Current Concepts of Animal Growth X
Metabolic and Cellular Regulation of Protein Deposition
Speakers and Topics:
Amino Acids: Regulators of Global and Specific mRNA Translation
Scot R. Kimball
Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine
A continuous supply of a complete complement of essential amino acids
is a prerequisite for maintenance of optimal rates of protein synthesis
in both liver and skeletal muscle. Deprivation of even a single essential
amino acid causes a decrease in the synthesis of essentially all cellular
proteins through an inhibition of the initiation phase of mRNA translation.
However, the synthesis of all proteins is not repressed equally. Specific
subsets of proteins, in particular those encoded by mRNAs containing a
5¹-terminal oligopyrimidine (TOP) motif, are affected to a much greater
extent compared to most proteins. The specific decrease in TOP mRNA translation
is a result of an inhibition of the ribosomal protein S6 kinase, S6K1,
and a concomitant decline in S6 phosphorylation. Interestingly, many TOP
mRNAs encode proteins involved in mRNA translation, such as elongation
factors eEF1A and eEF2, as well as the ribosomal proteins. Thus, deprivation
of essential amino acids not only directly and rapidly represses global
mRNA translation, but also potentially results in a reduction in the capacity
to synthesize protein.
Cellular control of protein degradation
Didier Attaix
Human Nutrition Research Center of Clermont-Ferrand and INRA, 63122 Ceyrat, FRANCE
A few years ago protein degradation was considered to be a global, non-selective
and poorly regulated metabolic process that was mainly involved in housekeeping
functions. This area of research has developed exponentially in the last
decade, and it is now clear that many major biological functions are controlled
by the breakdown of specific proteins. In this respect, the ubiquitin-proteasome-dependent
pathway is the most elaborate protein-degradation machinery known. The
formation of a polyubiquitin degradation signal is required for proteasome-dependent
proteolysis. Several families of enzymes, which may comprise hundreds of
members to achieve high selectivity, control this process. The substrates
tagged by polyubiquitin chains are then recognized by the 26S proteasome
and degraded into peptides. However, the 26S proteasome also recognizes
and degrades some non-ubiquitinated proteins. Indeed, several ubiquitin-
and/or proteasome-dependent systems degrade specific classes of substrates
and single proteins by alternative mechanisms and are presumably interconnected.
They may also interfere or cooperate with other proteolytic pathways.
Stress and Muscle Cachexia
Per-Olof Hasselgren
Department of Surgery, University of Cincinnati, Cincinnati, OH
One of the metabolic hallmarks of sepsis and severe injury is a catabolic
response in skeletal muscle. Muscle cachexia in these conditions is mainly
caused by increased protein breakdown, although inhibited protein synthesis
contributes to the negative nitrogen balance in skeletal muscle. Muscle
protein breakdown during sepsis and following severe injury mainly reflects
degradation of the myofibrillar proteins actin and myosin. There is evidence
that a calcium-calpain-dependent release of the myofilaments from the sarcomere
provides substrates for the ubiquitin-proteasome proteolytic pathway. Proteins
degraded by this pathway are ubiquitinated and subsequently degraded by
the 26S proteasome. Research in our laboratory has provided evidence that
the gene expression of calpains as well as various components of the ubiquitin
proteasome pathway is upregulated in skeletal muscle during sepsis and
following severe injury. In addition, proteasome inhibitors block the increase
in muscle protein breakdown seen in these conditions. Muscle cachexia in
patients with stress has important clinical implications because it can
prevent or delay ambulation, increasing the risk for thromboembolic complications
and prolonging rehabilitation. In addition, when respiratory muscles are
affected, there is an increased risk for pulmonary complications and a
need for prolonged ventilary support.
Developmental Regulation of Protein Metabolism
Teresa A. Davis
USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TX
Growth and development are characterized by high rates of protein turnover
that support rapid rates of protein accretion. The rate of protein deposition
varies among tissues, with the growth rate of the skeletal musculature
being amongst the highest. The efficiency with which dietary amino acids
are utilized for protein deposition is high in neonates and decreases as
the animal matures. This high efficiency is likely due to the enhanced
stimulation of protein synthesis after feeding. The rise in protein synthesis
in response to feeding and its developmental decline are more pronounced
in skeletal muscle than for other tissues. In the neonatal pig, the postprandial
rises in insulin and amino acids independently stimulate protein synthesis
in skeletal muscle, whereas amino acids are the principal anabolic stimulus
for liver. These developmental changes in protein synthesis are regulated
by alterations in the expression and activity of components of the signaling
pathway that controls the initiation of translation.
