Download Chapter 15 Regulation of Cell Number Normal and Cancer Cells

Chapter 15
Regulation of Cell Number
Normal and Cancer Cells
• The cell proliferation machinery
• Programmed cell death (apoptosis)
• Controlling cell proliferation and apoptosis
• Cancer: genetics of aberrant cell control
Apoptosis in Drosophila embryo
A) Wild-type, bright spots indicate cells that undergo cell apoptosis
B) Mutant which is defective for programmed cell death
Various combinations (complexes) of cyclins and CDKs act through the cell cycle
Cell proliferation
Cyclins and cyclin-dependent protein kinases are important for cell cycle control
Cyclins and Cyclin-Dependent Protein Kinases
The engines that drive progression from one step of the cell cycle to the next are a series of protein complexes
composed of two subunits: a cyclin and a cyclin-dependent protein kinase (abbreviated CDK). In every
eukaryote, there is a family of structurally and functionally related cyclin proteins. Cyclins are so named
because each is found only during one or another segment of the cell cycle. The onset of the appearance of a
specific cyclin is due to cell cycle-controlled transcription, in which the previously active cyclin-CDK complex
leads to the activation of a transcription factor that activates the transcription of this new cyclin. The
disappearance of a cyclin depends on three events: rapid inactivation of the activator of transcription of this
cyclin's gene (so that no new mRNA is produced), a high degree of instability of the cyclin mRNA (so that the
existing pool of mRNA is eliminated), and a high level of instability of the cyclin itself (so that the pool of cyclin
protein is destroyed).
Cyclin-dependent protein kinases also constitute a family of structurally and functionally related proteins.
Kinases are enzymes that add phosphate groups to target substrates; for protein kinases such as CDKs, the
substrates are proteins. CDKs are so named because their activities are regulated by cyclins and because they
catalyze the phosphorylation of specific serine and threonine residues of specific target proteins.
CDK targets
How does the phosphorylation of some target proteins control the cell cycle?
Phosphorylation initiates a chain of events that culminates in the activation of certain transcription
factors. These transcription factors promote the transcription of certain genes whose products are
required for the next stage of the cell cycle. Much of our knowledge of the cell cycle comes from
both genetic studies in yeast and from biochemical studies of cultured mammalian cells. A wellunderstood example is the Rb-E2F pathway in mammalian cells. Rb is the target protein of a
CDK-cyclin complex called Cdk2-cyclin A, and E2F is the transcription factor that Rb regulates.
From late M phase through the middle of G1, the Rb and E2F proteins are combined in a protein
complex that is inactive in promoting transcription. In late G1, the Cdk2-cyclin A complex is
produced and phosphorylates the Rb protein. This phosphorylation produces a change in the
shape of Rb such that it can no longer bind to the E2F protein. The free E2F protein is then able
to promote transcription of certain genes that encode enzymes vital for DNA synthesis. This
allows the next phase of the cell cycle (S phase) to proceed.
Rb and E2F are in fact representatives of two families of related proteins. In mammals, different
cyclin- CDK complexes are thought to selectively phosphorylate different proteins of the Rb
family, each of which in turn releases the specific E2F family member to which it is bound. The
different E2F transcription factors then promote the transcription of different genes that execute
different aspects of the cell cycle.
Message
Sequential activation of different CDK-cyclin complexes ultimately
controls progression of the cell cycle.
Yeast as a model for the cell cycle
The cell cycle of the budding yeast Saccharomyces cerevisiae. The scanning electron
micrograph shows cells at different points in the cell cycle, as indicated by different bud
sizes. The principal events in the cell cycle are shown.
The Apoptosis Pathway
In multicellular organisms, systems have evolved to
eliminate damaged (and, hence, potentially harmful)
cells through a self-destruct and disposal mechanism:
programmed cell death, or apoptosis. This selfdestruct
mechanism can be activated under many different
circumstances. Regardless, the events in apoptosis
seem to be the same. First, there is fragmentation of
the DNA of the chromosomes, disruption of organelle
structure, and loss of normal cell shape (apoptotic
cells become spherical). Then, the cells break up into
small cell fragments called apoptotic bodies that are
phagocytosed (literally, eaten up) by motile scavenger
cells.
