 |
|
|
 |
 |
Press Release: The 1986 Nobel Prize in Physiology or
Medicine
NOBELFÖRSAMLINGEN KAROLINSKA INSTITUTET
THE NOBEL ASSEMBLY AT
THE KAROLINSKA INSTITUTE13 October 1986
The Nobel Assembly at the
Karolinska Institute has today decided to award the Nobel Prize
in Physiology or Medicine for 1986 jointly to
Stanley
Cohen and Rita Levi-Montalcini
for their discoveries of
"growth factors".
Summary
The Nobel Prize in
Physiology or Medicine is awarded for discoveries which are of
fundamental importance for our understanding of the mechanisms which
regulate cell and organ growth. The pattern of cellular growth has
long been known, but it is the Italian developmental biologist
Rita Levi-Montalcini and the American biochemist Stanley
Cohen with their discovery of nerve growth factor
(NGF) and epidermal growth factor (EGF), respectively, who
could show how the growth and differentiation of a cell is
regulated. NGF and EGF were the first of many growth-regulating
signal substances to be discovered and characterized.
The
discovery of NGF and EGF has opened new fields of widespread
importance to basic science. As a direct consequence we may increase
our understanding of many disease states such as developmental
malformations, degenerative changes in senile dementia, delayed
wound healing and tumour diseases. The characterization of these
growth factors is therefore expected, in the near future, to result
in the development of new therapeutic agents and improved treatment
in various clinical diseases.
The Regulation of
Growth
Adult man consists of billions of cells. However,
he begins from a single cell which contains the genetic material
coding for the complete individual. The first cell divides, and in
the beginning, the daughter cells are identical. Soon, however, the
cells begin to exhibit slightly different characteristics. This
unique specialization of cells is termed differentiation. The
pattern for growth and differentiation has long been established but
the regulation of this development has remained unknown. It was only
during the past 2-3 decades with the discovery of growth factors
that our understanding of the regulation of cell's growth and
differentiation has begun to be clarified.
Today, we know
that cells communicate with each other via signal substances,
hormones. Initially it was believed that hormones were only produced
in specialized glands such as the pituitary, from which hormones
such as growth hormone, were released into the blood stream. It has
now become clear that many cell types synthesize signal substances
or hormones which have their effects both on their cell of origin as
well as on neighbouring cells. By this mechanism, cells can
influence the development of their neighbours.
The
scientists in the 1940's and 1950's already knew that the addition
of blood or organ extracts to cells in culture resulted in their
successful growth. They did not know, however, the identity of the
active substances, just as cancer researchers understood little of
the unregulated growth of tumour cells.
The Discovery
of NGF
The discovery of nerve growth factor (NGF)
in the beginning of the 1950's is a fascinating example of how a
skilled observer can create a concept out of apparent chaos. Until
this time, experimental neurobiologists did not understand how the
development of the nervous system was regulated to result in the
final complete innervation of the body. The investigation of NGF's
role in the development of the nervous system, as well as later, in
adult neural function, has been a lifelong dedication for Rita
Levi-Montalcini. Developmental biologist Rita
Levi-Montalcini, who in the beginning of 1950's moved from her
homeland Italy, to Viktor Hamburger's laboratory in St. Louis, USA,
showed in 1952 that when tumours from mice were transplanted to
chick embryos they induced potent growth of the chick embryo nervous
system, specifically sensory and sympathetic nerves. Since this
outgrowth did not require direct contact between the tumour and the
chick embryo, Rita Levi-Montalcini concluded that the tumour
released a nerve growth-promoting factor which had a selective
action on certain types of nerves. Following this discovery, Rita
Levi-Montalcini turned to a more sensitive cell culture system in
order to measure NGF activity in various extracts. NGF proved to be
an extremely potent biological substance. A sensory or sympathetic
nerve cell reacted within 30 seconds to the addition of minute
quantities of NGF. One billionth part of a gram of NGF per ml of
culture medium exerted a potent growth-promoting effect. A few
minutes after the addition of NGF, nerve fibres began to grow out
from the ganglion which after a day's exposure to NGF resembled a
sun surrounded by rays (Figure 1). This biological assay to detect
NGF paved the way for the next step in this pathway of discovery -
identification of the active nerve growth-promoting substance.
