differentiating into other types of cells. Stem and stem-like cells are more likely to cause intestinal tumors (Beyaz et al., 2016; Beyaz et al., 2021). Wnt/ β-catenin signaling plays a key role in intestinal epithelial homeostasis by promoting stem cell renewal. If Wnt signaling is overstimulated, stem cells divide uncontrollably. Mice with mutations in the Wnt signaling pathway develop polyps that eventually develop into colon cancer (Degirmenci et al., 2018).

Influence of inflammatory response on adult stem cell function

The function of ASCs has been established in the stem cell niche, which regulates stem cell survival and function by producing factors that directly act on stem cells (Ge et al., 2020). Inflammatory mediators, which are important niche components after tissue injury, influence the behavior of stem cells in various ways. Numerous studies have reported that the inflammatory microenvironment affects the biological characteristics and behavior of stem cells, both in vitro and in vivo (Table 1). For example, IL-1-induced mesenchymal stem cells (MSCs) show increased granulocyte colony-stimulating factor (G-CSF) expression through the type 1 IL-1 receptor, which results in decreased secretion of LPS-activated microglia inflammatory mediators, priming MSCs for in vitro transformation to an anti-inflammatory and pro-nutritional phenotype (Redondo-Castro et al., 2017). Moreover, a combination of four pro-inflammatory cytokines (IL-1 α, IL-13, TNF-α, and IFN- γ) secreted by T cells can promote the continuous expansion of muscle stem cells (MuSCs) in vitro for over 20 generations (Fu et al., 2015). IL-22 promotes ISC-mediated epithelial regeneration by inducing STAT3 phosphorylation (Lindemans et al., 2015). The inflammatory milieu mediated by IL-1 and TNF-α preconditioning initially stimulates the regeneration of gingival mesenchymal stem/progenitor cells (G-MSCs); this positive effect disappears as inflammation persists, followed by a short-term stimulatory effect on osteogenesis (Zhang et al., 2017). After exposing cultured MuSCs to the inflammatory mediator prostaglandin E2 (PGE2) for one day, the number of cells increased six-fold compared to the control group (Ho et al., 2017). Thus, these studies confirmed that inflammatory factors activate downstream stem cell pathways, enhancing their self-renewal and in vivo efficacy. In addition, when MSCs are exposed to an inflammatory environment, they release more functional growth factors, exosomes, and chemokines, thereby regulating the inflammatory microenvironment and promoting tissue regeneration (Liu et al., 2022). In vitro studies have confirmed that some inflammatory factors have profound effects on stem cell behavior, especially proliferation and immune phenotypes. Considering the stem cell division theory of cancer and the coexistence of stem cell proliferation and tumorigenicity, it is uncertain whether the inflammatory microenvironment, especially the influence of inflammatory factors on stem cell behavior, is beneficial or harmful to cancer formation. However, it is certain that stem cells retain their biological properties, such as self-renewal and tissue repair, after coming into contact with inflammatory factors. Partial inflammatory factors can maintain or even improve stem cell function; that is, inflammatory factor treatment indirectly maintains the integrity of the stem cell genome.

In vivo studies have confirmed that the tissue repair function of bone marrow MSCs in various inflammatory diseases depends on inflammation regulation and production of various growth factors (Shi et al., 2010; Wang et al., 2014). Thus, MSCs localized in damaged tissues can alleviate inflammation and promote tissue stem/progenitor and other resident cells to regenerate normal functioning cells and improve the tissue microenvironment by acting synergistically. For example, in vivo ISC turnover is heavily regulated by the crypt niche. Macrophages have been identified as a key component of intestinal crypts, and their production of PGE2 promotes Wnt/ β-catenin signaling by binding to the ISC prostaglandin E receptors (EP, also know as Ptger) 1 and EP4, which in turn promotes their self-renewal (Zhu et al., 2022). Furthermore, when mice were injected with PGE2 intramuscularly, muscle regeneration was significantly promoted and muscle strength increased. Conversely, when MuSC responses to PGE2 were inhibited, EP4 expression was reduced, or nonsteroidal anti-inflammatory drugs were administered to suppress PGE2 production, strength recovery was hampered (Ho et al., 2017). After intestinal injury, innate lymphocytes produce a large amount of IL-22, which directly targets ISCs, enhances the growth of small intestinal organoids, and promotes ISC proliferation and expansion (Lindemans et al., 2015). The interaction between ISCs and T cells in the local microenvironment has been elucidated, suggesting that T helper (Th) 1, Th2, and Th17 cells and their cytokines IFN-γ, IL-13, IL-17, and other pro-inflammatory signals can promote the differentiation of Lgr5+ISCs, while regulatory T cells (Treg) cells and their cytokine IL-10 promote ISC self-renewal (Biton et al., 2018). Skin-resident Tregs express high levels of the Notch ligand family member Jagged 1 (Jag1). Jag1 expression on Treg promotes hair follicle regeneration by enhancing hair follicle stem cell proliferation and differentiation (Ali et al., 2017). Additionally, impaired communication between aging keratinocytes and immune cells causes difficulty in wound healing in aging skin (Keyes et al., 2016). In summary, in vivo studies have confirmed the importance of inflammatory signaling in regulating tissue regeneration by promoting the proliferation and differentiation of tissue-resident stem cells. Both in vitro and in vivo studies have confirmed that the inflammatory environment profoundly affects the biological functions and behavior of stem cells, including enhanced self-renewal and tissue repair. Based on the stem cell division theory of cancer, cancer cells are derived from normal stem cells; therefore, the inflammatory microenvironment is a pivotal factor affecting cancer cell formation from the source. However, under different conditions, the microenvironments of stem cells are complex, and the influence of the inflammatory microenvironment on the function of stem cells and their development into cancer requires further exploration.

