Reduced fibulin-2 contributes to loss of basement membrane integrity and skin blistering in mice lacking integrin α3β1 in the epidermis

Proteomic analysis of liver fibrosis reveals EFEMP1 as a new modulator of focal adhesion and migration of hepatic stellate cells

Liver fibrosis is characterized by an excessive accumulation of extracellular matrix (ECM) leading to liver dysfunction. Proteomic approaches help to decipher ECM alterations during fibrosis progression. Using a decellularization method, we performed a proteomic analysis of 18 fibrotic human liver samples and identified 106 deregulated ECM proteins. Three members of the fibulin protein family (fibulin-2, -3, and -5) expressed by hepatic stellate cells were significantly associated with fibrosis progression. Integrative analyses of protein-protein interaction networks highlighted different functional annotations for these three fibulins. Gene silencing studies showed that unlike fibulin-2 (FBLN2), fibulin-3 (EFEMP1) depletion impaired focal adhesions, FAK phosphorylation, the fibronectin network, and cell migration. These findings are the first to demonstrate the critical involvement of fibulin-3 in the regulation of hepatic stellate cell focal adhesions and migration, emphasizing the intricate link between chronic liver disease progression and remodeling of the microenvironment.

Keratinocyte Integrin α3β1 Promotes Efficient Healing of Wound Epidermis

To date, studies of the role for epidermal integrin α3β1 in cutaneous wound re-epithelialization have produced conflicting results: wound studies in skin from global α3-null neonatal mice have implicated the integrin in promoting timely wound re-epithelialization, whereas studies in adult mice with constitutive, epidermal-specific α3β1 deletion have not. The objective of this study was to utilize a model of inducible α3β1 deletion in the epidermis to clarify the role of α3β1 in the healing of adult wounds. We utilized the recently developed transgenic K14Cre-ERT::α3flx/flx mice (ie, inducible α3 epidermal knockout), permitting us to delete floxed Itga3 alleles (α3flx/flx) from epidermis just prior to wounding with topical treatment of 4-hydroxytamoxifen. This allows for the elucidation of α3β1-dependent wound healing in adult skin, free from compensatory mechanisms that may occur after embryonic deletion of epidermal α3β1 in the widely used constitutive α3β1-knockout mouse. We found that re-epithelializing wound gaps are larger in inducible α3 epidermal knockout mice than in control mice, indicating delayed healing, and that epidermal integrin α3β1 promotes healing of wounds, at least in part by enhancing keratinocyte proliferation. This work provides essential rationale for future studies to investigate integrin α3β1 as a therapeutic target to facilitate wound healing.

FBLN2 is associated with basal cell markers Krt14 and ITGB1 in mouse mammary epithelial cells and has a preferential expression in molecular subtypes of human breast cancer

BACKGROUND

Fibulin-2 (FBLN2) is a secreted extracellular matrix (ECM) glycoprotein and has been identified in the mouse mammary gland, in cap cells of terminal end buds (TEBs) during puberty, and around myoepithelial cells during early pregnancy. It is required for basement membrane (BM) integrity in mammary epithelium, and its loss has been associated with human breast cancer invasion. Herein, we attempted to confirm the relevance of FBLN2 to myoepithelial phenotype in mammary epithelium and to assess its expression in molecular subtypes of human breast cancer.

METHODS

The relationship between FBLN2 expression and epithelial markers was investigated in pubertal mouse mammary glands and the EpH4 mouse mammary epithelial cell line using immunohistochemistry, immunocytochemistry, and immunoblotting. Human breast cancer mRNA data from the METABRIC and TCGA datasets from Bioportal were analyzed to assess the association of Fbln2 expression with epithelial markers, and with molecular subtypes. Survival curves were generated using data from the METABRIC dataset and the KM databases.

RESULTS

FBLN2 knockdown in mouse mammary epithelial cells was associated with a reduction in KRT14 and an increase in KRT18. Further, TGFβ3 treatment resulted in the upregulation of FBLN2 in vitro. Meta-analyses of human breast cancer datasets from Bioportal showed a higher expression of Fbln2 mRNA in claudin-low, LumA, and normal-like breast cancers compared to LumB, Her2 +, and Basal-like subgroups. Fbln2 mRNA levels were positively associated with mesenchymal markers, myoepithelial markers, and markers of epithelial-mesenchymal transition. Higher expression of Fbln2 mRNA was associated with better prognosis in less advanced breast cancer and this pattern was reversed in more advanced lesions.

CONCLUSION

With further validation, these observations may offer a molecular prognostic tool for human breast cancer for more personalized therapeutic approaches.

The epidermal integrin-mediated secretome regulates the skin microenvironment during tumorigenesis and repair

Integrins are cellular transmembrane receptors that physically connect the cytoskeleton with the extracellular matrix. As such, they are positioned to mediate cellular responses to microenvironmental cues. Importantly, integrins also regulate their own microenvironment through secreted factors, also known as the integrin-mediated secretome. Epidermal integrins, or integrins expressed by keratinocytes of the skin epidermis, regulate the cutaneous microenvironment through the contribution of matrix components, via proteolytic matrix remodeling, or by mediating factors like cytokines and growth factors that can promote support for nearby but distinct cells of the stroma, such as immune cells, endothelial cells, and fibroblasts. This role for integrins is enhanced during both pathological and repair tissue remodeling processes, such as tumor growth and progression and wound healing. This review will discuss examples of how the epithelial integrin-mediated secretome can regulate the tissue microenvironment. Although different epithelial integrins in various contexts will be explored, emphasis will be given to epidermal integrins that regulate the secretome during wound healing and cutaneous tumor progression. Epidermal integrin α3β1 is of particular focus as well, since this integrin has been revealed as a key regulator of the keratinocyte secretome.

Integrin mutations in blistering skin diseases and related genetically engineered mouse models

As major receptors for cellular adhesion, integrins in the epidermis are critical to maintain skin integrity. Integrins α6β4 and α3β1 are among the most highly and widely expressed integrins in the skin. Perhaps not surprisingly, mutation in subunits associated with these integrins cause variations of a blistering skin disease called junctional epidermolysis bullosa (JEB), which is characterized by blisters that form between the epidermis and dermis of the skin. This review highlights how the differences in structural roles and functions for these epidermal integrins lead to distinct JEB phenotypes resulting from their absence. Additionally, much has been learned by using genetically engineered mouse models, which are featured throughout the review, as they closely resemble the disorders of human patients that harbor analogous mutations.

