Endoglin haploinsufficiency promotes fibroblast accumulation during wound healing through Akt activation

Endothelial Activin Receptor-Like Kinase 1 (ALK1) Regulates Myofibroblast Emergence and Peritubular Capillary Stability in the Early Stages of Kidney Fibrosis

Renal tubulo-interstitial fibrosis is characterized by the excessive accumulation of extracellular matrix (ECM) in the tubular interstitium during chronic kidney disease. The main source of ECM proteins are emerging and proliferating myofibroblasts. The sources of myofibroblasts in the renal tubular interstitium have been studied during decades, in which the epithelial contribution of the myofibroblast population through the epithelial-to-mesenchymal (EMT) process was assumed to be the major mechanism. However, it is now accepted that the EMT contribution is very limited and other mechanisms such as the proliferation of local resident fibroblasts or the transdifferentiation of endothelial cells seem to be more relevant. Activin receptor-like kinase 1 (ALK1) is a type I receptor which belongs to the transforming growth factor beta (TGF-β) superfamily, with a key role in tissue fibrosis and production of ECM by myofibroblast. Predominantly expressed in endothelial cells, ALK1 also plays an important role in angiogenesis and vessel maturation, but the relation of these processes with kidney fibrosis is not fully understood. We show that after 3 days of unilateral ureteral obstruction (UUO), ALK1 heterozygous mice (Alk1^+/−) display lower levels of kidney fibrosis associated to a lower number of myofibroblasts. Moreover, Alk1^+/− mice have a lower degree of vascular rarefaction, showing improved peritubular microvasculature after UUO. All these data suggest an important role of ALK1 in regulating vascular rarefaction and emergence of myofibroblasts.

Endoglin in the Spotlight to Treat Cancer

A spotlight has been shone on endoglin in recent years due to that fact of its potential to serve as both a reliable disease biomarker and a therapeutic target. Indeed, endoglin has now been assigned many roles in both physiological and pathological processes. From a molecular point of view, endoglin mainly acts as a co-receptor in the canonical TGFβ pathway, but also it may be shed and released from the membrane, giving rise to the soluble form, which also plays important roles in cell signaling. In cancer, in particular, endoglin may contribute to either an oncogenic or a non-oncogenic phenotype depending on the cell context. The fact that endoglin is expressed by neoplastic and non-neoplastic cells within the tumor microenvironment suggests new possibilities for targeted therapies. Here, we aimed to review and discuss the many roles played by endoglin in different tumor types, as well as the strong evidence provided by pre-clinical and clinical studies that supports the therapeutic targeting of endoglin as a novel clinical strategy.

Effect of dermal fibroblasts and mesenchymal stem cells seeded on an amniotic membrane scaffold in skin regeneration: A case series

BACKGROUND

Skin wound healing has always been a challenging subject as it involves the coordinated functioning of various cells and molecules. Any disorder in wound healing can cause healing failure and result in chronic wounds. In this study, we hypothesized that co-cultured dermal fibroblasts (DFs) and Wharton's jelly mesenchymal stem cells (WJ-MSCs) seeded on an acellular amniotic membrane scaffold could be used to promote skin regeneration in chronic ulcers.

MATERIALS AND METHODS

In this case series, the chronic wounds of five diabetic patients aged between 30 and 60 years were treated with co-cultured WJ-MSCs and DFs seeded on an acellular amniotic membrane. Treatment was applied and the wound healing process was evaluated every three days for nine days, with the patients being subsequently followed up for one month. The wound healing percentage, time taken for the wound to heal, and wound size were monitored.

RESULTS

The mean wound healing rate (WHR) increased progressively in all lesions. The mean percentage of wound healing after transplantation of the biological scaffold enriched with WJ-MSCs and autologous DFs after treatment was 93.92%, respectively. The healing percentage significantly increased after three days; significant decreases in wound size and healing time were recorded after six and nine days of treatment, respectively (p < 0.002); and total skin regeneration and re-epithelialization were achieved by the ninth day of treatment. There were no side effects or complications.

CONCLUSION

Given the current problems and complications presented by chronic wounds, Novel Clinical approaches involving cell therapy and tissue engineering can be regarded as an attractive therapeutic option for the treatment of chronic and difficult-to-heal wounds.

