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Differential regulation of cyclooxygenase-2 expression by phytosphingosine derivatives, NAPS and TAPS, and its role in the NAPS or TAPS-mediated apoptosis.

We investigated the effect of novel phytosphingosine derivatives, N-acetyl phytosphingosine (NAPS) and tetra-acetyl phytosphingosine (TAPS), on induction of apoptosis in HaCaT cells in comparison with C2-ceramide. NAPS/TAPS effectively decreased cell viability in a dose dependent manner mainly due to apoptosis. An apoptosis expression array analysis showed that in the TAPS treated cells 13 genes including COX-2 encoding cyclooxygenase-2, the most induced by TAPS, were up-regulated while 23 others down-regulated. Therefore, we examined the mechanism underlying the altered expression of COX-2. Assays with inhibitors and antibodies against proteins involved in signal transduction demonstrated that NAPS and TAPS elevated COX-2 expression via tyrosine kinase, src, PI-3 kinase and PKC, followed by ERK activation. However, P38 was not involved in the NAPS-mediated COX-2 expression but in the TAPS-mediated. We further demonstrated by FACS analyses that NAPS- or TAPS-mediated apoptosis was greatly increased in cells treated with celecoxib, a selective COX-2 inhibitor. Inhibition of the ERK pathway apparently involved in the NAPS/TAPS-mediated COX-2 expression enhanced the NAPS/TAPS-mediated apoptosis, whereas inhibition of the P38 pathway did not. These results suggest that expression of COX-2 in the TAPS- or NAPS-treated cells may be increased to counteract the effect of those compounds on apoptosis.