RNA Editing Enzyme ADAR1 Emerges as Key Switch That Turns Cold Bladder Tumors Hot

Bladder cancer has long been one of the stubborn outliers in the immunotherapy revolution. While immune checkpoint inhibitors have transformed the treatment of melanoma, lung cancer and several other malignancies, many patients with bladder cancer fail to respond, and researchers have increasingly pointed the finger at the tumor microenvironment. These tumors are often described as immunologically cold, meaning they are shrouded in an immunosuppressive atmosphere that keeps cytotoxic immune cells out and allows malignant cells to grow largely unchecked. Now, a study published in the journal Molecular Cancer by a team at the First Affiliated Hospital of Nanjing Medical University offers a compelling explanation for why this happens and, more importantly, a strategy for reversing it. The researchers report that an RNA editing enzyme called ADAR1 acts as a gatekeeper of immune exclusion in bladder cancer, and that disabling it can convert a cold tumor into a hot one that responds robustly to PD-1 blockade therapy.

The enzyme at the center of the study, adenosine deaminase acting on RNA 1, or ADAR1, is one of the principal architects of the human transcriptome. It catalyzes the conversion of adenosine to inosine, a process known as A-to-I editing, in double-stranded RNA regions. Because cellular machinery reads inosine as guanosine, this editing can recode messenger RNA transcripts, alter RNA structures, and influence how RNAs are processed, translated or degraded. ADAR1 also plays a well-documented role in innate immunity, where it marks endogenous double-stranded RNA as self and prevents spurious activation of antiviral sensors. In cancer, elevated ADAR1 activity has been linked to tumor progression and immune evasion in several contexts, but its precise contribution to how bladder cancer evades immunotherapy had remained unclear until now.

Using a combination of whole transcriptome sequencing, whole genome sequencing, immunohistochemistry, and patient-derived materials, the team, led by corresponding authors Haiwei Yang, Qiang Lu and Xiao Yang, established that targeting ADAR1 in bladder cancer cells fundamentally reshapes the immune landscape of the tumor microenvironment. When ADAR1 was disabled in experimental models, two critical changes occurred. First, tumors became infiltrated by significantly more CD8-positive T cells, the cytotoxic soldiers of the adaptive immune system that are responsible for directly killing cancer cells. Second, tumor cells increased their expression of PD-L1, the checkpoint ligand through which cancers normally suppress T cell activity. That second change might at first seem counterproductive, but it is precisely what makes the tumors vulnerable to PD-1 blockade: a tumor expressing high levels of PD-L1 but crowded with CD8-positive T cells becomes an ideal target for checkpoint inhibitors, which release the molecular brakes on the waiting immune cells.

To dissect the mechanism, the researchers engineered a mutant form of ADAR1, designated ADAR1-E912A, that is defective in its RNA editing catalytic activity. Comparing cells carrying this editing-defective mutant with cells carrying wild-type ADAR1 allowed them to separate the consequences of the enzyme’s editing function from its other activities. The editing-defective mutant preserved the ability to promote CD8-positive T cell infiltration, and the team traced this effect to the chemokine CCL5, a signaling molecule known to recruit T cells to sites of inflammation and malignancy. Cells expressing the editing-defective mutant produced and secreted more CCL5, creating a chemotactic gradient that drew cytotoxic T lymphocytes into the tumor.

The molecular explanation for the increased CCL5 proved to be a matter of RNA stability. ADAR1 normally edits the messenger RNA encoding CCL5, and these edited transcripts are recognized by endonuclease V, an enzyme that cleaves inosine-containing RNA and thereby destines it for degradation. When ADAR1’s editing function was lost, CCL5 mRNA escaped this editing-dependent degradation pathway, remained stable for longer, and accumulated to higher levels within the cell. The result was a larger pool of CCL5 available for translation and secretion. In effect, ADAR1 acts as a post-transcriptional throttle on one of the most important T cell recruitment signals in the tumor, and switching off that throttle allows the tumor to summon the very immune cells it had been keeping at bay.

