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1
AN INTRODUCTION TO
ALPHA-FETOPROTEIN
AND THE GROWTH
INHIBITORY PEPTIDE
Akul Y. Mehta
INTRODUCTION:
Ever since the discovery of alpha-fetoprotein
(AFP) by Bergstrand and Czar in the year
1956, extensive research has been carried
out on this molecular wonder of nature to
elucidate its functions, its role and its
widespread presence. This fetal component
which was not commonly in adults was first
found as a post albumin migrating protein in
fetal serum. Its functions are varied from
development of the fetus, where it acts as a
major transport protein, to a serum marker in
cancer, and its uses in the detection of
congenital defects. The following discussion
takes a closer look at the molecule with
closer insight towards its structure, its role
played in the human body, its biochemical
functions, and the potential uses of the AFP
molecule.
PROTEINS
In order to understand the structure and
functions of AFP it is important to revise the
basics of protein structure and function.
Proteins are working molecules of a cell that
carry out the ‘program’ of activities encoded
onto them by genes. The ‘program’ which is
nothing but the cell function, requires the
coordinated effort of many different types of
protein which work in synchronous with each
other to provide the desired effect. [1]
Classification
of
Proteins:
Proteins are classified into several broad
classes based on the functional role played
by them within the body.
• Structural proteins- which provide
structural rigidity to the cell.
• Transport proteins- those which control
the flow of materials across the body and
cellular membranes e.g. albumin and AFP.
• Regulatory proteins- they act as sensors
and switches to control protein activity and
gene function.
• Signaling proteins- including cell-surface
receptors and other proteins that transmit
external signals to the cell interior.
• Motor proteins- they are those which
cause motion.
• Enzymes- specialized proteins which are
capable of catalyzing an incredible range of
intracellular and extracellular chemical
reactions.
Hierarchical
Proteins:
Structure
of
Although proteins are constructed by
polymerization of only 20 different amino
acids into linear chains, proteins carry out an
incredible array of diverse tasks. Only when
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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a protein is in its correct three dimensional
structure, or conformation is it able to
function efficiently. A key concept in
understanding how proteins work is thatfunction is derived from three dimensional
structure, and the three dimensional
structure is in turn specified by the amino
acid sequence. The structure of proteins can
be considered at four levels of organization
starting with their monomeric building blocks,
the amino acids as shown in Fig. 2
Fig.3 Peptide bonds (yellow) link the amide
nitrogen atom (blue) of one amino acid(aa)
with the carbonyl carbon atom (gray) of an
adjacent one in the linear polymers known
as peptides or polypeptides depending on
their lengths.
THE SECONDARY STRUCTURE:
Fig. 2 The linear sequence of amino acids
(primary structure) folds into helices or
sheets (secondary structure) which pack into
a globular or fibrous domain (tertiary
structure). Some individual proteins selfassociate into complexes (quaternary
structure) that can consist of tens to
hundreds of subunits (supramolecular
assemblies)
They are the core elements of protein
architecture.
The
various
spatial
arrangements resulting from the folding of
localized parts of a polypeptide chain are
referred to as the secondary structures.
When stabilizing hydrogen bonds are
formed between residues, parts of the
backbone fold into one or more well defined
periodic structures such as the alpha helix,
the beta sheet also called the beta pleated
sheet, and a set of turns (See Fig.4).
THE PRIMARY STRUCTURE:
It is the linear arrangement of the amino acid
sequences present in the protein. It is
nothing but the formation of a peptide bond
combining the carboxylic acid moiety of one
amino acid with an amino moiety of another
amino acid. Thus one end of the protein has
a free (unlinked) amino group (the Nterminus) and the other end has a free
carboxyl group (the C-terminus). The
sequence
of
a
protein
chain
is
conventionally written with its N-terminal
amino acid on the left and its C-terminal
amino acid on the right. A short chain of
amino acids (20-50 amino acid residues)
linked by peptide bonds and having a
definite sequence is called a peptide, while
longer chains are referred to as polypeptides
or proteins (up to 4000 amino acid residues).
Fig.3 shows the primary structure of a set of
amino acids and the formation of the peptide
bonds between them.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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together. Since the stabilizing forces are
weak the tertiary structure of a protein is not
rigidly fixed, but it undergoes continual and
minute fluctuation. This variation in structure
has important consequences in the function
and regulation of proteins.
There are two important substructures within
the tertiary structure which are of interest to
study:
Fig.4 Shows the Alpha Helix which is above
with the red backbone, and the beta pleated
sheet below with the blue backbone.
THE TERTIARY STRUCTURE:
The overall folding of the polypeptide chain
yields its tertiary structure. The tertiary
structure refers to the overall conformation
of a polypeptide chain-that is the three
dimensional arrangement of all its amino
acid residues. The tertiary structure is
primarily
stabilized
by
hydrophobic
interactions between the non-polar side
chains, by hydrogen bonds between the
polar side chains and by peptide bonds.
