Phosphodiesterase 4 in inflammatory diseases: Effects of apremilast in psoriatic blood and in dermal myofibroblasts through the PDE4/CD271 complex

Targeted synthetic pharmacotherapy for psoriatic arthritis: state of the art

INTRODUCTION

In recent years, different studies regarding psoriatic arthritis (PsA) have shown the pathogenetic role of dysfunction of signaling pathways involving the phosphodiesterase-4 enzyme and transcription factors or enzymes belonging to the kinase (JAK)-signal family pathway. These also represent the target of several drugs known as targeted synthetic disease-modifying antirheumatic drugs (tsDMARDs).

AREAS COVERED

The authors performed a systematic literature search using the PubMed database, as well as through retrieving data from randomized controlled trials, their post-hoc analysis, and pooled data analysis on the efficacy and safety profile of the PDE4 inhibitor (PDE4i), apremilast, and the inhibitors of JAK (JAKis), tofacitinib, filgotinib, baricitinib, and upadacitinib, in PsA.

EXPERT OPINION

In PsA, the PDE4i, apremilast, and the JAKi, tofacitinib, are effective across multiple clinical domains and have an acceptable tolerability profile, thus expanding the treatment options available for PsA patients. Apremilast and tofacitinib show several advantages mainly represented by their oral administration, a fast onset of action, and a short half-life. Data on tsDMARDs in PsA are still limited, and randomized trials and real-life studies are advocated.

Inhibition of PDE4/PDE4B improves renal function and ameliorates inflammation in cisplatin-induced acute kidney injury

Nephrotoxicity is a known clinical complication of cisplatin that limits the use of this potent antitumor drug. Cyclic nucleotide phosphodiesterases (PDEs) play complex roles in physiology and pathology. PDE4, which is a member of the PDE family, has four subtypes (PDE4A-PDE4D), and PDE4B plays an important role in inflammation. Thus, in the present study, we investigated the effect of PDE4/PDE4B inhibition on renal function and inflammation in a cisplatin nephrotoxicity model. In mice, cisplatin enhanced mRNA and protein expression of PDE4B in renal tubules. After treatment with the PDE4 inhibitor cilomilast, cisplatin-induced renal dysfunction, renal tubular injury, tubular cell apoptosis, and inflammation were all improved. Next, after silencing PDE4B in vivo, we observed a protective effect against cisplatin nephrotoxicity similar to that of the PDE4 inhibitor. In vitro, cisplatin-induced renal tubular cell death was strikingly ameliorated by the PDE4 inhibitor and PDE4B knockdown along with the blockade of the inflammatory response. Considering the known roles of some cell survival pathways in antagonizing insults, we examined levels of PDE4-associated proteins sirtuin 1, phosphatidylinositol 3-kinase, and phosphorylated AKT in cisplatin-treated renal tubular cells with or without cilomilast treatment. Strikingly, cisplatin treatment downregulated the expression of the above proteins, and this effect was largely abolished by the PDE4 inhibitor. Together, these findings indicate the beneficial role of PDE4/PDE4B inhibition in treating cisplatin nephrotoxicity, possibly through antagonizing inflammation and restoring cell survival signaling pathways.

Cyclic nucleotide phosphodiesterases: New targets in the metabolic syndrome?

Metabolic diseases have a tremendous impact on human morbidity and mortality. Numerous targets regulating adenosine monophosphate kinase (AMPK) have been identified for treating the metabolic syndrome (MetS), and many compounds are being used or developed to increase AMPK activity. In parallel, the cyclic nucleotide phosphodiesterase families (PDEs) have emerged as new therapeutic targets in cardiovascular diseases, as well as in non-resolved pathologies. Since some PDE subfamilies inactivate cAMP into 5'-AMP, while the beneficial effects in MetS are related to 5'-AMP-dependent activation of AMPK, an analysis of the various controversial relationships between PDEs and AMPK in MetS appears interesting. The present review will describe the various PDE families, AMPK and molecular mechanisms in the MetS and discuss the PDEs/PDE modulators related to the tissues involved, thus supporting the discovery of original molecules and the design of new therapeutic approaches in MetS.

