Progeny of Lgr5-expressing hair follicle stem cell contributes to papillomavirus-induced tumor development in epidermis. Oncogene. 2012; Sep 3. [Epub ahead of print]. [PMID: 22945646 DOI: 10.1038/onc.2012.375]

The Use of Retinoids for the Prevention and Treatment of Skin Cancers: An Updated Review

Retinoids are natural and synthetic vitamin A derivatives that are effective for the prevention and the treatment of non-melanoma skin cancers (NMSC). NMSCs constitute a heterogenous group of non-melanocyte-derived skin cancers that impose substantial burdens on patients and healthcare systems. They include entities such as basal cell carcinoma and cutaneous squamous cell carcinoma (collectively called keratinocyte carcinomas), cutaneous lymphomas and Kaposi’s sarcoma among others. The retinoid signaling pathway plays influential roles in skin physiology and pathology. These compounds regulate diverse biological processes within the skin, including proliferation, differentiation, angiogenesis and immune regulation. Collectively, retinoids can suppress skin carcinogenesis. Both topical and systemic retinoids have been investigated in clinical trials as NMSC prophylactics and treatments. Desirable efficacy and tolerability in clinical trials have prompted health regulatory bodies to approve the use of retinoids for NMSC management. Acceptable off-label uses of these compounds as drugs for skin cancers are also described. This review is a comprehensive outline on the biochemistry of retinoids, their activities in the skin, their effects on cancer cells and their adoption in clinical practice.

Neural, adipocyte and hepatic differentiation potential of primary and secondary hair follicle stem cells isolated from Arbas Cashmere goats

Background

Arbas Cashmere goats are excellent domestic breeds with high yields of wool and cashmere. Their wool and cashmere can bring huge benefits to the livestock industry. Our studies intend to more fully understand the biological characteristics of hair follicle stem cells (HFSCs) in order to further explore the mechanisms of wool and cashmere regular regeneration. And they have been increasingly considered as promising multipotent cells in regenerative medicine because of their capacity to self-renew and differentiate. However, many aspects of the specific growth characteristics and differentiation ability of HFSCs remain unknown. This study aimed to further explore the growth characteristics and pluripotency of primary hair follicle stem cells (PHFSCs) and secondary hair follicle stem cells (SHFCs).

Results

We obtained PHFSCs and SHFSCs from Arbas Cashmere goats using combined isolation and purification methods. The proliferation and vitality of the two types of HFSCs, as well as the growth patterns, were examined. HFSC-specific markers and genes related to pluripotency, were subsequently identified. The PHFSCs and SHFSCs of Arbas Cashmere goat have a typical cobblestone morphology. Moreover, the PHFSCs and SHFSCs express HFSC surface markers, including CD34, K14, K15, K19 and LGR5. We also identified pluripotency-associated gene expression, including SOX2, OCT4 and SOX9, in PHFSCs and SHFSCs. Finally, PHFSCs and SHFSCs displayed multipotent abilities. PHFSCs and SHFSCs can be directed to differentiate into adipocyte-like, neural-like, and hepatocyte-like cells.

Conclusions

In conclusion, this study confirmed that the biological characteristics and differentiation potential of PHFSCs and SHFSCs from Arbas Cashmere goats. These findings broaden and refine our knowledge of types and characteristics of adult stem cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03420-3.

MmuPV1-Induced Cutaneous Squamous Cell Carcinoma Arises Preferentially from Lgr5+ Epithelial Progenitor Cells

Murine papillomavirus, MmuPV1, causes natural infections in laboratory mice that can progress to squamous cell carcinoma (SCC) making it a useful preclinical model to study the role of papillomaviruses in cancer. Papillomavirus can infect cells within hair follicles, which contain multiple epithelial progenitor cell populations, including Lgr5+ progenitors, and transgenic mice expressing human papillomavirus oncogenes develop tumors derived from Lgr5 progenitors. We therefore tested the hypothesis that Lgr5+ progenitors contribute to neoplastic lesions arising in skins infected with MmuPV1 by performing lineage tracing experiments. Ears of 6-8-week-old Lgr5-eGFP-IRES-CreERT2/Rosa26LSLtdTomato mice were treated topically with 4-OH Tamoxifen to label Lgr5+ progenitor cells and their progeny with tdTomato and, 72 h later, infected with MmuPV1. Four months post-infection, tissue at the infection site was harvested for histopathological analysis and immunofluorescence to determine the percentage of tdTomato+ cells within the epithelial lesions caused by MmuPV1. Squamous cell dysplasia showed a low percentage of tdTomato+ cells (7%), indicating that it arises primarily from non-Lgr5 progenitor cells. In contrast, cutaneous SCC (cSCC) was substantially more positive for tdTomato+ cells (42%), indicating that cSCCs preferentially arise from Lgr5+ progenitors. Biomarker analyses of dysplasia vs. cSCC revealed further differences consistent with cSCC arising from LGR5+ progenitor cells.

