Chapter 080. Cancer Cell Biology and Angiogenesis (Part 9) ppt

Chapter 080. Cancer Cell Biology
and Angiogenesis
(Part 9)

Acetylation of the amino terminus of the core histones H3 and H4 induces
an open chromatin conformation that promotes transcription initiation. Histone
acetylases are components of coactivator complexes recruited to
promoter/enhancer regions by sequence-specific transcription factors during the
activation of genes (Fig. 80-4). Histone deacetylases (HDACs; at least 17 are
encoded in the human genome) are recruited to genes by transcriptional repressors
and prevent the initiation of gene transcription. Methylated cytosine residues in
promoter regions become associated with methyl-cytosine–binding proteins that
recruit protein complexes with HDAC activity. The balance between permissive
and inhibitory chromatin structure is therefore largely determined by the activity
of transcription factors in modulating the "histone code" and the methylation status
of the genetic regulatory elements of genes.
The pattern of gene transcription is aberrant in all human cancers, and in
many cases, epigenetic events are responsible. Unlike genetic events that alter
DNA primary structure (e.g., deletions), epigenetic changes are potentially
reversible and appear amenable to therapeutic intervention. In many human
cancers, including pancreatic cancer and multiple myeloma, the p16
Ink4a
promoter
is inactivated by methylation, thus permitting the unchecked activity of
CDK4/cyclin D and rendering pRB nonfunctional. In sporadic forms of renal,
breast, and colon cancer, the von Hippel–Lindau (VHL), breast cancer 1 (BRCA1),
and serine/threonine kinase 11 (STK11) genes, respectively, are epigenetically
silenced. Other targeted genes include the p15
Ink4b
CDK inhibitor, glutathione-S-
transferase (which detoxifies reactive oxygen species), and the E-cadherin

molecule (important for junction formation between epithelial cells). Epigenetic
silencing can occur in premalignant lesions and can affect genes involved in DNA
repair, thus predisposing to further genetic damage. Examples include MLH1 (mut
L homologue) in hereditary nonpolyposis colon cancer (HNPCC, also called
Lynch syndrome), which is critical for repair of mismatched bases that occur
during DNA synthesis, and 0
6
-methylguanine-DNA methyltransferase, which
removes alkylated guanine adducts from DNA and is often silenced in colon, lung,
and lymphoid tumors.
Many human leukemias have chromosomal translocations that code for
novel fusion proteins with enzymatic activities that alter chromatin structure. The
PML-RAR fusion protein, generated by the t(15;17) observed in most cases of
acute promyelocytic leukemia (APL), binds to promoters containing retinoic acid
response elements and recruits HDAC to these promoters, effectively inhibiting
gene expression. This arrests differentiation at the promyelocyte stage and
promotes tumor cell proliferation and survival. Treatment with pharmacologic
doses of all-trans retinoic acid (ATRA), the ligand for RARα, results in the release
of HDAC activity and the recruitment of coactivators, which overcomes the
differentiation block. This induced differentiation of APL cells has greatly
improved treatment of these patients and has provided a treatment paradigm for
the reversal of epigenetic changes in cancer. However, for other leukemia-
associated fusion proteins, such as AML-ETO and the MLL fusion proteins seen
in AML and ALL, no ligand is known. Therefore, efforts are ongoing to determine
the structural basis for interactions between translocation fusion proteins and
chromatin remodeling proteins, and to use this information to rationally design
small molecules that will disrupt specific protein-protein associations. Drugs that
block the enzymatic activity of HDAC are being developed. A number of different
chemical classes of HDAC inhibitors have demonstrated antitumor activity in
clinical studies against cutaneous T cell lymphoma (e.g., vorinostat) and some

solid tumors. HDAC inhibitors may target cancer cells via a number of
mechanisms including upregulation of death receptors (DR4/5, FAS, and their
ligands) and p21
Cip1/Waf1
, as well as inhibition of cell cycle checkpoints.
Major therapeutic efforts are also under way to reverse the
hypermethylation of CpG islands that characterizes many solid tumors. Drugs that
induce DNA demethylation, such as 5-aza-2'-deoxycytidine, can lead to
reexpression of silenced genes in cancer cells with restoration of function.
However, 5-aza-2'-deoxycytidine has limited aqueous solubility and is
myelosuppressive. Other inhibitors of DNA methyltransferases are in
development. In ongoing clinical trials, inhibitors of DNA methylation are being
combined with HDAC inhibitors. The hope is that by reversing coexisting
epigenetic changes, the deregulated patterns of gene transcription in cancer cells
will be at least partially reversed.
Aberrant signal transduction pathways activate a number of transcription
factors that promote tumor cell proliferation and survival. These include signal
transducer and activator of transcription (STAT)-3 and STAT5, NFκB, β-catenin
(a component of the APC tumor-suppressor pathway), the heterodimer of c-Jun
and Fos known as AP1, and c-Myc. The ability to target these transcription factors
therapeutically does not currently exist. However, structural and molecular
approaches may make it possible to identify small molecules that would inhibit
protein-protein interactions needed for transcription factor dimerization or
interaction with coactivator proteins. A small-molecule inhibitor has been
developed that blocks the association of Myc with its partner Max, thereby
inhibiting Myc-induced transformation. Many transcription factors are activated
by phosphorylation, which can be prevented by tyrosine- or serine/threonine
kinase inhibitors. The transcription factor NFκB is a heterodimer composed of p65
and p50 subunits that associate with an inhibitor, IκB, in the cell cytoplasm. In
response to growth factor or cytokine signaling, a multi-subunit kinase called IKK

(IκB-kinase) phosphorylates IκB and directs its degradation by the
ubiquitin/proteasome system. NFκB, free of its inhibitor, translocates to the
nucleus and activates target genes, many of which promote the survival of tumor
cells. Novel drugs called proteasome inhibitors block the proteolysis of IκB,
thereby preventing NFκB activation. For unexplained reasons, this is selectively
toxic to tumor cells. Further studies have indicated that the antitumor effects of
proteasome inhibitors are more complicated and involve the inhibition of the
degradation of multiple cellular proteins. Proteasome inhibitors [bortezomib
(Velcade)] have shown very significant activity in patients with multiple
myeloma, including partial and complete remissions. Inhibitors of IKK are also in
development, with the hope of more selectively blocking the degradation of IκB,
thus "locking" NFκB in an inhibitory complex and rendering the cancer cell more
susceptible to apoptosis-inducing agents.