Roles for epithelial integrin α3β1 in regulation of the microenvironment during normal and pathological tissue remodeling

Integrin receptors for the extracellular matrix activate intracellular signaling pathways that are critical for tissue development, homeostasis, and regeneration/repair, and their loss or dysregulation contributes to many developmental defects and tissue pathologies. This review will focus on tissue remodeling roles for integrin α3β1, a receptor for laminins found in the basement membranes (BMs) that underlie epithelial cell layers. As a paradigm, we will discuss literature that supports a role for α3β1 in promoting ability of epidermal keratinocytes to modify their tissue microenvironment during skin development, wound healing, or tumorigenesis. Preclinical and clinical studies have shown that this role depends largely on ability of α3β1 to govern the keratinocyte's repertoire of secreted proteins, or the "secretome," including 1) matrix proteins and proteases involved in matrix remodeling and 2) paracrine-acting growth factors/cytokines that stimulate other cells with important tissue remodeling functions (e.g., endothelial cells, fibroblasts, inflammatory cells). Moreover, α3β1 signaling controls gene expression that helps epithelial cells carry out these functions, including genes that encode secreted matrix proteins, proteases, growth factors, or cytokines. We will review what is known about α3β1-dependent gene regulation through both transcription and posttranscriptional mRNA stability. Regarding the latter, we will discuss examples of α3β1-dependent alternative splicing (AS) or alternative polyadenylation (APA) that prevents inclusion of cis-acting mRNA sequences that would otherwise target the transcript for degradation via nonsense-mediated decay or destabilizing AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). Finally, we will discuss prospects and anticipated challenges of exploiting α3β1 as a clinical target for the treatment of cancer or wound healing.

Keratinocyte integrin α3β1 induces expression of the macrophage stimulating factor, CSF-1, through a YAP/TEAD-dependent mechanism

The development of wound therapy targeting integrins is hampered by inadequate understanding of integrin function in cutaneous wound healing and the wound microenvironment. Following cutaneous injury, keratinocytes migrate to restore the skin barrier, and macrophages aid in debris clearance. Thus, both keratinocytes and macrophages are critical to the coordination of tissue repair. Keratinocyte integrins have been shown to participate in this coordinated effort by regulating secreted factors, some of which crosstalk to distinct cells in the wound microenvironment. Epidermal integrin α3β1 is a receptor for laminin-332 in the cutaneous basement membrane. Here we show that wounds deficient in epidermal α3β1 express less epidermal-derived macrophage colony-stimulating factor 1 (CSF-1), the primary macrophage-stimulating growth factor. α3β1-deficient wounds also have fewer wound-proximal macrophages, suggesting that keratinocyte α3β1 may stimulate wound macrophages through the regulation of CSF-1. Indeed, using a set of immortalized keratinocytes, we demonstrate that keratinocyte-derived CSF-1 supports macrophage growth, and that α3β1 regulates Csf1 expression through Src-dependent stimulation of Yes-associated protein (YAP)-Transcriptional enhanced associate domain (TEAD)-mediated transcription. Consistently, α3β1-deficient wounds in vivo display a substantially reduced number of keratinocytes with YAP-positive nuclei. Overall, our current findings identify a novel role for epidermal integrin α3β1 in regulating the cutaneous wound microenvironment by mediating paracrine crosstalk from keratinocytes to wound macrophages, implicating α3β1 as a potential target of wound therapy.

The Combination of Vascular Endothelial Growth Factor A (VEGF-A) and Fibroblast Growth Factor 1 (FGF1) Modified mRNA Improves Wound Healing in Diabetic Mice: An Ex Vivo and In Vivo Investigation

BACKGROUND

Diabetic foot ulcers (DFU) pose a significant health risk in diabetic patients, with insufficient revascularization during wound healing being the primary cause. This study aimed to assess microvessel sprouting and wound healing capabilities using vascular endothelial growth factor (VEGF-A) and a modified fibroblast growth factor (FGF1).

METHODS

An ex vivo aortic ring rodent model and an in vivo wound healing model in diabetic mice were employed to evaluate the microvessel sprouting and wound healing capabilities of VEGF-A and a modified FGF1 both as monotherapies and in combination.

RESULTS

The combination of VEGF-A and FGF1 demonstrated increased vascular sprouting in the ex vivo mouse aortic ring model, and topical administration of a combination of VEGF-A and FGF1 mRNAs formulated in lipid nanoparticles (LNPs) in mouse skin wounds promoted faster wound closure and increased neovascularization seven days post-surgical wound creation. RNA-sequencing analysis of skin samples at day three post-wound creation revealed a strong transcriptional response of the wound healing process, with the combined treatment showing significant enrichment of genes linked to skin growth.

CONCLUSION

f-LNPs encapsulating VEGF-A and FGF1 mRNAs present a promising approach to improving the scarring process in DFU.

ECM-Focused Proteomic Analysis of Ear Punch Regeneration in Acomys Cahirinus

In mammals, significant injury is generally followed by the formation of a fibrotic scar which provides structural integrity but fails to functionally restore damaged tissue. Spiny mice of the genus Acomys represent the first example of full skin autotomy in mammals. Acomys cahirinus has evolved extremely weak skin as a strategy to avoid predation and is able to repeatedly regenerate healthy tissue without scar after severe skin injury or full-thickness ear punches. Extracellular matrix (ECM) composition is a critical regulator of wound repair and scar formation and previous studies have suggested that alterations in its expression may be responsible for the differences in regenerative capacity observed between Mus musculus and A. cahirinus , yet analysis of this critical tissue component has been limited in previous studies by its insolubility and resistance to extraction. Here, we utilize a 2-step ECM-optimized extraction to perform proteomic analysis of tissue composition during wound repair after full-thickness ear punches in A. cahirinus and M. musculus from weeks 1 to 4 post-injury. We observe changes in a wide range of ECM proteins which have been previously implicated in wound regeneration and scar formation, including collagens, coagulation and provisional matrix proteins, and matricryptic signaling peptides. We additionally report differences in crosslinking enzyme activity and ECM protein solubility between Mus and Acomys. Furthermore, we observed rapid and sustained increases in CD206, a marker of pro-regenerative M2 macrophages, in Acomys, whereas little or no increase in CD206 was detected in Mus. Together, these findings contribute to a comprehensive understanding of tissue cues which drive the regenerative capacity of Acomys and identify a number of potential targets for future pro-regenerative therapies.

Matricellular Proteins in the Homeostasis, Regeneration, and Aging of Skin

Matricellular proteins are secreted extracellular proteins that bear no primary structural functions but play crucial roles in tissue remodeling during development, homeostasis, and aging. Despite their low expression after birth, matricellular proteins within skin compartments support the structural function of many extracellular matrix proteins, such as collagens. In this review, we summarize the function of matricellular proteins in skin stem cell niches that influence stem cells' fate and self-renewal ability. In the epidermal stem cell niche, fibulin 7 promotes epidermal stem cells' heterogeneity and fitness into old age, and the transforming growth factor-β-induced protein ig-h3 (TGFBI)-enhances epidermal stem cell growth and wound healing. In the hair follicle stem cell niche, matricellular proteins such as periostin, tenascin C, SPARC, fibulin 1, CCN2, and R-Spondin 2 and 3 modulate stem cell activity during the hair cycle and may stabilize arrector pili muscle attachment to the hair follicle during piloerections (goosebumps). In skin wound healing, matricellular proteins are upregulated, and their functions have been examined in various gain-and-loss-of-function studies. However, much remains unknown concerning whether these proteins modulate skin stem cell behavior, plasticity, or cell-cell communications during wound healing and aging, leaving a new avenue for future studies.

Fibulin-2 Facilitates Malignant Progression of Hepatocellular Carcinoma

BACKGROUND

Identification of biomarkers to assist in the clinical management of hepatocellular carcinoma represents an urgent requirement. Fibulin-2 is known to contribute to the development and progression of various cancer types. This research investigated the role of fibulin-2 in hepatocellular carcinoma and explored the possible mechanisms.

