Role of 1,25-dihydroxyvitamin D3 in alleviating hepatic steatosis: Targeting M1 macrophage polarization in non-alcoholic fatty liver disease

Abstract

Non-alcoholic fatty liver disease (NAFLD), a critical global health concern, continues to challenge medical researchers with limited treatment options. This letter examines on the study by Luo et al, demonstrating that vitamin D 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] improves hepatic steatosis in NAFLD by inhibiting M1 macrophage polarization via the vitamin D receptor-peroxisome proliferator-activated receptor gamma signaling pathway. This letter critically appraises these findings, comparing them to similar studies, and discusses their potential implications for treating NAFLD. Furthermore, we highlight future directions, including dose optimization and mechanistic studies.

Key Words: Vitamin D; Non-alcoholic fatty liver disease; M1 macrophage polarization; Vitamin D receptor pathway; Hepatic steatosis

Core Tip: This letter critically analyzes the study by Luo et al, on the effects of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] in reducing lipid accumulation in hepatocytes by inhibiting M1 macrophage polarization through the vitamin D receptor-peroxisome proliferator-activated receptor gamma pathway in non-alcoholic fatty liver disease (NAFLD). The study highlights how 1,25(OH)₂D₃ improves lipid metabolism and reduces inflammation, offering promising insights for NAFLD treatment. Future research should focus on dose optimization, detailed molecular mechanisms, and translating findings into human clinical trials to validate vitamin D's therapeutic potential in managing NAFLD.

TO THE EDITOR

Non-alcoholic fatty liver disease (NAFLD) is a chronic liver disorder closely associated with metabolic syndrome[1]. It affects nearly one-quarter of the global population and can progress to more severe conditions, such as non-alcoholic steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma[2,3]. Current therapeutic options for NAFLD are limited, necessitating novel interventions. Recent evidence suggests that vitamin D, specifically 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], may play a protective role in hepatic lipid metabolism and inflammation[4,5]. The therapeutic potential of 1,25(OH)₂D₃ in NAFLD stems from its multifaceted inflammatory modulation. By targeting key inflammatory pathways, including nuclear factor-kappa B (NF-κB), Toll-like receptors (TLRs), and the Janus kinase/signal transducers and activators of transcription cascades, 1,25(OH)₂D₃ comprehensively suppresses hepatic inflammation[6,7]. The molecule shifts macrophage polarization from pro-inflammatory M1 to anti-inflammatory M2 phenotypes, reduces pro-inflammatory cytokine production (TNF-α, IL-6, IL-1β), and attenuates oxidative stress through enhanced antioxidant enzyme expression[8]. Mechanistically, it operates via the vitamin D receptor (VDR) and the peroxisome proliferator-activated receptor gamma (PPARγ) signaling axis, interfering with inflammatory gene transcription, modifying adipokine secretion, and interrupting chronic inflammatory feedback loops[9]. By mitigating lipopolysaccharide-induced responses, reducing macrophage recruitment, and protecting hepatocytes from oxidative damage, 1,25(OH)₂D₃ offers a promising multimodal approach to addressing the complex inflammatory landscape of NAFLD[10]. The study by Luo et al[11] evaluates the effects of 1,25(OH)₂D₃ on lipid accumulation in hepatocytes and macrophage polarization, a key driver of inflammation in NAFLD.

Methods

Luo et al[11] used a murine model of NAFLD induced by a high-fat (HF) diet, supplemented with 1,25(OH)₂D₃, to evaluate its impact on hepatic steatosis and macrophage polarization. Male adult (aged 6-8 weeks) wild-type C57BL/6 mice were acquired from the Vital River Laboratory (China). Mice were fed either a regular normal control diet (15% kilocalories from fat) or a HF diet (60% kilocalories from fat) for a period of 16 weeks. For 1,25(OH)₂D₃ supplementation, HF diet-fed mice received 1,25(OH)₂D₃ (20 µg/kg, MedChemExpress, United States) or phosphate-buffered saline (Gibco, United States) by oral gavage every alternate day for 16 weeks. RAW264.7 macrophages and AML12 hepatocytes were cultured in a co-culture system to assess lipid accumulation and inflammatory responses. The VDR-PPARγ pathway, M1/M2 macrophage markers, and lipid metabolism genes were analyzed using qPCR, Western blot, and ELISA techniques.

