UDP-GlcNAc2-epimerase regulates cell surface sialylation and ceramide-induced cell death in human malignant lymphoma

Stress signals induce ceramide (cer) through sphingomyelinase activation, and metabolites of cer such as sphingosine (Sph) and sphingosine-1-phoshate (S-1-P) play a significant role in many biological processes. This study aimed to elucidate the association between the alteration in cell surface sialylation and ceramide-induced cell death in the human Burkitt's lymphoma cell line, HBL-8. The highly sialylated 3G3 clone was less sensitive to C6-ceramide-induced cell death. On the other hand, the hyposialylated 3D2 clone was more sensitive to C6-ceramide-induced cell death. Neuraminidase treatment or knockdown by siRNA of uridine diphosphate-N-acetylglucosamine 2-epimerase (UDP-GlcNAc2-epimerase), which is a key enzyme of sialic acid biosynthesis, enhanced the amount of cell death induced by C6-ceramide in the highly sialylated 3G3 clone. Sialic acid metabolic complementation assays using several precursors of sialic acid showed that cell surface resialylation by N-acetyl-D-mannosamine (ManNAc) inhibited C6-ceramide-induced cell death. The amount of cell death by C6-ceramide was enhanced after pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002 in both clones. In addition, clone 3G3 was less sensitive to Sph than the 3D2 clone. In conclusion, in human malignant lymphoma, ceramide and its metabolite-induced cell death is regulated by the amount of sialic acid on the cell surface which in turn is regulated by mRNA expression of UDP-GlcNAc2-epimerase.