Inflammation drives the generation of cancer stem cells

In some tumors, CSCs are considered to be a mutated version of non-malignant stem/progenitor cells and a precursor to differentiated cancer cells that can induce tumor formation and differentiation into various cell types within the tumor. Chronic inflammation, that is, secreted factors from stromal and immune cells, triggers signaling changes in stem/progenitor cells that abnormally activate stem cell-related molecular pathways and promote the transformation of non-malignant cells into CSCs, ultimately inducing tumor formation. The IL-6/JAK/STAT3 signaling axis plays a central role in regulating the generation of CSC in human breast cancer cell lines. Compared with other tumor cell types, IL-6/JAK2/STAT3 pathway is preferentially active in CD44+CD24- breast cancer cells, and JAK2 inhibition reduces their number and blocks the growth of xenograft (Marotta et al., 2011). Furthermore, IL-6 regulates the expression of CSC-related Oct-4 genes in non-CSC through the IL-6/JAK1/STAT3 signal transduction pathway (Kim et al., 2013). TNF- α induced inflammation promotes tumorigenesis and cancer progression and is a key signal for the generation of CSCs. For example, TNF-α triggers chromosomal instability in hepatocellular progenitors by modulating ubiquitin D and checkpoint kinase 2 and enhances the self-renewal of hepatocellular progenitors through the TNFR1/Src/STAT3 pathway, which collaboratively promotes the transformation of hepatocellular progenitors into hepatocellular carcinoma stem cells (Li et al., 2017). Additionally, TNF-α induces malignant transformation of ISCs by activating NF-κB and Wnt/ β-catenin pathways (Zhao et al., 2020). Overall, the cellular and molecular mechanisms of inflammatory induction of CSC generation have been gradually elucidated, providing a new molecular classification for the individualized treatment of cancer.

Inflammation maintains the malignant phenotype of cancer stem cells

The complex interaction between CSCs and their microenvironment can further regulate the growth of CSCs. Inflammatory cytokines, such as ILs, TNF, and chemokines, maintain the stemness state of CSCs in a variety of ways (Table 2). IL-6-mediated signal transduction has been extensively studied. For example, the transcription factor CCAA T/enhancer binding protein delta (C/EBP-δ) amplifies IL-6 and hypoxia-inducible factor 1 (HIF-1) signaling by directly targeting the IL-6 receptor (IL6RA), whose deletion or depletion reduces the expression of stem cell factors and stem cell markers, the formation of blobs, and self-renewal (Balamurugan et al., 2019). The expression of circATP5B in glioma stem cells (GSC) was significantly up-regulated and promoted the proliferation of GSC. Mechanically, circATP5B acts as a miR-185-5p sponge to up-regulate the expression of homeobox protein Hox-B5 (HOXB5). HOXB5 is overexpressed in gliomas and transcriptionally regulates IL6 expression (Zhao et al., 2021). IL-6 also promotes the cell stemness and invasiveness of glioblastoma by inhibiting the expression of miR-142-3p (Chiou et al., 2013). Alternatively, TNF-α-mediated signaling pathways have also been highlighted. Long-term exposure to TNF-α can increase the proportion of CSCs in oral squamous cell carcinoma, thereby enhancing its pellet-forming ability, stem cell transcription factor expression, and tumorigenicity (Lee et al., 2012). CircKPNB1 overexpression can promote the proliferation, invasion, and stem cell viability of GSC. Mechanistically, circKPNB1 regulates protein stability and nuclear translocation in SPI1. SPI1 promotes the malignant phenotype of GSC through TNF-α mediated NF-κB signaling (Jiang et al., 2022). Other inflammatory factors, such as PGE2, chemokine (C-C motif) ligand (CCL) 16, and IL-17(Wang et al., 2015a; Shen et al., 2021; Xiang et al., 2015). can promote the malignant phenotype of CSCs through relevant signaling pathways. Overall, the evidence suggests that inflammatory cytokines are involved in the mechanism of CSCs stemness, and targeting inflammatory cytokines may be a useful adjunct to cancer therapy.

