miR-24 and miR-205 expression is dependent on HPV onco-protein expression in keratinocytes

Versatile role of miR-24/24-1*/24-2* expression in cancer and other human diseases

MiRNAs (miRs) have been proven to be well-validated therapeutic targets. Emerging evidence has demonstrated that intricate, intrinsic and paradoxical functions of miRs are context-dependent because of their multiple upstream regulators, broad spectrum of downstream molecular targets and distinct expression in various tissues, organs and disease states. Targeted therapy has become an emerging field of research. One key for the development of successful miR-based/targeted therapy is to acquire integrated knowledge of its regulatory network and its association with disease phenotypes to identify critical nodes of the underlying pathogenesis. Herein, we systematically summarized the comprehensive role of miR-24-3p (miR-24), along with its passenger strands miR-24-1-5p* (miR-24-1) and miR-24-2-5p* (miR-24-2), emphasizing their microenvironment, intracellular targets, and associated gene networks and regulatory phenotypes in 18 different cancer types and 13 types of other disorders. MiR-24 targets and regulates numerous genes in various cancer types and enhances the expression of several oncogenes (e.g., cMyc, BCL2 and HIF1), which are challenging in terms of druggability. In contrast, several tumor suppressor proteins (p21 and p53) have been reported to be downregulated by miR-24. MiR-24 also regulates the cell cycle and is associated with numerous cancer hallmarks such as apoptosis, proliferation, metastasis, invasion, angiogenesis, autophagy, drug resistance and other diseases pathogenesis. Overall, miR-24 plays an emerging role in the diagnosis, prognosis and pathobiology of various diseases. MiR-24 is a potential target for targeted therapy in the era of precision medicine, which expands the landscape of targetable macromolecules, including undruggable proteins.

MicroRNA-182-5p and microRNA-205-5p as potential biomarkers for prognostic stratification of p16-positive oropharyngeal squamous cell carcinoma

BACKGROUND

MicroRNAs constitute promising biomarkers.

OBJECTIVE

The aim was to investigate diagnostic and prognostic implications of miR-182-5p and miR-205-5p in p16-positive and p16-negative oropharyngeal squamous cell carcinomas (OPSCCs).

METHODS

Expression of miR-182-5p, miR-205-5p were determined via quantitative real-time-PCR in fresh frozen tissues of 26 p16-positive, 19 p16-negative OPSCCs and 18 HPV-negative oropharyngeal controls. Associations between miRNA-expression, clinicopathological characteristics and prognosis were analyzed.

RESULTS

Higher miR-182-5p expression was associated with significant inferior disease-specific survival for p16-positive OPSCCs (HR = 1.98E+09, 95% CI 0-Inf; P= 0.028) and a similar trend was observed for p16-negative OPSCCs (HR = 1.56E+09, 95% CI 0-Inf; P= 0.051). Higher miR-205-5p expression was associated with an inferior progression-free survival (HR = 4.62, 95% CI 0.98-21.83; P= 0.034) and local control rate (HR = 2.18E+09, 95% CI 0-Inf; P= 0.048) for p16-positive OPSCCs.

CONCLUSIONS

Results indicate that miR-182-5p and miR-205-5p can further stratify patients with p16-positive OPSCC into prognostic groups.

Potential role of micro ribonucleic acids in screening for anal cancer in human papilloma virus and human immunodeficiency virus related malignancies

Despite advances in antiretroviral treatment (ART), human immunodeficiency virus (HIV) continues to be a major global public health issue owing to the increased mortality rates related to the prevalent oncogenic viruses among people living with HIV (PLWH). Human papillomavirus (HPV) is the most common sexually transmitted viral disease in both men and women worldwide. High-risk or oncogenic HPV types are associated with the development of HPV-related malignancies, including cervical, penile, and anal cancer, in addition to oral cancers. The incidence of anal squamous cell cancers is increasing among PLWH, necessitating the need for reliable screening methods in this population at risk. In fact, the currently used screening methods, including the Pap smear, are invasive and are neither sensitive nor specific. Investigators are interested in circulatory and tissue micro ribonucleic acids (miRNAs), as these small non-coding RNAs are ideal biomarkers for early detection and prognosis of cancer. Multiple miRNAs are deregulated during HIV and HPV infection and their deregulation contributes to the pathogenesis of disease. Here, we will review the molecular basis of HIV and HPV co-infections and focus on the pathogenesis and epidemiology of anal cancer in PLWH. The limitations of screening for anal cancer and the need for a reliable screening program that involves specific miRNAs with diagnostic and therapeutic values is also discussed.

