How a Single Transcription Factor Decides Whether Sunburned Skin Cells Live or Die

Every time skin is exposed to sunlight, ultraviolet B radiation slams into the DNA of keratinocytes, the workhorse cells that make up the epidermis. These cells respond with a well-mapped playbook: cell cycle checkpoints pause division, repair enzymes patch the damage, and if the injury is too severe, the cell dismantles itself through apoptosis. But a new study from North Carolina State University reveals that this canonical picture is incomplete. A transcription factor called C/EBPβ, long known as a regulator of inflammation and metabolism, turns out to act as a molecular gatekeeper that decides whether UVB-damaged skin cells quietly survive or die by a distinctly inflammatory form of cell death.

The research, published in Frontiers in Cell Death, shows that removing C/EBPβ from mouse keratinocytes dramatically sensitizes them to UVB-induced apoptosis. When the team silenced C/EBPβ in cultured keratinocytes using small interfering RNA and then exposed the cells to UVB light, cell death more than doubled compared with controls at multiple time points. Flow cytometry using Annexin V and propidium iodide staining confirmed that the dying cells were undergoing apoptosis, and immunoblotting revealed enhanced cleavage of caspase-3, the executioner enzyme that disassembles the cell from within. Strikingly, caspase-3 activation appeared in C/EBPβ-deficient cells at UVB doses far too low to kill normal keratinocytes, and a pan-caspase inhibitor abolished the death entirely, confirming that the process was genuinely apoptotic.

The execution pathway itself proved to be the extrinsic, or death-receptor, route rather than the intrinsic mitochondrial one. Caspase-8, the initiator caspase of extrinsic apoptosis, showed early and robust cleavage in the C/EBPβ-deficient cells, including the fully activated p18 subunit that was undetectable in controls. Downstream, caspase-8 cleaves the BH3-only protein Bid into its truncated form, tBid, which triggers mitochondrial outer membrane permeabilization and amplifies the caspase cascade. In the deficient keratinocytes, total Bid was markedly depleted, indicating active consumption by caspase-8. When the researchers knocked down Bid alongside C/EBPβ, or blocked tBid’s mitochondrial action with the small molecule inhibitor BI-6C9, the surge in caspase-3 activation collapsed. The death signal, in other words, flows through caspase-8 and Bid with remarkable dependence.

What was driving this extrinsic program? Because extrinsic apoptosis is normally initiated by external signals, the team turned to transcriptomics. They sequenced RNA from UVB-treated primary keratinocytes isolated from C/EBPβ knockout and wild-type newborn mice, identifying 1,732 differentially expressed genes. Gene set enrichment analysis delivered an unambiguous verdict: the Hallmark Interferon Alpha response and the Hallmark Interferon Gamma response were the two most positively enriched pathways in the knockout cells. Among the upregulated genes were numerous interferon-stimulated genes, including Xaf1, a pro-apoptotic factor known to sensitize cells to TNF-induced death by destabilizing the XIAP survival complex.

To test whether this interferon storm was merely a byproduct or the actual engine of cell death, the researchers ran a series of loss-of-function experiments. Silencing IFNAR1, the receptor subunit that transduces type I interferon signals, substantially reduced UVB-induced apoptosis in C/EBPβ-deficient keratinocytes. A pharmacological approach told the same story: BMS986165, an inhibitor of Tyk2, the Janus family kinase that sits immediately downstream of the interferon receptor, suppressed interferon-stimulated gene expression and blocked the induction of apoptosis. The conclusion was inescapable. Without C/EBPβ, keratinocytes respond to UVB not just with DNA repair but with a full-throttle interferon program, and that program is functionally required for the enhanced cell death.

