Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

Stem-cell states converge in multistage cutaneous squamous cell carcinoma development

Stem cells play a critical role in cancer development by contributing to cell heterogeneity, lineage plasticity, and drug resistance. We created gene expression networks from hundreds of mouse tissue samples (both normal and tumor) and integrated these with lineage tracing and single-cell RNA-seq, to identify convergence of cell states in premalignant tumor cells expressing markers of lineage plasticity and drug resistance. Two of these cell states representing multilineage plasticity or proliferation were inversely correlated, suggesting a mutually exclusive relationship. Treatment of carcinomas in vivo with chemotherapy repressed the proliferative state and activated multilineage plasticity whereas inhibition of differentiation repressed plasticity and potentiated responses to cell cycle inhibitors. Manipulation of this cell state transition point may provide a source of potential combinatorial targets for cancer therapy.

Mutations, Bottlenecks, and Clonal Sweeps: How Environmental Carcinogens and Genomic Changes Shape Clonal Evolution during Tumor Progression

The transition from a single, initiated cell to a full-blown malignant tumor involves significant genomic evolution. Exposure to carcinogens-whether directly mutagenic or not-can drive progression toward malignancy, as can stochastic acquisition of cancer-promoting genetic events. Mouse models using both carcinogens and germline genetic manipulations have enabled precise inquiry into the evolutionary dynamics that take place as a tumor progresses from benign to malignant to metastatic stages. Tumor progression is characterized by changes in somatic point mutations and copy-number alterations, even though any single tumor can itself have a high or low burden of genomic alterations. Further, lineage-tracing, single-cell analyses and CRISPR barcoding have revealed the distinct clonal dynamics within benign and malignant tumors. Application of these tools in a range of mouse models can shed unique light on the patterns of clonal evolution that take place in both mouse and human tumors.

Sniping Cancer Stem Cells with Nanomaterials

Cancer stem cells (CSCs) drive tumor initiation, progression, and therapeutic resistance due to their self-renewal and differentiation capabilities. Despite encouraging progress in cancer treatment, conventional approaches often fail to eliminate CSCs, necessitating the development of precise targeted strategies. Recent advances in materials science and nanotechnology have enabled promising CSC-targeted approaches, harnessing the power of tailoring nanomaterials in diverse therapeutic applications. This review provides an update on the current landscape of nanobased precision targeting approaches against CSCs. We elucidate the nuanced application of organic, inorganic, and bioinspired nanomaterials across a spectrum of therapeutic paradigms, encompassing targeted therapy, immunotherapy, and multimodal synergistic therapies. By examining the accomplishments and challenges in this potential field, we aim to inform future efforts to advance nanomaterial-based therapies toward more effective "sniping" of CSCs and tumor clearance.

Injury prevents Ras mutant cell expansion in mosaic skin

Healthy skin is a mosaic of wild-type and mutant clones1,2. Although injury can cooperate with mutated Ras family proteins to promote tumorigenesis3-12, the consequences in genetically mosaic skin are unknown. Here we show that after injury, wild-type cells suppress aberrant growth induced by oncogenic Ras. HrasG12V/+ and KrasG12D/+ cells outcompete wild-type cells in uninjured, mosaic tissue but their expansion is prevented after injury owing to an increase in the fraction of proliferating wild-type cells. Mechanistically, we show that, unlike HrasG12V/+ cells, wild-type cells respond to autocrine and paracrine secretion of EGFR ligands, and this differential activation of the EGFR pathway explains the competitive switch during injury repair. Inhibition of EGFR signalling via drug or genetic approaches diminishes the proportion of dividing wild-type cells after injury, leading to the expansion of HrasG12V/+ cells. Increased proliferation of wild-type cells via constitutive loss of the cell cycle inhibitor p21 counteracts the expansion of HrasG12V/+ cells even in the absence of injury. Thus, injury has a role in switching the competitive balance between oncogenic and wild-type cells in genetically mosaic skin.

All-Trans Retinoic Acid-Responsive LGR6 Is Transiently Expressed during Myogenic Differentiation and Is Required for Myoblast Differentiation and Fusion

All-trans retinoic acid (ATRA) promotes myoblast differentiation into myotubes. Leucine-rich repeat-containing G-protein-coupled receptor 6 (LGR6) is a candidate ATRA-responsive gene; however, its role in skeletal muscles remains unclear. Here, we demonstrated that during the differentiation of murine C2C12 myoblasts into myotubes, Lgr6 mRNA expression transiently increased before the increase in the expression of the mRNAs encoding myogenic regulatory factors, such as myogenin, myomaker, and myomerger. The loss of LGR6 decreased the differentiation and fusion indices. The exogenous expression of LGR6 up to 3 and 24 h after the induction of differentiation increased and decreased the mRNA levels of myogenin, myomaker, and myomerger, respectively. Lgr6 mRNA was transiently expressed after myogenic differentiation in the presence of a retinoic acid receptor α (RARα) agonist and an RARγ agonist in addition to ATRA, but not in the absence of ATRA. Furthermore, a proteasome inhibitor or Znfr3 knockdown increased exogenous LGR6 expression. The loss of LGR6 attenuated the Wnt/β-catenin signaling activity induced by Wnt3a alone or in combination with Wnt3a and R-spondin 2. These results indicate that LGR6 promotes myogenic differentiation and that ATRA is required for the transient expression of LGR6 during differentiation. Furthermore, LGR6 expression appeared to be downregulated by the ubiquitin-proteasome system involving ZNRF3.

