The Cell Cycle

Normal cells and abnormal (cancer) cells must complete the cell cycle in order to replicate. The cell cycle is an ordered sequence of cellular events regulated by chemical signals produced by genes. Some genes (proto-oncogenes) are responsible for stimulating cellular growth and division. Other genes (tumor suppressor genes) restrict cell cycle progression.

In many cases, genetic damage causes cells to lose their ability to produce or respond to the products of tumor suppressor genes. Such cells may embark on an uninhibited cycle of cell replication. Improved understanding of the cell cycle has allowed researchers to develop treatments that can inhibit each specific phase in the cycle and in some cases cause cells to self-destruct (apoptosis).

The CELL CYCLE diagram on the right depicts major landmarks on a cells journey to replication. Traditionally the eukaryotic cell cycle is divided into two alternating states; interphase and Mitosis. Shortly after mitosis, internal and external factors determine one of two outcomes for each daughter cell: rest or proliferation (replication). The typical human cell cycle is about 24 hours in length. The G1 phase can last about 11 hours, S phase about 8 hours, G2 phase about 4 hours, and M about 1 hour (Cooper 2000).

• Resting cells are considered to be outside of the proliferative cell cycle. Resting cells include two groups. The first group is differentiated cells that carry out all the normal functions of their cell type but do not engage in the activities required for proliferation. The second group, (e.g., stem cells) may be induced to differentiate or proliferate.
• Proliferating cells are those engaged in activities of replication. If they complete all phases of the cell cycle from G1 to mitosis the result is two daughter cells.

Interphase is the part of the cell cycle that produces the intracellular contents necessary to the production of a viable daughter cell. During interphase (G1, S, G2), the sequential progression from one phase to the next is associated with the rise and fall of cyclins and the activation of their cyclin-dependent kinases (CDK) (Tkacs 2020).

• Cyclin D accumulates in the G1 Phase. It drives progression from the G0 or G1 phase into the S phase and is often hyperactivated in many cancers:
  • Papillary thyroid carcinoma
  • Mantle cell lymphoma
  • Breast cancer (Stamatakos 2010).
• Cyclin E accumulates at the G1-S phase boundary and is degraded as cell progresses through S phase. Overexpression of this gene results in chromosome instability, and thus may contribute to tumorigenesis in
  • Breast cancer
  • Ovarian Cancer
  • Gastric cancer
  • Colorectal carcinomas
  • Melanomas
  • Non small cell lung carcinoma (Stamatakos 2010).
• Cyclin A is involved in phases S, G2, early M.  It is implicated in DNA replication and mitotic entry (Silva Cascales 2021). Guo et al. found high expression of cyclin A2 correlates with a better prognosis in patients with colorectal carcinoma. Conversly, low levels of cyclin A2 was found to induce DNA damage, inflammation, and activation of different regenerative pathways triggering dysplasia in the colonic epithelial cells of mice.
• Cyclin B expression is critical to the initiation of mitosis. It is involved in the disintegration of the nuclear membrane, assembly of the spindle apparatus and segregation of chromosomes (Petry 2016).

Interphase contains several checkpoints that ensure that the cell is competent to complete mitosis. If the cell does not meet the molecular and biologic requirements of a checkpoint, it may not progress beyond the checkpoint. Cells that are blocked at a checkpoint may be triggered to enter apoptosis.

• (G0) – In this "resting" phase, cells are carrying out their normal metabolic functions. They are not replicating and therefore not in cell cycle. Depending on the type of cell, this period can last for a few hours or until the cell dies. Some cells like terminally differentiated muscle cells and most nerve cells remain in G0 indefinitely. In the case of peripheral T-lymphocytes, most are in the G0 phase until they are induced to replicate by contact with an antigen presenting cell. When a G0 cell is signaled to replicate, it moves into the G1 phase.
• (G1) – During G1 phase, the cell responds to external signals including growth factors by synthesizing the cytoplasmic contents needed for two daughter cells. The cell will only pass the checkpoint if it is an appropriate size and has adequate energy reserves.  At this point, the cell also checks for DNA damage. A cell that does not meet all the requirements will not progress to the S phase.  G1 phases cells that do not progress to the S phase can delay progression to correct errors or enter G0. The G1 phase usually lasts about 11 hours.
  • G1/S checkpoint blocks cell-cycle proliferation until external growth factors and available nutrients induce production of sufficient activated cyclin-D+CDK 4 & 6 complexes. Cyclin-D+CDK 4 & 6 complexes release the retinoblastoma protein (Rb) from E2F transcription factors of the G1/S checkpoint. E2Fs are critical regulators of the cell cycle. They regulate every phase of the cell cycle by controlling the transcription of numerous target genes involved in DNA replication and cell cycle progression (Xie 2021).
  • In a majority of human cancers, RB1 function is suppressed during tumor progression (Linn 2021).
  • G1 phase-specific chemotherapeutic agents include: L-asparaginase and corticosteroids.
    