These stabilizing forces hold elements of the
secondary structure- alpha helices, beta
strands, turns and random coils- compactly
1. Motifs: They are particular combinations
of secondary structures. In some cases,
motifs are signatures for a specific function.
E.g. of motifs are
- the helix-loop-helix which is a calcium
binding motif found in more than hundred
calcium binding proteins.
- zinc-finger motif: it is an alpha helix and
two beta strands held together by a zinc ion.
This type of a motif is most commonly found
in proteins that bind to DNA or RNA such as
steroid hormone receptors.
- coiled coil: two or more alpha-helices orient
themselves around each other in a coiled
coil manner.
Fig. 5 gives a visual idea of how each of
these motifs looks.
Fig. 5 shows various kinds of motifs commonly found in proteins. (a) two helices connected with a
helix-loop-helix motif commonly found in calcium-binding and DNA-binding regulatory proteins. (b)
The zinc-finger motif which is present in many DNA-binding proteins that help regulate
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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transcription. (c) the parallel two stranded coiled-coil motif characterized by two alpha-helices
wound one around the other and is stabilized by interactions between hydrophobic side chains.
2. Domains: A domain is a compactly
folded region of a polypeptide. Often a
domain is characterized by some interesting
structural feature, such as, an unusual
abundance of a particular amino acid (e.g. a
proline rich domain), sequences common to
many proteins (e.g. the Epidermal Growth
Factor domain) or a particular secondarystructure domain (e.g. the zinc finger
domain).
Fig. 6 shows the quaternary structure of
haemoglobin which is a tetramer of two
alpha
and
two
beta
subunits.
Domains are sometimes defined in
functional terms on the basis of observations
that an activity of a protein is localized to a
particular region along the length of the
protein (e.g. DNA binding domain).
Experiment: Functional domains are often
identified experimentally by whittling down a
protein to its smallest active fragments with
the aid of proteases, enzymes that cleave
the polypeptide backbone- and then
checking individual fragments for particular
activity. Alternatively, the DNA, encoding a
particular protein, can be subjected to
mutagenesis so that segments of the
protein’s backbone are removed or changed.
The activity of the truncated or altered
protein product synthesized from the
mutated gene is then monitored and serves
as a source of insight about which part of
the protein is critical to its function.
THE QUARTENARY STRUCTURE
Multimeric proteins consist of two or more
polypeptides or subunits. Quaternary
structure of a protein describes the number
(stoichiometry) and relative positions of the
subunits in the multimeric proteins (e.g.
Haemoglobin is a tetramer consisting of two
alpha and two beta subunits as can be seen
in Fig.6) [1]
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ALPHA FETOPROTEIN
• History
• Structure
• Functions
HISTORY
Historically alpha fetoprotein’s presence was
observed in both normal fetuses as well as
in abnormal conditions. In 1956 Bergstrand
and Czar found a fetal component not
commonly found in adults which was first
detected as a post albumin migrating protein
in fetal serum using paper electrophoretic
techniques. Subsequently Masopust and
Kotal
assigned
to
the
unknown
developmental protein of Bergstrand and
Czar the name “fetoprotein”. Gitlin and coworkers devised the name “alpha-
fetoprotein” (AFP) for the electrophoretic
alpha-1 migrating human fetal protein.
Then in 1963 Abelev and co-workers found
a protein that migrated in the alpha-1 region
of an electrophoretogram in hepatoma
bearing mice. In 1965 Tatarinov described a
similar protein in sera of humans bearing
hepatomas. In the early 1970s, Brock and
co-workers reported elevated AFP levels in
human amniotic fluids and in maternal
serum that correlated with the presence of
neural tube defects in the fetus. In 1972
Gitlin and co-workers demonstrated that
AFP was synthesized by fetal liver and yolk
sac.
A
subsequent
study
indeed
demonstrated that AFP was synthesized by
a multitude of tissues, especially those of
gastrointestinal origin. [2]
Fig 7 showing a normal electrophoretogram of human serum and the alpha-1 region
STRUCTURE OF AFP
protein (DBP), AFP, and alpha-albumin
(alpha-ALB). [2]
Mammalian
AFP
is
a
single-chain
glycoprotein. Its molecular masses ranging
from 66 to 72kDa and a 3%-5%
carbohydrate (glycan) content. AFP is a
tumor associated fetal protein classified as a
member of a three-domain albuminoid gene
family that currently consists of four
members- Albumin (ALB), vitamin-D binding
When compared to albumin, AFP is quite
similar in that it also binds to and transports
a multitude of ligands, including bilirubin,
fatty acids, retinoids, steroids, heavy metals,
dyes, flavonoids, phytoestrogens, dioxins
and various other organic drugs. However,
unlike ALB, high concentrations of
hydrophobic ligands (such as estrogens and
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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fatty acids) have been reported to induce an
irreversible conformational change in the
tertiary structure of AFP. [2]
Structurally AFP is divided into 3-domains in
a U-shaped configuration. The albuminoid
gene family is structurally characterized by
cysteine residues that are folded into layers
that form loops dictated by disulphide
bridging. A hinge region is present in the
Domain 2 of AFP. The hinge concept
developed from the observation that HAFP
has two disulphide bridges fewer than
human albumin, providing it with means of
molecular flexibility. [4]
There also exists a carbohydrate side chain
attached to asparagine-232 in AFP. [5]
Fig 8 Molecular configurations of human AFP (Left) and human albumin (Right) based on the
predicted secondary structures. The amino acid residues participating in the formation of alpharespectively.