Key findings

The study found that 1,25(OH)₂D₃ supplementation significantly reduced lipid accumulation in hepatocytes, decreased the expression of proinflammatory M1 macrophage markers (iNOS, TNFα, IL-6), and improved lipid metabolism by modulating both lipid synthesis (SREBP1C, FASN) and breakdown (ACOX1, CPT1A) genes. These effects were mediated through the VDR-PPARγ pathway, as 1,25(OH)₂D₃ treatment inhibited fatty-acid-induced M1 macrophage polarization.

Additionally, the study reported improvements in liver function tests following 1,25(OH)₂D₃ supplementation. The treatment led to a reduction in liver and body weight, as well as a decrease in the expression of proinflammatory cytokines in the liver. These findings suggest that 1,25(OH)₂D₃ not only alleviates hepatic steatosis and inflammation but also positively impacts overall liver function, providing a comprehensive understanding of the interventions' impact on NAFLD.

Critical appraisal

Luo et al’s findings[11] align with previous studies highlighting vitamin D's anti-inflammatory and lipid-regulatory effects. For example, Zhang et al[9]. demonstrated that vitamin D reduces NAFLD severity by downregulating lipid biosynthesis pathways. However, the exact mechanisms linking vitamin D to macrophage polarization remain underexplored. While Luo et al[11] propose the VDR-PPARγ axis as the central pathway, other studies, such as those by Luo et al[12], suggest additional mediators like TLRs that could further modulate macrophage activity. Additionally, the study’s reliance on a murine model limits its direct applicability to human NAFLD treatment, as differences in macrophage activation may exist between species.

Potential limitations of the study include the use of a murine model, which may not fully replicate human NAFLD pathophysiology. The differences in macrophage activation and immune responses between mice and humans could affect the translatability of the findings. Furthermore, while the study focuses on the VDR-PPARγ pathway, it does not extensively explore other potential pathways and mediators, such as TLRs and NF-κB, which have been implicated in macrophage polarization and inflammation. Conflicting evidence from other studies suggests that these pathways might also play significant roles in NAFLD, indicating a need for broader mechanistic investigations.

Moreover, the study does not address the potential side effects of high-dose vitamin D supplementation, such as hypercalcemia, which could limit its clinical application. Future research should consider these limitations and explore the long-term safety and efficacy of vitamin D supplementation in diverse populations. Additionally, human clinical trials are necessary to validate the therapeutic potential of vitamin D in managing NAFLD and to determine optimal dosing strategies.

Future perspectives

The potential for 1,25(OH)₂D₃ to serve as a therapeutic for NAFLD opens exciting possibilities. However, future studies should explore several areas: (1) Dose optimization: Investigating the effects of different dosages of 1,25(OH)₂D₃ is essential for clinical application, as vitamin D supplementation at high doses may lead to hypercalcemia​. Dose-response studies should focus on identifying the optimal therapeutic dose. This involves maximizing efficacy while minimizing side effects. Key variables to consider include the duration of treatment, frequency of dosing, and patient-specific factors such as baseline vitamin D levels and liver function; (2) Mechanistic studies: Further elucidation of the molecular interactions between VDR and PPARγ in macrophage polarization and lipid metabolism is needed, with particular attention to cross-talk with other pathways like TLR and NF-κB. Understanding these interactions will help refine therapeutic strategies and identify potential biomarkers for treatment response; and (3) Human clinical trials: Translating these findings into clinical practice will require human trials that evaluate the long-term efficacy and safety of 1,25(OH)₂D₃ in NAFLD patients. Clinical trials should be designed to assess not only the impact on hepatic steatosis and inflammation but also on overall metabolic health and quality of life. Specific recommendations for clinical trials include stratifying participants based on disease severity, monitoring for potential adverse effects such as hypercalcemia, and evaluating the effects of 1,25(OH)₂D₃ in combination with other therapeutic agents to enhance efficacy.

Conclusions

Luo et al’s[11] study provides compelling evidence that 1,25(OH)₂D₃ reduces lipid accumulation in hepatocytes by inhibiting M1 macrophage polarization through the VDR-PPARγ pathway. This novel insight highlights the potential of vitamin D as a therapeutic target in NAFLD. However, further research is required to optimize dosing strategies, clarify molecular mechanisms, and validate these findings in human trials.