Dihomo-γ-Linolenic Acid: A Molecular Mediator of Inflammation and a Promising Target for the Therapy of Chronic Diseases

1. Introduction

Polyunsaturated fatty acids (PUFAs) are essential structural components of cell membranes and precursors of bioactive lipid mediators. They play a fundamental role in the regulation of inflammatory processes, immune response, metabolism, and homeostasis [1]. Despite significant progress in understanding the metabolism of PUFAs, the clinical significance of many of their representatives, in particular dihomo-γ-linolenic acid (DGLA, 20:3n-6), remains poorly understood. Traditionally, n-6 PUFAs have been associated with pro-inflammatory effects. However, accumulating data indicate that DGLA is an exception to this rule. It exhibits pronounced anti-inflammatory properties. Consequently, DGLA represents a pivotal signaling molecule that functions both as a potent anti-inflammatory mediator and as a clinically relevant biomarker. Its metabolism is significantly dependent on genetic variations (FADS) and nutritional factors, in particular zinc status. These combined factors highlight novel avenues for personalized therapeutic strategies.

2. Main Text

Unique metabolic pathway and antagonism with arachidonic acid. DGLA occupies a central position in the metabolic cascade of n-6 PUFAs. It is formed from gamma-linolenic acid (GLA) by the action of the elongase ELOVL5 [2]. In contrast to arachidonic acid (AA), which is the precursor of pro-inflammatory series-2 prostaglandins, series-4 leukotrienes and thromboxanes, DGLA is metabolized by cyclooxygenases (COX) and 15-lipoxygenase (15-LOX). As a result, it yields prostaglandin E1 (PGE1) and 15-hydroxyeicosatrienoic acid (15-HETrE), both of which possess anti-inflammatory, vasodilatory, and anti-aggregatory properties [3,4] (Figure 1).

Furthermore, DGLA competitively inhibits AA metabolism via COX and LOX, thereby suppressing the synthesis of pro-inflammatory eicosanoids [5]. This antagonism is critical for understanding the role of DGLA in the regulation of inflammatory homeostasis (Figure 2).

Clinical significance of DGLA as a prognostic biomarker. The strongest evidence for the clinical significance of DGLA comes from cardiovascular studies. In the OMEMI study of 1002 patients aged 70-82 years with a recent myocardial infarction, low serum phospholipid DGLA levels were an independent predictor of all-cause mortality during 2 years of follow-up (hazard ratio 0.47 comparing the highest three quartiles to the lowest quartile) [6]. The authors emphasize that this effect is likely due to reduced delta-6-desaturase activity. This is evidenced by the high LA/DGLA ratio, which was also associated with an increased risk of adverse cardiovascular events [6]. Similar results were obtained in a study by Ouchi et al. In this study, low DGLA levels predicted long-term mortality in patients with acute decompensated heart failure [7]. Thus, DGLA acts not just as a marker, but also as a potentially modifiable risk factor with prognostic value.

Therapeutic potential of DGLA in allergic diseases. The first randomized placebo-controlled human trial, conducted by Yokoi et al., demonstrated the efficacy of oral DGLA (314 mg/day) in alleviating symptoms of pollen-induced allergic rhinitis [8]. DGLA significantly reduced the severity of sneezing and nasal congestion compared with placebo [8]. The mechanism of this effect is likely to be related to the inhibition of mast cell degranulation and leukotriene synthesis, as confirmed by data from animal models of atopic dermatitis [9]. This study is an important step in confirming the therapeutic potential of DGLA in clinical practice.

Genetic variations (FADS) as moderators of the GLA/DGLA effect. A key factor influencing DGLA metabolism and the efficacy of GLA supplementation is a genetic polymorphism in the FADS1 gene, which encodes delta-5-desaturase. A study by Sergeant et al. showed that in carriers of the “inefficient” allele (TT), GLA supplementation resulted in significant accumulation of DGLA, whereas in carriers of the “efficient” genotype (GG), GLA was more actively converted to AA [10]. This is of critical importance for personalizing nutritional therapy, as high doses of GLA in individuals with the “efficient” genotype may increase pro-inflammatory AA levels, thereby nullifying potential benefits [10]. Therefore, genotyping of FADS1 is necessary to optimize the dosage and expected effect of GLA supplementation.

