PEPCK: a model of eukaryotic gene expression
PEPCK, phosphoenol pyruvate carboxykinase is traditionally viewed as a key enzyme
in gluconeogenesis, the conversion of pyruvate or lactate back to glucose. This
process is carried out in the liver typically during starvation to maintain blood glucose
levels. PEPCK and pyruvate carboxylase together reverse pryuvate kinase, one of the
irreversible steps in glycolysis.
PEPCK is also important in making glycerol. One may ask why is glycerol important?
Fatty acids are packaged into triglycerides with glycerol so 2 sites where it is really
important are the liver and adipose tissue. Pyruvate is converted to Oxaloacetate
(OAA) by pyruvate carboxylase then to PEP by, yours truly, PEPCK then up to
DiHydroxyAcetonePhosphate (DHAP) then to glycerol-3 P by glycerol-3
dehydrogenase. And this is all because adipose tissue doesn’t have glycerol kinase.
You need glycerol-3 phosphate to form triglycerides. This process is part of
lipogenesis.
H
H
C
OH
H
C
OH
O
H
C
O
P
H
O-
O-
Glycerol-3 phosphate
H
O
H
C
O
C
H
C
O
CO
H
C
O
CO
H
Triglyceride
This packaging is most active in adipose tissue when triglyceride is delivered to it
from the liver in VLDLs. The fatty acids are stripped off by lipoprotein lipase (an
enzyme which lives on the outside of the cell) and the fatty acids are transported
across the plasma membrane on carriers and then repackaged inside the adipocyte as
triglyceride. Some fatty acids will be synthesised in the adipocyte but most are made
or repackaged in the liver and sent to the adipocyte for storage.
Glycerol-3 P demand would be highest in the adipocyte in a fed state when there are
lots of fatty acids to be repackaged. Glycerol-3 P is made in the liver by glycerol
kinase, perhaps because that produced by glycolysis is being rapidly converted to
pyruvate for fatty acid synthesis and the glycerol can be recycled.
Overexpression studies: overexpress PEPCK in muscle and you have supermouse,
overexpress it in adipose tissue and you have an obese, diabetic mouse?
It needs to be up-regulated during fasting/starvation in the liver to maintain blood
glucose. Lactate and pyruvate back up to glc.
It is regulated totally by gene expression, no enzyme activation.
Let’s look at the available options for up-regulating our enzyme.
Transcription
Processing
Translation
Activation
In the normal fed state PEPCK mRNA in the liver is rapidly turned over. This is
typical for a sequence which is regulated at the level of gene expression. Why?
In the fed state insulin is released by the pancreas and this suppresses transcription of
PEPCK, the mRNA is unstable so very little PEPCK protein is synthesised. The
protein is also rapidly turned over??
During fasting, a glucocorticoid, cortisol is released and this steroid activates the
transcription of PEPCK in the liver. It also stabilises the mRNA. Cortisol, being a
steroid hormone and hydrophobic can slip through the plasma membrane where it
binds to a receptor, the GR or glucocorticoid receptor. The receptor steroid complex
then enters the nucleus and it binds to the GRE.
Starvation/fasting also stimulates the release of glucagon which binds to its receptor,
stimulates adenylyl cyclise which increases cAMP  PKA activated and this directly
or through a cell signal pathway phosphorylates a transcription factor, CREB which
enters the nucleus and binds to the CRE site.
PEPCK is up-regulated also during adipogenesis when immature fibroblasts
differentiate to form adipocytes. In this situation a ligand binds to PPARg which
dimerises with another transcription factor RAR then enters the nucleus, binding to
the PPARRE. In each case the activator binds to the DNA at a specific site then
interacts with the basal transcription machinery.
During starvation, although cortisol upregulates PEPCK in liver it acts to
downregulate its transcription and stability in adipocytes.