Fulvic acid (FA) is class of compounds of humic substances formed through the degradation of organic substances by chemical and biological processes. FA has been utilized in traditional Chinese medicine and possesses various pharmacological properties. Previously, we reported that FA extracted from solubilized excess sludge (SS-FA) had an inhibitory effect on β-hexosaminidase release in human leukemia basophilic (KU812) cells. In this study, we investigated the effects of SS-FA on the immediate-type allergic reaction and studied its possible mechanisms of action in KU812 cells following activation with phorbol myristate acetate (20 nmol L−1) plus calcium ionophore A23187 (1 μmol L−1) (PMACI). The inhibitory effect of SS-FA on degranulation in PMACI-stimulated KU812 cells was examined using histamine release assay. SS-FA significantly decreased the histamine release in KU812 cells at concentrations of 0.1–10.0 μg mL−1. To gain more information regarding the mechanism of the suppression of degranulation following SS-FA treatment, microarray was conducted to determine which genes were differentially expressed in response to SS-FA in PMACI-activated KU812 cells. From a total of 201 genes in the DNA chip, 28 genes were up-regulated and 173 genes were down-regulated in cells pretreated with SS-FA for 15 min and stimulated with PMACI. From the 71 genes that showed more than two fold change in expression, 16 genes were significantly down-regulated that were subjected to hierarchical clustering. SS-FA affected the expression of genes that were involved in the following pathways: signal transduction, cytokine–cytokine receptor interaction, immune response, cell adhesion molecules and IgE receptor β subunit response
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