Synergistic effect of phosphodiesterase 4 inhibitor and serum on migration of endotoxin-stimulated macrophages

Macrophage migration is an essential step in host defense against infection and wound healing. Elevation of cAMP by inhibiting phosphodiesterase 4 (PDE4), enzymes that specifically degrade cAMP, is known to suppress various inflammatory responses in activated macrophages, but the role of PDE4 in macrophage migration is poorly understood. Here we show that the migration of Raw 264.7 macrophages stimulated with LPS was markedly and dose-dependently induced by the PDE4 inhibitor rolipram as assessed by scratch wound healing assay. Additionally, this response required the involvement of serum in the culture medium as serum starvation abrogated the effect. Further analysis revealed that rolipram and serum exhibited synergistic effect on the migration, and the influence of serum was independent of PDE4 mRNA expression in LPS-stimulated macrophages. Moreover, the enhanced migration by rolipram was mediated by activating cAMP/exchange proteins directly activated by cAMP (Epac) signaling, presumably via interaction with LPS/TLR4 signaling with the participation of unknown serum components. These results suggest that PDE4 inhibitors, together with serum components, may serve as positive regulators of macrophage recruitment for more efficient pathogen clearance and wound repair.

New biologics and small molecules in inflammatory bowel disease: an update

Inflammatory bowel disease (IBD) is a spectrum of immune-mediated inflammatory disorders with a complex multifactorial pathogenesis, where different pathways may predominate in different individuals. This complexity will most likely require a panoply of drugs targeting different pathways if one wants to treat to steroid-free sustained remission and mucosal healing. Presently, the mainstay of medical management of IBD is based on 5-aminosalicylates, corticosteroids, thiopurines, methotrexate, antitumor necrosis factor, anti-alpha4 beta7 (α4β7) integrin and anti-interleukin (IL)-12/IL-23 therapies. The discovery of new pathways involved in the pathogenesis of IBD resulted in new drugs targeting Janus kinase/signal transducers and activators of transcription, IL-6, spingosine-1-phosphate, and phosphodiesterase 4, among others. These new therapies might result in more advantageous safety profiles. Several of these new drugs have already been successfully tested in other inflammatory disorders, such as psoriasis or rheumatoid arthritis. In this review, evidence from phase II and phase III randomized controlled clinical trials in patients with IBD involving new biologicals and small molecules are summarized.

Inhibition of phosphodiesterase-4 attenuates murine ulcerative colitis through interference with mucosal immunity

BACKGROUND AND PURPOSE

Ulcerative colitis (UC) is an aetiologically refractory inflammatory disease, accompanied by dysfunction of the epithelial barrier and intestinal inflammation. Phosphodiesterase-4 (PDE4) serves as an intracellular proinflammatory enzyme, hydrolyzing and inactivating cAMP. Though PDE4 inhibitors have been approved for pulmonary and dermatological diseases, the role of PDE4 inhibition in modulating mucosal immunity in the intestine remains ill-defined. This study was designed to explore whether PDE4 inhibition by apremilast exerts protective effects in dextran sulfate sodium-induced murine UC.

EXPERIMENTAL APPROACH

Intestinal inflammation and disease severity were evaluated by morphological, histopathological and biochemical assays, and in vivo imaging. Expression of inflammatory mediators, components of PDE4-mediated pathways in colon and macrophages were determined using quantitative real-time PCR, ELISA, Luminex assay, immunostaining, or western blotting, along with siRNA knockdown. Immune cells in mesenteric lymph nodes and colonic lamina propria were analysed by flow cytometry.

