Why Do Cancer Cells Keep Growing?
Why Do Cancer Cells Keep Growing?
Cancer cells keep growing because several normal controls over cell division and survival have been disrupted. They can keep growth signals active, ignore signals that tell them to stop, bypass cell-cycle checkpoints, resist senescence and cell death, and maintain their chromosome ends. Many also change how they use nutrients. Together, these changes allow cancer cells to keep producing new cells when normal cells would slow down, stop dividing, or die.
By Chris Morais, MSc, MPhil, PhD
- How Is Normal Cell Growth Usually Controlled?
- How Do Cancer Cells Keep Growth Signals Switched On?
- How Do Cancer Cells Ignore Signals That Normally Stop Division?
- How Do Cancer Cells Bypass Cell-Cycle Checkpoints?
- How Do Cancer Cells Avoid Senescence?
- How Do Cancer Cells Avoid Apoptosis?
- How Do Cancer Cells Keep Their Telomeres From Becoming Too Short?
- How Does Cancer Cell Metabolism Support Continued Growth?
- Why Does Uncontrolled Growth Not Mean Cancer Cells Divide as Fast as Possible?
- Why Do Some Cancer Cells Grow Faster Than Others?
How Is Normal Cell Growth Usually Controlled?
Normal cells do not divide simply because they are physically able to. In healthy tissue, cell division is strictly controlled by signals from outside the cell, conditions inside the cell, and the physiological needs of the surrounding tissue.
- Growth factors and receptors: Chemical signals called growth factors bind to specific receptor proteins located on the cell surface or inside the cell. The receptor relays this message through an intracellular chain of signaling proteins. If conditions are right, these signals move the cell into the cell cycle toward division.
- Inhibitory signals: Other chemical messengers do the exact opposite. They halt division when enough cells are already present, when the cell is damaged, or when the surrounding environment lacks necessary resources.
- Internal checkpoints: Cells rely on molecular checkpoints to inspect critical events before division is permitted to proceed.
The total number of cells in any tissue depends on the balance between cell production and cell loss. Normal tissues maintain their correct size and architecture by coordinating cell division, cell death, and periods when cells remain alive without dividing. Cancer fundamentally disrupts this balance[1].
How Do Cancer Cells Keep Growth Signals Switched On?
Cancer cells can acquire alterations that keep growth-promoting pathways abnormally active—a hallmark known as sustained proliferative signaling.[2]
Can Cancer Cells Produce Their Own Growth Signals?
Yes. Many cancer cells produce their own growth factors that bind to receptors on their own surface (autocrine signaling) or stimulate neighboring cancer cells.
This creates a self-stimulating growth loop:
- The cells become far less dependent on growth factors supplied by the surrounding normal tissue.
- Cancer cells can also release signals that recruit and alter nearby noncancerous cells, including fibroblasts and blood vessel cells.
- These recruited normal cells then release additional growth factors, creating a supportive local tumor microenvironment that promotes cancer cell survival and proliferation.
Can Growth-Signaling Proteins Stay Active Without an Outside Signal?
Yes. Mutations in genes that encode signaling proteins can keep parts of a growth pathway abnormally active even without the usual external growth signal:
- Mutated intracellular switches: A well-known example involves RAS proteins. Specific mutations can reduce the ability of RAS to switch off, allowing growth-promoting signals to remain abnormally active.
- Altered surface receptors: Mutations can alter receptor proteins so they continuously fire without needing a growth factor to bind to them.
- Gene amplification: A cell may produce excessive amounts of a receptor or signaling protein because the corresponding gene has been duplicated multiple times.
In each of these scenarios, the cancer cell frees itself from the strict external controls that normally govern tissue growth.
How Do Cancer Cells Ignore Signals That Normally Stop Division?
Normal cells continuously receive inhibitory signals that restrain growth and preserve tissue structure. Cancer cells evade these restraints by inactivating key tumor suppressor pathways.
A central regulator in this process is the retinoblastoma protein (RB):
- Under normal conditions, RB acts as a crucial gatekeeper at the G1 checkpoint, preventing a cell from entering the DNA-copying phase (S phase) of the cell cycle until all conditions are favorable.
- If the genes or regulatory proteins in the RB pathway are mutated or silenced, this molecular brake is weakened or lost entirely.
- As a result, the cell continues toward division even when clear stop signals are present.
Other growth-suppressing pathways rely on contact with neighboring cells and the extracellular matrix. Cancer cells do not have to eliminate every single inhibitory mechanism; disrupting a few central control nodes gives them a decisive growth advantage.
