An emerging role for cellular crosstalk in the cancer stem cell niche

Abstract

Although cumulative genetic and epigenetic changes in cancer cells are correlated with tumor malignancy, accumulating evidence supports that tumor cell-extrinsic mechanisms play an essential role in driving tumor progression. The tissue architecture surrounding tumor cells evolves during disease progression and becomes a significant barrier to cancer treatments. The functional traits of the tumor microenvironment (TME), either tumor suppressive or supportive, are defined by the distribution of various stromal cells and their sequential and reciprocal cellular interactions. Recent studies have uncovered a significant heterogeneity in stromal cells and identified specific subpopulations correlated with clinical outcomes, providing novel insights into the complex TME system that drives tumor progression and therapy resistance. Moreover, a small population of tumor cells with tumor-initiating and drug-resistant capabilities, cancer stem cells (CSCs), is maintained by the specialized TME, the so-called CSC niche. The crosstalk between CSCs and niche cells is an attractive avenue for identifying the vulnerability of difficult-to-treat cancers. Here, we review the recent advance in understanding TME biology and its impact on CSCs. We then focus on a newly identified niche signaling loop by which CSCs promote malignant progression and drug resistance of squamous cell carcinoma. The CSC niche is a promising research field that needs more attention and could facilitate the development of durable cancer treatment. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Keywords: CSC niche; IL-33; TGF-β; cancer progression; cancer stem cells (CSCs); cellular crosstalk; drug resistance; macrophages; tumor microenvironment (TME).

Figure 1.

The crosstalk between tumor cells and non-immune stromal cells and the TME conditions. Cellular interactions between tumor cells and the surrounding microenvironment influence tumor initiation, malignant progression, therapy resistance, and metastasis. This figure focuses on the heterogeneity of non-immune stromal cells and biochemical interactions. Fibroblasts have several key subtypes that interact with tumor cells, CSCs, and other stromal cells differentially. By secreting TGF-β and IL-1α, the tumor regulates CAF precursors’ differentiation into myofibroblastic CAFs and inflammatory CAFs, respectively. In addition to promoting tumor cell invasion, myofibroblastic CAFs contribute to immunosuppression by elevating OX40L and PD-L2 expression in CD4⁺ T cells and promoting differentiation of FoxP3⁺ regulatory T cells (Treg). In contrast, inflammatory CAFs induced by LIF-JAK/STAT signaling recruit myeloid cells and Treg. By secreting IL-6/8, CD10⁺CPR77⁺ CAFs generated by a self-reinforced C5a and NF-kB loop promote stemness and chemoresistance of CSCs. Ptgs2⁺ fibroblasts secrete metabolites, such as PGE2, induce the expansion of Sca1⁺ quiescent stem cells and tumor initiation through the Hippo-YAP pathway. Endothelial cells have more than 30 different states based on expression profiling. The tip and breach subtypes are predictive of poor prognosis and sensitive to VEGF-A blockade. Endothelial cells and CSCs form a feedback VEGF-A-Jagged1 loop through Notch signaling. The ECM produced by fibroblasts, tumor cells, and other stromal cells, provides the framework of the TME. CAFs and tumor cells secrete MMPs, such as MMP-2/9, to remodel the ECM. Some specific ECM proteins, including periostin, type I collagen, laminin-332, the 200 kDa hyaluronic acid (HA), and tenascin C (TNC), maintain and promote CSC phenotypes through activating various signaling pathways, such as Wnt, integrin/focal adhesion kinase (FAK), and NF-kB pathways.

Figure 2.

