Lipidomic analysis of facial skin surface lipid reveals the causes of pregnancy-related skin barrier weakness

Self-reported skin discomfort is a common problem during pregnancy, but it is not clear whether skin barrier function is altered in the process. Few studies have described the skin barrier function during pregnancy. In this work, we used highly sensitive and high-resolution ultra performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) to distinguish skin surface lipid (SSL) combined with multivariate analysis of lipids and metabolic changes to determine the relationship between SSL changes and skin physiology during pregnancy in order to better understand the skin condition of pregnant women. The results showed a significant reduction in the total lipid content in pregnant women. A total of 2270 lipids were detected, and the relative abundances of fatty acyls and glycerolipids were significantly reduced, while glycerophospholipids (GPs), sphingolipids, and saccharolipids was significantly increased in the pregnancy group. Multivariate data analysis indicated that 23 entities constituted the most important individual species responsible for the discrimination and phosphatidylcholine was the most abundant lipid in pregnancy group. In addition, compared to SSL profile of control group, it was observed that the average chain length of ceramides and fatty acids both decreased in SSL profile of pregnancy group. The main and most commonly affected pathway was that of GP pathways. These findings indicate that skin lipids are significantly altered in mid-pregnancy compared to the control group. Changes in ostrogen during pregnancy also make the skin more susceptible to inflammatory factors and lead to more fragile and susceptible skin, weakening the skin barrier along with the lipid alterations.

Pregnancy is a period closely related to various endocrines, metabolic, immune, and vascular changes in women's physiological and pathological skin changes 1 . More than 90% of pregnant women experience physiological changes in skin vasculature, pigmentation, appendages, and degeneration 2 . Pre-existing skin diseases (e.g. psoriasis, and atopic dermatitis [AD]), may worsen or improve during pregnancy, and pregnancy-specific skin diseases, such as intrahepatic cholestasis and pruritic urticarial papules, may occur 3 . Skin diseases during pregnancy have received extensive attention and research 4,5 and often occur in close association with, changes in skin condition 6 . Therefore, we speculated that the state of the skin changes and is prone to some skin diseases during pregnancy, but at the molecular level, the specific changes that take place remain unknown.
The state of the skin is closely related to the strength of the skin barrier and is also altered by changes in skin metabolism and microflora. Skin lipids maintain the skin barrier by creating a physicochemical barrier, transmitting a complex network of signals from the epidermis and altering the composition of the microbial skin flora [7][8][9] . In particular, healthy skin relies on a characteristic lipid profile to form a barrier 7,10,11 . Changes in the composition and content of skin lipids will affect the function of the skin barrier 12,13 . Studies have shown that defective skin barrier function is not simply a secondary consequence but rather a key factor in various skin diseases 9,14,15 . Variations in the eight main classes of lipids. Lipid entities were analysed based on differences between the two groups. The results identified 2270 lipids and separated them into eight major classes.
We calculated the relative content of each major lipid class. Figure 2a illustrates the differences between the eight classes by showing the relative content of each major lipid class in the two groups. In the pregnancy group, two main classes significantly (p < 0.05) decreased ( Relative content alterations in lipid subclasses. Analyses were then performed to identify the subclasses responsible for the significant changes in the five main classes mentioned above. The relative changes in subclasses from the five major classes were analysed. In the SSL profile of pregnant women, 16 subclasses differed significantly (p < 0.05), displaying the same change trend as the major class to which they belonged. As shown in

Identification of important individual lipids in the pregnant and control groups. OPLS-DA
was applied to identify the most important lipid individuals within the aforementioned subclasses, which were responsible for distinguishing pregnant samples from controls. The selection criteria were as follows: VIP > 3, p < 0.05, and fold change > 2. As shown in Table 1, 23 lipids were identified (for more information on the MS/ MS spectra and ion fragments of characteristic lipids, see Fig. S1). Among them, the phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylcholine (PC) contents were increased in the pregnancy group, and the triglyceride (TG) and diacylglycerol (DG) contents were decreased in the pregnancy group (Fig. 3a).
