February 15-18, 2009 Washington State Convention Center Seattle ...

DIETARY CARBOHYDRATE LEVEL AFFECTS EXPRESSION OF HEPATIC GLUCOSE
TRANSPORTERS IN RAINBOW TROUT (Oncorhynchus mykiss): IS RAINBOW TROUT
GLUT2 DIFFERENT FROM MAMMALIAN GLUT2?
Jon J. Amberg*, Gordon K. Murdoch, Barrie D. Robison, Madison S. Powell, Kenneth J. Rodnick, Rodney A. Hill
and Ronald W. Hardy
Department of Forestry and Natural Resources
Purdue University
West Lafayette, IN 47907 USA
jamberg@vandals.uidaho.edu
Little is known about the hepatic facultative glucose transporters (GLUT1 and GLUT2) in rainbow trout (Oncorhynchus
mykiss). Both, GLUT1 and GLUT2 are bi-directional transporters and are transcriptionally regulated by glucose. In mammals,
GLUT1 has been referred to as the basal glucose transport and GLUT2 senses blood glucose levels and then transports glucose
into hepatocytes and β-cells, which leads to the release of insulin. Thus, these transporters are significant factors in regulating
blood glucose levels in mammals but little is known about their role in fish. Rainbow trout are generally considered intolerant
to high levels of dietary carbohydrates and are inefficient at regulating blood glucose levels. This study used quantitative realtime
PCR (qRT-PCR) to evaluate GLUT1 and GLUT2 expression in response to different dietary carbohydrate levels as well as
using an in silico approach to compare amino acid sequences of GLUT2 proteins with other fish and mammalian species.
Rainbow trout were fed diets containing 0, 15, 25 or 35% starch for 12 weeks. Expression patterns of GLUT1 and GLUT2 in
fish were compared between dietary treatment groups using real-time PCR. To compare amino acid sequences of the GLUT2
proteins, we used software packages to identify putative transmembrane domains, glycosylation and phosphorylation sites
and compared the known functional residues and their locations in the amino acid sequences of mammalian and other piscine
GLUT2 with the rainbow trout GLUT2 sequence.
GLUT1 mRNA content was not affected by dietary carbohydrate. However, GLUT2 was down-regulated in fish fed the 25 or
35% carbohydrate diets. These diets resulted in significantly increased plasma glucose levels and high liver glycogen content
in the fish. Thus, when both plasma glucose levels and liver glycogen content are elevated, the rainbow trout liver decreases
GLUT2 expression that would minimize the uptake of glucose by the liver. This differs from the expression pattern of mammalian
GLUT2. Mammalian hepatic GLUT2 expression correlates with blood glucose levels, which allows for increased
hepatic glucose uptake and increased utilization of glucose as an energy source when blood glucose levels increase. These
results suggest there may be functional differences of rainbow trout GLUT2 than mammalian GLUT2. Preliminary in silico
analysis indicates that differences between the GLUT2 proteins may help to explain the rainbow trout’s intolerance to high
dietary carbohydrates.
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