The role of caspases in apoptosis.
A cascade of caspase activation leads to
the activation of the executioner
caspases. Cleavage of the zymogen
(inactive precursor) form of the
executioner caspase by another caspase
takes place at several aspartate residues,
producing several protein fragments. Two
of these fragments, the large and small
subunits, bind and form the enzymatically
active caspase. Through cleavage of a
series of target proteins (also by
cleavage at aspartate residues in the
target proteins), the various cellular
breakdown events take place, leading to
cell death and removal.
Caenorhabiditis elegans as a model for programmed cell death
Controlling cell proliferation and apoptosis
The control can be exerted by intracellular and extracellular signals.
Both positive and negative control loops exist.
Intracellular signalling
An example of negative control of cell cycle progression.
In mammals, the transition from the G1 to S phase requires the phosphorylation of Rb protein by the
CDK2-cyclin complex. In the presence of damaged DNA, p53 protein is induced, which in turn induces p21
protein.The elevated levels of p21 protein inhibit the protein kinase activity of the CDK2-cyclin complex.
When the damaged DNA has been repaired, p53 levels drop. In turn, p21 levels decrease, and the
inhibition of the CDK2-cyclin protein kinase activity is relieved, which allows Rb to be phosphorylated and
E2F to become an active transcription factor, permitting the cell to enter S phase.
Extracellular signals
A cell in a multicellular organism continually assesses its own internal status regarding
proliferation and survival. Nonetheless, the proliferative and survival abilities of a cell must
be subservient to the needs of the population of cells of which it is a member (populations
such as the entire early embryo, a tissue, or a body part such as a limb or an organ). For
example, in many adult organs, stem cells divide to produce replacement cells only when
there is a depletion of cell numbers. Without such homeostatic mechanisms, organs would
not be proportioned appropriately for the size of a given individual organism.
Mechanisms for cell-cell communication.
Many kinds of signals need to be transmitted between cells to coordinate virtually all aspects
of the development and physiology of complexmulticellular organisms. The major routes of
cell-cell communication are briefly outlined here.
All systems for intercellular communication have several components. A molecule called a
ligand is produced by secretion from signaling cells. Some ligands, called hormones, are longrange endocrine signals that are transmitted throughout the body by being released from
endocrine organs into the circulatory system. Hormones can act as master control switches
for many different tissues, which can then respond in a coordinated fashion. Other secreted
ligands act as paracrine signals; that is, they do not enter the circulatory system but act only
locally, in some cases only on immediately adjacent cells. We shall have more to say about
paracrine and endocrine signals in Chapter 16. Some ligands are proteins, whereas others are
small molecules such as steroids or vitamin D. Most (but not all) endocrine signals are small
molecules, such as the mammalian steroid hormones that are responsible for male (androgen)
or female (estrogen) sex-specific phenotypes. In contrast, most paracrine signals are
proteins. Here we focus on paracrine signaling through protein ligands.
Modes of intercellular signaling.
(a) Endocrine signals enter the circulatory system and can be received by distant target cells.
(b) Paracrine signals act locally and are received by nearby target cells.
Examples of transmembrane receptors.
(a) A receptor that passes through the cell membrane seven times. (b) Receptor tyrosine
kinase RTK), which has a single transmembrane domain. The extracellular domain binds to
ligand. The active site of the tyrosine kinase is in the cytoplasmic domain.
The consequences of ligand binding on RTK activity and the initiation of the signal
transduction cascade.
On dimerization of the RTK, autophosphorylation occurs. The ability of activated RTK to
initiate signal transduction is through two different routes: (a) by the activated RTK serving
as an anchor site for some signal transduction proteins and (b) by the activated RTK
phosphorylating other proteins.
An example of the G-protein activity cycle.
Ras is a member of the G-protein family. When Ras binds GDP, it does not signal. Through
direct interactions with Ras, another protein called Sos causes conformational changes in Ras
so that it preferentially binds GTP. The Ras-GTP complex is able to interact in turn with a
cytoplasmic serine/threonine kinase, activating its kinase activity, and thus transmits the
signal to the next step in the signal transduction pathway. When Ras-GTP is released from
Sos, it hydrolyzes GTP to GDP and reassumes the inactive Ras-GDP state.
