The ceramide [NP]/[NS] ratio in the stratum corneum is a potential marker for skin properties and epidermal differentiation

Background: Specific species of ceramides (Cer), major constituents of lipids in the stratum corneum (SC), are decreased and are correlated with SC barrier and water-holding functions in the skin of patients with atopic dermatitis (AD) or psoriasis (Pso). However, possible correlations between Cer subclass ratios and skin properties in barrier-disrupted skin and in healthy skin remain unclear. The objective of this study was to identify a new marker to evaluate skin properties and epidermal differentiation in SC not only in barrier-disrupted skin but also in healthy skin. Methods: The Cer subclass ratios in the SC of healthy control subjects and in patients with AD or Pso were evaluated. Correlations with candidate markers and facial skin features of healthy Japanese females (20-74 years old,n = 210) were investigated. Variations of markers during epidermal differentiation were studied in human epidermis and in cultured keratinocytes. Results: The ratios of Cer [NP]/[NS], Cer [NH]/[NS], Cer [NP]/[AS], Cer [NH]/[NS], Cer [NDS]/[AS], Cer [AH]/[AS] and Cer [EOP]/[AS] showed significant differences between non-lesional skin of AD patients and normal skin of healthy control subjects, as well as Pso patients and their healthy control subjects. The Cer [NP]/[NS] ratio was correlated with SC functional parameters (transepidermal water loss and capacitance) and with skin appearance (texture, scaling and color) even in the cheek skin of healthy female subjects. The Cer [NP]/[NS] ratio in the SC was approximately 18-times higher than in living keratinocytes, and it increased as they differentiated. Conclusions: The Cer [NP]/[NS] ratio in the SC is a potential marker for skin properties and epidermal differentiation in barrier-disrupted skin as well as in healthy skin.