helices, beta- sheets, beta-turns, and random coils are indicated by
The loops formed by disulfide bonding are filled in black. Stars indicate extra turns introduced in
human AFP at amino acid residues 195-198 and 504-507 where the probabilities of beta turn
occurrence were higher. Black squares represent the carbohydrate residues attached to
asparagine-232 in AFP. The first loop at the amino terminus in AFP is formed assuming cysteines
18 and 67participate in a disulfide linkage. [5]
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Fig 9 Primary and secondary structure amino acid sequence of HAFP. Note that the HAFP
molecule is composed of three domains in a U-shaped configuration. HAFP belongs to the
albuminoid gene family, which is structurally characterized by cysteine residues that are folded
into layers that form loops dictated by disulfide bridging. The hinge concept developed from the
observation that HAFP has two disulfide bridges fewer than human albumin, providing it with a
means of molecular flexibility. [4]
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FUNCTIONS OF AFP
FATTY ACID BINDING DURING
DEVELOPMENT
Presence of fatty acids on HAFP (human
AFP) was first reported in 1978,
enumerating six fatty acids namely- palmitic,
stearic, oleic, linoleic, arachidonic (AA), and
docosahexaenoic (DHA) acids. [2]
AFP plays a role in the intracellular delivery
of poly-unsaturated fatty acids (PUFA’s) into
developing cells.
Experiment- labeled AFP was found to enter
the cells via coated pits and receptosomes
and to move to tubular elements of the
transreticular portion of the golgi apparatus.
Fatty acids bound to AFP are transferred
into cells within 5 minutes at 37°C, and,
following fatty acid release, AFP can be
recycled back across the cell surface. In
short it followed receptor mediated
endocytosis.
Data revealed that the fatty acids bound to
AFP were mainly incorporated into cell
phospholipids and that 25%-40% of the
incorporated AFP was secreted and
released un-degraded after 60 minutes of
incubation.
The AFP first binds to an AFP cell surface
receptor, and then the fatty acid is
endocytosed and transferred within the cell
by a specific fatty acid binding protein.
During pregnancy and early infancy, the
biological role of HAFP in binding and
trafficking of PUFA’s is now well established.
Human AFP is known to both regulate and
facilitate the entry of fatty acids (especially
arachidonic acid and docosahexaenoic acid)
into cells undergoing differentiation. HAFP
reversibly binds docosahexaenoic acid with
high affinity and transports the fatty acid
mainly during the fetal, perinatal and
neonatal periods. HAFP itself undergoes
transplacental passage to the maternal
circulation and tissues.
Fatty Acid Binding Sites:
One major fatty acid binding site for long
chain fatty acids has been documented to lie
between residues 210 and 227 on HAFP on
domain 2. Lysine residues, especially
Lys-223, appears to be essential for the fatty
acid binding at this site. This is because
lysine contains a free amino group which
can form ionic bonds with the fatty acid
carboxylic acid functionality. The fatty acid
binding site at domain 1 residing at residues
36-69 shows an amino acid homology to
fatty acid synthetase.
Upon comparison of the domain 1 and
domain 2 fatty acid binding sequences a few
points of interest emerge:
First it can be seen that each of the 20
amino acid sequence stretches contain
three or more lysine residues, which are
essential for fatty acid binding.
Second, one or more lysines are located at
the amino-terminal side of the sequence
amino acid stretch.
Third, the crucial amino acid for complexing
to the carboxy group of the bound fatty acid
has been identified as lysine position 13-14
amino acids from the amino terminal lysine.
It must be noted that the third AFP fatty acid
binding site which resides on domain 3,
apparently overlaps or lies directly adjacent
to the documented estrogen-binding site on
RAFP (Rodent AFP), according to
competitive binding reports.
Binding, Spectral and immunological studies
have demonstrated that conformational
changes in the tertiary structures of rodent
and human AFP’s can be induced by a high
free fatty acid environment. The fatty acids
induced
a
rapid
and
reversible
conformational change in the AFP molecule.