METHODS

The expression of fibulin-2 in hepatocellular carcinoma was measured by bioinformatic analysis and confirmed by western blot and immunohistochemical staining in cell lines or patients' samples. The clinicopathologic features of hepatocellular carcinoma patients was analyzed. Cell viability assays were used to explore the role of fibulin-2 on proliferation in hepatocellular carcinoma. Western blot was conducted to uncover changes of protein expression of Ras-MEK-ERK1/2 pathway when Fibulin-2 was overexpressed or silenced. Flow cytometry analyses were used to determine the roles of fibulin-2 in the function of apoptosis and cell cycle. Subcutaneous xenograft mouse models showed the tumor growth pattern after fibulin-2 silence in vivo.

RESULTS

We reported the upregulation of fibulin-2 in most hepatocellular carcinoma tissues and cells lines. Fibulin-2 promoted the proliferation of hepatocellular carcinoma cells in vitro by regulating Ras-MEK-ERK1/2 signaling pathway, whereas knockdown of fibulin-2 incurred the opposite effect on proliferation. Consistently, knockdown of fibulin-2 resulted in increased apoptosis and induced growth arrest during the G0/G1 phase transition. In vivo xenograft assessment confirmed that knockdown of fibulin-2 inhibited hepatocellular carcinoma tumor growth.

CONCLUSIONS

Fibulin-2 exhibited tumor promotor activities in malignant progression of hepatocellular carcinoma. The results of the study highlighted the potential of fibulin-2 to be utilized as a promising biomarker and therapeutic target for hepatocellular carcinoma.

Matricellular proteins in cutaneous wound healing

Cutaneous wound healing is a complex process that encompasses alterations in all aspects of the skin including the extracellular matrix (ECM). ECM consist of large structural proteins such as collagens and elastin as well as smaller proteins with mainly regulative properties called matricellular proteins. Matricellular proteins bind to structural proteins and their functions include but are not limited to interaction with cell surface receptors, cytokines, or protease and evoking a cellular response. The signaling initiated by matricellular proteins modulates differentiation and proliferation of cells having an impact on the tissue regeneration. In this review we give an overview of the matricellular proteins that have been found to be involved in cutaneous wound healing and summarize the information known to date about their functions in this process.

Fibulin 2 Is Hypermethylated and Suppresses Tumor Cell Proliferation through Inhibition of Cell Adhesion and Extracellular Matrix Genes in Non-Small Cell Lung Cancer

Fibulins (FBLNs), interacting with cell adhesion receptors and extracellular matrix (ECM) components, play multiple roles in ECM structures and tissue functions. Abnormal expression of FBLN2, one of the fibulin family members, contributes to tumor initiation and development. However, the function of FBLN2 in human non-small cell lung cancer (NSCLC) has not yet been elucidated. In this study, we found that FBLN2 was downregulated in 9 out of 11 lung cancer cell lines compared to normal bronchial epithelial cells, which was associated with DNA hypermethylation. Primary lung squamous cell carcinoma expressed significantly more FBLN2 protein compared to adenocarcinoma (p = 0.047). Ectopic expression of FBLN2 led to decreased cell proliferation, migration and invasion, accompanied by inactivated MAPK/ERK and AKT/mTOR pathways, while FBLN2 siRNA knockdown resulted in an opposite biological behaviour in NSCLC cells. Additionally, overexpression of FBLN2 led to dysregulation of cell adhesion molecules, ECM markers and a panel of lysate/exosome-derived-microRNAs, which are involved in cell adhesion and ECM remodelling. Taken together, our data indicate that FBLN2 is methylated and exerts a tumor suppressor function through modulation of MAPK/ERK and AKT pathways and regulation of cell adhesion and ECM genes. Moreover, FBLN2 might be a potential biomarker for the sub-classification of NSCLC.

Role of Fibulins in Embryonic Stage Development and Their Involvement in Various Diseases

The extracellular matrix (ECM) plays an important role in the evolution of early metazoans, as it provides structural and biochemical support to the surrounding cells through the cell–cell and cell–matrix interactions. In multi-cellular organisms, ECM plays a pivotal role in the differentiation of tissues and in the development of organs. Fibulins are ECM glycoproteins, found in a variety of tissues associated with basement membranes, elastic fibers, proteoglycan aggregates, and fibronectin microfibrils. The expression profile of fibulins reveals their role in various developmental processes such as elastogenesis, development of organs during the embryonic stage, tissue remodeling, maintenance of the structural integrity of basement membrane, and elastic fibers, as well as other cellular processes. Apart from this, fibulins are also involved in the progression of human diseases such as cancer, cardiac diseases, congenital disorders, and chronic fibrotic disorders. Different isoforms of fibulins show a dual role of tumor-suppressive and tumor-promoting activities, depending on the cell type and cellular microenvironment in the body. Knockout animal models have provided deep insight into their role in development and diseases. The present review covers details of the structural and expression patterns, along with the role of fibulins in embryonic development and disease progression, with more emphasis on their involvement in the modulation of cancer diseases.

Epidermal Integrin α3β1 Regulates Tumor-Derived Proteases BMP-1, Matrix Metalloprotease-9, and Matrix Metalloprotease-3

As the major cell surface receptors for the extracellular matrix, integrins regulate adhesion and migration and have been shown to drive tumor growth and progression. Previous studies showed that mice lacking integrin α3β1 in the epidermis fail to form skin tumors during two-step chemical tumorigenesis, indicating a protumorigenic role for α3β1. Furthermore, genetic ablation of α3β1 in established skin tumors caused their rapid regression, indicating an essential role in the maintenance of tumor growth. In this study, analysis of immortalized keratinocyte lines and their conditioned media support a role for α3β1 in regulating the expression of several extracellular proteases of the keratinocyte secretome, namely BMP-1, matrix metalloprotease (MMP)-9, and MMP-3. Moreover, immunofluorescence revealed reduced levels of each protease in α3β1-deficient tumors, and RNA in situ hybridization showed that their expression was correspondingly reduced in α3β1-deficient tumor cells in vivo. Bioinformatic analysis confirmed that the expression of BMP1, MMP9, and MMP3 genes correlate with the expression of ITGA3 (gene encoding the integrin α3 subunit) in human squamous cell carcinoma and that high ITGA3 and MMP3 associate with poor survival outcome in these patients. Overall, our findings identify α3β1 as a regulator of several proteases within the secretome of epidermal tumors and as a potential therapeutic target.

Integrin α3β1 Represses Reelin Expression in Breast Cancer Cells to Promote Invasion

Simple Summary

Breast cancer remains the second leading cause of cancer-related deaths in women, and about 1 in 8 women in the United States develops invasive breast cancer in her lifetime. Integrin α3β1 has been linked to breast cancer progression, but mechanisms whereby it promotes tumor invasion remain unclear. The goal of our study was to determine how α3β1 drives invasion, towards exploiting this integrin as a therapeutic target for breast cancer. We found that α3β1 represses the expression of Reelin, a secreted glycoprotein that inhibits invasion and for which loss of expression is associated with poor prognosis in breast cancer. We also show that increased Reelin expression following RNAi-mediated suppression of α3β1 causes a significant decrease in breast cancer cell invasion. Our findings demonstrate a critical role for α3β1 in promoting cell invasion through repression of Reelin, highlighting the potential value of this integrin as a therapeutic target for breast cancer.