KEY RESULTS

Apremilast attenuated clinical features of UC, suppressing microscopic colon damage, production of inflammatory mediators, oxidative stresses, and fibrosis. Apremilast also promoted epithelial barrier function and inhibited infiltration of immune cells into inflamed tissues, through decreasing expression of chemokines and chemokine receptors. Furthermore, in UC, PDE4A, PDE4B, and PDE4D were highly expressed in colon. Apremilast not only inhibited PDE4 isoform expression but also activated PKA-CREB and Epac-Rap1 pathways and subsequently suppressed MAPK, NF-κB, PI3K-mTOR, and JAK-STAT-SOCS3 activation.

CONCLUSION AND IMPLICATIONS

Inhibition of PDE4 by apremilast protected against UC, by interfering with mucosal immunity. These findings represent a promising strategy for regulating intestinal inflammation.

Increased Levels of cAMP by the Calcium-Dependent Activation of Soluble Adenylyl Cyclase in Parkin-Mutant Fibroblasts

Almost half of autosomal recessive early-onset parkinsonism has been associated with mutations in PARK2, coding for parkin, which plays an important role in mitochondria function and calcium homeostasis. Cyclic adenosine monophosphate (cAMP) is a major second messenger regulating mitochondrial metabolism, and it is strictly interlocked with calcium homeostasis. Parkin-mutant (Pt) fibroblasts, exhibiting defective mitochondrial respiratory/OxPhos activity, showed a significant higher value of basal intracellular level of cAMP, as compared with normal fibroblasts (CTRL). Specific pharmacological inhibition/activation of members of the adenylyl cyclase- and of the phosphodiesterase-families, respectively, as well as quantitative reverse transcription polymerase chain reaction (RT-qPCR) analysis, indicate that the higher level of cAMP observed in Pt fibroblasts can contribute to a higher level of activity/expression by soluble adenylyl cyclase (sAC) and to low activity/expression of the phosphodiesterase isoform 4 (PDE4). As Ca²⁺ regulates sAC, we performed quantitative calcium-fluorimetric analysis, showing a higher level of Ca²⁺ in the both cytosol and mitochondria of Pt fibroblasts as compared with CTRL. Most notably, inhibition of the mitochondrial Ca²⁺ uniporter decreased, specifically the cAMP level in PD fibroblasts. All together, these findings support the occurrence of an altered mitochondrial Ca²⁺-mediated cAMP homeostasis in fibroblasts with the parkin mutation.

The phosphodiesterase type 4 inhibitor roflumilast suppresses inflammation to improve diabetic bladder dysfunction rats

PURPOSE

To demonstrate that phosphodiesterase type 4 (PDE4) inhibitors could potentially treat diabetic bladder dysfunction (DBD) through modulation of the systemic inflammatory response.

METHODS

In this 6-week study, 60 female Sprague-Dawley rats were divided into three groups: (i) vehicle-treated control rats; (ii) vehicle-treated streptozocin (STZ)-injected rats; and (iii) roflumilast-treated STZ-injected rats. Oral roflumilast (5 mg/kg/day) was administered during the last 4 weeks of STZ injection to induce diabetes in the test group. At 6 weeks, a urodynamic study was performed in each group. The expression of nuclear factor kappa B (NF-κB), tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, and IL-1β in detrusor smooth muscle (DSM) was analyzed using quantitative reverse transcription-polymerase chain reaction and Western blotting.

RESULTS

A significant decrease in bodyweight and significant increases in bladder weight and blood glucose level were observed in the diabetic rats and were not ameliorated by roflumilast treatment. Cystometry showed the increased bladder capacity, voiding volume, residual urine volume, and voiding interval in the diabetic rats and the prevention of these changes by roflumilast. These changes were accompanied by significantly enhanced expression of NF-κB, TNF-α, IL-6, and IL-1β in DSM tissue from diabetic rats. Furthermore, roflumilast attenuated the expression of inflammatory factors in DSM tissue.

CONCLUSIONS

Oral treatment with roflumilast in diabetic rats improves bladder function and inhibits the expression of inflammatory factors in DSM tissue, indicating that PDE4 is a potential therapeutic target for DBD.