Gene networks reveal stem-cell state convergence during preneoplasia and progression to malignancy in multistage skin carcinogenesis

Adult mammalian stem cells play critical roles in normal tissue homeostasis, as well as in tumor development, by contributing to cell heterogeneity, plasticity, and development of drug resistance. The relationship between different types of normal and cancer stem cells is highly controversial and poorly understood. Here, we carried out gene expression network analysis of normal and tumor samples from genetically heterogeneous mice to create network metagenes for visualization of stem-cell networks, rather than individual stem-cell markers, at the single-cell level during multistage carcinogenesis. We combined this approach with lineage tracing and single-cell RNASeq of stem cells and their progeny, identifying a previously unrecognized hierarchy in which Lgr6+ stem cells from tumors generate progeny that express a range of other stem-cell markers including Sox2, Pitx1, Foxa1, Klf5, and Cd44. Our data identify a convergence of multiple stem-cell and tumor-suppressor pathways in benign tumor cells expressing markers of lineage plasticity and oxidative stress. This same single-cell population expresses network metagenes corresponding to markers of cancer drug resistance in human tumors of the skin, lung and prostate. Treatment of mouse squamous carcinomas in vivo with the chemotherapeutic cis-platin resulted in elevated expression of the genes that mark this cell population. Our data have allowed us to create a simplified model of multistage carcinogenesis that identifies distinct stem-cell states at different stages of tumor progression, thereby identifying networks involved in lineage plasticity, drug resistance, and immune surveillance, providing a rich source of potential targets for cancer therapy.

High expression of LGR6 is a poor prognostic factor in esophageal carcinoma

BACKGROUND

Leucine-rich repeat-containing G-protein-coupled receptor 6 (LGR6) promotes carcinogenesis and progression in some cancer types. However, there are few reports of LGR6 expression in esophageal squamous cell carcinoma (ESCC). LGR6 expression and clinicopathological features in ESCC were investigated by RNAscope, a highly sensitive RNA in situ hybridization method.

METHODS

Appropriate tumors were selected from 41 cases of ESCC from which tissue microarrays were generated, and LGR6 expression was identified by RNAscope.

RESULTS

Thirty-seven patients had LGR6 expression. High LGR6 expression was observed in 17 cases and low LGR6 expression in 24 cases. LGR6 expression was significantly higher in high histological grade ESCC than in low histological grade ESCC (P = 0.0023). ESCC patients who received neoadjuvant chemotherapy had significantly higher LGR6 expression than those without neoadjuvant chemotherapy (P = 0.0109). Furthermore, high LGR6 expression showed a poorer prognosis than low LGR6 expression (log-rank test, P = 0.0365).

CONCLUSIONS

LGR6 may be a prognostic factor and a potential new therapeutic target in ESCC.

Loss of Epidermal Homeostasis Underlies the Development of Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) is one of the most common skin cancers. To develop targeted therapies for SCC, a comprehensive understanding of the disease through a systems approach is required. Here, we have collated and analyzed the literature on SCC and pathways that maintain skin homeostasis. Since, the loss of the Notch and the overactivation of the Wnt pathways in the epidermis cause SCC, we focused on these two pathways. We found that the two pathways are critical in maintaining epidermal homeostasis. Further, we found that the cancer stem cell (CSC) marker CD44 causes the transcription of SOX2, another CSC marker of SCC, activates the Wnt pathway, and blocks the Notch pathway. Similarly, the Wnt pathway causes the transcription of CD44 and SOX2 and blocks the Notch pathway. In this paper, we have discussed how the notch and the Wnt pathways affect epidermal homeostasis and the three CSCs (CD44, SOX2, and LGR6) affect the two pathways, linking the CSCs with epidermal homeostasis.

LGR6-dependent conditional inactivation of E-cadherin and p53 leads to invasive skin and mammary carcinomas in mice

Tissue-specific inactivation of E-cadherin combined with tumor suppressor loss leads to invasive and metastatic cancers in mice. While epidermal E-cadherin loss in mice induces squamous cell carcinomas, inactivation of E-cadherin in the mammary gland leads to invasive lobular carcinoma. To further explore the carcinogenic consequences of cell-cell adhesion loss in these compartments, we developed a new conditional mouse model inactivating E-cadherin (Cdh1) and p53 (Trp53) simultaneously in cells expressing the leucine-rich repeat-containing G-protein coupled receptor 6 (Lgr6), a putative epithelial stem cell marker in the skin and alveolar progenitor marker in the mammary gland. Compound Lgr6-CreERT2;Cdh1^F;Trp53^F female mice containing either heterozygous or homozygous Cdh1^F alleles were bred, and Lgr6-driven Cre expression was activated in pre-puberal mice using tamoxifen. We observed that 41% of the mice (16/39) developed mostly invasive squamous-type skin carcinomas, but also a non-lobular mammary tumor was formed. In contrast to previous K14cre or WAPcre E-cadherin and p53 compound models, no significant differences were detected in the tumor-free survival of Lgr6-CreERT2 heterozygous Cdh1^F/WT;Trp53^F/F versus homozygous Cdh1^F/F;Trp53^F/F mice (778 versus 754 days, p=0.5). One Cdh1^F homozygous mouse presented with lung metastasis that originated from a non-lobular and ERα negative invasive mammary gland carcinoma with squamous metaplasia. In total, 2/8 (25%) Cdh1^F heterozygous and 3/12 (25%) Cdh1^F homozygous mice developed metastases to lungs, liver, lymph nodes, or the gastro-intestinal tract. In conclusion, we show that inducible and conditional Lgr6-driven inactivation of E-cadherin and p53 in mice causes squamous cell carcinomas of the skin in approximately 40% of the mice and an occasional ductal-type mammary carcinoma after long latency periods.