The second mechanism the team uncovered concerns PD-L1, the protein through which tumors engage the PD-1 checkpoint on T cells. The researchers found that ADAR1 cooperates with DICER, the cytoplasmic enzyme that processes microRNA precursors into their mature forms, to facilitate the maturation of a specific microRNA, miR-377-3p. Intriguingly, this cooperation did not require ADAR1’s editing catalytic activity, indicating that the protein performs a scaffolding or partner role in the microRNA biogenesis pathway independent of its enzymatic function. Mature miR-377-3p, in turn, represses the expression of PD-L1. In tumors where ADAR1 is abundant, the resulting PD-L1 suppression helps malignant cells keep a low immunological profile even as they hold cytotoxic T cells outside. When ADAR1 is removed, this repression is lifted, PD-L1 rises to the tumor cell surface, and the tumor becomes, paradoxically, an excellent candidate for PD-1 blockade because it now displays the molecular handle that checkpoint drugs are designed to disengage.

Perhaps the most clinically significant element of the study is the identification of an existing drug that can be repurposed as an ADAR1 inhibitor. Fludarabine, a nucleoside analogue long used as a chemotherapeutic agent in hematologic malignancies, was found to suppress ADAR1 on two fronts simultaneously: it inhibited the enzyme’s editing activity and reduced its expression. This dual suppression reproduced the effects seen with genetic targeting of ADAR1. In experimental systems, fludarabine treatment promoted CD8-positive T cell infiltration into tumors, increased PD-L1 expression on tumor cells, and sensitized bladder cancer to PD-1 blockade therapy in both in vitro and in vivo models. The finding suggests that a drug already approved and widely characterized in the clinic could be combined with checkpoint inhibitors to extend their benefits to bladder cancer patients who currently derive little benefit from immunotherapy.

The strength of the study lies partly in the breadth of its experimental evidence. The team worked across multiple platforms, including engineered bladder cancer cell lines with distinct ADAR1 statuses, co-immunoprecipitation and RNA immunoprecipitation assays to map protein and RNA interactions, immunofluorescence combined with fluorescence in situ hybridization to localize molecular events within cells, and quantitative PCR to quantify transcript changes. In vivo, they employed the N-butyl-N-(4-hydroxybutyl) nitrosamine model, a well-established chemical carcinogenesis system for bladder cancer, alongside co-culture experiments with peripheral blood mononuclear cells and experiments using patient-derived organoids that retain features of the original tumors. This triangulation across cell culture, animal models and human-derived material lends considerable weight to the conclusion that ADAR1 targeting genuinely reprograms the tumor microenvironment rather than merely altering isolated molecular readouts.

The implications extend beyond bladder cancer itself. Cold tumors across many cancer types share the fundamental problem of T cell exclusion, and any mechanism that reliably converts cold tumors into hot ones is of broad interest to the oncology community. If ADAR1 functions as a similar gatekeeper in other malignancies, the combination of an ADAR1 inhibitor with PD-1 blockade could represent a generalizable strategy. At the same time, the work highlights the dual and sometimes opposing roles that a single RNA binding protein can play: one activity, the editing of CCL5 mRNA, suppresses immune recruitment, while another, editing-independent cooperation with DICER, suppresses PD-L1 expression. Disentangling these functions will be important as inhibitors are developed and deployed. Fludarabine itself carries a known toxicity profile, and its dosing and safety when combined with checkpoint inhibitors in solid tumors will require careful clinical evaluation. Nevertheless, by identifying a druggable molecular switch that simultaneously pulls immune cells into the tumor and exposes a checkpoint target on the tumor cell surface, the Nanjing team has provided a persuasive mechanistic blueprint for turning one of immunotherapy’s most resistant solid tumors into a far more vulnerable one.

Subject of Research: Targeting the RNA editing enzyme ADAR1 to reprogram the cold tumor microenvironment of bladder cancer and enhance PD-1 blockade immunotherapy

Article Title: Targeting ADAR1 reprograms cold tumors to hot and enhances immunotherapy in bladder cancer

Article References: Bai, K., Zhuang, J., Yu, H., Lv, J., Chen, Y., Jiang, L., Li, K., Yang, H., Lu, Q., & Yang, X. (2026). Targeting ADAR1 reprograms cold tumors to hot and enhances immunotherapy in bladder cancer. Molecular Cancer. https://doi.org/10.1186/s12943-026-02778-4

Image Credits: AI Generated

DOI: 10.1186/s12943-026-02778-4

Keywords: ADAR1, bladder cancer, RNA editing, PD-1 blockade, fludarabine, CD8-positive T cells, PD-L1, CCL5, miR-377-3p, cold tumors, tumor microenvironment, cancer immunotherapy