[2]
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Fig 10 shows the location of the fatty acid binding sites on the AFP molecule.
Fig 11 the amino acid sequences of the Fatty Acid Binding Sites seen in detail.
AFP IN DEVELOPING BRAIN
Human fetal brain tissues obtained at
autopsy
and
stained
by
immunohistochemical procedures revealed
positive AFP staining in nerve cells of the
cerebral wall, brain stem nuclei, and the
epithelial layers of the choroids plexus. The
presence of AFP within the cells of the
choroids plexus suggested that the fetal
protein was transudated from the blood to
the cerebrospinal fluid via a cellular route
across the choroids plexus epithelial layers.
A proposal was thus made that the presence
of AFP in the brain plays a role in neuronal
differentiation and/or development.
Large tissue areas and groups of cells in
many regions of the developing brain,
stained positively for AFP at various time
intervals during development. Intracellular
labeling localized AFP to the cytoplasm of
the neuronal cells and extending into these
cell’s axonic and dendritic extensions. AFP
was found mainly in the cytoplasm of
differentiating neurons at the axonal pole of
the cell and in the dendritic processes of
pyramidal cells.
Studies in a 9 week old fetal baboon brain
further demonstrated the presence of AFP in
neural tube and neural crest derivatives and
in the ventricles, which displayed an
intracytoplasmic staining pattern. The
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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staining localization of the baboon AFP
appeared to decline as myelination and glialcell development progressed. Uriel and coworkers proposed that the binding and
transport of poly-unsaturated fatty acids by
AFP could explain the presence of this fetal
protein in the developing nervous system.
As it is known that the myelin sheath of the
neuronal cells are made of a lipid bilayer
containing protein myelin, and this bilayer
requires PUFAs for development. [2]
AFP IN REPRODUCTIVE FLUIDS
Alpha fetoprotein has long been detected in
the biological fluids of the reproductive tract.
Electrophoretic studies of oviductal fluids
have demonstrated that the mammalian
oviduct secretes a variety of proteins such
as ALB, Beta-Globulins, and AFP. The
production of proteins in the human
oviductal fluid was found to be under
hormonal control and the levels of such
protein secretion were found to correlate
with the estrogen peak of the menstrual
cycle. The proteins derived from human
oviductal fluid have two sources: firstly
proteins originating from serum transudation,
and secondly those which are synthesized
and secreted by the uterine tubule mucosa.
One such secreted protein designated as
human oviductin-1 (HOV-1) displayed a
molecular mass of 54kDa and an isoelectric
pH of 4.5 and contained a carbohydrate
moiety.
Experiment: fresh donated human sperm
were incubated with
1) a mixture of Human Oviductal Fluid
(HOF) specific proteins
2) HOV-1
Then using indirect immunofluorescence,
the investigators studied the ability of
the HOF proteins to bind to the human
sperm. While the mixture of human
oviductal fluid proteins bound diffusely
over the entire surface of the sperm, cell
HOV-1 binding was restricted to the
head region of the sperm. The
investigators stated that the HOV-1
protein acted as an acrosome-stabilizing
factor, serving to prevent premature
acrosome activation.
So why are we talking so much about
HOV-1? In subsequent purifications of
HOV-1 protein, determination of amino acid
and carbohydrate composition of HOV-1
confirmed that HOV-1 was identical to HAFP.
However, it’s molecular mass suggested
that HOV-1 was a cut or truncated form of
AFP. Thus HOV-1 was a globular,
noncollagenous protein with carbohydrate
attachment via an N-glycosidic linkage
between
N-acetyl-glucosamine
and
asparagines as in the case of HAFP. It was
proposed that secreted AFP as a constituent
protein of Human oviductal fluid may serve
to mediate sperm survival and motility by
decreasing the response of the acrosomal
reaction so as to prolong sperm viability and
function. [2]
INTERACTION OF
ESTROGEN
DEVELOPMENT
AFP WITH
DURING
In human AFP two estrogen binding regions
are present. The first region lies between
amino acids 423-444, which represents a
major hydrophobic binding pocket on HAFP.
The second region is from amino acids 445480. Both the regions display overlapping
binding sites for fatty acids, diethyl
stilbesterol, retinoids, warfarin, coumarin
and other drugs (found by competitive
binding studies).
