Within the market demand for natural solutions, consumers seek products with skin-identical approaches to maintain and improve skin health and beauty. In response, formulators can develop products to improve skin function by focusing on activity in the surface layers of skin as well as stimulating activity from within. In this context, ceramides, which are present in skin and consist of N-acylated sphingoid bases,1 are well-established in the literature for their importance in skin barrier function and stratum corneum (SC) moisturization. However, less has been published on the effects of free sphingoid bases that are also present in skin. Phytosphingosine, typically an 18-carbon chain that incorporates a 2-amino-1,3, 4-triol for its lipid head group, is a free sphingoid base that constitutes part of the chemical antimicrobial barrier of the SC to help control infection. In addition, it can serve many other activities, which are the subject of this literature review.
Phytosphingosine as a PPAR Ligand
The anti-inflammatory and pro-differentiating benefits of phytosphingosine, which will be discussed later in this article, occur through its action as a peroxisomal proliferator activated receptor (PPAR) ligand. Nuclear hormone receptors exert their effects directly on genes by binding to DNA upon their activation by a receptor ligand.2 As an example, readers are likely familiar with the effects of retinoic acid and its mediation via the retinoid receptors, which are part of the superfamily of nuclear hormone receptors. A coordinate control of two families of transcription factors exists that mediates retinoic acid’s effects, namely the retinoid-x-receptor (RXR) and the retinoic acid receptor (RAR). The active transcription factor complex is formed by heterodimerization of the two receptors and binding of their retinoic acid metabolites.
PPARs are also members of this family and have been shown to be important in the regulation and catabolism of dietary fats,3 stimulation of epidermal differentiation,4–5 reduction of inflammation,6–7 and reduction of melanocyte proliferation8 and tyrosinase levels,9 together with reducing the signs of aging.10 Like the retinoid receptor complex, PPARs also form an active complex with RXR; this suggests that the combination of phytosphingosine and retinol might be advantageous since retinoid binding would activate the RXR receptor while phytosphingosine binding would activate the PPAR receptor. The dimerization of both activated receptor molecules would thus trigger highly specific molecular processes related to the above mentioned effects. These activities are regulated through one or more of the three isoforms of PPARs: PPARα, PPARδ and PPARγ. In human skin, epidermal differentiation is predominantly regulated by PPARδ and, to a lesser extent, by PPARα; inflammation through PPARα and some PPARγ; and melanocyte proliferation mainly via PPARγ. Fibroblast function is believed to be mediate via PPARα. Downie et al.11 used human chest sebaceous glands as 7-day cultured whole organs and were able to demonstrate that activators of PPARα and PPARγ inhibited the rate of sebaceous lipogenesis and reduced the synthesis of the sebum specific lipids squalane and triacylglycerol in human sebaceous glands. They concluded that since the suppression of sebum secretion is associated with reduced acne activity, the nuclear hormone receptors involved may open new avenues in the development of novel acne treatments. Most recently, Ottaviani et al.12 reported that increased expression of PPARs induced by squalene peroxides in HaCat keratinocytes leads to the down-regulation of inflammatory mediators.