Chapter 080. Cancer Cell Biology and Angiogenesis (Part 7) docx

Chapter 080. Cancer Cell Biology
and Angiogenesis
(Part 7)

Signaling Pathways Downstream of Rtks: Ras and PI3K
Several oncogene and tumor-suppressor gene products are components of
signal transduction pathways that emanate from RTK activation (Fig. 80-2). The
most extensively studied are the Ras/mitogen-activated protein (MAP) kinase
pathway and the phosphatidylinositol-3-kinase (PI3K) pathway, both of which
regulate multiple processes in cancer cells, including cell cycle progression,
resistance to apoptotic signals, angiogenesis, and cell motility. The development
of inhibitors of these pathways is an important avenue of anticancer drug
development.
Mutation of the Ras protooncogene occurs in 20% of human cancers and
results in loss of the response of oncogenic Ras to GTPase-activating proteins
(GAPs). The constitutively activated, GTP-bound Ras activates downstream
effectors including the MAP kinase and PI3K/Akt pathways. Cancers of the
pancreas, colon, and lung and AML harbor frequent Ras mutations, with the K-
Ras allele affected more commonly (85%) than N-Ras (15%); H-Ras mutations
are uncommon in human cancers. In addition, Ras activity in tumor cells can be
increased by other mechanisms, including upregulation of RTK activity and
mutation of GAP proteins (e.g., NF1 mutations in type I neurofibromatosis). Ras
proteins localize to the inner plasma membrane and require posttranslational
modifications, including addition of a farnesyl lipid moiety to the cysteine residue
of the carboxy-terminal CAAX-box motif. Inhibition of RAS farnesylation by
rationally designed farnesyltransferase inhibitors (FTIs) demonstrated encouraging
efficacy in preclinical models, most of which utilized oncogenic forms of H-Ras.
Despite this, clinical trials of FTIs in patients whose tumors harbor Ras mutations
have been disappointing, although some activity has been seen in AML. Upon
further study, it appears that in the presence of FTIs, lipid modification of the K-
and N-Ras proteins occurs by addition of a distinct lipid (geranylgeranyl) through

the action of geranylgeranyl transferase-I (GGT-I), which results in restoration of
Ras function. Thus, while FTIs are likely to have antitumor activity in select
human cancers, their mechanism of action appears to occur by inhibition of
farnesylation of proteins other than Ras, perhaps RhoB or Rheb (an activator of
mTOR). Oncologists anxiously await the development of bona fide Ras-targeted
therapeutics.
Effector pathways downstream of Ras are also targets of anticancer drug
efforts. Activation of the Raf serine/threonine kinase is induced by binding to Ras
and leads to activation of the MAP kinase pathway (Fig. 80-2). Two-thirds of
melanomas and 10% of colon cancers harbor activating mutations in the BRAF
oncogene, leading to constitutive activation of the downstream MAP/ERK kinase
(MEK) and extracellular signal-regulated kinases (ERK1/2). This results in the
phosphorylation of ERK's cytoplasmic and nuclear targets and alters the pattern of
normal cellular gene expression. Inhibitors of Raf kinases (e.g., sorafinib) have
entered clinical trials; their activity against tumors expressing mutant BRAF have
been disappointing as single agents, but they appear to increase the activity of
chemotherapy in some cases. Sorafinib also has significant activity against
VEGFRs, and this may account for its clinical activity observed in highly vascular
renal cell cancers (see below). Cells harboring mutant BRAF are highly sensitive
to MEK inhibition, providing another example of "oncogene addiction" (Fig. 80-
3).
Figure 80-3


Oncogene addiction and synthetic lethality: keys to discovery of new
anti-cancer drugs. Panel A. Normal cells receive environmental signals th
at
activate signaling pathways (pathways A, B, and C) that together promote G1 to S
phase transition and passage through the cell cycle. Inhibition of one pathway
(such as pathway A by a targeted inhibitor) has no significant effect due to

redundancy provi
ded by pathways B and C. In cancer cells, oncogenic mutations
lead over time to dependency on the activated pathway, with loss of significant
input from pathways B and C. The dependency or addiction of the cancer cell to
pathway A makes it highly vulnerabl
e to inhibitors that target components of this
pathway. Clinically relevant examples include Bcr-
Abl (CML), amplified
HER2/neu
(breast cancer), overexpressed or mutated EGF receptors (lung cancer),
and mutated BRAF (melanoma). Panel B. Genes are said to ha
ve a synthetic lethal
relationship when mutation of either gene alone is tolerated by the cell, but
mutation of both genes leads to lethality. Thus, in the example, mutant gene a
and
gene b have a synthetic lethal relationship, implying that the loss of on
e gene
makes the cell dependent on the function of the other gene. In cancer cells, loss of
function of a tumor-suppressor gene (wild-type designated gene A;
mutant
designated gene a
) may render the cancer cells dependent on an alternative
pathway of which gene B
is a component. As shown in the figure, if an inhibitor
of gene B
can be identified, this can cause death of the cancer cell, without
harming normal cells (which maintain wild-type function for gene A). High-
throughput screens can now be performed
using isogenic cell line pairs in which
one cell line has a defined defect in a tumor-

suppressor pathway. Compounds can
be identified that selectively kill the mutant cell line; targets of these compounds
have a synthetic lethal relationship to the tumor-s
uppressor pathway, and are
potentially important targets for future therapeutics. Note that this approach
allows discovery of drugs that indirectly target deleted tumor-
suppressor genes
and hence greatly expands the list of physiologically relevant cancer targets.