Expanding the Roles of Antimicrobial Peptides in Skin: Alarming and Arming Keratinocytes

IL-17A enhances vitamin D3-induced expression of cathelicidin antimicrobial peptide in human keratinocytes

Cathelicidin is strongly expressed in lesional skin in psoriasis and may play an important role as both an antimicrobial peptide and as an autoinflammatory mediator in this chronic skin disease. The mechanism of increased cathelicidin in psoriatic keratinocytes is not known, but recent observations have found that psoriasis has abundant Th17 cells that produce IL-17A and IL-22. We found that human keratinocytes stimulated with supernatants from T cells isolated from lesional psoriatic skin increased expression of cathelicidin when stimulated in the presence of 1,25-dihydroxyvitamin D(3) (1,25D(3)). This increase was signaled through the IL-17RA. In vitro, IL-17A, but not IL-22, enhanced cathelicidin mRNA and peptide expression in keratinocytes dependent on the presence of 1,25D(3). At the same time, coincubation with 1,25D(3) blocked induction of human beta-defensin 2 (HBD2), IL-6, and IL-8, which are other target genes of IL-17A. Act1, an adaptor associated with IL-17RA and essential for IL-17A signaling, mediated cathelicidin induction, as its suppression by small interfering RNA inhibited HBD2 and cathelicidin. Both, 1,25D(3) and IL-17A signaled cathelicidin induction through MEK-ERK. These results suggest that increased IL-17A in psoriatic skin increases cathelicidin through a vitamin D(3)-, Act1-, and MEK-ERK-dependent mechanism. Therapy targeting this cathelicidin-regulating system might be beneficial in patients suffering from psoriasis.

Double-stranded RNA induces an antiviral defense status in epidermal keratinocytes through TLR3-, PKR-, and MDA5/RIG-I-mediated differential signaling

Emerging evidence suggests an important role for human epidermal keratinocytes in innate immune mechanisms against bacterial and viral skin infections. The proinflammatory effect of viral infections can be mimicked by double-stranded RNA (dsRNA). Herein, we demonstrate that keratinocytes express all known dsRNA sensing receptors at a constitutive and inducible level, and that they use several downstream signaling pathways leading to a broad pattern of gene expression, not only proinflammatory and immune response genes under the control of NF-kappaB, but also genes under transcriptional control of IRF3. As a consequence, dsRNA, a stimulus for TLR3, protein kinase R (PKR), and the RNA helicases retinoic acid-inducible gene I (RIG-I) and MDA5, induces a status of antiviral defense in keratinocytes. Using inhibitors for the various dsRNA signaling pathways and specific small interfering RNA for TLR3, RIG-I, and MDA5, we demonstrated that in human keratinocytes, TLR3 seems to be necessary for NF-kappaB but not for IRF3 activation, whereas RIG-I and MDA5 are crucial for IRF3 activation. PKR is essential for the dsRNA response in both signaling pathways and thus represents the central antiviral receptor for dsRNA stimulation. Moreover, human keratinocytes up-regulate TLR7, the receptor for single-stranded RNA, in response to stimulation with dsRNA, which renders keratinocytes functionally responsive to the TLR7 agonist gardiquimod, a member of the imidazoquinoline antiviral immune response modifier family. Thus, in addition to building a physical barrier against infectious pathogens, keratinocytes are specially equipped with a full antiviral defense program that enables them to efficiently target viral infections of the skin.

The vitamin D pathway: a new target for control of the skin's immune response?

The surface of our skin is constantly challenged by a wide variety of microbial pathogens, still cutaneous infections are relatively rare. Within cutaneous innate immunity the production of antimicrobial peptides (AMPs) is a primary system for protection against infection. Many AMPs can be found on the skin, and these include molecules that were discovered for their antimicrobial properties, and other peptides and proteins first known for activity as chemokines, enzymes, enzyme inhibitors and neuropeptides. Cathelicidins were among the first families of AMPs discovered on the skin. They are now known to have two distinct functions; they have direct antimicrobial activity and will initiate a host cellular response resulting in cytokine release, inflammation and angiogenesis. Dysfunction of cathelicidin is relevant in the pathogenesis of several cutaneous diseases including atopic dermatitis where cathelicidin induction is suppressed, rosacea, where cathelicidin peptides are abnormally processed to forms that induce cutaneous inflammation and a vascular response, and psoriasis, where a cathelicidin peptide can convert self-DNA to a potent stimulus of an autoinflammatory cascade. Recent work has unexpectedly identified vitamin D3 as a major factor involved in the regulation of cathelicidin expression. Therapies targeting the vitamin D3 pathway and thereby cathelicidin may provide new treatment modalities in the management of infectious and inflammatory skin diseases.

