Corrigendum to "Investigation of tryptophan to kynurenine degradation in response to interferon-γ in melanoma cell lines" [Biochem. Biophys. Rep. 37 (2024), 101612].

[This corrects the article DOI: 10.1016/j.bbrep.2023.101612.].

Investigation of tryptophan to kynurenine degradation in response to interferon-γ in melanoma cell lines.

Background and aim: Melanoma is a fatal form of skin cancer that carries a grave prognosis if the cancer cells spread and form metastases. The Kynurenine (Kyn) pathway is activated by the enzyme indoleamine 2,3-dioxygenase 1 (IDO-1) and has been shown to have a role in tumour progression. We have previously shown that interferon-γ (IFN-γ) acts as an inducer of tryptophan (Trp) degradation to Kyn in keratinocytes of the basal layer in a 3D epidermis model. Before extending our reconstructed human epidermis model to not only contain keratinocytes but also fibroblasts and melanocytes/melanoma cells, we have in this study set out to investigate possible differences between primary adult melanocytes and six melanoma cell lines regarding the expression of the immune checkpoint inhibitors IDO-1 and programmed death ligand 1 (PD-L1) together with Kyn production. Methods: The melanocytes and melanoma cells were stimulated with 1-20 ng/ml of IFN-γ and the levels of Trp to Kyn degradation were monitored with high-performance liquid chromatography (HPLC). To analyze the viability of the cell types after IFN-γ treatment, an MTT assay was performed. mRNA quantity of IDO-1, PD-L1 and IFN-γ receptor (IFN-GR1) was analyzed with qPCR. Results: After 24 h, only the metastatic cell line WM-266-4 was affected by all concentrations of IFN-γ, whereas at 48 h, the higher IFN-γ concentrations gave a more pronounced effect on the viability in all cell types. Trp was detected at various levels in the culture medium from all cell types before and after IFN-γ treatment. The degradation to Kyn was detected in primary melanocytes, Mel Juso, and Mel Ho cell lines after 24 h of treatment and low levels of IFN-γ. However, the higher concentration of IFN-γ, 20 ng/ml, induced Kyn to various degrees in all cell types after 24 h. The change in mRNA quantity of IDO-1 and PD-L1 was similar in all cell types. Conclusion: To conclude, no significant difference in upregulation of the immune checkpoint inhibitors PD-L1 and IDO-1 was seen between primary tumour and metastatic melanoma. IFN-γ stimulation of melanocytes and different stages of melanoma cell lines resulted in an increased Kyn/Trp ratio in the more aggressive melanoma cells when a high concentration was used (20 ng/ml) but when a lower concentration of IFN-γ (5 ng/ml) was used an increased Kyn/Trp ratio were detected in media from primary melanocytes and early-stage melanoma.

Non-Invasive, Topical Sampling of Potential, Low-Molecular Weight, Skin Cancer Biomarkers: A Study on Healthy Volunteers.

Monitoring of low-molecular weight cancer biomarkers, such as tryptophan (Trp) and its derivative kynurenine (Kyn), might be advantageous to non-invasive skin cancer detection. Thus, we assessed several approaches of topical sampling of Trp and Kyn, in relation to phenylalanine (Phe) and tyrosine (Tyr), on the volar forearm of six healthy volunteers. The sampling was performed with three hydrogels (made of agarose or/and chitosan), hydrated starch films, cotton swabs, and tape stripping. The biomarkers were successfully sampled by all approaches, but the amount of collected Kyn was low, 20 ± 10 pmol/cm2. Kyn quantification was below LOQ, and thus, it was detected only in 20% of topical samples. To mitigate variability problems of absolute amounts of sampled amino acids, Tyr/Trp, Phe/Trp, and Phe/Tyr ratios were assessed, proving reduced inter-individual variation from 79 to 45% and intra-individual variation from 42 to 21%. Strong positive correlation was found between Phe and Trp, pointing to the Phe/Trp ratio (being in the 1.0-2.0 range, at 95% confidence) being least dependent on sampling materials, approaches, and sweating. This study leads to conclusion that due to the difficulty in quantifying less abundant Kyn, and thus the Trp/Kyn ratio, the Phe/Trp ratio might be a possible, alternative biomarker for detecting skin cancers.

Hydrogels and Cubic Liquid Crystals for Non-Invasive Sampling of Low-Molecular-Weight Biomarkers-An Explorative In Vivo Study.

The molecular composition of human skin is altered due to diseases, which can be utilized for non-invasive sampling of biomarkers and disease diagnostics. For this to succeed, it is crucial to identify a sampling formulation with high extraction efficiency and reproducibility. Highly hydrated skin is expected to be optimal for increased diffusion of low-molecular-weight biomarkers, enabling efficient extraction as well as enhanced reproducibility as full hydration represents a well-defined endpoint. Here, the aim was to explore water-based formulations with high water activities, ensuring satisfactory skin hydration, for non-invasive sampling of four analytes that may serve as potential biomarkers, namely tryptophan, tyrosine, phenylalanine, and kynurenine. The included formulations consisted of two hydrogels (chitosan and agarose) and two different liquid crystalline cubic phases based on the polar lipid glycerol monooleate, which were all topically applied for 2 h on 35 healthy subjects in vivo. The skin status of all sampling sites was assessed by electrical impedance spectroscopy and transepidermal water loss, enabling explorative correlations between biophysical properties and analyte abundancies. Taken together, all formulations resulted in the successful and reproducible collection of the investigated biomarkers. Still, the cubic phases had an extraction capacity that was approximately two times higher compared to the hydrogels.

