Low UVB Fluences Augment Microvesicle Particle Generation in Keratinocytes

The acid sphingomyelinase inhibitor imipramine enhances the release of UV photoproduct-containing DNA in small extracellular vesicles in UVB-irradiated human skin

Nucleic acids, lipids, and other cell components can be found within different types of extracellular vesicles (EVs), which include apoptotic bodies (ABs), large extracellular vesicles (LEVs), and small extracellular vesicles (SEVs). Release of LEVs from cells can be reduced by genetic or pharmacological inhibition of the enzyme acid sphinogomyelinase (aSMase), and indeed several studies have demonstrated a role for the clinically approved aSMase inhibitor imipramine in blocking LEV release, including in response to UVB exposure. Given that exposure of keratinocytes to UVB radiation results in the generation of UVR photoproducts in DNA that can subsequently be found in association with ABs and SEVs, we examined how imipramine impacts the release of extracellular DNA containing UVR photoproducts at an early time point after UVR exposure. Using several different model systems, including cultured keratinocytes in vitro, discarded human surgical skin ex vivo, and skin biopsies obtained from treated human subjects, these pilot studies suggest that imipramine treatment stimulates the release of CPD-containing, SEV-associated DNA. These surprising findings indicate that LEV and SEV generation pathways could be linked in UVB-irradiated cells and that imipramine may exacerbate the systemic effects of extracellular UVR-damaged DNA throughout the body.

Topical Photodynamic Therapy Generates Bioactive Microvesicle Particles: Evidence for a Pathway Involved in Immunosuppressive Effects

Although effective in treating actinic damage, topical photodynamic therapy (PDT) has been shown to be immunosuppressive through unknown mechanisms, which could potentially limit its effectiveness. Multiple types of environmental stressors, including PDT, can produce the immunosuppressive lipid mediator platelet-activating factor (PAF). Because PAF can produce subcellular microvesicle particles (MVPs), these studies tested whether PDT can generate PAF and MVP release and whether these are involved in PDT-induced immunosuppression. Previously, topical PDT using blue light and 5-aminolevulinic acid was found to be a potent stimulus for PAF production in mice and human skin explants and human patients, and we show that experimental PDT also generates high levels of MVP. PDT-generated MVPs were independent of the PAF receptor but were dependent on the MVP-generating enzyme acid sphingomyelinase. Patients undergoing topical PDT treatment to at least 10% of body surface area showed local and systemic immunosuppression as measured by inhibition of delayed-type hypersensitivity reactions. Finally, using a murine model of contact hypersensitivity, PDT immunosuppression was blocked by genetic and pharmacologic inhibition of acid sphingomyelinase and genetic inhibition of PAF receptor signaling. These studies describe a mechanism involving MVP through which PDT exerts immunomodulatory effects, providing a potential target to improve its effectiveness.

Platelet-activating factor and microvesicle particles as potential mediators for the toxicity associated with intoxicated thermal burn injury

Thermal burn injuries (TBIs) in patients who are alcohol-intoxicated result in greater morbidity and mortality. The systemic toxicity found in human patients, which includes both immediate systemic cytokine generation with multiple organ failure and a delayed systemic immunosuppression, has previously been replicated in mouse models combining ethanol and localized TBI. Though considerable insights have been provided with these models, the exact mechanisms for these pathologic effects are unclear. In this review, we highlight the roles of the lipid mediator platelet-activating factor (PAF) and subcellular microvesicle particle (MVP) release in response to intoxicated thermal burn injury (ITBI) as effectors in the pathology. Particularly, MVP is released from keratinocytes in response to PAF receptor (PAFR) activation due to excess PAF produced by ITBI. These subcellular particles carry and thus protect the metabolically labile PAF which enable binding of this potent lipid mediator to several key sites. We hypothesize that PAF carried by MVP can bind to PAFR within the gut, activating myosin light chain kinase (MLCK). The subsequent gut barrier dysfunction in response to MLCK activation then allows bacteria to invade the lymphatic system and, eventually, the bloodstream, resulting in sepsis and resultant dysregulated inflammation in multiple organs. PAF in MVP also activate the skin mast cell PAFR resulting in migration of this key effector cell to the lymph nodes to induce immunosuppression. This review thus provides a mechanism and potential therapeutic approaches for the increased toxicity and immunosuppressive outcomes of TBI in the presence of acute ethanol exposure.

UVB-Induced Microvesicle Particle Release and Its Effects on the Cutaneous Microenvironment

Ultraviolet B radiation (UVB) has profound effects on human skin that results in a broad spectrum of immunological local and systemic responses and is the major cause of skin carcinogenesis. One important area of study in photobiology is how UVB is translated into effector signals. As the skin is exposed to UVB light, subcellular microvesicle particles (MVP), a subtype of bioactive extracellular vesicles, are released causing a variety of local and systemic immunological effects. In this review, we highlight keratinocyte MVP release in keratinocytes in response to UVB. Specifically, Platelet-activating factor receptor agonists generated by UVB result in MVP released from keratinocytes. The downstream effects of MVP release include the ability of these subcellular particles to transport agents including the glycerophosphocholine-derived lipid mediator Platelet-activating factor (PAF). Moreover, even though UVB is only absorbed in the epidermis, it appears that PAF release from MVPs also mediates systemic immunosuppression and enhances tumor growth and metastasis. Tumor cells expressing PAF receptors can use this mechanism to evade chemotherapy responses, leading to treatment resistance for advanced cancers such as melanoma. Furthermore, novel pharmacological agents provide greater insight into the UVB-induced immune response pathway and a potential target for pharmacological intervention. This review outlines the need to more clearly elucidate the mechanism linking UVB-irradiation with the cutaneous immune response and its pathological manifestations. An improved understanding of this process can result in new insights and treatment strategies for UVB-related disorders from carcinogenesis to photosensitivity.

Evidence for microvesicle particles in UVB-mediated IL-8 generation in keratinocytes

Recent studies have implicated bioactive microvesicle particles (MVP) in the keratinocyte response to many environmental stressors, in partricular ultraviolet B radiation (UVB). The generation of MVP in response to UVB involves the Platelet-activating factor receptor (PAFR) and the enzyme acid sphingomyelinase (aSMase). As UVB generates some cytokines such as interleukin-8 (IL-8) in a PAFR-dependent manner, one question is if the production and release of IL-8 and MVP could be linked. Using the human keratinocyte-derived cell line HaCaT, the present in vitro studies indicate that pretreatment of HaCaT keratinocytes with PAFR agonist ester can synergize with low fluences of UVB to generate high levels of MVP as well as IL-8 protein. Treatment of cells with an aSMase pharmacologic inhibitor blocked both processes. These studies indicate the possibility that MVP could be involved in pathologic processes involving UVB-generated production of pro-inflammatory cytokines such as IL-8.