Review: The evolution of peptidergic signaling in Cnidaria and Placozoa, including a comparison with Bilateria

Bilateria have bilateral symmetry and are subdivided into Deuterostomia (animals like vertebrates) and Protostomia (animals like insects and mollusks). Neuropeptides occur in both Proto- and Deuterostomia and they are frequently structurally related across these two lineages. For example, peptides belonging to the oxytocin/vasopressin family exist in both clades. The same is true for the G protein-coupled receptors (GPCRs) of these peptides. These observations suggest that these neuropeptides and their GPCRs were already present in the common ancestor of Proto- and Deuterostomia, which lived about 700 million years ago (MYA). Furthermore, neuropeptides and their GPCRs occur in two early-branching phyla that diverged before the emergence of Bilateria: Cnidaria (animals like corals and sea anemones), and Placozoa (small disk-like animals, feeding on algae). The sequences of these neuropeptides and their GPCRs, however, are not closely related to those from Bilateria. In addition, cnidarian neuropeptides and their receptors are not closely related to those from Placozoa. We propose that the divergence times between Cnidaria, Placozoa, and Bilateria might be too long for recognizing sequence identities. Leucine-rich repeats-containing GPCRs (LGRs) are a special class of GPCRs that are characterized by a long N-terminus containing 10-20 leucine-rich domains, which are used for ligand binding. Among the ligands for LGRs are dimeric glycoprotein hormones, and insulin-like peptides, such as relaxin. LGRs have been found not only in Proto- and Deuterostomia, but also in early emerging phyla, such as Cnidaria and Placozoa. Humans have eight LGRs. In our current review, we have revisited the annotations of LGRs from the sea anemone Nematostella vectensis and the placozoan Trichoplax adhaerens. We identified 13 sea anemone LGRs and no less than 46 LGRs from T. adhaerens. All eight human LGRs appear to have orthologues in sea anemones and placozoans. LGRs and their ligands, therefore, have a long evolutionary history, going back to the common ancestor of Cnidaria and Placozoa.

Investigating the Interactions of Glioma Stem Cells in the Perivascular Niche at Single-Cell Resolution using a Microfluidic Tumor Microenvironment Model

The perivascular niche (PVN) is a glioblastoma tumor microenvironment (TME) that serves as a safe haven for glioma stem cells (GSCs), and acts as a reservoir that inevitably leads to tumor recurrence. Understanding cellular interactions in the PVN that drive GSC treatment resistance and stemness is crucial to develop lasting therapies for glioblastoma. The limitations of in vivo models and in vitro assays have led to critical knowledge gaps regarding the influence of various cell types in the PVN on GSCs behavior. This study developed an organotypic triculture microfluidic model as a means to recapitulate the PVN and study its impact on GSCs. This triculture platform, comprised of endothelial cells (ECs), astrocytes, and GSCs, is used to investigate GSC invasion, proliferation and stemness. Both ECs and astrocytes significantly increased invasiveness of GSCs. This study futher identified 15 ligand-receptor pairs using single-cell RNAseq with putative chemotactic mechanisms of GSCs, where the receptor is up-regulated in GSCs and the diffusible ligand is expressed in either astrocytes or ECs. Notably, the ligand-receptor pair SAA1-FPR1 is demonstrated to be involved in chemotactic invasion of GSCs toward PVN. The novel triculture platform presented herein can be used for therapeutic development and discovery of molecular mechanisms driving GSC biology.

LGR6 promotes glioblastoma malignancy and chemoresistance by activating the Akt signaling pathway

Chemoresistance is the primary cause of the poor outcome of glioblastoma multiforme (GBM) therapy. Leucine-rich repeat-containing G-protein coupled receptor 6 (LGR6) is involved in the growth and proliferation of several types of cancer, including gastric cancer and ovarian cancer. Therefore, the aim of the present study was to investigate the role of LGR6 in GBM malignancy and chemoresistance. Cell counting kit-8 and Matrigel®-Transwell assays were conducted to assess GBM cell viability and invasion. The effect of LGR6 on cell cycle progression and activation of Akt signaling was analyzed by performing propidium iodide staining and western blotting, respectively. The results demonstrated that LGR6, a microRNA-1236-3p target candidate, promoted GBM cell viability and invasion, and mediated temozolomide sensitivity in SHG-44 and U251 GBM cells. In addition, LGR6 triggered the activation of the Akt signaling pathway during GBM progression. Collectively, the results of the present study suggested that LGR6 promoted GBM malignancy and chemoresistance, at least in part, by activating the Akt signaling pathway. The results may aid with the identification of a novel therapeutic target and strategy for GBM.

DKK1 inhibits canonical Wnt signaling in human papillomavirus-positive penile cancer cells

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Elevated expression of Wnt pathway associated factors in HPV-positive penile cancer cells.

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lacking nuclear beta-catenin translocation indicated an actively abrogated Wnt signaling.

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elevated expression of the Wnt antagonist DKK1 in HPV-positive penile cancer cells.

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DKK1-driven autocrine Wnt pathway inhibition in penile cancer cells.

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DKK1⁺ penile cancers are with a higher frequency HPV⁺, less differentiated and grow more aggressively.

Penile squamous cell cancer (PSCC) is the most frequent penile malignant disease. Infections with human papillomaviruses (HPV) are a major etiologic driver of PSCC. However, the molecular details of the underlying carcinogenesis are understudied because of rare clinical specimens and missing cell lines. Here, we investigated if the expression of high-risk HPV16 oncogenes causes an augmentation of the Wnt pathway using unique HPV-positive penile cancer (PeCa) cell lines in monolayer and organotypic 3D raft cultures as well as tissue micro arrays containing clinical tissue specimens. The HPV oncoproteins enhanced the expression of Leucine-rich repeat-containing G-protein coupled receptor 6 (LGR6) and the HPV-positive PeCa cells expressed a signature of Wnt target and stemness-associated genes. However, the notable lack of nuclear β-catenin in vitro and in situ raised the question if the enhanced expression of Wnt pathway factors is tantamount to an active Wnt signaling. Subsequent TOP-flash reporter assays revealed Wnt signaling as absent and not inducible by respective Wnt ligands in PeCa cell lines. The HPV-positive PeCa cells and especially HPV-positive PeCa specimens of the tumor core expressed the Wnt antagonist and negative feedback-regulator Dickkopf1 (DKK1). Subsequent neutralization experiments using PeCa cell line-conditioned media demonstrated that DKK1 is capable to impair ligand-induced Wnt signaling. While gene expression analyses suggested an augmented and active canonical Wnt pathway, the respective signaling was inhibited due to the endogenous expression of the antagonist DKK1. Subsequent TMA stainings indicated Dkk1 as linked with HPV-positivity and metastatic disease progression in PeCa suggesting potential as a prognostic marker.

