A Possible Role of <i>cis</i> ‐8‐Octadecenoic Acid of the Sebum in Facial Skin Redness

Sebum on the skin surface is essential for maintaining the homeostasis and function of human skin; however, excessive sebum secretion and disturbances in sebum composition are known to cause various chronic inflammatory skin disorders [1]. Free fatty acids (FFAs), significant components of human sebum, consist mostly of saturated and monoenoic FFAs with chain lengths of 16 or 18 carbons [2]. Recently, we showed that the proportion of FFAs (C18:1) in sebum positively correlated with cheek redness in healthy individuals. Furthermore, oleic acid (C18:1, cis-9), one of the best-characterized sebum FFAs that induce interleukin-36γ expression, may be a link between skin redness and sebum [3]. In contrast, cis-8-octadecenoic acid (C18:1, cis-8) was first identified in 1974 as the most abundant isomer of sebum FFAs (C18:1) [4, 5]; however, its physiological functions remain elusive, likely owing to its lower commercial availability compared with other FFAs. Here, we investigated the relationship between cis-8-octadecenoic acid and cheek redness in healthy Japanese individuals, and its effects on interleukin-36γ expression in cultured human normal keratinocytes (see the online Supporting materials and methods). We first confirmed that cis-8-octadecenoic acid was the most abundant species among the four FFA (C18:1) isomers in skin sebum (Figure 1A). Next, we showed that the percentage composition of cis-8-octadecenoic acid in sebum was positively correlated with skin biophysical parameters associated with facial skin redness, that is, erythema index (EI) and a* values (Figure 1B,C), suggesting that the proportion of cis-8-octadecenoic acid in sebum is related to skin redness. Figure 1D shows representative images of the cheek of participants who had high or low degrees of skin redness and cis-8-octadecenoic acid proportion in the sebum. Recently, the ratio of the pro-inflammatory cytokine interleukin-36γ to the anti-inflammatory cytokine interleukin-37 in the stratum corneum was reported as a new skin inflammatory index that positively correlates with the degree of facial skin redness [6]. Therefore, to investigate the mechanism underlying the involvement of cis-8-octadecenoic acid in skin redness, we evaluated the dose- and time-dependent effects of cis-8-octadecenoic acid on the interleukin-36γ/interleukin-37 ratio in human epidermal keratinocytes. Addition of cis-8-octadecenoic acid to culture medium dose-dependently induced interleukin-36γ and interleukin-37 mRNA expression and the interleukin-36γ/interleukin-37 ratio, showing the highest elevation at dose of 150 μM compared with that of vehicle control (Figure 2A–C). Notably, oleic acid significantly increased interleukin-36γ mRNA expression and the interleukin-36γ/interleukin-37 ratio at dose of 50 μM, whereas cis-8-octadecenoic acid at this dose showed no significant induction (Figure 2A,C). These results indicated that cis-8-octadecenoic acid has a more moderate inflammatory effect than oleic acid. Time-course analyses showed that the cis-8-octadecenoic acid (100 or 150 μM)-induced enhancement of the interleukin-36γ/interleukin-37 ratio started after 3 h and reached maximal levels 24 h after treatment (Figure 2D). Previously, we demonstrated that N-methyl-D-aspartate (NMDA)-type glutamate receptors are involved in the oleic acid-induced interleukin-36γ/interleukin-37 ratio in human epidermal keratinocytes [3]. To investigate the involvement of NMDA-type glutamate receptors in the cis-8-octadecenoic acid-induced interleukin-36γ/interleukin-37 ratio, we examined the effects of MK801, a potent NMDA receptor antagonist, on interleukin-36γ and interleukin-37 mRNA expression in cis-8-octadecenoic acid-treated keratinocytes. As shown in Figure 2E, cis-8-octadecenoic acid-induced interleukin-36γ/interleukin-37 ratio was suppressed by MK801 in a dose-dependent manner. These results suggest that, similar to oleic acid, cis-8-octadecenoic acid upregulates the interleukin-36γ/interleukin-37 ratio in part through NMDA-type glutamate receptor, which may lead to increased skin redness. Although further studies are required, the variations in the binding affinity of cis-8-octadecenoic acid and oleic acid for the receptor may be attributed to their distinct inflammatory effects. In conclusion, to the best of our knowledge, our study is the first to show that cis-8-octadecenoic acid plays a role in facial skin redness. A. Kato, E. Watarai, T. Igarashi, and H. Yoshida designed the research. A. Kato, A. Kawamoto, E. Watarai, E. Shimizu, and T. Igarashi performed the research. A. Kato, A. Kawamoto, E. Shimizu, H. Tsujimura, E. Watarai, T. Igarashi, and H. Yoshida analyzed the data. A. Kato, E. Shimizu, and H. Yoshida wrote the manuscript. All the authors approved the final manuscript. The study was conducted under the Declaration of Helsinki and was approved by the Ethical Committee of Kao Corporation (approval no. D123-200512). The authors declare no conflicts of interest. The authors have nothing to report. Data S1. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.