Decreased expression of miR-24 in peripheral plasma of type 2 diabetes mellitus patients associated with diabetic foot ulcer

To examine the correlations of miR-24 expression in peripheral plasma with the onset of diabetic foot ulcer (DFU) and diabetic foot osteomyelitis (DFO) in type 2 diabetes mellitus (T2DM) patients and explore the clinical value of miR-24 as a potential biomarker for the diagnosis and treatment outcomes of DFU and DFO, a total of 60 newly diagnosed T2DM patients without DFU (T2DM group), 112 T2DM patients with DFU (DFU group), and 60 healthy controls (NC group) were included. DFU group were further divided into DFO group (n = 64) and non-DFO group (n = 48). MiR-24 levels were determined by quantitative real-time PCR, while clinical features and risk factors of DFU and DFO were explored. The expression level of miR-24 in T2DM and DFU group was significantly lower than in NC group (P < .05), and that in DFU group was significantly lower than in T2DM group (P < .01). Additionally, the level of miR-24 significantly decreased in DFO group compared to non-DFO group (P < .01). Moreover, it was negatively correlated with the amputation rate in DFU group (P = .043) and positively correlated with healing rate after 8 weeks (P = .036). The multivariate logistic regression analysis confirmed that a low expression of miR-24 was an independent risk factor for DFU and DFO. The ROC curve analysis indicated that the AUC of miR-24 for the diagnosis of DFU and DFO was 0.849 (95% CI, 0.618-0.879, P < .001) and 0.782 (95% CI, 0.595-0.813, P < .001). Thus, a decreased expression of miR-24 of T2DM patients was closely related to the occurrence, development and prognosis of DFU and DFO, suggesting the use of miR-24 as a potential biomarker for the prediction of DFU and DFO.

miR-34a-5p Inhibits Cell Proliferation, Migration and Invasion Through Targeting JAG1/Notch1 Pathway in HPV-Infected Human Epidermal Keratinocytes

Condyloma acuminate (CA) is a communicable disease caused by human papillomavirus (HPV). This study aimed to study the targeting relationship between miR-34a-5p and Jagged 1 (JAG1), as well as its regulatory effect in HPV-infected cells. Human keratinocyte HaCaT cells were infected with HPV16E6, and CA tissues were collected. The expression level of miR-34a-5p and JAG1 were detected in CA tissues and HPV-HaCaT cells. Cell proliferation, migration and invasion were respectively measured using 3-(4, 5)-dimethylthiahiazo-(-z-y1)-3, 5-diphenytetrazoliumromide (MTT), cell wound healing and Transwell assay. The potential binding sites of miR-34a-5p and JAG1 were predicted by website TargetScan, and confirmed using dual luciferase reporter gene assay. The proteins of Notch1 pathway-related were assessed using western blotting. The results showed that miR-34a-5p expression was decreased, and JAG1 expression was increased in CA tissues and HPV-HaCaT cells. Cell proliferation, migration and invasion were decreased when miR-34a-5p over-expression and JAG1 knock-down in HPV-HaCaT cells. Furthermore, miR-34a-5p had a targeting effect on JAG1. The expression level of Notch1, NICD, Hes1 and Hey1 were increased when miR-34a-5p knock-down. miR-34a-5p could inhibit cell development, and regulate the activity of Notch1 pathway through targeting JAG1 expression in HPV-infected keratinocytes.

Gene Expression Profiling of MicroRNAs in HPV-Induced Warts and Normal Skin

: Infection with the human papillomavirus (HPV) is a common occurrence among the global population, with millions of new cases emerging on an annual basis. Dysregulated microRNA (miRNA) expression is increasingly being identified to play a role in a number of different diseases, especially in the context of high-risk HPV infection. The present study investigated the miRNA expression profiles of warts induced by low-risk HPV. In warts, miR-27b, miR-24-1, miR-3654, miR-647, and miR-1914 were downregulated while miR-612 was upregulated compared to normal skin. Using miRTargetLink Human, experimentally supported evidence was obtained showing that miR-27b targeted the vascular endothelial growth factor C (VEGFC) and CAMP-responsive element binding protein 1 (CREB1) genes. The VEGFC and CREB1 genes have been reported to be involved in tumorigenesis and wart formation, respectively. Similarly, the oxidized low-density lipoprotein receptor 1 (OLR1) gene, which plays an important role in the humoral immunity of the skin, and the plexin D1 (PLXND1) gene, which is highly expressed in tumor vasculature, were both found to be common targets of miR-27b, miR-1914, and miR-612.

