Lgr5-expressing cells are sufficient and necessary for postnatal mammary gland organogenesis

CXCR4+ mammary gland macrophageal niche promotes tumor initiating cell activity and immune suppression during tumorigenesis

Tumor-initiating cells (TICs) share features and regulatory pathways with normal stem cells, yet how the stem cell niche contributes to tumorigenesis remains unclear. Here, we identify CXCR4+ macrophages as a niche population enriched in normal mammary ducts, where they promote the regenerative activity of basal cells in response to luminal cell-derived CXCL12. CXCL12 triggers AKT-mediated stabilization of β-catenin, which induces Wnt ligands and pro-migratory genes, enabling intraductal macrophage infiltration and supporting regenerative activity of basal cells. Notably, these same CXCR4+ niche macrophages regulate the tumor-initiating activity of various breast cancer subtypes by enhancing TIC survival and tumor-forming capacity, while promoting early immune evasion through regulatory T cell induction. Furthermore, a CXCR4+ niche macrophage gene signature correlates with poor prognosis in human breast cancer. These findings highlight the pivotal role of the CXCL12-CXCR4 axis in orchestrating interactions between niche macrophages, mammary epithelial cells, and immune cells, thereby establishing a supportive niche for both normal tissue regeneration and mammary tumor initiation.

LGR5 interacts with HSP90AB1 to mediate enzalutamide resistance by activating the WNT/β-catenin/AR axis in prostate cancer

BACKGROUND

Enzalutamide, a second-generation androgen receptor (AR) pathway inhibitor, is widely used in the treatment of castration-resistant prostate cancer. However, after a period of enzalutamide treatment, patients inevitably develop drug resistance. In this study, we characterized leucine-rich repeated G-protein-coupled receptor 5 (LGR5) and explored its potential therapeutic value in prostate cancer.

METHODS

LGR5 was screened by sequencing data of enzalutamide-resistant cell lines combined with sequencing data of lesions with different Gleason scores from the same patients. The biological function of LGR5 and its effect on enzalutamide resistance were investigated in vitro and in vivo. Glutathione-S-transferase (GST) pull-down, coimmunoprecipitation, Western blotting, and immunofluorescence assays were used to explore the specific binding mechanism of LGR5 and related pathway changes.

RESULTS

LGR5 was significantly upregulated in prostate cancer and negatively correlated with poor patient prognosis. Overexpression of LGR5 promoted the malignant progression of prostate cancer and reduced sensitivity to enzalutamide in vitro and in vivo. LGR5 promoted the phosphorylation of glycogen synthase kinase-3β (GSK-3β) by binding heat shock protein 90,000 alpha B1 (HSP90AB1) and mediated the activation of the Wingless/integrated (WNT)/β-catenin signaling pathway. The increased β-catenin in the cytoplasm entered the nucleus and bound to the nuclear AR, promoting the transcription level of AR, which led to the enhanced tolerance of prostate cancer to enzalutamide. Reducing HSP90AB1 binding to LGR5 significantly enhanced sensitivity to enzalutamide.

CONCLUSIONS

LGR5 directly binds to HSP90AB1 and mediates GSK-3β phosphorylation, promoting AR expression by regulating the WNT/β-catenin signaling pathway, thereby conferring resistance to enzalutamide treatment in prostate cancer.

Mammary stem cells: molecular cues, orchestrated regulatory mechanisms and its implications in breast cancer

Mammary stem cells (MaSCs), endowed with self-renewal and multilineage differentiation capabilities, are crucial for mammary gland development, function, and disease initiation. Recent advances in MaSCs biology research encompass molecular marker identification, regulatory pathway dissection, and microenvironmental crosstalk. This review synthesizes key progress and remaining challenges in MaSC research. Molecular profiling advances have identified key markers recently, such as Procr, Dll1, Bcl11b, and PD-L1. Central to their regulatory logic are evolutionarily conserved pathways, including Wnt, Notch, Hedgehog, and Hippo, which exhibit context-dependent thresholds to balance self-renewal and differentiation. Beyond intrinsic signaling, the dynamic interplay between MaSCs and their microenvironment, such as luminal-derived Wnt4, macrophage-mediated TNF-α signaling, and adrenergic inputs from sympathetic nerves, spatially orchestrates stem cell behavior. In addition, this review also discusses the roles of breast cancer stem cells (BCSCs) in tumorigenesis and therapeutic resistance, focusing on the molecular mechanisms underlying MaSC transformation into BCSCs. Despite progress, challenges remain: human MaSCs functional assays lack standardization, pathway inhibitors risk off-target effects, and delivery systems lack precision. Emerging tools like spatial multi-omics, organoids, and biomimetic scaffolds address these gaps. By integrating MaSCs and BCSCs biology, this review links mechanisms to breast cancer and outlines strategies to target malignancy to accelerate clinical translation.

20 years of stemness: From stem cells to hypertranscription and back

Transcriptional profiling of stem cells came of age at the beginning of the century with the use of microarrays to analyze cell populations in bulk. Since then, stem cell transcriptomics has become increasingly sophisticated, notably with the recent widespread use of single-cell RNA sequencing. Here, we provide a perspective on how an early signature of genes upregulated in embryonic and adult stem cells, identified using microarrays over 20 years ago, serendipitously led to the recent discovery that stem/progenitor cells across organs are in a state of hypertranscription, a global elevation of the transcriptome. Looking back, we find that the 2002 stemness signature is a robust marker of stem cell hypertranscription, even though it was developed well before it was known what hypertranscription meant or how to detect it. We anticipate that studies of stem cell hypertranscription will be rich in novel insights in physiological and disease contexts for years to come.

Functional and Biological Characterization of the LGR5Δ5 Splice Variant in HEK293T Cells

The regulator of the canonical Wnt pathway, leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5), is expressed in the stem cell compartment of several tissues and overexpressed in different human carcinomas. The isoform of the stem cell marker LGR5, named LGR5Δ5 and first described by our group, is associated with prognosis and metastasis in oral squamous cell carcinoma (OSCC) and soft tissue sarcoma (STS). In a proof-of-principle analysis, the function of LGR5Δ5 was investigated in HEK293T cells, a model cell line of the Wnt pathway, compared to full-length LGR5 (FL) expression. The CRISPR/CAS knockout of LGR5 and LGR4 (thereby avoiding the side effects of LGR4) resulted in a loss of Wnt activity that cannot be restored by LGR5Δ5 but by LGR5FL rescue. The ability to migrate was not affected by LGR5Δ5, but was reduced by LGR5FL overexpression. The CRISPR/CAS of LGR4 and 5 induced radiosensitization, which was enhanced by the overexpression of LGR5FL or LGR5Δ5. RNA sequencing analysis revealed a significant increase in the ligand R-spondin 1 (RSPO1) level by LGR5Δ5. Furthermore, LGR5Δ5 appears to be involved in the regulation of genes related to the cytoskeleton, extracellular matrix stiffness, and angiogenesis, while LGR5FL is associated with the regulation of collagens and histone proteins.

Modeling bladder cancer in the laboratory: Insights from patient-derived organoids

Bladder cancer (BCa) is the most common malignant tumor of the urinary system. Current treatments often have poor efficacy and carry a high risk of recurrence and progression due to the lack of consideration of tumor heterogeneity. Patient-derived organoids (PDOs) are three-dimensional tissue cultures that preserve tumor heterogeneity and clinical relevance better than cancer cell lines. Moreover, PDOs are more cost-effective and efficient to cultivate compared to patient-derived tumor xenografts, while closely mirroring the tissue and genetic characteristics of their source tissues. The development of PDOs involves critical steps such as sample selection and processing, culture medium optimization, matrix selection, and improvements in culture methods. This review summarizes the methodologies for generating PDOs from patients with BCa and discusses the current advancements in drug sensitivity testing, immunotherapy, living biobanks, drug screening, and mechanistic studies, highlighting their role in advancing personalized medicine.

