Pathological aggregation of α-synuclein (αSyn) is a hallmark of Parkinson’s disease (PD), but increasing evidence indicates that αSyn deposits extend beyond the central nervous system to peripheral tissues, including the skin. In PD patients, cutaneous αSyn accumulation has been associated with seborrheic dermatitis (SD), a chronic inflammatory condition linked to sebocyte dysfunction and altered sebum production. Marked differences in sebum composition between PD patients and healthy controls have been described. We aimed to investigate whether pathological αSyn could contribute to dysregulated sebum production and composition by assessing key markers of sebocyte differentiation and lipogenesis in human sebaceous gland cells and skin biopsies from PD patients (n = 5) and matched controls (n = 5) without clinically recorded SD. O-αSyn exposure of immortalized sebaceous gland cells (SZ95) altered the transcriptional programs related to sebocyte differentiation, inflammation, metabolism, and lipogenesis. Consistently, protein markers of mid-to-late sebocyte differentiation were increased. Upregulation of PLIN2, together with elevated PPARγ, SREBP1, SCD1, and FADS2, indicated progression toward a mature and lipid-producing phenotype characterized by enhanced accumulation of neutral lipids within lipid droplets. Lipidomic profiling revealed remodeling of multiple lipid classes, with increases in triglycerides, ceramides, and selected phospholipids. Increased expression of PLIN2 and PPARγ was also observed in sebaceous glands from PD skin biopsies compared to controls. Collectively, our findings indicate that O-αSyn exposure is associated with molecular changes consistent with altered sebocyte differentiation and lipid remodeling. These results support a potential association between αSyn and lipid dysregulation in sebocytes and suggest that these cells may represent unrecognized peripheral targets of αSyn.

Harlequin ichthyosis (HI) is the most severe form of autosomal recessive congenital ichthyosis (ARCI) with aberrant lipid transport in the epidermis leading to a severe barrier defect. It is caused by variants in ABCA12, a lipid transporter involved in the transport of glucosylceramides from the lamellar body to the lipid lamellae. The aim of this study was to explore further the effect of Janus kinase inhibitors (JAKis) on the epidermal barrier in in vitro and in vivo models of HI. Three-dimensional skin equivalents (3DSEs) were generated from human ABCA12 CRISPR-Cas9 knockout and wildtype keratinocytes and treated with the JAKi abrocitinib and upadacitinib (UPA) and bulk RNA sequencing (RNAseq) and lipidomics were performed. UPA was tested in an in vivo HI mouse model followed by haematoxylin and eosin, Nile red, ABCA12, glucosylceramide and immune cell staining. Bulk RNAseq on treated and control 3DSEs showed upregulation of genes involved in keratinocyte metabolism, differentiation and skin barrier function as well as downregulation in immune response genes post-JAKi. Lipidomic analysis showed upregulation of cholesterol sulfate and phosphatidylethanolamine post-JAKi. UPA was selected to test at a concentration of 5 mg kg−1 in 10% dimethyl sulfoxide/corn oil on adult male and female inducible Abca12flox/flox K14CreER and control mice (n = 4 per group, two repeats). One day after topical 4-OH-tamoxifen treatment, UPA or vehicle were administered via oral gavage for nine consecutive days before collecting back skin. The UPA-treated HI mice had significantly reduced epidermal thickness compared with controls. Nile red and glucosylceramide staining showed restoration of neutral and polar lipids and upper epidermal glucosylceramide in the UPA-treated mice compared with controls. A reduction in T cells and dendritic cells was also observed in the JAK1i-treated mice. These results suggest that UPA restores the skin barrier in severe ichthyosis and may be a promising treatment for this group of patients.

