Name: cdk4/6 blockers and cell cycle control
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 34 s
Channel: JoVE

During cell cycle transitions, Cdk activity is regulated by multiple proteins, ensuring controlled cell growth, complete DNA replication, and mitotic distribution of the chromosomes to daughter cells.
In the absence of regulatory proteins, an abnormal cell goes unchecked, leading to conditions such as cancer.
In a normal cell, Cdk activity is regulated through multiple mechanisms including cyclin degradation, inhibitory phosphorylation, and inhibitory conformational changes induced by Cdk inhibitor binding.
Cyclin levels are known to fluctuate during the cell cycle. Cdks are only active when bound to cyclin, therefore, cyclin degradation leaves Cdks inactive and unable to promote the transition to the next cell cycle stage.
In another mechanism, a kinase named Wee1, phosphorylates the active site in Cdk.&#160; This phosphorylation inhibits the activity of the cyclin-Cdk complex.
Additionally, Cdk inhibitors or CKIs, such as p16, p21, and p27, regulate Cdk activity through inhibitory conformational changes.
For instance, if DNA damage occurs during G1, p16 interacts with the cyclin-Cdk complex. This interaction causes a large structural rearrangement, detaching the bound cyclin, causing Cdk inactivation.&#160;
DNA damage in the G1 phase can also trigger the tumor suppressor protein, p53, to activate p21.
G1/S-Cdk and S-Cdk complexes are bound and inhibited by p21, arresting the cells in the G1 phase, and allowing enough time for DNA repair.
For the G1/S phase transition to take place, the cell is required to have all the resources needed for DNA replication. If the cell lacks the necessary resources, p27 binds to G1/S-Cdk and S-Cdk complexes, inhibiting the enzyme activity.
Once favorable conditions for cell cycle progression are met, p27 is degraded, restoring Cdk activity and promoting cell transition.

inhibition of cdk activity

Inhibition of Cdk Activity

The orderly progression of the cell cycle depends on the activation of Cdk protein by binding to its cyclin partner. However, the cell cycle must be restricted when undergoing abnormal changes. Most cancers correlate to the deregulated cell cycle, and since Cdks are a central component of the cell cycle, Cdk inhibitors are extensively studied to develop anticancer agents. For instance, cyclin D associates with several Cdks, such as Cdk 4/6, to form an active complex. The cyclin D-Cdk4/6 complex then phosphorylates and inactivates the tumor suppressor retinoblastoma protein (Rb) to promote the G1-to-S phase transition of the cell cycle. In normal cells, Rb protein is reactivated through the regulation of Cdk activity, thus, preventing abnormal cell cycle transitions. 
There are at least three known mechanisms by which Cdk activity is regulated- cyclin degradation, inhibitory phosphorylation, or binding of inhibitory proteins. Mutations that prevent these mechanisms lead to Cdk-mediated tumorigenesis.
Because Cdk 4/6 plays a substantial role in tumor formation, several Cdk inhibitors have been developed for clinical use. The most recent ones are selective for Cdk4 and Cdk6. There are at least three clinically approved Cdk 4/6 inhibitors: abemaciclib, ribociclib, and palbociclib. These inhibitors bind to the ATP pocket of Cdk 4 and 6, inactivating the Cyclin D-Cdk4/6 complexes, leading to Rb protein activation and subsequent cell cycle arrest. In some cases, the inhibitor-mediated cell cycle arrest causes an increase in apoptosis in tumor cells.
Inhibition of the cell cycle and subsequent programmed cell death are the most common mechanisms of Cdk4/6 inhibitors. However, a recent study in mouse models of breast cancer showed that Cdk4/6 inhibition could also lead to severe immunogenic effects. During the study, the Cdk inhibitor seemed to enhance tumor cells&#39; antigen-presenting ability, thereby allowing cytotoxic T cells to recognize and destroy the tumor cells.

