Abstract
GDP-fucose transporter plays a crucial role in fucosylation of glycoproteins by providing activated fucose donor, GDP-fucose, for fucosyltransferases in the lumen of the Golgi apparatus. Fucose-containing glycans are involved in many biological processes, which are essential for growth and development. Mutations in the GDP-fucose transporter gene cause leukocyte adhesion deficiency syndrome II, a disease characterized by slow growth, mental retardation and immunodeficiency. However, no information is available regarding its transcriptional regulation. Here, by using human cells, we show that TGF-β1 specifically induces the GDP-fucose transporter expression, but not other transporters tested such as CMP-sialic acid transporter, suggesting a diversity of regulatory pathways for the expression of these transporters. The regulatory elements that are responsive to the TGF-β1 stimulation are present in the region between bp −330 and −268 in the GDP-fucose transporter promoter. We found that this region contains two identical octamer GC-rich motifs (GGGGCGTG) that were demonstrated to be essential for the transporter expression. We also show that the transcription factor Sp1 specifically binds to the GC-rich motifs in vitro and Sp1 coupled with phospho-Smad2 is associated with the promoter region covering the Sp1-binding motifs in vivo using chromatin immunoprecipitation (ChIP) assays. In addition, we further confirmed that Sp1 is essential for the GDP-fucose transporter expression stimulated by TGF-β1 using a luciferase reporter system. These results highlight the role of TGF-β signaling in regulation of the GDP-fucose transporter expression via activating Sp1. This is the first transcriptional study for any nucleotide sugar transporters that have been identified so far. Notably, TGF-β1 receptor itself is known to be modified by fucosylation. Given the essential role of GDP-fucose transporter in fucosylation, the finding that TGF-β1 stimulates the expression of this transporter, suggests a possible intracellular link between the function of nucleotide sugar transporter and the TGF-β signaling pathway.
Highlights
Most secreted and membrane proteins, including ion channels, blood groups, growth factors, and their receptors in eukaryotes, are glycosylated in the lumen of the Golgi apparatus
Glycosylation requires activated sugar donors, nucleotide sugars, which are synthesized in cytosol in mammals and must be transported into the lumen of the Golgi apparatus where they serve as substrates for glycosyltransferases
Consistent with the roles of nucleotide sugar transporters (NSTs) in growth and development, we recently showed that NSTs are intimately involved in protein synthesis and secretion
Summary
Most secreted and membrane proteins, including ion channels, blood groups, growth factors, and their receptors in eukaryotes, are glycosylated in the lumen of the Golgi apparatus. These glycoproteins are modified with a variety of sugars including fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine and sialic acid. In humans and Caenorhabditis elegans, there are seven sugars in glycoconjugates, but 17–18 putative NSTs are predicted from their genomic databases [6,7]. This indicates that NSTs possess redundant functions by transporting overlapping substrates. The functional redundancy has been experimentally validated in C. elegans [8]
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