Drug Conjugates of Antagonistic R-Spondin 4 Mutant for Simultaneous Targeting of Leucine-Rich Repeat-Containing G Protein-Coupled Receptors 4/5/6 for Cancer Treatment

LGR4-6 (leucine-rich repeat-containing G-protein-coupled receptors 4, 5, and 6) are three related receptors with an upregulated expression in gastrointestinal cancers to various extents, and LGR5 is enriched in cancer stem cells. Antibody-drug conjugates (ADCs) targeting LGR5 showed a robust antitumor effect in vivo but could not eradicate tumors due to plasticity of LGR5-positive cancer cells. As LGR5-negative cancer cells often express LGR4 or LGR6 or both, we reasoned that simultaneous targeting of all three LGRs may provide a more effective approach. R-spondins (RSPOs) bind to LGR4-6 with high affinity and potentiate Wnt signaling. We identified an RSPO4 furin domain mutant (Q65R) that retains potent LGR binding but no longer potentiates Wnt signaling. Drug conjugates of a peptibody comprising the RSPO4 mutant and IgG1-Fc showed potent cytotoxic effects on cancer cell lines expressing any LGR in vitro and suppressed tumor growth in vivo without inducing intestinal enlargement or other adverse effects.

Expression of LGR5 in mammary myoepithelial cells and in triple-negative breast cancers

Lineage tracing in mice indicates that LGR5 is an adult stem cell marker in multiple organs, such as the intestine, stomach, hair follicles, ovary, and mammary glands. Despite many studies exploring the presence of LGR5 cells in human tissues, little is known about its expression profile in either human mammary tissue or pathological lesions. In this study we aim to investigate LGR5 expression in normal, benign, and malignant lesions of the human breast using RNA in situ hybridization. LGR5 expression has not been observed in normal lactiferous ducts and terminal duct lobular units, whereas LGR5-positive cells have been specifically observed in the basal myoepithelium of ducts in the regenerative tissues, ductal carcinoma in situ, and in ducts surrounded by invasive cancer cells. These findings suggest LGR5 marks facultative stem cells that are involved in post injury regeneration instead of homeostatic stem cells. LGR5 positivity was found in 3% (9 of 278 cases) of invasive breast cancers (BC), and it showed positive associations with higher histologic grades (P = 0.001) and T stages (P < 0.001), while having negative correlations with estrogen receptor (P < 0.001) and progesterone receptor (P < 0.001) expression. Remarkably, all LGR5-positive BC, except one, belong to triple-negative BC (TNBC), representing 24% (9 of 38 cases) of all of them. LGR5 histoscores have no correlations with EGFR, CK5/6, Ki-67, or P53 expression. Additionally, no β-catenin nuclear localization was observed in LGR5-positive BC, indicating that canonical Wnt pathway activation is less likely involved in LGR5 expression in BC. Our results demonstrate that LGR5 expression is induced in regenerative conditions in the myoepithelium of human mammary ducts and that its expression is only observed in TNBC subtype among all invasive BC. Further studies regarding the functional and prognostic impact of LGR5 in TNBC are warranted.

Genotype-Phenotype Relationships in the Context of Transcriptional Adaptation and Genetic Robustness

Genetic manipulations with a robust and predictable outcome are critical to investigate gene function, as well as for therapeutic genome engineering. For many years, knockdown approaches and reagents including RNA interference and antisense oligonucleotides dominated functional studies; however, with the advent of precise genome editing technologies, CRISPR-based knockout systems have become the state-of-the-art tools for such studies. These technologies have helped decipher the role of thousands of genes in development and disease. Their use has also revealed how limited our understanding of genotype-phenotype relationships is. The recent discovery that certain mutations can trigger the transcriptional modulation of other genes, a phenomenon called transcriptional adaptation, has provided an additional explanation for the contradicting phenotypes observed in knockdown versus knockout models and increased awareness about the use of each of these approaches. In this review, we first cover the strengths and limitations of different gene perturbation strategies. Then we highlight the diverse ways in which the genotype-phenotype relationship can be discordant between these different strategies. Finally, we review the genetic robustness mechanisms that can lead to such discrepancies, paying special attention to the recently discovered phenomenon of transcriptional adaptation. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Wnt-Independent LGR4-EGFR Signaling Axis in Cancer Metastasis

Leucine-rich repeat-containing G protein-coupled receptors 4, 5, and 6 (LGR4/5/6) play critical roles in development and cancer. The widely accepted mechanism is that these proteins, together with their R-spondin ligands, stabilize Wnt receptors, thus potentiating Wnt signaling. Here we show that LGR4 enhanced breast cancer cell metastasis even when Wnt signaling was deactivated pharmacologically or genetically. Furthermore, LGR4 mutants that cannot potentiate Wnt signaling nevertheless promoted breast cancer cell migration and invasion in vitro and breast cancer metastasis in vivo. Multiomic screening identified EGFR as a crucial mediator of LGR4 activity in cancer progression. Mechanistically, LGR4 interacted with EGFR and blocked EGFR ubiquitination and degradation, resulting in persistent EGFR activation. Together, these data uncover a Wnt-independent LGR4-EGFR signaling axis with broad implications for cancer progression and targeted therapy. SIGNIFICANCE: This work demonstrates a Wnt-independent mechanism by which LGR4 promotes cancer metastasis.See related commentary by Stevens and Williams, p. 4397.

Cellular heterogeneity and microenvironmental control of skin cancer

Healthy tissues harbour a surprisingly high number of cells that carry well-known cancer-causing mutations without impacting their physiological function. In recent years, strong evidence accumulated that the immediate environment of mutant cells profoundly impact their prospect of malignant progression. In this review, focusing on the skin, we investigate potential key mechanisms that ensure tissue homeostasis despite the presence of mutant cells, as well as critical factors that may nudge the balance from homeostasis to tumour formation. Functional in vivo studies and single-cell transcriptome analyses have revealed a tremendous cellular heterogeneity and plasticity within epidermal (stem) cells and their respective niches, revealing for example wild-type epithelial cells, fibroblasts or immune-cell subsets as critical in preventing cancer formation and malignant progression. It's the same cells, however, that can drive carcinogenesis. Therefore, understanding the abundance and molecular variation of cell types in health and disease, and how they interact and modulate the local signalling environment will thus be key for new therapeutic avenues in our battle against cancer.