Franz cells for facile biosensor evaluation: A case of HRP/SWCNT-based hydrogen peroxide detection via amperometric and wireless modes.

Reducing animal use in biosensor research requires broader use of in vitro methods. In this work, we present a novel application of Franz cells suitable for biosensor development and evaluation in vitro. The work describes how Franz cell can be equipped with electrodes enabling characterization of biosensors in close proximity to skin. As an example of a sensor, hydrogen peroxide biosensor was prepared based on horseradish peroxidase (HRP)/single-walled carbon nanotube (SWCNT)-modified textile. The electrode exhibited lower detection limit of 0.3 µM and sensitivity of 184 µA mM-1 cm-2. The ability of this biosensor to monitor H2O2 penetration through skin and dialysis membranes was evaluated in Franz cell setup in amperometric and wireless modes. The results also show that catalase activity present in skin is a considerable problem for epidermal sensing of H2O2. This work highlights opportunities and obstacles that can be addressed by assessment of biosensors in Franz cell setup before progressing to their testing in animals and humans.

Non-invasive skin sampling of tryptophan/kynurenine ratio in vitro towards a skin cancer biomarker.

The tryptophan to kynurenine ratio (Trp/Kyn) has been proposed as a cancer biomarker. Non-invasive topical sampling of Trp/Kyn can therefore serve as a promising concept for skin cancer diagnostics. By performing in vitro pig skin permeability studies, we conclude that non-invasive topical sampling of Trp and Kyn is feasible. We explore the influence of different experimental conditions, which are relevant for the clinical in vivo setting, such as pH variations, sampling time, and microbial degradation of Trp and Kyn. The permeabilities of Trp and Kyn are overall similar. However, the permeated Trp/Kyn ratio is generally higher than unity due to endogenous Trp, which should be taken into account to obtain a non-biased Trp/Kyn ratio accurately reflecting systemic concentrations. Additionally, prolonged sampling time is associated with bacterial Trp and Kyn degradation and should be considered in a clinical setting. Finally, the experimental results are supported by the four permeation pathways model, predicting that the hydrophilic Trp and Kyn molecules mainly permeate through lipid defects (i.e., the porous pathway). However, the hydrophobic indole ring of Trp is suggested to result in a small but noticeable relative increase of Trp diffusion via pathways across the SC lipid lamellae, while the shunt pathway is proposed to slightly favor permeation of Kyn relative to Trp.

Reverse Iontophoretic Extraction of Skin Cancer-Related Biomarkers.

Non-invasive methods for early diagnosis of skin cancer are highly valued. One possible approach is to monitor relevant biomarkers such as tryptophan (Trp) and kynurenine (Kyn), on the skin surface. The primary aim of this in vitro investigation was, therefore, to examine whether reverse iontophoresis (RI) can enhance the extraction of Trp and Kyn, and to demonstrate how the Trp/Kyn ratio acquired from the skin surface reflects that in the epidermal tissue. The study also explored whether the pH of the receiver medium impacted on extraction efficiency, and assessed the suitability of a bicontinuous cubic liquid crystal as an alternative to a simple buffer solution for this purpose. RI substantially enhanced the extraction of Trp and Kyn, in particular towards the cathode. The Trp/Kyn ratio obtained on the surface matched that in the viable skin. Increasing the receiver solution pH from 4 to 9 improved extraction of both analytes, but did not significantly change the Trp/Kyn ratio. RI extraction of Trp and Kyn into the cubic liquid crystal was comparable to that achieved with simple aqueous receiver solutions. We conclude that RI offers a potential for non-invasive sampling of low-molecular weight biomarkers and further investigations in vivo are therefore warranted.

Visualisation of H2O2 penetration through skin indicates importance to develop pathway-specific epidermal sensing.

Elevated amounts of reactive oxygen species (ROS) including hydrogen peroxide (H2O2) are observed in the epidermis in different skin disorders. Thus, epidermal sensing of H2O2 should be useful to monitor the progression of skin pathologies. We have evaluated epidermal sensing of H2O2 in vitro, by visualising H2O2 permeation through the skin. Skin membranes were mounted in Franz cells, and a suspension of Prussian white microparticles was deposited on the stratum corneum face of the skin. Upon H2O2 permeation, Prussian white was oxidised to Prussian blue, resulting in a pattern of blue dots. Comparison of skin surface images with the dot patterns revealed that about 74% of the blue dots were associated with hair shafts. The degree of the Prussian white to Prussian blue conversion strongly correlated with the reciprocal resistance of the skin membranes. Together, the results demonstrate that hair follicles are the major pathways of H2O2 transdermal penetration. The study recommends that the development of H2O2 monitoring on skin should aim for pathway-specific epidermal sensing, allowing micrometre resolution to detect and quantify this ROS biomarker at hair follicles.Graphical abstract.