Establishment of a Plasticity-Associated Risk Model Based on a SOX2- and SOX9-Related Gene Set in Head and Neck Squamous Cell Carcinoma

Recent studies highlighted SOX2 and SOX9 as key determinants for cancer-cell plasticity and demonstrated that cisplatin-induced adaptation in oral squamous cell carcinoma (SCC) is acquired by an inverse regulation of both transcription factors. However, the association between SOX2/SOX9-related genetic programs with risk factors and genetic or epigenetic alterations in primary head and neck SCC (HNSCC), and their prognostic value is largely unknown.Here, we identified differentially-expressed genes (DEG) related to SOX2 and SOX9 transcription in The Cancer Genome Atlas (TCGA)-HNSC, which enable clustering of patients into groups with distinct clinical features and survival. A prognostic risk model was established by LASSO Cox regression based on expression patterns of DEGs in TCGA-HNSC (training cohort), and was confirmed in independent HNSCC validation cohorts as well as other cancer cohorts from TCGA. Differences in the mutational landscape among risk groups of TCGA-HNSC demonstrated an enrichment of truncating NSD1 mutations for the low-risk group and elucidated DNA methylation as modulator of SOX2 expression. Gene set variation analysis (GSVA) revealed differences in several oncogenic pathways among risk groups, including upregulation of gene sets related to oncogenic KRAS signaling for the high-risk group. Finally, in silico drug screen analysis revealed numerous compounds targeting EGFR signaling with significantly lower efficacy for cancer cell lines with a higher risk phenotype, but also indicated potential vulnerabilities. IMPLICATIONS: The established risk model identifies patients with primary HNSCC, but also other cancers at a higher risk for treatment failure, who might benefit from a therapy targeting SOX2/SOX9-related gene regulatory and signaling networks.

Distinct tooth regeneration systems deploy a conserved battery of genes

BACKGROUND

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a 'lamina-less' tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

RESULTS

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

CONCLUSIONS

We propose that the expression domains described here delineate a highly conserved "successional dental epithelium" (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

Development of multiple cutaneous and follicular neoplasms associated with canine papillomavirus type 3 in a dog

A 12-year-old spayed English pointer dog developed multiple skin lesions including pigmented viral plaques, basal cell carcinomas, squamous cell carcinomas and trichoblastomas. Canine papillomavirus type 3 was detected in multiple lesions suggesting common aetiology.

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.

ALDH1A1 positive cells are a unique component of the tonsillar crypt niche and are lost along with NGFR positive stem cells during tumourigenesis

Interest into the cellular biology of human tonsillar crypts has grown in recent years because it is now known to be the site of origin of most human papilloma virus (HPV) induced oropharyngeal squamous cell carcinomas (OPSCC). Despite the interest, still relatively little is known regarding the cellular hierarchy and dynamics of this anatomical subsite. Here we evaluate normal tonsillar crypts for expression of putative stem cell markers. We found that ALDH1A1 was uniquely expressed in a subset of suprabasal tonsillar crypt epithelium. This cell population was unique from NGFR expressing cells, which were previously identified to have stem/progenitor activity in vitro. In vivo mitochondrial lineage tracing was consistent with a basal to luminal progression of cellular development. This provides support for NGFR cells as the resident stem/progenitor cells in tonsillar crypts, and suggests that the ALDH1A1 cells are not stem/progenitor cells, but merely a unique component of the crypt cellular microenvironment. Analysis of tumours found that both NGFR and ALDH1A1 are lost in HPV+ and HPV- tumours, while LGR5 expression is induced in the same tumours. These results identify a unique component of the tonsillar crypt epithelium-ALDH1A1 cells-and support a cellular model where NGFR+ cells are the long-lived progenitor cells within tonsillar crypts. They also provide evidence that NGFR and ALDH1A1+ cells are lost during tumourigenesis.

