Question

What are four signals that regulate cell division? Briefly explain how they are each involved in this control in preventing tumors.

Answer 1

CELL CYCLE REGULATORS

Studies on the cell cycle in several different experimental systems, lead to remarkable discoveries of cell cycle.

There are several innate and extrinsic systems are present in a cell to regulate the cell cycle.

Mainly cell cycles are regulated by following ways, (the second part of the question about cancer is briefly described under each title in bold.)

1. Cyclins and cyclin dependednt kinases (CDK)

2. Transcriptional control.

3. Ubiquitin- mediated, proteasome—dependent degradation

4. Other kinases and phosphatases.

Mitotic checkpoints ensure the regulation in-between different events

1. Cyclins and cyclin depended on kinases.

Cell cycle is regulated by a heterodimeric protein kinases called CDKs.

Major Cyclins and CDKS are mentioned below.

PHASE	CYCLINS	CDK
G1	D	4 and 6
G1/S	E	2
S	A	2
M	B & A	1

CDKs have no kinase activity unless they associated with a regulatory cyclin subunit

· Each CDK can associate with different cyclins, which determine the substrate specificity of the complex. Different types of cyclin-CDK complexes initiate different events. G1 CDKs and G1/S phase CDKs promote entry into the cell cycle, S phase CDKs trigger S phase, and mitotic CDKs initiate the events of mitosis. Each cyclin is only present and active during the cell cycle stage it promotes and hence restricts the kinase activity of the CDKs to which it binds to that cell cycle stage. Multiple mechanisms are there to ensure this. CDK activities oscillate during the cell cycle. Positive feedback (specific CDKs promote their own activation) and negative feedback mechanisms (indirectly CDKs promote their own inactivation) drive these oscillations.

The G1 cyclins connects the extracellular events with cell cycle. Their activity is regulated by signal transduction pathways that sense the presence of growth factors or cell proliferation inhibitory signals. (eg.: Nerve Growth Factor or NGF for nerve cells, Fibroblast Growth Fator (FGF) for fibroblast cells, Epidermal growth factor or EGF)

The level of G1 cyclins do not show strong fluctuation, as levels of other cyclins do. Instead, their levels gradually increase throughout the cell cycle in response to macromolecule biosynthesis and extracellular signals. ie… if growth factor present level of G1 cyclin Increase. Presence of G1 cyclins level of transition cyclins increase, also S phase cyclins Transition cyclins helps in the activation of S phase cyclins. During S phase mitotic cyclins will be synthesized The G1/S cyclins accumulate during late G1, when it reaches a peak levels, cells enter S phase, and decline during S phase.. The main function of cyclin E–CDK2 complexes, together with cyclin D–CDK4/6, is to trigger the G1–S phase transition. This transition, known as the START/restriction point. At this time, cells initiate DNA replication and centrosome duplication. S phase Cyclins are also synthesized along with G1 cyclins, but their levels remain high throughout S phase and decline during late mitosis. Mitotic Cyclins bind CDK1 to promote entry into and progression through mitosis. Mitotic Cyclin-CDK complexes are synthesized during S phase and G2, but they will be kept inactive till DNA synthesis is completed. Once activated, mitotic CDKs promote entry into mitosis by phosphorylating and activating many proteins to promote chromosome condensation, nuclear envelope breakdown, mitotic spindle formation, etc.

Tumar associated changes in the  Cyclins and  CDK leads to the cancer /tumor. For example the overexpression of cdk4 and cdk6 can be achieved through deregulated expression of D-type cyclins, loss of p16INK4a

2) Transcriptional control

Transcriptional control of the cyclins is one mechanism that ensures proper temporal expression of the cyclins. E2F transcription factor complex promotes transcription of

1.) G1/S phase cyclins

2.) transcription factors for the synthesis of mitotic cyclins.

Loss of function of RB leads to constitutive expression of E2F leads to tumor due to uncontrolled cell divisionLoss of function of RB leads to constitutive expression of E2F leads to tumor due to uncontrolled cell division

3) Ubiquitin- mediated, proteasome-dependent protein degradation restricts the activity of cyclins to the appropriate cell cycle stage.

Protein degradation is an irreversible process. So this will ensure that the cell cycle is moving forward and that cells cannot “go backward” in the cell cycle. Two ubiquitin-protein ligases operates in cell cycle for degradation of proteins.

1.) SCF (first letters of its constituents, Skp1, Cullin, and F-box proteins)

2.) Anaphase-promoting complex or cyclosome (APC/C). SCF controls the G1–S phase transition. APC/C 1. initiate chromosome segregation at the metaphase-anaphase transition. 2. degrades S phase and mitotic cyclins, to promote the exit from mitosis. SCF active throughout the cell cycle, but they can recognizes their substrates only when substrates are phosphorylated. APC/C-dependent protein degradation, substrates are recognizable throughout the cell cycle, but APC/C is activated by phosphorylation at the metaphase-anaphase transition through the action of mitotic CDKs. The substrate specificity of APC/C is determined by its association with one of two factors called Cdc20 and Cdh1. APC/C + Cdc20 ubiquitinylates proteins that inhibit chromosome segregation APC/C + Cdh1 degradation of mitotic cyclins for mitotic exit and geminin degradation. It is active throughout the late mitosis and during G1 for cyclin degradation and other mitotic regulators

Inactivation of APC/C or other ubiquitin ligases leads to the premature appearance of mitotic cyclins and an unscheduled initiation of DNA replication so ultimately results in cancer.

4) Other kinases and phosphatases.

Activatory and inhibitory phosphorylation for regulating CDK activity Phosphorylation of a threonine residue near the active site of the enzyme is required for CDK activity. This phosphorylation is mediated by the CDK-activating kinase (CAK). CAK is active throughout the cell cycle and phosphorylates CDK at 161th Thr. A dual specificity kinase called Wee1 gives two inhibitory phosphorylation

1.) tyrosine (Y15) 2.) threonine (T14) A dual specificity phosphatase called Cdc25 mediates dephosphorylation.

CDK inhibitors (CKIs) control cyclin-CDK activity CKIs bind to the cyclin-CDK complex and inhibit its activity.

Eg. INK4 (p15, p16, p14 ARF) – Binds to CDK4 & CDK6 and inhibit their binding with G1 cyclins.

2.) p21, p27 & p57 – binds and inhibit G1/S phase CDKs and S phase CDKs. So they must be degraded to initiate S phase.

3.) Sic1- inhibit G1/S phase CDKs in yeast cells 4.) Rb- inhibit E2F

Any alteration in the balanced equilibrium between kinases and phosphatases may result in the development and progression of various types of cancer. For example  Loss of phosphatases such as PTEN (phosphatase and tensin homologue deleted on chromosome 10),

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