In this study, we report the first cloning and characterization of a N-acetylneuraminic acid phosphate synthase gene from Drosophila melanogaster, an insect in the protostome lineage. The gene is ubiquitously expressed at all stages of Drosophila development and in Schneider cells. Similar to the human homologue, the gene encodes an enzyme with dual substrate specificity that can use either N-acetylmannosamine 6-phosphate or mannose 6-phosphate to generate phosphorylated forms of both the sialic acids, N-acetylneuraminic acid and 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid, respectively, when expressed in either bacterial or baculoviral expression systems. The identification of a functional sialic acid synthase in Drosophila indicates that insects have the biosynthetic capability to produce sialic acids endogenously. Although sialylation is widely distributed in organisms of the deuterstome lineage, genetic evidence concerning the presence or absence of sialic acid metabolism in organisms of the protostome lineage has been lacking. Homology searches of the Drosophila genome identified putative orthologues of other genes required for sialylation of glycoconjugates.
We investigated whether silibinin significantly affects gene expression, production and secretion of mucin from cultured airway epithelial cells. Confluent NCI-H292 cells were pretreated with silibinin for 30 min and then stimulated with epidermal growth factor (EGF), phorbol 12-myristate 13-acetate (PMA) or TNF-α for 24 h. The MUC5AC mucin gene expression and mucin protein production were measured by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). The effect of silibinin on TNF-α-induced activation of NF-κB p65 was also examined. Confluent primary rat tracheal surface epithelial (RTSE) cells were pretreated with adenosine triphosphate (ATP) for 5 min and then treated for 30 min in the presence of silibinin to assess the effect on mucin secretion using ELISA. The results were as follows: (i) silibinin inhibited the expression of the MUC5AC mucin gene induced by EGF, PMA or TNF-α from NCI-H292 cells; (ii) silibinin also inhibited the production of MUC5AC mucin protein induced by the same inducers from NCI-H292 cells; (iii) silibinin inhibited the activation of NF-κB p65 by TNF-α in NCI-H292 cells; (iv) silibinin significantly decreased ATP-induced mucin secretion from cultured RTSE cells. This result suggests that silibinin can regulate gene expression, production and secretion of mucin by directly acting on airway epithelial cells.
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