Liver glucose-6-phosphatase proteins in suckling and weaned grey seal pups: structural similarities to other mammals and relationship to nutrition, insulin signalling and metabolite levels

Abstract

Phocid seals have been proposed as models for diabetes because they exhibit limited insulin response to glucose, high blood glucose and increasing insulin resistance when fasting. Liver glucose-6-phosphatase (G6Pase) catalyses the final step in glucose production and is central to glucose regulation in other animals. G6Pase comprises a translocase (SLC37A4) and a catalytic subunit (G6PC). G6PC and SLC37A4 expression and activity are normally regulated by nutritional state and glucostatic hormones, particularly insulin, and are elevated in diabetes. We tested the hypotheses that (1) grey seal G6PC and SLC37A4 cDNA and predicted protein sequences differ from other species' at functional sites, (2) relative G6Pase protein abundances are lower during feeding than fasting and (3) relative G6Pase protein abundances are related to insulin, insulin receptor phosphorylation and key metabolite levels. We show that G6PC and partial SLC37A4 cDNA sequences encode proteins sharing 82-95 % identity with other mammals. Seal G6PC contained no differences in sites responsible for activity, stability or subcellular location. Several substitutions in seal SLC37A4 were predicted to be tolerated with low probability, which could affect glucose production. Suckling pups had higher relative abundance of both subunits than healthy, postweaned fasting pups. Furthermore, relative G6PC abundance was negatively related to glucose levels. These findings contrast markedly with the response of relative hepatic G6Pase abundance to feeding, fasting, insulin, insulin sensitivity and key metabolites in other animals, and highlight the need to understand the regulation of enzymes involved in glucose control in phocids if these animals are to be informative models of diabetes.

HSP70 Abundance and Antioxidant Capacity in Feeding and Fasting Gray Seal Pups: Suckling Is Associated with Higher Levels of Key Cellular Defenses

“…Seal pups also often have higher FFA levels than fasting individuals (Schweigert 1993). Here, FFA levels were 3.5-fold higher in suckling pups compared with fasting pups (Bennett et al 2013). Increased substrate supply to the tricarboxylic acid cycle increases free radical generation and could stimulate cellular defenses.…”

“…Oxidative stress could arise as a result of high levels of fuel availability. Suckling seal pups, including those in this study (Schweigert 1993;Hall 1998;Sakamoto et al 2009;Bennett et al 2013), have higher glucose levels than fasting individuals (∼10 mM vs. 7 mM). However, circulating glucose did not explain the variability in relative HSP70 abundance or antioxidant defenses.…”

“…Blubber thickness was included as a covariate to account for the large variability in body condition within and between groups. Glucose was included as a covariate because it differs between suckling pups and fasting pups (Bennett et al 2013). Individual was included as a random effect in each case.…”

“…Fasting seals also tend to have high blood glucose levels relative to those of terrestrial mammals of a similar size (Keith and Ortiz 1989;Nordøy and Blix 1991;Champagne et al 2005). Suckling pups often have elevated blood FFA and glucose levels (Schweigert 1993;Hall 1998;Sakamoto et al 2009;Bennett et al 2013). Sustained high levels of metabolic fuel, particularly glucose, FFAs, and ketones, are associated with oxidative tissue damage in non-fast-adapted species (Rebolledo and Actis Dato 2005;Marczuk-Krynicka et al 2009;Rains and Jain 2011).…”

“…As described previously (Bennett et al 2013), liver, blubber, and plasma samples were taken from fresh carcasses of suckling (n p 5) and healthy, weaned, fasting (n p 11) gray seal pups at Amet Island, Northumberland Strait, Nova Scotia, Canada, in January 2010. Sampling was approved by the Animal Care Committee at the Maurice Lamontagne Institute, Mont Joli, Quebec, Canada (protocol cpa 10-5), and animals were killed using approved methods under research notice IML-2010-005 by the Department of Fisheries and Oceans.…”