Clinical and molecular evaluation of therapy with the use of cyclosporine A in patients with psoriasis vulgaris

BACKGROUND

Psoriasis course involves increased secretion of pro-inflammatory cytokines, among others, a beta transforming growth factor (TGFβs) and its receptors. Cyclosporine A (CsA), an immunosuppressive medicine with the molecular mechanism of operation connected with the properties of cell cycle suppression, is often used in the treatment of severe forms of psoriasis. The efficacy of therapy is assessed based on the disease clinical progression indexes - Psoriasis Area and Severity Index (PASI), body surface area (BSA), and Dermatology Life Quality Index (DLQI). The aim of the study was the evaluation of the efficacy of the CsA treatment of patients with psoriasis vulgaris, based on the clinical parameters and an assessment of the expression profiles of TGFβs and TGFβRs, depending on the concurrent diabetes and metabolic syndrome.

METHODS

The group under study composed of 32 patients (15 with the metabolic syndrome, seven with diabetes) treated with CsA for 84 days. The molecular analysis included extraction of RNA, assessment of TGβF1-3, TGFβRI-III gene expression with the use of the RTqPCR method. The clinical assessment of the effects of this pharmacotherapy involved evaluation of the parameters: PASI, BSA, DLQI before therapy commencement, on the 42nd and 84th days of therapy.

RESULTS

A statistically significant change in the transcription activity of TGFβ1 in patients with and without diabetes (P = 0.018) and patients with and without metabolic syndrome (P = 0.023) was shown that on the 84th day of therapy.

CONCLUSIONS

TGFb1 may be claimed as the supplementary molecular marker to evaluate the efficacy of CsA therapy. It seems that systemic diseases have an effect on the efficacy of the applied pharmacotherapy and the course of psoriasis.

Phosphodiesterase-4 Inhibitors for the Treatment of Inflammatory Diseases

Phosphodiesterase-4 (PDE4), mainly present in immune cells, epithelial cells, and brain cells, manifests as an intracellular non-receptor enzyme that modulates inflammation and epithelial integrity. Inhibition of PDE4 is predicted to have diverse effects via the elevation of the level of cyclic adenosine monophosphate (cAMP) and the subsequent regulation of a wide array of genes and proteins. It has been identified that PDE4 is a promising therapeutic target for the treatment of diverse pulmonary, dermatological, and severe neurological diseases. Over the past decades, numerous PDE4 inhibitors have been designed and synthesized, among which roflumilast, apremilast, and crisaborole were approved for the treatment of inflammatory airway diseases, psoriatic arthritis, and atopic dermatitis, respectively. It is regrettable that the dramatic efficacies of a drug are often accompanied by adverse effects, such as nausea, emesis, and gastrointestinal reactions. However, substantial advances have been made to mitigate the adverse effects and obtain better benefit-to-risk ratio. This review highlights the dialectical role of PDE4 in drug discovery and the disquisitive details of certain PDE4 inhibitors to provide an overview of the topics that still need to be addressed in the future.

Targeted HFpEF therapy based on matchmaking of human and animal models

The diversity in clinical phenotypes and poor understanding of the underlying pathophysiology of heart failure with preserved ejection fraction (HFpEF) is the main reason why no effective treatments have been found yet. Targeted, instead of one size fits all, treatment seems the only promising approach for treating HFpEF. To be able to design a targeted, phenotype-specific HFpEF treatment, the matrix relating clinical phenotypes and underlying pathophysiological mechanisms has to be clarified. This review discusses the opportunities for additional evaluation of the underlying pathophysiological processes, e.g., to evaluate biological phenotypes on top of clinical routine, to guide us toward a phenotype-specific HFpEF treatment. Moreover, a translational approach with matchmaking of animal models to biological HFpEF phenotypes will be a valuable step to test the effectiveness of novel, targeted interventions in HFpEF. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/personalized-medicine-in-hfpef/ .