Inflammatory microenvironment reprograms cancer cell fate

Cytokines and growth factors produced during chronic inflammation may have multifunctional effects on tumor formation and growth, both directly on tumor cells and indirectly by promoting favorable conditions in the microenvironment. DNA damage is a bridge between chronic inflammation and cancer (Punt et al., 2016). During inflammation, reactive oxygen and nitrogen species are created to fight pathogens and stimulate tissue repair and regeneration; however, they also damage DNA, which in turn promotes mutations that lead to genomic instability that can initiate and promote cancer development (Kay et al., 2019; Colotta et al., 2009; Pikor et al., 2013). The ability of inflammation to reprogram cancer cell fate is linked to cancer development. Persistent inflammation promotes cancer development by activating the proliferation, survival, and metastasis of cancer cells. (Pikarsky et al., 2004; Kiraly et al., 2015) For example, CCL2-CCR2 signaling promotes cancer progression by supporting cancer cell proliferation and survival, inducing cancer cell migration and invasion, and stimulating inflammation and angiogenesis (Xu et al., 2021). IL-6 directly affects tumor growth by enhancing the proliferation and survival of malignant cells in multiple myeloma, non-Hodgkin’s lymphoma, and hepatocellular carcinoma (Aggarwal et al., 2006; He & Karin, 2011). It has been found that TNF-α and IL-1 β can activate the hypoxia signaling pathway in human liver cancer cells, regulate tumor cells, and directly affect tumor growth (Hellwig-Bürgel et al., 1999). Chronic inflammation caused by smoking can cause neutrophils to exhibit suicidal anti-infective behavior, which, in addition to awakening cancer cells that are dormant for years or even decades in patients with cancer, causes tumor recurrence (Albrengues et al., 2018). Conversely, anticancer compounds inhibit cancer initiation and progression by downregulating pro-inflammatory cytokine levels (Keshari et al., 2017; Barker et al., 2018; Chikara et al., 2018).

Inflammatory microenvironment promotes cancer cell metastasis

Epithelial-to-mesenchymal transition (EMT) is an embryonic process that loosens cell–cell adhesion complexes and confers enhanced migratory and invasive properties to cells, and it is exploited by cancer cells during metastasis. Cancer cells that undergo EMT are more aggressive and exhibit enhanced invasiveness, stem cell-like characteristics, and anti-apoptotic capabilities. Inflammation is a potent inducer of EMT in tumors; conversely, EMT can also stimulate cancer cells to produce pro-inflammatory factors (Suarez-Carmona et al., 2017). Researchers have comprehensively summarized the progression of inflammation and EMT in cancer cells (Suarez-Carmona et al., 2017). Briefly, the EMT/inflammatory axis promotes the aggressiveness of primary tumors, and EMT and inflammatory markers are associated with poor prognosis in multiple cancer patient cohorts. In addition to acting as a direct inducer of EMT in cancer cells, inflammation can act as an intermediate mediator of cancer development. For example, microenvironment dysregulation due to microbial interactions promotes tumor development. Microbes do not directly influence tumor behavior and require mediators to promote tumor development. The current view is that microorganisms regulate tumor immunity through their derived metabolites, toxins, antigens, and other substances; they regulate tumor cell metabolism and reshape the tumor microenvironment to promote tumor occurrence and progression. Inflammation acts as an intermediary in this process (Sepich-Poore et al., 2021). In addition, the regulatory balance between tumors and immunity is a cyclic process. For example, one study found that tumor cells secreted granulocyte-macrophage colony-stimulating factor to stimulate macrophages, which were activated and secreted CCL18, promoting tumor cell EMT and eventually leading to lung metastasis of breast cancer cells (Su et al., 2014). Altogether, the main cause of cancer death is not the primary tumor itself but the depletion of distant organs and tissue metastases. Although the mechanism underlying this process is unclear, inflammation-mediated EMT plays an important role.

In general, malignant tumors are difficult to treat because, in addition to the ability of cancer cells to proliferate, invade, and metastasize indefinitely, they can evade immune surveillance. A series of immunotherapeutic approaches based on immune evasion mechanisms have been developed and clinically applied in the past few decades. Unlike traditional chemoradiotherapy, immunotherapy mainly uses immune cells inside and outside the tumor microenvironment to identify and attack cancer cells (Yost et al., 2019), which theoretically makes immunotherapy more specific with few side effects. Nonetheless, downregulation of major histocompatibility complex class I antigen presentation, which frequently occurs in solid cancers, limits the effectiveness of these therapies. Research has confirmed that cells that appear to be in the quiescent phase are resistant to immune system attacks (Bruschini, Ciliberto & Mancini, 2020). The ability of long-lived stem cells to evade immune surveillance may be due to their mostly non-proliferating quiescent state, which may be an important feature of stem cells that develop into cancer.