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THE
ROLE OF EPIDERMAL GROWTH FACTOR RECEPTOR FAMILY IN THE MOLECULAR
PATHOGENESIS AND TREATMENT OF BREAST CANCER
K. T. Papazisis
Breast Cancer Biology Group, Cancer Research
UK, London SE1 9RT
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Keywords:
Breast cancer; HER; HER2; Herceptin; Protein tyrosine kinases |
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Receptor
Protein Tyrosine Kinases (rPTKs)
PTKs are a group of enzymes that catalyze the transfer of the g-phosphate
unit from ATP to a tyrosine residue. They are involved in the regulation
of cellular proliferation, differentiation and apoptosis. Human
genome encodes for more than 150 PTKs and at least 18 are well-characterized
oncogenes.
There are two distinct groups of protein tyrosine kinases: receptor
PTKs and non-receptor (cytoplasmic) PTKs. Receptor PTKs are glycoproteins
with an extracellular (aminoterminal) domain, a short transmembrane
domain and an intracellular (carboxytelic) domain. Intracellular
domain consists of specific subdomains for phosphorylation, interaction
with other proteins of the downstream signal transduction pathways
and the kinase subunit. All rPTKs (except Insulin-like Growth Factor
Receptor - IGFR) exist in the cellular membrane in the form of monomers.
Ligand association leads to receptor dimerization (or oligomerization),
eterophosphoryliosis of the cytoplasmic tails of the receptors and
activation of the downstream cascades by means of kinase activation
and "opening" of the adaptor modules that are capable to recruit
the components of signal transduction.
Epidermal
Growth Factor Receptor Famamily
There are four members of human epidermal growth factor receptor
(HER) family [HER-1 (EGFR or c-erbB), HER-2 (c-erbB-2), HER-3 and
HER-4] and several ligands. EGF is the ligand for HER-1, whilst
TGFa, neuregulins (NRGs), epiregulin (EPI), betacellulin (BTC),
and other related molecules interact with the other three receptors.
The interaction of ligand with the receptor induces conformational
changes of the later, which result to receptor dimerization, either
with a same receptor (homodimerization) or with another receptor
of the family (heretodimerization). Downstream messages include
PI3-kinase, the Grb2-ras pathway, PLC-g and MAPK pathway activation.
The outcome of these complex networks is the induction of cellular
proliferation and the inhibition of apoptosis.
The intracellular domain of HER family receptors has been described
in detail. There is a catalytic subunit and several SH2 (src homology
regions 2) and PTB (phosphotyrosine-binding) motifs. SH2 domains
are 100 amino acid-long motifs that interact with phosphotyrosine
residues depending on the 3-6 carboxy-terminal adjacent amino acids.
On the contrary, PTB domains interact with phosphotyrosines depending
on the amino-terminal adjacent sequence. Several adaptor proteins
are recruited to the receptors, like Grb2, Grb7, Shc, Crk and Gab1,
kinases like PI3K and Src, phospholipases (PLC-g1) and phosphatases
(SHP1 and SHP2). The net outcome of this complex network depends
on several factors, as the type of ligand (or ligands) that activated
the receptors, ligand concentration and the kind of dimer that was
formed (determined both by the ligand and by the degree of expression
of the receptor molecules).
It is interesting that HER2 (which has the most active PTK subunit)
has not any known ligand (and this way HER2/HER2 dimers cannot be
normally formed) but heterodimerizes with other receptors of the
family, when they bind to ligand. The dimerization with HER2 leads
to signal stabilization (i.e. prolongation) due to decreased intracellular
traffic of the dimers, increased affinity to the ligand and signal
amplification. On the other hand HER3 has not any kinase activity
(HER3/HER3 dimers can be formed but are inactive) but is the only
receptor of the HER family that can recruit PI3K effectively because
it has many p85 (the adaptor domain of PI3K) binding sites. This
way, the heterodimer HER2/HER3 is the most potent dimer in inducing
cellular proliferation and survival.
HER
family in breast cancer
HER2 is overexpressed in almost a third of breast cancer cases.
The overexpression of HER2 in other cancer types has been also documented
but only in breast cancer there is firm evidence for its role in
pathogenesis and therapy. On a cellular level, HER2 overexpression
can lead to increased cellular proliferation (by inducing cyclin
D expression and inhibiting p27KIP1 levels), decreased apoptosis
and resistance to cytotoxic treatment and reduced expression of
several adhesion molecules (a2-integrins, cadherins) that increases
the metastatic potential.
