Molecular Exercise Physiology
Skeletal Muscle Hypertrophy
Seminar 8
Henning Wackerhage
Task
Why was Robert Wadlow that
tall?
What would have happened if he
had a defect in the IGF-1 gene?
Answer
Why was Robert Wadlow that tall? Because he had an overexpression
of growth hormone.
What would have happened if he had a defect in the IGF-1 gene?
IGF-1 is a second messenger for growth hormone. He would have
been smaller.
Task
There is evidence that athletes dope with growth hormone. Is
growth hormone alone an effective treatment? Carry out a
literature search.
Answer
Claims for the anabolic effects of growth hormone: a case of
the emperor's new clothes?
Rennie MJ.
Faculty of Life Sciences, Old Medical School, University of Dundee,
Scotland, UK. m.j.rennie@dundee.ac.uk
This review examines the evidence that growth hormone has
metabolic effects in adult human beings. The conclusion is that
growth hormone does indeed have powerful effects on fat and
carbohydrate metabolism, and in particular promotes the metabolic
use of adipose tissue triacylglycerol. However, there is no proof that
net protein retention is promoted in adults, except possibly of
connective tissue. The overexaggeration of the effects of growth
hormone in muscle building is effectively promoting its abuse and
thereby encouraging athletes and elderly men to expose themselves
to increased risk of disease for little benefit.
Original IGF-1/somatomedin hypothesis
Researchers noted that injected growth hormone induced protein
synthesis in various tissues. In contrasts, if these tissues were
incubated with growth hormone outside the body, then protein
synthesis did not occur. The best explanation was that growth
hormone induced an intermediate hormone (termed somatomedin
and now IGF-1) that does would induce protein synthesis and growth.
IGF-1 is mainly released by the liver (Le Roith et al. 2001).
Growth hormone
IGF-1
Growth
IGF-1 splice variants
IGF-1Ec has been termed mechano-growth factor (MGF) to avoid
(or increase?) confusion
Muscle IGF-1 induces muscle hypertrophy
Coleman et al. (1995) generated transgenic mice where
IGF-1 was overexpressed only in skeletal muscle. This led
to large increases in the fibre area, indicating that IGF-1
could induce skeletal muscle hypertrophy. This confirmed
that IGF-1 could act via an autocrine/paracrine
mechanisms in skeletal muscle.
IGF-1
IGF-1
Table. Fibre area in µm2 in control and IGF-1 muscle transgenic
mice.
Fibre type
Control
IGF-1 muscle
transgenic
I
IIa
IId/x, IIb
675
1256
1975
1447
2435
3518
What regulates muscle IGF-1 expression?
Yang et al. (1996) found that stretch induced an increased
expression of an IGF-1 splice variant which they termed
mechano-growth factor (MGF).
Stretch
IGF-1 (MGF)
MGF and IGF in response to resistance exercise
Figure a shows that MGF mRNA increases significantly only in
young subjects in response to resistance. IGF-IEa does not change
significantly. The data appear a bit inconclusive and it is unclear
whether 2.5 h after resistance exercise was a time point where
MGF/IGF-1 expression was highest (Hameed et al. 2003).
Anabolic steroids induce IGF-1
Nandrolone (nan, anabolic steroid) increases IGF-1 expression in rat
skeletal muscle (Lewis et al. 2002).
Summary: Regulation of IGF-1 expression
Proinflammatory
cytokines (TNFa, Il-1)
Food intake
Resistance
training
?
?
Tes
Alternative
splicing
GH
GC
IGF-1
The expression of the IGF-1 gene is activated by testosterone (tes),
growth hormone (gh) and inhibited by glucocorticoids (gc). These
hormones bind to their receptor; e.g. testosterone to the androgen
receptor. Proinflammatory cytokines such as TNFa and interleukin-1
are increased in many situations associated with muscle wasting and
inhibit IGF-1 expression (reviewed by Frost and Lang 2003).
Task
IGF-1 is advertised as an anti-ageing treatment. Is IGF-1 alone
effective in inducing muscle growth and why might it be
advertised as an anti-ageing treatment?
Anwer
GH/IGF-1 knockout animals appear to live longer.
Animal models of genetic GH deficiencies such as Snell mice (Pit1 gene mutations) the Ames mice (PROP-1 gene mutation) and
the Laron mice (GH receptor gene knock-out) have a statistically
significant higher longevity compared to normal controls. On the
contrary, mice transgenic for GH and acromegalic patients
secreting high amounts of GH have premature death.
To better understand the role of the insulin-like receptors in
mammalian lifespan regulation and ageing, we explored the
phenotype of heterozygous IGF-I receptor (IGF1R) knockout
mice. Compared with control littermates these mutants live
longer without any obvious impairment of their health and
physiology, except a reduced glucose tolerance that we observed
in males.
Translation
(2) One amino acid corresponding
to the anticodon is bound to tRNA
(3) During translation, an
amino acid chain (peptide
chain, protein) is synthesized
by the ribosome. The mRNA
serves as a template.
