Caloric Restriction Makes Colorectal Tumors More Vulnerable to Radiation in Mice

Caloric restriction and short-term fasting made colorectal tumors in mice markedly more vulnerable to radiation, and did so through a specific molecular route: the cGAS-STING innate immune pathway. At the same time, the dietary interventions shielded normal intestinal tissue from the damage that radiotherapy typically inflicts. That dual effect, reported by researchers at Soochow University and collaborating institutions in the journal Cancer & Metabolism, is exactly what oncologists would want from a radiosensitizing strategy, but the findings so far come only from mice and cell cultures, and the authors themselves flag substantial obstacles before any clinical translation.

The experiment used 48 male BALB/c mice bearing subcutaneous CT-26 colorectal tumors. Once tumors reached roughly 60 cubic millimeters, the animals were divided into four groups of twelve: an untreated control, irradiation alone, seven days of 30% caloric restriction before irradiation, and a 24-hour fast before irradiation. The restricted group received 2.1 grams of food per day, corresponding to 70% of normal intake as established in a preliminary experiment. All irradiated mice received a single 4 Gy dose of abdominal X-rays, with the rest of the body protected by lead shielding. Mice were sacrificed on day nine after irradiation, when control-group tumors reached a predetermined endpoint size.

The results pointed clearly in one direction. Radiation alone suppressed tumor growth, but the combination with caloric restriction produced a more pronounced reduction in tumor size, and tumor weights in both the restricted and fasted groups were significantly lower than in controls. Under the microscope, tumors from the diet-plus-radiation groups showed less cellularity and more fibrosis and necrosis. Radiation reduced Ki67 staining, a marker of proliferation, and the dietary interventions deepened that reduction. The proportion of TUNEL-positive cells, which indicates apoptosis, climbed after radiation and rose further with either dietary pretreatment. Transmission electron microscopy revealed classic apoptotic morphology, including shrunken cells, nuclear condensation, and chromatin marginalization, and Western blotting showed higher levels of cleaved caspase-3, a central executioner of programmed cell death, in the calorie-restricted, irradiated tumors.

The mechanistic core of the study lies in how radiation damage is converted into an immune alarm. Radiation kills cells largely by causing DNA double-strand breaks, and fragments of DNA that escape into the cytoplasm are detected by the sensor cyclic GMP-AMP synthase, or cGAS. cGAS activates the stimulator of interferon genes, STING, which triggers inflammatory signaling through TBK1, IRF3, and NF-kappaB and initiates an antitumor immune response. In the irradiated tumors, caloric restriction further elevated cytosolic double-stranded DNA levels beyond what radiation achieved alone, and fasting raised levels of gamma-H2AX, a widely accepted marker of DNA damage. Protein levels of phosphorylated STING, TBK1, IRF3, and NF-kappaB were increased in the calorie-restricted group, and messenger RNA for interferon-beta and the chemokines Ccl5, Cxcl10, and Cxcl11 were upregulated, consistent with a stronger interferon-driven alarm signal.

That alarm appeared to reshape the tumor microenvironment. Flow cytometry showed that radiation increased the proportion of CD3-positive CD8-positive cytotoxic T lymphocytes within the tumors, an effect further enhanced by both dietary interventions. Among tumor-associated macrophages, radiation increased the M1, or antitumor, subset marked by CD86, and the diets amplified this shift. The two interventions diverged on the M2 side: calorie restriction significantly suppressed the pro-tumor CD206-positive M2 population, whereas fasting left relatively higher M2 levels. The authors attribute this difference to the distinct metabolic character of the two regimens, suggesting that sustained moderate restriction produces stable metabolic adaptation favoring an M1-skewed immune profile, while acute fasting triggers harsher stress responses that are less effective at blocking M2 polarization. Pro-inflammatory cytokine genes, including IL-6, IL-1 beta, and TNF-alpha, were also upregulated in the diet-treated groups.

