Sapienic Acid: Species-Specific Fatty Acid Metabolism of the Human Sebaceous Gland

Abstract

Hair follicle-associated sebaceous glands secrete sebum, a highly complex lipid mixture that covers the skin surface and hair shafts. The functional versatility of lipids, combined with the wide array of sebaceous lipid classes and aliphatic moieties, provide mammals with a substrate that facilitates adaptation to their diverse environments, including interaction with animals and microbes. Unique among the complexity of sebaceous lipids is sapienic acid, a 16 carbon monounsaturated fatty acid with an extremely rare position of the double bond, located between carbons 6 and 7 from the carboxyl terminal. Human sebum is the only documented location in the animal kingdom where sapienic acid is abundant and naturally occurring. It is produced by fatty acid desaturase 2 (FADS2), the same enzyme that is rate-limiting in the formation of polyunsaturated fatty acids. Multiple tissue-specific mechanisms are utilized in the human sebaceous gland in order to “repurpose” FADS2 for the production of sapienic acid, chief among which is the reduction of competing desaturase activity. Among mammals, human sebum has the highest amount of free fatty acids, of which sapienic acid is the most abundant monounsaturated fatty acid. Consistent with the role of fatty acids in modulating host-microbe interactions, sapienic acid has the highest antimicrobial activity among free fatty acids in human sebum, while also demonstrating selectivity for Staphylococcus aureus, an opportunistic pathogen. Increased infection by Staphylococcus aureus is associated with a reduction in sapienic acid in sebum of patients with atopic dermatitis, and topical application of sapienic acid is correlated with decreased bacterial load and amelioration of symptoms. Taken together, this strongly suggests that sapienic acid functions as a “first-line” component of the innate immune system at the cutaneous surface. The species-specific nature of sapienic acid in human sebum is related to the unique architecture of human skin and its microbial environment. Insight into pathogenesis of human skin disease will benefit from further investigation into the biochemistry of sapienic acid production in human sebaceous glands.