C. Buerger

Interleukin-1β interferes with epidermal homeostasis through induction of insulin resistance: implications for psoriasis pathogenesis

Response pathways of the metabolic and the immune system have been evolutionary conserved, resulting in a high degree of integrated regulation. Insulin is a central player in the metabolic system and potentially also in the homeostasis of the skin. Psoriasis is a frequent and often severe autoimmune skin disease, clinically characterized by altered epidermal homeostasis, of which the molecular pathomechanisms are only little understood. In this study, we have examined a potential role for insulin signaling in the pathogenesis of this disease. We show that IL-1β is present in high quantities in tissue fluid collected via microdialysis from patients with psoriasis; these levels are reduced under successful anti-psoriatic therapy. Our results suggest that IL-1β contributes to the disease by dual effects. First, it induces insulin resistance through p38MAPK (mitogen-activated protein kinase), which blocks insulin-dependent differentiation of keratinocytes, and at the same time IL-1β drives proliferation of keratinocytes, both being hallmarks of psoriasis. Taken together, our findings point toward insulin resistance as a contributing mechanism to the development of psoriasis; this not only drives cardiovascular comorbidities, but also its cutaneous phenotype. Key cytokines inducing insulin resistance in keratinocytes and kinases mediating their effects may represent attractive targets for novel anti-psoriatic therapies.

Mammalian target of rapamycin and its downstream signalling components are activated in psoriatic skin.

Mammalian target of rapamycin (mTOR) signalling integrates signals leading to cellular growth, proliferation and differentiation. Disturbance of this tightly regulated interplay leads to malignancies, as reflected by altered mTOR signalling in epidermal tumours. As psoriatic keratinocytes also show features of perturbed cell growth and differentiation, the question arises as to whether mTOR signalling also plays a role in the pathogenesis of psoriasis.

Endothelial cells are highly heterogeneous at the level of cytokine-induced insulin resistance

The endothelial wall plays a crucial role in various diseases as it serves as the barrier between circulatory system and organ tissue. Inflammation‐driven insulin resistance and subsequent endothelial dysfunction represent a pathomechanism in cardiovascular diseases such as atherosclerosis and myocardial infarction. It was recently suggested that insulin resistance also contributes to the pathogenesis of psoriasis, a chronic inflammatory disease of the skin. However, it is not clear whether similar mechanisms at the endothelium contribute to the disease. In this study, we ask which endothelial cells are most suitable to address this question. We investigated the insulin response of four cell types (primary cells and cell lines) representing different vascular beds (micro‐ and macrovascular cells) in the presence of different pro‐inflammatory cytokines. All four cell types used responded well to insulin; however, the ability to become resistant to insulin due to an inflammatory stimulus by cytokines involved in psoriasis (e.g. IL‐1β, IL‐12, IL‐17, IL‐23 and TNF‐α) was very heterogeneous and could not be attributed to the differential expression of the cognate cytokine receptors. We conclude that this disparity is due to the different origins and properties of the endothelial cells used. Thus, endothelial cells should be carefully selected for the purpose of the respective study, particularly when it comes to analysing the pathogenesis of a disease and the search of new molecular targets for innovative therapies.

Inflammation dependent mTORC1 signaling interferes with the switch from keratinocyte proliferation to differentiation

Psoriasis is a frequent and often severe inflammatory skin disease, characterized by altered epidermal homeostasis. Since we found previously that Akt/mTOR signaling is hyperactivated in psoriatic skin, we aimed at elucidating the role of aberrant mTORC1 signaling in this disease. We found that under healthy conditions mTOR signaling was shut off when keratinocytes switch from proliferation to terminal differentiation. Inflammatory cytokines (IL-1β, IL-17A, TNF-α) induced aberrant mTOR activity which led to enhanced proliferation and reduced expression of differentiation markers. Conversely, regular differentiation could be restored if mTORC1 signaling was blocked. In mice, activation of mTOR through the agonist MHY1485 also led to aberrant epidermal organization and involucrin distribution. In summary, these results not only identify mTORC1 as an important signal integrator pivotal for the cells fate to either proliferate or differentiate, but emphasize the role of inflammation-dependent mTOR activation as a psoriatic pathomechanism.

Hypotonic stress response of human keratinocytes involves LRRC8A as component of volume-regulated anion channels

The barrier function of the human epidermis is constantly challenged by environmental osmotic fluctuations. Hypotonic stress triggers cell swelling, which is counteracted by a compensatory mechanism called regulatory volume decrease (RVD) involving volume‐regulated anion channels (VRACs). Recently, it was discovered that VRACs are composed of LRRC8 heteromers and that LRRC8A functions as the essential VRAC subunit in various mammalian cell types; however, the molecular identity of VRACs in the human epidermis remains to be determined. Here, we investigated the expression of LRRC8A and its role in hypotonic stress response of human keratinocytes. Immunohistological staining showed that LRRC8A is preferentially localized in basal and suprabasal epidermal layers. RNA sequencing revealed that LRRC8A is the most abundant subunit within the LRRC8 gene family in HaCaT cells as well as in primary normal human epidermal keratinocytes (NHEKs). To determine the contribution of LRRC8A to hypotonic stress response, we generated HaCaT‐ and NHEK‐LRRC8A knockout cells by using CRISPR‐Cas9. I− influx assays using halide‐sensitive YFP showed that LRRC8A is crucially important for mediating VRAC activity in HaCaTs and NHEKs. Moreover, cell volume measurements using calcein‐AM dye further revealed that LRRC8A also substantially contributes to RVD. In summary, our study provides new insights into hypotonic stress response and suggests an important role of LRRC8A as VRAC component in human keratinocytes.