Abstract

Integrin α3β1, a cell adhesion receptor for certain laminins, is known to promote breast tumor growth and invasion. Our previous gene microarray study showed that the RELN gene, which encodes the extracellular glycoprotein Reelin, was upregulated in α3β1-deficient (i.e., α3 knockdown) MDA-MB-231 cells. In breast cancer, reduced RELN expression is associated with increased invasion and poor prognosis. In this study we demonstrate that α3β1 represses RELN expression to enhance breast cancer cell invasion. RELN mRNA was significantly increased upon RNAi-mediated α3 knockdown in two triple-negative breast cancer cell lines, MDA-MB-231 and SUM159. Modulation of baseline Reelin levels altered invasive potential, where enhanced Reelin expression in MDA-MB-231 cells reduced invasion, while RNAi-mediated suppression of Reelin in SUM159 cells increased invasion. Moreover, treatment of α3β1-expressing MDA-MB-231 cells with culture medium that was conditioned by α3 knockdown MDA-MB-231 cells led to decreased invasion. RNAi-mediated suppression of Reelin in α3 knockdown MDA-MB-231 cells mitigated this effect of conditioned-medium, identifying secreted Reelin as an inhibitor of cell invasion. These results demonstrate a novel role for α3β1 in repressing Reelin in breast cancer cells to promote invasion, supporting this integrin as a potential therapeutic target.

The Human Epidermal Basement Membrane: A Shaped and Cell Instructive Platform That Aging Slowly Alters

One of the most important functions of skin is to act as a protective barrier. To fulfill this role, the structural integrity of the skin depends on the dermal-epidermal junction-a complex network of extracellular matrix macromolecules that connect the outer epidermal layer to the underlying dermis. This junction provides both a structural support to keratinocytes and a specific niche that mediates signals influencing their behavior. It displays a distinctive microarchitecture characterized by an undulating pattern, strengthening dermal-epidermal connectivity and crosstalk. The optimal stiffness arising from the overall molecular organization, together with characteristic anchoring complexes, keeps the dermis and epidermis layers extremely well connected and capable of proper epidermal renewal and regeneration. Due to intrinsic and extrinsic factors, a large number of structural and biological changes accompany skin aging. These changes progressively weaken the dermal-epidermal junction substructure and affect its functions, contributing to the gradual decline in overall skin physiology. Most changes involve reduced turnover or altered enzymatic or non-enzymatic post-translational modifications, compromising the mechanical properties of matrix components and cells. This review combines recent and older data on organization of the dermal-epidermal junction, its mechanical properties and role in mechanotransduction, its involvement in regeneration, and its fate during the aging process.

EMILIN proteins are novel extracellular constituents of the dentin-pulp complex

Odontoblasts and pulp stroma cells are embedded within supramolecular networks of extracellular matrix (ECM). Fibrillin microfibrils and associated proteins are crucial constituents of these networks, serving as contextual scaffolds to regulate tissue development and homeostasis by providing both structural and mechanical properties and sequestering growth factors of the TGF-β superfamily. EMILIN-1, -2, and -3 are microfibril-associated glycoproteins known to modulate cell behaviour, growth factor activity, and ECM assembly. So far their expression in the various cells of the dentin-pulp complex during development, in the adult stage, and during inflammation has not been investigated. Confocal immunofluorescence microscopy and western blot analysis of developing and adult mouse molars and incisors revealed an abundant presence of EMILINs in the entire dental papilla, at early developmental stages. Later in development the signal intensity for EMILIN-3 decreases, while EMILIN-1 and -2 staining appears to increase in the pre-dentin and in the ECM surrounding odontoblasts. Our data also demonstrate new specific interactions of EMILINs with fibulins in the dentin enamel junction. Interestingly, in dentin caries lesions the signal for EMILIN-3 was significantly increased in inflamed odontoblasts. Overall our findings point for the first time to a role of EMILINs in dentinogenesis, pulp biology, and inflammation.

Integrin α3β1 on Tumor Keratinocytes Is Essential to Maintain Tumor Growth and Promotes a Tumor-Supportive Keratinocyte Secretome

The development of integrin-targeted cancer therapies is hindered by incomplete understanding of integrin function in tumor cells and the tumor microenvironment. Previous studies showed that mice with epidermis-specific deletion of the α3 integrin subunit fail to form skin tumors during two-step chemical tumorigenesis, indicating a protumorigenic role for integrin α3β1. Here, we generated mice with tamoxifen-inducible, epidermis-specific α3 knockout to determine the role of α3β1 in the maintenance of established tumor cells and/or the associated stroma. Genetic ablation of α3 in established skin tumors caused their rapid regression, indicating that α3β1 is essential to maintain tumor growth. Although reduced proliferation and increased apoptosis were observed in α3β1-deficient tumor cells, these changes followed a robust increase in stromal apoptosis. Furthermore, macrophages and fibulin-2 levels were reduced in stroma following α3 deletion from tumor cells. Mass spectrometric analysis of conditioned medium from immortalized keratinocytes showed that α3β1 regulates a substantial fraction of the keratinocyte secretome, including fibulin-2 and macrophage CSF1; RNA in situ hybridization showed that expression of these two genes was reduced in tumor keratinocytes in vivo. Our findings identify α3β1 as a regulator of the keratinocyte secretome and skin tumor microenvironment and as a potential therapeutic target.

Laminin 332 in cancer: When the extracellular matrix turns signals from cell anchorage to cell movement

Laminin 332 is crucial in the biology of epithelia. This large extracellular matrix protein consists of the heterotrimeric assembly of three subunits - α3, β3, and γ2 - and its multifunctionality relies on a number of extracellular proteolytic processing events. Laminin 332 is central to normal epithelium homeostasis by sustaining cell adhesion, polarity, proliferation, and differentiation. It also supports a major function in epithelial tissue formation, repair, and regeneration by buttressing cell migration and survival and basement membrane assembly. Interest in this protein increased after the discovery that its expression is perturbed in tumor cells, cancer-associated fibroblasts, and the tumor microenvironment. This review summarizes current knowledge regarding the established involvement of the laminin 332 γ2 chain in tumor invasiveness and discusses the role of its α3 and β3 subunits.

Roles of Fibulin-2 in Carcinogenesis

Fibulin-2, an extracellular matrix (ECM) protein expressed in normal epithelia, is a kind of fibulin which is associated with basement membranes (BM) and elastic ECM fibers. The role of fibulin-2 has been recognized as an oncogene. The upregulation of fibulin-2 correlates with cancer development and progression. Furthermore, the upregulation of fibulin has been detected in ovarian cancer and stomach adenocarcinoma. However, the downregulation of fibulin has been detected in different intestinal and respiratory tumor cells. Additional studies have revealed that the role of fibulin-2 in carcinogenesis is context dependent and is caused by the interaction of fibulin proteins such as cell surface receptors and other ECM proteins, including integrins and syndecans. The present study summarizes the role of fibulin in carcinogenesis and its underlying molecular mechanism.

