Fasting Diet Triggers Anti-Tumor Immune Response in Macrophages via Protein Recycling
A fasting-mimicking diet activates IFNβ secretion in tumor macrophages by degrading the immune suppressor Trex1 through a NRF1-driven proteasome pathway.
Summary
Researchers discovered that a fasting-mimicking diet (FMD) boosts anti-tumor immunity by activating a molecular recycling program inside tumor-associated macrophages (TAMs). Under fasting conditions, the transcription factor NRF1 ramps up proteasome activity, which tags and destroys the immune-suppressing protein Trex1 via ubiquitin-dependent proteolysis. With Trex1 out of the way, mitochondrial DNA accumulates and activates the cGAS-STING signaling pathway, triggering release of interferon-beta (IFNβ) — a potent anti-tumor cytokine. When NRF1 was knocked out specifically in myeloid cells, Trex1 built up, blocked mtDNA sensing, and blunted the anti-tumor response. These findings reveal a new mechanism by which dietary restriction reprograms immune cells in the tumor microenvironment.
Detailed Summary
**Why This Matters:** The fasting-mimicking diet (FMD) is already being investigated in cancer clinical trials for its ability to sensitize tumors to therapy and modulate immunity. Yet until now, little was known about how FMD specifically reprograms tumor-associated macrophages (TAMs) — the dominant immune cells in most solid tumors. TAMs are often co-opted by tumors to suppress anti-cancer immunity, making them a critical but underexplored therapeutic target.
**What Was Studied:** This study used subcutaneous MC38 colorectal tumor models in wild-type and myeloid-specific NRF1 knockout (Mye-NFE2L1−/−) mice, with or without FMD and a pharmacological Trex1 inhibitor. In vitro, bone marrow-derived macrophages (BMDMs) were exposed to fasting medium and MC38 tumor supernatant to generate BMDM-derived TAMs (B-TAMs). The researchers profiled gene expression via RNA-seq, quantified IFNβ by ELISA, tracked protein ubiquitination and Trex1 levels by Western blot and co-immunoprecipitation, and measured mitochondrial DNA dynamics using qPCR and ChIP-qPCR.
**Key Results:** FMD induced energy stress in TAMs that activated nuclear translocation of NRF1, which transcriptionally upregulated proteasome subunit genes, enhancing ubiquitin-proteasome system (UPS) activity. This led to the targeted ubiquitination and degradation of Trex1 — a nuclease that normally degrades cytosolic DNA and suppresses innate immune sensing. With Trex1 degraded, mitochondrial DNA (mtDNA) accumulated in the cytosol, where it activated cGAS-STING signaling and drove robust IFNβ secretion. In tumors from Mye-NFE2L1−/− mice, this pathway was broken: without NRF1, proteasome activity was impaired, Trex1 accumulated, bound and degraded mtDNA, and IFNβ production was markedly reduced — correlating with faster tumor growth and blunted FMD efficacy. Combining FMD with a Trex1 inhibitor partially restored IFNβ signaling in NRF1-deficient settings.
**Implications:** This study reveals for the first time a complete mechanistic axis — FMD → NRF1 → UPS → Trex1 degradation → cGAS-STING → IFNβ — operating in TAMs to reshape the tumor microenvironment. It positions NRF1 as a metabolic checkpoint linking nutritional state to innate immune activation, and identifies Trex1 as a druggable node that, when inhibited, can mimic or amplify the immunostimulatory effects of dietary fasting in tumors.
**Caveats:** The study is primarily conducted in mouse colorectal tumor models and in vitro systems. Translation to human TAMs and diverse tumor types remains to be validated. The therapeutic window and clinical feasibility of combining FMD with Trex1 inhibition in patients has not been explored.
Key Findings
- FMD activates NRF1 nuclear entry in TAMs, upregulating proteasome subunit genes and UPS activity.
- NRF1-driven UPS degrades Trex1 via ubiquitination, de-repressing the cGAS-STING-IFNβ innate immune axis.
- FMD increases cytosolic mtDNA in TAMs, serving as a cGAS ligand to amplify IFNβ secretion.
- Myeloid-specific NRF1 knockout abolishes Trex1 degradation, blocks IFNβ, and attenuates FMD anti-tumor effects.
- Pharmacological Trex1 inhibition partially rescues IFNβ signaling and anti-tumor immunity in NRF1-deficient TAMs.
Methodology
Subcutaneous MC38 murine colorectal tumor models were established in wild-type and myeloid-specific NRF1 knockout C57BL/6 mice, treated with FMD and/or Trex1 inhibitor. In vitro studies used BMDMs exposed to fasting medium and tumor supernatant, with readouts including RNA-seq, ELISA, Western blot, Co-IP, qPCR, ChIP-qPCR, and immunofluorescence. The design is primarily mechanistic and preclinical.
Study Limitations
All key experiments were conducted in mouse models (MC38 colorectal tumors) and in vitro macrophage systems, limiting direct translation to human cancer. The study does not address whether FMD duration, cycling frequency, or tumor type affects the NRF1-Trex1-IFNβ axis. Long-term safety and efficacy of combining FMD with Trex1 inhibition in patients has not been evaluated.
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