Differences in miRNA differential expression in whole blood between horses with sarcoid regression and progression

Background

Currently no methods are available to predict the clinical outcome of individual horses with equine sarcoid (ES) disease.

Objective

To investigate if whole blood microRNA (miRNA) profiles can predict the long‐term development of ES tumors.

Animals

Five horses with regression and 5 with progression of ES lesions monitored over 5‐7 years and 5 control horses free of ES for at least 5 years.

Methods

For this cohort study, RNA extracted from whole blood samples from the regression, progression, and control groups was used for high throughput sequencing. Known and novel miRNAs were identified using miRDeep2 and differential expression analysis was carried out by the DESeq2 algorithm. Target gene and pathway prediction as well as enrichment and network analyses were conducted using TarBase, mirPath, and metaCore from GeneGo.

Results

Fourteen miRNAs were differentially expressed between regression and progression groups after accounting for the control condition: 4 miRNAs (28.6%) were upregulated and 10 miRNAs (71.4%) were downregulated with >2‐fold change. Seven of the 10 downregulated miRNAs are encoded in an miRNA cluster on equine chromosome 24, homologous to the well‐known 14q32 cluster in humans. Their target genes show enrichment for pathways involved in viral carcinogenesis.

Conclusions and Clinical Importance

Whole blood miRNA expression profiles are associated with long‐term ES growth in horses and warrant further validation as prognostic biomarkers in a larger study cohort. Deregulation of miRNAs on equine chromosome 24 might represent a trigger for ES development.

Cross-talk Signaling between HER3 and HPV16 E6 and E7 Mediates Resistance to PI3K Inhibitors in Head and Neck Cancer

Human papillomavirus (HPV) type 16 is implicated in approximately 75% of head and neck squamous cell carcinomas (HNSCC) that arise in the oropharynx, where viral expression of the E6 and E7 oncoproteins promote cellular transformation, tumor growth, and maintenance. An important oncogenic signaling pathway activated by E6 and E7 is the PI3K pathway, a key driver of carcinogenesis. The PI3K pathway is also activated by mutation or amplification of PIK3CA in over half of HPV(+) HNSCC. In this study, we investigated the efficacy of PI3K-targeted therapies in HPV(+) HNSCC preclinical models and report that HPV(+) cell line- and patient-derived xenografts are resistant to PI3K inhibitors due to feedback signaling emanating from E6 and E7. Receptor tyrosine kinase profiling indicated that PI3K inhibition led to elevated expression of the HER3 receptor, which in turn increased the abundance of E6 and E7 to promote PI3K inhibitor resistance. Targeting HER3 with siRNA or the mAb CDX-3379 reduced E6 and E7 abundance and enhanced the efficacy of PI3K-targeted therapies. Together, these findings suggest that cross-talk between HER3 and HPV oncoproteins promotes resistance to PI3K inhibitors and that cotargeting HER3 and PI3K may be an effective therapeutic strategy in HPV(+) tumors.Significance: These findings suggest a new therapeutic combination that may improve outcomes in HPV(+) head and neck cancer patients. Cancer Res; 78(9); 2383-95. ©2018 AACR.

Dysregulation of cellular microRNAs by human oncogenic viruses - Implications for tumorigenesis

Infection with certain animal and human viruses, often referred to as tumor viruses, induces oncogenic processes in their host. These viruses can induce tumorigenesis through direct and/or indirect mechanisms, and the regulation of microRNAs expression has been shown to play a key role in this process. Some human oncogenic viruses can express their own microRNAs; however, they all can dysregulate the expression of cellular microRNAs, facilitating their respective life cycles. The modulation of cellular microRNAs expression brings consequences to the host cells that may lead to malignant transformation, since microRNAs regulate the expression of genes involved in oncogenic pathways. This review focus on the mechanisms used by each human oncogenic virus to dysregulate the expression of cellular microRNAs, and their impact on tumorigenesis.

miR clusters target cellular functional complexes by defining their degree of regulatory freedom

