dr mohammad s alanazi, msc, phd molecular biology ksu cell cycle control, defects and apoptosis 1 st...
TRANSCRIPT
Dr Mohammad S Alanazi, MSc, PhDMolecular Biology
KSU
Cell Cycle Control, Defects and Apoptosis
1st Lecture
2
The Cell Cycle Is an Ordered
Series of Events
Leading to Replication of
Cells
Cell-Cycle Control in Mammalian Cells
3
Dr Gihan Gawish
Regulated Protein Phosphorylation and
Degradation Control Passage through the Cell Cycle
4 Amphibian and invertebrate eggs and early embryos
from synchronously fertilized eggs provide sources of
extracts for biochemical studies of cell-cycle events.
The isolation of yeast cell-division cycle (cdc) mutants
led to the identification of genes that regulate the
cell cycle
Diverse Experimental Systems Have Been Used to Identify and Isolate Cell-Cycle Control Proteins
5
Dr Gihan Gawish
Isolation of wild-type cell-division cycle (CDC) genes from S. cerevisiae cells carrying temperature-sensitive mutations in these genes
6
In multicellular organisms, cell replication is controlled by a complex
network of signaling pathways that integrate signals from the extracellular
environment with intracellular cues about cell size and developmental
program.
Polypeptide growth factors called mitogens stimulate cultured
mammalian cells to cycle. Once cycling cells pass the restriction point,
they can enter the S phase and complete S, G2, and mitosis in the
absence of growth factors.
Mammalian Restriction Point is Analogous to start in Yeast Cells
7
Multiple Cdks and Cyclins Regulate Passage of Mammalian Cells through
the Cell Cycle
Experimental demonstration that cyclin D is required for passage through the restriction point in the mammalian cell cycle
8
Multiple Cdks and Cyclins Regulate Passage of Mammalian Cells through
the Cell Cycle
Activity of mammalian
Cdkcyclin complexes
through the course of
the cell cycle in G0 cells
induced to divide by
treatment with growth
factors The width of the colored
bands is approximately
proportional to the protein
kinase activity of the
indicated complexes. Cyclin
D refers to all three D-type
cyclins.
9
Dr Gihan Gawish
Cell-Cycle Progression is Blocked by
DNA Damage and p53: DNA Damage
Checkpoints
10
The Cell-Cycle Control System
11
GENERAL NAME FUNCTIONS AND COMMENTS
Protein kinases and protein
phosphatases that modify Cdks
Cdk-activating kinase (CAK)
phosphorylates an activating site in Cdks
Wee1 kinase phosphorylates inhibitory sites in Cdks; primarily involved in controlling entry into mitosis
Cdc25 phosphataseremoves inhibitory phosphates from Cdks; three family members (Cdc25A, B, C) in mammals; Cdc25C is the activator of Cdk1 at the onset of mitosis
Cdk inhibitory proteins (CKIs)
Sic1 (budding yeast) suppresses Cdk activity in G1; phosphorylation by Cdk1 triggers its destruction
p27 (mammals)suppresses G1/S-Cdk and S-Cdk activities in G1; helps cells to withdraw from cell cycle when they terminally differentiate; phosphorylation by Cdk2 triggers its ubiquitylation by SCF
p21 (mammals) suppresses G1/S-Cdk and S-Cdk activities following DNA damage in G1; transcriptionally activated by p53
p16 (mammals) suppresses G1-Cdk activity in G1; frequently inactivated in cancer
Ubiquitin ligases and their activators
SCFcatalyzes ubiquitylation of regulatory proteins involved in G1 control, including CKIs (Sic1 in budding yeast, p27 in mammals); phosphorylation of target protein usually required for this activity
APCcatalyzes ubiquitylation of regulatory proteins involved primarily in exit from mitosis, including Securin and M-cyclins; regulated by association with activating subunits
Cdc20APC-activating subunit in all cells; triggers initial activation of APC at metaphase-to- anaphase transition; stimulated by M-Cdk activity
Hct1 maintains APC activity after anaphase and throughout G1; inhibited by Cdk activity
Gene regulatory proteins
E2Fpromotes transcription of genes required for G1/S progression, including genes encoding G1/S cyclins, S-cyclins, and proteins required for DNA synthesis; stimulated when G1-Cdk phosphorylates Rb in response to extracellular mitogens
p53promotes transcription of genes that induce cell cycle arrest (especially p21) or apoptosis in response to DNA damage or other cell stress; regulated by association with Mdm2, which promotes p53 degradation
The Major Cell-cycle Regulatory Proteins
12
In multicellular organisms, cells that are no longer needed or are a threat to the
organism are destroyed by a tightly regulated cell suicide process known as programmed
cell death, or apoptosis.
Apoptosis is mediated by proteolytic enzymes called caspases, which trigger cell death by
cleaving specific proteins in the cytoplasm and nucleus.
Caspases exist in all cells as inactive precursors, or procaspases, which are usually
activated by cleavage by other caspases, producing a proteolytic caspase cascade.
The activation process is initiated by either extracellular or intracellular death signals,
which cause intracellular adaptor molecules to aggregate and activate procaspases.
Caspase activation is regulated by members of the Bcl-2 and IAP protein families.
Programmed Cell Death (Apoptosis)
13 Dr Gihan Gawish
(A) Each suicide protease
is made as an inactive
proenzyme (procaspase),
which is usually activated
by proteolytic cleavage by
another member of the
caspase family
(B) Each activated
caspase molecule can
cleave many procaspase
molecules, thereby
activating them, and these
can then activate even
more procaspase
molecules.
The caspase cascade involved in apoptosis
14 Dr Gihan Gawish
Procaspases Are Activated by Binding to Adaptor Proteins
15
The factors that promote organ or organism growth can be operationally divided into three major classes:
Mitogens, which stimulate cell division, primarily by relieving intracellular
negative controls that otherwise block progress through the cell cycle.
Growth factors, which stimulate cell growth (an increase in cell mass) by
promoting the synthesis of proteins and other macromolecules and by
inhibiting their degradation.
Survival factors, which promote cell survival by suppressing apoptosis.
Extracellular Control of Cell Division, Cell Growth, and Apoptosis
The extracellular signal molecules that regulate cell size and cell number are generally either soluble secreted proteins, proteins bound to the surface of cells, or components of the extracellular
matrix.