Terc is dispensable for most of the short-term HPV16 oncogene-mediated phenotypes in mice

High-risk human papillomaviruses (HPVs) have been shown in vitro to impinge on telomere homeostasis in a number of ways. However, the in vivo interaction of viruses with the telomere homeostasis apparatus has not been previously explored. Since E6 and E7 are the main viral oncogenes and key for viral replication, we have explored here the short-term phenotypes of the genes in the context of defective telomere homeostasis. We examined the short-term phenotypes of E6 and E7 in a context where the Terc component of the telomerase holoenzyme was knocked out. We determined that Terc was dispensable for most oncogene-mediated phenotypes. Surprisingly, E7-mediated reduction of label retaining cells was found to be in part dependent on the presence of Terc. Under the conditions examined here, there appears to be no compelling evidence Terc is required for most short-term viral oncogene mediated phenotypes. Further studies will elucidate its role in longer-term phenotypes.

Deregulation of the pRb-E2F4 axis alters epidermal homeostasis and favors tumor development

E2F/RB activity is altered in most human tumors. The retinoblastoma family of proteins plays a key role in regulating the progression of the cell cycle from the G1 to S phases. This is achieved through negative regulation of E2F transcription factors, important positive regulators of cell cycle entry. E2F family members are divided into two groups: activators (E2F1-E2F3a) and repressors (E2F3b-E2F8). E2F4 accounts for a large part of the E2F activity and is a main E2F repressor member in vivo. Perturbations in the balance from quiescence towards proliferation contribute to increased mitotic gene expression levels frequently observed in cancer. We have previously reported that combined Rb1-Rbl1 or Rb1-E2f1 ablation in epidermis produces important alterations in epidermal proliferation and differentiation, leading to tumor development. However, the possible roles of E2F4 in this context are still to be determined. Here, we show the absence of any discernible phenotype in the skin of mice lacking of E2f4. In contrast, the inducible loss of Rb1 in the epidermis of E2F4-null mice produced multiple skin abnormalities including altered differentiation and proliferation, spontaneous wounds, carcinoma in situ development and stem cell perturbations. All these phenotypic alterations are associated with extensive gene expression changes, the induction of c-myc and the Akt activation. Moreover the whole transcriptome analyses in comparison with previous models generated also revealed extensive changes in multiple repressive complexes and in transcription factor activity. These results point to E2F4 as a master regulator in multiple steps of epidermal homeostasis in Rb1 absence.

Co-expression of Lgr5 and CXCR4 characterizes cancer stem-like cells of colorectal cancer

Therapies designed to target cancer stem cells (CSCs) in colorectal cancer (CRC) may improve treatment outcomes. Different markers have been used to identify CSCs or CSC-like cells in CRC, but the enrichment of CSCs using these markers has yet to be optimized. We recently reported the importance of Lgr5-positive CRC cells in cancer growth. Here, we studied the possibility of using Lgr5 and CXCR4 as CSC markers for CRC. We detected high Lgr5 and CXCR4 levels in stage IV CRC specimens. Both high Lgr5 and CXCR4 levels were associated with poor prognosis in stage IV CRC patients. In vitro, Lgr5+CXCR4-, CXCR4+Lgr5- and Lgr5+CXCR4+ cells were purified in human CRC cell lines and examined for their CSC properties. We found that compared to the unsorted cells, CXCR4+Lgr5-, Lgr5+CXCR4-, and Lgr5+/CXCR4+ cells showed significantly greater cancer mass after subcutaneous transplantation, greater tumor sphere formation, higher resistance to chemotherapy, and higher incidence of tumor formation after serial adoptive transplantation into NOD/SCID mice. Taken together, our data suggest that the combined use of Lgr5 and CXCR4 may facilitate the enrichment of CSCs in CRC, and that treating Lgr5+/CXCR4+ CRC cells may improve the outcome of CRC therapy.