Using the two paralog miR-23∼27∼24 clusters as an example and combining experimental and clinical data in a systematical approach to microRNA (miR) function and dysregulation, a complex picture of their roles in cancer is drawn. Various findings appear to be contradictory to a larger extent and cannot be fully explained by the classical regulatory network models and feedback loops that are mainly considered by one-to-one regulatory interactions of the involved molecules. Here, we propose an extended model of the regulatory role of miRs that, at least, supplements the usually considered single/oligo-target regulation of certain miRs. The cellular availability of the participating miR members in this model reflects an upper hierarchy level of intracellular and extracellular environmental influences, such as neighboring cells, soluble factors, hypoxia, chemotherapeutic drugs, and irradiation, among others. The novel model is based on the understanding of cellular functional complexes, such as for apoptosis, migration, and proliferation. These complexes consist of many regulatory components that can be targeted by miR cluster members to a different extent but may affect the functional complex in different ways. We propose that the final miR-related effect is a result of the possible degree of regulatory freedom provided by the miR effects on the whole functional complex structure. This degree of regulatory freedom defines to which extent the cellular functional complex can react in response to regulatory triggers, also understood as sensitization (more regulatory response options) or de-sensitization (less regulatory response options) of the system rather than single molecules.

The human papillomavirus E7 oncoprotein as a regulator of transcription

High-risk human papillomaviruses (HPVs) encode oncoproteins which manipulate gene expression patterns in the host keratinocytes to facilitate viral replication, regulate viral transcription, and promote immune evasion and persistence. In some cases, oncoprotein-induced changes in host cell behavior can cause progression to cancer, but a complete picture of the functions of the viral oncoproteins in the productive HPV life cycle remains elusive. E7 is the HPV-encoded factor most responsible for maintaining cell cycle competence in differentiating keratinocytes. Through interactions with dozens of host factors, E7 has an enormous impact on host gene expression patterns. In this review, we will examine the role of E7 specifically as a regulator of transcription. We will discuss mechanisms of regulation of cell cycle-related genes by E7 as well as genes involved in immune regulation, growth factor signaling, DNA damage responses, microRNAs, and others pathways. We will also discuss some unanswered questions about how transcriptional regulation by E7 impacts the biology of HPV in both benign and malignant conditions.

Human papillomaviruses in epigenetic regulations

Human Papillomaviruses (HPVs) are double-stranded DNA viruses, that infect epithelial cells and are etiologically involved in the development of human cancer. Today, over 200 types of human papillomaviruses are known. They are divided into low-risk and high-risk HPVs depending on their potential to induce carcinogenesis, driven by two major viral oncoproteins, E6 and E7. By interacting with cellular partners, these proteins are involved in interdependent viral and cell cycles in stratified differentiating epithelium, and concomitantly induce epigenetic changes in infected cells and those undergoing malignant transformation. E6 and E7 oncoproteins interact with and/or modulate expression of many proteins involved in epigenetic regulation, including DNA methyltransferases, histone-modifying enzymes and subunits of chromatin remodeling complexes, thereby influencing host cell transcription program. Furthermore, HPV oncoproteins modulate expression of cellular micro RNAs. Most of these epigenetic actions in a complex dynamic interplay participate in the maintenance of persistent infection, cell transformation, and development of invasive cancer by a considerable deregulation of tumor suppressor and oncogenes. In this study, we have undertaken to discuss a number of studies concerning epigenetic regulations in HPV-dependent cells and to focus on those that have biological relevance to cancer progression.

miR-24 regulates CDKN1B/p27 expression in prostate cancer

BACKGROUND

MicroRNAs (miRNAs) are small, non-coding RNA molecules with an important role in cancer. In prostate cancer, several miRNAs are expressed abnormally suggesting they may be useful markers for diagnosis, prognosis, and potential therapeutic intervention in this disease. However, the contribution of individual miRNAs to the development and progression of this disease remains poorly understood. This study investigated the role of miR-24, which has not been extensively studied in relation to prostate cancer.

METHODS

We used PCR to investigate the expression of miR-24 in a panel of prostate cancer cell-lines and in a series of clinical prostate biopsy specimens. The biological significance of miR-24 expression in prostate cancer cells was assessed by a series of in vitro bioassays and the effect on proposed targets p27 (CDKN1B) and p16 (CDK2NA) was investigated.

RESULTS

We showed that miR-24 expression was significantly lower in prostate cancer cell lines compared to a normal prostate epithelial cell line. Decreased expression of miR-24 was also more frequently observed in both needle core and prostatectomy tumor tissue relative to matched normal tissue. Low miR-24 expression correlated with high PSA serum levels and other markers of increased prostate cancer progression. Importantly, over-expression of miR-24 inhibited cell cycle, proliferation, migration, and clonogenic potential of prostate cancer cells, as well as inducing apoptosis. p27 and p16 were confirmed as targets of miR-24 in prostate cancer cells and a significant inverse correlation between miR-24 and p27 was revealed in clinical prostatectomy specimens.

