ARTICLE
Auteur(s) : R Rosell, M Cuello, F Cecere, M Santarpia, N
Reguart, E Felip, M Taron
Catalan Institute of Oncology, Hospital Germans Trias i Pujol,
Badalona, Barcelona, Spain
Chemotherapy in non-small-cell lung cancer (NSCLC) has reached a
plateau, with no evidence of substantial improvement in survival. A
performance status of 0 is the most significant prognostic factor,
though ERCC1 mRNA expression is closely linked to cisplatin
resistance. The economic impact of novel targeted therapies has not
yet been evaluated, and their overall benefit is still meager.
Activating mutations in tyrosine kinases have emerged as a new
paradigm for predicting response and outcome. Growing evidence
indicates that EGFR deletions and L858R mutations are strong
predictors of dramatic responses to gefitinib and erlotinib.
Table 1 highlights some of the most relevant findings in lung
cancer molecular biology that can pave the way for individualized
treatment based on predictive markers; this approach will also
contribute to optimizing the cost-effectiveness of novel targeted
therapies.
Predictive markers for customized chemotherapy
( Table 1 )Between 1993 and 1999, 1436
patients with stage IV or IIIB NSCLC with effusion were treated
with platinum-based doublets (involving either paclitaxel,
docetaxel, vinorelbine or gemcitabine). The response rates and
median survival times were 20% and 8.2 months. One- and two-year
survivals were 33% and 11%, respectively [1]. In a multivariate
analysis, lower performance status (PS) PS 1 versus 0 was
identified as one prognostic factor. The salient finding of the
Eastern Cooperative Oncology Group (ECOG) trial was that no
survival differences were observed between any of the different
platinum-based doublets, with a modest benefit for chemotherapy in
NSCLC. However, reality shows that survival can vary significantly
between individual patients with some surviving years and others
succumbing to their disease within a few months.
Cisplatin resistance is associated with increased expression of
the excision repair cross-complementing 1 (ERCC1) gene. Cancer
tissues from ovarian cancer patients whose tumors were clinically
resistant to therapy showed greater levels of ERCC1 mRNA [2].
We carried out a study to examine the role of ERCC1 mRNA levels
in advanced NSCLC patients treated with gemcitabine plus cisplatin.
Patients with low ERCC1 mRNA levels attained a response rate of
52%, while in those with high levels, the response rate was 36%.
This difference was not significant; however, when we used a cutoff
of 5.8 for ERCC1 expression, median survival was 15 months for
patients with low levels and only 5 months for those with high
levels (p < 0.001) [3].
Based on these findings, we carried out an ERCC1 mRNA customized
chemotherapy trial. More than 400 patients have been included and
randomized to the control or the experimental arm. The control arm
received docetaxel plus cisplatin, and patients in the experimental
arm received either the same combination of docetaxel plus
cisplatin if their ERCC1 mRNA levels were low or docetaxel plus
gemcitabine if their levels were high. The preliminary results on
264 patients [4] showed that the response rate for patients with
low ERCC1 levels was 56.6% while for patients in the control arm it
was 40.4% (p = 0.02). When patients in the control arm were split
according to the ERCC1 levels, those with low levels had a response
rate of 47.3%, while those with high levels had a response rate of
26.1%. The logistic regression model for tumor progression
indicated a significant improvement for patients randomized to
docetaxel plus cisplatin based on low ERCC1 levels. Although the
results are still preliminary, time to progression and survival
adjusted for age are significantly in favor of the group with low
ERCC1 levels.
