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ENHANSEMENT
OF PROTEIN TURNOVER (PROTEIN REGENERATION) |
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there is an age-related decline in the rates of protein
synthesis and protein degradation and, therefore, and
age-related decrease in the rate of protein turnover.
This is true in most individual tissue types and organs
in the body as well as for the total protein turnover
in the whole body. An exception may be the lung, where
there may be no decline in the rate of protein turnover.
In addition to using protein turnover to recycle amino
acids, turnover permits adjustments in enzyme and structural
proteins and receptors and modulators as well as for destruction
of damaged proteins (e.g., oxidative damage) and replacement
with normal proteins.
there is an age-related increase in the concentration
of abnormal proteins due to post-translational modification
errors (the main metabolic cause of abnormal proteins)
and in oxidatively damaged proteins. With CR, there is
less age-related increase in abnormal proteins form both
sources (i.e., post-transnational modifications and oxidative
damage). There is an intracellular protein complex that
specifically degrades oxidatively damaged proteins. This
complex is called a "proteasome".
CR produces an increase in the rate of protein
synthesis and an increase in the rate of protein degradation,
and therefore causes an increase in the rate of protein
turnover. The age-related decline in the rate
of protein turnover is not stopped or slowed by CR, but
since young CR animals have 30%-40% greater protein turnover
and the age-related decline in the rate of protein turnover
is essentially the same in CR and in AL animals, at any
age, CR animals always have substantially higher rates
of protein turnover than do AL animals. The benefit of
higher rates of protein turnover include faster responses
when alterations in proteins are needed (e.g., receptors
or regulators) and faster elimination of damaged proteins
(e.g., oxidative damage). This may be one mechanism by
which CR increases ML and LS. Other types of "problematic"
proteins can develop as a result of metabolic errors (e.g.,
gene mutations, errors in transcription, mRNA processing,
translation, or post-transnational processing of proteins
as well as damage caused by environmental factors (e.g.,
bacteria, radiation, heat, toxins).
CR may increase ML and LS in many animals by increasing
the synthesis of protects substances such as heat shock
proteins, which increase protection against a variety
of adverse factors in saddition to elevated temperature
.
In most organs (exept skeletal muscles and lung) caloric
restriction leads to an increase in protein synthesis,
degradation and turnover compared to age-matched fully
fed controls.
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VA Palo Alto Health Care System, Palo
Alto, CA, USA; University of Massachusetts, Amherst, MA,
USA.
The effects of prolonged caloric
restriction on protein kinetics in lean subjects has not
been previously investigated. PURPOSE: To test the hypotheses
that 21 days of caloric restriction (CR) in lean subjects
would a) result in significant losses of lean mass despite
a suppression in leucine turnover and oxidation, and b)
negatively impact exercise performance. METHODS: Nine
young, normal weight men (23+/-5 y, 78.6+/-5.7 kg, VO2peak:
45.2+/-7.3 ml(.)kg(-1)(.)min(-1),mean+/-SD) were underfed
by 40% of the calories required to maintain body weight
(BW) for 21 days and lost 3.8+/-0.3 kg BW and 2.0+/-0.4
kg lean mass. Protein intake was kept at 1.2 g(.)kg(-1)(.)day(-1).
Leucine kinetics were measured using KIC reciprocal pool
model in the post-absorptive state during rest and 50
minutes of exercise (EX) at 50% of VO2peak. Body composition,
basal metabolic rate (BMR) and exercise performance were
measured throughout the intervention. RESULTS: At rest,
leucine flux (~131 micromol(.)kg(-1)(.)hr(-1)) and oxidation
(Rox; ~19 micromol(.)kg(-1)(.)hr(-1)) did not differ pre-
and post- CR. During EX, leucine flux (129+/-6 vs. 121+/-6)
and Rox (54+/-6 vs. 46+/-8)were lower following CR than
pre-CR. Nitrogen balance was negative throughout the intervention
(~3.0gN(.)d(-1)) and BMR declined from 1898+/-262 kcal(.)d(-1)
to 1670+/-203. Aerobic performance (VO2peak, endurance
cycling) was not impacted by CR, but arm flexion endurance
decreased by 20%. CONCLUSIONS: Three weeks of caloric
restriction reduced leucine flux and oxidation during
exercise in normal weight young men. However, despite
negative nitrogen balance and loss of lean mass, whole
body exercise performance was well maintained in response
to CR.
