GLUCOSYLCERAMIDE METABOLISM IS REGULATED DURING NORMAL AND HORMONALLYSTIMULATED EPIDERMAL BARRIER DEVELOPMENT IN THE RAT

Glucosylceramides, delivered to the stratum corneum interstices by exo cytosis of lamellar body contents, are enzymatically hydrolyzed to cer amides, which are major components of the lipid lamellar bilayers that mediate epidermal barrier function. Because this conversion is critic al for permeability barrier homeostasis in the adult animal, in this s tudy we measured the charges in activities of the enzymes responsible for the synthesis of glucosylceramide and its conversion to ceramide, UDP-glucose:ceramide glucosyltransferase (GC synthase) and beta-glucoc erebrosidase (beta-GlcCer'ase), respectively, during fetal barrier for mation. In epidermis from rats of gestational age 17-21 days, GC synth ase activity peaked on day 19, prior to barrier competence, whereas be ta-GlcCer'ase activity rose throughout barrier formation, exhibiting a 5-fold increase over this time period. beta-GlcCer'ase protein rose i n parallel with activity, as did mRNA levels. Enzyme activities in ski n explants from 17-day fetal rats, incubated up to 4 days in hormone- and serum-free media, paralleled those measured at corresponding time points in utero. Incubation with hormones that accelerate barrier deve lopment had minimal effects on GC synthase activity, whereas beta-GlcC er'ase activity was significantly increased after 1 or 2 days in cultu re. Finally, inhibition of beta-GlcCer'ase with conduritol B epoxide p revented barrier development in vitro and was accompanied by abnormali ties in the lamellar bilayer ultrastructure of the stratum corneum. Th ese data indicate that both synthesis and hydrolysis of glucosylcerami de are regulated during fetal development. Furthermore, the enzymatic hydrolysis of glucosylceramide to ceramide is essential for fetal barr ier ontogenesis.