Mgat5, a regulator of metabolic homeostasis in mice.

摘要

Golgi beta1,6N-acetylglucosaminyltransferase V (Mgat5) produces beta1,6GIcNAc-branched N-glycans on glycoproteins, which increases their affinity for galectins and opposes loss from cell surface to constitutive endocytosis. Oncogenic transformation increases Mgat5 expression, and enhances sensitivities to growth factors, cell motility and tumour metastasis. Therefore, I explored growth and tissue renewal of Mgat5-/- mice, and found that they are hypoglycaemic, resistant to weight-gain, hypersensitive to fasting, display increased oxidative respiration, reduced fecundity and decreased serum leptin. These phenotypes are similar to that of calorie restricted (CR) rodents; however, unlike CR animals, Mgat5-/- mice have accelerated loss of muscle and bone mass and reduced lifespan. CR-induced longevity involve downregulation of the PI3K/Akt/TOR pathway, downstream of many growth factor receptors including EGF, PDGF and IR. In this regard, Mgat5-/- primary MEFs and macrophages are suppressed in MAPK activation in response to EGF, PDGF, IGF, and FCS. However, muscle satellite cells and bone marrow osteoblasts revealed a FCS-induced signalling imbalance, favouring Smad2/3 activation over Erk, which may contribute to reduced growth, self-renewal and longevity of the Mgat5-/- animals.;Aged CR rodents exhibit decreased oxidative damage, implicating either decreased ROS production or elevated antioxidant activity. I demonstrated that Mgat5-/- primary MEFs have decreased glucose uptake and mitochondrial ROS. Unlike WT control, PyMT Mgat5-/- cells fail to down regulate glucose transport, protein synthesis, ROS and activation of Erk and Akt following serum withdrawal. PyMT transformation reveals that Mgat5-deficiency not only impairs positive growth signalling, but also suppresses negative growth regulation, allowing for proliferation and tumourigenicity. Mgat5 participates upstream of PyMT signalling through N-glycan modification of growth factor receptors, but also downstream of Akt signalling through its dependence on glucose uptake for supply of UDP-GlcNAc. Therefore, Mgat5 enhances cellular signalling and growth, and metabolism and longevity in the animal.;Pten opposes PI3K/Akt signalling and is a potent tumour suppressor. Mgat5 +/- Pten+/- and Mgat5-/- Pten +/- mutant mice showed a small increase in lifespan compared with Pten+/- mice, due to an apparent delay in tumour progression. Primary Pten+/-Mgat5-/- MEFs are normalised for Akt activation and cell spreading, and Pten+/- MEFs have enhanced L-PHA staining, suggesting that Mgat5 structures are regulated by P13K signalling.