Streptococcal Collagen-like Protein 1 Binds Wound Fibronectin: Implications in Pathogen Targeting

Group A Streptococcus (GAS) infections are responsible for significant morbidity and mortality worldwide. The outlook for an effective global vaccine is reduced because of significant antigenic variation among GAS strains worldwide. Other challenges in GAS therapy include the lack of common access to antibiotics in developing countries, as well as allergy to and treatment failures with penicillin and increasing erythromycin resistance in the industrialized world. At the portal of entry, GAS binds to newly deposited extracellular matrix, which is rich in cellular fibronectin isoforms with extra domain A (EDA, also termed EIIIA) via the surface adhesin, the streptococcal collagen-like protein 1 (Scl1). Recombinant Scl1 constructs, derived from diverse GAS strains, bind the EDA loop segment situated between the C and C' β-strands. Despite the sequence diversity in Scl1 proteins, multiple sequence alignments and secondary structure predictions of Scl1 variants, as well as crystallography and homology modeling studies, point to a conserved mechanism of Scl1-EDA binding. We propose that targeting this interaction may prevent the progression of infection. A synthetic cyclic peptide, derived from the EDA C-C' loop, binds to recombinant Scl1 with a micromolar dissociation constant. This review highlights the current concept of EDA binding to Scl1 and provides incentives to exploit this binding to treat GAS infections and wound colonization.

Homeostasis, regeneration and tumour formation in the mammalian epidermis

The epidermis is the outer covering of the skin and provides a protective interface between the body and the environment. It is well established that the epidermis is maintained by stem cells that self-renew and generate differentiated cells. In this review, we discuss how recent technological advances, including single cell transcriptomics and in vivo imaging, have provided new insights into the nature and plasticity of the stem cell compartment and the differing roles of stem cells in homeostasis, wound repair and cancer.

Keratinocyte Integrin α3β1 Promotes Secretion of IL-1α to Effect Paracrine Regulation of Fibroblast Gene Expression and Differentiation

After cutaneous injury, keratinocytes secrete paracrine factors that regulate wound cell functions; dysregulation of this signaling can lead to wound pathologies. Previously, we established that keratinocyte integrin α3β1 promotes wound angiogenesis through paracrine stimulation of endothelial cells. We hypothesize here that α3β1-dependent paracrine signaling from keratinocytes regulates the differentiation state of myofibroblasts. We report that epidermal α3-knockout mice exhibit more wound myofibroblasts and fewer cyclooxygenase 2 (Cox-2)-positive dermal cells than controls. We also found that conditioned medium from α3-expressing mouse keratinocytes (MKα3⁺), but not from α3-null MK cells (MKα3^-), induces expression of Cox-2 in fibroblasts in a time- and dose-dependent manner and that this induction is mediated by IL-1α. Compared with MKα3^- cells, MKα3⁺ cells secrete more IL-1α and less IL-1RA, a natural IL-1 receptor antagonist. Treatment with an IL-1α neutralizing antibody, recombinant IL-1RA, or IL-1 receptor-targeting small interfering RNA suppresses MKα3⁺ conditioned medium-dependent induction of Cox-2 expression in fibroblasts. Finally, active recombinant IL-1α is sufficient to induce Cox-2 in fibroblasts and to inhibit transforming growth factor-β-induced α-SMA expression. Our findings support a role for keratinocyte integrin α3β1 in controlling the secretion of IL-1α, a paracrine factor that regulates the wound myofibroblast phenotype.

Fibulin-2 is required for basement membrane integrity of mammary epithelium

Fibulin-2 (FBLN2) is a secreted extracellular matrix glycoprotein which has been associated with tissue development and remodelling. In the mouse mammary gland, FBLN2 can be detected during ductal morphogenesis in cap cells and myoepithelial cells at puberty and early pregnancy, respectively. In an attempt to assign its function, we knocked down Fbln2 in the mouse mammary epithelial cell line EpH4. FBLN2 reduction led to an increase in the size of spheroidal structures when compared to scrambled control shRNA-transduced cells plated on Matrigel matrix. This phenotype was associated with a disruption of the collagen IV sheath around the epithelial spheroids and downregulation of integrin β1, suggesting a role for FBLN2 in stabilizing the basement membrane (BM). In contrast to mice, in normal adult human breast tissue, FBLN2 was detected in ductal stroma, and in the interlobular stroma, but was not detectable within the lobular regions. In tissue sections of 65 breast cancers FBLN2 staining was lost around malignant cells with retained staining in the neighbouring histologically normal tissue margins. These results are consistent with a role of FBLN2 in mammary epithelial BM stability, and that its down-regulation in breast cancer is associated with loss of the BM and early invasion.

Extracellular Interactions between Fibulins and Transforming Growth Factor (TGF)-β in Physiological and Pathological Conditions

Transforming growth factor (TGF)-β is a multifunctional peptide growth factor that has a vital role in the regulation of cell growth, differentiation, inflammation, and repair in a variety of tissues, and its dysregulation mediates a number of pathological conditions including fibrotic disorders, chronic inflammation, cardiovascular diseases, and cancer progression. Regulation of TGF-β signaling is multifold, but one critical site of regulation is via interaction with certain extracellular matrix (ECM) microenvironments, as TGF-β is primarily secreted as a biologically inactive form sequestrated into ECM. Several ECM proteins are known to modulate TGF-β signaling via cell–matrix interactions, including thrombospondins, SPARC (Secreted Protein Acidic and Rich in Cystein), tenascins, osteopontin, periostin, and fibulins. Fibulin family members consist of eight ECM glycoproteins characterized by a tandem array of calcium-binding epidermal growth factor-like modules and a common C-terminal domain. Fibulins not only participate in structural integrity of basement membrane and elastic fibers, but also serve as mediators for cellular processes and tissue remodeling as they are highly upregulated during embryonic development and certain disease processes, especially at the sites of epithelial–mesenchymal transition (EMT). Emerging studies have indicated a close relationship between fibulins and TGF-β signaling, but each fibulin plays a different role in a context-dependent manner. In this review, regulatory interactions between fibulins and TGF-β signaling are discussed. Understanding biological roles of fibulins in TGF-β regulation may introduce new insights into the pathogenesis of some human diseases.

Requirement for and polarized localization of integrin proteins during Drosophila wound closure

Wound reepithelialization is an evolutionarily conserved process in which skin cells migrate as sheets to heal the breach and is critical to prevent infection but impaired in chronic wounds. Integrin heterodimers mediate attachment between epithelia and underlying extracellular matrix and also act in large signaling complexes. The complexity of the mammalian wound environment and evident redundancy among integrins has impeded determination of their specific contributions to reepithelialization. Taking advantage of the genetic tools and smaller number of integrins in Drosophila, we undertook a systematic in vivo analysis of integrin requirements in the reepithelialization of skin wounds in the larva. We identify αPS2-βPS and αPS3-βPS as the crucial integrin dimers and talin as the only integrin adhesion component required for reepithelialization. The integrins rapidly accumulate in a JNK-dependent manner in a few rows of cells surrounding a wound. Intriguingly, the integrins localize to the distal margin in these cells, instead of the frontal or lamellipodial distribution expected for proteins providing traction and recruit nonmuscle myosin II to the same location. These findings indicate that signaling roles of integrins may be important for epithelial polarization around wounds and lay the groundwork for using Drosophila to better understand integrin contributions to reepithelialization.