CONCLUSIONS

These findings provide evidence that miR-24 has a tumor suppressor role in prostate cancer and also targets p27 and p16 in prostate cancer cells. We propose that it may be a useful progression biomarker or focus of therapeutic intervention for this disease.

MicroRNAs Expressed during Viral Infection: Biomarker Potential and Therapeutic Considerations

MicroRNAs (miRNAs) are short sequences of noncoding single-stranded RNAs that exhibit inhibitory effects on complementary target mRNAs. Recently, it has been discovered that certain viruses express their own miRNAs, while other viruses activate the transcription of cellular miRNAs for their own benefit. This review summarizes the viral and/or cellular miRNAs that are transcribed during infection, with a focus on the biomarker and therapeutic potential of miRNAs (or their antagomirs). Several human viruses of clinical importance are discussed, namely, herpesviruses, polyomaviruses, hepatitis B virus, hepatitis C virus, human papillomavirus, and human immunodeficiency virus.

Concise review: custodians of the transcriptome: how microRNAs guard stemness in squamous epithelia

At the core of every dynamic epithelium resides a population of carefully regulated stem cells ensuring its maintenance and balance. The complex mammalian epidermis is no exception to this rule. The last decade has delivered a wealth of knowledge regarding the biology of adult stem cells, but questions still remain regarding the intricate details of their function and maintenance. To help address these gaps, we turn to the small, single-stranded RNA molecules known as microRNAs. Since their discovery, microRNAs have provided us with novel insights and ground-breaking impulses to enhance our understanding of the biological sciences. Due to their unique role in post-transcriptional regulation, microRNAs are essential to cutaneous biology as well as the epidermal stem cell. By serving as buffers to balance between epithelial stemness, proliferation, and differentiation, microRNAs play essential roles in the maintenance of cutaneous stem cells and their transition out of the stem cell compartment. Following an updated overview of microRNA biology, we summarize the current knowledge of the role of microRNAs in cutaneous stem cells, focusing on three major players that have dominated the recent literature: miR-205, miR-203, and miR-125b. We then review clinical applications, discussing the potential of microRNAs as therapeutic targets in regenerative and oncological stem cell-based medicine.

Novel Functions of the Human Papillomavirus E6 Oncoproteins

Human papillomaviruses (HPVs) infect the epidermis as well as mucous membranes of humans. They are the causative agents of anogenital tract and some oropharyngeal cancers. Infections begin in the basal epithelia, where the viral genome replicates slowly along with its host cell. As infected cells begin to differentiate and progress toward the periphery, the virus drives proliferation in cells that would otherwise be quiescent. To uncouple differentiation from continued cellular propagation, HPVs express two oncoproteins, HPV E6 and E7. This review focuses on high-risk α-HPV E6, which in addition to supporting viral replication has transforming properties. HPV E6 promotes p53 degradation and activates telomerase, but the multifaceted oncoprotein has numerous other functions that are highlighted here.

Model systems to study the life cycle of human papillomaviruses and HPV-associated cancers

The prevalent human papillomaviruses (HPVs) infect either cutaneous or mucosal epithelium. Active Infections lead to epithelial hyperprolifeation and are usually cleared in healthy individuals within a year. Persistent infections in the anogenital tracts by certain high-risk genotypes such as HPV-16, HPV-18 and closely related types, can progress to high grade dysplasias and carcinomas in women and men, including cervical, vulva, penile and anal cancers. A significant fraction of the head and neck cancers are also caused by HPV-16. The viral oncogenes responsible for neoplastic conversion are E6 and E7 that disrupt the pathways controlled by the two major tumor suppressor genes, p53 and members of pRB family. Because HPV cannot be propagated in conventional submerged monolayer cell cultures, organotypic epithelial raft cultures that generate a stratified and differentiated epithelium have been used to study the viral life cycle. This article describes several systems to examine aspects of the viral productive phase, along with the advantages and limitations. Animal model systems of HPV carcinogenesis are also briefly described.