BMP Receptor Inhibition Enhances Tissue Repair in Endoglin Heterozygous Mice

Hereditary hemorrhagic telangiectasia type 1 (HHT1) is a severe vascular disorder caused by mutations in the TGFβ/BMP co-receptor endoglin. Endoglin haploinsufficiency results in vascular malformations and impaired neoangiogenesis. Furthermore, HHT1 patients display an impaired immune response. To date it is not fully understood how endoglin haploinsufficient immune cells contribute to HHT1 pathology. Therefore, we investigated the immune response during tissue repair in Eng+/- mice, a model for HHT1. Eng+/- mice exhibited prolonged infiltration of macrophages after experimentally induced myocardial infarction. Moreover, there was an increased number of inflammatory M1-like macrophages (Ly6Chigh/CD206-) at the expense of reparative M2-like macrophages (Ly6Clow/CD206+). Interestingly, HHT1 patients also showed an increased number of inflammatory macrophages. In vitro analysis revealed that TGFβ-induced differentiation of Eng+/- monocytes into M2-like macrophages was blunted. Inhibiting BMP signaling by treating monocytes with LDN-193189 normalized their differentiation. Finally, LDN treatment improved heart function after MI and enhanced vascularization in both wild type and Eng+/- mice. The beneficial effect of LDN was also observed in the hind limb ischemia model. While blood flow recovery was hampered in vehicle-treated animals, LDN treatment improved tissue perfusion recovery in Eng+/- mice. In conclusion, BMPR kinase inhibition restored HHT1 macrophage imbalance in vitro and improved tissue repair after ischemic injury in Eng+/- mice.

Transforming growth factor-β in tissue fibrosis

TGF-β is extensively implicated in the pathogenesis of fibrosis. In fibrotic lesions, spatially restricted generation of bioactive TGF-β from latent stores requires the cooperation of proteases, integrins, and specialized extracellular matrix molecules. Although fibroblasts are major targets of TGF-β, some fibrogenic actions may reflect activation of other cell types, including macrophages, epithelial cells, and vascular cells. TGF-β–driven fibrosis is mediated through Smad-dependent or non-Smad pathways and is modulated by coreceptors and by interacting networks. This review discusses the role of TGF-β in fibrosis, highlighting mechanisms of TGF-β activation and signaling, the cellular targets of TGF-β actions, and the challenges of therapeutic translation.

Botulinum toxin type A prevents the phenotypic transformation of fibroblasts induced by TGF‑β1 via the PTEN/PI3K/Akt signaling pathway

Hypertrophic scar (HS) is a common type of dermatosis. Botulinum toxin type A (BTXA) can exert an anti-HS effect; however, the regulatory mechanisms underlying this effect remain unclear. Thus, the aim of this study was to examine the effects of BTXA on phosphatase and tensin homolog deleted on chromosome ten (PTEN) expression and the fibroblast phenotypic transformation induced by transforming growth factor (TGF)-β1, which is an important regulatory factor involved in the process of HS. For this purpose, fibroblasts were treated with various concentrations of BTXA and then treated with 10 ng/ml of TGF-β1 with gradient concentrations of BTXA. The proliferation and apoptosis of fibroblasts were measured by cell counting kit-8 assay (CCK-8) and flow cytometry, respectively. PTEN methylation was analyzed by methylation-specific PCR (MSP) and DNA methyltransferase (DNMT) activity was determined using a corresponding kit. RT-qPCR and western blot analysis were performed to detect the transcription and translation levels. The results revealed that BTXA suppressed the proliferation and increased the apoptosis of fibroblasts treated with TGF-β1 in a dose-dependent manner. BTXA in combination with TGF-β1 suppressed the expression of molecules related to the extracellular matrix (ECM), epithelial-mesenchymal transition (EMT) and apoptosis. BTXA reduced the PTEN methylation level and downregulated the expression levels of methylation-associated genes. BTXA also inhibited the phosphorylation of phosphoinositide 3-kinase (PI3K) and Akt. On the whole, the findings of this study indicate that BTXA may inhibit fibroblast phenotypic transformation by regulating PTEN methylation and the phosphorylation of related pathways. The findings of this study can provide a theoretical basis for HS treatment.

