Introduction
Oncogenes and Tumor Suppressor Genes
Overexpression of cyclin D1
Loss of p16 Function
Loss of signalling Contributes to abnormal cell proliferation and malignancy
Summary
Questions
2. The entry of cells into the cell cycle from
a quiescent state and the progression of cells around
the cycle are precisely controlled events.
This assures that cellular growth and the coordination
of DNA synthesis with cell-size increase
and cytokinesis are monitored and do not fall out of
regulated synchrony.
Once a cell progresses past a certain point in late G1,
called the restriction point
It becomes irreversibly committed to entering the S
phase and replicating its DNA
INTRODUCTION
3. Cyclins, cyclin-dependent kinases (Cdks), and the
Rb protein are all elements of the control system that
regulate passage through the restriction point.
The ability of these proteins to check cell-cycle
progression, and hold cells in quiescence or even lead
cells to commit suicide unless conditions are
appropriate, means that they can prevent cells from
becoming cancerous.
Altered regulation of expression of at least one cyclin as
well as mutation of several proteins that negatively
regulate passage through the restriction point can be
oncogenic .
4.
5. Passage fromG1 to S Phase Is Controlled by Proto-
Oncogenes andTumor-Suppressor Genes
The expression of D-type cyclins in mammals is
induced by many extracellular substances called
growth factors or mitogens.
These cyclins (D1, D2, and D3) assemble with their
partners Cdk4 and Cdk6 to generate catalytically
active kinases.
Mitogen withdrawal prior to passage through
the restriction point leads to accumulation of p27 or
p16, which bind to and inhibit cyclin D –
dependent kinase activity, thus causing G1 arrest.
6. Another key player in cell-cycle control is the
Rb protein
Its unphosphorylated form, Rb
binds transcription factors collectively called E2F
Thereby prevents E2F-mediated transcriptional
activation of many genes whose products are
required for DNA synthesis, such as DNA
polymerase.
Also, the Rb-E2F complex acts as a transcriptional
repressor for many of these same genes.
7. Phosphorylation of Rb protein midway through
G1 causes it to dissociate from E2F, allowing E2F to
induce synthesis of these DNA replication enzymes,
which irreversibly commits the cell to DNA synthesis.
Rb phosphorylation is initiated by an active Cdk4 –
cyclin D complex and is completed by other cyclin-
dependent kinases.
Most tumors contain an oncogenic mutation of one of
the genes in this pathway such that the cells are
propelled into the S phase in the absence of the proper
extracellular growth signals that regulate Cdk activity.
8. Rb/E2F
Stimulation of G0 cells with mitogens induces expression of Cdk4, Cdk6, D-
type cyclins and E2F transcription factors (E2Fs), all encoded by delayed-
response genes
Interaction of E2Fs with hypophosphorylated Rb protein initially inhibits
E2F activity.
When signaling from mitogens is sustained, the resulting Cdk4 – cyclin D
and Cdk6 – cyclin D complexes (Cdk4/6 – cyclin D) initiate the
phosphorylation of Rb, converting some E2F to the active form.
Active E2F then stimulates its own synthesis and the synthesis of Cdk2 and
cyclin E. Cdk2 – cyclin E further stimulates Rb phosphorylation releasing
more E2F activity.
These processes result in positive feedback loops (blue arrows) leading to a
rapid rise in both E2F and Cdk2 – cyclin E activity as the cell approaches the
G1→S transition.
9. Overexpression of Cyclin D1
Gene amplification or a chromosome translocation that
places cyclin D1 under control of an
inappropriate promoter leads to overexpression of this
cyclin in many human cancers, indicating that it can
function as an oncoprotein.
That cyclin D1 overexpression can directly cause cancer
was shown by studies with a transgenic mouse in which
the cyclin D gene was placed under control of an enhancer
specific for mammary ductal cells
Initially the ductal cells underwent hyperproliferation, and
eventually breast tumors developed in these transgenic
mice.
Amplification of the cyclin D1 gene and concomitant
overexpression of the cyclin D1 protein is common in
human breast cancer.
10. Loss of p16 Function
The group of proteins that function as cyclin
kinase inhibitors (CKIs) play an important role in
regulating the cell cycle
Mutations,especially deletions of the p16 gene, that
inactivate the ability of p16 to inhibit cyclin D –
dependent kinase activity are common in several
human cancers.
Loss of p16 would mimic cyclin D1 overexpression,
leading to Rb hyperphosphorylation and release of
active E2F transcription factor
Thus p16 normally acts as a tumor suppressor.