|
 |
|
Figure 1. The classical biological assay
for the measurement of NGF which was developed by Rita
Levi-Montalcini. Sensory ganglion dissected from chick embryo is
cultured in the presence of extract to be measured. Nerve fibre
outgrowth from the chick ganglion is determined after 24 hours. The
lowest concentration of the extract which causes a halo of nerve
fibre outgrowth (right hand side figure) contains 1 biological unit
of NGF. This is equivalent to a concentration of approximately 10
nanograms NGF/ml culture medium (10 ng=1/100,000 of 1 milligram).
The left hand side ganglion has been incubated without NGF being
present in the medium and is in the process of dying.
The figure
has been published in Scientific American 1979, 240, p. 48.
The Characterization of NGF
In 1953,
biochemist Stanley Cohen, joined the research group in St.
Louis. Three years later they had purified a nerve growth-promoting
extract from mouse tumour which contained both protein and nucleic
acids. To determine which of these components was active, Stanley
Cohen added snake venom containing a high concentration of a nucleic
acid-degrading enzyme. To his surprise, the snake venom contained
more nerve growth-promoting activity than the tumour itself. When
added alone to the incubation medium, the snake venom induced an
enormous outgrowth of sympathetic nerve fibres. The group followed
up this unexpected finding by systematically searching for the
presence of NGF in various tissues. In 1958, they discovered another
rich source of NGF - a salivary gland in the male mouse.
With the help of snake venom and salivary gland extract,
Stanley Cohen was now able to purify NGF and produce antibodies
against NGF. Just as NGF induced nerve growth, so did the addition
of the NGF antibodies to the incubation medium inhibit it. The
chemical structure of NGF is now known to consist of 118 amino
acids. Two such chains join together to build a biologically active
molecule.
The advances were a milestone in developmental
neurobiology. For the first time, chemically defined signal
substances could be used in the search for mechanisms which
regulated neural development.
NGF is Necessary for
the Survival of Nerve Cells
NGF is found in mammals,
birds, reptiles, amphibians and fishes. Many cell types in these
species synthesize and release NGF during development. The growth of
nerve fibres is stimulated via this mechanism. Nerve fibres grow
towards the source of NGF which is taken up in the fibre ends and
transported back to the nerve cell body. One can imagine that
tissues lure nerve fibres to them by sending out NGF. An excess of
sensory and sympathetic nerve cells is produced during development.
Only those which establish contact with a target organ producing NGF
survive. With modern gene technology it has now been possible to
characterize the NGF gene in man and animals. Hybridization
techniques identifying the messenger RNA for NGF have mapped the
tissues that synthesize NGF. Gene technology also makes possible
recombinant NGF, which opens the way for its future application in
clinical medicine.
The Discovery of
EGF
During the course of his studies of NGF Stanley
Cohen observed an unexpected acceleration of development when he
injected salivary gland extract to newborn mice. The mice displayed
precocious eyelid opening and tooth eruption. The explanation was
that the salivary gland extract contained another growth factor
apart from NGF. Cohen termed this substance epidermal growth
factor (EGF) because it could stimulate the proliferation of
epithelial cells in skin and cornea. He raised antibodies against
EGF as he previously had against NGF.
In the following years
Cohen purified and determined the amino acid sequence of EGF (Figure
2). For the first time scientists had a factor available which
stimulated epithelial cell growth and allowed studies of the growth
process. Cohen and his coworkers found that EGF enhanced a cascade
of events including stimulation of glucose and amino acid transport,
activation of protein synthesis and initiation of DNA synthesis and
cell replication. In later studies both Cohen and others have shown
that EGF stimulates the growth of a large variety of cells including
fibroblasts, liver cells, and vascular cells as well as endocrine
cells the thyroid, ovaries and pituitary glands.
|
 |
|
Figure 2. The amino acid sequence of EGF
with placement of disulfide bonds.