The crosstalk between tumor cells and immune cells in the TME. The TME is composed of heterogeneous immune cells. Here, we summarize some of the major cell types. Various mechanisms suppress the cytotoxic activity of CD8⁺ T cells. T cell-intrinsic expression of immune checkpoint molecules, such as PD-1 and CTLA4, suppress anti-tumor immunity. Tumor-derived VEGF-A also induces PD-1 expression in CD8⁺ T cells. CD4⁺ T cells can also kill tumor cells in an MHC-II-dependent manner. Although TGF-β suppresses T cell activity in general, TGF-β can also upregulate CD80 expression in CSCs and CD80 binds to CTLA4 to suppress anti-tumor responses. Bone marrow-derived myeloid cells are abundantly recruited into the tumor. The MDSCs could differentiate into M-MDSCs and PMN-MDSCs subtypes, which are more similar to TAMs and neutrophils, respectively. Tissue inflammation causes neutrophils to release NET proteases, which cleave laminin in the basement membrane and activate integrin signaling in dormant tumor cells, promoting metastatic tumor formation. Tumor-derived retinoic acid (RA) suppresses monocyte differentiation into dendritic cells, resulting in an increase in TAMs. Macrophages have two interchangeable states, classically activated phenotype (M1) and alternatively activated phenotype (M2). PD-L1 expressed in tumor cells also suppresses macrophage differentiation toward M1 by inhibiting PD-1. M2 macrophages induce angiogenesis and promote metastasis. Periostin secreted from tumor cells also supports M2 macrophage differentiation. CSCs establish an IL-33–TGF-β signaling loop with newly identified FcεRIα⁺ macrophages to maintain their stemness.

Figure 3.

The TGF-β signaling pathway, SCC mouse model, and models of IL-33 extracellular release. (A) A summary of the TGF-β signaling pathway. TGF-β is translated as larger polypeptides and proteolytically cleaved in TGF-β-producing cells. After secretion, TGF-β is sequestered in the ECM as a latent complex. The release of TGF-β ligand is regulated by diverse mechanisms, including protease-mediated cleavage of latent associated protein (LAP), low pH, reactive oxygen species, and conformational changes of LAP by Arg-Gly-Asp (RGD)-binding integrins [69]. TGF-β binds and brings together two transmembrane serine–threonine kinases, TGF-β receptor I (TβRI) and II (TβRII). The receptor complex propagates the signal by phosphorylating intracellular substrates. The canonical downstream effectors are SMAD2 and SMAD3. Phosphorylated SMAD2/3 can complex with SMAD4, translocate to the nucleus, and form an active transcription factor by binding to the specific DNA sequence called SMAD-binding elements (SBEs). Additionally, TGF-β activates SMAD-independent, non-canonical pathways, including PI3K/Akt, MAPK (ERK, JNK, and p38), and Rho/Rho kinase pathways. (B) Schematic illustration of epidermis-specific lentiviral transduction and HRAS-driven SCC mouse model. Lentiviral vectors (LVs) are injected into the amniotic fluid surrounding the TetO-Hras, Rosa-YFP transgenic mouse embryo to transduce the epidermal progenitor cells. Postnatally, doxycycline administration induces HRAS-driven SCC (green), in which TGF-β-responding tumor cells are detected by the reporter protein expression (red). (C) Regulation of the extracellular release of IL-33. After necrotic cell death, full-length IL-33 is released from the nuclei of IL-33-producing cells. Additionally, a cell death-independent, mechanical stress-induced release of IL-33 has been reported [70]. (D) Our group found that NRF2^Hi, TGF-β-responding tumor cells in invasive SCC show cytoplasmic translocation of IL-33, which was correlated with the accumulation of FcεRIα⁺ and the IL-33 receptor ST2⁺ macrophages in the stroma.

Figure 4.

A model of CSC-niche interactions during invasive SCC progression. (A) CSCs, either TGF-β-responding or NRF2-activated, release IL-33 as a short-distant signal, which promotes the differentiation of ST2⁺ immature myeloid cells into FcεRIα⁺ alternatively activated macrophages. (B) FcεRIα⁺ macrophages express high levels of TGF-β and produce ‘TGF-β rich’ niche microenvironments in the proximity to CSCs. The niche sustains paracrine TGF-β signaling necessary to maintain slow-cycling CSCs and promote their invasive and drug-resistant properties. Moreover, TGF-β-responding CSCs upregulate IL-33 expression and activate the extracellular release of IL-33 through the NRF2-mediated antioxidant response. This self-reinforcing IL-33-TGF-β niche signaling loop is a crucial mechanism of malignant progression and drug resistance of SCC.