The screening of α-glucosidase inhibitory peptides from β-conglycinin and hypoglycemic mechanism in HepG2 cells and zebrafish larvae

Inhibition of carbohydrate digestive enzymes is a key focus across diverse fields, given the prominence of α-glucosidase inhibitors as preferred oral hypoglycaemic drugs for diabetes treatment. β-conglycinin is the most abundant functional protein in soy; however, it is unclear whether the peptides produced after its gastrointestinal digestion exhibit α-glucosidase inhibitory properties. Therefore, we examined the α-glucosidase inhibitory potential of soy peptides. Specifically, β-conglycinin was subjected to simulated gastrointestinal digestion by enzymatically cleaving it into 95 peptides with gastric, pancreatic and chymotrypsin enzymes. Eight soybean peptides were selected based on their predicted activity; absorption, distribution, metabolism, excretion and toxicity score; and molecular docking analysis. The results indicated that hydrogen bonding and electrostatic interactions play important roles in inhibiting α-glucosidase, with the tripeptide SGR exhibiting the greatest inhibitory effect (IC50 = 10.57 μg/mL). In vitro studies revealed that SGR markedly improved glucose metabolism disorders in insulin-resistant HepG2 cells without affecting cell viability. Animal experiments revealed that SGR significantly improved blood glucose and decreased maltase activity in type 2 diabetic zebrafish larvae, but it did not result in the death of zebrafish larvae. Transcriptomic analysis revealed that SGR exerts its anti-diabetic and hypoglycaemic effects by attenuating the expression of several genes, including Slc2a1, Hsp70, Cpt2, Serpinf1, Sfrp2 and Ggt1a. These results suggest that SGR is a potential food-borne bioactive peptide for managing diabetes.

Niche inflammatory signals control oscillating mammary regeneration and protect stem cells from cytotoxic stress

Stem cells are known for their resilience and enhanced activity post-stress. The mammary gland undergoes frequent remodeling and is subjected to recurring stress during the estrus cycle, but it remains unclear how mammary stem cells (MaSCs) respond to the stress and contribute to regeneration. We discovered that cytotoxic stress-induced activation of CD11c+ ductal macrophages aids stem cell survival and prevents differentiation. These macrophages boost Procr+ MaSC activity through IL1β-IL1R1-NF-κB signaling during the estrus cycle in an oscillating manner. Deleting IL1R1 in MaSCs results in stem cell loss and skewed luminal differentiation. Moreover, under cytotoxic stress from the chemotherapy agent paclitaxel, ductal macrophages secrete higher IL1β levels, promoting MaSC survival and preventing differentiation. Inhibiting IL1R1 sensitizes MaSCs to paclitaxel. Our findings reveal a recurring inflammatory process that regulates regeneration, providing insights into stress-induced inflammation and its impact on stem cell survival, potentially affecting cancer therapy efficacy.

Long-term maintenance of human endometrial epithelial organoids and their stem cell properties

The endometrium undergoes dynamic changes throughout the menstrual cycle and pregnancy, which is unique to primates. Endometrium remodeling is essential for the implantation and nutritional support of the conceptus. Despite this, the role of uterine glands in driving endometrial tissue remodeling is still poorly understood. To address this, a 3-dimensional culture system was used to generate endometrial epithelial organoids from human endometrium biopsies. These organoids are genetically stable, long-term expandability. They reproduce some functions of uterine glands in vivo. The epithelial organoids exhibit characteristics of stem cells, with the proportion of stem cells increasing with culture time and passage number. Long-term maintenance of organoids strongly expressed stemness related genes accompanied by a decrease expression in mature epithelial gene, which suggests the organoids had switched from a mature stage to a progenitor stage. Thus we proposed the possible markers for epithelial progenitors. Meanwhile, long-term cultured organoids exhibit an increase in the proportion of luminal epithelial stem cells, accompanied by a decrease of glandular epithelial stem cells. Organoids also show hormone responsiveness, reflecting the various stages of the menstrual cycle and early pregnancy.

A genomic perspective of the aging human and mouse lung with a focus on immune response and cellular senescence

BACKGROUND

The aging lung is a complex process and influenced by various stressors, especially airborne pathogens and xenobiotics. Additionally, a lifetime exposure to antigens results in structural and functional changes of the lung; yet an understanding of the cell type specific responses remains elusive. To gain insight into age-related changes in lung function and inflammaging, we evaluated 89 mouse and 414 individual human lung genomic data sets with a focus on genes mechanistically linked to extracellular matrix (ECM), cellular senescence, immune response and pulmonary surfactant, and we interrogated single cell RNAseq data to fingerprint cell type specific changes.

RESULTS

We identified 117 and 68 mouse and human genes linked to ECM remodeling which accounted for 46% and 27%, respectively of all ECM coding genes. Furthermore, we identified 73 and 31 mouse and human genes linked to cellular senescence, and the majority code for the senescence associated secretory phenotype. These cytokines, chemokines and growth factors are primarily secreted by macrophages and fibroblasts. Single-cell RNAseq data confirmed age-related induced expression of marker genes of macrophages, neutrophil, eosinophil, dendritic, NK-, CD4+, CD8+-T and B cells in the lung of aged mice. This included the highly significant regulation of 20 genes coding for the CD3-T-cell receptor complex. Conversely, for the human lung we primarily observed macrophage and CD4+ and CD8+ marker genes as changed with age. Additionally, we noted an age-related induced expression of marker genes for mouse basal, ciliated, club and goblet cells, while for the human lung, fibroblasts and myofibroblasts marker genes increased with age. Therefore, we infer a change in cellular activity of these cell types with age. Furthermore, we identified predominantly repressed expression of surfactant coding genes, especially the surfactant transporter Abca3, thus highlighting remodeling of surfactant lipids with implications for the production of inflammatory lipids and immune response.

CONCLUSION

We report the genomic landscape of the aging lung and provide a rationale for its growing stiffness and age-related inflammation. By comparing the mouse and human pulmonary genome, we identified important differences between the two species and highlight the complex interplay of inflammaging, senescence and the link to ECM remodeling in healthy but aged individuals.

Genetic heterogeneity of liver cancer stem cells

Cancer cell heterogeneity is a serious problem in the control of tumor progression because it can cause chemoresistance and metastasis. Heterogeneity can be generated by various mechanisms, including genetic evolution of cancer cells, cancer stem cells (CSCs), and niche heterogeneity. Because the genetic heterogeneity of CSCs has been poorly characterized, the genetic mutation status of CSCs was examined using Exome-Seq and RNA-Seq data of liver cancer. Here we show that different surface markers for liver cancer stem cells (LCSCs) showed a unique propensity for genetic mutations. Cluster of differentiation 133 (CD133)-positive cells showed frequent mutations in the IRF2, BAP1, and ERBB3 genes. However, leucine-rich repeat-containing G protein-coupled receptor 5-positive cells showed frequent mutations in the CTNNB1, RELN, and ROBO1 genes. In addition, some genetic mutations were frequently observed irrespective of the surface markers for LCSCs. BAP1 mutations was frequently observed in CD133-, CD24-, CD13-, CD90-, epithelial cell adhesion molecule-, or keratin 19-positive LCSCs. ASXL2, ERBB3, IRF2, TLX3, CPS1, and NFATC2 mutations were observed in more than three types of LCSCs, suggesting that common mechanisms for the development of these LCSCs. The present study provides genetic heterogeneity depending on the surface markers for LCSCs. The genetic heterogeneity of LCSCs should be considered in the development of LCSC-targeting therapeutics.

