Cathelicidin LL-37 Activates Human Keratinocyte Autophagy through the P2X₇, Mechanistic Target of Rapamycin, and MAPK Pathways

Ionotropic purinergic receptor 7 (P2X7) channel structure and pharmacology provides insight regarding non-nucleotide agonism

P2X7 is a member of the Ionotropic Purinergic Receptor (P2X) family. The P2X family of receptors is composed of seven (P2X1-7), ligand-gated, nonselective cation channels. Changes in P2X expression have been reported in multiple disease models. P2Xs have large complex extracellular domains that function as receptors for a variety of ligands, including endogenous and synthetic agonists and antagonists. ATP is the canonical agonist. ATP affinity ranges from nanomolar to micromolar for most P2XRs, but P2X7 has uniquely poor ATP affinity. In many physiological settings, it may be difficult to achieve the millimolar extracellular ATP concentrations needed for P2X7 channel activation; however, channel function is implicated in pain sensation, immune cell function, cardiovascular disease, cancer, and osteoporosis. Multiple high-resolution P2X7 structures have been solved in apo-, ATP-, and antagonist-bound states. P2X7 structural data reveal distinct allosteric and orthosteric antagonist-binding sites. Both allosteric and orthosteric P2X7 antagonists are well documented to inhibit ATP-evoked channel current. However, a growing body of evidence supports P2X7 activation by non-nucleotide agonists, including extracellular histone proteins and human cathelicidin-derived peptides (LL-37). Interestingly, P2X7 non-nucleotide agonism is not inhibited by allosteric antagonists, but is inhibited by orthosteric antagonists. Herein, we review P2X7 function with a focus on the efficacy of available pharmacology on P2X7 channel current activation by non-nucleotide agonists in effort to understand agonist/antagonist efficacy, and consider the impact of these data on the current understanding of P2X7 in physiology and disease given these limitations of P2X7-selective antagonists and incomplete knockout mouse models.

Cathelicidin LL-37 promotes wound healing in diabetic mice by regulating TFEB-dependent autophagy

Diabetic patients often experience impaired wound healing. Human cathelicidin LL-37 possesses various biological functions, such as anti-microbial, anti-inflammatory, and pro-wound healing activities. Autophagy has important effects on skin wound healing. However, little is known about whether LL-37 accelerates diabetic wound healing by regulating autophagy. In the study, we aimed to investigate the role of autophagy in LL-37-induced wound healing and uncover the underlying mechanisms involved. A full-thickness wound closure model was established in diabetic mice to evaluate the effects of LL-37 and an autophagy inhibitor (3-MA) on wound healing. The roles of LL-37 and 3-MA in regulating keratinocyte migration were assessed using transwell migration and wound healing assays. The activation of transcription factor EB (TFEB) was measured using western blotting and immunofluorescence (IF) assays of its nuclear translocation. The results showed that LL-37 treatment improved wound healing in diabetic mice, whereas these effects were reversed by 3-MA. In vitro, 3-MA decreased the effects of LL-37 on promoting HaCat keratinocyte migration in the presence of high glucose (HG). Mechanistically, LL-37 promoted TFEB activation and resulted in subsequent activation of autophagy, as evidenced by increased nuclear translocation of TFEB and increased expression of ATG5, ATG7, and beclin 1 (BECN1), whereas these changes were blocked by TFEB knockdown. As expected, TFEB knockdown damaged the effects of LL-37 on promoting keratinocyte migration. Collectively, these results suggest that LL-37 accelerates wound healing in diabetic mice by activating TFEB-dependent autophagy, providing new insights into the mechanism by which LL-37 promotes diabetic wound healing.

Calcitriol modulates epidermal tight junction barrier function in human keratinocytes

BACKGROUND

The aberrant expression of tight junction (TJ) proteins play an important role in several diseases with impaired skin barriers, including atopic dermatitis, psoriasis, and chronic wounds. The evidence provided thus far suggests an important role of calcitriol in skin homeostasis. However, it is not known whether calcitriol improves the impaired skin barrier.

OBJECTIVE

To investigate the effect of calcitriol on TJ barrier function in human primary keratinocytes.

METHODS

Normal human primary keratinocytes were stimulated with calcitriol, and the expression of TJ-related proteins was measured by real-time PCR and Western blotting. Immunofluorescence was used to examine the intercellular distribution of TJ-related proteins. TJ barrier function was assessed by the transepithelial electrical resistance (TER) assay.

