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Ceramide-Dependent Regulation of Human Epidermal Keratinocyte CD1d Expression during Terminal Differentiation¹

Rita Fishelevich^*, Alla Malanina^*, Irina Luzina, Sergei Atamas, Miriam J. Smyth, Steven A. Porcelli and Anthony A. Gaspari^2,*,¶

^* Department of Dermatology, Division of Rheumatology, Department of Medicine, and Cancer Center, University of Maryland School of Medicine, Baltimore, MD 21201; Department of Microbiology and Immunology and Department of Medicine, Albert Einstein College of Medicine, New York, NY 10461; and ^¶ Research Service, Veteran’s Administration Medical Center, Baltimore, MD 21201

Human keratinocytes (KC), when cultured under conditions to remain undifferentiated or to terminally differentiate, changed their cellular distribution of CD1d. As studied by confocal microscopy, undifferentiated KC had a pool of cytoplasmic CD1d, whereas after terminal differentiation, this molecule localized in the cell membrane, which recapitulates CD1d expression in vivo. A comparison of undifferentiated and differentiated cultured KC did not reveal any differences in the association with ₂-microglobulin, invariant chain of class II MHC, or patterns of glycosylation, suggesting that these biochemical properties are not regulating the cellular distribution of CD1d. Time-course studies of CD1d gene expression indicated that KC slowly increased gene expression with CaCl₂-induced terminal differentiation. Increased CD1d gene expression was dependent on ceramide synthesis, because fumonisin B1, a ceramide synthetase inhibitor, blocked the increase in CD1d gene expression during terminal differentiation. Similarly, exogenous ceramide or the ceramidase inhibitor, B13, induced CD1d gene expression by undifferentiated, but not terminally differentiated, KC. A protein kinase C- (PKC-) inhibitor (a pseudosubstrate oligopeptide), but not a PKC- inhibitor, significantly decreased CD1d gene expression by undifferentiated or ceramide-stimulated cultured, undifferentiated KC. As expected, downstream signaling events of PKC- (JNK phosphorylation and NF-B accumulation in the nucleus) were also attenuated. The calcineurin phosphatase inhibitor cyclosporine A, which blocks KC terminal differentiation, also blocked CD1d gene expression by cultured KC. In conclusion, this novel function of cellular ceramides extends the importance of this class of biologically active lipids beyond that of terminal differentiation and barrier function in normal human skin.

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