Wnt signaling from Gli1-expressing apical stem/progenitor cells is essential for the coordination of tooth root development

Stem cell regulation plays a crucial role during development and homeostasis. Here, an essential source of Wnts from Gli1⁺ stem/progenitor cells was identified in the murine molar. Loss of Wnt production in Gli1⁺ apical stem/progenitor cells led to loss of Axin2 at the root apex, mis-regulation of SOX9, loss of BMP and Hh signaling, and truncation of root development. In the absence of Wnt signals, the root epithelium lost its integrity and epithelial identity. This phenotype could be partially mimicked by loss of Sox9 in the Gli1 population. Stabilization of Wnt signaling in the apical papilla led to rapid unordered differentiation of hard tissues and fragmentation of the epithelial root sheath. Wnt signaling from Gli1⁺ stem/progenitor cells, therefore, orchestrates root development, coordinating mesenchymal and epithelial interactions via SOX9 to regulate stem/progenitor cell expansion and differentiation. Our results demonstrate that disparate stem/progenitor cell populations are unified in their fundamental signaling interactions.

A Guide Toward Multi-scale and Quantitative Branching Analysis in the Mammary Gland

The mammary gland consists of a bilayered epithelial structure with an extensively branched morphology. The majority of this epithelial tree is laid down during puberty, during which actively proliferating terminal end buds repeatedly elongate and bifurcate to form the basic structure of the ductal tree. Mammary ducts consist of a basal and luminal cell layer with a multitude of identified sub-lineages within both layers. The understanding of how these different cell lineages are cooperatively driving branching morphogenesis is a problem of crossing multiple scales, as this requires information on the macroscopic branched structure of the gland, as well as data on single-cell dynamics driving the morphogenic program. Here we describe a method to combine genetic lineage tracing with whole-gland branching analysis. Quantitative data on the global organ structure can be used to derive a model for mammary gland branching morphogenesis and provide a backbone on which the dynamics of individual cell lineages can be simulated and compared to lineage-tracing approaches. Eventually, these quantitative models and experiments allow to understand the couplings between the macroscopic shape of the mammary gland and the underlying single-cell dynamics driving branching morphogenesis.

Gonadal Sex Differentiation and Ovarian Organogenesis along the Cortical-Medullary Axis in Mammals

In most mammals, the sex of the gonads is based on the fate of the supporting cell lineages, which arises from the proliferation of coelomic epithelium (CE) that surfaces on the bipotential genital ridge in both XY and XX embryos. Recent genetic studies and single-cell transcriptome analyses in mice have revealed the cellular and molecular events in the two-wave proliferation of the CE that produce the supporting cells. This proliferation contributes to the formation of the primary sex cords in the medullary region of both the testis and the ovary at the early phase of gonadal sex differentiation, as well as to that of the secondary sex cords in the cortical region of the ovary at the perinatal stage. To support gametogenesis, the testis forms seminiferous tubules in the medullary region, whereas the ovary forms follicles mainly in the cortical region. The medullary region in the ovary exhibits morphological and functional diversity among mammalian species that ranges from ovary-like to testis-like characteristics. This review focuses on the mechanism of gonadal sex differentiation along the cortical-medullary axis and compares the features of the cortical and medullary regions of the ovary in mammalian species.

Clone wars: From molecules to cell competition in intestinal stem cell homeostasis and disease

The small intestine is among the fastest self-renewing tissues in adult mammals. This rapid turnover is fueled by the intestinal stem cells residing in the intestinal crypt. Wnt signaling plays a pivotal role in regulating intestinal stem cell renewal and differentiation, and the dysregulation of this pathway leads to cancer formation. Several studies demonstrate that intestinal stem cells follow neutral drift dynamics, as they divide symmetrically to generate other equipotent stem cells. Competition for niche space and extrinsic signals in the intestinal crypt is the governing mechanism that regulates stemness versus cell differentiation, but the underlying molecular mechanisms are still poorly understood, and it is not yet clear how this process changes during disease. In this review, we highlight the mechanisms that regulate stem cell homeostasis in the small intestine, focusing on Wnt signaling and its regulation by RNF43 and ZNRF3, key inhibitors of the Wnt pathway. Furthermore, we summarize the evidence supporting the current model of intestinal stem cell regulation, highlighting the principles of neutral drift at the basis of intestinal stem cell homeostasis. Finally, we discuss recent studies showing how cancer cells bypass this mechanism to gain a competitive advantage against neighboring normal cells.

Stem cells in the gut rapidly renew themselves through processes that cancer cells co-opt to trigger tumor development. Gabriele Colozza from the Institute of Molecular Biotechnology in Vienna, Austria, and colleagues review how a network of critical molecular signals and competition for limited space help to regulate the dynamics of stem cells in the intestines. The correct balance between self-renewal and differentiation is tightly controlled by the so-called Wnt signaling pathway and its inhibitors. Competition between dividing cells in the intestinal crypts, the locations between finger-like protrusions in the gut where stem cells are found, provides another protective mechanism against runaway stem cell growth. However, intestinal cancer cells, thanks to their activating mutations, bypass these safeguards to gain a survival advantage. Drugs that target these ‘super-competitive’ behaviors could therefore help combat tumor proliferation.

The role of the Wnt signalling pathway in the energy metabolism of bone remodelling

OBJECTIVES

Bone remodelling is necessary to repair old and impaired bone caused by aging and its effects. Injury in the process of bone remodelling generally leads to the development of various bone diseases. Energy metabolism plays crucial roles in bone cell formation and function, the disorder of which will disrupt the balance between bone formation and bone resorption.

MATERIALS AND METHODS

Here, we review the intrinsic interactions between bone remodelling and energy metabolism and the role of the Wnt signalling pathway.

RESULTS

We found a close interplay between metabolic pathways and bone homeostasis, demonstrating that bone plays an important role in the regulation of energy balance. We also discovered that Wnt signalling is associated with multiple biological processes regulating energy metabolism in bone cells.

CONCLUSIONS

Thus, targeted regulation of Wnt signalling and the recovery of the energy metabolism function of bone cells are key means for the treatment of metabolic bone diseases.

Differential epithelial and stromal LGR5 expression in ovarian carcinogenesis

Lgr5 has been identified as a marker of the stem/progenitor cells in the murine ovary and oviduct by lineage tracing. However, little is known regarding LGR5 expression or its functional significance in human ovary tissues. Here, using RNA in situ hybridization and/or immunohistochemistry, we thoroughly investigated LGR5 expression in normal human ovaries, fallopian tubes and various ovarian tumors. We discovered that LGR5 expression is negligible in the human ovary surface epithelium, whereas ovarian stromal cells normally express low levels of LGR5. Remarkably, fallopian tube epithelium, inclusion cysts and serous cystadenomas with a Müllerian phenotype expressed high levels of LGR5, and LGR5 expression was restricted to PAX8⁺/FOXJ1^- secretory cells of the tubal epithelium. Strong stromal LGR5 expression without epithelial LGR5 expression was consistently observed in the path from serous cystadenoma to serous borderline tumor to low grade serous carcinoma (LGSC). Unlike LGSC, high grade serous carcinoma (HGSC), clear cell carcinoma, endometrioid carcinomas displayed various epithelial-stromal LGR5 expression. Notably, high levels of LGR5 expression were observed in serous tubal intraepithelial carcinoma, which slightly declined in invasive HGSC. LGR5 expression was significantly associated with improved progression-free survival in HGSC patients. Moreover, in vitro assays demonstrated that LGR5 expression suppressed tumor proliferation and migratory capabilities. Taken together, these findings indicate a tumor-suppressive role for LGR5 in the progression of HGSC.