Site 1 estrogen binding site on the HAFP is
a high affinity binding site located at
residues 424-439. The 5 amino acids crucial
to binding on RAFP were found to be
glycine-425, methionine-427, isoleucine-430,
alanine-432, and threonine-433. These
amino acids match precisely on the MAFP
residues
glycine-428,
methionine-430,
isoleucine-433, alanine-434, and threonine435. In humans three out of the five of these
amino acids show substitutions namely
glycine to arginine, isoleucine to threonine,
and serine to alanine. Amino acid
sequences of human and rodent AFPs were
compared to an estrogen binding region on
the human estrogen receptor (HER:
residues 419-431). It was seen that 4 out of
5 of these crucial amino acids for binding
estrogen on rodent AFP matched and
aligned to those on HER. The remaining
hydrophobic amino acids in human and
rodent AFP are also present in HER, namely
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
11
the leucines and valine. In human AFP
multiple (five) alanines have replaced many
of the crucial amino acids present in both
RAFP and HER. However a single alanine
retained in all species is probably significant
for binding. Thus, this 15-amino acid region
on the various AFPs seems to represent the
high-affinity binding site for rodent and to a
lesser extent human AFP.
The Site 2 estrogen binding site on the AFP
molecule is a low affinity binding site located
from amino acids 454-468 on the HAFP.
The site is highly hydrophobic (leucines,
isoleucines,
alanines).
Various
AFP
molecules display a common cysteine,
unlike HER, but otherwise they show similar
arginine and histidine positioning. In both the
AFPs and HER, the composition and
placement (position) of leucines, isoleucines,
arginines, and histidine as the dominant
amino acid are shared. The HER segment is
devoid of glycines and cysteine in its
secondary binding site, in contrast to human
and rodent AFPs. Cysteine does not seem
to be required for estrogen binding at the
secondary site.
Thus AFP site 1 is primarily committed to
estrogen binding and ligation. And it is
proposed that site 2 is thought to serve as a
docking site for proteins of heat-shock
proteins (HSPs) family, such as HSP-70 and
HSP-90 as in HER. Such docking sites for
HSP’s are also known to be involved in
protein folding/unfolding activities. Misfolded
proteins are believed to be the cause of
various diseases such as Alzheimer’s, and
neural tube defects such as Down’s
syndrome. [2]
Fig. 12 Shows the Estrogen binding sites present on the AFP molecule
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Fig. 13 Shows amino acid sequences of the estrogen binding sites present on HER(human
estrogen receptor); RAFP (rat alpha-fetoprotein); MAFP (mouse alpha-fetoprotein); the numbers
indicate amino acid sequence numbers of the protein. [2]
AFP AND INSULIN:
In the late 1990’s studies of AFP-derived
peptide fragments from the amino-terminal
side of AFP Domain 1 were found to have
glucose/insulin related activities. A synthetic
peptide, duplicating amino acids 13-19
(LDSYQC) of HAFP Domain 1, was reported
to influence glucose uptake by human red
blood cells is a clinical laboratory. The
LDSYQC peptide was found to stimulate the
entry of glucose into red blood cells from
insulin-dependant diabetic children invitro. It
is of interest that an amino acid sequence
on the insulin alpha chain (residues 17-21)
has been found to be structurally
homologous to the AFP-derived LDSYQC
sequence. [2]
Fig. 14 shows the insulin like segment present in Domain-1 of the AFP molecule.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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AFP AND THYROID HORMONES
In developing and neoplastic cell cultures
AFP and thyroid hormones display an
inverse relationship in that cells exposed to
increasing concentrations of triiodothyronine
(T3)
show
decreased
secretion
of
measurable AFP. In vitro studies have
demonstrated that thyroxine (T4) treated
newborn rodents displayed depressed
serum AFP levels that were attributed to
blockage of hepatic AFP synthesis. In
human newborn studies, a reciprocal
relationship between serum AFP levels and
congenital hypothyroidism also exists. Thus
AFP and thyroid hormone levels have
consistently
displayed
an
inverse
relationship in both invitro and in vivo model
systems. [2]
AFP
AND
CELL
SEQUENCES
ADHESION
Intercellular and cell-extracellular matrix
(ECM) interactions are of great significance
in many biological processes of growth,
apoptosis, differentiation and cell migration,
as well as cancer cell invasion and
dissemination
or
metastasis.
These
functions are mediated by many cell
adhesion molecules and cell surface
receptors. Several families of adhesion
molecules have been identified and their
synthesis and expression on the cell
membrane studied in relation to the invasive
and metastatic phenotype. Results of
studies
on tumor metastasis have
demonstrated that cell adhesion plays an
important role in various steps of the
metastatic cascade and that dysregulation of
adhesion mechanisms contributes to the
formation of metastasis. On the other hand,
certain adhesive interactions may diminish
the metastatic process. For example,
adhesion molecules that promote homotypic
cell adhesion among homotypic tumor cells
in a primary tumor site will likely diminish the
metastatic potential. In that context, down
regulation of these adhesion molecules has
been shown to correlate with a higher
propensity of tumor cells to detach from the
primary site and to spread. On the other
hand up regulation of other adhesion
molecules correlates with a higher potential
of tumor cells to metastasize. Preferential
adhesion of metastatic tumor cells to
vascular endothelial cells of certain organs
has been demonstrated in experimental
tumors and it may be explained in part by
the organ-specific expression pattern of
some adhesion molecules.