The orderly progression of the cell cycle depends on the activation of Cdk protein by binding to its cyclin partner. However, the cell cycle must be ...

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JoVE Core

Molecular Biology

Unit 2

Cell Proliferation

KEY TERMS AND CONCEPTS

cdk4/6 blockers and cell cycle control

Cdk4/6 blockers help control the cell cycle by stopping the signals that push cells from G1 into S phase. Cdks, or cyclin-dependent kinases, must bind cyclins to become active, and cyclin D often pairs with Cdk4/6 to form this active complex. The cyclin D-Cdk4/6 complex then phosphorylates the retinoblastoma protein, or Rb, which normally acts as a tumor suppressor.

When Rb is inactivated, cells can move into the next stage of division. In normal cells, Cdk activity is carefully regulated so Rb can be reactivated and abnormal cell-cycle transitions are prevented. Cdk activity can be controlled in at least three ways: cyclin degradation, inhibitory phosphorylation, or binding of inhibitory proteins. Mutations that block these controls can lead to Cdk-driven tumor formation.

Because Cdk4/6 has a major role in tumor growth, several inhibitors have been developed for clinical use. The approved Cdk4/6 inhibitors include abemaciclib, ribociclib, and palbociclib. These drugs bind to the ATP pocket of Cdk4 and Cdk6, shut down the Cyclin D-Cdk4/6 complex, activate Rb, and cause cell-cycle arrest.

In some tumor cells, this arrest also increases apoptosis, or programmed cell death. Cell-cycle arrest followed by apoptosis is one of the most common effects of Cdk4/6 inhibitors. A recent mouse study in breast cancer also found that Cdk4/6 inhibition may have strong immunogenic effects. In that study, the inhibitor appeared to improve tumor cells' antigen-presenting ability, which helped cytotoxic T cells recognize and destroy the cancer cells.

Cdk4/6 Blockers and Cell Cycle Control

what is the cell cycle?

Cell Growth and Division in the Cell Cycle

The cell cycle is the process that cells use to grow and divide. It helps living things make new cells for growth and repair. In many cases, it is also the way single cells reproduce. The cycle has two main parts: interphase and cell division. During interphase, the cell grows and prepares for division. During cell division, one cell splits into new cells. This sequence keeps cells in a repeating pattern of growth, copying, and splitting.

interphase

Interphase in the Cell Cycle

Interphase is the part of the cell cycle when a cell grows and prepares to divide. It is a major stage in the life of a cell, and it includes the phases G1, S, and G2. During G1, the cell grows and carries out its normal functions. In the S phase, the cell copies its DNA. During G2, the cell continues to grow and gets ready for mitosis, the stage when the cell divides. Interphase takes up most of the cell cycle. It gives the cell time to build materials, make a copy of its genetic...

the cell cycle control system

How Cell Division Is Controlled

The cell cycle is the organized process that controls cell division. It helps one cell divide into two daughter cells with chromosomes that match the parent cell. This process is essential for growth from a single-cell zygote and for tissue renewal in adult multicellular eukaryotes. Cell division also supports repair and replacement in the body. In bone marrow, stem cells divide to make new blood cells. When cell division happens without control, it can lead to cancer and many genetic diseases.

positive regulator molecules

Cyclins and Cdks in Cell Cycle Control

Cyclins and cyclin-dependent kinases help control how cells move through the cell cycle. These positive regulator molecules keep mitotic cell division on schedule so daughter cells can match the parent cell. When this control fails, errors in DNA replication or in the distribution of genetic material can create mutant cells that pass those changes to every new cell they form. Cell cycle checkpoints limit the spread of these abnormal cells. A checkpoint is a control point at a specific stage of...

