TUMOR SUPPRESSOR GENES AND GROWTH INHIBITORY PATHWAYS

Loss of TSG function is an important step in lung carcinogenesis
process and usually both alleles need to be inactivated.
Generally, loss of heterozygosity (LOH) inactivates one allele
through chromosomal deletion or translocation, and point
mutation, epigenetic or transcriptional silencing inactivates the
second allele. 73,74 In lung cancer, commonly inactivated TSGs
include TP53 , RB1 , CDKN2A , FHIT , RASSF1A , and PTEN .
The p53 Pathway TP53 (17p13) encodes a phosphoprotein
that prevents accumulation of genetic damage in daughter
cells. In response to cellular stress, p53 induces the expression
of downstream genes such as cyclin-dependent kinase (CDK)
inhibitors that regulate cell cycle checkpoint signals, causing
the cell to undergo G1 arrest and allowing DNA repair or
apoptosis. 74 p53 inactivating mutations are the most common
alterations in cancer, especially lung cancer, where 17p13 frequently
demonstrates hemizygous deletion and mutational inactivation
in the remaining allele. 75–77 Regulation of p53 can
occur through the oncogene MDM2, which reduces p53 levels
through degradation, and the p14 ARF isoform of CDKN2A,
which acts as a tumor suppressor by inhibiting MDM2. As such,
the genes that encode MDM2 and p14 ARF are altered in lung
cancer with amplification of MDM2 seen in 6% of NSCLCs 78
and loss of p14 ARF expression in approximately 40% and 65%
of NSCLCs and SCLCs, respectively. 79,80 Restoration of p53
expression in vivo has been achieved with p53 gene therapy of
lung cancer patients in a subpopulation of tumor cells. 81
The CDKN2A/RB Pathway The CDKN2A-RB1
pathway controls G1- to S-phase cell cycle progression.
Hypophosphorylated retinoblastoma (RB) protein, encoded
by RB1, halts the G1/S-phase transition by binding to the
transcription factor E2F1. This tumor-suppressing effect
can be inhibited by hyperphosphorylation of RB by CDKCCND1
complexes (complexes between CDK4 or CDK6
and CCND1), and in turn, formation of CDK-CCND1
complexes can be inhibited by CDNK2A. 82 Nearly all constituents
of the CDKN2A/RB pathway have been shown to
be altered in lung cancer through mutations (CDK4 and
CDKN2A), deletions (RB1 and CDKN2A), amplifications
(CDK4 and CCDN1), methylation silencing (CDKN2 A
and RB1), and phosphorylation (RB). 83–88
Chromosome 3p TSGs Loss of one copy of chromosome
3p is one of the most frequent and early events in human cancer,
found in 96% of lung tumors and 78% of lung preneoplastic
lesions. 89 Mapping of this loss identified several genes
with functional tumor-suppressing capacity including FHIT
(3p14.2), RASSF1A, TUSC2 (also called FUS1), and semaphorin
family members SEMA3B and SEMA3F (all at 3p21.3),
and RAR (3p24). In addition to LOH or allele loss, some of
these 3p genes (FHIT, RASSF1A, SEMA3B, and RAR ) often
exhibit decreased expression in lung cancer cells by means of epigenetic
mechanisms such as promoter hypermethylation. 90–94
Additionally, FHIT, RASSF1A, TUSC2, and SEMA3B will reduce
growth when reintroduced into lung cancer cells. FHIT,
located in the most common fragile site in the human genome
(FRA3B), has been shown to induce apoptosis in lung cancer. 95
RASSF1A can induce apoptosis, as well as stabilize microtubules,
and affect cell cycle regulation. 96 The tumor-suppressing
effect of TUSC2 is thought to occur through inhibition of protein
tyrosine kinases such as EGFR, PDGFR, c-Abl , c-Kit, and
AKT 97 as well as inhibition of MDM2-mediated degradation of
p53. 98 The candidate TSG SEMA3B encodes a secreted protein
that can decrease cell proliferation and induce apoptosis when
reexpressed in lung, breast, and ovarian cancer cells 90,91,99,100 in
part, by inhibiting the AKT pathway. 101 Another family member,
SEMA3F may inhibit vascularization and tumorigenesis
by acting on VEGF and ERK1/2 activation, 102,103 and RAR
exerts its tumor- suppressing function by binding retinoic acid,
thereby limiting cell growth and differentiation.
LKB1 The serine/threonine kinase LKB1 (also called STK11)
is inactivated in approximately 30% of lung cancers and often
correlates with KRAS activation, 104 resulting in the promotion of
cell growth. It functions as a TSG by regulating cell polarity, differentiation,
and metastasis and can regulate cell metabolism. 105
It has also been reported to inhibit the mTOR pathway. 106
EPIGENETIC REGULATION
Genetic abnormalities are associated with changes in DNA
sequence; however, epigenetic events may lead to changes in gene
expression without any changes in DNA sequence and therefore,
the latter are potentially reversible. 107 Aberrant promoter
hypermethylation is an epigenetic change that occurs early in
lung tumorigenesis and is found both in genes that normally
undergo methylation in response to aging, as well as in genes
that normally remain unmethylated regardless of age. 108 Gains
of DNA methylation in a normally unmethylated promoter
region of a gene results in silencing of gene transcription and
is therefore a common method for the inactivation of TSGs.
In lung cancer, many genes have been found to be silenced by
promoter hypermethylation (summarized in Table 5.1). They
include genes involved in tumor suppression, tissue invasion,
DNA repair, detoxification of tobacco carcinogens, and differentiation.
Recent advances in whole-genome microarray profiling
have allowed researchers to globally study DNA methylation
patterns in lung cancer, the results of which have led to suggestions
that the role of methylation in lung tumorigenesis has
been underestimated. 109–112 Restoration of expression of epigenetically
silenced genes is a new targeted therapeutic approach.
Histone deacetylation is an example of epigenetic change that
can inhibit gene expression. Histone deacetylase (HDAC) inhibitors
are being studied for the treatment of lung cancer and
function by reversing gene silencing through inhibiting histone
deacetylation