Patient-derived organoids model cervical tissue dynamics and viral oncogenesis in cervical cancer

Cervical cancer is a common gynecological malignancy often caused by high-risk human papillomavirus. There is a paucity of human-derived culture systems to study the cervical epithelium and the cancers derived thereof. Here we describe a long-term culturing protocol for ecto- and endocervical epithelia that generates 3D organoids that stably recapitulate the two tissues of origin. As evidenced for HSV-1, organoid-based cervical models may serve to study sexually transmitted infections. Starting from Pap brush material, a small biobank of tumoroids derived from affected individuals was established that retained the causative human papillomavirus (HPV) genomes. One of these uniquely carried the poorly characterized HPV30 subtype, implying a potential role in carcinogenesis. The tumoroids displayed differential responses to common chemotherapeutic agents and grew as xenografts in mice. This study describes an experimental platform for cervical (cancer) research and for future personalized medicine approaches.

Distinct transcriptomic landscapes of cutaneous basal cell carcinomas and squamous cell carcinomas

The majority of non-melanoma skin cancer (NMSC) is cutaneous basal cell carcinoma (BCC) or squamous cell carcinoma (SCC), which are also called keratinocyte carcinomas, as both of them originate from keratinocytes. The incidence of keratinocyte carcinomas is over 5 million per year in the US, three-fold higher than the total incidence of all other types of cancer combined. While there are several reports on gene expression profiling of BCC and SCC, there are significant variations in the reported gene expression changes in different studies. One reason is that tumor-adjacent normal skin specimens were not included in many studies as matched controls. Furthermore, while numerous studies of skin stem cells in mouse models have been reported, their relevance to human skin cancer remains unknown. In this report, we analyzed gene expression profiles of paired specimens of keratinocyte carcinomas with their matched normal skin tissues as the control. Among several novel findings, we discovered a significant number of zinc finger encoding genes up-regulated in human BCC. In BCC, a novel link was found between hedgehog signaling, Wnt signaling, and the cilium. While the SCC cancer-stem-cell gene signature is shared between human and mouse SCCs, the hair follicle stem-cell signature of mice was not highly represented in human SCC. Differential gene expression (DEG) in human BCC shares gene signature with both bulge and epidermal stem cells. We have also determined that human BCCs and SCCs have distinct gene expression patterns, and some of them are not fully reflected in current mouse models.

The epigenetic regulator Mll1 is required for Wnt-driven intestinal tumorigenesis and cancer stemness

Wnt/β-catenin signaling is crucial for intestinal carcinogenesis and the maintenance of intestinal cancer stem cells. Here we identify the histone methyltransferase Mll1 as a regulator of Wnt-driven intestinal cancer. Mll1 is highly expressed in Lgr5⁺ stem cells and human colon carcinomas with increased nuclear β-catenin. High levels of MLL1 are associated with poor survival of colon cancer patients. The genetic ablation of Mll1 in mice prevents Wnt/β-catenin-driven adenoma formation from Lgr5⁺ intestinal stem cells. Ablation of Mll1 decreases the self-renewal of human colon cancer spheres and halts tumor growth of xenografts. Mll1 controls the expression of stem cell genes including the Wnt/β-catenin target gene Lgr5. Upon the loss of Mll1, histone methylation at the stem cell promoters switches from activating H3K4 tri-methylation to repressive H3K27 tri-methylation, indicating that Mll1 sustains stem cell gene expression by antagonizing gene silencing through polycomb repressive complex 2 (PRC2)-mediated H3K27 tri-methylation. Transcriptome profiling of Wnt-mutated intestinal tumor-initiating cells reveals that Mll1 regulates Gata4/6 transcription factors, known to sustain cancer stemness and to control goblet cell differentiation. Our results demonstrate that Mll1 is an essential epigenetic regulator of Wnt/β-catenin-induced intestinal tumorigenesis and cancer stemness.

Intestinal cancer stem cells (CSC) are associated with colon cancer. Here, the authors show that Wnt/beta-catenin signalling in CSC requires the epigenetic regulator Mll1 to promote stemness and tumourigenesis in murine and human colon cancer models.

High expression of CD34 and α6-integrin contributes to the cancer-initiating cell behaviour in ultraviolet-induced mouse skin squamous cell carcinoma

Squamous cell carcinoma caused by ultraviolet light exposure represents over 40% of all malignant diseases. It is one of the most commonly found human tumours. Tumour mass within squamous cell carcinoma consists of various cell types, including cancer-initiating cells that are responsible for tumour progression, metastasis and chemoresistance and implicated in clinical relapse. In the present study, we aimed to characterise whether the cell population with high CD34 and α6-integrin expression behave as cancer-initiating cells within ultraviolet-induced squamous cell carcinoma in mouse skin. CD34^highα6-integrin^high compared to CD34^lowα6-integrin^high cells isolated from ultraviolet-induced squamous cell carcinoma could propagate effectively by displaying greater tumour initiating and self-renewal abilities. Our study suggests that CD34^highα6-integrin^high cells act as initiators upon ultraviolet-induced skin squamous cell carcinoma.