Background/Objectives: Sebocytes, the primary cell type in sebaceous glands (SGs), produce a lipid mixture called sebum that is released onto the skin surface and is required for skin homeostasis. The lipid receptor Peroxisome Proliferator-Activated Receptor gamma (PPARγ) regulates sebocyte proliferation and lipid synthesis and is involved in acne development. As inhibition of PPARγ has been shown to reduce insulin-induced lipogenesis and Akt/mTOR signalling in SZ95 sebocytes, we here investigated the effects of PPARγ deletion on lipid homeostasis and autophagic stress responses and how the secretomes affect dermal fibroblasts. Methods: SZ95 sebocytes wildtype (WT) and PPARγ knockout (KO) were shifted to low serum and EGF-deficient conditions permissive for autophagy. Untargeted and targeted HPLC-MS/MS analyses were used to analyze native and oxidized lipids, respectively. Protein levels of LC3I/II and p62 were assessed using immunoblots and immunofluorescence microscopy to investigate the autophagic flux. Dermal fibroblasts were exposed to conditioned media. Results: In low serum culture media, KO SZ95 sebocytes displayed significantly altered levels of 23 lipid classes. We observed a significant increase in ether-linked fatty acids as components of complex lipids and detected elevated levels of phospholipid hydroperoxides and aldehydolipids in the KO sebocytes. KO SZ95 sebocytes failed to show the typical responses to lipoxidative stress, such as elevated p62 crosslinking or inclusion body formation, and had reduced LC3II/I ratios as compared to WT cells. PPARγ KO conditioned media promoted a trend towards an inflammatory fibroblast phenotype. Conclusions: These findings suggest that PPARγ in sebocytes may alter the lipidome, elevate redox stress, and affect the autophagic machinery, which could cause accumulation of oxidized lipids and other potentially harmful compounds in sebocytes.

With age, human skin undergoes a progressive decline in essential functions, including barrier protection, immunity, and wound healing capacity, which underlie many age-related skin diseases. Skin aging is not only driven by chronological aging, but also strongly influenced by extrinsic stressors, notably ultraviolet radiation, pollutants, and diet. Thus, understanding the complex interplay between these intrinsic and extrinsic factors is essential for developing strategies to preserve skin health across the lifespan. Given the growing appreciation for the physiologic differences between humans and animal models, more advanced in vitro and ex vivo models are needed to dissect the human-specific mechanisms of skin aging and test emerging therapies. In this review, we summarize the major hallmarks of human skin aging and provide an overview of current in vitro modeling approaches that capture both intrinsic and environmental aging mechanisms. We highlight recent advances in complex 3D in vitro systems — including full-thickness human skin equivalents, organoids, and microphysiological platforms — and discuss how these emerging models can be leveraged to interrogate aging biology and support translational research. Together, these developments pave the way for more predictive and mechanistically informed tools to study skin aging and to accelerate the development of next-generation therapeutic and preventive strategies.

Abstract Introduction and aims Steatocystoma multiplex (SM) is a genodermatosis usually caused by KRT17 variants, presenting at puberty with multiple inflamed cysts that significantly affect quality of life. These cysts resemble sebaceous ducts and contain sebocytes in the wall. We previously observed sebaceous differentiation in the epidermis overlying early cysts, but the molecular pathways remain unknown. Historical thin-layer chromatography suggested that cyst contents resemble sebum, yet modern characterization has not been attempted. Our aim was to investigate SM pathogenesis using spatial transcriptomics and lipidomics of cysts. Methods We used Visium HD spatial transcriptomics on formalin-fixed paraffin-embedded samples from early cysts, established cyst wall and overlying epidermis in five patients (four men, 1 woman; aged 21–83 years). Analyses included receptor–ligand interactions (CellPhoneDB), trajectory (Monocle), and transcription factor profiling (Decoupler). Lipidomic profiling of cyst contents, sebum and stratum corneum was performed via mass spectrometry–coupled gas and liquid chromatography, with two normal skin controls. Results We identified a strong interaction between S100A8/9 and CD36, a regulator of cell lipid uptake, in the epidermis overlying an early cyst showing sebaceous differentiation. Sebocyte reclustering and trajectory analysis identified a sebocyte subcluster specific to cysts expressing keratinization genes such as CARD18, CDSN and ASPRV1. Transcription factor analysis demonstrated that early keratinocytes show activation of the Wnt pathway. Lipidomic cyst content analysis revealed two phenotypes with three of five resembling sebum and two of five resembling stratum corneum. Sphingoid bases and N-acylethanoamines were also identified which are infrequently seen in normal sebum. Conclusions Early cyst formation may involve alarmin–CD36 interactions, Wnt–β-catenin activation, and sebaceous differentiation in the epidermis. We identified two lipid profiles and two lipid classes rarely seen in normal sebum. Future work will validate these findings with multiplex imaging and pathway targeting in cell models.