The second domain of AFP contains short
peptide sequences common to extra cellular
matrix(ECM)
proteins
(like
laminins,
fibronectin, collagen etc) bearing cellular
adhesion motifs (CAMs). These short
sequences are not found commonly in
Albumin, Alpha-Albumin and DBP. Adhesive
macromolecules
(such
as
laminins,
fibronectin etc) have a potential utility in
developmental state and disease state
involving growth, developmental state and
disease
state
involving
growth,
differentiation, cell-migration and tumor
metastasis. Using synthetic peptides derived
from ECM’s, the functional significance of
such short signal peptide sequences has
been identified from one or more domains of
these macromolecules. On inspection of the
matched peptide sequences, it is observed
that frequent amino acid matches of AFP
with CAM-like sequences occur in the
second domain of AFP (amino acids 190394). The physiological functions of
adhesion sequences of CAMs and the cell
types involved are listed in the table below.
Some of these activities include cell
adhesion, migration, differentiation, growth,
neurite outgrowth, tumor spread, enzyme
activity, angiogenesis, and heparin, fibrin,
collagen interactions. Some adhesion
proteins are known to contain two signals of
opposing activities e.g. YIGSR and SIKVAV
of laminin. This demonstrates that additional
fine-tuning controls are in place on the same
molecule to even further regulate adhesion
activities. Two such sites may not be
simultaneously available on a protein due to
ligand and or conformational masking, and
could progressively be unveiled during a
cascade such as clotting anticlotting network.
Such a diverse array of peptide recognition
signals on AFP suggests that AFP might
share functional properties with adhesion
molecules. It is proposed that AFP may play
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
14
a role in fine-tuning the architectural
interstitial growth patterns in developing
organisms. However all adhesion sites may
not be concurrently accessible since AFP is
thought to express different functions at
various developmental stages. Some sites
may be unmasked or made conformationally
available during progressive stages of
embryonic and fetal development depending
upon ligand binding and stress condition of
the protein.
[3]
Fig. 15 shows the locations of some cell-adhesion motif sequences present in the AFP molecule.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
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Table 1 Cell Adhesion Sequences in Extracellular Matrix Proteins Compared With Sequences on
AFP, Albumin, Vitamin-D Binding Protein, and Alpha Albumin. [3]
THE GROWTH INHIBITORY
PEPTIDE
(as a temporary halt may be required until
fetal homeostasis is achieved in the
stress/shock condition).
Alpha fetoprotein known largely as a growth
promoting agent, also possesses a growth
inhibitory motif recently identified as an
occult segment of the molecule. The 34
amino-acid stretch termed the growth
inhibitory peptide (GIP) has been chemically
synthesized, purified and characterized. The
GIP segment lies buried in a molecular
crevice and can be exposed following a
conformational change in HAFP. The hidden
segment is an amino acid stretch on AFP
that can potentially be exposed by extreme
ligand concentrations and possibly by stress
or shock conditions during fetal development
HISTORY OF THE GIP
In the early 1970’s and 1980’s extensive
studies were carried out to determine the
growth regulatory properties of AFP. Studies
involving anti-AFP antibodies were carried
out. When mouse AFP was used as a
control for the antibody experiments, it was
found that the fetal protein, incubated with
estradiol (E2) in 130-fold molar excess was
capable of growth regulation in the
prepuberal mouse uterus.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
16
In 1983 the first report of an AFP-E2
complex-induced inhibition of rat mammary
tumors appeared. During 1983-1985 reports
detailed and documented use of immature
mouse uterine bioassay to measure and
quantify the inhibition of growth in normal
tissues.
At
the
same
time,
E2
addition/depletion experiments showed that
minute amounts of AFP complexed with E2
were capable of blocking E2-supported
growth of MTW9A breast tumors in rats. It
was further proposed that AFP might be
present in adult cancer tissues as a means
to regulate growth.
In 1990 it was found that both mouse AFP
and HAFP activated by complexing with E2
were capable of growth inhibition of MCF-7
human breast cancer xenografts. During this
period an AFP “cassette” concept was
proposed in which isolated stretches of
amino acids on the AFP molecule were
predicted to possess biological activities of
their own. Thus scientists isolated many
peptides from the AFP molecule and tested
them for biological activities.
These studies led to the identification,
chemical synthesis of a 34-amino acid
segment from the third domain of HAFP
which was found to be a disrupter of
endocrine associated growth in the rodent
bioassay. Subsequent findings served to
confirm that this amino acid stretch was an
active site of E2 regulatory growth on the
native AFP molecule. This 34 amino acid
sequence was then designated as the
Growth Inhibitory Peptide (GIP). [4]
STRUCTURE OF THE GIP
The 34-amino acid GIP was also termed the
P149. Synthetic fragments of which have
been designated as P149a (the amino
terminal 12 amino acids), P149b (the
hydrophobic midpiece consisting of 14
amino acids) and P149c (an eight amino
acid peptide located near the carboxyl
terminus). The molecular mass of the
peptide
determined
by
mass
spectrophotometry was found to be 3573Da.