What causes hidradenitis suppurativa?

Hidradenitis suppurativa (HS)--a rather common, very chronic and debilitating inflammatory skin appendage disorder with a notoriously underestimated burden of disease--has long been a playground for the high priests of nomenclature: Ask a bunch of eminent dermatologists and skin pathologists to publicly share their thoughts on what causes HS, and they will soon get entrenched in a heated debate on whether this historical term is a despicable misnomer. Fortunately, the recently founded Hidradenitis Suppurativa Foundation (HSF; http://www.hs-foundation.org), to which EXP DERMATOL serves as home journal, has broken with this unproductive tradition and has encouraged publication of the current CONTROVERSIES feature. This is exclusively devoted to discussing the pathobiology of this chronic neutrophilic folliculitis of unknown origin. Although traces of terminological bickering remain visible, it does the HS experts in our virtual debate room credit that they engage in a constructive and comprehensive dissection of potential pathogenesis pathways that may culminate in the clinical picture we know under the competing terms HS or acne inversa. These experts sketch more often complementary than mutually exclusive pathogenesis scenarios, and the outlines of a conceivable consensus on the many open pathobiology questions begin to emerge in these CONTROVERSIES. Hopefully, this heralds a welcome new tradition: to get to the molecular heart of HS pathogenesis, which can only be achieved by a renaissance of solid basic HS research, as the key to developing more effective HS therapy.

Antimicrobial peptides and the skin immune defense system

Our skin is constantly challenged by microbes but is rarely infected. Cutaneous production of antimicrobial peptides (AMPs) is a primary system for protection, and expression of some AMPs further increases in response to microbial invasion. Cathelicidins are unique AMPs that protect the skin through 2 distinct pathways: (1) direct antimicrobial activity and (2) initiation of a host response resulting in cytokine release, inflammation, angiogenesis, and reepithelialization. Cathelicidin dysfunction emerges as a central factor in the pathogenesis of several cutaneous diseases, including atopic dermatitis, in which cathelicidin is suppressed; rosacea, in which cathelicidin peptides are abnormally processed to forms that induce inflammation; and psoriasis, in which cathelicidin peptide converts self-DNA to a potent stimulus in an autoinflammatory cascade. Recent work identified vitamin D3 as a major factor involved in the regulation of cathelicidin. Therapies targeting control of cathelicidin and other AMPs might provide new approaches in the management of infectious and inflammatory skin diseases.

Vitamin D regulation of cathelicidin in the skin: toward a renaissance of vitamin D in dermatology?

1,25-Dihydroxyvitamin D(3), the active form of vitamin D, is a major regulator of the expression of the cationic antimicrobial peptide cathelicidin, not only in monocytes but also in epidermal keratinocytes. The involvement of cathelicidin in wound healing and skin diseases as diverse as psoriasis, rosacea, and atopic dermatitis may create new opportunities for the use of vitamin D in dermatology.

The epidermal growth factor receptor system in skin repair and inflammation

The epidermal growth factor (EGF) family comprises multiple mediators such as transforming growth factor-alpha, amphiregulin, heparin binding-EGF, and epiregulin, which are crucially involved in the tissue-specific proliferation/differentiation homeostasis. Typically, they act in an autocrine and paracrine manner on their specific cell membrane receptor and mount an effective reparative response to any attack to biophysical integrity. In addition, the EGFR can be activated by transactivation from a variety of G-protein-coupled receptors, integrins, and cytokine receptors, so that it acts as the major transducer of disparate cell functions, including changes in proliferation rate, cellular shape, attachment and motility, and regulation of proinflammatory activation. However, numerous experimental observations indicate that the different EGFR ligands are not redundant, but may rather provide distinct and specific contributions to keratinocyte functions. Importantly, increasing evidence now suggests that the EGFR pathway has a major impact on the inflammatory/immune reactions of the skin, in the apparent effort of enhancing innate immune defense while opposing overactivation of keratinocyte pro-inflammatory functions. This review covers the molecular mechanisms and functions activated by this major growth factor system in the regulation of keratinocyte biology and focuses on the complex contribution of EGFR signaling to the inflammatory processes in the skin.