Lgr5+ stem cells and their progeny in mouse epidermis under regimens of exogenous skin carcinogenesis, and their absence in ensuing skin tumors

Actively proliferating Lgr5+ skin stem cells are found deep in the hair follicle (HF). These cells renew the HF and drive its expansion in anagen phase. Their long residence and continuous mitotic activity make them prime candidates to transform into skin tumor-initiating cells. This was investigated by subjecting Lgr5-EGFP-Ires-CreERT2/R26R-LacZ mice (haired and hairless) to chemical and UV carcinogenic regimens. In the course of these regimens Lgr5+ cells (EGFP+) remained exclusively located in HFs, and in deep-seated cysts of hairless skin. In haired mice, progeny of Lgr5+ stem cells (LacZ+ after a pulse of tamoxifen) appeared in the interfollicular epidermis upon UV-induced sunburn and in TPA-induced hyperplasia. In hairless mice the progeny remained located in deep-seated cysts and in HF remnants. Progeny in hairless skin was only detected interfollicularly at a late stage, in between outgrowing tumors. Lgr5+ stem cells were absent in the ultimate tumor masses, and no tumor appeared to be a (clonal) expansion of Lgr5+ cells (52 tumors with tamoxifen at the start of carcinogenesis, 42 tumors with tamoxifen late during tumor outgrowth). In contrast to CD34/K15+ quiescent bulge stem cells, actively proliferating Lgr5+ stem cells do therefore not appear to be tumor drivers in experimental skin carcinogenesis.

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

The G-protein-coupled receptors Lgr4/5/6 are Wnt signalling mediators, but their functions in squamous carcinomas (SCCs) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2- and Lgr6-EGFP-CreERT2- Rosa26/Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in vivo in SCCs. We identify, by single molecule in situ hybridisation and cell sorting, rare Lgr6-positive cells in immortalised keratinocytes, and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation, with expanded lineage tracing from Lgr6⁺ epidermal stem cells. Surprisingly, Lgr6 germline knockout mice are predisposed to SCC development, by a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline loss of function mutations in WNT pathway genes RSPO1 or LGR4, who show increased susceptibility to squamous tumour development.

Changing Stem Cell Dynamics during Papillomavirus Infection: Potential Roles for Cellular Plasticity in the Viral Lifecycle and Disease

Stem cells and cellular plasticity are likely important components of tissue response to infection. There is emerging evidence that stem cells harbor receptors for common pathogen motifs and that they are receptive to local inflammatory signals in ways suggesting that they are critical responders that determine the balance between health and disease. In the field of papillomaviruses stem cells have been speculated to play roles during the viral life cycle, particularly during maintenance, and virus-promoted carcinogenesis but little has been conclusively determined. I summarize here evidence that gives clues to the potential role of stem cells and cellular plasticity in the lifecycle papillomavirus and linked carcinogenesis. I also discuss outstanding questions which need to be resolved.

Behavior of leucine-rich repeat-containing G-protein coupled receptor 5-expressing cells in the reprogramming process

It remains unclear what cells are proper for the generation of induced pluripotent stem cells (iPSCs). Leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5) is well known as a tissue stem cell and progenitor marker, both of which are reported to be sensitive to reprogramming. In the present study, we examined the reprogramming behavior of Lgr5-expressing cells (Lgr5+ cells). First, we compared reprogramming behavior using mouse Lgr5+ and Lgr5 negative (Lgr5-) hair follicles (HFs). The number of alkaline phosphatase staining-positive cells was lesser in a well of Lgr5+ HFs than in Lgr5- HFs; however, the ratio of Nanog+ SSEA1+ cells in the cell mixture derived from Lgr5+ HFs was much higher than that from Lgr5- HFs. Lgr5+ cells could be induced from mouse embryonic fibroblasts (MEFs) after transduction with Yamanaka factors. As shown in HFs, the progeny of Lgr5+ cells arising from MEFs highly converted into Nanog+ cells and did not form Nanog- colonies. The progeny represented the status of the late reprogramming phase to a higher degree than the nonprogeny. We also confirmed this using human Lg5+ cells. Our findings suggest that the use of Lgr5+ cells will minimize sorting efforts for obtaining superior iPSCs.

Recruiting endogenous stem cells: a novel therapeutic approach for erectile dysfunction

Transplanted stem cells (SCs), owing to their regenerative capacity, represent one of the most promising methods to restore erectile dysfunction (ED). However, insufficient source, invasive procedures, ethical and regulatory issues hamper their use in clinical applications. The endogenous SCs/progenitor cells resident in organ and tissues play critical roles for organogenesis during development and for tissue homeostasis in adulthood. Even without any therapeutic intervention, human body has a robust self-healing capability to repair the damaged tissues or organs. Therefore, SCs-for-ED therapy should not be limited to a supply-side approach. The resident endogenous SCs existing in patients could also be a potential target for ED therapy. The aim of this review was to summarize contemporary evidence regarding: (1) SC niche and SC biological features in vitro; (2) localization and mobilization of endogenous SCs; (3) existing evidence of penile endogenous SCs and their possible mode of mobilization. We performed a search on PubMed for articles related to these aspects in a wide range of basic studies. Together, numerous evidences hold the promise that endogenous SCs would be a novel therapeutic approach for the therapy of ED.