BRCA1 levels and cisplatin resistance
BRCA1 was overexpressed in the cisplatin-resistant breast cancer
cell line MCF7 [5]. BRCA1 is a component of multiple DNA repair
pathways and functions as a molecular determinant of response to a
range of cytotoxic chemotherapeutic agents. It has been
demonstrated that BRCA1 abrogates the apoptotic phenotype induced
by a range of DNA-damaging agents, including cisplatin, etoposide
and bleomycin, and induces dramatic responses to a range of
antimicrotubule agents, including paclitaxel and vinorelbine. These
landmark findings indicate that BRCA1 functions as a differential
regulator of chemotherapy-induced apoptosis [6]. Sporadic cancers,
such as breast, ovarian and NSCLC, can have the BRCA1 function
abrogated by methylation or other mechanisms. In addition,
methylation of FANCF has been observed in these tumors. These
characteristics, known as “BRCAness”, increase sensitivity to
cisplatin and related DNA cross-linking agents and may increase
resistance to antimicrotubule drugs [7]. Recently, it has been
shown that BRCA1 or BRCA1 dysfunction profoundly sensitizes cells
to the inhibition of poly(ADP-ribose) polymerase (PARP) [8]. We
have observed that BRCA1 mRNA expression closely correlates with
ERCC1 mRNA expression and that BRCA1 mRNA expression predicts
outcome in locally advanced NSCLC patients treated with
neo-adjuvant gemcitabine plus cisplatin followed by surgery. Median
survival has not been reached in patients with the lowest BRCA1
mRNA levels, while survival was very poor in patients with the
highest levels [9]. These findings are along the same lines as
preclinical data, indicating that patients with high BRCA1 levels
could respond more favorably, not to non-cisplatin regimens but to
antimicrotubule drugs. Although gemcitabine is a neutral drug for
the BRCA1 mRNA effect, we have observed that elevation of ERCC1 and
BRCA1 is closely related to high levels of ribonucleotide
reductase, which is one of the principal mechanisms of resistance
to gemcitabine [10]. With the exception of ribonucleotide
reductase, the mechanisms of gemcitabine resistance have not been
explored in the clinical setting. Experimental evidence indicates
that increased expression levels of human equilibrative nucleoside
transporter 1 (hENT1) are associated with gemcitabine sensitivity
while decreased levels of deoxycytidine kinase (dCK) predict
acquired resistance to gemcitabine in NSCLC cells [11].
Table 1 Areas of research in lung cancer
|
. BRCA mRNA expression
|
|
. K-ras mutations
|
|
. ERCC1 and SEMA3B single nucleotide polymorphisms
|
|
. Mutations in tyrosine kinases
|
|
. Acquired resistance in sensitive EGFR mutations
|
|
. Erythropoietin receptor
|
|
. Estrogen and progesterone receptors
|
|
. MicroRNAs
|
|
. Wnt signaling pathway
|
K-ras mutations
Adjuvant vinorelbine plus cisplatin increased survival in patients
with stage II NSCLC. Median survival was 41 months in the
observation group and 80 months in the chemotherapy group (hazard
ratio [HR] = 0.59; p =0.004). However, adjuvant chemotherapy
did not confer survival advantage in patients whose tumors had ras
mutations (HR = 0.95; p = 0.87) [12]. Pooled data on K-ras
mutations indicate that it is mutated in 20% of NSCLC, mainly in
adenocarcinoma and linked to poor prognosis. p21 overexpression, as
evaluated by immunohistochemistry, was not conclusively related to
prognosis [13]. The implications of K-ras mutations in predicting
response to EGFR tyrosine kinase (TK) inhibitors are described
below.
Single nucleotide polymorphisms (SNPs) in ERCC1 and SEMA3B
SNPs are found in nearly all human DNA repair genes that have been
investigated. The repair genotype can be very complex in an
individual, resulting in several variant peptides between the
respective repair complexes. It has been suggested that DNA repair
function and cancer risk are significantly modulated by additive
and even multiplicative effects of various variant alleles [14].
Two common SNPs of ERCC1, codon 118 C/T and C8092A, are well
recognized. The codon 118 C/T SNP is associated with differential
mRNA levels. The C8092A SNP, located in the 3′-untranslated region
of the gene, may affect ERCC1 mRNA stability. A significant
observation has been observed between C8092A SNP and survival in
cisplatin-treated NSCLC patients. Median survival was 22.3 months
for patients with the C/C genotype 13.4 months for those with C/A
or A/A (p = 0.006), suggesting that any copy of the A allele is
associated with poor outcome [15]. In addition, carriers of at
least one A allele had a significantly increased risk of grade 3 or
4 gastrointestinal toxicity (adjusted odds ratio = 2.33; p = 0.03)
[16]. No statistically significant association was found for the
codon 118 SNP; however, shorter survival was observed in patients
homozygous for the variant C/C [15].
