The role of ceramides in the pathophysiology of diabetic retinopathy: Mechanisms and Implications for therapeutic strategies

Abstract

Diabetic retinopathy (DR) is a leading cause of vision impairment among individuals with diabetes, driven by microvascular damage and retinal degeneration. Recent evidence implicates ceramides—bioactive sphingolipids—as critical mediators in DR pathogenesis. These lipids influence critical cellular processes, including apoptosis, inflammation, and oxidative stress. Emerging data suggests that ceramide accumulation exacerbates retinal neurodegeneration in DR by modulating key molecular pathways. This review synthesizes current knowledge on the role of ceramides in DR development and progression, examines the factors influencing their activity, and explores the therapeutic potential of targeting ceramide metabolism. By integrating recent findings, this article aims to provide insights for future research and clinical strategies in DR management.

1. Introduction

Diabetic retinopathy (DR) is one of the most common complications of diabetes and a leading cause of vision loss worldwide.^1–3 It is a progressive retinal disease characterized by microvascular damage, retinal ischemia, and neurodegeneration.^4–6 Early detection and treatment are crucial to prevent irreversible vision impairment.

Ceramides, key components of cell membrane lipids, have gained attention for their role in the pathophysiology of various diseases, including DR. They regulate fundamental cellular processes such as apoptosis, inflammation, and oxidative stress. ⁷ Emerging research suggests ceramide accumulation plays a pivotal role in DR progression by modulating molecular pathways involved in retinal neurodegeneration and vascular dysfunction.^8–11

The mechanisms by which ceramides influence DR are complex. Ceramide accumulation impairs mitochondrial function, promotes oxidative stress, and activates pro-inflammatory pathways, all of which contribute to retinal damage.^12,13 For instance, a recent study has demonstrated that elevated levels of pro-inflammatory long-chain ceramides (e.g., C16-ceramide) correlate with DR severity, while protective very long-chain ceramides (e.g., C26-ceramide) are reduced under diabetic conditions. ¹⁴ These molecular processes highlight ceramide as a promising target for therapeutic intervention in DR. The finding positions ceramides as promising therapeutic targets in DR.

Current DR treatments primarily focus on glycemic control and managing vascular complications using anti-VEGF injections. ¹⁵ However, these approaches mainly address symptoms rather than underlying molecular causes. Therefore, exploring novel therapeutic strategies targeting ceramide metabolism may offer more effective and long-term solutions to prevent or slow DR progression.

This review aims to explore the role of ceramides in DR pathogenesis, summarize the underlying molecular mechanisms, and discuss potential therapeutic strategies targeting ceramide accumulation.

2. Biological characteristics of ceramides

Ceramides are a class of sphingolipids involved in various biological functions, including cell signaling, apoptosis, and inflammation. ¹⁶ They contribute to membrane rigidity and the formation of lipid microdomains, which are essential for cellular communication and signaling. In the context of DR, ceramide-induced alterations in cell signaling may exacerbate retinal microvascular damage and neurodegeneration. The biological activity of ceramides is influenced by their structure, particularly fatty acid chain length and degree of saturation. Consequently, ceramides participate in diverse cellular processes such as proliferation, differentiation, and apoptosis, playing important roles in both health and disease^17,18 (Figure 1).

Figure 1.

Structural and functional characteristics of ceramides in membrane dynamics. (a) Hydrophobicity and hydrogen bonding of ceramides. (b) Three-dimensional structure of ceramides. (c) Interaction of ceramides with biological membranes. (d) The sphingolipid rheostat and its regulation of cellular processes.