RESULTS

We demonstrated that calcitriol increased the expression levels of TJ-related proteins, including claudin-4, claudin-7, occludin, and zonula occludens (ZO)- 1. Calcitriol enhanced the distribution of TJ-related proteins at cellcell borders and induced the phosphorylation of pathways involved in the regulation of TJ barrier function, such as atypical protein kinase C (aPKC), Ras-related C3 botulinum toxin substrate 1 (Rac1), phosphoinositide 3-kinase (PI3K), and protein kinase B (Akt), as evidenced by the effects of specific inhibitors on the above pathways. Indeed, we confirmed that calcitriol enhanced TER in keratinocyte monolayers.

CONCLUSION

These findings showed that calcitriol could modify the expression of keratinocyte TJ proteins, contributing to the maintenance of homeostatic barrier function.

Skin barrier-inflammatory pathway is a driver of the psoriasis-atopic dermatitis transition

As chronic inflammatory conditions driven by immune dysregulation are influenced by genetics and environment factors, psoriasis and atopic dermatitis (AD) have traditionally been considered to be distinct diseases characterized by different T cell responses. Psoriasis, associated with type 17 helper T (Th17)-mediated inflammation, presents as well-defined scaly plaques with minimal pruritus. AD, primarily linked to Th2-mediated inflammation, presents with poorly defined erythema, dry skin, and intense itching. However, psoriasis and AD may overlap or transition into one another spontaneously, independent of biological agent usage. Emerging evidence suggests that defects in skin barrier-related molecules interact with the polarization of T cells, which forms a skin barrier-inflammatory loop with them. This loop contributes to the chronicity of the primary disease or the transition between psoriasis and AD. This review aimed to elucidate the mechanisms underlying skin barrier defects in driving the overlap between psoriasis and AD. In this review, the importance of repairing the skin barrier was underscored, and the significance of tailoring biologic treatments based on individual immune status instead of solely adhering to the treatment guidelines for AD or psoriasis was emphasized.

Rosacea pathogenesis and therapeutics: current treatments and a look at future targets

Rosacea is a chronic inflammatory skin condition associated with a significant health and economic burden from costs and loss of productivity due to seeking medical treatment. The disease encompasses multiple phenotypic manifestations involving a complex and multi-variate pathogenesis. Although the pathophysiology of rosacea is not completely understood, ongoing research is continually elucidating its mechanisms. In this review, current concepts of rosacea pathogenesis will be addressed which involve skin barrier and permeability dysfunction, the innate and adaptive immune systems, and the neurovascular system. More specifically, the cathelicidin pathway, transient potential receptor channels, mast cells, and the NLRP3 inflammasome pathway are various targets of current pharmacologic regimens. Future therapies may seek different mechanisms to act on current treatment targets, like the potential use of JAK/STAT inhibitors in ameliorating skin barrier dysfunction or TLR antagonists in alleviating cathelicidin mediated inflammation. Other potential treatments aim for entirely different molecular targets such as microvesicle particle mediated local and systemic inflammation. Ultimately rosacea is associated with a significant health and economic burden which warrants deeper research into its pathogenesis and resultant new treatment discovery.

The P2X7 Receptor in Oncogenesis and Metastatic Dissemination: New Insights on Vesicular Release and Adenosinergic Crosstalk

The tumor niche is an environment rich in extracellular ATP (eATP) where purinergic receptors have essential roles in different cell subtypes, including cancer, immune, and stromal cells. Here, we give an overview of recent discoveries regarding the role of probably the best-characterized purinergic receptor in the tumor microenvironment: P2X7. We cover the activities of the P2X7 receptor and its human splice variants in solid and liquid cancer proliferation, dissemination, and crosstalk with immune and endothelial cells. Particular attention is paid to the P2X7-dependent release of microvesicles and exosomes, their content, including ATP and miRNAs, and, in general, P2X7-activated mechanisms favoring metastatic spread and niche conditioning. Moreover, the emerging role of P2X7 in influencing the adenosinergic axis, formed by the ectonucleotidases CD39 and CD73 and the adenosine receptor A2A in cancer, is analyzed. Finally, we cover how antitumor therapy responses can be influenced by or can change P2X7 expression and function. This converging evidence suggests that P2X7 is an attractive therapeutic target for oncological conditions.