(1) Transfer RNA (tRNA) has an anticodon
that only binds to a particular mRNA codon
AUCUUAACCUCCCCAGCAGCUGGGACUACAGCCACGCGCCACUGCAC
mRNA
PKB pathway
Researchers at the School of Life Sciences have made numerous
contributions to elucidating the PKB pathway which is involved not
also in mediating muscle hypertrophy but also cardiac hypertrophy,
diabetes, cancer and many other growth situations. The key
researchers on this pathway are shown below (I have probably
omitted one or two). Before looking at the structure of the pathway,
you will see the results of an experiment where PKB was activated
on muscle to show that active PKB can cause hypertrophy.
Sir Philip Cohen
Dario Alessi
Chris Proud
Peter Downes
PKB activation causes hypertrophy
PKB transfection
In this experiment, an italian group of researchers have injected an
activated PKB DNA constructs into a muscle and the construct has
been taken up by the fibres that are stained light. PKB kinase activity
was very high in these fibres and led to a dramatic hypertrophy
compared to the much smaller fibres that are unstained (Pallafacchina
et al. 2002).
The whole picture
IGF-1 or insulin
PI3K
PDK1
PKB
mTOR
p70S6k
4E-BP1 P
S6 P
P
eIF4E
P
Translation
activation
Ribosome
Skeletal muscle fibre
Protein synthesis and p70 S6k responses are slow
(a)
(b)
Rates of (a) protein synthesis and (b) p70 S6k activity in rat muscle
in sedentary control (sed; open bars) and exercised (ex; solid bars)
groups of rats studied after resistance exercise (Hernandez et al.
2000). The data show that p70 S6k activity increases first after 6 h
and that p70 S6k activity and protein synthesis are increased 12 and
24 after resistance exercise.
AMPK, amino acids and mTOR signalling
IGF-1, insulin
PKB
High
energy
turnover,
hypoxia
AMPK
mTOR
raptor
Amino
acids
Translation,
protein
synthesis
Skeletal muscle fibre
Task
Why might glucose increase protein synthesis?
Answer
High glucose increases insulin which may stimulate protein
synthesis.
Stimulating muscle growth via the PKB pathway:
Doping and the clever way
The message of all the slides is: If you manage to activate the PKB
pathway, growth will occur. Some athletes try to activate the PKB
pathway with doping agents. They are:
1) Human growth hormone (Is it effective? See Rennie 2003);
2) Insulin (see BBC report on next slide)
3) Clenbuterol (Katrin Krabbe)
Obviously, testosterone and androgenic steroids will also affect IGF1 expression in muscle and thus will probably affect the PKB
pathway.
The natural and clever way is resistance training (which increases
MGF and possibly IGF-1) and protein (which, when digested will
increase the amino acid concentration). However, the timing is
important. Two papers suggest that the proteins have to be taken
directly after resistance exercise.
Cross-sectional area of
quadriceps femoris
Eat protein directly after resistance exercise!
Two groups of old subjects (70-80 years) performed a period of
endurance training. Both groups received a gel containing 10 g protein
(from skimmed milk and soybean), 7 g carbohydrate and 3.3 g lipid
either directly after exercise (P0) or 2 h after exercise (P2). Only
ingestion directly after exercise caused hypertrophy (Esmarck et al.
2001).
Muscle protein synthesis in response to amino acids
In this important study by Cutherbertson, Smith and Rennie, young
and elderly subjects were given up to 40 g of essential amino acids
(EAA). The data show that the maximal rate of protein synthesis is
lower in the elderly.
Young
Elderly
FSR (% h-1)
0.12
0.09
0.06
0.03
0.00
0
10
20
EAA (g)
30
40
Task
You can achieve a maximal stimulation of protein synthesis with
10 to 20 g of amino acids. How many eggs, pints of milk, steaks
is that?
Protein supplements are heavily advertised in body building
magazines. Are high amounts of protein needed? Are they
effective? Are they dangerous?
Answer
You can achieve a maximal stimulation of protein synthesis with
10 to 20 g of essential amino acids. How many eggs, pints of
milk, steaks is that?
10 g of EAA is roughly 20 g of total amino acids (and thus
protein).
7 g of protein per egg
8 g per cup of milk
30 g fillet steak
(3) Autocrine,
paracrine IGFs
(4) Systemic IGFs
IGF-1/MGF binding
to receptor
(5)
PI3K
(2) IGF-1
precursor
splicing into
IGF-1, MGF
PDK1
(6)
PKB
(1) Stretch,
other signals
(7) High
energy
turnover,
hypoxia
AMPK
mTOR
raptor
p70S6k
4E-BP1 P
IGF promoter
Nucleus
(9) eIF3
and
P
translation
activation
(8)
Amino
acids
(10)
Translation
of ribosomal
eIF4E
proteins
Ribosome Peptide/protein
S6 P
Skeletal muscle fibre
The End