LGR6 Promotes Tumor Proliferation and Metastasis through Wnt/β-Catenin Signaling in Triple-Negative Breast Cancer

Leucine-rich-repeat-containing G protein-coupled receptor 6 (LGR6) has been identified as the stem cell marker in multiple normal tissues and malignancies. Previous studies implicated paradoxical functions of LGR6 as a tumor-suppressor gene or oncogene given to the specific context. To explore the exact role of LGR6 in triple-negative breast cancer (TNBC) that never has been studied before, in this study, we assessed LGR6 expression levels by RT-PCR and immunohistochemistry. LGR6 stable expressing/silenced cells were established, and functional assays on tumor proliferation, as well as metastasis, were conducted both in vitro and in vivo. Here, we found that LGR6 was overexpressed in TNBC, which correlated with poor disease-free and overall survivals. Functional assays both in vitro and in vivo showed that LGR6 promotes tumor proliferation and metastasis. LGR6 also increased the ability of tumor spheroid formation. Underlying mechanism exploration further revealed that the oncogenic role of LGR6 might be associated with the Wnt/β-catenin pathway. In conclusion, our findings first proved that LGR6 acts as an oncogene in (TNBC), indicating that LGR6 might be a potential therapeutic target for TNBC treatment.

Altering MYC phosphorylation in the epidermis increases the stem cell population and contributes to the development, progression, and metastasis of squamous cell carcinoma

cMYC (MYC) is a potent oncoprotein that is subject to post-translational modifications that affect its stability and activity. Here, we show that Serine 62 phosphorylation, which increases MYC stability and oncogenic activity, is elevated while Threonine 58 phosphorylation, which targets MYC for degradation, is decreased in squamous cell carcinoma (SCC). The oncogenic role of MYC in the development of SCC is unclear since studies have shown in normal skin that wild-type MYC overexpression can drive loss of stem cells and epidermal differentiation. To investigate whether and how altered MYC phosphorylation might affect SCC development, progression, and metastasis, we generated mice with inducible expression of MYC^WT or MYC^T58A in the basal layer of the skin epidermis. In the T58A mutant, MYC is stabilized with constitutive S62 phosphorylation. When challenged with DMBA/TPA-mediated carcinogenesis, MYC^T58A mice had accelerated development of papillomas, increased conversion to malignant lesions, and increased metastasis as compared to MYC^WT mice. In addition, MYC^T58A-driven SCC displayed stem cell gene expression not observed with MYC^WT, including increased expression of Lgr6, Sox2, and CD34. In support of MYC^T58A enhancing stem cell phenotypes, its expression was associated with an increased number of BrdU long-term label-retaining cells, increased CD34 expression in hair follicles, and increased colony formation from neonatal keratinocytes. Together, these results indicate that altering MYC phosphorylation changes its oncogenic activity—instead of diminishing establishment and/or maintenance of epidermal stem cell populations like wild-type MYC, pS62-MYC enhances these populations and, under carcinogenic conditions, pS62-MYC expression results in aggressive tumor phenotypes.

Integrin α3β1 in hair bulge stem cells modulates CCN2 expression and promotes skin tumorigenesis

Although hair bulge stem cells are not the cancer cells-of-origin, they contribute to two-stage DMBA/TPA skin carcinogenesis in an α3β1-dependent manner.

Epidermal-specific deletion of integrin α3β1 almost completely prevents the formation of papillomas during 7,12-Dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA) two-stage skin carcinogenesis. This dramatic decrease in tumorigenesis was thought to be due to an egress and premature differentiation of α3β1-depleted hair bulge (HB) stem cells (SCs), previously considered to be the cancer cells-of-origin in the DMBA/TPA model. Using a reporter mouse line with inducible deletion of α3β1 in HBs, we show that HB SCs remain confined to their niche regardless of the presence of α3β1 and are largely absent from skin tumors. However, tumor formation was significantly decreased in mice deficient for α3β1 in HB SCs. RNA sequencing of HB SCs isolated from short-term DMBA/TPA–treated skin showed α3β1-dependent expression of the matricellular protein connective tissue growth factor (CCN2), which was confirmed in vitro, where CCN2 promoted colony formation and 3D growth of transformed keratinocytes. Together, these findings show that HBs contribute to skin tumorigenesis in an α3β1-dependent manner and suggest a role of HB SCs in creating a permissive environment for tumor growth through the modulation of CCN2 secretion.

Canine Epithelial Skin Tumours: Expression of the Stem Cell Markers Lgr5, Lgr6 and Sox9 in Light of New Cancer Stem Cell Theories

Evidence is accumulating that tumour development is driven by cancer stem cells (CSCs). In order to understand the presence and potential contribution of stem cells (SCs) as tumour-initiating cells in canine cutaneous tumours, we selected three putative SC markers (Lgr5, Lgr6 and Sox9) and investigated their expression pattern, level of protein and mRNA expression, in 43 canine hair follicle (HF) and 18 canine cutaneous epidermal tumours by immunohistochemistry and qRT-PCR, using normal skin samples as controls. Lgr5 protein expression was not detected in epidermal and HF tumours; however, Lgr5 mRNA overexpression was evident in some HF tumours. Sox9 was expressed in several tumour cases, both at the protein and mRNA level. The Lgr6 antibody tested was not suitable for formalin-fixed paraffin-embedded tissue samples, but Lgr6 gene showed higher expression in several samples of both HF and epidermal tumours compared with normal skin. Significantly higher mRNA expression levels of the three SC markers were found in trichoblastomas (TB) compared with basal cell carcinomas (BCC). The present results indicated that canine HF and epidermal tumours might have common tumour-initiating cells. The mRNA expression of the three selected SC markers, especially Lgr5, could be potentially useful in the distinction between canine TB and BCC.

Regeneration of pulpo-dentinal-like complex by a group of unique multipotent CD24a+ stem cells

Dental pulp is critical to maintain the vitality of a tooth. Regeneration of pulpo-dentinal complex is of great interest to treat pulpitis and pulp necrosis. In this study, through three-dimensional spheroid culture, a group of unique multipotent stem cells were identified from mouse dental papilla called multipotent dental pulp regenerative stem cells (MDPSCs). MDPSCs exhibited enhanced osteogenic/odontogenic differentiation capabilities and could form regenerative dentin and neurovascular-like structures that mimicked the native teeth in vivo. Further analysis revealed that CD24a was the bona fide marker for MDPSCs, and their expansion was highly dependent on the expression of a key transcriptional factor, Sp7. Last, CD24a⁺ cells could be detected in primary dental papilla in mice and human, suggesting that MDPSCs resided in their native niches. Together, our study has identified a previously unidentified group of multipotent pulp regenerative stem cells with defined molecular markers for the potential treatment of pulpitis and pulp necrosis.

