The cancer stem cell niche: how essential is the niche in regulating stemness of tumor cells?

Abstract

Cancer stem cells (CSCs) are tumor cells that have the principal properties of self-renewal, clonal tumor initiation capacity, and clonal long-term repopulation potential. CSCs reside in niches, which are anatomically distinct regions within the tumor microenvironment. These niches maintain the principle properties of CSCs, preserve their phenotypic plasticity, protect them from the immune system, and facilitate their metastatic potential. In this perspective, we focus on the CSC niche and discuss its contribution to tumor initiation and progression. Since CSCs survive many commonly employed cancer therapies, we examine the prospects of targeting the niche components as preferable therapeutic targets.

Fig. 1. Models of carcinogenesis

Models are exemplified for an epithelial tissue. Hierarchical model of carcinogenesis: Normal stem cells have limited proliferative capacity and give rise to progenitor cells that proliferate and differentiate into various types of cells. If a normal stem cell escapes regulation, it becomes a cancer stem cell, which can self-renew and produce cancer progenitor cells. If a normal progenitor cell escapes regulation, it becomes cancer progenitor cells, which can give rise to poorly differentiated cells. If those cells are generated from different types of cancer progenitor cells, they might form different subtypes of tumors with limited proliferative capacity. Due to plasticity (red arrows), the progenitor cells and some of the differentiated cells can de-differentiate to become CSCs again. Either CSCs from normal stem cells or from cancer progenitor cells initiate and sustain aggressive tumor growth, and the cells-of-origin for these two types of tumors are either CSCs (blue arrow) or cancer progenitor cells (purple arrow) respectively. Stochastic model of carcinogenesis: Healthy epithelial cells develop an oncogenic mutation (yellow strikes) that forms hyperplasia. Some of the hyperplastic cells can become the cells-of-origin developing additional oncogenic mutations and transform into tumor cells. Under multiple clonal evolutions (colonies shown with various colors), aggressive tumors can form. Some mutations can lead to a stem cell like permissive epigenome and thus create cancer progenitor cells. This process reconciles the stochastic model with the hierarchical model. However, if the hyperplastic cells develop non-oncogenic mutations (green strikes), they will not transform into tumor cells although they may continue to proliferate. If healthy epithelial cells initially undergo non-oncogenic mutations (green strikes), they can overcome such mutations and maintain a healthy tissue.

Fig. 2. The molecular and cellular basis of the cross talk between CSCs and their niches

Cancer stem cell (CSCs) are metastatic cancer cells that can self-renew. Their plasticity and dormancy correlates with their therapeutic resistance. By secreting CXCL12, IL6, and IL8, mesenchymal stem cells (MSCs) promote cancer cell stemness through upregulating NF-κB while CSCs secrete IL6 to attract more MSCs. They also produce the antagonist, Gremlin 1, to promote the undifferentiated state. Surrounding tumor cells produce IL4 to accumulate T_H2, which produces TNFα to upregulate the NF-κB signaling pathway and facilitates a pro-tumor microenvironment. In such a microenvironment, tumor cells produce M-CSF, GM-CSF and G-CSF to induce expansion of tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), tumor-associated neutrophils (TANs) and dendritic cells (DCs). TAM produces TNFα and TGF-β to promote NF-κB dependent or TGF-β dependent EMT and thus enhance CSC plasticity. TGF-β can also directly interact with NF-κB signaling pathways to further enhance cancer cell stemness. In addition, TGF-β produced by TAMs accumulates Treg cells. In addition, TAM, T_Reg and the hypoxic environment inhibit immunosurveillance by inhibiting CD8⁺ T cell and NK cells cytotoxicity and macrophage phagocytosis. A subset of anti-tumor stimulatory DCs necessary for T-cell-mediated tumor rejection is kept away from the niche. Furthermore, hypoxia increases ROS, which promotes cell survival and induces EMT through the TGF-β signaling pathway. Both hypoxia and ROS induce CSCs to express HIF-1α, directly promote EMT. Moreover, hypoxia also inhibits cell proliferation by downregulating c-Myc expression, enhancing stemness. Hypoxia further promotes cancer cell stemness by promoting an undifferentiated state through TGF-β and WNT signaling pathways. Under hypoxia, endothelial cells produce TF to promote angiogenesis. CSCs and cancer-associated fibroblasts (CAFs) produce CXCL12 to promote angiogenesis, and hypoxia causes both CSCs and endothelial cells to produce VEGF, which further induces angiogenesis. CAFs produce TNC and HGF to enhance WNT and NOTCH signaling for CSC maintenance. CAFs also produce MMP2, 3, and 9. Along with the MMP10 produced by CSCs, those MMPs promote ECM degradation and remodeling which enhances EMT and CSC state. Of note, this figure does not provide spatial information as to the exact localization of CSCs and niche cells.