Altered expression of microRNA-23a in psoriatic arthritis modulates synovial fibroblast pro-inflammatory mechanisms via phosphodiesterase 4B

OBJECTIVES

To investigate the functional role of miR-23a in synovial fibroblasts (SFC) activation in psoriatic arthritis (PsA).

METHODS

Differential expression of the miR-23a-27a-24-2 cluster was identified by real-time quantitative PCR in PsA synovial tissue and peripheral blood mononuclear cells (PBMC) compared to osteoarthritis (OA) and correlated with disease activity. For regulation experiments, PsA synovial fibroblasts (SFC) were cultured with Toll-like receptor (TLR) ligands and pro-inflammatory cytokines. PsA SFC were transfected with a miR-23a inhibitor to assess the functional effect on migration, invasion and expression of pro-inflammatory meditators. The direct interaction between miR-23a and predicted target mRNA, phosphodiesterase 4B (PDE4B), was examined by luciferase reporter gene assay, with the expression and regulation confirmed by RT-PCR and western blot. A PDE4 inhibitor was used to analyse the function of PDE4B signalling in both miR-23a and Poly(I:C)-induced PsA SFC activation.

RESULTS

Synovial tissue expression of miR-23a was lower in PsA compared to OA and correlated inversely with disease activity and synovitis. TLR activation via Poly(I:C) and LPS, but not Pam3CSK4, significantly decreased miR-23a expression, with no significant effect observed in reponse to stimulation with pro-inflammatory cytokines. Decreased miR-23a expression enhanced PsA SFC migration, invasion and secretion of IL-6, IL-8, MCP-1, RANTES and VEGF. We identified PDE4B as a direct target of miR-23a and demonstrated enhanced mRNA and protein expression of PDE4B in anti-miR-23a transfected PsA SFC. Poly(I:C) and/or miR-23a-induced migration and enhanced cytokine expression was suppressed by the blockade of PDE4 signalling.

CONCLUSIONS

In PsA, dysregulated miR-23a expression contributes to synovial inflammation through enhanced SFC activation, via PDE4B signalling, and identifies a novel anti-inflammatory mechanism of PDE4 blockade.

Variances in the mRNA expression profile of TGF-β1-3 isoforms and its TGF-βRI-III receptors during cyclosporin a treatment of psoriatic patients

Introduction

Psoriasis is a chronic, immunologic, multi-factor inflammatory skin disease, strongly associated with a higher level of a number of cytokines, such as isoforms of transforming growth factor β (TGF-β1–3) and its receptors (TGF-βRI–III). One of the most popular and important drugs used to treat this disease is cyclosporin A (CsA).

Aim

The aim of this study was to investigate the expression of genes encoding the transforming growth factor (TGF)-β isoforms and receptors of the cytokine TGF-βRs in psoriatic patients during an 84-day long observation of the effects of cyclosporin A therapy. It made an attempt to determine the usefulness of testing mRNA expression of TGF-β1–3 and its receptors TGF-βRI–III as the supplementary molecular markers of lost sensitivity to the medicine.

Material and methods

The study group consisted of 32 patients with psoriasis (20 men and 12 women) treated with cyclosporin A. The changes in expression patterns of TGF-β1-3 and TGF-βRI-III were performed by real-time quantitative reverse transcription PCR (RTqPCR).

Results

The expression of TGF-β1-3 and TGF-βRI-III were detected in the whole period of therapy with CsA. Changes in transcriptional activities of TGF-β1–3 and TGF-βRI–III during pharmacotherapy were observed. Differences in the expression of these genes were found before and after 42 and 84 days of using CsA.

Conclusions

The changes in expression profiles of TGF-β1-3 and TGF-βRI-III during CsA therapy can be a useful molecular marker of lost sensitivity to the medicine.

Roflumilast, type 4 phosphodiesterase inhibitor, attenuates inflammation in rats with ulcerative colitis via down-regulation of iNOS and elevation of cAMP

BACKGROUND

Roflumilast (Rof), a phosphodiesterase 4 (PDE4) inhibitor, has been shown to be an effective agent in inflammatory diseases and marketed for chronic obstructive pulmonary disease.