Pten loss in Lgr5+ hair follicle stem cells promotes SCC development

Accumulating data support that tissue stem cells give rise to cancer cells. Hair follicle stem cells (HFSCs) undergo cyclic quiescence and activation and may sever as the origin of cutaneous squamous cell carcinoma (SCC). Pten is a tumor suppressor gene that is frequently mutated in hereditary cancer syndromes such as Cowden disease, which is featured with papillomatosis in cutaneous tissues and hyperkeratosis in the acral region of the skin. Additionally, mice with keratinocyte-specific Pten deficiency (k5-Pten^-/- mice) show epidermal hyperplasia and spontaneous tumor formation. However, the impact of Pten mutation in HFSCs, such as in Lgr5⁺ HFSCs, on SCC formation is unclear.

Methods: We established experiments with wildtype and Lgr5-CreER; Pten^flox/flox mice, and used DMBA/TPA two-stage skin carcinogenesis model to explore the effect of Pten loss in Lgr5⁺ HFSCs of 3 weeks old mice in skin carcinogenesis. In vitro experiments (cell culture and protein expression analysis) are employed to investigate molecular mechanisms involved.

Results: Pten loss in Lgr5⁺ HFSCs promoted SCC formation, which was attenuated in TNF^-/- mice. Notably, β-catenin loss in Lgr5⁺ HFSCs decreased the formation of SCC. In addition, Pten loss in cultured epidermal stem cells upregulated the levels of both phospho-Akt and β-catenin.

Conclusion: Pten loss in Lgr5⁺ cells induced Akt/β-catenin signaling, and SCCs can subsequently be raised as progeny from these primed Lgr5⁺ stem cells.

Silencing LGR6 Attenuates Stemness and Chemoresistance via Inhibiting Wnt/β-Catenin Signaling in Ovarian Cancer

Leucine-rich-repeat-containing G protein-coupled receptors (LGRs) have been widely found to be implicated with development and progression in multiple cancer types. However, the clinical significance and biological functions of LGR6 in ovarian cancer remains unclear. In this study, LGR6 expression was mainly examined by immunohistochemistry. Functional assays in vitro and animal experiments in vivo were carried out to explore the effect of LGR6 on cancer stem cell (CSC) characteristics and chemotherapeutic responses in ovarian cancer cells. Luciferase assays and GSEA were used to discern the underlying mechanisms contributing to the roles of LGR6 in ovarian cancer. Here, we reported that LGR6 was upregulated in ovarian cancer, which positively correlated with poor chemotherapeutic response and progression survival in ovarian cancer patients. Loss-of-function assays showed that downregulating LGR6 abrogated the CSC-like phenotype and chemoresistance in vitro. More importantly, silencing LGR6 improved the chemoresistance of ovarian cancer cells to cisplatin in vivo. Mechanistic investigation further revealed that silencing LGR6 inhibited stemness and chemoresistance by repressing Wnt/β-catenin signaling. Collectively, our results uncover a novel mechanism contributing to LGR6-induced chemotherapeutic resistance in ovarian cancer, providing the evidence for LGR6 as a potential therapeutic target in ovarian cancer.

Akermanite bioceramic enhances wound healing with accelerated reepithelialization by promoting proliferation, migration, and stemness of epidermal cells

Reepithelialization is an important step of wound healing, which is mainly completed by proliferation and migration of epidermal cells. Akermanite is a Ca-, Mg-, and Si-containing bioceramic. This study evaluated the effects of Akermanite on wound healing and investigated the mechanisms. Using scald burn mice models, we demonstrated that local Akermanite treatment significantly accelerated wound healing by increasing reepithelialization and the stemness of epidermal cells. Epidermal cells were cultured in medium containing Akermanite extracts to explore the cellular mechanism of reepithelialization. Akermanite promoted the cell proliferation and migration, maintaining more cells in the S and G₂ /M phases of the cell cycle. An additional study showed that Akermanite enhanced the expressions of integrinβ1, Lgr4, Lgr5, and Lgr6, which are specific molecular markers of epidermal stem cells, accompanied by the activation of the Wnt/β-catenin pathway. These results suggested that Akermanite accelerated reepithelialization by increasing the proliferation, migration, and stemness of epidermal cells in a manner related to the Wnt/β-catenin pathway, which might contribute, at least partially, to accelerated wound healing by Akermanite therapy.

R-spondin signaling as a pivotal regulator of tissue development and homeostasis

R-spondins (Rspos) are cysteine-rich secreted glycoproteins which control a variety of cellular functions and are essential for embryonic development and tissue homeostasis. R-spondins (Rspo1 to 4) have high structural similarity and share 60% sequence homology. It has been shown that their cysteine-rich furin-like (FU) domain and the thrombospondin (TSP) type I repeat domain are essential for initiating downstream signaling cascades and therefore for their biological functions. Although numerous studies have unveiled their pivotal role as critical developmental regulators, the most important finding is that Rspos synergize Wnt signaling. Recent studies have identified novel receptors for Rspos, the Lgr receptors, closely related orphans of the leucin-rich repeat containing G protein-coupled receptors, and proposed that Rspos potentiate canonical Wnt signaling via these receptors. Given that Wnt signaling is one of the most important developmental signaling pathways that controls cell fate decisions and tissue development, growth and homeostasis, Rspos may function as key players for these processes as well as potential therapeutic targets. Here, I recapitulate the Wnt signaling and then outline the biological role of Rspos in tissue development and homeostasis and explore the possibility that Rspos may be used as therapeutic targets.