Metabolic pathway analysis. The lipids from Table 1 were imported into MetaboAnalyst 4.0 (http://www. metab oanal yst.ca/) to explore the metabolic pathways for SSL changes during pregnancy. The metabolic pathway analysis module identified five metabolic pathways associated with skin conditions during pregnancy (Fig. 3b). These metabolic pathways included GP metabolism, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, arachidonic acid metabolism, linoleic acid metabolism, and alpha-linolenic acid metabolism. GP metabolism   www.nature.com/scientificreports/

Average ceramides (Cers) and fatty acids (FAs) chain length.
It is well known that the long acyl chain in Cers is necessary to form tightly wrapped impermeable lipid lamellae. The skin barrier has a high affinity for the chain lengths of Cer and FAs [20][21][22] . We observed a significant reduction in mean Cer and FA chain length in the pregnancy group compared to the control group (Fig. 4). The relevant Cer subclasses sre shown in Table S2. In the pregnancy group, significantly decreased levels of unsaturated free fatty acids (FFAs) and saturated FFAs increased, but no significant changes were observed. In conclusion, the observed changes in lipid composition as well as reductions in Cer, and FFA chain lengths in the pregnancy group contributed to the altered lipid organization and reduced skin barrier function.

Discussion
This study was conducted to confirm whether changes in the skin barrier during pregnancy is associated with SSL changes. Research has shown that approximately 60-80% of women first develop eczema during pregnancy, and over 50% experience skin deterioration 23 . An impaired skin barrier, which increases skin susceptibility and, thereby results in immune activation, has been documented in atopic eczema 24,25 . Trans Epidermal Water Loss (TEWL) refers to the total amount of water loss through the skin, which can be used as an index of skin barrier function for non-invasive measurement. A previous study found that TEWL increases in pregnancy and the postnatal period 6 . Lipids significantly affect the skin state through different mechanisms, such as maintaining the barrier properties of the skin and acting as signaling molecules to transmit intercellular signals 7,26 . Lipidomics employ mass spectrometry to analyse and characterise lipid species in biological systems on a large scale 18,19,27 .
In this study, we used lipidomics method to study the changes of lipids on the facial skin surface of women in the second trimester. The study found significant lipid changes on the skin surface of the pregnancy group (Fig. 2), which may be related to the fragile skin barrier of pregnant women. Overall, the total SSL content in the pregnancy group was lower and significantly different than that in the control group (p < 0.01), presumably due to the weakened skin barrier function during pregnancy (see Figure S2). We identified a total of 2270 lipids and found that among the eight main classes of lipids, the relative abundances of GPs, SPs, and SLs increased significantly in the pregnancy group while those of FAs and GLs decreased significantly. Based on the significant trends in these five main classes of lipids, we screened 16 subclasses of lipids and analysed them under the screening criteria of VIP > 3, p < 0.05, and fold change > 2. Twenty-three individual lipid species that contributed significantly to the group differences were selected, and PC was the most abundant lipid in the pregnancy group. The obtained GP metabolic pathway included two lipids, PC and PE, both of which made important contributions to the group differences. Furthermore, we observed a decrease in Cers average chain length and an increase in FFA in the SSL profile of the pregnancy group compared to that of the control group.
Women have the highest levels of serum ostrogen during pregnancy. In normal pregnancy, the female body produces large amounts of ostrogen and progesterone. The concentrations of serum estrone and oestradiol increases by approximately 50-fold compared to their maximum pre-pregnancy values, and estriol increases by approximately 1000-fold 28 . Because of these changes during pregnancy, the skin becomes more sensitive and fragile due to increased sensitivity to external stimuli 29 . Topical ostrogen is known to inhibit sebum production, and ostrogen-containing contraceptives are used in the treatment of female acne 30 . Loss of ostrogen after menopause has been shown to lead to reduced sebum production 31 . There are several hypotheses that elucidate the mechanism by which ostrogen inhibits sebum production. These include the idea that ostrogen directly antagonizes androgenic activity, regulates genes associated with lipogenesis, and inhibits androgen production by gonadal tissue via a negative feedback loop 32 . Both receptors for ostrogen, ERα and Erβ, are expressed in basal and differentiated sebaceous cells, suggesting that the site of ostrogen action in the sebaceous gland is quite important 33 . In a study monitoring the skin condition at all stages of the menstrual cycle, it was found that blood progesterone levels peaked between days 15 and 28 while the skin barrier was weakest between days 22 and 26 34 . www.nature.com/scientificreports/ An early study found a significant decrease in sebum secretion in the middle of gestation (28-32 weeks) with slight fluctuations throughout the pregnancy, which is consistent with our results 35 . Therefore, we speculate that due to changes in ostrogen during pregnancy, there is a decrease in SSL secretion and weakening of the skin barrier. However, some epidemiological investigation 4,[36][37][38] have found that acne is also a common condition during pregnancy. It is worth noting that acne occurs most often in the third trimester of pregnancy due to increased sebum secretion from maternal androgens, which does not contradict our results (the sampling population was women in the second trimester of pregnancy). GP metabolism is significantly affected in a variety of diseases associated with pregnancy, such as preeclampsia 39 , recurrent miscarriage 40 , placental syncytiotrophoblast microvesicles in adverse pregnancy 41 , and early in utero growth restriction of breast milk and macrosomic babies 42 , which indicates that the process of pregnancy may cause a disturbance in the metabolism of GPs. Some GPs are involved in signaling pathways for cell survival and inflammation and affect membrane-associated proteolysis 43 . Alterations in GP metabolism also occur in psoriasis and lead to defects in skin fibroblasts and breakdown of the skin barrier in patients with neutral lipid storage disease with ichthyosis 44,45 .