The 34 amino acid sequence was found to
exhibit complex aggregation behavior. Upon
solubilization, the peptide formed trimers;
however, at high concentrations (8mg/ml)
the trimers clustered into large aggregates.
At 0.2mg/ml the trimers slowly dissociated to
form dimers (by intrapeptide disulphide
bonds). It was found that the trimeric form
was active and showed growth inhibitory
activity while the dimeric form was lacking
growth inhibitory activity. [4]
Fig. 16 the three domains of human alpha-fetoprotein are shown in a bar configuration showing
the 590 amino acid full-length protein. The telescoping segment displayed, from amino acids
#445–477, is depicted in its single letter code amino acid sequence together with the three
fragment
constituents
of
P149
comprising
P149a,
P149b,
and
P149c.
[6]
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
17
Fig. 17 Shows the GIP segment highlighted on the AFP molecule.
Fig. 18 shows the cyclized form of the GIP segment.
EFFECT OF GIP ON MOUSE ASCITES
MAMMARY TUMOR
The growth suppression of the mammary
ascites tumor by P149 was determined in
non-estrogen supplemented assay which
measured tumor cell growth and ascites
accumulation of 6WI-1 mammary tumors
transplanted in mice (isografts). Tumor cells
inoculated were 0.3x106 cells. Mice
inoculated with 6WI-1 mammary cells were
injected with a previously determined
optimal dose of 1.0 µg peptide per day or
saline for 11 days. On day 12 following
inoculation, the total accumulated ascites
fluid volume and tumor cells in the peritoneal
cavity of each animal were harvested and
the total ascites cell count was determined.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
18
It was observed that the dose of 1µg/day of
P149 for 11 days suppressed the tumor
associated body weight gain and nor was
there any increase in the ascites fluid
volume. These mice further went on to live
an additional 30 days before finally they
were sacrificed. The tests were then also
carried out on various peptide fragments of
the P149 peptide. It was found that the P149
or its fragment P149c, but not P149a or
P149b
significantly
suppressed
accumulation of both cells and ascites. [6]
Fig. 19 The serial transplantation in NYALAR mice of isograft 6WI-1 murine mammary tumor
ascites cells is depicted at 3 different dose inocula. A1 is non-tumor treated animal. B1 is the
ascites laden breast tumor inoculated mouse which has 0.3×106 cells inoculated. C1 is the group
of mice which has the ascites breast cancer cells inoculated but is treated with 1.0 µg of peptide
per day. [6]
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
19
EFFECT OF GIP ON CELL LINES
An experiment was carried out in which the
GIP was challenged against 60 different cell
culture lines representing a variety of human
cancers. In a 6 day proliferation assay, the
P149 peptide was seen to suppress growth
in 38 out of 60 cancer cell lines,
representing nine different cancer cell types
including prostate, breast, and ovarian
cancers. It was seen that the GIP peptide
was seen to be active against both E2dependant and E2-independent growth.
The table below shows the list of human
tumor cell cultures and the growth
suppressive screening results of HAFPderived peptide (P149) [4]
Table 2 the growth-suppressive (cytostatic) screening results of HAFP-derived peptide (P149) for
multiple types of human tumor cell cultures [4]
PROPOSED MECHANISM OF ACTION
Although the exact mechanism of action has
not as yet been elucidated, a proposal has
been made as to how GIP acts as a growth
inhibitor. Following administration, E2 serves
as a growth-promoting agent in immature
tissues such as the uterus, or in
transformational
E2-dependant
breast
cancer cells. Likewise, administration of the
GIP results in homing of this peptide to
tissue areas of up-regulated growth, as seen
in estrogen-targeted cells 24 hours later. By
means of receptor-mediated endocytosis via
integral cell membrane proteins, the GIP
might gain access through the plasma
membrane
while
encountering
the
membrane-associated caveolae. Since the
estrogen receptor (ER) has been reported to
localize to the inner membrane portion of the
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
20
caveolae, it is conceivable that the GIP
binds to the membrane-tethered ER, a
peptide-to-ER binding interaction that has
been demonstrated in vitro.
It has been shown that ER alpha, soon after
translation on the ribosome, translocates to
the plasma membrane caveolae and resides
there, attached as a complex with the SHc
adaptor/scaffold protein. The caveolae or
lipid rafts have been shown to be essential
for insulin-like growth factor-1 receptor (IFG1R) signaling during the induction of cell
activation, proliferation, and differentiation.