How Do Cancer Cells Bypass Cell-Cycle Checkpoints?
The cell cycle is the ordered sequence of events through which a cell prepares its components, duplicates its DNA, and divides. Checkpoints function as quality-control gates that pause the cycle if critical requirements are not met:
- G1/S checkpoint: Pauses the cycle if DNA damage is detected, allowing time for repair before replication begins.
- G2/M checkpoint: Checks that DNA replication is complete and that serious DNA damage has been repaired before mitosis begins.
- Spindle assembly checkpoint: Ensures that chromosomes are properly aligned and attached to the spindle apparatus before they are pulled apart.
These checkpoints depend on tightly regulated networks of proteins, including cyclins, cyclin-dependent kinases (CDKs), RB, and the master tumor suppressor p53.
Cancer cells frequently carry genetic changes that disable these checkpoints. They may overexpress cyclins that drive the cycle forward, lose CDK inhibitors, or mutate p53. By bypassing these checkpoints, cancer cells divide under conditions of severe DNA damage or cellular stress that would cause a normal cell to pause or die[3].
How Do Cancer Cells Avoid Senescence?
Cellular senescence is a stable, long-lasting state in which a cell remains metabolically active but stops dividing. Senescence acts as a critical barrier against tumor development and can be triggered by several forms of stress:
- Extensive or unrepairable DNA damage.
- Excessive, abnormal growth signaling (oncogene-induced senescence).
- Critically shortened telomeres.
When normal cells experience these stresses, pathways involving p53, RB, and CDK inhibitors such as p16 and p21 can produce long-lasting growth arrest.
Cancer cells can overcome this barrier when pathways involving p53, RB, and related regulators are disrupted. Escaping senescence allows damaged, abnormal cells to continue proliferating. While non-dividing senescent cells can still be found within tumor tissue, the emergence of cancer requires that malignant clones evade the growth limits of senescence.
How Do Cancer Cells Avoid Apoptosis?
Apoptosis is a highly regulated form of programmed cell death. It removes cells that are no longer needed or are seriously damaged, usually without triggering the inflammation associated with uncontrolled cell rupture.
For a tumor to expand, producing new cells is not sufficient; a significant proportion of those cells must also survive. Cancer cells develop resistance to apoptosis through several mechanisms[4]:
- Loss of p53 function: Normal p53 responds to signals produced when severe DNA damage is detected and can activate genes that promote apoptosis.
- Upregulation of anti-apoptotic proteins: Many cancer cells produce elevated levels of survival proteins (such as BCL-2), which block the release of death-inducing factors from mitochondria.
- Downregulation of pro-apoptotic proteins: Cells may reduce or silence proteins that directly initiate the caspase cascade of cell destruction.
- Constitutive survival signaling: Continuous signaling through pathways like PI3K/AKT provides constant survival signals that suppress apoptosis.
These alterations do not make cancer cells completely indestructible, but they raise the threshold required to trigger cell death, allowing severely damaged cells to persist and produce daughter cells.
How Do Cancer Cells Keep Their Telomeres From Becoming Too Short?
Telomeres are specialized, repetitive DNA sequences located at the ends of linear chromosomes. They protect chromosome ends from degradation and prevent them from being mistaken for broken DNA strands.
In most normal human somatic cells, telomeres shorten slightly with each round of cell division because standard DNA polymerases cannot fully replicate the very ends of linear DNA molecules. After many divisions, telomeres can become critically short, triggering replicative senescence or, if that barrier is bypassed, chromosome instability and crisis. This process sets a natural limit on the proliferative lifespan of normal cells.
Normal Cells: Successive Divisions → Progressive Telomere Shortening → Senescence / Crisis
Cancer Cells: Telomerase Activation / ALT → Telomere Maintenance → Replicative Immortality
What Role Does Telomerase Play in Cancer Cells?
To achieve indefinite proliferation, approximately 85% to 90% of cancers reactivate or upregulate telomerase—an enzyme complex that synthesizes and adds telomeric DNA repeats back onto chromosome ends.
Telomerase activity is typically low or absent in mature somatic tissues. While telomerase does not transform a cell by itself, its activation removes the natural limit on cell division, granting the already abnormal clone replicative immortality.
About 10% to 15% of cancers maintain their telomeres through a mechanism called Alternative Lengthening of Telomeres (ALT), showing that cancer cells can bypass telomere shortening in more than one way.