LGR5 regulates pro-survival MEK/ERK and proliferative Wnt/β-catenin signalling in neuroblastoma

LGR5 is a marker of normal and cancer stem cells in various tissues where it functions as a receptor for R-spondins and increases canonical Wnt signalling amplitude. Here we report that LGR5 is also highly expressed in a subset of high grade neuroblastomas. Neuroblastoma is a clinically heterogenous paediatric cancer comprising a high proportion of poor prognosis cases (~40%) which are frequently lethal. Unlike many cancers, Wnt pathway mutations are not apparent in neuroblastoma, although previous microarray analyses have implicated deregulated Wnt signalling in high-risk neuroblastoma. We demonstrate that LGR5 facilitates high Wnt signalling in neuroblastoma cell lines treated with Wnt3a and R-spondins, with SK-N-BE(2)-C, SK-N-NAS and SH-SY5Y cell-lines all displaying strong Wnt induction. These lines represent MYCN-amplified, NRAS and ALK mutant neuroblastoma subtypes respectively. Wnt3a/R-Spondin treatment also promoted nuclear translocation of β-catenin, increased proliferation and activation of Wnt target genes. Strikingly, short-interfering RNA mediated knockdown of LGR5 induces dramatic Wnt-independent apoptosis in all three cell-lines, accompanied by greatly diminished phosphorylation of mitogen/extracellular signal-regulated kinases (MEK1/2) and extracellular signal-regulated kinases (ERK1/2), and an increase of BimEL, an apoptosis facilitator downstream of ERK. Akt signalling is also decreased by a Rictor dependent, PDK1-independent mechanism. LGR5 expression is cell cycle regulated and LGR5 depletion triggers G1 cell-cycle arrest, increased p27 and decreased phosphorylated retinoblastoma protein. Our study therefore characterises new cancer-associated pathways regulated by LGR5, and suggest that targeting of LGR5 may be of therapeutic benefit for neuroblastomas with diverse etiologies, as well as other cancers expressing high LGR5.

HPV disease transmission protection and control

Human papillomaviruses (HPVs) represent a large collection of viral types associated with significant clinical disease of cutaneous and mucosal epithelium. HPV-associated cancers are found in anogenital and oral mucosa, and at various cutaneous sites. Papillomaviruses are highly species and tissue restricted, and these viruses display both mucosotropic, cutaneotropic or dual tropism for epithelial tissues. A subset of HPV types, predominantly mucosal, are also oncogenic and cancers with these HPV types account for more than 200,000 deaths world-wide. Host control of HPV infections requires both innate and adaptive immunity, but the viruses have developed strategies to escape immune detection. Viral proteins can disrupt both innate pathogen-sensing pathways and T-cell based recognition and subsequent destruction of infected tissues. Current treatments to manage HPV infections include mostly ablative strategies in which recurrences are common and only active disease is treated. Although much is known about the papillomavirus life cycle, viral protein functions, and immune responsiveness, we still lack knowledge in a number of key areas of PV biology including tissue tropism, site-specific cancer progression, codon usage profiles, and what are the best strategies to mount an effective immune response to the carcinogenic stages of PV disease. In this review, disease transmission, protection and control are discussed together with questions related to areas in PV biology that will continue to provide productive opportunities of discovery and to further our understanding of this diverse set of human viral pathogens.

Wnt/β-catenin pathway in tissue injury: roles in pathology and therapeutic opportunities for regeneration

The Wnt/β-catenin pathway is an evolutionarily conserved set of signals with critical roles in embryonic and neonatal development across species. In mammals the pathway is quiescent in many organs. It is reactivated in response to injury and is reported to play complex and contrasting roles in promoting regeneration and fibrosis. We review the current understanding of the role of the Wnt/β-catenin pathway in injury of various mammalian organs and discuss the current advances and potential of Wnt inhibitory therapeutics toward promoting tissue regeneration and reducing fibrosis.-Bastakoty, D., Young, P. P. Wnt/β-catenin pathway in tissue injury: roles in pathology and therapeutic opportunities for regeneration.