Adipose extracellular matrix promotes skin wound healing by inducing the differentiation of adipose‑derived stem cells into fibroblasts

Fibroblasts are the major effector cells of skin wound healing. Adipose-derived stem cells can differentiate into fibroblasts under certain conditions. In the present study, it was hypothesized that adipose-derived stem cells (ADSCs) could be induced by the adipose extracellular matrix (ECM) to differentiate into fibroblasts in order to promote skin wound healing. First, flow cytometry was used to detect the ratio of fibroblasts and relative expression of the fibroblast markers cytokeratin 19 (CK19) and vimentin in ADSCs. Then, the effect of the adipose ECM during the differentiation of ADSCs into fibroblasts was investigated by detecting the total amount of collagen fibers and degree of fibrosis, and the proliferation and cell cycle of differentiated fibroblasts, using the MTT assay and flow cytometry analysis respectively. Finally, a mouse skin wound model was established and treated with PBS, ADSC suspension or ECM + ADSCs to compare wound healing rate and expression of collagen I and collagen III by immunohistochemistry. Following induction of ADSCs with the adipose ECM, more fibroblasts were found, expression of CK19 and vimentin increased, and a greater degree of fibrosis occurred, which revealed the positive effect of the adipose ECM on the differentiation of ADSCs into fibroblasts. In addition, the induced fibroblasts had enhanced proliferation activity, with more cells in the S phase and fewer in the G2/M phase. The in vivo experiment indicated that the ECM produced by the ADSCs had a faster wound healing rate and increased expression of collagen I and collagen III compared with mice injected with PBS or ADSCs alone, which verified that ADSCs induced by the adipose ECM had a positive effect on skin wound healing. The present study demonstrated that the adipose ECM in combination with ADSCs may be a novel therapeutic target for the repair of skin injury, due to the ability of the adipose ECM to induce the differentiation of ADSCs into fibroblasts and to facilitate the wound healing process.

Endoglin haploinsufficiency is associated with differential regulation of extracellular matrix production during skin fibrosis and cartilage repair in mice

Transforming growth factor (TGF)-β is a multifunctional growth factor with potent pro-fibrotic effects. Endoglin is a TGF-β co-receptor that strongly regulates TGF-β signaling in a variety of cell types. Although aberrant regulation of TGF-β signaling is known to play a key role in fibrotic diseases such as scleroderma and impaired cartilage repair, the significance of endoglin function in regulating these processes is poorly understood. Here we examined whether endoglin haploinsufficiency regulates extracellular (ECM) protein expression and fibrotic responses during bleomycin induced skin fibrosis and surgically induced osteoarthritis, using endoglin-heterozygous (Eng+/-) mice and wild-type (Eng+/+) littermates. Skin fibrosis was induced by injecting mice intradermally with bleomycin or vehicle. Osteoarthritis was induced surgically by destabilization of medial meniscus. Dermal thickness, cartilage integrity and ECM protein expression were then determined. Eng+/- mice subjected to bleomycin challenge show a marked decrease in dermal thickness (P < 0.005) and reduced collagen content and decreased collagen I, fibronectin, alpha-smooth muscle actin levels as compared to Eng+/+ mice, both under basal and bleomycin treated conditions. Eng+/- mice undergoing surgically induced osteoarthritis show no differences in the degree of cartilage degradation, as compared to Eng+/+ mice, although chondrocytes isolated from Eng^+/- display markedly enhanced collagen II levels. Our findings suggest that endoglin haploinsufficiency in mice ameliorates bleomycin-induced skin fibrosis suggesting that endoglin represents a pro-fibrotic factor in the mouse skin. However, endoglin haploinsufficiency does not protect these mice from surgically indiced cartilage degradation, demonstrating differential regulation of endoglin action during skin and cartilage repair.

Leucine-rich α-2 glycoprotein promotes lung fibrosis by modulating TGF-β signaling in fibroblasts