OBJECTIVE

This study was conducted to examine the potential anti-inflammatory effects of Rof in dextran sulphate sodium (DSS)-induced ulcerative colitis (UC) in rats and to investigate the molecular mechanisms underlying these effects.

METHODS

Forty male Wistar rats were divided into four groups: normal control, colitis group (rats received 5% DSS in their drinking water continuously for 7 days), Rof group, and sulfasalazine (SLZ) group. The Rof (5 mg/kg) and SLZ (500 mg/kg) groups underwent pretreatment with DSS one week ahead of DSS challenge and parallel with DSS. Colitis was determined by assessing colon length, weight loss, histologic colon score, quantifying the concentration of tumor necrosis factor alpha (TNF-α), nitric oxide (NO), cyclic adenosine monophosphate (cAMP), myeloperoxidase (MPO) activity and inducible nitric oxide synthase (iNOS) gene expression in colon tissue.

RESULTS

Rof attenuated the severity of colitis as evidenced by increased colon length, prevention of body weight loss, and improved colon histologic score compared to DSS group. Rof also suppressed the inflammatory response induced in DSS colitis group by decreasing colon concentration of TNF-α, NO and MPO activity and down- regulation of iNOS gene expression. The level of cAMP was increased by Rof compared to DSS group. The obtained results of Rof were comparable to those exerted by SLZ.

CONCLUSION

These findings revealed the beneficial effects of Rof in alleviating inflammation in DSS colitis.

Treatment of obesity-associated overactive bladder by the phosphodiesterase type-4 inhibitor roflumilast

PURPOSE

To prove that phosphodiesterase type-4 inhibitors could potentially treat obesity-associated overactive bladder through modulation of the systemic inflammatory response.

METHODS

In this 12-week study, 90 female Sprague-Dawley rats were divided into three groups: (1) vehicle-treated normal diet (ND)-fed rats; (2) vehicle-treated high-fat diet (HFD)-fed rats; and (3) roflumilast-treated HFD-fed rats. Oral roflumilast (5 mg/kg/day) was administered during the last 4 weeks of HFD feeding in the test group. At 12 weeks, a urodynamic study was performed in ten rats of each group. Bladder tissue was extracted, the bladder mucosa was separated under microscopy, and bladder detrusor smooth muscle (DSM) expression of TNF-α, interleukin (IL)-6, IL-1β, and nuclear factor kappa B (NF-κB) were analyzed using Western blotting and quantitative reverse transcription-polymerase chain reaction (qRT-PCR).

RESULTS

Bodyweights of the HFD-fed rats significantly increased and were not ameliorated by roflumilast treatment. Cystometry evidenced augmented frequency and non-void contractions in obese rats that were also prevented by roflumilast. These alterations were accompanied by a markedly increased expression of TNF-α, IL-6, IL-1β, and NF-κB in DSM of obese rats. Furthermore, roflumilast decreased expression of inflammatory factors in DSM.

CONCLUSIONS

Oral treatment with roflumilast in rats fed an HFD restores normal bladder function and downregulates expression of inflammatory factors in the bladder.

The phosphodiesterase 4 inhibitor apremilast inhibits Th1 but promotes Th17 responses induced by 6-sulfo LacNAc (slan) dendritic cells

BACKGROUND

The phosphodiesterase 4 (PDE4) inhibitor apremilast increases cellular cAMP levels and has proven effective in the treatment of psoriasis and psoriasis arthritis. We recently described 6-sulfo LacNAc dendritic cells (slanDCs) as immature DCs in blood and as a subset of inflammatory dermal DCs in psoriasis with a pronounced capacity to produce proinflammatory cytokines and to program Th17/Th1 T cell responses.

OBJECTIVE

The aim of this study was to investigate possible immune regulatory effects of the PDE4 inhibitor apremilast on slanDCs.

METHODS

In vitro studies were performed analyzing the effects of apremilast on the proinflammatory function of slanDCs and their capacity to induce Th1/Th17-biased T cell responses.