How Does Cancer Cell Metabolism Support Continued Growth?
A rapidly proliferating cell requires more than energy in the form of ATP; it requires vast quantities of raw building materials (nucleotides, lipids, and amino acids) to assemble new organelles, membranes, and genetic material for daughter cells.
Cancer cells systematically reprogram their metabolic pathways to fulfill these biosynthetic demands[5]:
- Aerobic glycolysis (the Warburg effect): Many cancer cells increase glucose uptake and use glycolysis to convert more glucose to lactate even when oxygen is available. This pathway can provide energy while also supplying intermediate molecules that help the cell make proteins, lipids, nucleotides, and other materials needed for growth.
- Continued mitochondrial function: Cancer cells do not abandon their mitochondria. Most malignant cells maintain functional mitochondrial oxidative phosphorylation and rely on the tricarboxylic acid (TCA) cycle to process alternative fuels, generate ATP, and produce metabolic intermediates.
- Amino acid use: Many tumors increase their use of glutamine and other amino acids to replenish materials used by the TCA cycle and support the production of new cell components.
These metabolic changes help provide both the energy and the raw materials needed for continued cancer cell growth.
Why Does Uncontrolled Growth Not Mean Cancer Cells Divide as Fast as Possible?
"Uncontrolled growth" refers to a breakdown in regulatory timing and tissue constraints, not necessarily an extreme speed of division.
Key realities of tumor kinetics include:
- Cell cycle duration: Some normal cells—such as rapidly dividing blood-cell progenitors in the bone marrow, hair follicle cells, and cells lining the intestine—can divide as fast as or faster than cells in some cancers.
- Net growth dynamics: A tumor increases in volume because cell production exceeds cell death. Even if individual cells divide slowly, a population expands steadily if its members resist apoptosis and clearance.
- Intratumoral heterogeneity: Within a single solid tumor, only a fraction of cells are actively moving through the cell cycle at any given moment. Others remain quiescent in G0, divide slowly due to hypoxia and nutrient deprivation, or undergo spontaneous necrosis in poorly vascularized areas.
- Growth deceleration: Many tumors grow more slowly as they enlarge because limited blood supply, reduced oxygen and nutrients, and increased cell loss can restrict further expansion.[6]
Why Do Some Cancer Cells Grow Faster Than Others?
The rate of tumor progression varies widely between different cancer types and among different regions of the same tumor.
This variation is determined by several interacting biological factors:
| Biological Factor | Influence on Tumor Growth Rate |
|---|---|
| Cancer-Promoting Changes | Some combinations of genetic changes produce stronger growth or survival signals than others. |
| Degree of Checkpoint Loss | Loss of important checkpoint controls can allow cells to continue through the cell cycle despite DNA damage or other stress. |
| Vascularization (Angiogenesis) | Tumors with better access to blood vessels may receive more oxygen and nutrients, which can support continued growth. |
| Immune Evasion | The immune system can recognize and destroy some abnormal cells; cancer cells that evade these responses may gain a survival advantage. |
| Clonal Evolution | As different subclones arise within a tumor, those with greater growth or survival advantages may expand more successfully than their neighbors. |
Cancer is therefore dangerous not because its cells divide at extraordinary physical speed, but because their proliferation is uncoupled from the homeostatic signals that protect normal tissues.
Morais C. Why Do Cancer Cells Keep Growing? [Internet]. BiologyWithin.com; 2026 Sep 12. Available from: https://www.biologywithin.com/2026/09/why-do-cancer-cells-keep-growing.html
- National Cancer Institute. What Is Cancer?. Updated 2021.
- Hanahan D. Hallmarks of Cancer: New Dimensions. Cancer Discov. 2022;12(1):31-46. doi:10.1158/2159-8290.CD-21-1059.
- Cooper GM. Components of the Cell-Cycle Control System. In: The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.
- Lodish H, Berk A, Zipursky SL, et al. The Molecular Basis of Cancer-Cell Behavior. In: Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.
- American Association for Cancer Research (AACR). Cancer Hallmarks: Reprogramming Cellular Metabolism. Published 2022.
- Alberts B, Johnson A, Lewis J, et al. Universal Mechanisms of Cell-Cycle Division and Proliferation. In: Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.
Note on External Links: External links were checked and accessible when this article was published. Their content and availability may change over time and are outside the control of BiologyWithin.com.
Chris is a biologist dedicated to making complex biological and physiological sciences clear, rigorous, and accessible.
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
Comments
Post a Comment