TGF‐β has an important role in fibrotic diseases, including idiopathic pulmonary fibrosis (IPF). Detailed analysis of TGF‐β signaling in pulmonary fibrosis at the molecular level is needed to identify novel therapeutic targets. Recently, leucine‐rich alpha‐2 glycoprotein (LRG) was reported to function as a modulator of TGF‐β signaling in angiogenesis and tumor progression. However, the involvement of LRG in fibrotic disorders, including IPF, has not yet been investigated. In this study, we investigated the role of LRG in fibrosis by analyzing LRG knockout (KO) mice with bleomycin‐induced lung fibrosis, an animal model of pulmonary fibrosis. The amount of LRG in the lungs of wild‐type (WT) mice was increased by bleomycin administration prior to fibrosis development. In LRG KO mice, lung fibrosis was significantly suppressed, as indicated by attenuated Masson's trichrome staining and lower collagen content than those in WT mice. Moreover, in the lungs of LRG KO mice, phosphorylation of Smad2 was reduced and expression of α‐SMA was decreased relative to those in WT mice. In vitro experiments indicated that LRG enhanced the TGF‐β‐induced phosphorylation of Smad2 and the expression of Serpine1 and Acta2, the downstream of Smad2, in fibroblasts. Although endoglin, an accessory TGF‐β receptor, is essential for LRG to promote TGF‐β signaling in endothelial cells during angiogenesis, we found that endoglin did not contribute to the ability of LRG to enhance Smad2 phosphorylation in fibroblasts. Taken together, our data suggest that LRG promotes lung fibrosis by modulating TGF‐β‐induced Smad2 phosphorylation and activating profibrotic responses in fibroblasts.

Circulating soluble endoglin modifies the inflammatory response in mice

Inflammation is associated with every health condition, and is an important component of many pathologies such as cardiovascular diseases. Circulating levels of soluble endoglin have been shown to be higher in the serum of patients with cardiovascular diseases with a significant inflammatory component. The aim of this study was to evaluate the implication of circulating soluble endoglin in the inflammatory response. For this purpose, a transgenic mouse expressing human soluble endoglin (sEng+) was employed, and three different inflammatory approaches were used to mimic inflammatory conditions in different tissues. This study shows that control sEng+ mice have a normal inflammatory state. The lung and kidney injury induced by the inflammatory agents was reduced in sEng+ mice, especially the intra-alveolar and kidney infiltrates, suggesting a possible reduction in inflammation induced by soluble endoglin. To deepen into this possible effect, the leukocyte number in the bronchoalveolar lavage and air pouch lavage was evaluated and a significant reduction of neutrophil infiltration in LPS-treated lungs and ischemic kidneys from sEng+ with respect to WT mice was observed. Additionally, the mechanisms through which soluble endoglin prevents inflammation were studied. We found that in sEng+ animals the increment of proinflammatory cytokines, TNFα, IL1β and IL6, induced by the inflammatory stimulus was reduced. Soluble endoglin also prevents the augmented adhesion molecules, ICAM, VCAM and E-selectin induced by the inflammatory stimulus. In addition, vascular permeability increased by inflammatory agents was also reduced by soluble endoglin. These results suggest that soluble endoglin modulates inflammatory-related diseases and open new perspectives leading to the development of novel and targeted approaches for the prevention and treatment of cardiovascular diseases.

Absence of K-Ras Reduces Proliferation and Migration But Increases Extracellular Matrix Synthesis in Fibroblasts

The involvement of Ras-GTPases in the development of renal fibrosis has been addressed in the last decade. We have previously shown that H- and N-Ras isoforms participate in the regulation of fibrosis. Herein, we assessed the role of K-Ras in cellular processes involved in the development of fibrosis: proliferation, migration, and extracellular matrix (ECM) proteins synthesis. K-Ras knockout (KO) mouse embryonic fibroblasts (K-ras(-/-) ) stimulated with transforming growth factor-β1 (TGF-β1) exhibited reduced proliferation and impaired mobility than wild-type fibroblasts. Moreover, an increase on ECM production was observed in K-Ras KO fibroblasts in basal conditions. The absence of K-Ras was accompanied by reduced Ras activation and ERK phosphorylation, and increased AKT phosphorylation, but no differences were observed in TGF-β1-induced Smad signaling. The MEK inhibitor U0126 decreased cell proliferation independently of the presence of K-ras but reduced migration and ECM proteins expression only in wild-type fibroblasts, while the PI3K-AKT inhibitor LY294002 decreased cell proliferation, migration, and ECM synthesis in both types of fibroblasts. Thus, our data unveil that K-Ras and its downstream effector pathways distinctively regulate key biological processes in the development of fibrosis. Moreover, we show that K-Ras may be a crucial mediator in TGF-β1-mediated effects in this cell type. J. Cell. Physiol. 231: 2224-2235, 2016. © 2016 Wiley Periodicals, Inc.