SEMA3B belongs to a large family of secreted, transmembrane and
membrane-associated semaphoring proteins characterized by a
conserved, cysteine-rich, 500 amino-acid “sema” domain. SEMA3B is
located in the LUCA region of chromosome 3p21.3, which is
frequently deleted in human lung cancer. Protein homology between
the SEMA domain of SEMA3B and the MET and RON oncogenes suggest
that SEMA3B can antagonize the VEGF pathway. A SNP has been
reported at codon 415 of SEMA3B, leading to a substitution of Thr →
Ile (T415I). The variant Ile allele occurs in African-American and
Latino-American control subjects but not in Caucasian subjects.
Possessing either the heterozygous or homozygous variant genotype
confers a > 40% reduced relative risk of lung cancer in
Latino-Americans, controlling for other lung cancer risk factors
[17].
Another member of the sema family, the SEMA3F gene, was
originally isolated from a recurrent 3p21.3 homozygous deletion in
small-cell lung cancer cells. Expression of SEMA3F and SEMA3B is
reduced in various lung cancer cell lines. It has recently been
demonstrated that SEMA3F is frequently methylated, and chromatin
remodeling through histone deacetylase inhibition is sufficient to
activate SEMA3F expression [18].
Gene mutations as predictive markers for molecular therapy
Inhibition of TKs by selective small molecule inhibitors is
emerging as a new strategy for treatment of hematologic
malignancies and solid tumors, including leukemias,
gastrointestinal stromal cell tumors and NSCLC. Determination of
EGFR expression is not sufficient to predict sensitivity to EGFR TK
inhibitors. The identification of somatic mutations in the TK
domain of the EGFR gene represents the most important molecular
marker of sensitivity to EGFR TK inhibitors [19-21]. In a recent
study, neither EGFR nor p-EGFR protein expression was correlated
with gefitinib response in chemorefractory NSCLC patients.
Expression of downstream markers, like p-Erk, was negatively
associated with response. Those without p-Erk staining had a
response rate of 38% and those with 1+ staining had 14%, while
there was no responder among patients with 2+ staining.
Furthermore, tumors with positive p-Akt and negative p-Erk nuclear
expression exhibited the best response (60%). Patients with
positive p-Akt tended to show prolonged time to progression (6.8
versus 2.5 months; p = 0.05) and significantly prolonged survival,
regardless of p-Erk expression. The authors speculate that tumors
with PI3K/Akt as a preferential downstream pathway are more
susceptible to gefitinib whereas those with Ras/Raf/Erk are more
resistant [22].
Intriguingly, gefitinib-resistant adenocarcinoma cell line
populations have increased Akt phosphorylation (not inhibited by
gefitinib), reduced PTEN protein expression and loss of the EGFR
gene mutation, when compared with parental cell lines [23].
Second-line erlotinib has yielded a response rate of 8.9%, in
contrast with less than 1% in the placebo group. Progression-free
survival was 2.2 versus 1.8 months, with median survival 6.7 versus
4.7 months. Responses were higher in females, adenocarcinomas and
never-smokers. There were no differences according to EGFR
expression in the total of 731 patients examined [24]. However, in
a multivariate analysis of the same study, EGFR expression by
immunostaining was associated with better response [25].
In addition, amplification or high polysomy of the EGFR gene
(seen in 33 of 102 patients treated with gefitinib) and high
protein expression (seen in 58 of 98 patients) were significantly
associated with better response (36 versus 3%; p < 0.001), time
to progression (9 versus 2.5 months; p < 0.001) and survival
(18.7 versus 7 months; p = 0.03). EGFR mutations (seen in 15 of 89
patients) were also significantly related to response and time to
progression, but the association with survival was not
statistically significant, and 40% of patients with mutations had
progressive disease. In the multivariate analysis, only high EGFR
gene copy number remained significantly associated with better
survival [26]. EGFR mutations have been examined in 68
gefitinib-treated chemorefractory NSCLC patients from the United
States, Europe and Asia. Responses were observed in 94% of patients
harboring EGFR mutations, in contrast with 12.6% with wild-type
EGFR (p < 0.001) [27]. These results mirror accumulated data
from the three seminal studies of EGFR mutations (reviewed in
[28]), in which 81% of NSCLC patients with EGFR mutations attained
an objective response whereas none of 29 non-responders had
mutations. In the Taron study [27], patients harboring EGFR
mutations attained dramatic and durable responses with
disappearance of brain metastases and other metastatic lesions.
