Characterization of the oxytocin system in primary human dermal fibroblasts and keratinocytes

Abstract

The aim of this doctoral thesis was to characterize the oxytocin (OXT) system in human skin and to analyze its functional activity and relevance in cutaneous homeostasis. Apart from a recent study in 2012 showing OXT expression and release by epidermal keratinocytes (1), the OXT system in human skin has not been explored so far.The results obtained in the present study show that both OXT and its receptor are expressed by primary human dermal fibroblasts and keratinocytes. OXT induced dose-dependent calcium fluxes in both cell types, demonstrating that the OXT receptor (OXTR) is functional. In addition, analyses of dermal fibroblasts show that expression of the OXT system can be enhanced by its own components, suggesting its regulation in a positive feedback loop. Previous studies have reported on increased OXT levels in plasma after massage-like stroking in rats (3), as well as hugs (4) and suckling (2) in humans. Here, in an in vivo study, tactile stimulation of the skin increased OXT concentrations in suction blister fluids. While former studies reported only on elevated systemic OXT release, this study demonstrates local cutaneous OXT release. In summary, these data show that the OXT system can be modulated in vitro and in vivo in human skin cells.This work further reveals a functional role for the OXT system in the modulation of cutaneous proliferation and structure. OXT decreased proliferation of dermal fibroblasts and keratinocytes in a dose-dependent manner. Moreover, continuous OXT treatment reduced the epidermal layer thickness in a 3-D skin model. In the same model, OXT increased the expression of the terminal differentiation marker loricrin in the epidermis. Whether the reduction in epidermal layer thickness was due to OXT-mediated alteration of proliferation or of cell differentiation remains to be elucidated.The OXT system appears also to be involved in neuroendocrine, oxidative and inflammatory stress responses of the skin. Corticotropin-releasing hormone (CRH) and corticosterone are central mediators of the systemic/cutaneous neuroendocrine stress response. OXTR knockdown in dermal fibroblasts resulted in an increased expression of CRH and its receptor. Additionally, OXT treatment of keratinocytes decreased the release of corticosterone. Therefore, it is hypothezised that the OXT system counteracts neuroendocrine stress in the skin. Furthermore, OXTR knockdown in dermal fibroblasts and keratinocytes led to elevated levels of reactive oxygen species and reduced levels of glutathione, pointing to a cytoprotective role of the cutaneous OXT system. This is also assumed to apply to inflammation, as OXTR-depleted keratinocytes exhibited an increased release of the pro-inflammatory cytokines IL6, CCL5 and CXCL10.Numerous skin disorders are associated with abnormal proliferation and stress responses with atopic dermatitis as one of the most prominent representatives. Therefore, it was examined whether the OXT system might be aberrantly expressed in atopic compared to healthy skin. In fact, a downregulation of the OXT system in peri-lesional and lesional atopic skin was detected.Taken together, the presented findings indicate that the OXT system modulates key processes which are dysregulated in atopic dermatitis, including proliferation, inflammation and oxidative stress responses. In conclusion, the data of this thesis suggest that the OXT system is a novel neuroendocrine mediator in human skin homoeostasis and relevant to stressed skin conditions like atopic dermatitis.