Cdk4/6 blockers help control the cell cycle by stopping the signals that push cells from G1 into S phase. Cdks, or cyclin-dependent kinases, must bind cyclins to become active, and cyclin D often pairs with Cdk4/6 to form this active complex. The cyclin D-Cdk4/6 complex then phosphorylates the retinoblastoma protein, or Rb, which normally acts as a tumor suppressor. When Rb is inactivated, cells can move into the next stage of division. In normal cells, Cdk activity is carefully regulated so...

s-cdk initiates dna replication

Cdk Control of DNA Replication Timing

Cdk control of DNA replication timing helps cells copy DNA once each cycle. The cell cycle has four main stages: G1, S, G2, and M. In G1, the cell grows. In S phase, it copies its DNA. In G2, it prepares to divide. In M phase, mitosis divides the cell content into two daughter cells. In eukaryotes, DNA replication starts at many sites on the chromosomes called origins of replication. These origins switch between two states during the cell cycle. In G1, a multiprotein pre-replicative complex,...

m-cdk drives transition into mitosis

Cell Cycle Kinases Trigger Entry into Mitosis

Cell cycle kinases help control the move from G2 into M phase. Checkpoints act as safeguards and gatekeepers, so the cell cycle can continue in good conditions and slow down or stop when problems appear. This regulation is called the cell cycle control system. Cyclin-dependent kinases, or Cdks, work with cyclins to manage cell cycle transitions. A key example is M-Cdk, which is made of Cdk1 bound to M cyclin. M-Cdk helps drive the shift from G2 to M phase by preparing the cell for mitosis. M...

mitogens and the cell cycle

EGF Signaling and Cancer Cell Growth

Epidermal growth factor, or EGF, is a mitogen that helps control cell division. Its receptor, EGFR, sits in the cell membrane and sends a growth signal when EGF binds to it. EGFR is part of the ErbB family of tyrosine kinase receptors, and this signaling helps decide whether a cell will keep proliferating. In healthy tissue, the number of EGFR molecules on the cell surface is tightly regulated. This balance helps cell division match the tissue’s needs. In cancer cells, however, EGFR is often...

replicative cell senescence

Telomeres, Telomerase, and Tumor Control

Telomeres and telomerase help control how many times a cell can divide. Telomeres are short, repetitive DNA sequences at the ends of chromosomes. They are covered by proteins that form a protective cap. In many human body cells, this cap gets a little shorter after each round of cell division. This shortening is part of replicative senescence, the point at which a cell stops dividing permanently. Embryonic stem cells make telomerase, an enzyme that adds telomeric repeats and supports repeated...

abnormal proliferation

Ras and MYC in Uncontrolled Cell Growth

Ras and MYC can drive abnormal cell proliferation, which is uncontrolled cell growth. Under normal conditions, most adult cells stay non-proliferative unless they receive internal or external signals to replace lost cells. In abnormal proliferation, cell division continues even after those signals stop. This persistent growth can lead to tumors. The tumor forms from damaged cells that keep copying themselves. Those cells pass the damage to daughter cells and to later generations of daughter...

cells coordinate growth and proliferation

cells-coordinate-growth-and-proliferation

Cell Size Control in Growth and Muscle

Cell size control helps cells keep their shape, function, and fitness. Many dividing cells grow and then double their mass before they divide. This coordination depends on checkpoints between cell cycle phases, which help the cell stay in balance. When checkpoint control fails, cells may grow without proper control. That loss of coordination can lead to uncontrolled growth and reduced cellular function. In adult tissues, non-dividing cells also adjust size based on metabolic state. Adult...

key terms and concepts

Learn how Cdk4/6 inhibitors stop cell-cycle progression, reactivate Rb, and are used in cancer treatment, with examples like palbociclib and ribociclib.

Cdk4/6 Blockers and Cell Cycle Control | JoVE K12

molecular biology

JoVE Core Molecular Biology showcases the molecular processes of the cell through clear and concise animated video lessons. Additionally, in scientist-in-action videos, these concepts' application is demonstrated through real experiments in modern laboratories.

Cdk4/6 Blockers and Cell Cycle Control

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