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

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.

Effect of lactic acid isomers on keratinocyte ceramide synthesis, stratum corneum lipid levels and stratum corneum barrier function.

Alpha-hydroxy acids are effective agents for the treatment of skin xerosis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectively. To date, the relative efficacy of the different isomers of lactic acid has not been evaluated and the mode of action of lactic acid in improving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xerosis. In vitro, lactic acid enhanced the production of ceramides by keratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorporated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactate. In vivo, lactic acid increased the levels of stratum corneum ceramides. Whereas, lotions containing L-lactic acid resulted in the greatest increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The increases in stratum corneum ceramide levels following lactic acid treatment also led to improvements in stratum corneum barrier function, measured by transepidermal water loss following a challenge to the skin with SLS and in the regression phase of a moisturization efficacy study. Significant improvements in barrier function and resistance to the appearance of skin xerosis were observed following L-lactic acid and D,L-lactic acid, but not following D-lactic acid treatment. From these results we believe that lactic acid, particularly the L isomer, stimulates ceramide biosynthesis leading to increased stratum corneum ceramide levels which results in superior lipid barrier and a more effective resistance against xerosis.

Adult age differences in free recall as a function of basis of organization and method of presentation.

To examine age-related differences in the discovery of intralist relationships, young and elderly adults were presented a free-recall list in either the conventional successive single-item format or in a whole-list display. A list that could be organized by associative or rhyming intralist relationships was used to test the levels-of-processing model of memory as an explanation of age differences in recall. Young adults recalled more base-words, associates, and rhymes than elderly subjects on immediate free and cued tests and on an uncued test one week later. Elderly subjects showed less utilization of both semantic and nonsemantic intralist relationships. Age did not interact with method of presentation. Recall and organization deficits occurred for elderly adults even though they were less anxious than young adults.