Seborrheic dermatitis (SD) is a chronic inflammatory skin condition often involving the sebaceous-rich areas, characterized by erythematous scaly lesions. It is frequently observed in individuals with immune dysregulation, suggesting the interplay between the immune system and disease development. An altered immune environment leads to an exaggerated inflammatory response with the activation of innate immunity, involving the participation of mast cells, γδ T cells, and the NOD–LRR–pyrin-domain-containing protein 3 (NLRP3) inflammasome. This review aims to assess the complex relationship between Malassezia and the immune system in the pathogenesis of SD. We will explore how an impaired immune response predisposes the skin to Malassezia overgrowth and infection. We will examine the role of adaptive immunity, particularly T helper cells, in driving chronic inflammation in SD. All actors involved, whether part of innate or adaptive immunity, are responsible for the release of pro-inflammatory cytokines, which contribute to the progression of the disease. Therapeutic strategies aimed at the modulation of the immune response in SD have been tested in clinical trials evaluating the efficacy of immunomodulatory treatments in the management of SD. This review synthesizes insights from immunological studies and clinical trials to present an in-depth analysis of the immune mechanisms underpinning SD, thereby proposing targeted therapeutic strategies for its management.

Sex hormones regulating the menstrual cycle influence sebaceous gland cell lipogenesis and the feeling of skin oiliness or dryness on the face. The aim of this study was to elucidate sebaceous lipogenesis in females during the menstrual cycle and define their facial sebum composition. Sebum was sampled from cheeks and foreheads in 38 Chinese women, 19 with sebometry ≤ 70 μg/cm2 (low sebometry group, LS), and 19 with sebometry ≥ 150 μg/cm2 (high sebometry group, HS), in the ovulation phase (OP) and in the early luteal phase (ELP). In addition, the follicular phase (FP) and the late luteal phase (LLP) were examined within the HS group. Sebum lipid classes were quantified by GCMS and LCMS. The HS skin type was characterized by presenting more sebum lipids on the cheeks and the forehead than the LS skin type, respectively. In the HS subgroup, multivariate analysis of forehead sebum data was applied to the amounts assessed at FP, OP, ELP, and LLP. Our data detected a fluctuation of facial sebogenesis during the menstrual cycle.

Neurodegenerative diseases (NDDs), including Parkinson's disease and Alzheimer's disease, are major age-related disorders characterized by progressive neuronal degeneration and a decline in cognitive and motor functions. Managing NDDs poses an increasing healthcare challenge as the global population ages. The onset of NDDs is linked to protein misfolding, oxidative stress, dysfunction of mitochondria and lysosomes, and neuroinflammation. Clinical manifestations of NDDs only appear after substantial neuronal damage has already occurred. This underscores the urgent need for accessible tissue biomarkers to enable early diagnosis, disease monitoring and assessment of therapeutic efficacy. The skin has emerged as a valuable peripheral indicator of neurodegeneration, sharing embryological origin, gene expression profiles, protein alterations and cellular dysfunctions with the brain. Notably, pathological protein deposits, which are hallmarks of NDDs, such as beta-amyloid, tau proteins, and oligomeric alpha-synuclein, have been observed in the skin. Increasing evidence links NDDs with various pathological skin conditions, including melanoma and inflammatory diseases. This review aims to explore the potential of the skin as a window into neurodegenerative processes at an early stage, before clinical signs arise. The main advantages of using skin as a source of NDD biomarkers are its accessibility and the minimally invasive sampling methods such as stratum corneum collection, sebum and volatile compounds analysis, and biopsies. Immunohistochemistry and omics approaches applied to skin samples provide valuable insights into NDD pathophysiology and facilitate biomarker discovery for early diagnosis and disease monitoring. NDDs are multisystemic disorders and new findings in skin research highlight the value of peripheral tissues for investigating central nervous system alterations enabling earlier neuroprotective interventions.

