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N-Linked glycosylation (N-glycosylation) of proteins has long been associated with oncogenesis, but not until recently have the molecular mechanisms underlying this relationship begun to be unraveled. Here, we review studies describing how dysregulation of the N-glycosylation-regulating gene, DPAGT1, drives oral cancer. DPAGT1 encodes the first and rate-limiting enzyme in the assembly of the lipid-linked oligosaccharide precursor in the endoplasmic reticulum and thus mediates N-glycosylation of many cancer-related proteins. DPAGT1 controls N-glycosylation of E-cadherin, the major epithelial cell-cell adhesion receptor and a tumor suppressor, thereby affecting intercellular adhesion and cytoskeletal dynamics. DPAGT1 also regulates and is regulated by Wnt/β-catenin signaling, impacting the balance between proliferation and adhesion in homeostatic tissues. Thus, aberrant induction of DPAGT1 promotes a positive feedback network with Wnt/β-catenin that represses E-cadherin-based adhesion and drives tumorigenic phenotypes. Further, modification of receptor tyrosine kinases (RTKs) with N-glycans is known to control their surface presentation via the galectin lattice, and thus increased DPAGT1 expression likely contributes to abnormal activation of RTKs in oral cancer. Collectively, these studies suggest that dysregulation of the DPAGT1/Wnt/E-cadherin network underlies the etiology and pathogenesis of oral cancer., (© The Author 2014. Published by Oxford University Press.)

The dolichol pathway serves in the synthesis of the dolichol-linked oligosaccharide precursor for protein N-glycosylation. Recently, we reported that mRNAs of genes that function at the early steps in the dolichol pathway in yeast, ALG7, ALG1 and ALG2, were co-ordinately induced following growth stimulation of G0-arrested cells in a manner similar to that of the transcripts of the early growth response genes (Kukuruzinska, M.A. and Lennon, K. Glycobiology, 4, 437-443, 1994). To determine whether the entire dolichol pathway was co-ordinately regulated with growth, we examined the expression of genes functioning late in the pathway, including two genes encoding oligosaccharyltransferase subunits, at two critical control points in the G1 phase of cell cycle: G0/G1 and START. We show that early in G1, at the G0/G1 transition point, the late ALG genes and the two oligosaccharyltransferase-encoding genes examined were regulated co-ordinately with the early ALG genes: they were downregulated upon exit from the mitotic cell cycle into G0, and they were induced following growth stimulation in the absence of de novo protein synthesis. All the dolichol pathway genes produced transcripts with short half-lives that were rapidly stabilized in the presence of cycloheximide. In contrast, cell division arrest late in G1, at START, was accompanied by a selective downregulation of only the first dolichol pathway gene, ALG7, and not of the genes functioning later in the pathway. These results indicate that, depending on their position in G1, cells either co-ordinately or differentially regulate the dolichol pathway genes.

The Saccharomyces cerevisiae ALG7, ALG1 and ALG2 genes, whose products function early in the dolichol pathway of protein N-glycosylation, are essential for cell viability, and perturbation in their expression causes G1-specific cell cycle arrest. Here, we show that expression of the ALG7, ALG1 and ALG2 genes is coordinately regulated at the G0/G1 transition point in the yeast life cycle. Carbon starvation, which induces cells to exit from the G1 stage of the mitotic cycle into G0, resulted in a time-dependent decrease in the levels of the early ALG genes' mRNAs. Accordingly, addition of glucose, which stimulates the G0-arrested cells to resume proliferation, resulted in a rapid induction of their mRNAs. Cycloheximide alone also induced the early ALG transcripts, albeit to a much lower extent than glucose. Simultaneous addition of glucose and cycloheximide caused superinduction of these mRNAs, indicating that more than one control level was involved in their activation. Consistent with this, rapid degradation of ALG7, ALG1 and ALG2 mRNAs was completely abolished in the presence of cycloheximide. These data suggest that in yeast, the early N-glycosylation genes are regulated in a manner similar to that of the early growth-response genes.

In the yeast Saccharomyces cerevisiae, the alg2 mutation causes temperature-sensitive growth and abnormal accumulation of the lipid-linked oligosaccharide Man2GlcNAc2-PP-Dol (Jackson et al., Arch. Biochem. Biophys., 272, 203-209, 1989; Huffaker and Robbins, Proc. Natl. Acad. Sci. USA, 80, 7466-7470, 1983). A gene having the function and genomic location of ALG2 was cloned from libraries based on the multicopy plasmid YEp24 and on the centromere plasmid YCp50. Alg2 mutants transformed with plasmids containing ALG2 regained the capacity to grow and to synthesize lipid-linked oligosaccharides normally at the previously non-permissive temperature. ALG2 was essential for viability in haploid and diploid yeast. The ALG2 gene was transcribed into a single mRNA of 1.7 kb in size. The stability of ALG2 mRNA, assessed after thermal inactivation of RNA polymerase II in an rpb1-1 mutant (Herrick et al., Mol. Cell. Biol., 10, 2269-2284, 1990) was very low, with a t1/2 of < 5 min. The ALG2 transcript accumulation was growth dependent, and it was at least an order of magnitude lower in stationary phase cells compared to exponentially growing cells. The putative translation product of ALG2 contained a potential dolichol recognition domain similar to that found in all three glycosyltransferases of the lipid-linked pathway that have been sequenced.(ABSTRACT TRUNCATED AT 250 WORDS)