Only 16% of responders with wild-type EGFR, compared to 81% of
responders with EGFR mutations, are still alive, and median
survival has not been reached for this group. In a sub-group of
patients, the response rate for patients with increased gene copy
numbers was 45%, in contrast with 89% for patients with EGFR
mutations (p = 0.02). The response rate was 100% for patients with
both increased gene copy numbers and EGFR mutations. Interestingly,
patients with both EGFR mutations and low levels of EGFR or
caveolin-1 mRNA had a median survival of 13 months, whereas median
survival has not been reached for those patients with EGFR
mutations and high levels of EGFR or caveolin-1 mRNA [27]. Another
study has confirmed the low rate and short duration of response in
Spanish second-line gefitinib-treated patients with wild-type EGFR
[29]. Time to progression for patients with mutations was 12.3
months, compared to 3.6 months for those with wild-type EGFR (p =
0.002). In all studies of EGFR mutations to date [27-36], the
majority of the mutations were in-frame deletions in exon 19 and
the missense mutation L858R in exon 21. Only one study examined the
role of EGFR mutations in patients treated with first-line
gefitinib [30]. Seventeen patients (19%) harboring EGFR mutations
had an impressive time to progression of 21.7 months, in contrast
with 1.8 months for those without mutations (p < 0.001). Median
survival was a landmark 30.5 months for patients with mutations in
comparison with 6.6 months for those with wild-type EGFR (p <
0.001). P-Akt expression was not associated with response, time to
progression or survival [30].
Table 2( Table 2 ) shows recent
salient findings with regard to EGFR mutations. Some evidence
suggests that deletions can predict better response than the
missense mutation L858R [31]. In addition, other studies report
that the L858R mutation is more frequent in women than in men [32,
33]. Intriguingly, squamous cell carcinoma patients with EGFR
mutations did not respond to gefitinib treatment [34]. Table 3(
Table 3 ) shows the geographical and
ethnic frequencies of EGFR mutations found in several studies
[29-32, 34-36]. The lowest frequency was found in African-American
patients [36], and the different frequencies found between patients
from Maryland (6.3%) and Minnesota (20.3%) can be attributed to the
higher proportion of African-Americans from Maryland [36] (table
3). A large-scale study of EGFR mutations was initiated by the
Spanish Lung Cancer Group in April 2005; in the first three months,
tumors from 260 patients were examined, with striking differences
in frequencies according to smoker status (5% in active smokers,
35% in ex-smokers, and 60% in never-smokers) [Reguart N, personal
communication].
In a study of paclitaxel plus carboplatin with or without
erlotinib (Tribute) [37], the influence of EGFR mutations was
examined in 274 patients. Patients with EGFR-mutant tumors in the
erlotinib arm attained better outcome than those receiving
chemotherapy alone (response: 53 versus 21%; time to progression:
12.5 versus 6.6 months, respectively). A detrimental effect of
K-ras mutations was observed for patients in the erlotinib arm,
with a response rate of 8%, compared to 23% for those receiving
chemotherapy alone. Time to progression and survival were also
shorter for patients with K-ras mutations receiving erlotinib than
for those receiving chemotherapy alone (time to progression: 3.4
versus 6 months; survival: 4.4 versus 13.5 months) [37] (table
4( Table 4 )). Numerous EGFR mutations
have also been found in sporadic and hereditary breast cancers, in
the form of point mutations; neither deletions nor the L858R
mutation were observed [38]. Mutations were found more frequently
in hereditary breast cancer than in sporadic breast cancer, which
can be attributed to genomic instability stemming from disruption
of DNA repair capacity by defects in BRCA1 and BRCA2 [38]. This
raises the hypothesis that the better survival observed with
chemotherapy alone in patients with EGFR mutations [37] may be due
to the underlying DNA repair defects that can accompany EGFR
mutations. One of the most salient aspects of primary resistance to
EGFR TK inhibitors is the presence of K-ras mutations [3], which
was originally described by Pao et al. [39], who found that K-ras
mutations were associated with lack of response to gefitinib or
erlotinib.
