UP-REGULATION OF GLUCOSYLCERAMIDE SYNTHASE EXPRESSION AND ACTIVITY DURING HUMAN KERATINOCYTE DIFFERENTIATION

During keratinocyte differentiation, the glycolipid, glucosylceramide (GlcCer), is thought to be synthesized, stored in intracellular lamell ar granules and eventually extruded into the intercellular space where GlcCer is hydrolyzed to ceramide, a major component of the epidermal permeability barrier. Previous studies showed that GlcCer synthase (GC S) activity increases during keratinocyte differentiation; however, th e mechanism by which GCS activity is regulated was not established. In the present study, we prepared anti-peptide antibodies and amplified cDNA probes based on the cDNA sequence for human GCS (Ichikawa, S., Sa kiyama, H., Suzuki, G., Hidari, K. I.-P. J., and Hirabayashi, Y. (1996 ) Proc. Natl. Acad. Sci, U. S. A. 93, 4638-4643) in order to study GCS expression during keratinocyte differentiation. Confluent human kerat inocytes in culture were induced to terminally differentiate by elevat ion of Ca+2 in the medium without exogenous hormones or growth factors . GlcCer synthesis assayed in situ using a fluorescent ceramide analog increased similar to 5-fold during keratinocyte differentiation, peak ing at day 6. Fluorescence microscopy studies of living keratinocytes showed that fluorescent ceramide and/or its metabolites accumulated in the Golgi in undifferentiated cells but targeted to unique vesicular structures that may be derived from the trans-Golgi region. Expression of both GCS mRNA, a similar to 3.8-kilobase transcript on Northern bl ots, and GCS protein, a similar to 38-kDa polypeptide detected by West ern blotting, increased dramatically (similar to 5-fold) during differ entiation, reaching a maximum at about day 8. These results suggest th at GCS is up-regulated at the transcriptional level during keratinocyt e differentiation and provide the first direct evidence for GCS up-reg ulation in any cell type.