CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 4, July/August 2011
178
AFRICA
Discussion
We previously found a significant increase in body weight and
fasting blood glucose levels for 18- to 20-week-old db/db female
mice versus matched controls.
8
For the current study, 2D-PAGE
illustrated differences between control and obese hearts for
female mice that we broadly categorised into two groups, i.e.
related to energy metabolism and contraction/cytoskeleton.
Here we identified changes in several proteins that play a role
in mitochondrial energy metabolism, although only three, i.e.
ATP synthase D chain, ubiquinol cytochrome-C reductase core
protein 1 and electron transfer flavoprotein subunit alpha were
identified as real changes.
ATP synthase D chain peptide levels were up-regulated for
the db/db mouse versus controls. ATP synthase D chain is a non-
enzymatic component of the F
0
channel of the F
1
F
0
ATP synthase
that links the flow of protons (from within the inter-mitochon-
drial membrane space) through its F
0
channel to ATP synthesis
that occurs on F
1
. We propose that this may represent an adaptive
mechanism by the obese female heart to increase proton transfer
and thereby enhance mitochondrial ATP production. In agree-
ment with this notion, we found a marked induction of electron
transfer flavoprotein subunit alpha. The latter usually serves as
an electron acceptor for dehydrogenases, and plays a role in the
transfer of electrons along the mitochondrial ETC.
Conversely, we found a decrease in peptide levels of ubiquinol
cytochrome-C reductase core protein 1, a subunit of complex III
of the electron transfer chain that catalyses transfer of electrons
from coenzyme Q to cytochrome C. These data suggest that
despite attempts by hearts from the obese animals to augment
mitochondrial ATP production, decreased ubiquinol cytochrome-
C reductase core protein 1 peptide levels likely contribute to
impaired mitochondrial ATP production. In agreement, we previ-
ously found diminished cardiac respiratory capacity in female
db/db mice compared to matched controls.
8
It is likely that ATP
synthase D chain and electron transfer flavoprotein subunit alpha
may also be down-regulated with prolonged persistence of the
diabetic phenotype.
We also found coordinated down-regulation of key contrac-
tile/cytoskeletal proteins in the obese heart, i.e.
α
-smooth muscle
actin,
α
-cardiac actin, MHC
α
and MyBP-C. Together these
peptides play a crucial role in ensuring sustained myocardial
contractile function and cytoskeletal support.
9
The contractile
proteins identified in this study were associated with the isolated
mitochondrial fraction and likely represent proteins closely asso-
ciated with interfibrillar mitochondria. These data are consistent
with previous work that found an MHC isoform switch during
the onset of diabetes, i.e. decreased MHC
α
and increased MHC
β
expression.
10
MyBP-C is a thick filament-associated protein and
provides an additional regulatory step to myocardial contrac-
tion.
11
MyBP-C gene mutations can cause hypertrophic cardio-
myopathy,
11
while its absence (cMyBP-C null mice) significantly
attenuates
in vivo
left ventricular function.
12
Together our data are in agreement with an earlier study that
found increased contractile dysfunction with older db/db female
mice (aged 16–18 weeks).
13
We therefore propose that it is likely
that lower contractile protein expression may indeed contribute
to impaired contractile function observed in the diabetic heart.
However, additional studies are required to confirm these chang-
es within the cytosolic fraction of hearts from diabetic animals.
Conclusion
This study found that diabetic mouse hearts displayed altered
expression of mitochondrial metabolic peptides together with
the coordinated down-regulation of several cardiac contractile/
cytoskeletal proteins. We propose that attenuated contractile
protein expression may contribute to the onset of diabetic cardio-
myopathy.
The authors acknowledge the contributions of the biological mass spec-
trometry and proteomics facility in the Department of Biological Sciences,
University of Warwick. Research work was supported by the National
Research Foundation and the South African Medical Research Council
(awarded to MFE).
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