Cardio-miRNAs and onco-miRNAs: circulating miRNA-based diagnostics for non-cancerous and cancerous diseases

Cardiovascular diseases and cancers are the leading causes of morbidity and mortality in the world. MicroRNAs (miRNAs) are short non-coding RNAs that primarily repress target mRNAs. Here, miR-24, miR-125b, miR-195, and miR-214 were selected as representative cardio-miRs that are upregulated in human heart failure. To bridge the gap between miRNA studies in cardiology and oncology, the targets and functions of these miRNAs in cardiovascular diseases and cancers will be reviewed. ACVR1B, BCL2, BIM, eNOS, FGFR3, JPH2, MEN1, MYC, p16, and ST7L are miR-24 targets that have been experimentally validated in human cells. ARID3B, BAK1, BCL2, BMPR1B, ERBB2, FGFR2, IL6R, MUC1, SITR7, Smoothened, STAT3, TET2, and TP53 are representative miR-125b targets. ACVR2A, BCL2, CCND1, E2F3, GLUT3, MYB, RAF1, VEGF, WEE1, and WNT7A are representative miR-195 targets. BCL2L2, ß-catenin, BIM, CADM1, EZH2, FGFR1, NRAS, PTEN, TP53, and TWIST1 are representative miR-214 targets. miR-125b is a good cardio-miR that protects cardiomyocytes; miR-195 is a bad cardio-miR that elicits cardiomyopathy and heart failure; miR-24 and miR-214 are bi-functional cardio-miRs. By contrast, miR-24, miR-125b, miR-195, and miR-214 function as oncogenic or tumor suppressor miRNAs in a cancer (sub)type-dependent manner. Circulating miR-24 is elevated in diabetes, breast cancer and lung cancer. Circulating miR-195 is elevated in acute myocardial infarction, breast cancer, prostate cancer and colorectal adenoma. Circulating miR-125b and miR-214 are elevated in some cancers. Cardio-miRs and onco-miRs bear some similarities in functions and circulation profiles. miRNAs regulate WNT, FGF, Hedgehog and other signaling cascades that are involved in orchestration of embryogenesis and homeostasis as well as pathogenesis of human diseases. Because circulating miRNA profiles are modulated by genetic and environmental factors and are dysregulated by genetic and epigenetic alterations in somatic cells, circulating miRNA association studies (CMASs) within several thousands of cases each for common non-cancerous diseases and major cancers are necessary for miRNA-based diagnostics.

Human papillomavirus induced transformation in cervical and head and neck cancers

Human papillomavirus (HPV) is one of the most widely publicized and researched pathogenic DNA viruses. For decades, HPV research has focused on transforming viral activities in cervical cancer. During the past 15 years, however, HPV has also emerged as a major etiological agent in cancers of the head and neck, in particular squamous cell carcinoma. Even with significant strides achieved towards the screening and treatment of cervical cancer, and preventive vaccines, cervical cancer remains the leading cause of cancer-associated deaths for women in developing countries. Furthermore, routine screens are not available for those at risk of head and neck cancer. The current expectation is that HPV vaccination will prevent not only cervical, but also head and neck cancers. In order to determine if previous cervical cancer models for HPV infection and transformation are directly applicable to head and neck cancer, clinical and molecular disease aspects must be carefully compared. In this review, we briefly discuss the cervical and head and neck cancer literature to highlight clinical and genomic commonalities. Differences in prognosis, staging and treatment, as well as comparisons of mutational profiles, viral integration patterns, and alterations in gene expression will be addressed.

Not miR-ly micromanagers: the functions and regulatory networks of microRNAs in mammalian skin

The microRNA (miRNA) pathway is a widespread mechanism of post-transcriptional gene regulation in eukaryotic cells. In animals, each miRNA species can regulate hundreds of protein-coding genes, resulting in pervasive functions for miRNAs in numerous cellular processes. Since the identification of the first mammalian miRNA, the function of miRNAs in mammals has been a topic of great interest, both because of the versatile roles of miRNAs in biological systems, as well as the clinical potential of these regulatory RNAs. With well-defined cell lineages and the availability of versatile tools for both in vivo and in vitro studies, mammalian skin has emerged as an important system in which to examine miRNAs' functions in adult tissues. In this review, we discuss recent insights into the functions and regulatory networks of miRNAs in mammals, with a specific focus on murine skin development as a model system. We first introduce functional analyses of the miRNA biogenesis pathway in the skin, then highlight the functions of individual miRNAs in skin development, followed by an examination of miRNA roles in skin stress responses. We finish with a discussion of miRNA regulatory networks and emphasize future challenges and emerging technologies that permit the genome-wide study of miRNA functions and regulatory mechanisms in mammalian skin.