One pathway for RTK signaling.
Raf, MEK, and MAP kinase are three cytoplasmic
protein kinases that are sequentially activated in
the signal transduction cascade.
Positive extracellular control of
apoptosis.
A molecule such as Apaf is activated
through binding of FasL (Fas ligand),
anchored on the outside of an adjacent cell.
FasL binds to the Fas transmembrane
receptor on the cell that will undergo
apoptosis. Apaf activation in turn causes
proteolysis and activation of the initiator
caspase. A series of caspases are then
proteolysed and activated in turn,
ultimately leading to apoptosis of the cell.
The inputs into control of cell number
Cancer: genetics of aberrant cell control
Message
Tumors arise through a series of sequential mutational
events that lead to a state of uncontrolled proliferation.
The Relations between Function of the Wild-Type Protein Product and the Types of
Tumor-Promoting Mutations That Can Arise in the Genes Encoding Those Products
Type of wild-type protein function
Properties of tumor-promoting mutations
Promotes cell cycle progression
Oncogene (gain of function)
Inhibits cell cycle progression
mutierte Tumor-suppressor gene (loss of
function)
Promotes apoptosis
mutierte Tumor-suppressor gene (loss of
function)
Inhibits apoptosis
Oncogene (gain of function)
Promotes DNA repair
Tumor-suppressor gene (loss of function)
Oncogenes and mutated Tumor-suppressor genes
Two general kinds of mutations are associated with tumors:
oncogene mutations and mutations in tumor-suppressor genes.
Oncogenes are mutated alleles of proto-oncogenes. Proto-oncogenes are positive
regulators of the cell cycle. Oncogenes are mutated in such a way that the
proteins that they encode are activated in tumor cells; they usually represent
dominant mutant alleles. A tumor cell will typically be heterozygous for an
oncogene mutation and its normal allelic counterpart, the proto-oncogene.
Mutated Tumor-suppressor genes are mutated alles of negative regulators (for
example p53) of the cell cycle. They can represent dominant or recessive alleles.
For recessive mutations, the tumor cell will lack any copy of the corresponding
wild-type allele.
The p53 tumor-suppressor gene
a link between cell cycle and apoptosis
A very important recessive tumor-promoting mutation has identified the p53 gene as a tumorsuppressor gene. Mutations in p53 are associated with many types of tumors, and estimates
are that more than 50 % of human tumors lack a functional p53 gene. The active p53 protein
is a transcriptional regulator that is activated in response to DNA damage. Activated wildtype p53 serves double duty, preventing progression of the cell cycle until the DNA damage is
repaired and, under some circumstances, inducing apoptosis. In the absence of a functional
p53 gene, the p53 apoptosis pathway does not become activated, and the cell cycle progresses
even in the absence of DNA repair. This progression elevates the overall frequency of
mutations,
The Ras oncoprotein.
(a) The ras oncogene differs from the wild
type by a single base pair, producing a Ras
oncoprotein that differs from the wild type in
one amino acid, at position 12 in the ras open
reading frame. (b) The effect of this missense
mutation is to create a Ras oncoprotein that
cannot hydrolyze GTP to GDP. Because of this
defect, the Ras oncoprotein remains in the
active Ras-GTP complex and continuously
activates the down-stream serine/threonine
kinase
Messages
Oncogenes encode oncoproteins‚ deregulated forms of proteins
whose wild-type function is to participate in the positive control
of the cell cycle or in the negative control of apoptosis.
Dominant oncogenes contribute to the oncogenic state by causing
a protein to be expressed in an activated form or in the wrong cells
(ectopic expression).
Retinoblastoma, a cancer of the retina.
The mutational origin of retinal tumors in hereditary
and sporadic retinoblastoma. Recessive rb alleles of
the RB gene lead to tumor development.
The major pathways that are mutated to contribute to cancer formation and progression.
The main events that contribute to tumor formation are increased cell proliferation and cell
survival (decreased apoptosis). The pathways in red are susceptible to gain-of-function
oncogene mutation. The pathways in blue are susceptible to loss-of-function tumorsuppressor gene mutation.