Decreased Phospholipase D (PLD) Activity in Ceramide-Induced Apoptosis of Human Keratinocyte Cell Line HaCaT

Ceramide is recognized as an intracellular lipid second messenger, which induces various kinds of cell function including apoptosis. To evaluate the competence of ceramide on the keratinocyte apoptosis, we examined effects of a cell-permeable ceramide, N-acetylsphingosine (C 2 -ceramide), on a human keratinocyte cell line, HaCaT. C 2 -ceramide induced a distinct apoptosis in HaCaT cells in a time-dependent manner, as inferred by morphologic hallmarks of apoptosis such as bleb formation, cell body shrinkage, nuclear chromatin condensation, and inter-nucleosomal DNA fragmentation. In sharp contrast, an inactive C 2 -ceramide, dihydroC 2 -ceramide, which lacks the 4–5trans double bond, failed to induce the apoptosis. The apoptotic HaCaT cells induced by C 2 -ceramide showed a significant suppression of phospholipase D (PLD) activity, regardless of the presence or absence of guanosine 5'-0-(3-thiotriphosphate) (GTPgS). This indicates that C 2 -ceramide inhibits both GTPγS dependent and GTPγS independent PLD. The membrane associated GTPγS dependent PLD activity was stimulated by recombinant adenosine diphosphate-ribosylation factor. The adenosine diphosphate-ribosylation factor dependent and independent PLD activities were inhibited by C 2 -ceramide in a concentration dependent manner, but not by the inactive C 2 -ceramide. The concentration of C 2 -ceramide to inhibit the membrane associated PLD activity was comparable with that required for apoptosis induction in HaCaT cells. It was thus suggested that downregulation of PLD activity may be involved in the mechanism underlying C 2 -ceramide induced keratinocyte apoptosis.