SLUG is a key regulator of epithelial-mesenchymal transition in pleomorphic adenoma

The histogenesis of pleomorphic adenoma (PA) of the salivary glands remains controversial. PAs are characterized by the transition of epithelial cells to spindled mesenchymal cells, known as epithelial-mesenchymal transition (EMT). The present study aimed to identify a major EMT-inducing transcription factor (EMT-TF) in PAs. Real-time PCR analysis of SNAIL, SLUG, ZEB1, and TWIST1 demonstrated that only SLUG was significantly upregulated in normal salivary glands and PAs. Combined in situ hybridization for SLUG and multiplex immunohistochemistry for CK19 and P63 revealed that SLUG was specifically expressed in the myoepithelial cells of normal salivary glands. In PAs, SLUG was expressed in neoplastic myoepithelial cells and stromal cells but not in the luminal cells lining the inner layers of tumor glands. SLUG expression showed no correlation with PLAG1 expression, and in vitro experiments demonstrated that PLAG1 suppression in primary cultured PA cells or PLAG1 overexpression in HEK 293 T cells did not affect SLUG levels, indicating that PLAG1 was not involved in the upregulation of SLUG in PAs. The suppression of SLUG expression in cultured PA cells resulted in a morphology change to a less elongated shape and attenuated tumor growth. In addition, SLUG downregulation led to increased E-cadherin and decreased N-cadherin and vimentin expression levels along with decreased migratory activity in cultured PA cells. These findings suggest that SLUG is a major TF that can induce EMT in PAs. In summary, SLUG is specifically and highly expressed in the myoepithelial cells and stromal cells of PAs and is a key regulator of EMT in PAs.

Procr functions as a signaling receptor and is essential for the maintenance and self-renewal of mammary stem cells

The protein C receptor (Procr) has been implicated as a stem cell surface marker in several tissues. It is unknown whether Procr acts as a functional signaling receptor in stem cells. Here, by conditional knockout in mammary stem cells (MaSCs), we demonstrate that Procr is essential for mammary gland development and homeostasis. Through proteomics profiling, we identify that, upon stimulation by the ligand protein C, Procr interacts with heat shock protein 90 (HSP90AA1) via its short cytoplasmic tail, recruiting Src and IGF1R to the complex at the plasma membrane. We show that Procr acts as a signaling receptor of protein C in regulation of MaSCs through HSP90, Src, and IGF1R in vitro. In vivo, IGF1R deletion in MaSCs displays similar phenotypes to Procr deletion. These findings illustrate the essential role of Procr signaling in MaSC maintenance, shedding light onto the molecular regulation by Procr in tissue stem cells.

Gpr125 is a unifying hallmark of multiple mammary progenitors coupled to tumor latency

Gpr125 is an orphan G-protein coupled receptor, with homology to cell adhesion and axonal guidance factors, that is implicated in planar polarity and control of cell movements. By lineage tracing we demonstrate that Gpr125 is a highly specific marker of bipotent mammary stem cells in the embryo and of multiple long-lived unipotent basal mammary progenitors in perinatal and postnatal glands. Nipple-proximal Gpr125+ cells express a transcriptomic profile indicative of chemo-repulsion and cell movement, whereas Gpr125+ cells concentrated at invasive ductal tips display a hybrid epithelial-mesenchymal phenotype and are equipped to bind chemokine and growth factors and secrete a promigratory matrix. Gpr125 progenitors acquire bipotency in the context of transplantation and cancer and are greatly expanded and massed at the pushing margins of short latency MMTV-Wnt1 tumors. High Gpr125 expression identifies patients with particularly poor outcome within the basal breast cancer subtype highlighting its potential utility as a factor to stratify risk.

Gpr125 has emerged as a specific marker of mammary stem cells and basal progenitors. Here they show that Gpr125 cells congregate at ductal tips during morphogenesis and amass at tumor margins, and that high Gpr125 predicts early tumor onset and poor outcome in basal breast cancer.

The Wnt Pathway: From Signaling Mechanisms to Synthetic Modulators

The Wnt pathway is central to a host of developmental and disease-related processes. The remarkable conservation of this intercellular signaling cascade throughout metazoan lineages indicates that it coevolved with multicellularity to regulate the generation and spatial arrangement of distinct cell types. By regulating cell fate specification, mitotic activity, and cell polarity, Wnt signaling orchestrates development and tissue homeostasis, and its dysregulation is implicated in developmental defects, cancer, and degenerative disorders. We review advances in our understanding of this key pathway, from Wnt protein production and secretion to relay of the signal in the cytoplasm of the receiving cell. We discuss the evolutionary history of this pathway as well as endogenous and synthetic modulators of its activity. Finally, we highlight remaining gaps in our knowledge of Wnt signal transduction and avenues for future research. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Runx1-enhancer element eR1 identified lineage restricted mammary luminal stem cells

Mammary gland homeostasis is maintained by adult tissue stem-progenitor cells residing within the luminal and basal epithelia. Dysregulation of mammary stem cells is a key mechanism for cancer development. However, stem cell characterization is challenging because reporter models using cell-specific promoters do not fully recapitulate the mammary stem cell populations. We previously found that a 270-basepair Runx1 enhancer element, named eR1, marked stem cells in the blood and stomach. Here, we identified eR1 activity in a rare subpopulation of the ERα-negative luminal epithelium in mouse mammary glands. Lineage-tracing using an eR1-CreERT2 mouse model revealed that eR1+ luminal cells generated the entire luminal lineage and milk-secreting alveoli – eR1 therefore specifically marks lineage-restricted luminal stem cells. eR1-targeted-conditional knockout of Runx1 led to the expansion of luminal epithelial cells, accompanied by elevated ERα expression. Our findings demonstrate a definitive role for Runx1 in the regulation of the eR1-positive luminal stem cell proliferation during mammary homeostasis. Our findings identify a mechanistic link for Runx1 in stem cell proliferation and its dysregulation in breast cancer. Runx1 inactivation is therefore likely to be an early hit in the cell-of-origin of ERα+ luminal type breast cancer.

Coordinate control of basal epithelial cell fate and stem cell maintenance by core EMT transcription factor Zeb1

Maintenance of undifferentiated, long-lived, and often quiescent stem cells in the basal compartment is important for homeostasis and regeneration of multiple epithelial tissues, but the molecular mechanisms that coordinately control basal cell fate and stem cell quiescence are elusive. Here, we report an epithelium-intrinsic requirement for Zeb1, a core transcriptional inducer of epithelial-to-mesenchymal transition, for mammary epithelial ductal side branching and for basal cell regenerative capacity. Our findings uncover an evolutionarily conserved role of Zeb1 in promoting basal cell fate over luminal differentiation. We show that Zeb1 loss results in increased basal cell proliferation at the expense of quiescence and self-renewal. Moreover, Zeb1 cooperates with YAP to activate Axin2 expression, and inhibition of Wnt signaling partially restores stem cell function to Zeb1-deficient basal cells. Thus, Zeb1 is a transcriptional regulator that maintains both basal cell fate and stem cell quiescence, and it functions in part through suppressing Wnt signaling.