Stem Cell-Associated Marker Expression in Canine Hair Follicles

Functional hair follicle (HF) stem cells (SCs) are crucial to maintain the constant recurring growth of hair. In mice and humans, SC subpopulations with different biomarker expression profiles have been identified in discrete anatomic compartments of the HF. The rare studies investigating canine HF SCs have shown similarities in biomarker expression profiles to that of mouse and human SCs. The aim of our study was to broaden the current repertoire of SC-associated markers and their expression patterns in the dog. We combined analyses on the expression levels of CD34, K15, Sox9, CD200, Nestin, LGR5 and LGR6 in canine skin using RT-qPCR, the corresponding proteins in dog skin lysates, and their expression patterns in canine HFs using immunohistochemistry. Using validated antibodies, we were able to define the location of CD34, Sox9, Keratin15, LGR5 and Nestin in canine HFs and confirm that all tested biomarkers are expressed in canine skin. Our results show similarities between the expression profile of canine, human and mouse HF SC markers. This repertoire of biomarkers will allow us to conduct functional studies and investigate alterations in the canine SC compartment of different diseases, like alopecia or skin cancer with the possibility to extend relevant findings to human patients.

Retinoic acid metabolism proteins are altered in trichoblastomas induced by mouse papillomavirus 1

Skin cancer burden is significant as treatment costs have skyrocketed to $8.1 million annually and some forms metastasize, such as cutaneous squamous cell carcinoma (cSCC) and melanoma. cSCC is caused by altered growth factor signaling induced by chemical carcinogens, ultraviolet light (UV) exposure, and infections with papillomaviruses (PVs). One of the few options for preventing cSCC in high-risk patients is oral retinoids. While much is understood about retinoid treatments and metabolism in mouse models of chemically and UV exposure induced cSCC, little is known about the role of retinoids in PV-induced cSCC. To better understand how retinoid metabolism is altered in cSCC, we examined the expression of this pathway in the newly discovered mouse papillomavirus (MmuPV1), which produces trichoblastomas in dorsal skin but not cSCC. We found significant increases in a rate-limiting enzyme involved in retinoic acid synthesis and retinoic acid binding proteins, suggestive of increased RA synthesis, in MmuPV1-induced tumors in B6.Cg-Foxn1(nu)/J mice. Similar increases in these proteins were seen after acute UVB exposure in Crl:SKH1-Hr(hr) mice and in regressing pre-cancerous lesions in a chemically-induced mouse model, suggesting a common mechanism in limiting the progression of papillomas to full blown cSCC.

Role of Lgr5-positive cells in colorectal cancer

The molecular regulation of the growth of colorectal cancer (CRC) cells is not completely understood. Here, we report expression of Lgr5, a stem cell marker for the intestine and hair follicle, in some of the CRC cells in the patients. To determine the role of Lgr5-positive cells in the tumorigenesis of CRCs, we prepared an adeno-associated virus (AAV) that carries diphtheria toxin fragment A (DTA) under the control of Lgr5 promoter (AAV-pLgr5-DTA). Transduction of several CRC cell lines with this virus selectively killed Lgr5-positive cells, resulting in significant inhibition of the CRC cell growth in vitro and in vivo. Thus, our data highlight a potential role of Lgr5-positive cells in the tumorigenesis of CRCs, and suggest that treating these Lgr5-positive cells in CRCs may substantially improve the outcome of CRC therapy.

Loss of Snail2 favors skin tumor progression by promoting the recruitment of myeloid progenitors

Snail2 is a zinc finger transcription factor involved in driving epithelial to mesenchymal transitions. Snail2 null mice are viable, but display defects in melanogenesis, gametogenesis and hematopoiesis, and are markedly radiosensitive. Here, using mouse genetics, we have studied the contributions of Snail2 to epidermal homeostasis and skin carcinogenesis. Snail2 (-/-) mice presented a defective epidermal terminal differentiation and, unexpectedly, an increase in number, size and malignancy of tumor lesions when subjected to the two-stage mouse skin chemical carcinogenesis protocol, compared with controls. Additionally, tumor lesions from Snail2 (-/-) mice presented a high inflammatory component with an elevated percentage of myeloid precursors in tumor lesions that was further increased in the presence of the anti-inflammatory agent dexamethasone. In vitro studies in Snail2 null keratinocytes showed that loss of Snail2 leads to a decrease in proliferation indicating a non-cell autonomous role for Snail2 in the skin carcinogenic response observed in vivo. Bone marrow (BM) cross-reconstitution assays between Snail2 wild-type and null mice showed that Snail2 absence in the hematopoietic system fully reproduces the tumor behavior of the Snail2 null mice and triggers the accumulation of myeloid precursors in the BM, blood and tumor lesions. These results indicate a new role for Snail2 in preventing myeloid precursors recruitment impairing skin chemical carcinogenesis progression.

