Gf. Davies et al., THE PHOSPHORYLATION STATE OF THE CAMP RESPONSE ELEMENT-BINDING PROTEIN IS DECREASED IN DIABETIC RAT-LIVER, Archives of biochemistry and biophysics, 323(2), 1995, pp. 477-483
Phosphoenolpyruvate carboxykinase (PEPCK) is the rate-limiting enzyme
of gluconeogenesis. This metabolically important enzyme is unique in t
hat it has no known allosteric modifiers, and all of the regulation of
its activity is exerted at the level of gene expression. The expressi
on of the PEPCK gene in liver is elevated in most forms of diabetes, a
nd plays a major contributory role in the hyperglycemia characteristic
of this disease. In this study, we initiated studies to determine the
molecular basis for the increased PEPCK gene expression in diabetes.
RNase protection assays of RNA isolated from control, streptozotocin-i
nduced diabetic, and insulin-treated diabetic rat liver indicated that
PEPCK mRNA levels are elevated two- to threefold in diabetic rat live
r compared to controls. Nuclear run-on assays indicated that the incre
ased PEPCK mRNA levels can be fully accounted for by changes in the tr
anscription rate of the gene, We next initiated characterization of th
e cAMP response element binding protein (CREB) in diabetic rat liver,
since it is known to play a major role in mediating the basal transcri
ptional activity of the PEPCK gene as well as the cAMP-dependent stimu
lation of PEPCK gene transcription, the latter through the phosphoryla
tion of serine 133 of CREB, Western blot analysis of nuclear lysates p
repared from rat livers indicated that CREB protein levels in diabetic
rat liver nuclei were similar to those of controls. However, using an
antibody which specifically recognizes the serine 133-phosphorylated
form of CREB, we found that the levels of phospho-CREB were significan
tly decreased in diabetic rat liver, an effect which insulin treatment
reversed. This observation suggests that overexpression of the PEPCK
gene in diabetes is not linked to the cAMP signaling system in liver.
(C) 1995 Academic Press, Inc.