Notably, the screened differential lipids PC, PE are also involved in the metabolism of GPs, and the PC content was the highest among the 23 differential lipids. PC and PE are the main phospholipid components of the cell membrane, with PC accounting for 50% of the total phospholipids and PE content comprising approximately 20-30% 46 . PC is the main phospholipid in mammalian cell membranes and plays important roles in necrosis, signal transduction, protein sorting and apoptosis 47 . PC is the primary source of many second messengers (such as arachidonic acid and DAG) and is often further metabolized into other signaling elements 48 . PC also serve as a mitogen needed for growth factors-induced DNA synthesis 48 . PE is often involved in oxidative phosphorylation, membrane fusion, and mitochondrial biogenesis and is sensitive to inflammatory stimuli 49,50 . PE also plays a pivotal role in cell division to ensure proper cytoplasmic division. PE-rich domains are exposed at the cleavage groove on the cell surface to regulate interactions between the contractile ring and plasma membrane 51 . In the final stages of cytoplasmic division in some cells, PE appears to be required for the breakdown of actin filaments; in other words, cells that lack PE cannot complete division and separation 52 . It has been found that PC and PE contents increase under particulate matter 2.5 exposure, resulting in metabolic disorders of related lipids and thereby inducing inflammatory reactions and atherosclerosis acceleration 53 . In addition, we found that the ratio of PC/PE was significantly higher in the pregnancy group than in the control group. Studies have found that malignant tumor cells with metastasis have a higher PC/PE ratio than those without metastasis 54 . This is associated with increased synthesis of cell membranes and accelerated replication of tumor cells. It has also been shown that PC, PE, and PC/PE ratios are significantly higher in prostate cancer tissue than in benign prostate tissue 55 . Based on the above discussion, we speculate that an increase in PC and PE contributes to the growth and division of skin cells during pregnancy while also being more sensitive to inflammation. Thus, the PC/PE ratio increase during pregnancy may be associated with accelerated membrane integrity as well as increased energy generation.
Interestingly, compared to our previous experiments, in the study of SSL differences between men and women, women had higher levels of GPs, such as PG, PC, PE, PS, PA, and PI 17 . The levels of PC, PE, and PS in this study were higher in pregnant women than in non-pregnant women. This finding appears to indicate that the levels of certain GPs are correlated with female status, which may be related to ostrogen levels in women; however, further experiments are needed to verify the hypothesis. During pregnancy, the mother's immune response is extensively altered to allow the foetus to better attach to the mother 56 . Hormonal changes during pregnancy also alter the cytokine profile, resulting in a preference for Th2-type cytokines (interleukin [IL]-4, IL-5, IL-10, and IL-13) that contribute to the maintenance of fetal survival 3 . This local immune change appears to translate into peripheral effects. Studies have shown that the levels of both regulatory T cell cytokines (IL-10 and sTNFRII) and Th1-type cytokines (IL-2, IL-12, and IL-27) are elevated in mid-pregnancy 57 . Alterations in immunity will also affect the synthesis and proportion of lipids. IL-13 inhibits FFA elongases (Elongation of very long chain fatty acids [ELOVL] 3 and 6) in a STAT6-dependent manner and downregulates expression in mouse skin lesions 58 . IL-4 downregulates mRNAs encoding sphingomyelinase (SMase) and glucocerebrosidase (GBA) and upregulates mRNA encoding acid ceramidases, resulting in lower Cer levels 59 . Th2 cytokines significantly reduce the expression levels of the epidermal lipid metabolism enzyme-encoding genes SMase, GBA and Elovl1, resulting in significant downregulation of specific Cer levels 60 . These immune factor-induced changes in lipids are closely associated with increased TEWL and AD phenotypes 61,62 . Therefore, we inferred that lipid disturbance along with alterations in the immune system work together to weaken the immune and physical barriers of the skin during pregnancy.