The IGF-1 receptors were reportedly located
in caveolar plasma membranes and are
intimately involved in signal transduction
cascades. Binding of the GIP to the ER
could involve the steroid receptor segment
region that recognizes the co-activator NRbox. It is conceivable that signal
transduction, especially via MAP kinases,
could also be blunted at this point.
Alternatively, once the GIP has complexed
with the ER, a cytoskeletal shape change
could occur due to the juxtaposition of the
ER with tubulin and actin. The ER-GIP
complex might then be transported through
the cell via the cytoskeletal network to the
perinuclear endoplasmic reticulum (EPR)
and the entire complex could position itself
near the entrance of the nuclear pore. The
EPR is known to be contiguous with the
perinuclear region of the cytoplasm.
Immunofluorescent micrographs presently
demonstrate such a perinuclear location for
the GIP in the uterine tissue sections. Since
the GIP has sequence identity to both
nuclear Lamin B and an inner-membrane
protein of the nuclear pore, it is possible that
the ER/GIP complex can be translocated
and positioned at or near the nuclear pore
channel to impede (gate) further entry of
complex into the nucleoplasm. In effect, the
ER would be denied contact with the
transcriptional machinery of the nucleus,
thus
inhibiting
immediate-early
gene
activation. [7]
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
21
Fig. 20An artist’s conception of a possible cell penetration route of growth inhibitory peptide after
approaching the plasma membrane at an angle. During the cell membrane insertion process,
peptides are capable of interacting with integral membrane receptors/proteins to affect signal
transduction. In this proposed model, P149 (shown along the extended arrow) might encounter
and bind the estrogen receptor-alpha (ER-alpha) tethered to the insulin growth factor-1 receptor
(IGF-1R)-SHc adaptor/scaffold complex. When incoming E2 binds normally to this tripartite
complex via ER-alpha, the ER-alpha is released, after which it attaches to the cytoskeleton (grey
background threads) for transcytosis to the perinuclear region of the endoplasmic reticulum and
enters the nuclear pore region (see text). The ER-alpha-GIP bound complexes might blunt this
response, either at the cell membrane or by plugging the nuclear pore.
Heterochromatin = nuclear material consisting of DNA, histones, and associated proteins. Circled
P = phosphorylation sites; MAPK= mitosis activated protein kinases; p110 and p85=
phosphoinositol-3 kinase proteins; Lamins A, B, C = nucleoskeletal proteins near the nucleopores;
HSP- 90, 70 = heat shock proteins that interact with the cytoskeleton. [7]
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE
22
CONCLUSION
Alpha-fetoprotein is truly an amazing protein
which is found in our bodies. The myriad of
functions it performs in our bodies have still
only been discovered in part. Many more
theories have been released on it’s uses in
the human body, but who knows what more
functions this molecule performs. Further
research behind this molecule is thus
mandatory in order to understand the
complete working of the molecule inside the
body. The discovery of the GIP from the
AFP has lead to the opening of the use of
AFP and it’s peptides in development of
cures for cancer. Furthermore the structure
of the GIP itself may provide vital
information for development of new drugs
for a cure for cancer. I would like to
conclude by saying that this truly is a
superman molecule which finds use in
almost every area of molecular biology.
REFERENCES
1. Harvey Lodish [et al.]: Protein Structure
and Function. In: Molecular Cell Biology (5th
ed.). New York, W. H. Freeman and
Company, 2003, p.59.
2. Mizejewski GJ. Biological Roles of AlphaFetoprotein During Pregnancy and Perinatal
Development. Exp Biol Med 229:439-463,
2004.
3.
Mizejewski
GJ.
Alpha-fetoprotein
structure and function: relevance to isoforms,
epitopes, and conformational variants. Exp
Biol Med 226:377-408, 2001.
4. Mizejewski GJ, MacColl R: Alphafetoprotein growth inhibitory peptides:
Potential leads for cancer therapeutics. Mol
Cancer Ther 2: 1243–1255, 2003.
5. Morinaga T, Sakai M, Wegmann TG,
Tomoaki T. Primary structures of human
AFP and its mRNA. Proc Natl Acad Sci USA
80:4604– 4608, 1983.
6. Meuhlemann M., Miller KD, Dauphinee M,
Mizejewski GJ. Review of Growth Inhibitory
Peptide as a Biotherapeutic agent for tumor
growth, adhesion, and metastasis. Cancer
and Metastasis Reviews 24:441-467,2005.
7. Mizejewski G, Smith G, Butterstein G:
Review and Proposed Action of alphafetoprotein growth inhibiting peptides as
estrogen and cytoskeletal-associated factors.
Intl Journal Cell Biology 28: 913–933, 2004.
AN INTRODUCTION TO ALPHA-FETOPROTEIN AND THE GROWTH INHIBITORY PEPTIDE