In the complex landscape of radiation-induced injury, the gastrointestinal acute radiation syndrome (GI-ARS) stands as a critical and often fatal consequence, marked by severe impairment of intestinal function. Recent advancements in understanding the molecular underpinnings of GI-ARS have shed light on the pivotal role of ceramides, bioactive lipids that orchestrate cellular fate in the irradiated gut. Ceramide synthesis occurs primarily through two main pathways: a de novo biosynthetic route and the catabolism of sphingomyelin by sphingomyelinases. These enzymes, classified into acidic, neutral, and alkaline forms, initiate a cascade leading to ceramide accumulation, which profoundly influences cell survival and apoptosis, especially within the microvasculature of the intestinal lamina propria.
Acid sphingomyelinase (ASM), encoded by the SMPD1 gene, emerges as a critical mediator in this context. Expressed prominently in Paneth and endothelial cells of the small intestine, ASM activity is modulated by several signaling pathways, including Fas/Apo-1, CD28, and interleukin-1, as well as by environmental stressors such as ionizing radiation. The enzymatic conversion of sphingomyelin to ceramide by ASM within irradiated endothelial cells acts as a gateway to programmed cell death, or apoptosis, underscoring the lipid’s role in initiating GI tract injury post-irradiation.
Complementing the activity of ASM, the family of neutral sphingomyelinases (nSMases), particularly nSMase2 localized to the Golgi apparatus and plasma membrane, contributes to ceramide generation in irradiated tissues. Activation of nSMases is similarly induced by pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and interferon-gamma (IFN-γ). Their swift response to irradiation and cytokine signaling propels endothelial apoptosis, creating a multifaceted lipid-driven pathway that exacerbates GI tract damage.
The apoptotic effects triggered by ceramide accumulation unfold through intricate biochemical signaling. Ceramide suppresses the phosphoinositide-3-kinase (PI3K) and serine/threonine kinase Akt pathways, pivotal for cell survival, thereby activating BAD, a proapoptotic BCL-2 family member. Concurrently, ceramide promotes protein phosphatase 2A (PP2A) activity, leading to dephosphorylation and inactivation of the anti-apoptotic protein BCL-2. Beyond these signaling events, ceramides facilitate the formation of specialized mitochondrial microdomains that support pore formation by BAX and BAK proteins, culminating in mitochondrial outer membrane permeabilization (MOMP) and irreversible commitment to apoptosis.
Seminal animal studies have illustrated the direct correlation between radiation-induced ceramide production, microvascular apoptosis in the intestinal lamina propria, and mortality associated with GI-ARS. Paris and colleagues documented how lethal doses of radiation induce microvascular endothelial cell apoptosis in the early hours post-exposure, preceding epithelial cell death within crypt regions. These findings suggest that endothelial demise is the critical initial event triggering the cascade leading to the GI syndrome, rather than the epithelial cell loss that had previously garnered more focus.
Corroborating these findings, experiments involving genetically modified mice deficient in ASM provided compelling evidence of causality. When exposed to whole-body irradiation, ASM-null mice exhibited a marked reduction in microvascular endothelial apoptosis, preservation of crypt-villus structure, and significantly enhanced survival compared to wild-type counterparts. Intriguingly, despite this endothelial protection, epithelial cell apoptosis occurred unabated, highlighting a dichotomy in the cellular responses orchestrated by ceramide pathways in the GI tract.
Further dissection of apoptotic regulators revealed the nuanced roles of the pro-apoptotic proteins BAX and BAK. Studies featuring mice deficient in these proteins indicated a substantial decrease in endothelial apoptosis and improved crypt survival, similar to ASM-null models. However, loss of BAX or BAK did not significantly affect the apoptosis of epithelial crypt cells. These insights reinforce the concept that endothelial cell death is a principal early driver of GI damage post-radiation, and that modulating these lipid-mediated pathways may selectively spare critical stem cell niches.
Conversely, the role of BAX in certain mouse genetic backgrounds, such as C57BL strains, appears less impactful. Although loss of BAX reduced endothelial apoptosis, it had only a marginal effect on overall survival following irradiation, underscoring the complexity of genetic determinants and their influence on radiation responses. This observation aligns with additional studies using Cre-lox systems to selectively delete BAX and BAK in endothelial lineages, confirming the nuanced interplay of apoptotic mediators in GI syndrome pathogenesis.