Akt signaling leads to stem cell activation and promotes tumor development in epidermis

Hair follicle stem cells (HF-SCs) alternate between periods of quiescence and proliferation, to finally differentiate into all the cell types that constitute the hair follicle. Also, they have been recently identified as cells of origin in skin cancer. HF-SCs localize in a precise region of the hair follicle, the bulge, and molecular markers for this population have been established. Thus, HF-SCs are good model to study the potential role of oncogenic activations on SC physiology. Expression of a permanently active form of Akt (myrAkt) in basal cells leads to Akt hyperactivation specifically in the CD34(+)Itga6(H) population. This activation causes bulge stem cells to exit from quiescence increasing their response to proliferative stimuli and affecting some functions such as cell migration. HF-SC identity upon Akt activation is preserved; in this sense, increased proliferation does not result in stem cell exhaustion with age suggesting that Akt activation does not affect self-renewal an important aspect for normal tissue maintenance and cancer development. Genome-wide transcriptome analysis of HF-SC isolated from myrAkt and wild-type epidermis underscores changes in metabolic pathways characteristic of cancer cells. These differences manifest during a two-step carcinogenesis protocol in which Akt activation in HF-SCs results in increased tumor development and malignant transformation.

New insights into skin stem cell aging and cancer

Adult tissue homoeostasis requires continual replacement of cells that are lost due to normal turnover, injury and disease. However, aging is associated with an overall decline in tissue function and homoeostasis, suggesting that the normal regulatory processes that govern self-renewal and regeneration may become impaired with age. Tissue-specific SCs (stem cells) lie at the apex of organismal conservation and regeneration, ultimately being responsible for continued tissue maintenance. In many tissues, there are changes in SC numbers, or alteration of their growth properties during aging, often involving imbalances in tumour-suppressor- and oncogene-mediated pathways. Uncovering the molecular mechanisms leading to changes in SC function during aging will provide an essential tool to address tissue-specific age-related pathologies. In the present review, we summarize the age-related alterations found in different tissue SC populations, highlighting recently identified changes in aged HFSCs (hair-follicle SCs) in the skin.

Immune status, strain background, and anatomic site of inoculation affect mouse papillomavirus (MmuPV1) induction of exophytic papillomas or endophytic trichoblastomas

Papillomaviruses (PVs) induce papillomas, premalignant lesions, and carcinomas in a wide variety of species. PVs are classified first based on their host and tissue tropism and then their genomic diversities. A laboratory mouse papillomavirus, MmuPV1 (formerly MusPV), was horizontally transmitted within an inbred colony of NMRI-Foxn1(nu)/Foxn1nu (nude; T cell deficient) mice of an unknown period of time. A ground-up, filtered papilloma inoculum was not capable of infecting C57BL/6J wild-type mice; however, immunocompetent, alopecic, S/RV/Cri-ba/ba (bare) mice developed small papillomas at injection sites that regressed. NMRI-Foxn1(nu) and B6.Cg-Foxn1(nu), but not NU/J-Foxn1(nu), mice were susceptible to MmuPV1 infection. B6 congenic strains, but not other congenic strains carrying the same allelic mutations, lacking B- and T-cells, but not B-cells alone, were susceptible to infection, indicating that mouse strain and T-cell deficiency are critical to tumor formation. Lesions initially observed were exophytic papillomas around the muzzle, exophytic papillomas on the tail, and condylomas of the vaginal lining which could be induced by separate scarification or simultaneous scarification of MmuPV1 at all four sites. On the dorsal skin, locally invasive, poorly differentiated tumors developed with features similar to human trichoblastomas. Transcriptome analysis revealed significant differences between the normal skin in these anatomic sites and in papillomas versus trichoblastomas. The primarily dysregulated genes involved molecular pathways associated with cancer, cellular development, cellular growth and proliferation, cell morphology, and connective tissue development and function. Although trichoepitheliomas are benign, aggressive tumors, few of the genes commonly associated with basal cell carcinoma or squamous cells carcinoma were highly dysregulated.