Metabolism supplied a second line of attack. Cancer cells rely heavily on aerobic glycolysis, the Warburg effect, and enhanced glycolysis has been implicated in radioresistance. In the irradiated tumors, protein levels of the glycolytic enzymes HK2 and LDHA were already reduced, and the dietary interventions pushed key players down further: GLUT1 declined in both diet groups, LDHA fell additionally with fasting, and tumor lactate concentrations dropped markedly with calorie restriction. Enzymes of fatty acid metabolism, including FASN, ACC, HSL, and SCD1, which can serve as an alternative energy supply when glycolysis falters, were also suppressed. The researchers additionally observed signs of impaired mitophagy, the cellular process that clears damaged mitochondria, with fewer autophagosomes and altered levels of LC3, Beclin1, P62, and Parkin, suggesting that radiation-damaged mitochondria accumulated in the diet-pretreated tumors and released the mitochondrial DNA that fuels cGAS-STING signaling.

Crucially, the same interventions that hurt the tumor helped the healthy gut. Caloric restriction and fasting restored colon length, which radiation shortened, and partially restored tight junction proteins such as occludin and claudin-1 that maintain the intestinal barrier. Histological damage in the jejunum, including reduced villus height and crypt depth, was ameliorated, with calorie restriction increasing villus length. To probe this further, the team isolated intestinal crypts from mice and grew them as organoids, exposing some to glucose-free medium to mimic caloric restriction. Radiation ruptured organoids and shrank their surface area, while glucose-free pretreatment alleviated the damage, increased the proportions of proliferating Ki67-positive and E-cadherin-positive cells, and raised the percentage of budding organoids, a readout of intestinal stem cell regeneration.

In cultured cells, the contrast between tumor and normal tissue became even sharper. Glucose-free pretreatment before irradiation worsened mitochondrial membrane potential loss, elevated cytosolic mitochondrial DNA, and increased reactive oxygen species in CT-26 cancer cells, while preserving mitochondrial function, reducing mitochondrial DNA release, and lowering ROS in IEC-6 normal intestinal epithelial cells. Glucose deprivation also suppressed glycolytic enzyme levels and reduced oxygen consumption and lactate output in the cancer cells without affecting the normal cells. Clonogenic and viability assays confirmed that glucose-free pretreatment reduced the survival of irradiated CT-26 cells but increased the survival of irradiated IEC-6 cells.

The dependency on cGAS-STING was tested directly. When the researchers silenced STING with siRNA or blocked it with the inhibitor H-151, the radiosensitizing effect of glucose deprivation on CT-26 cells disappeared, along with the upregulation of interferon-beta and the chemokines Ccl5 and Cxcl10. Conversely, activating STING with the agonist MSA-2 abolished the protective effect of glucose deprivation in IEC-6 cells. Notably, ROS production triggered by radiation was not altered by any of the STING manipulations, placing ROS upstream of the pathway rather than downstream of it.

The authors are candid about the limits of their work. The tumor model was subcutaneous rather than orthotopic, so it does not reproduce the microenvironment of clinical colorectal tumors; only a single tumor model was tested; the dietary intervention restricted total calories without separating the effects of energy deficit from macronutrient composition; and STING was inhibited only in vitro, not in the living tumor microenvironment. The mouse findings also involved precise, controlled feeding regimens that cannot be assumed to translate to patients undergoing radiotherapy. Still, the convergence of tumor sensitization, immune activation, metabolic suppression, and concurrent intestinal protection in one preclinical system makes caloric restriction a candidate adjuvant worth pursuing, and the authors call for additional preclinical and translational studies to optimize intervention regimens and validate safety before any clinical application.

Subject of Research: Preclinical study of caloric restriction as a radiosensitizer for colorectal tumors via the cGAS-STING pathway

Article Title: Caloric restriction enhances radiosensitivity of colorectal tumors through the cGAS-STING pathway activation

Article References: Zhang, L.-L., Qin, L.-Q., Ding, L., Shan, W.-Y., Zhao, Y., Zhang, Y.-Q., Gu, J., Zhao, L., Hui, K.-Y., Zhang, Y.-S., & Xu, J.-Y. (2026). Caloric restriction enhances radiosensitivity of colorectal tumors through the cGAS-STING pathway activation. Cancer & Metabolism, 14(1), Article 33. https://doi.org/10.1186/s40170-026-00449-y

Image Credits: AI Generated

DOI: 10.1186/s40170-026-00449-y

Keywords: caloric restriction, fasting, colorectal cancer, radiotherapy, cGAS-STING, radiosensitivity, tumor microenvironment, CD8 T cells, macrophage polarization, glycolysis, intestinal organoids, preclinical study