The role of R-spondin proteins in cancer biology

R-spondin (RSPO) proteins constitute a family of four secreted glycoproteins (RSPO1-4) that have appeared as multipotent signaling ligands. The best-known molecular function of RSPOs lie within their capacity to agonize the Wnt/β-catenin signaling pathway. As RSPOs act upon cognate receptors LGR4/5/6 that are typically expressed by stem cells and progenitor cells, RSPO proteins importantly potentiate Wnt/β-catenin signaling especially within these proliferative stem cell compartments. Since multiple organs express LGR4/5/6 receptors and RSPO ligands within their stem cell niches, RSPOs can exert an influential role in stem cell regulation throughout the body. Inherently, over the last decade a multitude of reports implicated the deregulation of RSPOs in cancer development. First, RSPO2 and RSPO3 gene fusions with concomitant enhanced expression have been identified in colon cancer patients, and proposed as an alternative driver of Wnt/β-catenin hyperactivation that earmarks cancer in the colorectal tract. Moreover, the causal oncogenic capacity of RSPO3 overactivation has been demonstrated in the mouse intestine. As a paradigm organ in this field, most of current knowledge about RSPOs in cancer is derived from studies in the intestinal tract. However, RSPO gene fusions as well as enhanced RSPO expression have been reported in multiple additional cancer types, affecting different organs that involve divergent stem cell hierarchies. Importantly, the emerging oncogenic role of RSPO and its potential clinical utility as a therapeutic target have been recognized and investigated in preclinical and clinical settings. This review provides a survey of current knowledge on the role of RSPOs in cancer biology, addressing the different organs implicated, and of efforts made to explore intervention opportunities in cancer cases with RSPO overrepresentation, including the potential utilization of RSPO as novel therapeutic target itself.

Dual recombinase action in the normal and neoplastic mammary gland epithelium

We developed a transgenic mouse line that expresses the codon-optimized Flp recombinase under the control of the MMTV promoter in luminal epithelial cells of the mammary gland. In this report, we demonstrate the versatile applicability of the new MMTV-Flp strain to manipulate genes in a temporally and spatially controlled manner in the normal mammary gland, in luminal-type mammary tumors that overexpress ERBB2, and in a new KRAS-associated mammary cancer model. Although the MMTV-Flp is expressed in a mosaic pattern in the luminal epithelium, the Flp-mediated activation of a mutant Kras^G12D allele resulted in basal-like mammary tumors that progressively acquired mesenchymal features. Besides its applicability as a tool for gene activation and cell lineage tracing to validate the cellular origin of primary and metastatic tumor cells, we employed the MMTV-Flp transgene together with the tamoxifen-inducible Cre recombinase to demonstrate that the combinatorial action of both recombinases can be used to delete or to activate genes in established tumors. In a proof-of-principle experiment, we conditionally deleted the JAK1 tyrosine kinase in KRAS-transformed mammary cancer cells using the dual recombinase approach and found that lack of JAK1 was sufficient to block the constitutive activation of STAT3. The collective results from the various lines of investigation showed that it is, in principle, feasible to manipulate genes in a ligand-controlled manner in neoplastic mammary epithelial cells, even when cancer cells acquire a state of cellular plasticity that may no longer support the expression of the MMTV-Flp transgene.

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.

Discovery of an Orally Bioavailable Small-Molecule Inhibitor for the β-Catenin/B-Cell Lymphoma 9 Protein-Protein Interaction

Aberrant activation of Wnt/β-catenin signaling is strongly associated with many diseases including cancer invasion and metastasis. Small-molecule targeting of the central signaling node of this pathway, β-catenin, is a biologically rational approach to abolish hyperactivation of β-catenin signaling but has been demonstrated to be a difficult task. Herein, we report a drug-like small molecule, ZW4864, that binds with β-catenin and selectively disrupts the protein-protein interaction (PPI) between B-cell lymphoma 9 (BCL9) and β-catenin while sparing the β-catenin/E-cadherin PPI. ZW4864 dose-dependently suppresses β-catenin signaling activation, downregulates oncogenic β-catenin target genes, and abrogates invasiveness of β-catenin-dependent cancer cells. More importantly, ZW4864 shows good pharmacokinetic properties and effectively suppresses β-catenin target gene expression in the patient-derived xenograft mouse model. This study offers a selective chemical probe to explore β-catenin-related biology and a drug-like small-molecule β-catenin/BCL9 disruptor for future drug development.

Single cell transcriptome atlas of mouse mammary epithelial cells across development

Background

Heterogeneity within the mouse mammary epithelium and potential lineage relationships have been recently explored by single-cell RNA profiling. To further understand how cellular diversity changes during mammary ontogeny, we profiled single cells from nine different developmental stages spanning late embryogenesis, early postnatal, prepuberty, adult, mid-pregnancy, late-pregnancy, and post-involution, as well as the transcriptomes of micro-dissected terminal end buds (TEBs) and subtending ducts during puberty.

Methods

The single cell transcriptomes of 132,599 mammary epithelial cells from 9 different developmental stages were determined on the 10x Genomics Chromium platform, and integrative analyses were performed to compare specific time points.

Results

The mammary rudiment at E18.5 closely aligned with the basal lineage, while prepubertal epithelial cells exhibited lineage segregation but to a less differentiated state than their adult counterparts. Comparison of micro-dissected TEBs versus ducts showed that luminal cells within TEBs harbored intermediate expression profiles. Ductal basal cells exhibited increased chromatin accessibility of luminal genes compared to their TEB counterparts suggesting that lineage-specific chromatin is established within the subtending ducts during puberty. An integrative analysis of five stages spanning the pregnancy cycle revealed distinct stage-specific profiles and the presence of cycling basal, mixed-lineage, and 'late' alveolar intermediates in pregnancy. Moreover, a number of intermediates were uncovered along the basal-luminal progenitor cell axis, suggesting a continuum of alveolar-restricted progenitor states.

Conclusions

This extended single cell transcriptome atlas of mouse mammary epithelial cells provides the most complete coverage for mammary epithelial cells during morphogenesis to date. Together with chromatin accessibility analysis of TEB structures, it represents a valuable framework for understanding developmental decisions within the mouse mammary gland.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01445-4.

Exploiting within-breed variability in the autochthonous Reggiana breed identified several candidate genes affecting pigmentation-related traits, stature and udder defects in cattle

Autochthonous cattle breeds constitute important reservoirs of genetic diversity. Reggiana is an Italian local cattle breed reared in the north of Italy for the production of a mono‐breed Parmigiano–Reggiano cheese. Reggiana cattle usually have a classical solid red coat colour and pale muzzle. As part of the strategies designed for the sustainable conservation of this genetic resource, we investigated at the genome‐wise level the within‐breed detected variability of three pigmentation‐related traits (intensity of red coat colour, based on three classes – light/diluted, normal and dark; spotted patterns/piebaldism that sometime emerge in the breed; muzzle colour – pink/pale, grey and black), stature, presence/absence and number of supernumerary teats and teat length. A total of 1776 Reggiana cattle (about two‐thirds of the extant breed population) were genotyped with the GeneSeek GGP Bovine 150k SNP array and single‐marker and haplotype‐based GWASs were carried out. The results indicated that two main groups of genetic factors affect the intensity of red coat colour: darkening genes (including EDN3 and a few other genes) and diluting genes (including PMEL and a few other genes). Muzzle colour was mainly determined by MC1R gene markers. Piebaldism was mainly associated with KIT gene markers. Stature was associated with BTA6 markers upstream of the NCAPG–LCORL genes. Teat defects were associated with TBX3/TBX5, MCC and LGR5 genes. Overall, the identified genomic regions not only can be directly used in selection plans in the Reggiana breed, but also contribute to clarifying the genetic mechanisms involved in determining exterior traits in cattle.

LGR5 enhances the osteoblastic differentiation of MC3T3-E1 cells through the Wnt/β-catenin pathway

Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) is a Wnt-associated gene that contributes to cell proliferation and self-renewal in various organs. LGR5 is expressed in Ewing sarcoma, and LGR5-overexpressing mesenchymal stem cells promote fracture healing. However, the effects of LGR5 on osteoblastic differentiation remain unclear. The aim of the present study was to explore the function of LGR5 in osteoblastic differentiation. LGR5 was overexpressed or knocked down in the MC3T3-E1 pre-osteoblastic cell line via lentiviral transfection and its function in osteoblastic differentiation was investigated. The mRNA expression levels of the osteoblast differentiation markers alkaline phosphatase (ALP), osteocalcin and collagen type I a1 were determined, and ALP and Alizarin red staining were performed. In addition, the effects of LGR5 modulation on β-catenin and the expression of target genes in the Wnt pathway were investigated. The results revealed that the overexpression of LGR5 promoted osteoblastic differentiation. This was associated with enhancement of the stability of β-catenin and its levels in the cell nucleus, which enabled it to activate Wnt signaling. By contrast, the inhibition of LGR5 decreased the osteogenic capacity of MC3T3-E1 cells. These results indicate that LGR5 is a positive regulator of osteoblastic differentiation, whose effects are mediated through the Wnt/β-catenin signaling pathway. This suggests suggesting that the regulation of LGR5/Wnt/β-catenin signaling has potential as a therapy for osteoporosis.