Acquired resistance to gefitinib or erlotinib has also been
described. In addition to a primary drug-sensitive EGFR mutation, a
secondary mutation in exon 20, leading to the substitution Thr →
Met at position 790 (T790M), has been reported [40, 41]. One
patient with a gefitinib-sensitive EGFR mutation relapsed after two
years of complete remission, and the tumor biopsy at relapse
revealed the presence of the T790M mutation [40] (table 4).
Importantly, however, in human NSCLC cell lines harboring
gefitinib-sensitive EGFR mutations, secondary resistance is rarely
associated with the presence of the T790M mutation. Drug-resistant
clones showed persistence of the sensitive EGFR mutation without
new mutations in ERBB2, p53, K-ras or PTEN [42]. The authors
describe a novel mechanism of acquired gefitinib resistance through
altered receptor trafficking. They found that increased
internalization of ligand-bound EGFR in resistant cells was linked
to potential dissociation of the gefitinib-EGFR complex at the low
pH of intracellular vesicles [42]. Caveolin-1 mRNA levels can also
help to explain the responses associated with EGFR mutations [27].
(Space limitations make it impossible to include here all the
studies on EGFR mutations.)
Overexpression of ERBB3 has also been related to gefitinib
sensitivity in NSCLC cell lines. ERBB3 may be used to couple EGFR
to the PI3K/Akt pathway in gefitinib-sensitive NSCLC cell lines
harboring wild-type and mutant EGFR [43] (table 4). ERB-B2 has also
been shown to sensitize low-EGFR NSCLC cell lines to growth
inhibition by gefitinib [44, 45].
Mutations in other TKs have also been described, including ERBB2
mutations, originally found in 5 of 51 lung adenocarcinomas (10%)
[46]. Nevertheless, ERBB2 mutations were found at a much lower
frequency: 11 of 671 NSCLCs (1.6%). All mutations were in-frame
insertions in exon 20 and were more frequent in never-smokers
(3.2%) and in adenocarcinomas (2.8%) [47] (table 4). No ERBB2
mutations were found in Korean lung adenocarcinoma patients
[48].
Activating mutations of PDGFRa have been found in
gastrointestinal stromal tumors with wild-type KIT [49]. These
mutations could be found in other tumors that are sensitive to
imatinib and other small molecule drugs that inhibit PDGFRα kinase
activity. PDGFRa activation loop (exon 18) mutations have been
identified in only one of 45 NSCLCs examined [Santarpia M, personal
communication] (table 4).
PIK3CA missense mutations in exons 9 and 20 have been described
in breast, colon and brain tumors [50] and were found in one of 24
lung cancers (4%) [51]. Recently, PIK3CA mutations were not found
in any of 100 NSCLCs [Santarpia M, personal communication] (table
4). PIK3CA mutations are especially common in breast cancer
patients and intriguingly have been observed more frequently in
clear cell than in other ovarian cancers [52]. Clear cell ovarian
cancers also have higher ERCC1 mRNA levels than other ovarian tumor
histologies [53]. These observations lead us to speculate that
tumors with PIK3CA mutations may be associated with elevated levels
of ERCC1 mRNA and cisplatin resistance.
Erythropoietin sensitivity is seen in polycythemia vera, and
recently a mutation in JAK2 has been found in the majority of
polycythemia vera patients [54]. JAK2 is an upstream molecule
directly linked to erythropoietin receptor (EpoR) signaling. mRNA
transcripts of EpoR have been detected in the majority of resected
NSCLC patients. This finding is particularly relevant since
recombinant human Epo used to treat anemia can stimulate cancer
cell survival, angiogenesis and promotion of tumor growth. Epo and
EpoR expression could increase cisplatin resistance [55].
c-Met/HGF signaling also plays a role in tumor growth,
metastasis and angiogenesis. C-Met mutations have been identified
in small and non-small-cell lung cancer in the juxtamembrane domain
(R988C and T1010I) and the semaphorin domain (E168D). These
mutations could be a therapeutic target for novel small molecule
specific inhibitors of c-Met, such as SU11274 [56] (table 4).
