At the cellular level, a bad sunburn is not just a splash of red on the skin, it is a cascading molecular alarm that can tip healthy tissue toward cancer. A new study in Nature Communications pinpoints a single RNA binding protein, YTHDF2, as a crucial brake on this process, revealing how ultraviolet B (UVB) light ramps up inflammation and nudges skin cells toward tumor formation.
Working with human keratinocytes, skin cancer cell lines, and mouse models, researchers at the University of Chicago show that YTHDF2 controls the fate of a small non coding RNA called U6. When this control is lost after UV exposure, U6 accumulates in the wrong place inside the cell and activates an innate immune sensor, Toll like receptor 3 (TLR3), driving chronic inflammation and, over time, UV induced skin tumors.
A Molecular Gatekeeper For Sunburn Inflammation
The team focused on N6 methyladenosine (m6A), a common chemical tag on RNA that is written by enzymes such as METTL16 and read by proteins like YTHDF2. In this case, the key target is U6 small nuclear RNA, best known as a core component of the spliceosome but also found, transiently, outside the nucleus. The study shows that METTL16 adds m6A to U6, and YTHDF2 then recognizes this modified U6 and promotes its decay, keeping its levels in check.
“When we removed YTHDF2 from skin cells, we saw that UV-triggered inflammation was much worse,” He said. “This suggests that the YTHDF2 protein plays a key role in suppressing inflammatory responses.”
RNA sequencing in YTHDF2 depleted keratinocytes revealed broad upregulation of inflammatory pathways, including TNF, IL 17, interleukin signaling, and the TLR3 cascade. In both human HaCaT keratinocytes and normal human epidermal keratinocytes, knocking down YTHDF2 boosted expression of classic inflammatory mediators such as TNF alpha, IL 6, COX 2, and IL 1 beta after UVB exposure, and increased baseline inflammatory gene expression in A431 skin cancer cells.
In mice engineered to lack Ythdf2 specifically in the skin, UVB produced thicker epidermis, more immune cell infiltration, and heightened inflammatory responses compared with wild type animals. Over chronic UVB exposure, these Ythdf2 deficient mice developed more tumors, including squamous cell carcinoma, demonstrating that the protein ordinarily suppresses both inflammation and tumor initiation in vivo.
Self RNA Becomes A Danger Signal In The Wrong Compartment
Mechanistically, the work reveals that U6 behaves as a self derived danger signal when its metabolism and trafficking are disrupted. YTHDF2 binds m6A modified U6 and promotes its decay through recruitment of the CNOT1 containing CCR4 NOT deadenylase complex. When YTHDF2 or METTL16 are knocked down, U6 becomes more stable, its levels rise, and more of it appears in the cytoplasm and within endosomes.
“Our study uncovers a new layer of biological regulation, a surveillance system through YTHDF2 that helps protect the body from excessive inflammation and inflammatory damage,” He said.
Within endosomes, U6 snRNA binds directly to the LRR21 domain of TLR3, distinct from the LRR20 domain that recognizes the synthetic agonist poly I C. This interaction activates TLR3 dependent inflammatory programs and drives cytokine production. Importantly, the authors show that YTHDF2 and U6 both localize to endosomes via the RNA transporter SIDT2 and dynamin dependent endocytosis. When YTHDF2 is present, it competes with TLR3 for m6A modified U6 and promotes its degradation, limiting TLR3 activation. When YTHDF2 is absent, U6, especially its m6A modified fraction, binds more readily to TLR3 and amplifies inflammatory signaling.
UVB exposure undermines this protective system in two ways. First, it rapidly induces dephosphorylation of YTHDF2 at serine 39, which weakens its interaction with CNOT1, reduces endosomal localization, and slows U6 decay. Second, UVB promotes p62 dependent autophagic degradation of YTHDF2, reducing protein levels even as YTHDF2 mRNA rises. In keratinocytes and in mouse skin, YTHDF2 protein dips at the same time that U6 levels and inflammatory gene expression peak after UVB, then recover later as inflammation subsides.
The downstream consequences extend beyond basic immunology. In A431 skin cancer cells, loss of YTHDF2 increases proliferation and migration in vitro, and accelerates tumor growth in xenograft models, effects that are blunted by knocking down U6 or TLR3, or by pharmacologically blocking COX 2 with celecoxib. Human skin squamous cell carcinoma samples show reduced YTHDF2 expression compared with normal epidermis, with lower levels in more advanced tumors, suggesting that this YTHDF2 U6 TLR3 axis is perturbed in clinical disease as well.
Together, the study reframes sunburn as a failure of self RNA surveillance. By reading an m6A mark on U6 and escorting this RNA through endosomal checkpoints, YTHDF2 prevents a housekeeping molecule from masquerading as viral RNA and lighting up inflammatory alarms that can, over time, fuel skin cancer. Understanding this axis, the authors argue, could open new therapeutic paths that target RNA modifications or YTHDF2 activity to dampen harmful inflammation without shutting down the immune system entirely.
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