Uncovering the stem cell hierarchy by genetic lineage tracing in the mammary gland

The mammary gland is the distinct feature that gives the name to the class of mammals and distinguishes them from other animals. Functionally, the mammary gland is a secretory organ which main role is to produce milk to nourish the offspring. Organogenesis of the mammary gland starts during embryogenesis but occurs mainly after birth at puberty under the influence of hormonal cues. Throughout the adult life as well as during pregnancy, the mammary gland shows a remarkable regenerative ability, thus constituting an excellent model for studying stem cell biology. Although the mammary gland consists of a relatively simple epithelial structure with a luminal and a basal cell layers, these are indeed composed by distinct subsets of mammary epithelial cells. Flow cytometry and transplantation assay have identified several subpopulations of stem and/or progenitor cells in the mammary gland. Yet, physiological and developmental relevant information can only be obtained when investigating the stem cell hierarchy in the intact mammary gland. Genetic lineage tracing studies have offered unprecedented levels of information regarding the organization of the stem cell compartment and possible role of resident stem and/or progenitor cells at different stages of the mammary gland organogenesis. These studies, although creating a passionate debate, highlight the existence of heterogeneous stem cell compartment, where bipotent as well as unipotent mammary stem cells seems to co-exist. Genetic lineage tracing experiments provide relevant information on stem cells that are key for understanding both normal development as well as associated pathologies in human. It holds the promise of providing new insights into the cell-of-origin and heterogeneity of breast tumorigenesis.

Linking EMT programmes to normal and neoplastic epithelial stem cells

Epithelial stem cells serve critical physiological functions in the generation, maintenance and repair of diverse tissues through their ability to self-renew and spawn more specialized, differentiated cell types. In an analogous fashion, cancer stem cells have been proposed to fuel the growth, progression and recurrence of many carcinomas. Activation of an epithelial-mesenchymal transition (EMT), a latent cell-biological programme involved in development and wound healing, has been linked to the formation of both normal and neoplastic stem cells, but the mechanistic basis underlying this connection remains unclear. In this Perspective, we outline the instances where aspects of an EMT have been implicated in normal and neoplastic epithelial stem cells and consider the involvement of this programme during tissue regeneration and repair. We also discuss emerging concepts and evidence related to the heterogeneous and plastic cell states generated by EMT programmes and how these bear on our understanding of cancer stem cell biology and cancer metastasis. A more comprehensive accounting of the still-elusive links between EMT programmes and the stem cell state will surely advance our understanding of both normal stem cell biology and cancer pathogenesis.

Wnt signaling and mammary stem cells

Wnt signaling is an important morphogenetic signaling pathway best known for its essential role in determining embryonic cell fates; it is often activated to re-specify fetal cells or to maintain the lineage flexibility of somatic stem cells. In this review, we consider the role of this pathway in the remarkable process of differentiation, growth and morphogenesis of the mammary gland during embryogenesis, ductal outgrowth and pregnancy. Specifically, mammary stem cells are compared with stem cells from other tissues, to identify commonalities and differences. Wnt signaling is known to be required to maintain the bipotent basal stem cell present in adult mammary ductal trees, however, the absence of this stem cell has little effect on growth or morphogenesis, and Wnt signaling is not induced during the ductal/alveolar expansion during pregnancy. The evidence for pre-determined hierarchies of mammary epithelial cells is reviewed, together with the role of signaling between mixtures of specified mammary epithelial cells in the maintenance of Wnt-dependent clonagenic stem cells. The dazzling variety of Wnt signaling components expressed by mammary epithelial cells is presented, along with some potential stromal sources of Wnt proteins that may be important starting points for the induction of plasticity in the epithelium.

The Cellular Organization of the Mammary Gland: Insights From Microscopy

Despite rapid advances in our knowledge of the cellular heterogeneity and molecular regulation of the mammary gland, how these relate to 3D cellular organization remains unclear. In addition to hormonal regulation, mammary gland development and function is directed by para- and juxtacrine signaling among diverse cell-types, particularly the immune and mesenchymal populations. Precise mapping of the cellular landscape of the breast will help to decipher this complex coordination. Imaging of thin tissue sections has provided foundational information about cell positioning in the mammary gland and now technological advances in tissue clearing and subcellular-resolution 3D imaging are painting a more complete picture. In particular, confocal, light-sheet and multiphoton microscopy applied to intact tissue can fully capture cell morphology, position and interactions, and have the power to identify spatially rare events. This review will summarize our current understanding of mammary gland cellular organization as revealed by microscopy. We focus on the mouse mammary gland and cover a broad range of immune and stromal cell types at major developmental stages and give insights into important tissue niches and cellular interactions.

PTEN-mediated AKT/β-catenin signaling enhances the proliferation and expansion of Lgr5+ hepatocytes

Rationale: Compelling evidence suggests that Lgr5+ hepatocytes repair liver damage by promoting the regeneration of hepatocytes and ductal cells in the case of liver injury. The PTEN-mediated AKT/β-catenin signaling plays a key role in the regulation of innate immune regulation in the liver. However, the signaling pathways that control Lgr5+ hepatocyte proliferation in the liver remain unclear.

Methods: In order to assess the involvement of PTEN-mediated AKT/β-catenin signaling in the expansion of Lgr5+ hepatocytes upon liver injuries, the Lgr5-CreER; Rosa-mTmG lineage tracing system was used to target Lgr5+ hepatocytes.

Results: The tracing of Lgr5+ hepatocytes showed that PTEN deletion and β-catenin activation significantly promoted the proliferation of Lgr5+ hepatocytes. In converse, the simultaneous inhibition of PTEN and β-catenin limited Lgr5+ hepatocyte proliferation in the liver. Our findings provide an insight into understanding how PTEN-mediated AKT/β-catenin signaling regulates the proliferation of Lgr5+ hepatocytes.

Conclusion: The outcomes can improve the application potential of Lgr5+ hepatocytes in the treatment of liver injury diseases and provide a new treatment option for liver cancer.

Microfluidic Organoids-on-a-Chip: Quantum Leap in Cancer Research

Organ-like cell clusters, so-called organoids, which exhibit self-organized and similar organ functionality as the tissue of origin, have provided a whole new level of bioinspiration for ex vivo systems. Microfluidic organoid or organs-on-a-chip platforms are a new group of micro-engineered promising models that recapitulate 3D tissue structure and physiology and combines several advantages of current in vivo and in vitro models. Microfluidics technology is used in numerous applications since it allows us to control and manipulate fluid flows with a high degree of accuracy. This system is an emerging tool for understanding disease development and progression, especially for personalized therapeutic strategies for cancer treatment, which provide well-grounded, cost-effective, powerful, fast, and reproducible results. In this review, we highlight how the organoid-on-a-chip models have improved the potential of efficiency and reproducibility of organoid cultures. More widely, we discuss current challenges and development on organoid culture systems together with microfluidic approaches and their limitations. Finally, we describe the recent progress and potential utilization in the organs-on-a-chip practice.