11. Loss of TGFβ SignalingContributes to Abnormal Cell
Proliferation and Malignancy
Until now we have considered growth factors that
mainly stimulate proliferation of different types of
cells
Tumorderived growth factor β (TGFβ) is secreted by
most body cells and has a diverse range of
biological activities.
Most relevant to the present discussion however,is
the ability of TGFβ to inhibit the growth of many
types of cells, including most epithelial and immune
system cells.
12. Loss of TGFβ-mediated growth inhibition
contributes to the development and progression of
a variety of tumors.
TGFβ signals through the sequential activation of
two cell-surface receptors, termed type I and type
II,both of which have intrinsic
serine/threonine protein kinase activity.
Binding of TGFβ induces formation of a complex of
the type I and type II receptors and phosphorylation
and activation of the type I receptor by the type II
receptor kinase.
13. Smad proteins are the key intracellular signal
transducers in the pathway downstream from the
TGFβ receptors.
The ligand-activated type I receptor phosphorylates
conserved serines at the C-terminus of either
Smad2 or Smad3, which enables them to bind to
one or more molecules of Smad4, a common
partner for all phosphorylated Smads involved in
signaling by both TGFβ and bone morphogenic
proteins.
14. These Smad complexes then enter the nucleus and
activate transcription of a variety of genes.
One important gene induced by TGFβ encodes p15.
This G1 cyclin-kinase inhibitor displaces p27 from
the Cdk4-cyclin D complex, freeing p27 to bind to
and inhibit the Cdk2- cyclin E complex, which is
required for entry into the S phase.
Thus by inducing expression of p15, TGFβ causes
the cell to arrest in G1.
15.
16. Many tumors contain inactivating mutations in
either the TGFβ receptors or the Smad proteins, and
thus are resistant to growth inhibition by TGFβ.
Most human pancreatic cancers contain a deletion
in the gene encoding Smad4; this mutation-defined
gene was originally called DPC (deleted in
pancreatic cancer).
Retinoblastoma, colon and gastric cancer,
hepatoma, and some T- and B-cell malignancies are
unresponsive to TGFβ growth inhibition.
17. This loss of responsiveness correlates with loss of type
I or type II TGFβ receptors; responsiveness to TGFβ can
be restored by recombinant expression of the
“missing” protein
Mutations in Smad2 also commonly occur in several
types of human tumors.
The TGFβ signaling pathway also induces expression of
the genes encoding many extracellular matrix proteins,
such as collagens, and plasminogen activator inhibitor-1
(PAI-1), an inhibitor of a serum protease that degrades
many matrix proteins.
18. An inability to synthesize these proteins may
contribute to metastasis, allowing tumor cells
to “escape,” since less matrix will be made by
the cells and matrix that is present may be
degraded by inappropriately activated
proteases.
19. SUMMARY
1. Overexpression of the protooncogene encoding cyclin
D1 or loss of the tumor-suppressor genes encoding
p16 and Rb can cause inappropriate, unregulated
passage through the restriction point in late G1, a key
element in cell-cycle control. Such abnormalities are
common in human tumors.
2. TGFβ induces expression of p15, leading to arrest in
G1, and synthesis of extracellular matrix proteins such
as collagens and plasminogen activator inhibitor-1.
3. Loss of TGFβ receptors or Smad 4, a characteristic of
many human tumors, abolishes TGFβ signaling. This
promotes cell proliferation and development of
malignancy.
20. QUESTIONS
1.The entry of cells into the cell cycle called a _______state
(quiescent/G0)
2.The point in late G1 is called as_________
(restriction point)
3.The are two elements of the control system that regulate
passage through the restriction point?
(Cyclins, cyclin-dependent kinases (Cdks), and the Rb protein )
4. Cyclins (D1, D2, and D3) assemble with their partners Cdk4
and Cdk6 to generate catalytically active kinases(True /false)
True
21. 5. Rb phosphorylation is initiated by
Active Cdk4 – cyclin D complex
6. Overexpression of cyclin D2 is responsiple for many human
cancers True/false?
(False-cyclin D1)
7.Abbreviation of CKIs is____
Cyclin-kinase inhibitors
8. p16 normally acts as a____
Tumor suppressor.
9. Abbreviation of TGFβ is______
Tumorderived growth factor β
10.Loss of TGFβ signaling contributes to_________
Abnormal cell proliferation and
malignancy
22. 11. TGFβ signals pass through the ____and _____cell
surface receptor
Type I and Type II
12.Type I receptors is activated by _____
Type II receptors
13.Give a example of extracellular matrix protein
Collagens and plasminogen activator inhibitor-1.
14. Which proteins are the key intracellular signal
transducers in the pathway downstream from the TGFβ
receptors
Smad
15.Name any one mportant gene induced by TGFβ?
p15