New therapeutic strategies: delta-5-desaturase inhibition. An alternative approach to modulating the DGLA/AA pathway is the target inhibition of delta-5-desaturase (D5D) via small molecules or RNA interference (RNAi). As shown in the review by Pang et al., inhibition of D5D not only reduces AA formation but also redirects DGLA metabolism to 8-hydroxyoctanoic acid (8-HOA). This is a unique anticancer agent that induces apoptosis of cancer cells [11]. This strategy is considered promising for the treatment of colorectal, pancreatic, and breast cancers [11]. D5D inhibition is an example of how knowledge of the molecular pathways of PUFA metabolism can be used to develop fundamentally new classes of drugs. Thus, a comprehensive therapeutic approach to modulate DGLA metabolism may include nutritional support (GLA supplementation), dietary correction (LA reduction), and targeted pharmacological interventions (D5D inhibition) (Figure 3).

The role of trace elements and dietary balance. The activity of desaturases, particularly delta-6-desaturase – which is critical for GLA synthesis – is significantly modulated by zinc (Zn) status. As noted by Knez et al., zinc deficiency leads to a decrease in delta-6-desaturase activity. This is manifested in an increase in the LA/DGLA ratio, which is proposed as a new biomarker of zinc status [12]. In addition, the modern Western diet is characterized by excessive consumption of LA and a high n-6/n-3 ratio (up to 20-50:1). This creates a competitive advantage for the formation of pro-inflammatory mediators from AA [1]. The optimal n-6/n-3 ratio recommended for reducing inflammation is 4-5:1 [1]. Analysis of modern clinical studies confirms the multifaceted role of DGLA as a prognostic biomarker and therapeutic agent. The main results of key studies are summarized in the following table (Table 1).

The table summarizes the results of key prospective cohort and randomized controlled trials demonstrating the dual clinical significance of DGLA. The first cluster of data (Nilsen et al., Ouchi et al.) demonstrates the prognostic value of the molecule: low serum DGLA levels and a low DGLA/AA ratio are independent predictors of increased long-term all-cause mortality in patients with acute cardiovascular pathology (myocardial infarction, decompensated heart failure). The second cluster reflects the therapeutic potential: exogenous use of DGLA supplements significantly reduces the symptoms of allergic rhinitis (Yokoi et al.). The study by Sergeant et al. highlights the mechanism of genetic moderation: the effectiveness of nutritional support with DGLA precursors critically depends on the polymorphism of the FADS1 gene, which determines the rate of further conversion of DGLA into pro-inflammatory arachidonic acid and dictates the need for genetic screening.

3. Conclusions

Dihomo-γ-linolenic acid is a unique molecule among n-6 PUFAs. It combines the properties of a structural component of membranes, a precursor of anti-inflammatory eicosanoids, and a clinically relevant biomarker. Accumulating evidence suggests that its deficiency is associated with increased mortality in cardiovascular diseases. Therapeutic use of DGLA demonstrates efficacy in allergic conditions. The efficacy of GLA (precursor of DGLA) supplementation is significantly modulated by genetic variations in FADS1. This justifies the need for genotyping to personalize nutritional support. New therapeutic approaches, in particular D5D inhibition, open up prospects for the use of the DGLA metabolic pathway in anticancer therapy. An integrated approach that takes into account genetic, nutritional (zinc status), and dietary factors (n-6/n-3 ratio) is key to realizing the therapeutic potential of DGLA. Further research should be aimed at developing standardized protocols for determining DGLA levels, assessing the long-term effectiveness of its use, and creating selective modulators of its metabolism.