Acid ceramidase ASAH1 is a key regulator of epidermal ceramide levels and composition

Maintenance of appropriate ceramide levels and composition in the stratum corneum of the epidermis is essential for skin barrier function. Although ceramide homeostasis is regulated by both synthesis and degradation, the extent of ceramide degradation in the epidermis, as well as the ceramidase responsible for this degradation, has thus far remained unclear. Here, we found that the acid ceramidase ASAH1 is strongly expressed in differentiated human keratinocytes. To investigate its role, we generated ASAH1 KO cells using immortalized human keratinocytes and analyzed their ceramide levels. Under differentiation conditions, ASAH1 KO keratinocytes exhibited a marked accumulation of ceramide classes composed of sphingosine (S) or dihydrosphingosine (dS) and nonhydroxy fatty acid (N) (ceramides NS and NdS). In contrast, ceramides with (an) additional hydroxyl group(s)-such as those containing phytosphingosine (P) or 6-hydroxysphingosine (H) and N or α-hydroxy fatty acid (A) (ceramides NP, NH, AP, and AH)-showed a moderate or no increase. Similar results were obtained upon treatment with SABRAC, a specific ASAH1 inhibitor. In vitro enzyme assays revealed that ASAH1 exhibited strong activity toward NS and NdS, weak activity toward NP and NH, and no activity toward AP. These results indicate that ASAH1-mediated ceramide class-dependent degradation occurs in differentiated human keratinocytes. This degradation likely plays an important role in maintaining appropriate ceramide levels and class composition in the stratum corneum, thereby contributing to the integrity of the skin barrier.