Receptor tyrosine kinases (RTKs) are cell surface receptors that bind polypeptide growth factors, cytokines, and hormones. They regulate normal cellular processes but also play a critical role in cancer development and progression. There are approximately 20 classes of RTKs that exist as single or multimeric complexes and activate intracellular signaling pathways through autophosphorylation following ligand binding. Mutations in RTKs and their downstream effectors can lead to uncontrolled cell growth by constitutively activating growth signaling pathways. Several RTK inhibitors have been developed for cancer treatment, including those that target specific kinases as well as multi-kinase inhibitors.

1. M.Prasad Naidu
MSc Medical Biochemistry, Ph.D,.

2.  Receptor tyrosine kinases (RTK)s are the high-
affinity cell surface receptors for many
polypeptide growth factors, cytokines,
and hormones.
 Of the 90 unique tyrosine kinase genes identified
in the human genome, 58 encode receptor tyrosine
kinase proteins.
 Receptor tyrosine kinases have been shown not
only to be key regulators of normal cellular
processes but also to have a critical role in the
development and progression of many types
of cancer.

3.  RTK Approximately 20 different
RTK classes have been identified.[3]
 class I (EGF receptor family)(ErbB
family)
 RTK class II (Insulin
receptor family)
 RTK class III (PDGF receptor family)
 RTK class IV (FGF receptor family)
 RTK class V (VEGF
receptors family)
 RTK class VI (HGF receptor family)
 RTK class VII (Trk receptor family)
 RTK class VIII (Eph receptor family)
 RTK class IX (AXL receptor family
 RTK class IX (AXL receptor family)
 RTK class X (LTK receptor family)
 RTK class XI (TIE receptor family)
 RTK class XII (ROR receptor family)
 RTK class XIII (DDR
receptor family)
 RTK class XIV (RET receptor family)
 RTK class XV (KLG receptor family)
 RTK class XVI (RYK
receptor family)
 RTK class XVII (MuSK
receptor family)

4.  RTKs Most are single subunit receptors but
some exist as multimeric complexes, e.g.,
the insulin receptor that forms disulfide-linked
dimers in the absence of hormone; moreover,
ligand binding to the extracellular domain
induces formation of receptor dimers.
 Each monomer has a single
hydrophobic transmembrane-
spanning domain composed of 25-38 amino
acids, an extracellular N-terminal region, and
an intracellular C-terminal region.

6.  The extracellular N-terminal region exhibits a variety
of conserved elements including immunoglobulin (Ig)-
like or epidermal growth factor (EGF)-like domains,
fibronectin type III repeats, or cysteine-rich regions
that are characteristic for each subfamily of RTKs;
 these domains contain primarily a ligand-binding site,
which binds extracellular ligands, e.g., a
particular growth factor or hormone.
 The intracellular C-terminal region displays the highest
level of conservation and comprises catalytic domains
responsible for the kinase activity of these receptors,
which catalyses receptor autophosphorylation and
tyrosine phosphorylation of RTK substrates.

7. Kinase activity
 In biochemistry, a kinase is a type of enzyme that
transfers phosphate groups (see below) from high-
energy donor molecules, such as ATP to specific
target molecules (substrates); the process is
termed phosphorylation.
 The opposite, an enzyme that removes phosphate
groups from targets, is known as a phosphatase.
 Kinase enzymes that specifically phosphorylate
tyrosine amino acids are termedtyrosine kinases.

8.  When a growth factor binds to the extracellular
domain of an RTK, its dimerization is triggered
with other adjacent RTKs.
 Dimerization leads to a rapid activation of the
protein's cytoplasmic kinase domains, the first
substrate for these domains being the receptor
itself.
 The activated receptor as a result then becomes
autophosphorylated on multiple specific
intracellular tyrosine residues

9. Signal transduction
 The phosphorylation of specific tyrosine residues within the
activated receptor creates binding sites for Src homology
2 (SH2) domain- and phosphotyrosine binding (PTB)
domain-containing proteins.
 Specific proteins containing these domains
include Src and phospholipase Cγ. Phosphorylation and
activation of these two proteins on receptor binding lead to
the initiation of signal transduction pathways.
 Other proteins that interact with the activated receptor act
as adaptor proteins and have no intrinsic enzymatic activity
of their own.
 These adaptor proteins link RTK activation to
downstream signal transduction pathways, such as
the MAP kinase signalling cascade.

15.  Activation of the insulin-receptor Tyr kinase by autophosphorylation.
 (a) In the inactive form of the Tyr kinase domain
 (PDB ID 1IRK), the activation loop (blue) sits in the active site, and
 none of the critical Tyr residues (black and red ball-and-stick structures)
 are phosphorylated. This conformation is stabilized by hydrogen
 bonding between Tyr1162 and Asp1132. (b) When insulin binds to
 the chains of insulin receptors, the Tyr kinase of each subunit of
 the dimer phosphorylates three Tyr residues (Tyr1158, Tyr1162, and
 Tyr1163) on the other subunit (shown here; PDB ID 1IR3). (Phosphoryl
 groups are depicted here as an orange space-filling phosphorus
 atom and red ball-and-stick oxygen atoms.) The effect of introducing
 three highly charged P –Tyr residues is to force a 30 Å change
 in the position of the activation loop, away from the substrate-binding
 site, which becomes available to bind to and phosphorylate a target
 protein, shown here as a red arrow

20.  Membrane rafts and caveolae sequester groups
of signaling proteins in small regions of the
plasma membrane, enhancing their interactions
and making signaling more efficient.