RESULTS

Increasing cAMP levels in slanDCs by PDE4 inhibition strongly reduced production of IL-12 and TNF-α. In line with these findings, co-culture experiments with apremilast-pulsed slanDCs and allogeneic T cells either from psoriasis patients or healthy controls, revealed a significant reduction of IFN-γ production and expression of the transcription factor T-bet. In parallel, production of IL-23 and IL-1ß by slanDCs was increased and co-cultured T cells revealed a largely augmented IL-17 production and an upregulated RORyt expression.

CONCLUSIONS

We here demonstrate anti-inflammatory as well as Th17-promoting effects of apremilast when studying blood precursors of human inflammatory dermal dendritic cells. In the concert of the broad anti-inflammatory effects of apremilast on keratinocytes, fibroblasts and endothelial cells, the dual effect on slan⁺ inflammatory dermal DCs should be taken into account and may constrain therapeutic responses.

p75 Neurotrophin Receptor in the Skin: Beyond Its Neurotrophic Function

p75 neurotrophin receptor (p75^NTR), also known as CD271, is the low-affinity receptor that, together with the tyrosine kinase receptor tropomyosin-receptor kinase (Trk), mediate neurotrophin (NT) functions. Beside their classic role in skin innervation, NT and their receptors constitute a complex cutaneous network associated with a number of autocrine and paracrine activities. In this context, the role of p75^NTR is becoming more and more important. This review will focus on the intriguing functions of p75^NTR in healthy and diseased skin. First, p75^NTR counterbalances the proliferative and survival activities of its cognate receptor Trk by inducing keratinocyte apoptosis. In addition, p75^NTR identifies an early transit-amplifying (TA) keratinocyte population and plays a critical role in keratinocyte stem cell transition to its progeny as well as in epidermal differentiation. p75^NTR is absent in psoriatic TA cells, thus rendering these cells resistant to apoptosis. On the other hand, p75^NTR infection restores NT-induced apoptosis in psoriatic keratinocytes. Taken together, these results provide evidence for a critical role of p75^NTR in epidermal homeostasis, while its lack may account for the TA defect in psoriasis. While the issue of p75^NTR as a marker of melanoma initiating cells is still to be solved, there is strong evidence that downregulation of this receptor is a precondition to melanoma invasion and metastasis in vitro and in vivo. All in all, this review points to p75^NTR as a major actor in both physiologic and pathologic conditions at the skin level.

Upregulation of ANGPTL6 in mouse keratinocytes enhances susceptibility to psoriasis

Psoriasis is a chronic inflammatory skin disease marked by aberrant tissue repair. Mutant mice modeling psoriasis skin characteristics have provided useful information relevant to molecular mechanisms and could serve to evaluate therapeutic strategies. Here, we found that epidermal ANGPTL6 expression was markedly induced during tissue repair in mice. Analysis of mice overexpressing ANGPTL6 in keratinocytes (K14-Angptl6 Tg mice) revealed that epidermal ANGPTL6 activity promotes aberrant epidermal barrier function due to hyperproliferation of prematurely differentiated keratinocytes. Moreover, skin tissues of K14-Angptl6 Tg mice showed aberrantly activated skin tissue inflammation seen in psoriasis. Levels of the proteins S100A9, recently proposed as therapeutic targets for psoriasis, also increased in skin tissue of K14-Angptl6 Tg mice, but psoriasis-like inflammatory phenotypes in those mice were not rescued by S100A9 deletion. This finding suggests that decreasing S100A9 levels may not ameliorate all cases of psoriasis and that diverse mechanisms underlie the condition. Finally, we observed enhanced levels of epidermal ANGPTL6 in tissue specimens from some psoriasis patients. We conclude that the K14-Angptl6 Tg mouse is useful to investigate psoriasis pathogenesis and for preclinical testing of new therapeutics. Our study also suggests that ANGPTL6 activation in keratinocytes enhances psoriasis susceptibility.