• (S) – In S phase, the cell "begins the enormous task of duplicating each of the 3.2 billion base pairs that make up its DNA. Massive replication factories scattered throughout the genome are activated and begin unwinding the double helix and building new DNA molecules at a blistering rate of 500 nucleotides per minute with an error rate of one nucleotide in a billion. In prokaryotes, replication begins from a single site and continues until it terminates at the end of the genome. If eukaryotes were to use an identical strategy, however, it would take on the order of several days to completely replicate its significantly larger genome. Therefore, eukaryotic cells initiate replication from multiple locations referred to as replication origins through out each chromosome (Takeda 2005)."
  • Daughter DNA is replicated (Click) from each parent template DNA strand. Upon completion of the S phase, each chromosome is composed of two DNA double-helix molecules joined at a centromere, forming a chromatid. The S phase lasts about 8 hours.
  • S phase-specific agents include: azacitidine, cytarabine, decitabine, doxorubicin, fludarabine, 5-fluorouracil, gemcitabine, hydroxyurea, mercaptopurine. methotrexate, procarbazine, thioguanine.
    
• (G2) - This phase involves production of additional protein, RNA and the mitotic spindle necessary for mitosis. G2 lasts about 4 hours
  • G2/M checkpoint restricts progression to M phase if errors are found in the duplicated chromosomes. Significant errors may trigger apoptosis
    • G2 phase-specific agents include: bleomycin, etoposide, irinotecan, topotecan.
• Mitosis (M) Mitosis is the biological sum of four phases: Prophase, Metaphase, Anaphase and Telophase. When mitosis is complete, genetically equivalent chromosomes exists at opposing poles of the cell. Cytokinesis divides the mother cell into two daughter cells. The M phase usually lasts about one hour.
  • M phase-specific agents include: docetaxel, paclitaxel, vinblastine, vincristine, vinorelbine, ixabepilone, estramustine

Many chemotherapy agents are cell cycle specific (CCS) drugs that exert their major cytotoxic effect during a specific phase of proliferation. Typically CCS drugs have less effect on cells in the resting state (GO). CCS drugs are administered in the minimum effective concentration via continuous dosing methods rather bolus to maximize the time exposure of cells within the continuum of the targeted phase.

Cell cycle non-specific (CCNS) drugs like cyclophosphamide or platinum based cisplatin act against cells in all phases of the cell cycle, including the GO or resting phase. CCNS drugs are dose dependent (>dose = >kill rate). They are most effective and least toxic when given in high dose bolus. Large solid tumors with a greater fraction of cells in resting cells may be treated more effectively by CCNS agents.

During which phase is DNA replicated?

GO
G1
S
G2
M

The body has a sophisticated system for maintaining normal tissue repair and growth. It is thought that the body responds to a feedback system that signals a G0 (stem cell) to enter the G-1 phase of the cell cycle in response to tissue damage. In cancer, this feedback system doesn’t work normally and cancer cells enter the cell cycle independent of the body’s feedback system. Cancer is a disease in which cells don’t respond to the normal mechanisms that control normal cell replication and death. Four characteristics of cancer cells that are not found in normal cells include:

• uncontrolled cell replication,
• decreased cell differentiation (the maturation of cells into separate functions),
• the ability to invade surrounding tissues, and
• the ability to metastasize to other organs and tissues.

INSTANT FEEDBACK:

The cell kill theory proposes that a set percentage of cells are killed with each dose of chemotherapy.

Reference

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Ghelli Luserna Di Rorà, A., Ghetti, M., Ledda, L. et al.(2021)  Exploring the ATR-CHK1 pathway in the response of doxorubicin-induced DNA damages in acute lymphoblastic leukemia cells. Cell Biol Toxicol (2021). https://doi.org/10.1007/s10565-021-09640-x

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