Unique structural motif supports mannosylphospho dolichol synthase: an important angiogenesis regulator

Mannosylphospho dolichol synthase (DPMS) catalyzes the transfer reaction GDP-mannose + Dol-P <--> Dol-P-Man + GDP, a 'key step' in the assembly of lipid-linked oligosaccharide (LLO) and a pre-requisite for asparagine-linked (N-linked) protein glycosylation. DPMS is present from a protozoan parasite to human, and its sequence carries a cAMP-dependent phosphorylation motif. We have evaluated the involvement of DPMS in angiogenesis, an essential physiological event during the growth of breast and other solid tumors. It has been observed that enhancers of intracellular cAMP accelerated the capillary endothelial cell proliferation by reducing the cell cycle duration. Reduced Con A to WGA fluorescence ratio indicated high level complex type N-glycans on the cell surface. This was supported by upregulated LLO biosynthesis in cells stimulated either with a beta-agonist isoproterenol or other cAMP enhancer, such as 8Br-cAMP, forskolin, cholera toxin, or prostaglandin E1. The turnover (t((1/2))) of LLO was also increased. Increased LLO biosynthesis correlated extremely well with the DPMS activity in cells treated with 8Br-cAMP. High DPMS activity in isoproterenol-treated cells was not due to an increased gene expression because actinomycin D failed to block the upregulation. cDNA cloning of capillary endothelial cell Dpm1 gene and the deduced amino acid sequence identified a PKA motif in capillary endothelial cell DPMS. Thus, it has been concluded that increased DPMS activity through protein phosphorylation is a driving force for angiogeneis. Its abolition, however, led to cell arrest in G1 and induction of apoptosis.