Constitutional absence of epithelial integrin α3 impacts the composition of the cellular microenvironment of ILNEB keratinocytes

Integrin α3β1, a major epidermal adhesion receptor is critical for organization of the basement membrane during development and wound healing. Integrin α3 deficiency leads to interstitial lung disease, nephrotic syndrome and epidermolysis bullosa (ILNEB), an autosomal recessive multiorgan disease characterized by basement membrane abnormalities in skin, lung and kidney. The pathogenetic chains from ITGA3 mutation to tissue abnormalities are still unclear. Although integrin α3 was reported to regulate multiple extracellular proteins, the composition of the extracellular compartment of integrin α3-negative keratinocytes has not been resolved so far. In a comprehensive approach, quantitative proteomics of deposited extracellular matrix, conditioned cultured media as well as of the intracellular compartment of keratinocytes isolated from an ILNEB patient and from normal skin were performed. By mass spectrometry-based proteomics, 167 proteins corresponding to the GO terms "extracellular" and "cell adhesion", or included in the "human matrisome" were identified in the deposited extracellular matrix, and 217 in the conditioned media of normal human keratinocytes. In the absence of integrin α3, 33% and 26% respectively were dysregulated. Dysregulated proteins were functionally related to integrin α3 or were known interaction partners. The results show that in the absence of integrin α3 ILNEB keratinocytes produce a fibronectin-rich microenvironment and make use of fibronectin-binding integrin subunits αv and α5. The most important results were validated in monolayer and organotypic coculture models. Finally, the in vivo relevance of the most dysregulated components was demonstrated by immunostainings of skin, kidney and lung samples of three ILNEB patients.

Opposing Roles of Epidermal Integrins α3β1 and α9β1 in Regulation of mTLD/BMP-1-Mediated Laminin-γ2 Processing during Wound Healing

Proteolytic processing of the laminin-γ2 chain is a hallmark of basement membrane maturation in the skin. Integrin α3β1, a major receptor for epidermal adhesion to laminin-332, is critical for proper basement membrane organization during skin development and wound healing. Previously, we identified a role for α3β1 in promoting the processing of laminin-γ2 in cultured keratinocytes in vitro and in wound epidermis in vivo. In this study we identify the Bmp1 gene, which encodes variants of the mTLD/BMP-1 metalloproteases, as a critical regulator of α3β1-dependent laminin-γ2 processing, thereby expanding the role of this integrin in controlling the secretion by the epidermis of factors that modulate the tissue microenvironment. Because our previous studies identified another epidermal integrin, α9β1, as a suppressive regulator of α3β1-dependent wound angiogenesis, we investigated whether α9β1 has a similar cross-suppressive effect on the ability of α3β1 to promote basement membrane organization. Here, we show that, rather than a cross-suppressive role, α9β1 has an opposing role in basement membrane assembly/maturation through reduced laminin-γ2 processing via mTLD/BMP-1. Although α3β1 promotes this process during wound healing, α9β1 has an inhibitory role, suggesting that regulation of basement membrane assembly requires a complex interplay between these distinct epidermal integrins.

Extracellular matrix contribution to skin wound re-epithelialization

The ability of skin to act as a barrier is primarily determined by cells that maintain the continuity and integrity of skin and restore it after injury. Cutaneous wound healing in adult mammals is a complex multi-step process that involves overlapping stages of blood clot formation, inflammation, re-epithelialization, granulation tissue formation, neovascularization, and remodeling. Under favorable conditions, epidermal regeneration begins within hours after injury and takes several days until the epithelial surface is intact due to reorganization of the basement membrane. Regeneration relies on numerous signaling cues and on multiple cellular processes that take place both within the epidermis and in other participating tissues. A variety of modulators are involved, including growth factors, cytokines, matrix metalloproteinases, cellular receptors, and extracellular matrix components. Here we focus on the involvement of the extracellular matrix proteins that impact epidermal regeneration during wound healing.

Loss of fibulin-2 expression is involved in the inhibition of breast cancer invasion and forms a new barrier in addition to the basement membrane

Previous studies have demonstrated that fibulin-2 may facilitate cancer cell invasion and metastasis during tumor progression. In the present study, immunohistochemical analyses of fibulin-2 and collagen IV expression in 35 patients with breast cancer were performed to define their localization and association with breast cancer tissue. Fibulin-2 was revealed to be expressed in all tissues surrounding the breast ducts and blood vessels in normal breast tissue, while its expression was not integrated in invasive ductal carcinoma or terminal duct-lobular unit. In malignant breast tissue, collagen IV was integrated around the duct, while fibulin-2 was expressed around collagen IV and was incomplete. These results demonstrated that fibulin-2 was associated with breast cancer invasion. Fibulin-2 expression decreased prior to basement membrane (BM) degradation, indicating that fibulin-2 forms an additional barrier around the BM. Therefore, it was proposed that fibulin-2 composes the general BM, which differs from the traditional BM. These results provide insight into extracellular matrix components and the involvement of fibulin-2 in tumor invasion and metastasis. Fibulin-2 was involved in the process of breast cancer development. It performed an important role in prevention of cancer cell penetration and metastasis.

Suppression of integrin α3β1 by α9β1 in the epidermis controls the paracrine resolution of wound angiogenesis

The development of novel therapies to promote wound healing is hindered by our poor understanding of how different integrins function together in the epidermis. Longmate et al. show that cross-suppression by integrins within the epidermis controls paracrine signals that regulate wound angiogenesis. Integrin α9β1 suppresses the proangiogenic functions of α3β1 during late-stage wound healing, leading to the normalization of blood vessel density in the wound bed.

Development of wound therapies is hindered by poor understanding of combinatorial integrin function in the epidermis. In this study, we generated mice with epidermis-specific deletion of α3β1, α9β1, or both integrins as well as keratinocyte lines expressing these integrin combinations. Consistent with proangiogenic roles for α3β1, α3-null keratinocytes showed reduced paracrine stimulation of endothelial cell migration and survival, and wounds of epidermis-specific α3 knockout mice displayed impaired angiogenesis. Interestingly, α9β1 in keratinocytes suppressed α3β1-mediated stimulation of endothelial cells, and wounds of epidermis-specific α9 knockout mice displayed delayed vascular normalization and reduced endothelial apoptosis, indicating that α9β1 cross-suppresses α3β1 proangiogenic functions. Moreover, α9β1 inhibited α3β1 signaling downstream of focal adhesion kinase (FAK) autoactivation at the point of Src-mediated phosphorylation of FAK Y861/Y925. Finally, α9β1 cross-suppressed many α3β1-dependent genes, including the gene that encodes MMP-9, which we implicated as a regulator of integrin-dependent cross talk to endothelial cells. Our findings identify a novel physiological context for combinatorial integrin signaling, laying the foundation for therapeutic strategies that manipulate α9β1 and/or α3β1 during wound healing.

