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Violet light induces apoptosis via the reactive oxygen species-mitochondrial signaling pathway in chordoma

This study demonstrates that violet light induces apoptosis in chordoma cells by increasing reactive oxygen species levels and triggering mitochondrial damage, suggesting a promising new therapeutic approach for this radiotherapy-resistant tumor.

Original authors: Shunsuke Tamaki, Toshihiko Nishisho, Makoto Takeuchi, Shunichi Toki, Shinji Kawaguchi, Hiroaki Manabe, Keisuke Nishidono, Koichi Sairyo

Published 2026-07-12

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Shunsuke Tamaki, Toshihiko Nishisho, Makoto Takeuchi, Shunichi Toki, Shinji Kawaguchi, Hiroaki Manabe, Keisuke Nishidono, Koichi Sairyo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a stubborn, slow-growing monster called a chordoma. It's a rare bone tumor that likes to hide deep in the spine or skull, often growing so big and tangled with nerves before anyone notices it that surgeons can't get it all out. Once it's there, it's tough to defeat with standard weapons like radiation or chemotherapy; it often just says, "Nope, I'm staying," and comes back.

But what if we could zap it with a specific color of light? That's exactly what a team of researchers at Tokushima University and Shizuoka Cancer Center decided to test. They didn't use a giant laser or a sledgehammer; they used a gentle, low-power glow from a light-emitting diode (LED), kind of like a very specific nightlight for cells.

The Color Test: Finding the Magic Bullet

The scientists set up a little experiment with two types of chordoma cells (named MUG-chor1 and U-CH1). They shined four different colors of light on them: red, green, blue, and violet.

Think of the cells as little factories. The researchers wanted to see which color of light would make the factory stop working.

Red, Green, and Blue lights? The cells didn't care much. They kept chugging along, dividing, and growing almost as if the lights weren't even there.
Violet light? This was the game-changer. When the cells were bathed in violet light (specifically peaking at 405 nm), they started to panic. The more time they spent in the violet glow, the more they slowed down. After 24 hours of exposure, the cells' survival rate dropped to about 40%. That's a massive drop compared to the other colors, which barely made a dent.

The researchers also cranked up the brightness of the violet light to 0.7, 0.9, and 1.7 mW/cm². They found that the brighter the light (up to 1.7 mW/cm²), the more the cells struggled to survive. It was a clear "dose-dependent" effect: more light meant more trouble for the tumor.

How the Light Wins: The "Oxidative Stress" Bomb

So, how does a gentle violet light take down a tough tumor? It doesn't burn them up with heat. The team checked the temperature and found that even after 24 hours of shining, the cells only got about 1°C warmer than the room they were in. That's basically the same temperature as a cup of tea left on a counter. So, it wasn't a heat attack; it was a chemical one.

Here's the cool part: The violet light acted like a trigger for a chemical bomb inside the cell called Reactive Oxygen Species (ROS).

The Analogy: Imagine the cell is a quiet library. The violet light is like someone throwing a handful of glitter bombs (ROS) into the room. Suddenly, the library is chaotic. The glitter (ROS) starts damaging the shelves and the books.
The Damage: The researchers saw that the violet light caused a huge spike in these "glitter bombs" all over the cell, but especially inside the mitochondria. Mitochondria are the cell's power plants. When the glitter bombs hit the power plant, the lights flickered, the energy production crashed, and the whole system started to fail.

The Final Blow: The Cell's Self-Destruct Button

Once the mitochondria were damaged, the cell didn't just give up; it hit the self-destruct button. This process is called apoptosis, or programmed cell death.

The violet light caused the cell's "power plant" to leak a protein called cytochrome c.
This leak turned on a chain reaction of enzymes called caspases (specifically caspase-9).
Think of caspases as the scissors that cut the cell's life support. Once they started cutting, the cell couldn't stop. It rounded up, floated away, and died.