The figure has been published
in J. Biol. Chem. 1973, 248, p. 7670.
The presence of
specific binding sites, termed receptors, on the surface of target
cells was a prerequisite for the action of EGF. These receptors
catch EGF and the EGF-receptor complex is taken into the cell. One
important step in the events leading to the biological action of EGF
is the phosphorylation of a specific amino acid tyrosine on the EGF
receptor.
The finding of tyrosine-specific
autophosphorylation of the receptor was a breakthrough in our
understanding of how a signal from the outside reaches the inside of
the cell. It has later been shown that this event is a general
pathway through which many growth factors mediate their effects.
The knowledge of the regulation of normal cell growth has
provided new insights into cell transformation and tumour growth.
Studies on certain virus-induced tumours have led to the discoveries
of special genes, called oncogenes, which play a role in the
transformation of cells. Amongst these oncogenes there is one, which
codes for the synthesis of a protein with homology to the
EGF-receptor. Another oncogene product shows similarity with a later
discovered growth factor derived from the blood platelets (PDGF). As
often occurs, increased knowledge of normal events has led to an
improved understanding of disease.
Search for Other
Growth Factors
During the last decade, several growth
factors have been isolated and characterized by different research
groups. For example, somatomedin or insulin-like growth
factor, which mediates the growth-promoting effect of growth
hormone, was isolated from human plasma. Platelet derived growth
factor (PDGF) was isolated and shown to stimulate growth of
mesenchymal cells. Interleukin-2 was isolated and shown to
promote growth of the lymphocytes of the immune system. From
tumours, endothelial cell growth factor (ECGF) was isolated
and shown to have similarities to the fibroblast growth
factor (FGF).
All research groups who discovered "new"
growth factors have followed in the tracks of Levi-Montalcini and
Cohen. In the research area of growth factors and their biological
action, Levi-Montalcini and Cohen have created a scientific school
with an increasing number of followers.
Clinical
Application of NGF and EGF
Clarification of the
mechanisms regulating the growth and survival of cells as well as
their differentiation are of great interest to basic science.
However, this knowledge can be expected to improve our understanding
of the pathogenesis of several clinical disorders such as
malformation and errors of development, degenerative changes such as
occur in senile dementia, delayed wound healing, muscle dystrophy as
well as tumour diseases.
An important future field of NGF
therapy is the possibility of enhancing the reparative process after
damage to nerves in the periphery as well as in the central nervous
system. The recent finding of NGF in the brain has raised great
expectation. An important pathway in the brain with acetylcholine as
a transmittor substance seems to be sensitive to NGF.
In
studies performed on animals, EGF has been shown to enhance the
healing of wounds in both skin and cornea. Limited amounts of human
EGF have restricted its use in humans. Only now that it is possible
to produce recombinant EGF have clinical trials been started. A
future application of EGF to enhance the repair of wounds in skin
and cornea by, for example, local application after damage or
surgery can be anticipated. Autotransplantation of skin would be
improved if epithelial cells could be quickly cultivated outside the
body with the help of EGF. It would also be possible that
antagonists to EGF or antibodies to the EGF-receptors on the cell
surface could be useful in the treatment of tumours in which
derangement of EGF or the EGF-receptor is involved in their
transformation.
References
- Rita Levi-Montalcini & Pietro Calissano:
The Nerve-Growth Factor. Scientific American 1979, 240, pp.
44-53.
- B Alberts, D Bray, J Lewis, M Raff, K Roberts
& J D Watson, Eds.: Molecular Biology of the Cell. Garland
Publ. Incorp., New York and London, 1983.
|
|
|
|
|