Muscle Wasting and Protein Metabolism
Carmen Castaneda Sceppa
Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University
Accelerated muscle proteolysis is the primary cause of muscle wasting
in many catabolic diseases such as diabetes mellitus, renal and liver failure,
HIV infection and AIDS, and cancer. In individuals with catabolic diseases,
as it is the case of fasting states (anorexia and starvation), protein
breakdown increases while protein synthesis declines resulting in negative
muscle protein balance. The pathway responsible for accelerated proteolysis
in catabolic conditions is the ubiquitin-proteosome dependent system. Muscle
proteolysis increases under conditions of acidosis, up-regulation of branched-chain
ketoacid dehydrogenase, the presence of catabolic hormones (glucocorticoids,
thyrotoxic states), insulin resistance, and multiple cytokines (interlukin-1
and 6 and tumor necrosis factor). In contrast, factors that suppress muscle
proteolysis and wasting leading to a state of adaptation include dietary
protein deficiency with adequate energy intake, use of anabolic agents,
and resistance exercise training. The understanding of the biochemical
adaptation that reduce protein degradation and improve nitrogen balance
are important for the development of effective therapies to combat muscle
wasting and improve protein homeostasis with catabolic illnesses.
Exercise and Protein Metabolism
Robert R. Wolfe
University of Texas Medical Branch and Shriners Burns Hospital, Galveston, Texas
Resistance exercise training produces an anabolic effect on muscle,
yet during exercise muscle protein synthesis is not stimulated, and muscle
protein breakdown may be accelerated. The anabolic response begins after
the exercise is completed. Muscle protein synthesis is elevated by about
one hour after exercise, and remains elevated for as long as 48 hours.
A simultaneous increase in muscle protein breakdown blunts the effect of
the stimulation of synthesis on the net protein balance. Net muscle protein
balance is improved after exercise, but remains negative unless nutrients
are ingested. Thus, the "anabolic" response to resistance exercise occurs
in the fed state, where ingested amino acids are incorporated into muscle
protein to a greater extent than when ingested at rest.
Hormonal Regulation of Regional and Tissue Protein Turnover
Sreekumaran Nair
Endocrinology Unit, Mayo Clinic, Rochester, MN
Hormones are major regulators of protein turnover in humans. Whole body
protein balance and tissue concetrations of specific proteins are determined
by the balance between synthesis and degradation of proteins. Most of the
hormonal actions are tissue and protein specific and the same hormone may
inhibit synthesis of one protein but directly or indirectly stimulate synthesis
of another protein. Hormonal effects are targeted at different levels of
regulation of protein synthesis and degradation. Examples of hormonal imbalance
resulting in profound changes in protein turnover is evident in type I
diabetes. Insulin deficiency results in elevated glucagon and growth hormone
levels in human. Insulin deficiency has been shown to result in profound
muscle wasting by a net increase in muscle protein breakdown. High glucagon
causes increased metabolic rate and accelerated leucine oxidation thus
contributing to the catabolic state in diabetes. Other catabolic hormones
such as cortisol also may contribute to catabolism. Insulin also is a key
hormone involved in regulating the trafficking of amino acids across organs
especially making amino acids available for synthesizing essential proteins
in between meals. Recent interest has focused on insulin specific effects
on certain proteins with specific functions such as mitochondrial proteins.
Hormones also act in conjunction with substrates which modulates hormonal
effect on protein turnover.
Nutritional Regulation of Protein Metabolism
Peter J. Garlick
Department of Surgery, State University of New York at Stony Brook, Stony Brook, NY
Protein homeostasis depends on the balance between protein synthesis
and protein degradation. In muscle of growing animals, feeding is accompanied
by an increase in protein synthesis, resulting in net protein deposition.
This has been shown to depend on amino acid supply and insulin secretion.
In contrast, in the liver, protein deposition after feeding results mainly
from an inhibition of protein degradation. At the whole body level, increasing
nutrient intake in human preterm neonates has been shown to be associated
with increased protein deposition, resulting from enhanced protein synthesis.
In healthy adults there is no growth, but there is a need to retain protein
after meals to counter the protein loss that occurs postabsorptively. The
adult rat shows little stimulation of muscle protein synthesis by food
intake or insulin infusion, whereas in human muscle the responses to nutrients
or insulin remain controversial. Pathological conditions such as trauma
and infection are characterized by muscle protein loss and a decrease in
muscle protein synthesis, and much effort has been spent on strategies
for reversing muscle wasting by nutritional and pharmacological means.
However, nutritional support, even when supplemented with branched chain
amino acids or glutamine, have not yet been shown to be effective.