BMP9 Crosstalk with the Hippo Pathway Regulates Endothelial Cell Matricellular and Chemokine Responses

Endoglin is a type III TGFβ auxiliary receptor that is upregulated in endothelial cells during angiogenesis and, when mutated in humans, results in the vascular disease hereditary hemorrhagic telangiectasia (HHT). Though endoglin has been implicated in cell adhesion, the underlying molecular mechanisms are still poorly understood. Here we show endoglin expression in endothelial cells regulates subcellular localization of zyxin in focal adhesions in response to BMP9. RNA knockdown of endoglin resulted in mislocalization of zyxin and altered formation of focal adhesions. The mechanotransduction role of focal adhesions and their ability to transmit regulatory signals through binding of the extracellular matrix are altered by endoglin deficiency. BMP/TGFβ transcription factors, SMADs, and zyxin have recently been implicated in a newly emerging signaling cascade, the Hippo pathway. The Hippo transcription coactivator, YAP1 (yes-associated protein 1), has been suggested to play a crucial role in mechanotransduction and cell-cell contact. Identification of BMP9-dependent nuclear localization of YAP1 in response to endoglin expression suggests a mechanism of crosstalk between the two pathways. Suppression of endoglin and YAP1 alters BMP9-dependent expression of YAP1 target genes CCN1 (cysteine-rich 61, CYR61) and CCN2 (connective tissue growth factor, CTGF) as well as the chemokine CCL2 (monocyte chemotactic protein 1, MCP-1). These results suggest a coordinate effect of endoglin deficiency on cell matrix remodeling and local inflammatory responses. Identification of a direct link between the Hippo pathway and endoglin may reveal novel mechanisms in the etiology of HHT.

The role of endoglin in kidney fibrosis

Tubulointerstitial fibrosis and glomerulosclerosis, are a major feature of end stage chronic kidney disease (CKD), characterised by an excessive accumulation of extracellular matrix (ECM) proteins. Transforming growth factor beta-1 (TGF-β1) is a cytokine with an important role in many steps of renal fibrosis such as myofibroblast activation and proliferation, ECM protein synthesis and inflammatory cell infiltration. Endoglin is a TGF-β co-receptor that modulates TGF-β responses in different cell types. In numerous cells types, such as mesangial cells or myoblasts, endoglin regulates negatively TGF-β-induced ECM protein expression. However, recently it has been demonstrated that 'in vivo' endoglin promotes fibrotic responses. Furthermore, several studies have demonstrated an increase of endoglin expression in experimental models of renal fibrosis in the kidney and other tissues. Nevertheless, the role of endoglin in renal fibrosis development is unclear and a question arises: Does endoglin protect against renal fibrosis or promotes its development? The purpose of this review is to critically analyse the recent knowledge relating to endoglin and renal fibrosis. Knowledge of endoglin role in this pathology is necessary to consider endoglin as a possible therapeutic target against renal fibrosis.

Potential role of Akt signaling in chronic kidney disease

Renal fibrosis, particularly tubulointerstitial fibrosis, is the common final outcome of almost all chronic kidney diseases. However, the mechanisms involved in the development of renal fibrosis are poorly understood. The Akt (also known as protein kinase B, PKB) family is serine/threonine protein kinases that play critical roles in regulating growth, proliferation, survival, metabolism and other cellular activities. Cytokines, high-glucose medium, transforming growth factor-β1 or advanced glycation end-products activate Akt in different renal cells. Increased Akt activation has been found in experimental tubulointerstitial fibrosis. In addition, Akt activation is also an important node in diverse signaling cascades involved in kidney damage. These data give evidence for a role for Akt in renal fibrosis, but no reviews are available on the role of Akt in the process. Thus, our aim is to review the role of Akt activation and signaling in renal fibrosis.