Estrogen and progesterone receptors in NSCLC
It is well-known that a significant number of NSCLCs overexpress
estrogen and progesterone receptors and that crosstalk between
estrogen receptors and EGFR pathways is common. Experimentally, it
has been demonstrated that EGFR protein expression was
downregulated in response to estrogen and upregulated in response
to fulvestrant (an estrogen receptor antagonist), suggesting that
the EGFR pathway is activated when estrogen is depleted in NSCLC
[57]. This important finding paves the way for clinical trials of
EGFR TK inhibitors with aromatase inhibitors.
Progesterone receptor immunoreactivity was observed in 46.5% of
228 NSCLC patients and was associated with better clinical outcome.
Positive progesterone receptor status was more frequently seen in
males, stage I disease and adenocarcinomas [58].
Table 2 Clinical and therapeutic findings associated
with EGFR mutations
|
. EGFR mutations predict response and survival [31]
|
|
. Time to progression with gefitinib in patients with EGFR
mutations: 21 months;
|
|
with chemotherapy: 5 months [30]
|
|
. Long-lasting response in almost all EGFR mutant NSCLCs
[27-29]
|
|
. Deletions in exon 19 predict better response than L858R mutation
[31]
|
|
. Dominance of exon 19 in males and exon 21 in females [32]
|
|
. L858R more frequent in females [33]
|
|
. No response in non-adenocarcinomas with EGFR mutations [34]
|
Table 3 Geographical and ethnic frequencies of EGFR
mutations
|
Frequency
|
Country
|
References
|
|
19%
|
Korea
|
[30]
|
|
56%
|
Japan
|
[31]
|
|
32%
|
Japan
|
[32]
|
|
61%
|
Taiwan
|
[34]
|
|
55%
|
Taiwan
|
[35]
|
|
18%
|
Italy
|
[36]
|
|
20%
|
Mayo, USA
|
[36]
|
|
6.3%
|
Maryland, USA
|
[36]
|
|
14%
|
Caucasians
|
[31]
|
|
2.4%
|
African-Americans
|
[31]
|
|
12%
|
Spain
|
[31]
|
Table 4 Tyrosine kinase and other gene alterations for
targeted therapy in NSCLC
|
Tyrosine kinase
|
Activating alterations
|
Targeted therapy
|
References
|
|
ERBB1 (EGFR)
|
Deletion LREA L858R
|
gefitinib erlotinib HKI-272
|
[26-32, 34]
|
|
Acquired resistance: T790M
|
|
[40, 41]
|
|
No other mutations
|
|
[42]
|
|
|
|
|
|
ERBB3 12q21
|
Overexpression
|
gefitinib, erlotinib
|
[43, 45]
|
|
|
|
|
|
ERBB2 (HER-2)
|
Overexpression
|
trastuzuumab
|
[44]
|
|
17q21
|
Mutations exon 20
|
|
[46, 47]
|
|
|
|
|
|
PDGFRα4q12
|
Mutations exon 18
|
imatinib
|
Santarpia M
|
|
Rare
|
|
(personal comm.)
|
|
|
|
|
|
JAK2 9p24
|
V617F
|
|
[54]
|
|
Polycythemia vera
|
|
|
|
|
|
|
|
c-Met
|
Juxtamembrane domain:
|
SU11274
|
[56]
|
|
R988C, T1010I
|
|
|
|
Sema domain
|
|
|
|
|
|
|
|
K-ras mutations
|
Codon 12
|
Negative effect of erlotinib
|
[37]
|
|
|
No effect
|
[39]
|
|
|
|
|
|
PIK3CA
|
E542K, ES45K
|
Small molecule inhibitors
|
[51]
|
|
H1047
|
|
Santarpia M
|
|
Rare
|
|
(personal comm.)
|
|
Absent
|
|
|
MicroRNAs
MicroRNAs (miRNAs) and short interfering RNAs (siRNAs), processed
by the type III double-stranded RNase Dicer, function in an
RNA-based mechanism of gene silencing. MiRNA genes are expressed as
primary transcripts (pri-miRNAs) that are longer than pre-miRNAs.