Effect of IFN-γ on the kynurenine/tryptophan ratio in monolayer-cultured keratinocytes and a 3D reconstructed human epidermis model.

BACKGROUND: Interferon-gamma (IFN-γ) represents a potent inducer for keratinocyte inflammatory and immune activation in vitro. Since tryptophan (trp) conversion to kynurenine (kyn) is involved in inflammation, the topical kyn/trp ratio may serve as a biomarker of skin inflammation. However, the trp metabolism in keratinocytes exposed to IFN-γ is not yet fully understood. OBJECTIVE: The aim of this study was to establish a human epidermis model in order to quantify cytokine and kyn/trp secretion from IFN-γ stimulated cells and tissues. Moreover, to compare the cell response of 2D-cultured keratinocytes and the 3D epidermis model. METHODS: Polycarbonate filters were used on which primary keratinocytes could attach and stratify in order to form the typical layers of reconstructed human epidermis (RHE). After IFN-γ treatment, secretion of kyn/trp was measured by high performance liquid chromatography. Gene and protein expression of indoleamine 2,3-dioxygenase 1 (IDO) was analyzed with real-time PCR and immunohistochemistry. The secretion of cytokines was quantified with ELISA. RESULTS: Trp catabolism to kyn was significantly increased (P < 0.01) in the 2D culture in response to IFN-γ treatment. Before kyn secretion, IDO was strongly upregulated (P < 0.001). IFN-γ treatment also increased the secretion of IL-6 and IL-8 from the keratinocytes. In the RHE, IDO was upregulated by IFN-γ, and kyn secretion could be detected. Interestingly, IDO expression was only present in the basal cells of the RHE. CONCLUSION: Our results suggest that IFN-γ acts as an inducer of trp degradation preferentially in undifferentiated keratinocytes, indicated by the IDO expression in the basal layer of the RHE.

Optimization of sample preparation for transporter protein quantification in tissues by LC-MS/MS.

BACKGROUND: Reproducible quantification of drug transporter protein expression in tissues is important for predicting transporter mediated drug disposition. Many mass-spectrometry based transporter protein quantification methods result in high variability of the estimated transporter quantities. Therefore, we aimed to evaluate and optimize mass spectrometry-based quantification method for drug transporter proteins in tissues. MATERIALS AND METHODS: Plasma membrane (PM) proteins from mouse tissues were isolated by applying three extraction protocols: commercial plasma membrane extraction kit, tissue homogenization by Potter-Elvehjem homogenizer in combination with sucrose-cushion ultracentrifugation, and PM enrichment with Tween 40. Moreover, five different protein digestion protocols were applied on the same PM fraction. PM isolation and digestion protocols were evaluated by measuring the amount of transporter proteins by liquid chromatography-tandem mass spectrometry in selected reaction monitoring mode. RESULTS: Mouse liver homogenization by Potter-Elvehjem homogenizer in combination with sucrose-cushion ultracentrifugation and PM enrichment with Tween 40 resulted in two times higher transporter protein quantity (Breast cancer resistance protein (Bcrp) 18.0 fmol/µg protein) in comparison with the PM samples isolated by extraction kit (Bcrp 9.8 fmol/µg protein). The evaluation of protein digestion protocols revealed that the most optimal protocol for PM protein digestion is with Lys-C and trypsin, in combination with trypsin enhancer and heat denaturation. Overall, quantities of Bcrp and Na+/K + ATPase proteins evaluated in mouse liver and kidney cortex by using our optimized PM isolation method, as well as, established digestion protocol were two to three times higher than previously reported and coefficient of variation (CV) for technical replicates was below 10%. CONCLUSION: We have established an improved transporter protein quantification methodology by optimizing PM isolation and protein digestion procedures. The optimized procedure resulted in a higher transporter protein yield and improved precision.

Ex-Vivo Force Spectroscopy of Intestinal Mucosa Reveals the Mechanical Properties of Mucus Blankets.

Mucus is the viscous gel that protects mucosal surfaces. It also plays a crucial role in several diseases as well as in mucosal drug delivery. Because of technical limitations, mucus properties have mainly been addressed by in-vitro studies. However, this approach can lead to artifacts as mucus collection can alter its structure. Here we show that by using an implemented atomic force microscope it is possible to measure the interactions between micro-particles and mucus blankets ex-vivo i.e., on fresh excised mucus-covered tissues. By applying this method to study the small intestine, we were able to quantify the stiffness and adhesiveness of its mucus blanket at different pH values. We also demonstrate the ability of mucus blankets to bind and attract particles hundreds of µm away from their surface, and to trap and bury them even if their size is as big as 15 µm.