2.1. Structure and function of ceramides

Ceramides consist of a sphingosine backbone linked to a fatty acid via an amide bond, forming a hydrophobic structure that integrates into lipid bilayers. Their ability to form hydrogen bonds and create gel-like domains within membranes facilitates the clustering of signaling molecules and receptors, thereby amplifying cellular responses to external stimuli.^18,19 The balance of ceramide metabolism and signaling is dynamically regulated by enzymes such as sphingomyelinases and ceramidases. ²⁰

2.2. Role of ceramides in cell signaling

Ceramides act as signaling molecules mediating various cellular responses. They function as pro-apoptotic signals, inducing cell death in response to oxidative stress or chemotherapeutic agents. This apoptotic function is linked to the “sphingolipid rheostat,” where the balance between ceramide and its metabolite sphingosine-1-phosphate (S1P) determines cell fate: ceramide promotes apoptosis, while S1P supports survival and proliferation. ²¹ Ceramides also modulate inflammatory responses by regulating immune cell activity and are implicated in diseases such as cancer and diabetes. ¹⁷ Dysregulated ceramide metabolism contributes to vascular dysfunction and inflammation in DR, highlighting their importance in cellular homeostasis and disease pathogenesis.

3. Role of ceramides in diabetic retinopathy mechanisms

In preclinical models of DR, ceramides have been shown to contribute to inflammation, neuronal apoptosis, and microvascular dysfunction (Figure 2). ²² Concurrently, in patients with DR, vitreous ceramide profiles show elevated levels of pro-inflammatory long-chain ceramides and reduced levels of protective very long-chain ceramides. ¹⁴ This ceramide imbalance promotes retinal damage and disease progression, making ceramide pathways attractive targets for therapeutic intervention.

Figure 2.

Mechanisms of ceramide-induced pathology in diabetic retinopathy.

3.1. Ceramide-induced endothelial dysfunction and vascular damage

Under hyperglycemic conditions, ceramide accumulation leads to breakdown of the blood–retinal barrier (BRB) and increased vascular permeability.^23,24 In retinal endothelial cells, elevated ceramide levels promote the formation of ceramide-rich lipid rafts, which recruit inflammatory molecules and adhesion proteins such as ICAM-1 and VCAM-1, facilitating leukocyte adhesion and infiltration into retinal vessels.^14,25–29 This inflammatory cascade results in endothelial cell apoptosis, vascular leakage, and exacerbated retinal ischemia. Additionally, ceramide induces endothelial dysfunction by activating protein phosphatase 2A (PP2A), which inhibits nitric oxide (NO) production. ³⁰

In the early stages of DR (non-proliferative DR, NPDR), ceramide contributes to endothelial dysfunction and pericyte loss, two hallmark early pathological events. Ceramide promotes pericyte apoptosis via oxidative and endoplasmic reticulum stress pathways,^31,32 leading to capillary destabilization and increased vascular permeability. As DR progresses to the proliferative stage (PDR), chronic retinal ischemia upregulates vascular endothelial growth factor (VEGF), which drives pathological neovascularization. ³³ These newly formed vessels are fragile and prone to hemorrhage, ³⁴ further compromising retinal integrity.

3.2. Pro-inflammatory effects in the retina

Ceramide is a key mediator of retinal inflammation in DR. Its accumulation activates the nuclear factor kappa B $NF-\kappa B$ signaling pathway, leading to transcription of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6.^12,35 Ceramide also activates retinal microglia, which release inflammatory mediators via the NLRP3 inflammasome and MAPK pathways.^36–38 These cytokines increase vascular permeability, promote leukocyte recruitment, and compromise BRB integrity.

3.3. Oxidative stress, and retinal neurodegeneration

Ceramide exacerbates neuronal damage in DR by promoting mitochondrial dysfunction and reactive oxygen species (ROS) production. Under hyperglycemia, ceramide accumulation disrupts mitochondrial function, creating an imbalance between ROS generation and antioxidant defenses.^39,40 This is particularly evident in retinal ganglion cells (RGCs) and photoreceptors, which are highly susceptible to metabolic stress.^37,41

Ceramide-induced oxidative stress activates NF-κB, leading to expression of pro-inflammatory cytokines and adhesion molecules, which promote leukostasis and capillary occlusion.^42,43 Additionally, elevated ceramide levels activate stress-activated protein kinases (JNK and p38 MAPK), mediating apoptotic signaling. ⁴⁴ A vicious cycle ensues, as ROS further stimulate ceramide synthesis, perpetuating cellular damage. ⁴⁵ This interplay underscores the need for combined therapeutic strategies targeting both ceramide metabolism and oxidative stress.^46,47