MicroRNA-21 regulates hTERT via PTEN in hypertrophic scar fibroblasts

BACKGROUND

As an important oncogenic miRNA, microRNA-21 (miR-21) is associated with various malignant diseases. However, the precise biological function of miR-21 and its molecular mechanism in hypertrophic scar fibroblast cells has not been fully elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

Quantitative Real-Time PCR (qRT-PCR) analysis revealed significant upregulation of miR-21 in hypertrophic scar fibroblast cells compared with that in normal skin fibroblast cells. The effects of miR-21 were then assessed in MTT and apoptosis assays through in vitro transfection with a miR-21 mimic or inhibitor. Next, PTEN (phosphatase and tensin homologue deleted on chromosome ten) was identified as a target gene of miR-21 in hypertrophic scar fibroblast cells. Furthermore, Western-blot and qRT-PCR analyses revealed that miR-21 increased the expression of human telomerase reverse transcriptase (hTERT) via the PTEN/PI3K/AKT pathway. Introduction of PTEN cDNA led to a remarkable depletion of hTERT and PI3K/AKT at the protein level as well as inhibition of miR-21-induced proliferation. In addition, Western-blot and qRT-PCR analyses confirmed that hTERT was the downstream target of PTEN. Finally, miR-21 and PTEN RNA expression levels in hypertrophic scar tissue samples were examined. Immunohistochemistry assays revealed an inverse correlation between PTEN and hTERT levels in high miR-21 RNA expressing-hypertrophic scar tissues.

CONCLUSIONS/SIGNIFICANCE

These data indicate that miR-21 regulates hTERT expression via the PTEN/PI3K/AKT signaling pathway by directly targeting PTEN, therefore controlling hypertrophic scar fibroblast cell growth. MiR-21 may be a potential novel molecular target for the treatment of hypertrophic scarring.

The stimulatory effects of Stewartia koreana extract on the proliferation and migration of fibroblasts and the wound healing activity of the extract in mice

Stewartia koreana (S. koreana) has been used in the treatment of inflammatory diseases, such as acute gastroenteritis and aches, in Korean folk medicine and has been reported to have a number of biological activities, such as anti-inflammatory activity and the promotion of angiogenesis. In this study, we aimed to determine the effects of S. koreana extract (SKE) and its components on dermal fibroblast growth and migration, and to investigate the wound healing activity of the extract in mice. In vitro experiments revealed that the numbers of SKE-treated cells increased by approximately 2.5-‑ and 3.7-fold with 50 and 100 µg/ml of SKE, respectively. 5-bromo-2'-deoxy-uridine (BrdU) incorporation was also increased in the SKE-treated cells by 2.3-fold. SKE promoted the migration of human skin fibroblasts and, among the isolated compounds, hyperin increased the proliferation and migration of the fibroblasts to almost the same degree as SKE. Western blot analysis demonstrated that SKE stimulated the MEK/ERK1/2 and PI3K/Akt signaling pathways. In in vivo experiments, the SKE-treated wound lesions of mice decreased by approximately 7% in diameter after 2 days of treatment with SKE compared with the wound lesions on the 1st day of the experiment. On the 9th day of treatment, the diameter of the lesions was further reduced by approximately 83% in the SKE-treated wound areas compared with the wound areas on the 1st day of treatment. Our results demonstrate that methanol extracts of S. koreana leaves promote the proliferation and migration of skin fibroblasts and possess effective wound healing activity through the activation of the MEK/ERK1/2 and PI3K/Akt signaling pathways. Hyperin was identified as an active compound responsible for the stimulation of fibroblast growth and migration.

Impaired wound repair in adult endoglin heterozygous mice associated with lower NO bioavailability

Endoglin (Eng) is a transmembrane glycoprotein that is mainly expressed in endothelial cells, but it is also present in the epidermis and skin appendages. To address the role of Eng in cutaneous wound healing, we compared the kinetics of reepithelialization in Eng heterozygous null (Eng(+/-)) mice and their normal littermates (Eng(+/+)) following skin wounds. The wound area was significantly larger in Eng(+/-) than in Eng(+/+) mice from 2 to 8 days after injury; overall wound closure was delayed by 1 to 2 days. In Eng(+/-) mice, keratinocytes at the wound edges exhibited impaired proliferation but were more migratory, as shown by their elongated morphology and increased keratin 17 expression. Inhibition of nitric oxide (NO) synthesis delayed healing in Eng(+/+) but not in Eng(+/-) mice. Administration of the NO donor LA-803 accelerated wound closure in Eng(+/-) mice, with no effect on normal littermates. The acute stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA) enhanced Eng expression in mouse epidermal keratinocytes in vivo and in vitro associated with hyperproliferation. Similarly, the skin of Eng(+/-) mice failed to mount a hyperplastic response to acute stimulation with TPA. These results demonstrate an important involvement of Eng in wound healing that is associated with NO bioavailability.