Fig. 3. CSC niches in the primary tumor and metastasis

In the primary tumor, hypoxia develops within the tumor mass due to impaired vascularization, and ROS is increased. Both hypoxia and ROS upregulate the CSC stress signaling pathways to enhance cancer cell survival and maintain cancer cell stemness. At the same time, MSCs and CSCs produce angiogenic factors to stimulate angiogenesis. In the primary tumor, various chemokines and cytokines are secreted to recruit MDSCs, TAMs, and TANs. These pro-tumorigenic and pro-metastatic cells suppress the cytotoxic functions of NK cells and CD8⁺ T cells and inhibit immunosurveillance. Treg cells are accumulated by TAMs to further down-regulate T cell cytotoxicity. TAMs, CAFs, newly generated blood vessels, and other stromal cells accumulate at the invasive front CAFs secrete M-CSF to turn on TAMs’ pro-angiogenic switch. TAMs suppress anti-angiogenic factor expression and secrete VEGF-A and WNT to promote angiogenesis. CAF derived CXCL12 triggers the EGF-M-CSF loop in which cancer cells stimulate TAMs to produce EGF by secreting M-CSF while the activated EGF receptor on CSCs increases their invasiveness. By physically contacting with the platelets, CSCs undergo epithelial-to-mesenchymal (EMT) and become MetSC. At the invasive front, WNT, NOTCH, TNF-α, TGF-β, and other cytokines secreted by tumor stroma support the survival of MetSCs. Meanwhile, TAMs and CSCs release exosomes to prepare the potential metastatic sites for the survival of arriving tumor cells, and various stromal cells and released factors help establish metastatic niches at distant sites. Exosomes also facilitate multidrug resistance (MDR) in tumor cells. In the blood vessels, platelets surround and prevent MetSCs from dying in the harsh and foreign environment. Clusters of tumor cells in the blood vessels secrete M-CSF and EGF family members to direct macrophage and MetSCs to the sites of metastasis. After successful extravasation and seeding of metastatic niches, MetSCs potentially undergo mesenchymal-to-epithelial transition (MET) to become CSCs, which can become dormant or grow metastases in three types of metastatic niche sites. The CSCs can hijack normal stem cell niches established by MSCs. The normal stem cell niche has various factors like TGF-β and various cells to maintain the stemness of CSCs and support their survival. In the niche, CSCs can upregulate EMT pathways in the surrounding nontumorigenic cells and transform them into CSCs to further support the CSCs to colonize the new niche. Primary CSCs can also manipulate distant tissue niches to create a metastatic niche for their future arrival. The primary tumor sends off VEGF-A, TGF-β, TNF-α and LOX, which induce chemotactic protein S100A expression and extracellular matrix remodeling in the metastatic sites, which creates the pre-metastatic niche. Newly formed blood vessels express fibronectin and VCAM to attract inflammatory monocytes (IM) to secrete MMPs for metastatic growth. In the niche, integrins facilitate the migration of arriving CSCs, which is maintained by periostin and TNC upregulate while LOX and S100A actively recruit MDSCs to promote metastatic growth. CSCs initiate their metastatic outgrowth around blood capillaries created by perivascular niches enriched in angiocrine factors like VEGF-A. Surrounding TANs also potentially enhance MetSCs settlement by producing neutrophil extracellular traps (NETs). As the niche is established, CSCs recruit TAMs, CAFs, and other stromal cells to establish the paracrine loops to supply CSCs with TNF-α, TGF-β, and ILs for CSC maintenance. At the meantime, the surrounding stromal cells secrete MMPs and cathepsins to further break down the ECM, which in turn releases TGF-β and various growth factors like VEGF-A, to allow tumor expansion.