We also observed a decreased Cer chain length in the pregnancy group, which was consistent with our previous research on acne 18,19 , as well as a decrease in FA chain length, which is also reflected in adolescent acne patients 63 , a slight decrease in unsaturated FFAs content, and a slight increase in saturated FFAs content. When long-chain Cers decrease and short-chain Cers increase, the skin barrier is known to become defective 58 . Cers are the main lipids in the stratum corneum, accounting for approximately one-third of lipid molecules or 50% of their weight. Cers contain a large number of hydrophilic groups, which have a good affinity for water. Cer promote epidermal hydration and enhance epidermal cell cohesion. They also form a bedplate bilayer structure in the keratinocytes, which effectively prevents epidermal water from escaping. Cer can penetrate deep into the epidermis, prompting the skin to regain the ability to retain water, so they have an outstanding moisturizing effect. In addition, Cers can initiate aging cells, promote epidermal cell division and basal layer cell regeneration, enhance the role of the skin barrier (or isolation protection), and prevent the invasion of external irritants. Although FA carbon chains were reduced in the pregnancy group, the FA content did not increase ( Figure S2), suggesting that the reduction in carbon chains was not used for oxidation reactions and that long-chain FA were the majority of the contents in both groups. Meanwhile, the long-chain FA content in the pregnancy group was significantly lower than that in the control group (p < 0.5). Long-chain FAs protect the body as a component of www.nature.com/scientificreports/ the skin barrier. FAs are extended by the addition of two carbons through a four-step process (condensation, reduction, dehydration, and reduction), and the rate-limiting step of the FA elongation cycle is a condensation step catalyzed by seven elongases (Elovl1to-7). Li et al. 64 found that Elovl4-deficient mice were deficient in long-chain FA-derived Cers, resulting in defective skin permeability barrier function and neonatal mortality. Mice with Elovl1 gene deletion exhibited severe epidermal barrier defects and died shortly after birth. In these mice, lipid lamellar formation was largely impaired 65 . In addition, Cer in the epidermis contains extremely long FAs. Reduced levels of epidermal Cer and changes in FA chain length can lead to several skin diseases. Elovl1 is a key determinant of epidermal Cer chain length and is differentially regulated in a tissue-specific manner 65 . This explains the commonality between the changes in FA chain length and Cer chain length, which collectively weaken the skin barrier. An early study showed that FA chain length affects the phase behavior of cholesterol/ Cer/FA mixtures, and the lipid phase behavior of the three lipid mixtures is very similar to that of the intact cuticle with only long-chain FFAs 66 . The decrease in the average chain length of Cer and the decrease in the expression of FA synthetase (e.g. elongase) leads to a significant shortening of the FFA chain length, which is also reflected in AD 67 . When long-chain FAs are replaced by short-chain FAs in the intercellular space of the healthy human stratum corneum, the laminar structure of the skin tissue is affected, resulting in skin barrier dysfunction. When the FFAs in lipid mixtures with simulated sebum fractions are changed from short-chains to long-chains, the mixtures become more structurally robust, and their stability is enhanced. In addition, studies have shown that FFAs can bind to newly synthesized PC and PE and FFA are important drivers of PC synthesis in human skin fibroblasts 68 . These results further corroborate that lipid disorders weaken the skin barrier in women during pregnancy. Current research on pregnant women has focused on pigmentation, pruritus and changes in plasma lipids. For the first time, we investigated lipid changes in the facial skin of pregnant women and found significant differences in lipid composition compared to that of the non-pregnant control group, presumably related to GP metabolism. Although most skin changes subside after pregnancy, the weakening skin barrier during pregnancy calls for greater attention to skin care and careful cosmetic product selection. Our study provides insights for better skin management during pregnancy.