Sox4 links tumor suppression to accelerated aging in mice by modulating stem cell activation

Sox4 expression is restricted in mammals to embryonic structures and some adult tissues, such as lymphoid organs, pancreas, intestine, and skin. During embryogenesis, Sox4 regulates mesenchymal and neural progenitor survival, as well as lymphocyte and myeloid differentiation, and contributes to pancreas, bone, and heart development. Aberrant Sox4 expression is linked to malignant transformation and metastasis in several types of cancer. To understand the role of Sox4 in the adult organism, we first generated mice with reduced whole-body Sox4 expression. These mice display accelerated aging and reduced cancer incidence. To specifically address a role for Sox4 in adult stem cells, we conditionally deleted Sox4 (Sox4^cKO) in stratified epithelia. Sox4^cKO mice show increased skin stem cell quiescence and resistance to chemical carcinogenesis concomitantly with downregulation of cell cycle, DNA repair, and activated hair follicle stem cell pathways. Altogether, these findings highlight the importance of Sox4 in regulating adult tissue homeostasis and cancer.

Emerging role for leucine-rich repeat-containing G-protein-coupled receptors LGR5 and LGR4 in cancer stem cells

The concept of cancer stem cells has gained considerable interest in the last few decades, partly because of their potential implication in therapy resistance. However, the lack of specific cellular surface markers for these cells has impeded their isolation, making the characterization of this cellular subpopulation technically challenging. Recent studies have indicated that leucine-rich repeat-containing G-protein-coupled receptor 4 and 5 (LGR4 and LGR5) expression in multiple organs may represent a global marker of adult stem cells. This review aims to give an overview of LGR4 and LGR5 as cancer stem cell markers and their function in development.

Shared mechanisms in stemness and carcinogenesis: lessons from oncogenic viruses

A rise in technologies for epigenetic reprogramming of cells to pluripotency, highlights the potential of understanding and manipulating cellular plasticity in unprecedented ways. Increasing evidence points to shared mechanisms between cellular reprogramming and the carcinogenic process, with the emerging possibility to harness these parallels in future therapeutics. In this review, we present a synopsis of recent work from oncogenic viruses which contributes to this body of knowledge, establishing a nexus between infection, cancer, and stemness.

Skin Tumors Rb(eing) Uncovered

The Rb1 gene was the first bona fide tumor suppressor identified and cloned more than 25 years ago. Since then, a plethora of studies have revealed the functions of pRb and the existence of a sophisticated and strictly regulated pathway that modulates such functional roles. An emerging paradox affecting Rb1 in cancer connects the relatively low number of mutations affecting Rb1 gene in specific human tumors, compared with the widely functional inactivation of pRb in most, if not in all, human cancers. The existence of a retinoblastoma family of proteins pRb, p107, and p130 and their potential unique and overlapping functions as master regulators of cell cycle progression and transcriptional modulation by similar processes, may provide potential clues to explain such conundrum. Here, we will review the development of different genetically engineered mouse models, in particular those affecting stratified epithelia, and how they have offered new avenues to understand the roles of the Rb family members and their targets in the context of tumor development and progression.

The HPV16 oncogenes cause aberrant stem cell mobilization

Human Papilloma Virus related epithelial cancers have been speculated to derive from virus-infected tissue stem cells. Stem cells also are thought to provide a reservoir of latently infected cells that can persist for long periods. In this study we have examined the effects of HPV16 E6 and E7 oncogenes on multipotent epithelial stem cells, using in vivo systems. Our results show that expression of HPV16 oncogenes reduces the number of bulge label-retaining cells within hair follicles at telogen suggesting aberrant mobilization, a result supported by increased mobilization upon acute anagen induction. Importantly the loss of relative quiescence, a hallmark feature of stem cells, occurs in the absence of a reduction in other stem cell markers. This points to an atypical stem cell compartment in the context of E6 and E7 expression. We hypothesize that this aberrant compartment may have important roles in the viral life cycle and/or ensuing carcinogenesis.

Progress in understanding the relationship between Lgr5 and colorectal cancer

Leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5), a member of the G protein-coupled receptor family, contains 17 leucine-rich repeats and N- and C-terminal cysteine-rich flanking domains. Lgr5 is a marker for stem cells in the stomach, small intestine, colon, and hair follicles. Recently, Lgr5 has been identified as a target of Wnt signaling. As a potential marker for colorectal cancer stem cells, Lgr5 might be responsible for the initiation and progression of colorectal cancer. In this article we review recent progress in understanding the relationship between Lgr5 and colorectal cancer.