Mammary development in the embryo and adult: new insights into the journey of morphogenesis and commitment

The mammary gland is a unique tissue and the defining feature of the class Mammalia. It is a late-evolving epidermal appendage that has the primary function of providing nutrition for the young, although recent studies have highlighted additional benefits of milk including the provision of passive immunity and a microbiome and, in humans, the psychosocial benefits of breastfeeding. In this Review, we outline the various stages of mammary gland development in the mouse, with a particular focus on lineage specification and the new insights that have been gained by the application of recent technological advances in imaging in both real-time and three-dimensions, and in single cell RNA sequencing. These studies have revealed the complexity of subpopulations of cells that contribute to the mammary stem and progenitor cell hierarchy and we suggest a new terminology to distinguish these cells.

Immune privilege of skin stem cells: What do we know and what can we learn?

The skin forms a barrier that prevents dehydration and keeps us safe from pathogens. To ensure proper function, the skin possesses a myriad of stem cell populations that are essential for maintenance and repair upon damage. In order to protect, the skin is also an active immunological site, with abundant resident immune cells and strong recruitment of even more immune cells during wounding or infection. Such active and strong immunity makes the skin susceptible to a diverse spectrum of autoimmune diseases, such as vitiligo and alopecia areata. Conversely, despite constant immune surveillance, the skin is also a tissue where frequent malignancies occur, which suggests that immune evasion must also take place. Skin stem cells play a crucial role during both regeneration and tumorigenesis. How immune cells, and in particular T cells, interact with skin stem cells and the implications this crosstalk has in skin disease (both autoimmunity and cancer) is not fully understood. Uncovering the mechanisms governing immune-stem cells interactions in the skin is critical for the development of new therapeutic strategies to safeguard susceptible cells during autoimmunity and, conversely, to improve cancer immunotherapy. Here, I will discuss how distinct skin stem cell populations are attacked by, or conversely, cloaked from immune cells, and the implications their differences have in autoimmunity and cancer.

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.

Single-Cell Chromatin Analysis of Mammary Gland Development Reveals Cell-State Transcriptional Regulators and Lineage Relationships

Technological improvements enable single-cell epigenetic analyses of organ development. We reasoned that high-resolution single-cell chromatin accessibility mapping would provide needed insight into the epigenetic reprogramming and transcriptional regulators involved in normal mammary gland development. Here, we provide a single-cell resource of chromatin accessibility for murine mammary development from the peak of fetal mammary stem cell (fMaSC) functional activity in late embryogenesis to the differentiation of adult basal and luminal cells. We find that the chromatin landscape within individual cells predicts both gene accessibility and transcription factor activity. The ability of single-cell chromatin profiling to separate E18 fetal mammary cells into clusters exhibiting basal-like and luminal-like chromatin features is noteworthy. Such distinctions were not evident in analyses of droplet-based single-cell transcriptomic data. We present a web application as a scientific resource for facilitating future analyses of the gene regulatory networks involved in mammary development.

Non-canonical Fzd7 signaling contributes to breast cancer mesenchymal-like stemness involving Col6a1

Mesenchymal-like stemness is characterized by epithelial-mesenchymal transition (EMT). Breast cancer (BC) cell mesenchymal-like stemness is responsible for distal lung metastasis. Interrogation of databases showed that Fzd7 was closely associated with a panel of mesenchymal-related genes and a panel of stemness-related genes. Fzd7 knockdown in mesenchymal-like MDA-MB-231 and Hs578T cells reduced expression of Vimentin, Slug and Zeb1, induced an epithelial-like morphology, inhibited cell motility, impaired mammosphere formation and decreased Lgr5⁺ subpopulation. In contrast, Fzd7 overexpression in MCF7 cells resulted in opposite changes. Fzd7 knockdown delayed xenograft tumor formation, suppressed tumor growth, and impaired lung metastasis. Mechanistically, Fzd7 combined with Wnt5a/b and modulated expression of phosphorylated Stat3 (p-STAT3), Smad3 and Yes-associated protein 1 (Yap1). Moreover, Fzd7-Wnt5b modulated expression of collagen, type VI, alpha 1 (Col6a1). Both Wnt5b knockdown and Col6a1 knockdown disrupted BC cell mesenchymal phenotype and stemness. Taken together, Fzd7 contributes to BC cell EMT and stemness, inducing tumorigenesis and metastasis, mainly through a non-canonical Wnt5b pathway. Col6a1 is implicated in Fzd7-Wnt5b signaling, and mediates Fzd7-Wnt5b -induced mesenchymal-like stemness.

Integrating single-cell RNA-sequencing and functional assays to decipher mammary cell states and lineage hierarchies

The identification and molecular characterization of cellular hierarchies in complex tissues is key to understanding both normal cellular homeostasis and tumorigenesis. The mammary epithelium is a heterogeneous tissue consisting of two main cellular compartments, an outer basal layer containing myoepithelial cells and an inner luminal layer consisting of estrogen receptor-negative (ER⁻) ductal cells and secretory alveolar cells (in the fully functional differentiated tissue) and hormone-responsive estrogen receptor-positive (ER⁺) cells. Recent publications have used single-cell RNA-sequencing (scRNA-seq) analysis to decipher epithelial cell differentiation hierarchies in human and murine mammary glands, and reported the identification of new cell types and states based on the expression of the luminal progenitor cell marker KIT (c-Kit). These studies allow for comprehensive and unbiased analysis of the different cell types that constitute a heterogeneous tissue. Here we discuss scRNA-seq studies in the context of previous research in which mammary epithelial cell populations were molecularly and functionally characterized, and identified c-Kit⁺ progenitors and cell states analogous to those reported in the recent scRNA-seq studies.

Parity reduces mammary repopulating activity but does not affect mammary stem cells defined as CD24 + CD29/CD49fhi in mice

BACKGROUND

Breast cancer (BCa) mortality is decreasing with early detection and improvement in therapies. The incidence of BCa, however, continues to increase, particularly estrogen-receptor-positive (ER +) subtypes. One of the greatest modifiers of ER + BCa risk is childbearing (parity), with BCa risk halved in young multiparous mothers. Despite convincing epidemiological data, the biology that underpins this protection remains unclear. Parity-induced protection has been postulated to be due to a decrease in mammary stem cells (MaSCs); however, reports to date have provided conflicting data.

METHODS

We have completed rigorous functional testing of repopulating activity in parous mice using unfractionated and MaSC (CD24^midCD49f^hi)-enriched populations. We also developed a novel serial transplant method to enable us to assess self-renewal of MaSC following pregnancy. Lastly, as each pregnancy confers additional BCa protection, we subjected mice to multiple rounds of pregnancy to assess whether additional pregnancies impact MaSC activity.

RESULTS

Here, we report that while repopulating activity in the mammary gland is reduced by parity in the unfractionated gland, it is not due to a loss in the classically defined MaSC (CD24⁺CD49f^hi) numbers or function. Self-renewal was unaffected by parity and additional rounds of pregnancy also did not lead to a decrease in MaSC activity.

CONCLUSIONS

Our data show instead that parity impacts on the stem-like activity of cells outside the MaSC population.

Organoid models for mammary gland dynamics and breast cancer

The mammary gland is a highly dynamic tissue that undergoes repeated cycles of growth and involution during pregnancy and menstruation. It is also the site from which breast cancers emerge. Organoids provide an in vitro model that preserves several of the cellular, structural, and microenvironmental features that dictate mammary gland function in vivo and have greatly advanced our understanding of glandular biology. Their tractability for genetic manipulation, live imaging, and high throughput screening have facilitated investigation into the mechanisms of glandular morphogenesis, structural maintenance, tumor progression, and invasion. Opportunities remain to enhance cellular and structural complexity of mammary organoid models, including incorporating additional cell types and hormone signaling.