The p53-upregulated modulator of apoptosis (PUMA) has been heavily studied for its function in radiation-induced cell death. Investigations employing PUMA-null mice demonstrated that while epithelial apoptosis was suppressed after high-dose irradiation, endothelial apoptosis remained unaffected. This differential regulation correlated with improved survival in mutant mice, suggesting that PUMA-driven epithelial apoptosis critically influences GI-ARS outcomes. However, the significant non-apoptotic loss of endothelial cells post-irradiation complicates this narrative, implying additional, perhaps necrotic mechanisms contribute to endothelial dysfunction and disease progression.
In a translational leap, Rotolo and colleagues engineered a monoclonal antibody targeting ceramide, which effectively inhibited the formation of ceramide-rich microdomains on endothelial plasma membranes. Administration of this antibody curtailed endothelial apoptosis in the lamina propria, enhanced survival of crypt base columnar (CBC) stem cells, and dramatically reduced mortality due to GI syndrome. These striking results phenocopied those seen in ASM-null mice and reinforced the causative link between ceramide accumulation, endothelial damage, and lethal intestinal injury following radiation.
Fibroblast growth factor 2 (FGF2), a well-characterized mitogen with wide-ranging roles in cell proliferation, survival, and differentiation, has been implicated as an endogenous radioprotective agent within the gut. Studies in murine models have shown that FGF2 expression is selectively induced in the intestinal lamina propria post-irradiation. Exogenous administration of recombinant FGF2 before radiation exposure decreases microvascular apoptosis and bolsters crypt epithelial cell survival, effectively mitigating GI syndrome lethality. While FGF2’s interactions with various fibroblast growth factor receptors (FGFRs), particularly FGFR3 isoforms, have demonstrated anti-apoptotic effects in epithelial cells, its protective mechanisms in endothelial populations remain to be fully elucidated.
The advent of organ-on-a-chip technologies has afforded unprecedented insights into the cellular dynamics underpinning radiation injury. Using a human gut-on-a-chip platform, researchers irradiated constructs containing both epithelial and endothelial cells, recapitulating the architectural and functional features of the intestine. Consistent with in vivo findings, endothelial apoptosis was observed prominently within 24 hours, predating epithelial cell death detected at 48 hours post-exposure. Moreover, chips lacking endothelial cells exhibited substantially less villus atrophy after irradiation, compellingly positioning the endothelium as a primary target and mediator of radiation-induced intestinal injury.
Despite extensive focus on apoptotic pathways, research reveals that GI-ARS pathogenesis cannot be solely attributed to p53-dependent apoptosis. Intriguingly, p53-null mice, although exhibiting diminished epithelial apoptosis, display heightened sensitivity to GI irradiation, manifesting in reduced survival. Similar susceptibility is noted in p21-deficient models, implicating the vital role of cell cycle checkpoints in DNA damage repair and survival. The prevailing interpretation is that loss of p53/p21-mediated controls precipitates premature mitotic entry with unrepaired DNA lesions, driving mitotic catastrophe rather than apoptosis as the dominant mode of tissue destruction after radiation.
In summary, a coherent body of evidence from genetic, biochemical, and bioengineering studies converges on a unifying hypothesis: endothelial cell apoptosis, driven by ceramide accumulation and its downstream signaling, constitutes an early and essential event precipitating gastrointestinal acute radiation syndrome. Therapeutic interventions targeting ceramide synthesis or function, such as ASM inhibition or ceramide monoclonal antibodies, alongside FGF2-based protective strategies, hold promise for mitigating radiation-induced intestinal damage. The incorporation of advanced models like the gut-on-a-chip has significantly expanded the understanding of cellular interactions in GI injury, highlighting endothelial cells as critical determinants of radiosensitivity.
This refined perspective challenges traditional views that primarily attribute GI-ARS to epithelial crypt cell death, emphasizing instead the vascular endothelium’s central role. Understanding the ceramide-driven apoptotic pathways offers new avenues for intervention, with potential to transform clinical management of radiation injuries. As research progresses, integrating molecular, genetic, and tissue-engineering approaches will be paramount to developing efficacious countermeasures against GI-ARS, representing a beacon of hope for patients exposed to high-dose radiation in therapeutic or accidental settings.
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Subject of Research: Gastrointestinal acute radiation syndrome (GI-ARS) and the molecular mechanisms, specifically focusing on ceramide-mediated endothelial apoptosis in radiation-induced intestinal injury.
Article Title: Gastrointestinal acute radiation syndrome: current knowledge and perspectives.
Article References:
Freeman, M.L. Gastrointestinal acute radiation syndrome: current knowledge and perspectives.
Cell Death Discov. 11, 235 (2025). https://doi.org/10.1038/s41420-025-02525-6
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41420-025-02525-6