Free fatty acids (FFAs) are essential components of skin surface lipids (SSLs), which consist of a blend of epidermal and sebaceous lipids. The sebum component of SSLs is abundant in seborrheic body areas, which are rich in sebaceous glands (SGs). Sebum FFAs exhibit unique characteristics, including a prevalence of C14-C18 chain lengths, terminal branching, and a double bond at the O6 position. Notably, over one-third of the sebaceous FAs contain a single double bond. Deregulated synthesis of monounsaturated FAs (MUFAs) can lead to changes in sebum composition, which contribute to the pathogenesis of inflammatory skin conditions such as acne vulgaris, rosacea, psoriasis, and seborrheic dermatitis. A comprehensive characterization of the different isomers of unbound MUFAs distinguished by the position of the carbon-carbon double bond, has not been achieved in human sebum. Therefore, we aimed to develop an analytical strategy to differentiate the various carbon-carbon double bond positions in FFAs. Our methodology combines the Paterno`-B & uuml;chi reaction, utilizing 2-acpy, with liquid chromatography and tandem mass spectrometry (LC-MS/MS). This strategy was initially optimized on standard FFAs and subsequently applied to human sebum. The method enabled the characterization of seventeen MUFAs with C14-C18 chain lengths and two C18 polyunsaturated FAs, namely linoleic and sebaleic acid. The prevalence of sapienic acid (C16:1n-10) among MUFAs supports the dominance of the O6 desaturation pathway catalyzed by the FADS2 enzyme in human SG. Additionally, the assay proved comparable abundance of MUFAs and PUFAs in sebum from males and females.

IntroductionAbnormalities of keratinocyte differentiation and impairment of permeability barrier are features of inflammatory skin diseases driven by Th1/Th17 and Th2 immune response, such as psoriasis and atopic dermatitis. We aimed at identifying the signature of the Th1/Th17 and Th2 environments on keratinocytes, focusing on the expression of genes involved in the lipid metabolism and profiles of abundance of lipid metabolites.MethodsHuman immortalized keratinocytes in prodifferentiative conditions induced by increasing calcium concentration, and 3D epidermal equivalents were treated with mixtures either of TNF-α and IL-17A plus Th1-related cytokines (IL-1α, IL-6) or of Th2 cytokines (IL-4, IL-13). The expression of genes involved in epidermal differentiation and lipid metabolism was evaluated by RT-PCR at 2, 4 and 7 days of treatment. The protein levels of early and late keratinocyte differentiation markers were assessed. The lipid composition was investigated by GCMS and LCMS.ResultsBoth Th1/Th17 and Th2 cytokine mixtures changed the expression of genes involved in the metabolism of fatty acids (FAs), i.e., FAS, FADS2, SCD1, and ALOX12B. Th1/ Th17 downregulated the ELOVL3 gene, which is implicated in the FAs elongation, while the mRNA levels of ABCA12 and HMGCR, genes involved in lipids transport and cholesterol synthesis, respectively, were decreased with both cytokine mixtures. DEGS1 and DEGS2, key enzymes in the ceramide synthesis, were downregulated and upregulated in the Th1/Th17 and Th2 environments, respectively. The mRNA expression of CERS3, which synthesizes ceramides containing long chain FAs, was increased by Th1/Th17 cytokines. Both Th1/ Th17 and Th2 cytokine mixtures lowered the CERS6 mRNA levels in differentiated keratinocytes. Effects specific to Th1/Th17 or Th2 cytokines were observed on freely extractable cell lipids. Th1/Th17 cytokines significantly inhibited the high calcium-induced synthesis of phospholipids (PCs, PEs, SMs), and short-chain ceramides, while the synthesis of ceramides with medium to long carbon chains was upregulated. Th2 cytokines caused a generalized decrement of free FAs, including long-chain ones. In contrast to 2D cultures, the 3D epidermal equivalents allowed the identification of altered profiles of acyland hexosyl-ceramides.ConclusionThe different effects exerted by Th1/Th17 and Th2 cytokines support, at least in part, the features of lipid barrier alterations specific to psoriasis or atopic dermatitis.