LGR5 in breast cancer and ductal carcinoma in situ: a diagnostic and prognostic biomarker and a therapeutic target

BACKGROUND

Novel biomarkers are required to discern between breast tumors that should be targeted for treatment from those that would never become clinically apparent and/or life threatening for patients. Moreover, therapeutics that specifically target breast cancer (BC) cells with tumor-initiating capacity to prevent recurrence are an unmet need. We investigated the clinical importance of LGR5 in BC and ductal carcinoma in situ (DCIS) to explore LGR5 as a biomarker and a therapeutic target.

METHODS

We stained BC (n = 401) and DCIS (n = 119) tissue microarrays with an antibody against LGR5. We examined an LGR5 knockdown ER^- cell line that was orthotopically transplanted and used for in vitro colony assays. We also determined the tumor-initiating role of Lgr5 in lineage-tracing experiments. Lastly, we transplanted ER^- patient-derived xenografts into mice that were subsequently treated with a LGR5 antibody drug conjugate (anti-LGR5-ADC).

RESULTS

LGR5 expression correlated with small tumor size, lower grade, lymph node negativity, and ER-positivity. ER⁺ patients with LGR5^high tumors rarely had recurrence, while high-grade ER^- patients with LGR5^high expression recurred and died due to BC more often. Intriguingly, all the DCIS patients who later died of BC had LGR5-positive tumors. Colony assays and xenograft experiments substantiated a role for LGR5 in ER^- tumor initiation and subsequent growth, which was further validated by lineage-tracing experiments in ER^- /triple-negative BC mouse models. Importantly, by utilizing LGR5^high patient-derived xenografts, we showed that anti-LGR5-ADC should be considered as a therapeutic for high-grade ER^- BC.

CONCLUSION

LGR5 has distinct roles in ER^- vs. ER⁺ BC with potential clinical applicability as a biomarker to identify patients in need of therapy and could serve as a therapeutic target for high-grade ER^- BC.

A STAT3 of Addiction: Adipose Tissue, Adipocytokine Signalling and STAT3 as Mediators of Metabolic Remodelling in the Tumour Microenvironment

Metabolic remodelling of the tumour microenvironment is a major mechanism by which cancer cells survive and resist treatment. The pro-oncogenic inflammatory cascade released by adipose tissue promotes oncogenic transformation, proliferation, angiogenesis, metastasis and evasion of apoptosis. STAT3 has emerged as an important mediator of metabolic remodelling. As a downstream effector of adipocytokines and cytokines, its canonical and non-canonical activities affect mitochondrial functioning and cancer metabolism. In this review, we examine the central role played by the crosstalk between the transcriptional and mitochondrial roles of STAT3 to promote survival and further oncogenesis within the tumour microenvironment with a particular focus on adipose-breast cancer interactions.

Progress in the application of organoids to breast cancer research

Breast cancer is the most common cancer diagnosed in women. Breast cancer research is currently based mainly on animal models and traditional cell culture. However, the inherent species gap between humans and animals, as well as differences in organization between organs and cells, limits research advances. The breast cancer organoid can reproduce many of the key features of human breast cancer, thereby providing a new platform for investigating the mechanisms underlying the development, progression, metastasis and drug resistance of breast cancer. The application of organoid technology can also promote drug discovery and the design of individualized treatment strategies. Here, we discuss the latest advances in the use of organoid technology for breast cancer research.

R-spondin-mediated WNT signaling potentiation in mammary and breast cancer development

The mammary gland is a secretory organ, which develops as a network of growing epithelial ducts composed of luminal and basal cells that invade the surrounding adipose tissue through a series of developmental cycles. Mammary stem cells (MaSCs) maintain an accurate tissue homeostasis, and their proliferation and cell fate determination are regulated by multiple hormones and local factors. The WNT pathway plays a critical role in controlling the enormous tissue expansion and remodeling during mammary gland development through the maintenance and differentiation of MaSCs, and its deregulation has been implicated in breast cancer (BC) initiation and progression. The R-spondins (RSPOs) are four secreted proteins that strongly enhance target cell sensitivity to WNT ligands. Moreover, leucine-rich repeat-containing G-protein-coupled receptors (LGRs) 4-6 are considered obligate high-affinity receptors for RSPOs and have been described as stem cell markers. Importantly, elevated RSPO expression has been recently identified in several tumor types from patients, including BC, and it has been reported that they play a significant role in mammary tumor progression in experimental models. In this review, exploring our present knowledge, we summarize the role of the RSPO-LGR axis as a WNT-enhancing signaling cascade in the MaSC compartment and during the normal and neoplastic mammary gland development. In addition, we include an updated expression profile of the RSPOs and their action mediators at the cell membrane, the LGRs, and the ubiquitin-ligases ZNRF3/RNF43, in different BC subtypes. Finally and based on these data, we discuss the significance of tumor-associated alterations of these proteins and their potential use as molecular targets for detection and treatment of BC.

Epithelial-adipocyte interactions are required for mammary gland development, but not for milk production or fertility

To investigate the role of adipose tissue in reproductive function and mammary gland development and function, we have examined lipodystrophic (LD) mice. LD mice of both sexes are sterile, but fertility can be restored with leptin injections. Mammary glands from lipodystrophic mice were rudimentary and lacked terminal end buds. Leptin-injected LD mice were able to become pregnant, showed normal pregnancy-associated glandular proliferation despite a smaller glandular area, were able to produce a small amount of milk that had grossly normal content of milk proteins and neutral lipids, but could not sustain pups to weaning. In order to separate the individual requirements for 1) adipokines such as leptin, 2) estradiol, and 3) physical epithelial-adipocyte interactions, we performed a series of experiments with both lipodystrophic mice and ob (obese mice with a mutation in the lep gene encoding the adipokine leptin) mice that received either estradiol treatment or preadipocyte transplant. The resulting fat pad did not rescue the defect in mammary gland development in lipodystrophic mice. The defect also could not be rescued with estradiol pellets. Ob/ob mice, like LD mice, lack leptin and estradiol, but retain adipose tissue. Ob mice have defective mammary gland development. However, in striking contrast to what was observed in lipodystrophic mice, reconstitution of a WT fat pad in ob mice rescued the defect in mammary gland development. Estradiol treatment did not rescue mammary gland development in ob mice. Therefore direct interaction between mammary gland epithelia and adipocytes is a requirement for full invasion and expansion of the gland, but is not required for glandular proliferation during pregnancy and milk production.

Aberrant WNT/CTNNB1 Signaling as a Therapeutic Target in Human Breast Cancer: Weighing the Evidence

WNT signaling is crucial for tissue morphogenesis during development in all multicellular animals. After birth, WNT/CTNNB1 responsive stem cells are responsible for tissue homeostasis in various organs and hyperactive WNT/CTNNB1 signaling is observed in many different human cancers. The first link between WNT signaling and breast cancer was established almost 40 years ago, when Wnt1 was identified as a proto-oncogene capable of driving mammary tumor formation in mice. Since that discovery, there has been a dedicated search for aberrant WNT signaling in human breast cancer. However, much debate and controversy persist regarding the importance of WNT signaling for the initiation, progression or maintenance of different breast cancer subtypes. As the first drugs designed to block functional WNT signaling have entered clinical trials, many questions about the role of aberrant WNT signaling in human breast cancer remain. Here, we discuss three major research gaps in this area. First, we still lack a basic understanding of the function of WNT signaling in normal human breast development and physiology. Second, the overall extent and precise effect of (epi)genetic changes affecting the WNT pathway in different breast cancer subtypes are still unknown. Which underlying molecular and cell biological mechanisms are disrupted as a result also awaits further scrutiny. Third, we survey the current status of targeted therapeutics that are aimed at interfering with the WNT pathway in breast cancer patients and highlight the importance and complexity of selecting the subset of patients that may benefit from treatment.