The team proved this by using a special "stop" button. They added a chemical called Z-VAD that blocks caspases. When they did this, the violet light couldn't kill the cells anymore. The cells survived! This proved that the light kills the cells specifically by turning on this caspase scissors chain.

They also used another chemical, NAC, which acts like a vacuum cleaner for the "glitter bombs" (ROS). When they used NAC, the violet light lost its power. The cells didn't die. This confirmed that the "glitter bombs" (ROS) were the essential first step that led to the mitochondria failing and the cell dying.

What About Moving and Invading?

Tumors are scary because they don't just sit there; they move and invade other tissues. The researchers tested if the violet light stopped the chordoma cells from migrating (moving) or invading (breaking through barriers).

Migration: In a "wound healing" test, where they watched how fast cells moved to fill a gap, the violet-light group moved much slower than the normal group.
Invasion: In a test where cells tried to crawl through a tough mesh (Matrigel), the violet-light group was stopped dead in their tracks. They simply couldn't break through.

The Catch and The Future

So, is this a magic cure? The paper suggests it's a very promising new approach, but there are some big "buts."

The Depth Problem: Violet light is great at killing cells, but it's a shy traveler. It can only penetrate about 1 mm into skin. Chordomas, however, are often deep inside the body, near the spine or skull. You can't just shine a flashlight on your back and expect it to reach the tumor.
The Solution? The authors suggest that instead of shining light from the outside, doctors might one day implant a tiny, wireless LED inside the body, right next to the tumor, after surgery. This would let the light do its work without having to travel through miles of skin and muscle.
The Status: Right now, this is all in vitro, meaning it happened in a petri dish in a lab. The researchers are very clear that this is a "suggestion" of a new path, not a finished cure. They need to test it in living animals (in vivo) to see if it works in a real body.

The Bottom Line

This study suggests that violet light (405 nm) can be a powerful weapon against chordoma cells by creating a chemical chaos (ROS) that breaks the cell's power plant and forces it to self-destruct. It's not a heat burn, and it's not a magic wand that works from the outside of the body yet. But for a tumor that is usually hard to treat and hard to remove, the idea of using a tiny, implantable light source to finish the job is a fascinating new possibility that scientists are now eager to explore further.

Technical Summary: Violet Light Induces Apoptosis via the Reactive Oxygen Species-Mitochondrial Signaling Pathway in Chordoma

Problem Statement
Chordoma is a rare, primary malignant bone tumor arising from embryonic notochordal tissue, predominantly located in the axial skeleton (skull base, spine, sacrum). Despite being slow-growing, chordomas are often diagnosed at advanced stages with significant local infiltration into vascular and nerve bundles, rendering complete surgical resection difficult. Consequently, local recurrence is a major clinical issue. Current adjuvant therapies, including radiotherapy and chemotherapy, often yield poor responses due to drug resistance, and the 5-year survival rate remains between 45% and 67%. There is an urgent need for novel therapeutic strategies to improve prognosis for this treatment-resistant malignancy. While photobiomodulation (PBM) using light-emitting diodes (LEDs) has shown antitumor effects in other malignancies (e.g., synovial sarcoma, melanoma), its efficacy and mechanisms in chordoma remain unexplored.

Methodology
The study utilized two human chordoma cell lines, MUG-chor1 and U-CH1, cultured under standard conditions. The researchers employed a Teleopto LED array system to deliver continuous, low-intensity light irradiation (1.7 mW/cm²) from the bottom of multi-well plates within a CO₂ incubator. Four distinct wavelengths were tested: red (630 nm), green (525 nm), blue (470 nm), and violet (VL, 405 nm).