3.4. Ceramide and retinal cell apoptosis

Ceramide is a potent inducer of apoptosis in retinal cells, contributing to both endothelial and neuronal cell death in DR. In retinal endothelial cells, ceramide-induced apoptosis leads to microvascular dropout and ischemia. ⁴⁸ In neurons, including RGCs and photoreceptors, ceramide promotes apoptosis through mitochondrial membrane permeabilization, cytochrome c release, and caspase activation.^12,49 Ceramide-induced oxidative stress exacerbates this process by reducing glutathione levels. ⁵⁰

Ceramide also indirectly promotes neuronal apoptosis by activating astrocytes and microglia, leading to neuroinflammation.^51,52 For example, the activation of microglial cells can cause the release of proinflammatory cytokines not only exacerbating retinal cell death, but also disrupting the blood retinal barrier, further disrupting neurodegenerative effects in DR. ⁵³ Targeting ceramide-induced apoptosis has shown promise in preclinical studies. For example, inhibiting ceramide synthesis or using ceramide antagonists reduces retinal cell death and preserves vascular and neuronal structures.^54,55 FTY720, which modulates ceramide synthesis, has demonstrated neuroprotective effects against retinal degeneration. ⁵⁴ Similarly, inhibition of acid sphingomyelinase (ASM) protects the retina from ischemic damage.^55,56

3.5. Neurovascular coupling disruption

Ceramide disrupts neurovascular coupling, exacerbating retinal ischemia and vascular dysfunction in advanced DR. Accumulation of ceramide alters endothelial signaling, impairing vasodilation and blood flow regulation.^57,58 For instance, ceramide induces pyroptosis in endothelial cells, leading to increased vascular permeability. ⁵⁹ Concurrently, retinal neurodegeneration reduces the ability to regulate blood flow, further aggravating ischemia.^60,61 This bidirectional relationship creates a vicious cycle, as neuronal degeneration promotes vascular dysfunction and vice versa.^58,60,62,63 Restoring neurovascular coupling by targeting ceramide metabolism may improve oxygen and nutrient delivery to retinal tissues, mitigating both neural and vascular damage.^64,65

4. Therapeutic strategies targeting ceramides in diabetic retinopathy

Given the multifaceted role of ceramides in DR pathogenesis, they represent promising therapeutic targets. This section discusses current and emerging strategies to modulate ceramide activity in DR.

4.1. Current therapies and their limitations

Current DR treatments primarily target vascular complications. Anti-VEGF agents (e.g., bevacizumab, ranibizumab) reduce vascular leakage and inhibit pathological neovascularization.^66,67 They have been shown to improve visual acuity and reduce diabetic macular edema (DME). ⁶⁸ N-acetylcysteine (NAC) replenishes glutathione and reduces retinal inflammation and apoptosis in experimental DR models.^69–71 Other antioxidants, such as vitamin E and selenium, have also been investigated and demonstrated antioxidant properties that may counteract oxidative stress in DR.^72,73

Laser photocoagulation remains a standard treatment for PDR, reducing severe vision loss risk by ablating ischemic retina and decreasing VEGF production. Evidence from the Early Treatment Diabetic Retinopathy Study (ETDRS) demonstrated that laser photocoagulation reduces the risk of severe vision loss in patients with high-risk proliferative diabetic retinopathy. ⁷⁴ The procedure stabilizes the retinal environment by reducing retinal ischemia and the subsequent generation of angiogenic factors such as VEGF. ⁷⁵ Steroid therapies (e.g., dexamethasone implants) reduce inflammation and vascular permeability in DME but are associated with side effects such as increased intraocular pressure and cataract formation.^76,77

Despite their efficacy, these therapies have limitations. Anti-VEGF agents require frequent intravitreal injections, posing compliance and safety concerns. Laser therapy can cause retinal scarring and peripheral vision loss. Corticosteroids carry risks of ocular hypertension and cataracts. Moreover, these treatments mainly address vascular components, leaving metabolic and neurodegenerative aspects largely untreated. These limitations highlight the need for novel therapies targeting ceramide pathways.

