Fig. 4.

Proposed mechanism of Vit C-mediated increase in ceramide production in human keratinocyte (KC). Vit C supplementation increases epidermal ceramide production by three pathways: i) increased de novo ceramide synthesis pathway due to activations of serine palmitoyltransferase (SPT) and ceramide synthase (CerS); ii) sphingomyelin (SM) hydrolysis via substrate regulation of constitutively expressed sphingomyelinases (SMases); iii) increased sphingosine-1-phosphate (S1P) hydrolysis by sphingosine-1-phosphate phosphatase (SPPase) to produce sphingosine (SO), followed by ceramide synthesis by CerS.