In breast cancer patients, increased HER2 expression is usually
the result of gene amplification (usually an increased copy number
up to ´10) that results to protein overexpression (´10- ´100) in
the cytoplasmic membrane. However there are single-copy overexpressors
as well as patients with gene amplification without protein overexpression.
The significance of the two later observations is not clear, but
some studies have shown that gene amplification without protein
overexpression is also a bad prognostic indicator, possibly due
to amplification of adjacent genes (as topoisomerase-IIa). Finally,
some patients express an alternatively spliced form that lacks a
small part just carboxytelic to the transmembrane domain that leads
to constant HER2 dimerization and activation. The proteolytical
cleavage of the extracellular domain can also lead to constant dimerization
and activation whilst the extracellular domain can be detected (by
immunoabsorbent assays) in the serum and used as a tumor marker.
In the clinical setting, HER2 overexpression is a bad prognostic
indicator leading to
1) Decreased disease-free survival and
2) Decreased overall survival in node-positive patients.
It is also a predictive factor because it results in
1) Resistance to CMF chemotherapy
2) Resistance to antiestrogen hormonal treatment
3) A better response to anthracyclin-containing regimens (possibly
because of the co-expression of the target protein for anthracyclins
topoisomerase-IIa)
4) Response to a new anti-HER2 monoclonal antibody treatment (trastuzumab,
Herceptin®)
However, because HER2 is not usually signaling in the absence of
the other HER family members, the oncogenic potential of HER2 depends
largely on the kind of heterodimers that forms with HER1,3 and 4.
Among HER2-positive breast cancer patients, those with overexpression
of HER1 or/and HER3 are the ones that belong to worse prognosis
group. The overexpression of HER4 on the other hand seems to improve
the prognosis of HER2-positive patients. Much work is yet to be
done in analyzing by detail the effect of the HER-family differential
expression in breast cancer, using large-scale studies with standardized
and approved techniques.
HER2-targeted therapy for breast cancer
Trastuzumab (Herceptin, Roche) was developed as a humanized anti-HER2
monoclonal antibody that targets the extracellular (juxta-membrane)
domain of HER2. Trastuzumab treatment results in
• Inhibition of HER2 expression (inducing endocytosis of the
HER2/HERx/trastuzumab complex and subsequently proteolytic cleavage
of HER2)
• Inhibition of HER2 phosphoryliosis
• Induction of HER3 cleavage when it is in dimers with HER2
• Increased expression of p27KIP1 and decreased expression
of cyclin D1 (resulting in inhibition of proliferation in G0/G1).
The decreasing on cyclin D1 expression leaves unbound p27KIP1 to
bind and inactivate cyclin E also, resulting in G1 arrest.
• Inhibition of Rb phosphorylation
• Inhibition of extracellular cleavage of HER2
• Reduction in VEGF expression
• Antigen-depended cell cyctotoxicity (ADCC) against HER2+ve
cells
In clinical terms, these effects can be translated to a higher response
rate, longer disease-free survival and possibly to a longer overall
survival as well. The combination of trastuzumab with chemotherapeutic
drugs (taxanes as paclitaxel and docetaxel, cis-platinum, etc) is
proven synergistic in vitro and in vivo.
However, only one third of HER2+ve patients will eventually respond
and benefit from herceptin treatment in the clinical practice. An
explanation for this is the overexpression and activation of other
"parallel" signal transduction pathways (HER1, FGFR1) or the independence
of the proliferation and survival signal from the receptor regulation,
due to mutations in downstream signals that render them unresponsive
to HER2 downregulation. On the other hand, HER2+ve tumors do not
represent a single group, since expression of the other three members
of the HER family is largely influencing the final outcome. In this
way, HER3+ve patients or patients with active PI3K (as can be demonstrated
with anti p-akt immunohistochemistry) are more likely to respond
to herceptin treatment. There is still a long way to identify the
patients with a higher probability to respond to anti-HER2 treatment,
to standardize the most effective ways of anti-HER treatment and
combinations with other therapeutic modalities and to improve anti-HER
treatment possibly by using small molecule PTK inhibitors with high
anti-HER potency.
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