4.2. Targeting ceramide pathways

4.2.1. Inhibition of ceramide synthesis

Inhibiting key enzymes in ceramide synthesis, such as serine palmitoyltransferase (SPT) and dihydroceramide desaturase (DES1), reduces ceramide levels and improves cellular function in preclinical studies.^78,79 Sirtuin-1 (SIRT1) activation has also been shown to modulate ceramide metabolism in experimental models and has been proposed as a potential therapeutic approach. ⁸⁰

In preclinical investigations, myriocin, an SPT inhibitor, has attracted considerable attention among various strategies to inhibit ceramide synthesis. In rodent models of DR, myriocin reduces retinal inflammation and apoptosis by inhibiting de novo ceramide synthesis, and prevents ceramide-induced inflammatory cell recruitment and blood-retinal barrier breakdown.^81,82 Myriocin also restores insulin signaling and prevents apoptosis in these models, suggesting a multifaceted therapeutic approach.^54,83 However, these findings are derived exclusively from animal and in vitro studies; to date, the clinical efficacy and safety of myriocin in patients with DR have not been evaluated in human trials.

4.2.2. Modulation of ceramide metabolism

Pharmacological strategies to reduce ceramide levels include ceramidase activators and sphingosine kinase enhancers. S1P analogs are being investigated as anti-inflammatory agents for ocular diseases. ⁸⁴ Adeno-associated virus-mediated expression of ceramide synthases restores retinal structure and function in mouse models of retinal dystrophy. ⁸⁵

Inhibition of acid sphingomyelinase (ASM) reduces ceramide generation, preventing mitochondrial dysfunction and retinal inflammation. ¹⁴ Dietary docosahexaenoic acid, which inhibits ASM, protects against diabetic retinal vascular degeneration. ⁸⁶ ASM inhibition also preserves endothelial integrity and reduces apoptosis. ⁸⁷

4.2.3. Targeting ceramide-induced inflammation

Non-steroidal anti-inflammatory drugs (NSAIDs) have shown potential in reducing ceramide-induced inflammation. However, systemic NSAID administration is associated with gastrointestinal and other systemic side effects, while topical ophthalmic NSAIDs carry risks of corneal epithelial damage and, in rare cases, corneal melting.^88,89 These safety concerns have limited their routine use in DR. More targeted approaches include inhibitors of ceramide transfer protein (CERT), which facilitates ceramide transport from the endoplasmic reticulum to the Golgi apparatus. CERT inhibitors such as (1R,3S)-HPA-12 and E16A reduce ceramide accumulation and limit its harmful effects on vascular function. ⁹⁰ CERT inhibition also reduces cytokine release and preserves BRB integrity.^91,92

4.2.4. Anti-ceramide immunotherapy

Anti-ceramide antibodies have been shown to prevent ceramide-rich platform formation and reduce retinal endothelial cell apoptosis in preclinical DR models. Both intravitreal and systemic administration of these antibodies have demonstrated protective effects, suggesting potential for early intervention in non-proliferative DR.^14,28,37,41

5. Challenges and future directions

Despite the promise of ceramide-targeted therapies, several challenges remain. First, ceramides are involved in essential cellular processes, and indiscriminate modulation may cause toxicity. Selective targeting of pathogenic ceramide species (e.g., C16-ceramide) while preserving protective species (e.g., C26-ceramide) is crucial. Second, the safety and long-term effects of these therapies in humans are unknown. Third, effective delivery to the retina remains a challenge; nanoparticle- and liposome-based systems may offer targeted, controlled release with improved bioavailability and reduced side effects.

6. Conclusion

Ceramides play a critical role in DR pathophysiology, mediating inflammation, apoptosis, and microvascular dysfunction. Preclinical evidence suggests that ceramide-targeted therapies, including synthesis inhibitors, metabolism modulators, and anti-ceramide antibodies, hold promise for early intervention and disease modification. However, clinical evidence supporting these therapies in patients with DR remains limited, and further research is needed to translate these preclinical findings into safe and effective clinical treatments.

ORCID iDs

Xiaoji Niu https://orcid.org/0000-0001-9580-7517

Jinping Hu https://orcid.org/0009-0004-8910-6254