To evaluate antitumor effects, the following assays were conducted:

Cell Viability: Measured via CCK-8 assay at 8, 16, and 24 hours post-irradiation across varying intensities and durations.
Migration and Invasion: Assessed using wound healing and Transwell Matrigel invasion assays, respectively.
Apoptosis Detection: Quantified using Annexin V/Propidium Iodide (PI) flow cytometry and Western blotting for cleaved Poly (ADP-ribose) polymerase (PARP).
Mechanistic Pathway Analysis:
- Reactive Oxygen Species (ROS): Intracellular and mitochondrial ROS levels were measured using CellROX Green and MitoSOX Red fluorescent probes via flow cytometry and microscopy.
- Mitochondrial Function: Mitochondrial membrane potential (MMP) was assessed using JC-1 dye; oxygen consumption rates (OCR) were measured via Seahorse XF Cell Mito Stress Assay.
- Protein Expression: Western blotting analyzed levels of Bcl-2, Bax, cytochrome c, caspase-9, and heme oxygenase-1 (HO-1).
Pathway Verification: The study utilized specific inhibitors: Z-VAD-FMK (a pan-caspase inhibitor) to block apoptosis and N-acetyl-L-cysteine (NAC) to scavenge ROS, observing their impact on cell viability and mitochondrial function under VL irradiation.
Control for Thermal Effects: Temperature measurements confirmed that the irradiation caused less than a 1°C rise, ensuring observed effects were due to light photobiology rather than thermal damage.

Key Results

Wavelength Specificity: Among the four wavelengths tested, only violet light (405 nm) significantly inhibited chordoma cell proliferation in a time- and dose-dependent manner. Red, green, and blue light showed no significant cytotoxic effects. At 24 hours, VL reduced cell viability to approximately 40% for both cell lines.
Inhibition of Metastatic Potential: VL irradiation significantly suppressed cell migration (wound healing) and invasion (Transwell assay) compared to non-irradiated controls.
Induction of Apoptosis: Flow cytometry and Western blotting confirmed that VL irradiation triggered apoptosis, evidenced by increased Annexin V/PI positive cells and elevated levels of cleaved PARP.
ROS Generation: VL irradiation caused a significant, time-dependent increase in total intracellular ROS and, specifically, mitochondrial ROS. This was accompanied by an upregulation of HO-1, an oxidative stress response protein.
Mitochondrial Dysfunction: VL exposure led to a collapse of mitochondrial membrane potential (shift from JC-1 aggregates to monomers) and a suppression of mitochondrial respiration (reduced basal respiration, ATP production, and maximal respiration).
Signaling Pathway: The study established a causal chain:
- VL irradiation $\rightarrow$ Increased ROS $\rightarrow$ Mitochondrial damage (MMP collapse, Cytochrome c release) $\rightarrow$ Activation of the intrinsic apoptotic pathway (Upregulation of Bax, Downregulation of Bcl-2, Activation of Caspase-9) $\rightarrow$ Apoptosis.
- Inhibition Validation: The addition of the ROS scavenger (NAC) or the caspase inhibitor (Z-VAD-FMK) significantly reversed the cytotoxic effects of VL, restored cell viability, and prevented apoptosis, confirming that ROS-mediated mitochondrial dysfunction and caspase activation are essential for the observed cell death.

Significance and Claims
The paper claims to demonstrate, for the first time, that low-intensity violet light (405 nm) LED irradiation exerts potent antitumor effects on chordoma cells in vitro. The study elucidates that this effect is not thermal but photobiological, mediated specifically through the generation of reactive oxygen species that trigger the intrinsic mitochondrial apoptotic pathway.

The authors position this finding as a potential new treatment avenue for chordoma, a tumor notoriously resistant to conventional therapies. They suggest that while the limited tissue penetration depth of violet light (~1 mm) precludes external irradiation for deep-seated tumors, the technology could be viable as an adjunctive therapy. Specifically, they propose the feasibility of implanting miniaturized, wireless LED sources directly into the tumor bed post-resection to prevent local recurrence. The study concludes that these in vitro results provide a foundation for future in vivo research to validate photobiomodulation as a therapeutic option for difficult-to-treat bone tumors.