In Aging Cell, researchers have published an explanation of how neutrophil extracellular traps (NETs) worsen UVB damage and how inhibiting them alleviated this damage in a mouse model.
When defenders just make things worse
Immune activation is a common response to damage, as the immune system often perceives such damage as a pathogenic attack. Activation of immune system components in the absence of actual pathogens is known as sterile inflammation, and it is a frequent subject of aging research; chronic sterile inflammation is known as inflammaging.
Damage done by UVB radiation from the sun causes an immune reaction that brings neutrophils into the skin. Exposed to this radiation themselves, these immune cells react by flooding the area with inflammatory cytokines, recruiting even more neutrophils and creating systemic inflammation that can even affect the kidneys [1].
NETs are created when neutrophils destroy themselves in a controlled demolition that releases proteins and DNA from their nuclei, and as their name suggests, NETs trap and kill pathogens. However, in sterile inflammation conditions, excessive NETs lead to autoimmune disorders, including rheumatoid arthritis [2], and previous work has found that reducing NETs alleviates UVB-related skin damage in mice [3]. This research builds upon that work, focusing on PAD4, a nuclear enzyme that is required for NET formation [4].
Stopping NETs at their source
The researchers first examined tissue samples derived from the lesions of human patients with actinic keratosis or chronic actinic dermatitis, two conditions that are related to sun damage of the skin. Compared to a control group, these samples had increased numbers of NETs.
They then began experimenting on mice, exposing the skin of wild-type Black 6 mice to UVB radiation and giving some of the exposed mice GSK484, a compound that inhibits PAD4. Compared to the untreated mice exposed to UVB, the GSK484-treated group had much less visible skin damage and reduced skin erosion, confirming previous research [5]. While it did not fully protect the mice from UVB damage, skin thickening, inflammatory biomarkers, and measurements of oxidative stress were all significantly reduced compared to the untreated group.
Cellular death by apoptosis was reduced as well. Fibroblasts and keratinocytes normally die in this way when exposed to UVB, but the rate of apoptosis, according to multiple biomarkers, was substantially reduced when GSK484 was administered.
RNA sequencing was performed to determine the pathways involved. The JNK branch of the well-known inflammation pathway MAPK, which was previously reported to be involved in NET-related damage [6] and was found to be linked to UVB damage in this study, was strongly inhibited by GSK484.
HaCaT cells, a line of immortalized human skin cells, were used in another experiment. The researchers exposed neutrophils to UVB radiation in order to generate NETs, which were then delivered to these cells. At 4 milligrams per liter of NETs, the HaCaT cells became unable to proliferate, began expressing inflammatory cytokines, experienced oxidative stress, and activated the JNK pathway, leading to apoptosis. However, exposing NETs to DNAse I before delivery degraded them, significantly reducing their negative effects on HaCaT cells.
A receptor may be a target
The researchers then investigated CCDC25, a gene in keratinocytes that responds to NET exposure. HaCaT cells that had their RNA expression of this gene silenced barely reacted to NETs, having no measurable increase in the JNK pathway and only slightly increased apoptosis.
These results were confirmed in mice. Using an adeno-associated virus (AAV) to block CCDC25 in UVB-exposed mice yielded similar results to GSK484 treatment: inflammation was significantly decreased, additional skin thickness was reduced, and markers of oxidative stress were lessened compared to the UVB-exposed and untreated control group.
This research opens up new avenues for potential treatments that might reduce long-term sun damage. While stopping the radiation itself can only be done through direct preventative measures, the immune system’s role in worsening this damage may be a treatable target. Preventing unnecessary NET formation may help curb inflammaging and reduce skin damage.
Literature
[1] Skopelja-Gardner, S., Tai, J., Sun, X., Tanaka, L., Kuchenbecker, J. A., Snyder, J. M., … & Elkon, K. B. (2021). Acute skin exposure to ultraviolet light triggers neutrophil-mediated kidney inflammation. Proceedings of the National Academy of Sciences, 118(3), e2019097118.
[2] Fousert, E., Toes, R., & Desai, J. (2020). Neutrophil extracellular traps (NETs) take the central stage in driving autoimmune responses. Cells, 9(4), 915.
[3] Inaba, I., Hiramoto, K., Yamate, Y., Morita, A., Tsutsumi, T., Yasuda, H., & Sato, E. F. (2024). Inhibiting neutrophil extracellular traps protects against ultraviolet B-induced skin damage: effects of Hochu-ekki-to and DNase I. International Journal of Molecular Sciences, 25(3), 1723.
[4] Mutua, V., & Gershwin, L. J. (2021). A review of neutrophil extracellular traps (NETs) in disease: potential anti-NETs therapeutics. Clinical reviews in allergy & immunology, 61(2), 194-211.
[5] Inaba, I., Hiramoto, K., Yamate, Y., Morita, A., Tsutsumi, T., Yasuda, H., & Sato, E. F. (2024). Inhibiting neutrophil extracellular traps protects against ultraviolet B-induced skin damage: effects of Hochu-ekki-to and DNase I. International Journal of Molecular Sciences, 25(3), 1723.
[6] Hu, Z., Hua, X., Mo, X., Chang, Y., Chen, X., Xu, Z., … & Song, J. (2023). Inhibition of NETosis via PAD4 alleviated inflammation in giant cell myocarditis. IScience, 26(7).
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