Pri-miRNAs are trimmed into 70 nucleotide pre-miRNA forms, mainly
in the nucleus. After this initial processing, the pre-miRNAs are
exported to the cytoplasm and are cleaved to generate the final
products of 22 nucleotides by Dicer. The miRNAs were originally
described as miR-1 to miR-33 [59]. miR-15 and miR-16 are commonly
downregulated in chronic lymphocytic leukemia [60], while miR-143
and miR-145 are downregulated in colorectal cancer [61]. The
Caenorhabditis elegans let-7 miRNA is highly preserved through the
species, and reduced let-7 expression has been associated with
shorter survival in resected NSCLCs [62]. Dicer expression levels
were reduced in a fraction of NSCLCs, especially in non-squamous
histologies, conferring poor prognosis in completely resected
patients (HR, 17.6) [63].
Wnt signaling in NSCLC
Ectopic wingless (Wnt) signaling is involved in breast cancer stem
cells [64]. Wnt signaling is required for stem cell maintenance and
is emerging as a critical pathway in lung carcinogenesis. Growing
evidence indicates that in NSCLC, the Wnt pathway is activated
upstream of beta-catenin. Overexpression of Wnt effectors such as
disheveled and repression of Wnt antagonists like Wnt inhibitory
factor (WIF)-1 play a crucial role. The components of the Wnt
pathway have been characterized and represent key targets for
potential therapeutic agents. It has been demonstrated
experimentally that blocking components of the Wnt pathway is
feasible and is an important way to inhibit the signaling required
for cancer stem cells. Wnt glycoproteins comprise a family of
extracellular signaling ligands. In the resting state of the Wnt
pathway, beta-catenin is phosphorylated by glycogen synthase kinase
b (GSK3b) in a multiprotein complex involving adenomatous polyposis
coli (APC) and Axin. Following beta-catenin phosphorylation, it is
rapidly degraded. Upon the binding of the Wnt ligand to the
frizzled membrane receptor, the Wnt signal transduction pathway is
activated. This results in the inactivation of GSK3b and thus an
increase in the cytoplasmic pool of beta-catenin. Then this stable
form of beta-catenin becomes translocated to the cell nucleus where
it interacts with members of the T cell factor/lymphocyte enhancer
factor (TCF/LEF) family of transcription factors to promote
expression of Wnt-responsive genes [65].
Recently, overexpression of Wnt-1 was observed in NSCLC cell
lines and primary cancer tissues [66]. Blockade of Wnt signaling
with siRNA or a specific monoclonal antibody induced apoptosis in
vitro. The monoclonal anti-Wnt-1 antibody also suppressed tumor
growth in vivo [66].
Conclusion
Several lines of evidence indicate that multiple genetic
disturbances found in human cancer cell lines and in the tumors of
NSCLC patients can be incorporated as predictive markers for
response and improved survival with chemotherapy regimens currently
used. These markers can also be used to identify subgroups of
patients that can have a dramatic response when treated with novel
targeted therapies. The paradigm is the impressive response to
gefitinib or erlotinib in the presence of EGFR TK mutations
responses two or three times greater than what can be attained with
chemotherapy. The use of rapid and sensitive PCR assays for
diagnostic screening [67], coupled with a greater accessibility to
tumor tissue from lung cancer patients, will help facilitate the
comprehensive application of this knowledge and could lead to
improved survival and optimal use of health resources. Along these
lines, the Spanish Lung Cancer Group has undertaken a large-scale
study in which EGFR mutations are being examined in newly-diagnosed
lung adenocarcinoma patients; those harboring EGFR mutations
receive erlotinib, and those with wild-type EGFR receive customized
chemotherapy.
Acknowledgements
Research supported in part by Spanish Ministry of Health grants
provided through Red Temática de Investigación Cooperativa de
Centros de Cáncer (CO-010), by La Fundació Badalona Contra el
Càncer, and by La Fundació Carvajal. The authors thank Renée
O’Brate for assistance with the manuscript and Lourdes Franquet and
Francisca Gonzalez for technical assistance.
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