The lipid composition of the epidermis plays a critical role in the skin's barrier function, and defects in lipid synthesis or assembly can cause a spectrum of skin diseases, ranging from dry skin to severe ichthyoses. The aim of this study was to develop an in vitro model of human skin with tunable inhibition of lipid synthesis. Human N/TERT keratinocytes were engineered to express doxycycline-inducible short hairpin RNAs targeting ceramide synthase 3, which is essential for synthesis of ultralong-chain ceramides and skin barrier function. We show that 3-dimensional human skin equivalents with induced knockdown of ceramide synthase 3 display normal stratification and terminal differentiation but have reduced Nile red staining for polar lipids. Further analysis of the lipidome by mass spectrometry confirmed a significant reduction in specific classes of ceramides and ceramide chain length in the ceramide synthase 3-depleted human skin equivalents. We also show that ceramide synthase 3 knockdown is reversible upon removal of doxycycline and can be used to study recovery and repair of epidermal lipids. Together, these findings provide an overall strategy for genetically regulating the lipid composition within human skin models and establish a tunable in vitro model of ceramide deficiency.

Atopic dermatitis (AD) is a chronic inflammatory skin disorder exacerbated by Staphylococcus aureus colonization. The specific factors that drive S. aureus overgrowth and persistence in AD remain poorly understood. This study analyzed skin barrier functions and microbiome diversity in lesional (LE) and non-lesional (NL) forearm sites of individuals with severe AD compared to healthy control subjects (HS). Notable differences were found in transepidermal water loss, stratum corneum hydration, and microbiome composition. Cutibacterium was more prevalent in HS, while S. aureus and S. lugdunensis were predominantly found in AD LE skin. The results highlighted that microbial balance depends on inter-species competition. Specifically, network analysis at the genus level demonstrated that overall bacterial correlations were higher in HS, indicating a more stable microbial community. Notably, network analysis at the species level revealed that S. aureus engaged in competitive interactions in NL and LE but not in HS. Whole-genome sequencing (WGS) showed considerable genetic diversity among S. aureus strains from AD. Despite this variability, the isolates exhibited convergence in key phenotypic traits such as adhesion and biofilm formation, which are crucial for microbial persistence. These common phenotypes suggest an adaptive evolution, driven by competition in the AD skin microenvironment, of S. aureus and underscoring the interplay between genetic diversity and phenotypic convergence in microbial adaptation.

Derangement of the epidermal barrier lipids and dysregulated immune responses are key pathogenic features of atopic dermatitis (AD). The Th2-type cytokines interleukin IL-4 and IL-13 play a prominent role in AD by activating the Janus Kinase/Signal Transduction and Activator of Transcription (JAK/STAT) intracellular signaling axis. This study aimed to investigate the role of JAK/STAT in the lipid perturbations induced by Th2 signaling in 3D epidermal equivalents. Tofacitinib, a low-molecular-mass JAK inhibitor, was used to screen for JAK/STAT-mediated deregulation of lipid metabolism. Th2 cytokines decreased the expression of elongases 1, 3, and 4 and serine-palmitoyl-transferase and increased that of sphingolipid delta(4)-desaturase and carbonic anhydrase 2. Th2 cytokines inhibited the synthesis of palmitoleic acid and caused depletion of triglycerides, in association with altered phosphatidylcholine profiles and fatty acid (FA) metabolism. Overall, the ceramide profiles were minimally affected. Except for most sphingolipids and very-long-chain FAs, the effects of Th2 on lipid pathways were reversed by co-treatment with tofacitinib. An increase in the mRNA levels of CPT1A and ACAT1, reduced by tofacitinib, suggests that Th2 cytokines promote FA beta-oxidation. In conclusion, pharmacological inhibition of JAK/STAT activation prevents the lipid disruption caused by the halted homeostasis of FA metabolism.