22.  Proteins in the transforming growth factor
superfamily use receptors that have serine/
threonine kinase activity and associate with
proteins from the Smad family, which are gene-
specific transcription factors .
 This superfamily includes transforming growth
factor (TGF-), a cytokine/hormone involved in
tissue repair, immune regulation, and cell
proliferation, and bone morphogenetic proteins
(BMPs), which control proliferation,
differentiation, and cell death during
development.

25.  The JAK-STAT transduction mechanism for the erythropoietinReceptor
 Binding of erythropoietin (EPO) causes dimerization of the EPO
receptor, which allows the soluble Tyr kinase JAK to bind to the internal
domain of the receptor and phosphorylate it on several Tyr residues.
 The STAT protein STAT5 contains an SH2 domain and binds to the P –
Tyr residues on the receptor, bringing the receptor into proximity with
JAK.
 Phosphorylation of STAT5 by JAK allows two STAT molecules to
dimerize, each binding the other’s P –Tyrresidue.
 Dimerization of STAT5 exposes a nuclear localization sequence (NLS)
that targets STAT5 for transport into the nucleus.
 In the nucleus, STAT causes the expression of genes controlled by EPO.
 A second signaling pathway is also triggered by autophosphorylation of
JAK that is associated with EPO binding to its receptor.
 The adaptor protein Grb2 binds P –Tyr in JAK and triggers the MAPK
cascade, as in the insulin system .

26.  The ErbB protein family
or epidermal growth
factor receptor (EGFR)
family is a family of
four structurally related
receptor tyrosine
kinases. Insufficient
ErbB signaling in
humans is associated
with the development
of neurodegenerative
diseases, such
as multiple
sclerosis and Alzheimer'
s Disease
 Excessive ErbB
signaling is associated
with the development
of a wide variety of
types of solid tumor.
ErbB-1 and ErbB-2 are
found in many
human cancersand their
excessive signaling may
be critical factors in the
development
and malignancy of
these tumors

27.  Vascular endothelial growth factor (VEGF) is
one of the main inducers of endothelial
cell proliferation and permeability of blood
vessels. Two RTKs bind to VEGF at the cell
surface, VEGFR-1 (Flt-1) and VEGFR-2
(KDR/Flk-1)

32.  Oncogene-encoded defective EGF receptor.
The product of the erbB oncogene (the ErbB
protein) is a truncated version of the normal
receptor for epidermal growth factor (EGF).
 Its intracellular domain has the structure
normally induced by EGF binding but the
protein lacks the extracellular binding site for
EGF.
 Unregulated by EGF, ErbB continuously
signals cell division

33.  Some oncogenes encode surface receptors with
defective or missing signal-binding sites such that
their intrinsicTyr kinase activity is unregulated.
 For example, the protein ErbB is essentially
identical to the normal receptor for epidermal
growth factor, except that ErbB lacks the amino-
terminal domain that normally binds EGF and as
a result sends the “divide” signal whether EGF is
present or not.
 Mutations in erbB2, the gene for a receptor Tyr
kinase related to ErbB, are commonly associated
with cancers of the glandular epithelium in breast,
stomach, and ovary.

35.  Mutant forms of the G protein Ras are common in
tumor cells.
 The ras oncogene encodes a protein with normal GTP
binding but no GTPase activity.
 The mutant Ras protein is therefore always in its
activated (GTP-bound) form, regardless of the
signals arriving through normal receptors.
 The result can be unregulated growth.
 Mutations in ras are associated with 30% to 50% of
lung and colon carcinomas and more than 90% of
pancreatic carcinomas.

37.  Drugs that target the inactive conformation
of a specific protein kinase and prevent its
conversion to the active form may have a
higher specifity of action.
 For eg; monoclonal antibodies . They
eliminate receptor kinase activity by
preventing dimerisation or by causing their
removal from cell surface.

38.  erlotinib: targets (non
small cell lung cancer)
small molecule kinase
inhibitor.
 Imatinibmesylate:
100% effective in early
stage CML.
 TRASTUZUMAB,
CETUXIMAB are
monoclonal
antibodies that target
HER2/neu, EGF- R,
and VEGF-R that are
in clinical use for
certain types of
cancer.(lung cancer,
large intestinal
cancers)

39.  Because many cell division signalling systems
involve more than one protein kinase ,
inhibitors that act on several protein kinases
may be useful in the treatment of cancer
 eg;: sunitinib and sorafenib target several
protein kinases including VEGR-R and
PDGF-R used for treatment of GI stromal
tumors and advanced renal cell carcinoma.