CERAMIDES ARE TRANSPORTED THROUGH THE GOLGI-APPARATUS IN HUMAN KERATINOCYTES IN-VITRO

The intercellular lipid sheets of the stratum corneum constitute the e pidermal permeability barrier that permits terrestrial life. Although lamellar granules are known to deliver the precursors of the stratum c orneum lipids into the intercellular spaces, their site of origin rema ins unknown. Lamellar granules have characteristics of both secretory granules and lysosomes, which are known to originate from the Golgi ap paratus in other cell types. Glucosylceramides, a major component of l amellar granule contents and the precursors of stratum corneum ceramid es, have been found to be synthesized primarily in the early compartme nts of the Golgi apparatus in other cell types. We have investigated t he transport and metabolism of a fluorescently labeled ceramide in hum an keratinocyte cultures using laser-scanning confocal microscopy and lipid analysis. We found that ceramide is metabolized to glucosylceram ide and sphingomyelin as it passes through the Golgi apparatus and the metabolites are then delivered to the plasma membrane. Cold temperatu re, Brefeldin A, and monensin, all known to inhibit transport from the Golgi to the plasma membrane, prevented ceramide metabolites from app earing at the plasma membrane. Because glucosylceramides are one of th e most important lipid constituents of lamellar granules, these result s support the hypothesis that the Golgi is the origin of lamellar gran ules.