Induction of Ceramide Glucosyltransferase Activity in Cultured Human Keratinocytes

Ceramides are the major component of the extracellular lipids that comprise the epidermal permeability barrier. They are derived from glucosylceramides (GlcCer) upon their extrusion from lamellar granules into the extracellular space in the upper layers of the epidermis. To better understand the regulation of the unique pathway for ceramide production in epidermis, we have studied the activity of the enzyme responsible for GlcCer synthesis, ceramide glucosyltransferase (CerGlc transferase), during keratinocyte culture differentiation. Human keratinocyte cultures were expanded in low calcium keratinocyte growth medium (KGM) and then switched to either normal calcium KGM (nKGM) or “complete” Dulbecco's modified Eagle's medium/Ham's F-12 (3:1) supplemented with 10% fetal bovine serum (cDMEM). At 7 and 10 days after the medium switch, electron microscopy revealed that cDMEM cultures were more fully differentiated morphologically and contained numerous lamellar granules. The GlcCer/DNA content of cDMEM cultures increased to 6 times that of day 0 cultures and was nearly 4 times greater than that of nKGM cultures, whereas the total lipid/DNA content of cDMEM cultures increased to only 1.8 times that of day 0 cultures and was ∼1.2 times that of nKGM cultures. CerGlc transferase activity/DNA increased 6 times in cDMEM cultures but