We characterize functional roles of a newly discovered chemical,
sphingosylphosphorylcholine (SPC), in the epidermis by elucidating the
biological effect of SPC on human keratinocytes in culture. The intracellular
calcium level of human keratinocytes was increased by incubation with SPC, but not with
sphingosine (SS) or
sphingomyelin (SM). The addition of SPC,
sphingosine 1-phosphate (SSP), or SS to human keratinocytes
at 10 microM concentrations also significantly suppressed
DNA synthesis, and SPC, but not SSP, or SS increased the activities of membrane-bound and soluble
transglutaminases (TGases). Reverse transcription polymerase chain reaction (RT-PCR) of TGase transcripts revealed that SPC treatment
at 10 microM concentrations increased the expression of TGase 1
mRNA. The increased activity of soluble TGase was accompanied by the concomitant activation of
cathepsin D as revealed by the increased ratio of mature active form to inactive intermediate form of the
protease. Pretreatment of human keratinocytes with
pepstatin, a protease inhibitor, blocked the increase in soluble TGase activity induced by treatment with SPC. Consistently, SPC treatment at 1-10 microM concentrations stimulated the cornified envelope formation. These findings suggest that SPC plays an important role in the altered keratinization process of epidermis in
skin diseases with high expression of
sphingomyelin deacylase, such as
atopic dermatitis.