TGF-β responding tumor cells – a CSC population of squamous cell carcinomas
Transforming growth factor-βs (TGF-βs) are a family of evolutionally conserved cytokines and an integral component of the cellular crosstalk between normal stem cells and the niches (Oshimori & Fuchs, Cell Stem Cell 2012, Review). To understand cellular crosstalk and signaling pathways that regulate CSCs in early tumor development, we need an experimental system to induce tumorigenesis from a native (stem) cell population at a clonal level, detect activation of signaling pathways in situ, and analyze their functional significance in developing tumors. Through an ultrasound-guided in utero lentiviral microinjection, we previously generated a novel mouse model of HRAS-driven SCC that allows us to label and lineage trace tumor cells responding to TGF-β. We showed that TGF-β–responding tumor cells accumulate at the tumor-stroma interface and give rise to invasive areas of SCC. The progenies of TGF-β-responding tumor cells exhibited aberrant differentiation and phenotypes resembling epithelial-mesenchymal transition (EMT). Moreover, lineage-tracing studies demonstrated that TGF-β-responding tumor cells drive cancer recurrence after cisplatin chemotherapy, suggesting that TGF-β signaling is a critical determinant for the emergence of drug-resistant cancer stem cells (CSCs). Therefore, understanding the mechanisms by which TGF-β endows tumor cells with malignant properties could potentially offer a novel strategy to destabilize CSCs.
Tumor-initiating stem cells drive an IL-33–TGF-β signaling loop for the niche formation
The CSC niche is thought to evolve continuously in tumor initiation and progression and upon drug treatment and recurrence. The mechanism by which the CSC–niche interaction emerges in the course of tumor development is poorly understood. Solid tumors recruit immune cells in the stroma and create favorable conditions for their growth and survival. However, not much is known about how tumor-initiating cells regulate the localization and function of supportive immune cells in their spatial proximity.
TGF-β responding tumor cells are spatially associated with localized TGF-β expression in the adjacent stroma (Fig. X). Therefore, the mechanisms that lead to “TGF-β rich” tumor microenvironments may underlie the development of CSC–niche interactions and potentially be exploited as a new target for destabilizing CSCs. We hypothesized that tumor-initiating cells might send a specific signaling molecule to the adjacent stroma to induce a supporting niche. We identified interleukin-33 (IL-33) as the most highly upregulated cytokine in TGF-β responding tumor cells. IL-33 was required for invasive progression and drug resistance of squamous cell carcinomas. Mechanistically, IL-33 induces the accumulation of a subset of tumor-associated macrophages expressing the IL-33 receptor ST2 and the high-affinity IgE receptor (FcεRIα) close to TGF-β responding cells. In turn, these macrophages alternatively activated and created a TGF-β-rich niche microenvironment, inducing paracrine TGF-β signaling to CSCs and further upregulating IL-33 expression. Moreover, the abrogation of the pathway or depletion of the niche macrophages reduced the rate of invasive tumor progression and chemo-resistance.
Therapy-resistant CSCs are considered major culprits in cancer treatment failure. We unveiled the cellular and molecular basis for forming a CSC niche that promotes malignant tumor progression and drug resistance. The discovery of the IL-33–TGF-β signaling loop between tumor-initiating cells and tumor-associated macrophages provides mechanistic insights into self-reinforcing CSC–niche interactions, which could be a potential target for destabilizing CSCs to improve cancer treatment efficacy.
ADAP1, a predictor of poor survival in early-stage HNSCC, promotes tumor progression
The progression to invasive squamous cell carcinoma is associated with the high risk of recurrence and metastasis, but the critical determinants of its progression remain elusive. Because genes upregulated in CSCs can be predictive of poor patient outcomes, we sought CSC signature genes with prognostic value. We identified ADAP1 (ArfGAP with dual pleckstrin homology domains 1) as a strong predictor of poor survival of Stage I/II HNSCC patients.
ADAP1 facilitates the activity of the small GTPase ADP-ribosylation factor 6 (ARF6) to hydrolyze GTP to GDP. Notably, ARF family proteins do not have detectable intrinsic GTPase activity. Thus, GTPase-activating proteins (GAPs), such as ADAP1, are crucial for ARF6 function. ARF6 regulates endocytic membrane trafficking and the internalization and externalization of various membrane proteins, including growth factor receptors, integrins, and matrix metalloproteases. ARF6 has been implicated in tumor development and metastasis.
We found that ADAP1 promotes invasive tumor progression by facilitating cell migration and the breakdown of the basement membrane. Consistent with this, squamous cell carcinomas developing on Adap1-deficient mice had fewer invading cells in the stroma compared to control tumors. Our study showed that ADAP1 is a critical mediator of CSC invasion and might be exploited for the treatment of high-risk SCC.
Cancer stem cells of salivary gland carcinoma
Salivary gland carcinomas are diverse cancer, likely reflecting differences in the tissue- and cell-of-origin. There are substantial differences in malignancy rates among the major salivary glands, including the parotid gland (PG), submandibular gland (SMG), and sublingual gland (SLG). The malignancy rates of tumors arising in the PG and SMG are about 25% and 40%, respectively, whereas approximately 80%-90% of SLG tumors are malignant diseases with poor prognosis. It remains unclear what factors are associated with extremely high malignancy rates in SLG.
Using an in utero microinjection methodology, we transduced the placodes of mouse salivary glands with lentiviruses and studied the process of HRAS-driven salivary gland tumorigenesis. We found that HMGA2 (High-mobility group AT-hook 2) expression marks the tumor onset. Notably, an established (cancer) stem cell marker SOX2 expresses in the SLG, but not in the other glands. SOX2+ stem/progenitor cells were expanded in SLG tumors, but not in tumors arising in the PG and SMG. Since SOX2 plays a pivotal role in tumor initiation, cancer stem cell maintenance, and epithelial-mesenchymal transition (EMT). Indeed, we found that invasive progression of SLG tumors was mediated by EMT and our data supported that TGF-β signaling drives EMT.
Salivary gland malignancies are relatively rare types of head and neck cancers; thus, not much is known about their etiology and potential therapeutic targets. Moreover, there are few mouse models which offer limited ability to study the processes of malignant tumor development from native tissues. Our de novo tumor mouse model provides a new opportunity to explore the biology of salivary gland tumorigenesis, which will advance our understanding of the mechanistic basis of malignant progression and may help combat this highly heterogeneous cancer.