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Department of Biochemistry, College
of Medicine, Dongguk University, Kyungju, Kyungpook 780-714,
Korea.
Calorie restriction (CR) has been
shown to improve peripheral insulin resistance and type
2 diabetes in animal models. However, the exact mechanism
of CR on GLUT4 expression and translocation in insulin-sensitive
tissues has not been well elucidated. In the present study,
we examine the effect of CR on the expression of glucose
transporter 4 (GLUT4), GLUT4 translocation, and glucose
transport activity in adipose tissue from Otsuka Long-Evans
Tokushima Fatty (OLETF) rat and control (LETO) rats. CR
(70% of satiated group) ameliorated hyperglycemia and
improved impaired glucose tolerance (IGT) in OLETF rats.
In skeletal muscle, the expression levels of GLUT4 and
GLUT1 were not significantly different between LETO and
OLETF rats, and were not affected by CR. By contrast,
the expression level of GLUT4 was markedly decreased in
the adipose tissue of OLETF rats, but was dramatically
increased by CR. The GLUT4 recruitment stimulated by insulin
was also improved in OLETF rat adipocytes by CR. The insulin-stimulated
2-deoxyglucose (2DG) uptake was significantly increased
in adipocytes from the CR OLETF rats, as compared with
the satiated OLETF rats. Taken together, these results
suggest that CR improves whole body glucose disposal and
insulin resistance in OLETF rats, and that these effects
may associate with the increased adipocyte-specific GLUT4
expression.
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Department of Molecular Biology and
Biochemistry, Rutgers, The State University of New Jersey,
New-Bruns Wick, NJ, USA.
Oxidative damage to cellular macromolecules
has been postulated to be a major contributor to the ageing
of diverse organisms. Oxidative damage can be limited
by maintaining high anti-oxidant defenses and by clearing/repairing
damage efficiently. Protein turnover is one of the main
routes by which functional proteins are maintained and
damaged proteins are removed. Protein turnover rates decline
with age, which might contribute to the accumulation of
damaged proteins in ageing cells. Interestingly, protein
turnover rates are maintained at high levels in caloric
restricted animals. Whether changes in protein turnover
are a cause or a consequence of ageing is not clear, and
this question has not been a focal point of modern ageing
research. Here we survey work on protein turnover and
ageing and suggest that powerful genetic models such as
the nematode Caenorhabditis elegans are well suited for
a thorough investigation of this long-standing question.
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Department of Biochemistry, School
of Pharmaceutical Sciences, Toho University, Funabashi,
Chiba, 274-8510 Japan.
Many reports have been published
on the effects of lifelong dietary restriction (DR) on
a variety of parameters such as life span, carcinogenesis,
immunosenescence, memory function, and oxidative stress.
There is, however, limited available information on the
effect of late onset DR that might have potential application
to intervene in human aging. We have investigated the
effect of DR initiated late in life on protein and protein
degradation. Two months of DR in 23.5-month-old mice significantly
reduced heat-labile altered proteins in the liver, kidney,
and brain. DR reversed the age-associated increase in
the half-life of proteins, suggesting that the dwelling
time of the proteins is reduced in DR animals. In accordance
with this observation, the activity of proteasome, which
is suggested to be responsible for degradation of altered
proteins, was found increased in the liver of rats 30
months of age subjected to 3.5 months of DR. Thus, DR
can increase turnover of proteins, thereby possibly attenuating
potentially harmful consequences by altered proteins.