Integrin-mediated regulation of epidermal wound functions

During cutaneous wound healing, keratinocyte proliferation and migration are critical for re-epithelialization. In addition the epidermis secretes growth factors, cytokines, proteases, and matricellular proteins into the wound microenvironment that modify the extracellular matrix and stimulate other wound cells that control the inflammatory response, promote angiogenesis and facilitate tissue contraction and remodeling. Wound keratinocytes express at least seven different integrins-the major cell adhesion receptors for the extracellular matrix-that collectively control essential cell-autonomous functions to ensure proper re-epithelialization, including migration, proliferation, survival and basement membrane assembly. Moreover, it has become evident in recent years that some integrins can regulate paracrine signals from wound epidermis that stimulate other wound cells involved in angiogenesis, contraction and inflammation. Importantly, it is likely that abnormal integrin expression or function in the epidermis contributes to wound pathologies such as over-exuberant healing (e.g., hypertrophic scar formation) or diminished healing (e.g., chronic wounds). In this review, we discuss current knowledge of integrin function in the epidermis, which implicates them as attractive therapeutic targets to promote wound healing or treat wound pathologies. We also discuss challenges that arise from the complex roles that multiple integrins play in wound epidermis, which may be regulated through extracellular matrix remodeling that determines ligand availability. Indeed, understanding how different integrin functions are temporally coordinated in wound epidermis and which integrin functions go awry in pathological wounds, will be important to determine how best to target them clinically to achieve maximum therapeutic benefit. Graphical abstract In addition to their well-characterized roles in keratinocyte adhesion, migration and wound re-epithelialization, epidermal integrins play important roles in modifying the wound microenvironment by regulating the expression and secretion of growth factors, extracellular proteases, and matricellular proteins that stimulate other wound cells, including vascular endothelial cells and fibroblasts/myofibroblasts.

Simvastatin Increases Fibulin-2 Expression in Human Coronary Artery Smooth Muscle Cells via RhoA/Rho-Kinase Signaling Pathway Inhibition

The composition and structure of the extracellular matrix (ECM) in the vascular wall and in the atherosclerotic plaque are important factors that determine plaque stability. Statins can stabilize atherosclerotic plaques by modulating ECM protein expression. Fibulins are important components of the ECM. We evaluated the in vitro effect of simvastatin on the expression of fibulin-1, -2, -4 and -5 in human coronary artery smooth muscle cells (SMCs) and the mechanisms involved. Cells were incubated with simvastatin (0.05–1 μM), mevalonate (100 and 200 μM), geranylgeranyl pyrophosphate (GGPP) (15 μM), farnesyl pyrophosphate (FPP) (15 μM), the Rho kinase (ROCK) inhibitor Y-27632 (15 and 20 μM), the Rac-1 inhibitor (another member of Rho family) NSC23766 (100 μM), arachidonic acid (a RhoA/ROCK activator, 25–100 μM) and other fatty acids that are not activators of RhoA/ROCK (25–100 μM). Gene expression was analyzed by quantitative real-time PCR, and fibulin protein levels were analyzed by western blotting and ELISA. Simvastatin induced a significant increase in mRNA and protein levels of fibulin-2 at 24 hours of incubation (p<0.05), but it did not affect fibulin-1, -4, and -5 expression. Mevalonate and GGPP were able to reverse simvastatin’s effect, while FPP did not. In addition, Y-27632, but not NSC23766, significantly increased fibulin-2 expression. Furthermore, activation of the RhoA/ROCK pathway with arachidonic acid decreased fibulin-2 mRNA. Simvastatin increased mRNA levels and protein expression of the ECM protein fibulin-2 through a RhoA and Rho-Kinase-mediated pathway. This increase could affect the composition and structure of the ECM.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

Fibulin-2 is a key mediator of the pro-neurogenic effect of TGF-beta1 on adult neural stem cells

Transforming growth factor beta 1 (TGF-beta1), an anti-inflammatory cytokine, has been shown to have pro-neurogenic effects on adult Neural Stem Cells (aNSC) from the dentate gyrus and in vivo models. Here, we expanded the observation of the pro-neurogenic effect of TGF-beta1 on aNSC from the subventricular zone (SVZ) of adult rats and performed a functional genomic analysis to identify candidate genes to mediate its effect. 10 candidate genes were identified by microarray analysis and further validated by qRT-PCR. Of these, Fibulin-2 was increased 477-fold and its inhibition by siRNA blocks TGF-beta1 pro-neurogenic effect. Curiously, Fibulin-2 was not expressed by aNSC but by a GFAP-positive population in the culture, suggesting an indirect mechanism of action. TGF-beta1 also induced Fibulin-2 in the SVZ in vivo. Interestingly, 5 out of the 10 candidate genes identified are known to interact with integrins, paving the way for exploring their functional role in adult neurogenesis. In conclusion, we have identified 10 genes with putative pro-neurogenic effects, 5 of them related to integrins and provided proof that Fibulin-2 is a major mediator of the pro-neurogenic effects of TGF-beta1. These data should contribute to further exploring the molecular mechanism of adult neurogenesis of the genes identified and the involvement of the integrin pathway on adult neurogenesis.

Targeted Inhibition of PAI-1 Activity Impairs Epithelial Migration and Wound Closure Following Cutaneous Injury

Objective: Aberrant plasminogen activator inhibitor-1 (PAI-1) expression and activity have been implicated in bleeding disorders, multiorgan fibrosis, and wound healing anomalies. This study details the physiological consequences of targeted PAI-1 functional inhibition on cutaneous injury repair. Approach: Dorsal skin wounds from FVB/NJ mice, created with a 4 mm biopsy punch, were treated topically with the small-molecule PAI-1 antagonist tiplaxtinin (or vehicle control) for 5 days and then analyzed for markers of wound repair. Results: Compared to controls, tiplaxtinin-treated wounds displayed dramatic decreases in wound closure and re-epithelialization. PAI-1 immunoreactivity was evident at the migratory front in all injury sites indicating these effects were due to PAI-1 functional blockade and not PAI-1 expression changes. Stimulated HaCaT keratinocyte migration in response to recombinant PAI-1 in vitro was similarly attenuated by tiplaxtinin. While tiplaxtinin had no effect on keratinocyte proliferation, cell cycle progression, or apoptosis, it effectively reduced collagen deposition, the number of Ki-67⁺ fibroblasts, and incidence of differentiated myofibroblasts (i.e., smooth muscle α-actin immunoreactive cells), but not fibroblast apoptosis. Innovation: The role for PAI-1 in hemostasis and fibrinolysis is established; involvement of PAI-1 in cutaneous wound healing, however, remains unclear. This study tests the effect of a small-molecule PAI-1 inhibitor in a murine model of skin wound repair. Conclusion: Loss of PAI-1 activity significantly impaired wound closure. Re-epithelialization and fibroblast recruitment/differentiation were both reduced in tiplaxtinin-treated mice. Therapies directed at manipulation of PAI-1 expression and/or activity may have applicability as a treatment option for chronic wounds and scarring disorders.

Crucial role of posttranslational modifications of integrin α3 in interstitial lung disease and nephrotic syndrome

Interstitial lung disease, nephrotic syndrome and junctional epidermolysis bullosa is an autosomal recessive multiorgan disorder caused by mutations in the gene for the integrin α3 subunit (ITGA3). The full spectrum of manifestations and genotype-phenotype correlations is still poorly characterized. Here, we uncovered the disease-causing role and the molecular mechanisms underlying a homozygous ITGA3 mutation leading to the single amino acid substitution, p.R463W. The patient suffered from respiratory distress and episodes of cyanosis with onset in the first week of life and had a nephrotic syndrome. Although there was no clinical evidence for cutaneous fragility, the analysis of a skin sample and of skin epithelial cells enabled the direct assessment of the authentic mutant protein. We show that the mutation altered the conformation of the extracellular β-propeller domain of the integrin α3 subunit preventing correct processing of N-linked oligosaccharides, heterodimerization with β1 integrin and maturation through cleavage into heavy and light chains in the Golgi. Confocal microscopy demonstrated that the mutant protein accumulated intracellularly, but it was not present in focal adhesions or on the cell membrane as shown by flow cytometry. These findings highlight that single amino acid changes in the integrin α3 subunit may crucially alter the structure and complex processing of this integrin, completely preventing its functionality. The present report also underscores that ITGA3 mutations may account for atypical cases solely with early onset respiratory and renal involvement.