Background: Variation in sebum composition and diet contribute to the development of acne in adolescents. The composition of sebum depends upon multiple factors including dietary habits. There are limited reports on effects of diet on sebum composition.

17-β-estradiol, involved in mesothelioma pathogenesis, and its precursors were explored as potential biomarkers for the early diagnosis of mesothelioma. Using enzyme-linked immunosorbent assay(ELISA) for 17-β-estradiol and ultra-high performance liquid chromatography/tandem mass spectrometry(UHPLC-MS/MS) for 19 17-β-estradiol precursors, a comprehensive analysis of 20steroid hormones was conducted in the serum of mesothelioma patients(n=67), asbestos-exposed healthy subjects(n=39), and non-asbestos-exposed healthy subjects(n=35). Bioinformatics analysis explored three potential serum biomarkers: 17-β-estradiol, DHEA-S, and androstenedione. The results revealed significant differences in 17-β-estradiol levels between mesothelioma patients and both non-asbestos-exposed and asbestos-exposed healthy subjects. No significant variations in serum 17-β-estradiol levels were observed among mesothelioma patients at different stages, suggesting its potential as an early diagnostic marker. 17-β-estradiol levels were similar in mesothelioma patients with environmental and occupational asbestos exposure, while males with occupational asbestos exposure exhibited significantly higher levels of 17-β-estradiol compared to females. Significant reduction in androstenedione and an increase in DHEA-S were observed in asbestos-exposed individuals compared to non-asbestos-exposed individuals. The analysis of DHEA-S-androstenedione-17-β-estradiol signature score showed an increase in asbestos-exposed individuals and mesothelioma patients compared to non-asbestos-exposed individuals, and this score effectively distinguished between the groups. The Cancer Genome Atlas data was utilized to analyze the expression of 5-α-reductase1 and hydroxysteroid-17β-dehydrogenase2 genes. The findings indicated that mesothelioma patients with elevated gene values for 5-α-reductase1 and hydroxysteroid-17β-dehydrogenase2 have a worse or better prognosis on overall survival, respectively. In conclusion, this study suggests 17-β-estradiol, DHEA-S, and androstenedione as biomarkers for mesothelioma risk and early diagnosis of mesothelioma in asbestos-exposed individuals, aiding timely intervention and improved care.

Atopic dermatitis (AD) is a composite disease presenting disruption of the skin permeability barrier (SPB) in the stratum corneum (SC). Recent evidence supports derangement of the sebaceous gland (SG) activity in the AD pathomechanisms. The objective of this study was to delineate profiles of both sebaceous and epidermal lipids and of aminoacids from SG-rich (SGR) and SG-poor (SGP) areas in AD. Both sebum and SC were sampled from SGR areas, while SC was sampled also from SGP areas in 54 adult patients with AD, consisting of 34 and 20 subjects, respectively with and without clinical involvement of face, and in 44 age and sex-matched controls. Skin biophysics were assessed in all sampling sites. Disruption of the SBP was found to be associated with dysregulated lipidome. Abundance of sapienate and lignocerate, representing, respectively, sebum and the SC type lipids, were decreased in sebum and SC from both SGR and SGP areas. Analogously, squalene was significantly diminished in AD, regardless the site. Extent of lipid derangement in SGR areas was correlated with the AD severity. The abundance of aminoacids in the SC from SGR areas was altered more than that determined in SGP areas. Several gender-related differences were found in both controls and AD subgroups. In conclusion, the SG activity was differently compromised in adult females and males with AD, in both SGR and SGP areas. In AD, alterations in the aminoacidome profiles were apparent in the SGR areas. Lipid signatures in association with aminoacidome and skin physical properties may serve the definition of phenotype clusters that associate with AD severity and gender.