Likewise, DR in old rats reduced carbonylated proteins
in liver mitochondria, although the effect was not observed
in cytosolic proteins. Fasting induced apoA-IV synthesis
in the liver of young mice for efficient mobilization
of stored tissue fats, while it occurred only marginally
in the old. DR for 2 months from 23 months of age partially
restored inducibility of this protein, suggesting the
beneficial effect of DR. Taking all these findings together,
it is conceivable that DR conducted in old age can be
beneficial not only to retard age-related functional decline
but also to restore functional activity in young rodents.
Interestingly, recent evidence that involves DNA array
gene _expression analysis supports the findings on the
age-related decrease in protein turnover and its reversion
by late-onset DR.
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Department of Biochemistry, University
of California, Riverside, Riverside, CA 92521, USA.
Differential 'fuel usage' has been
proposed as a mechanism for life-span extension by caloric
restriction (CR). Here, we report the effects of CR, initiated
after weaning, on metabolic enzyme gene _expression 0,
1.5, 5, and 12 h after feeding of 24-month-old mice. Plasma
glucose and insulin were reduced by approximately 20 and
80%. Therefore, apparent insulin sensitivity, as judged
by the glucose to insulin ratio, increased 3.3-fold in
CR mice. Phosphoenolpyruvate carboxykinase mRNA and activity
were transiently reduced 1.5 h after feeding, but were
20-100% higher in CR mice at other times. Glucose-6-phosphatase
mRNA was induced in CR mice and repressed in control mice
before, and for 5 h following feeding. Feeding transiently
induced glucokinase mRNA fourfold in control mice, but
only slightly in CR mice. Pyruvate kinase and pyruvate
dehydrogenase activities were reduced approximately 50%
in CR mice at most times. Feeding induced glutaminase
mRNA, and carbamyl phosphate synthetase I and glutamine
synthase activity (and mRNA). They were each approximately
twofold or higher in CR mice. These results indicate that
in mice, CR maintains higher rates of gluconeogenesis
and protein catabolism, even in the hours after feeding.
The data are consistent with the idea that CR continuously
promotes the turnover and replacement of extrahepatic
proteins.
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Department of Biochemistry, University
of California, Riverside 92521, USA.
Our studies show that dietary caloric
restriction (CR) alters the _expression of key metabolic
enzymes in a manner consistent with an increased rate
of extrahepatic protein turnover and renewal during aging.
Of the key hepatic gluconeogenic enzyme genes affected
by CR, glucose 6-phosphatase mRNA increased 1.7- and 2.3-fold
in young and old CR mice. Phosphoenolpyruvate carboxykinase
mRNA increased 2-fold in young mice, and its mRNA and
activity increased 2.5- and 1.7-fold in old mice. These
changes indicate that CR enhances the enzymatic capacity
for gluconeogenesis. The carbon required for gluconeogenesis
appears to be generated from peripheral protein turnover.
Muscle glutamine synthetase mRNA increased 1.3- and 2.1-fold
in young and old CR mice, suggesting increased disposal
of nitrogen and carbon derived from protein catabolism
for energy. mRNA for the key liver nitrogen disposal enzymes
glutaminase, carbamyl phosphate synthase I, and tyrosine
aminotransferase were increased by 2.4-, 1.8-, and 1.8-fold
in CR mice. Consistent with increased hepatic nitrogen
disposal, hepatic glutamine synthetase mRNA and activity
were each decreased about 40% in CR mice. Together, these
and our other published data suggest that CR enhances
and maintains protein turnover, and thus protein renewal,
into old age. These effects are likely to resist the well-documented
decline in whole body protein renewal with age. Enhanced
renewal may reduce the level of damaged and toxic proteins
that accumulate during aging, contributing to the extension
of life span by CR.
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Department of Pharmacology, University
of North Texas Health Science Center at Fort Worth, 76107,
USA.