WNT Signaling in Cancer Immunosurveillance

Deregulated WNT signaling has been shown to favor malignant transformation, tumor progression, and resistance to conventional cancer therapy in a variety of preclinical and clinical settings. Accumulating evidence suggests that aberrant WNT signaling may also subvert cancer immunosurveillance, hence promoting immunoevasion and resistance to multiple immunotherapeutics, including immune checkpoint blockers. Here, we discuss the molecular and cellular mechanisms through which WNT signaling influences cancer immunosurveillance and present potential therapeutic avenues to harness currently available WNT modulators for cancer immunotherapy.

The transcriptional coactivator WBP2 primes triple-negative breast cancer cells for responses to Wnt signaling via the JNK/Jun kinase pathway

The transcriptional coactivator WW domain-binding protein 2 (WBP2) is an emerging oncogene and serves as a node between the signaling protein Wnt and other signaling molecules and pathways, including epidermal growth factor receptor, estrogen receptor/progesterone receptor, and the Hippo pathway. The upstream regulation of WBP2 is well-studied, but its downstream activity remains unclear. Here, we elucidated WBP2's role in triple-negative breast cancer (TNBC), in which Wnt signaling is predominantly activated. Using RNAi coupled with RNA-Seq and MS analyses to identify Wnt/WBP2- and WBP2-dependent targets in MDA-MB-231 TNBC cells, we found that WBP2 is required for the expression of a core set of genes in Wnt signaling. These included AXIN2, which was essential for Wnt/WBP2-mediated breast cancer growth and migration. WBP2 also regulated a much larger set of genes and proteins independently of Wnt, revealing that WBP2 primes cells to Wnt activity by up-regulating G protein pathway suppressor 1 (GPS1) and TRAF2- and NCK-interacting kinase (TNIK). GPS1 activated the c-Jun N-terminal kinase (JNK)/Jun pathway, resulting in a positive feedback loop with TNIK that mediated Wnt-induced AXIN2 expression. WBP2 promoted TNBC growth by integrating JNK with Wnt signaling, and its expression profoundly influenced the sensitivity of TNBC to JNK/TNIK inhibitors. In conclusion, WBP2 links JNK to Wnt signaling in TNBC. GPS1 and TNIK are constituents of a WBP2-initiated cascade that primes responses to Wnt ligands and are also important for TNBC biology. We propose that WBP2 is a potential drug target for JNK/TNIK-based precision medicine for managing TNBC.

Down-regulation of LGR6 promotes bone fracture recovery using bone marrow stromal cells

OBJECTIVE

The Leucine-rich repeat-containing G-protein coupled receptor 6 (LGR6) is a well-known marker of stem cells. In present study, we aimed to further explore the effects of LGR6 on promoting osteogenic differentiation of bone marrow stromal cells (BMSCs) and bone healing.

METHODS

Flow cytometry assay was used to determine the expression of BMSCs surface markers, and western blot was performed to detect the LGR6 protein expression. The osteogenic differentiation of BMSCs was qualified using ALP and ARS staining. Protein expression of osteogenic genes (ALP, Collagen I, Runx2 and OCN) were evaluated using western blot. In vivo, BMSCs transfected with sh-LGR6 or LGR6 cDNA were injected into the fracture site to establish rat fracture healing model. X-ray system and hematoxylin-eosin (HE) staining were conducted to observe the fracture recovery. Biomechanical test was performed to detect the changes of maximum load, elastic modules and bone mineral density.

RESULTS

In BMSCS, CD90 and CD44 were positively expressed, while CD11b was negatively expressed. Expression level of LGR6 gradually decreased with the osteogenic differentiation of BMSCs. The osteogenic genes expression level during the osteogenic differentiation significantly increased with the down-regulation of LGR6. In vivo, 8 weeks after injection, rats treated with LGR6 knocked-down BMSCs showed increased number of fibroblasts. Maximum load, elastic modulus and the bone mineral density were enhanced with the knocking-down of LGR6.

CONCLUSION

Inhibition of LGR6 promoted the osteogenic differentiation of BMSCs in vitro. Moreover, transplantation of LGR6-knockout BMSCs in rat models contributes to a better recovery after the fracture.

Cancer: the dark side of wound healing

Complex multicellular organisms have evolved sophisticated mechanisms to rapidly resolve epithelial injuries. Epithelial integrity is critical to maintaining internal homeostasis. An epithelial breach represents the potential for pathogen ingress and fluid loss, both of which may have severe consequences if not limited. The mammalian wound healing response involves a finely tuned, self-limiting series of cellular and molecular events orchestrated by the transient activation of specific signalling pathways. Accurate regulation of these events is essential; failure to initiate key steps at the right time delays healing and leads to chronic wounds, while aberrant initiation of wound healing processes may produce cell behaviours that promote cancer progression. In this review, we discuss how wound healing pathways co-opted in cancer lose their stringent regulation and become compromised in their reversibility. We hypothesize on how the commandeering of wound healing 'master regulators' is involved in this process, and also highlight the implications of these findings in the treatment of both chronic wounds and cancer.

Pathogenesis of Skin Carcinomas and a Stem Cell as Focal Origin

UV radiation in sunlight has long been recognized as the main exogenous cause of skin carcinomas. We present a brief historical perspective on the progress in understanding the pathogenesis of skin carcinomas, and recent advances. Sun-exposed skin carries numerous UV-related mutations, and skin carcinomas rank among the tumors with the highest mutational loads. In this multitude of mutations only a few are crucial in driving the tumor. Some are known from hereditary (skin) cancer syndromes and other recurrent ones have been validated in transgenic mice. Considering the continuous renewal of the epidermis, the question arises whether the lifelong residing stem cells are the main targets in skin carcinogenesis, a multistep process that would require ample time to evolve. Therefore, classic quiescent stem cells have been studied as potential tumor-initiating cells, as well as more recently discovered actively dividing stem cells (either Lgr5+ or Lgr6+). Interesting differences have emerged between experimental UV and two-stage chemical carcinogenesis, e.g., the latter appears to originate from follicular stem cells, in contrast to the former.