Ceramide potentiates, but sphingomyelin inhibits, vitamin D-induced keratinocyte differentiation: comparison between keratinocytes and HL-60 cells

Abstract Differentiation of epidermal keratinocytes and leukemia HL-60 cells induced by 1,25-dihydroxyvitamin D [1,25(OH)₂D] has been reported to be mediated, at least in part, by increases in cellular ceramide levels. Ceramides produced by 1,25(OH)₂D-induced sphingomyelin (SM) hydrolysis also contribute to the permeability barrier lipids in keratinocytes. Exogenously supplied SM is taken up by mammalian cells, including keratinocytes, and is incorporated into cellular pools. However, the effects of exogenously added SM on keratinocyte differentiation have not been studied. Therefore, in this study, we compared exogenously added SM with a cell-permeable ceramide for their ability to stimulate keratinocyte differentiation induced by 1,25(OH)₂D. Both short-chain ceramide (C2-cer) and SM stimulated the differentiation and inhibited the proliferation of HL-60 cells. As expected, this effect was potentiated by 1,25(OH)₂D. However, SM inhibited the differentiation and stimulated the proliferation of keratinocytes. While C2-cer potentiated the effects of 1,25(OH)₂D, SM reversed the effects of 1,25(OH)₂D on keratinocytes. The ratio of SM to ceramide was significantly different between keratinocytes and HL-60 cells. While the SM level of HL-60 cells were twice that of keratinocytes, keratinocytes contained ten times more ceramides than HL-60 cells, resulting in a ceramide/SM ratio 17 times higher in keratinocytes. Thus, we identified similarities and significant differences in the sphingolipid-mediated cell signaling pathway between keratinocytes and HL-60 cells. While SM stimulated HL-60 cell differentiation, presumably by incorporation into SMase-accessible membrane pools, it inhibited keratinocyte differentiation. In keratinocytes, SM was possibly incorporated into a different cellular pool (barrier lipid pool) or altered membrane phospholipid metabolism and membrane fluidity.