The age-associated increase in
oxidative damage in ad libitum-fed mice is attenuated
in mice fed calorically restricted (CR) diets. The objective
of this study was to determine if this effect results
from a slowing of age-related accumulation of oxidative
damage, or from a reversible decrease of oxidative damage
by caloric restriction. To address these possibilities,
crossover studies were conducted in C57BL/6 mice aged
15 to 22 months that had been maintained, after 4 months
of age, on ad libitum (AL) or a 60% of AL caloric regimen.
One half of the mice in these groups were switched to
the opposite regimen of caloric intake for periods up
to 6 weeks, and protein oxidative damage (measured as
carbonyl concentration and loss of sulfhydryl content)
was measured in homogenates of brain and heart. In AL-fed
mice, the protein carbonyl content increased with age,
whereas the sulfhydryl content decreased. Old mice maintained
continuously under CR had reduced levels of protein oxidative
damage when compared with the old mice fed AL. The effects
of chronic CR on the carbonyl content of the whole brain
and the sulfhydryl content of the heart were fully reversible
within 3-6 weeks following reinstatement of AL feeding.
The effect of chronic CR on the sulfhydryl content of
the brain cortex was only partially reversible. The introduction
of CR for 6 weeks in the old mice resulted in a reduction
of protein oxidative damage (as indicated by whole brain
carbonyl content and cortex sulfhydryl), although this
effect was not equivalent to that of CR from 4 months
of age. The introduction of CR did not affect the sulfhydryl
content of the heart. Overall, the current findings indicate
that changes in the level of caloric intake may reversibly
affect the concentration of oxidized proteins and sufhydryl
content. In addition, chronic restriction of caloric intake
also retards the age-associated accumulation of oxidative
damage. The magnitude of the reversible and chronic effects
appears to be dependent upon the tissue examined and the
nature of the oxidative alteration.
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Institute of Human Ageing, University
of Liverpool, UK.
Rates of protein synthesis (measured
in vivo) and growth of the small intestine were studied
as a function of age in ad libitum fed (control) and chronic
dietary-restricted rats. At weaning, the fractional rates
of synthesis in the mucosal and muscularis externa and
serosal layers of the small intestine of control animals
were similarly high (90-100% per day). Although these
rates subsequently declined with age in the muscularis
externa and serosa, they remained constant in the mucosa.
Restricted feeding (50% reduced intake), when imposed
from weaning onwards, significantly extends the maximum
life span of rodents. However, the change in nutritional
status slows the accumulation of protein, RNA, and DNA
in both layers of the small intestine. Although underfeeding
did not prevent the age-related fall in muscularis externa
and serosal protein synthesis, significantly higher rates
(both fractional and per ribosome) were found when compared
age for age with controls. Mucosal fractional synthetic
rates were similarly increased by the reduced food intake.
These changes in protein turnover in the small intestine
are consistent with the higher rates of whole body turnover
previously observed in chronically underfed rats.
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Changes in whole body growth, nucleic
acids, and protein turnover have been studied in conjunction
with ageing and chronic dietary restriction. Normal developmental
changes between weaning and senescence included progressive
decreases in the fractional rates of growth, protein synthesis,
and protein breakdown; the decline in the synthetic rate
correlating with decreases in the ribosomal capacity.
Dietary intervention was imposed at weaning and involved
pair feeding to 50% of the ad libitum food intake. Although
this regime slowed whole body growth by retarding the
developmental decline in protein turnover, growth was
extended into the second and third years of life. The
dietary-induced increase in longevity resulting from a
retardation of the ageing process(es) appears therefore
to be associated with an enhanced turnover of proteins
during the major portion of the life span of dietary restricted
rats. These observations are strange as up to 50% of basal
metabolic rate may be due to the energy requirements for
protein synthesis. So an increased protein synthesisi
in caloric restricted animals becomes difficult to reconcile
with the sharply decreased energy availability in these
seme animals, althouh on a lean body mass basis, these
may be no decrease. (from Weindruch & Walford 1988).
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