Integrin α3β1 signaling through MEK/ERK determines alternative polyadenylation of the MMP-9 mRNA transcript in immortalized mouse keratinocytes

Integrin α3β1 is highly expressed in both normal and tumorigenic epidermal keratinocytes where it regulates genes that control cellular function and extracellular matrix remodeling during normal and pathological tissue remodeling processes, including wound healing and development of squamous cell carcinoma (SCC). Previous studies identified a role for α3β1 in immortalized and transformed keratinocytes in the regulation of genes that promote tumorigenesis, invasion, and pro-angiogenic crosstalk to endothelial cells. One such gene, matrix metalloproteinase-9 (MMP-9), is induced by α3β1 through a post-transcriptional mechanism of enhanced mRNA stability. In the current study, we sought to investigate the mechanism through which α3β1 controls MMP-9 mRNA stability. First, we utilized a luciferase reporter assay to show that AU-rich elements (AREs) residing within the 3’-untranslated region (3’-UTR) of the MMP-9 mRNA renders the transcript unstable in a manner that is independent of α3β1. Next, we cloned a truncated variant of the MMP-9 mRNA which is generated through usage of an alternative, upstream polyadenylation signal and lacks the 3’-UTR region containing the destabilizing AREs. Using an RNase protection assay to distinguish “long” (full-length 3’-UTR) and “short” (truncated 3’-UTR) MMP-9 mRNA variants, we demonstrated that the shorter, more stable mRNA that lacks 3’-UTR AREs was preferentially generated in α3β1-expressing keratinocytes compared with α3β1-deficient (i.e., α3-null) keratinocytes. Moreover, we determined that α3β1-dependent alternative polyadenylation was acquired by immortalized keratinocytes, as primary neonatal keratinocytes did not display α3β1-dependent differences in the long and short transcripts. Finally, pharmacological inhibition of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway in α3β1-expressing keratinocytes caused a shift towards long variant expression, while Raf-1-mediated activation of ERK in α3-null keratinocytes dramatically enhanced short variant expression, indicating a role for ERK/MAPK signaling in α3β1-mediated selection of the proximal polyadenylation site. These findings identify a novel mode of integrin α3β1-mediated gene regulation through alternative polyadenylation.

Fibulin-2 is involved in early extracellular matrix development of the outgrowing mouse mammary epithelium

Cell-matrix interactions control outgrowth of mammary epithelium during puberty and pregnancy. We demonstrate here that the glycoprotein fibulin-2 (FBLN2) is strongly associated with pubertal and early pregnant mouse mammary epithelial outgrowth. FBLN2 was specifically localized to the cap cells of the terminal end buds during puberty and to myoepithelial cells during very early pregnancy (days 2-3) even before morphological changes to the epithelium become microscopically visible, but was down-regulated thereafter. Exposure to exogenous oestrogen (E2) or E2 plus progesterone (P) increased Fbln2 mRNA expression in the pubertal gland, indicating hormonal control. FBLN2 was co-expressed and co-localised with the proteoglycan versican (VCAN) and co-localised with laminin (LN), while over-expression of FBLN2 in HC-11 cells increased cell adhesion to several extracellular matrix proteins including LN and fibronectin, but not collagens. Mammary glands from Fbln2 knockout mice showed no obvious phenotype but increased fibulin-1 (FBLN1) staining was detected, suggesting a compensatory mechanism by other fibulin family members. We hypothesise that similar to embryonic aortic smooth muscle development, FBLN2 and VCAN expression alters the cell-matrix interaction to allow mammary ductal outgrowth and development during puberty and to enable epithelial budding during pregnancy.

Regulation of fibulin-2 gene expression by integrin α3β1 contributes to the invasive phenotype of transformed keratinocytes

The laminin-binding integrin α3β1 is highly expressed in epidermal keratinocytes, where it regulates both cell-autonomous and paracrine functions that promote wound healing and skin tumorigenesis. However, the roles for α3β1 in regulating gene expression programs that control the behaviors of immortalized or transformed keratinocytes remain underexplored. In the current study, we used a microarray approach to identify genes that are regulated by α3β1 in immortalized keratinocytes. α3β1-Responsive genes included several genes that are involved in extracellular matrix proteolysis or remodeling, including fibulin-2 and secreted protein acidic and rich in cysteine. However, α3β1-dependent induction of specific target genes was influenced by the genetic lesion that triggered immortalization, as α3β1-dependent fibulin-2 expression occurred in cells immortalized by either SV40 large T antigen or p53-null mutation, whereas α3β1-dependent expression of secreted protein acidic and rich in cysteine occurred only in the former cells. Interestingly, quantitative PCR arrays did not reveal strong patterns of α3β1-dependent gene expression in freshly isolated primary keratinocytes, suggesting that this regulation is acquired during immortalization. p53-null keratinocytes transformed with oncogenic RasV12 retained α3β1-dependent fibulin-2 expression, and RNAi-mediated knockdown of fibulin-2 in these cells reduced invasion, although not their tumorigenic potential. These findings demonstrate a prominent role for α3β1 in immortalized/transformed keratinocytes in regulating fibulin-2 and other genes that promote matrix remodeling and invasion.

Integrin Regulation of Epidermal Functions in Wounds

Significance: Integrins are bidirectional signaling receptors for extracellular matrix that regulate both inside-out signaling that controls keratinocyte-mediated changes to the wound microenvironment and outside-in signaling that controls keratinocyte responses to microenvironmental changes. As such, integrins represent attractive therapeutic targets for treatment of chronic wounds or general promotion of wound healing. Advances in wound management are particularly important as the elderly and diabetic populations within the United States continue to grow. Recent Advances: Although integrins are best known for mediating cell adhesion and migration, integrins in wound epidermis also control cell survival, proliferation, matrix remodeling, and paracrine crosstalk to other cellular compartments of the wound. Importantly, the concept of targeting integrins in the clinic has been established for treatment of certain cancers and other diseases, laying the groundwork for similar exploitation of integrins as targets to treat chronic wounds. Critical Issues: Despite their attractiveness as therapeutic targets, integrins have complex roles in wound healing that are impacted by both their own expression and a highly dynamic wound microenvironment that determines ligand availability. Therefore, identifying relevant integrin ligands in the wound and understanding both distinct and overlapping functions that different integrins play in the epidermis will be critical to determine their precise roles in wound healing. Future Directions: Future research should focus on gaining a thorough understanding of the highly coordinated functions of different integrins in wound epidermis, and on determining which of these functions go awry in pathological wounds. This focus should facilitate development of integrin-targeting therapeutics for treating chronic wounds.