Interferon, however, was only half of the story. Ingenuity Pathway Analysis of the sequencing data, which predicts upstream regulators from expression patterns, ranked tumor necrosis factor alpha as the single most significant activated upstream regulator in the UVB-treated knockout cells. Quantitative PCR showed roughly a threefold increase in Tnfa transcripts, and ELISA measurements of the culture medium revealed approximately 2.5-fold more secreted TNF-α protein compared with wild-type controls. When the researchers pretreated the knockout keratinocytes with a TNF-α neutralizing antibody before UVB exposure, apoptosis dropped significantly. TNF-α, it seems, is not an innocent bystander but a direct murder weapon in this pathway.

The two arms of the response are also intertwined. Pretreating C/EBPβ-deficient keratinocytes with the Tyk2 inhibitor BMS986165 not only silenced interferon-stimulated genes but also slashed TNF-α transcript levels after UVB exposure, suggesting that interferon signaling feeds forward into TNF-α production. This kind of cross-priming, in which basal type I interferon activity heightens a cell’s responsiveness to other cytokines, has been documented in monocytes and macrophages, and the new data extend it to skin. The authors propose that C/EBPβ normally restrains the interferon response, and that its absence leaves keratinocytes pre-primed so that UVB damage tips them into a synergistic interferon-TNF death spiral.

The findings carry weight well beyond sunburn biology. Type I interferon and TNF-α are central effectors of innate immunity, and their chronic or misregulated activation underlies autoimmune diseases such as cutaneous lupus erythematosus, in which patients’ skin cells are notoriously hypersensitive to UVB. The authors note that a short interferon burst after UVB is protective, dampening skin inflammation, whereas sustained interferon expression in lupus exacerbates disease through heightened cytokine secretion and UVB-triggered flares. Understanding how C/EBPβ restrains these pathways could illuminate why lupus patients burn so easily and point toward ways to modulate the response therapeutically.

There is also an oncological dimension. Previous work from the same laboratory showed that deleting C/EBPβ from established mouse skin tumors triggers interferon upregulation, increased apoptosis, and tumor regression, a synthetic lethal event. Meanwhile, cancer immunotherapy strategies are actively exploring deliberate activation of the cGAS-STING DNA-sensing pathway to provoke interferon-driven tumor cell death. The new study adds mechanistic texture to these observations by identifying the extrinsic apoptotic machinery, caspase-8, Bid, and TNF-α, as the downstream effectors that connect interferon signaling to actual cell killing in skin.

What emerges is a revised model of how skin handles solar DNA damage. Alongside the canonical checkpoint-and-repair response, UVB activates a non-canonical inflammatory branch in which DNA fragments released from the nucleus and mitochondria can be sensed as danger signals, igniting interferon production. C/EBPβ, abundantly expressed in epidermal keratinocytes, appears to hold this branch in check, buying cells time to repair. When the factor is lost, the inflammatory program runs hot, TNF-α accumulates, death receptors engage, and caspase-8 carves the cell apart. The next questions, the authors say, are how precisely C/EBPβ regulates the interferon response after UVB, and whether tuning its activity up or down could restore proper cell death regulation in inflammatory skin disease or be exploited to eliminate cancer cells that have learned to survive.

Subject of Research: How C/EBPβ regulates interferon and TNF signaling to control keratinocyte apoptosis after UVB-induced DNA damage

Article Title: C/EBPβ deficiency enhances keratinocyte apoptosis after UVB-induced DNA damage via regulation of the type I IFN and TNF responses

Article References: Tobin, E. E., Sharma, A., Kros, S. T., Jima, D. D., Gray, S. C., & Hall, J. R. (2025). C/EBPβ deficiency enhances keratinocyte apoptosis after UVB-induced DNA damage via regulation of the type I IFN and TNF responses. Frontiers in Cell Death, 4, Article 1658598. https://doi.org/10.3389/fceld.2025.1658598

Image Credits: AI Generated

DOI: 10.3389/fceld.2025.1658598

Keywords: C/EBPβ, keratinocytes, UVB radiation, apoptosis, type I interferon, TNF-alpha, caspase-8, Bid, DNA damage response, skin cancer, cutaneous lupus, innate immunity