Biol. Pharm. Bull. 29(8) 1575—1579 (2006)

sphingomyelin (SM) that have different polar head groups, a sugar chain and phosphocholine, respectively. It has been reported that GSLs are involved in cell growth, differentiation, and adhesion. SM is defined as a precursor for ceramide and sphingosine that are involved in intracellular signaling pathways for cytokines. In pathogenesis, GSLs are receptors for various types of virulent agents such as bacteria and viruses. Among GSLs, some gangliosides, GSLs containing sialic acids are dominant receptors for influenza viruses. Influenza viruses attach to the receptor molecules such as gangliosides and/or glycoproteins on the host cell surface and enter the cells. In the infected cells, viral genome is replicated and viral proteins are synthesized by host translational systems. These materials are carried and assembled along vesicular trafficking systems in the cells. Finally matured viral particles go out from the host cells by the action of neuraminidase, a viral glycoprotein. Recently, several lines of evidence demonstrated new aspects of cellular function of sphingolipids. They are characteristically enriched in microdomains on the cellular membranes together with cholesterol and are indispensable for trafficking of secretary vesicles to the apical surface of epithelial cells, such as Madin-Darby canine kidney (MDCK) cells. It is also well known that transfected hemagglutinin (HA) is a specific marker for subcellular organelles related with trafficking vesicles to the apical surface, such as trans-Golgi network. The association of transmembrane domain of the transfected HA with sphingolipids in vesicular membrane is critically involved in the trafficking of this molecule to the apical surface. The participation of sphingolipid-enriched microdomains is, however undefined in the infection, especially the maturation of influenza viruses in the host cells. In this study, we used two different inhibitors of sphingolipid metabolism to address the issue. The treatment with fumonisin B1 (FB1) suppresses all complex sphingolipids with polar head groups including GSLs and SM in the cells. d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) inhibits the synthesis of GSLs alone, but not SM. By additional treatment of MDCK cells with the inhibitors post viral adsorption, but not pretreatment alone, we observed significant reduction of viral infection. Compositional analysis of cellular sphingolipids and immunocytochemical staining strongly suggest the involvement of sphingolipids in the propagation of influenza viruses in the host cells.