4.1. Gene Expression Levels and Protein Levels
The differentiation of preadipocytes into adipocytes is tightly regulated by transcription factors, including PPARγ and C/EBP, required for growth arrest [12, 14]. Two studies examined delphinidin's impact on PPARγ. In the first, 3T3-L1 cells treated with delphinidin (25-100 µM) reduced PPARγ mRNA and protein levels dose-dependently [10]. In the second, delphinidin suppressed intermediate PPARγ markers [8]. However, our study showed no significant change in PPARγ protein or mRNA levels, suggesting no anti-adipogenic effect, possibly due to lower dosage compared to other studies. C/EBPβ, a key transcription factor in early adipocyte differentiation [14], was examined in one study, which found that delphinidin significantly downregulated C/EBPβ and C/EBPδ expression in 3T3-L1 cells [8]. PPARγ and C/EBPα expression are followed by the expression of adiponectin and fatty acid-binding protein 4 (FABP4), which are responsible for terminal differentiation and formation of the fat cell phenotype [14]. While two studies suggest delphinidin as a potential anti-obesity agent due to its suppression of adipogenesis and mesenchymal stem cell adipogenesis [15], our data did not show a difference in FABP4 expression. Further studies are needed to confirm its effect.
CIDEA regulates beige adipocytes and it is associated with genes as PGC-1α and UCP1 involved in thermogenesis, during the transformation of WAT into BAT [16]. Although no studies explore delphinidin's effect on CIDEA expression, anthocyanin oligomers significantly up-regulated CIDEA [15]. UCP1, another key player in thermogenesis [17], lacks delphinidin-specific research, however anthocyanidins have shown potential in boosting energy expenditure by reducing mitochondrial respiration and dissipation of the mitochondrial proton gradient [18]. In a recent study, after the treatment of beige-like adipocytes with anthocyanin-rich black soybean, the level of UCP1 in mitochondria was fivefold higher than that in WAT [19]. On the contrary, current research demonstrated that there was not a significant difference after treatment for UCP1 protein concentration. However, we observed the highest UCP1 expression levels were in 5µM delphinidin-exposed cells during the differentiation stage. This result suggests delphinidin induced browning in 3T3-L1 adipocytes, especially in the early phase of adipogenesis, but not in a dose-dependent manner. However, the current study is the first that uses only delphinidin and it is limited to demonstrating the preventive role of delphinidin as a thermogenic compound in adipocytes.
PGC1-α, PPARγ, and PRDM16 are known crucial regulators of browning and UCP1 mRNA expression [17, 20]. PGC1-α increased oxygen consumption via mitochondrial activation and its expression increased in BAT [21]. There are no studies on the effect of delphinidin on PGC1-α, however, there are limited studies evaluating the anthocyanidin effect on this protein. In a recent study, the expression of PGC1-α was dose-dependently increased by anthocyanin treatment (500 mg/kg/day) cell culture of BAT from rats fed anthocyanins compared to that in BAT from non-fed rats [19]. Similarly, 3T3-L1 cells increased PGC-1α protein levels after treatment with anthocyanin oligomers [15]. In the present research, the protein concentration of PGC1-α was dose-dependently increased during maturation. However, delphinidin did not induce significant changes in PGC1α gene expression at any stage. PRDM16 is one of the key transcription factors that regulate the thermogenic gene program in brown and beige adipocytes [22]. PPARγ deacetylation is strictly connected with the up-regulation of PRDM16, which promotes browning [23]. To our knowledge, this is the first study to show the effect of delphinidin on PRDM16. There is only one study on the treatment of anthocyanin oligomers in 3T3-L1 that showed a significantly increased expression of the PRDM16 gene and proteins [15]. According to this study, this research showed an increase level of PRDM16 during the maturation process, especially at high dose of delphinidin. Our results suggest that delphinidin has browning effect on 3T3-L1 cells. It is necessary to conduct more studies to demonstrate the preventive role of delphinidin as a thermogenic compound in adipocytes.
4.2. Triglyceride Levels
In vitro and in vivo studies have shown that treatment with delphinidin had a great inhibitory effect on intracellular triglyceride accumulation in a dose-dependent manner. The triglyceride accumulation in HepG2 cells was reduced by 50% with 100 µM and 59% with 180 µM of delphinidin [24]. Another study showed that 25, 50, and 100 µM of delphinidin reduced intracellular triglyceride accumulation in 3T3-L1 cells by 30%, 38%, and 58%, respectively [8]. Black soybean seed coat extract reduced triglyceride levels significantly within a high-fat diet and streptozotocin (STZ)-induced diabetic mice [25]. However, some studies did not show an effect of delphinidin on triglyceride accumulation [24, 26, 27]. In the present study, intracellular triglyceride accumulation was significantly reduced by delphinidin. However, triglyceride did not reduce in a dose-dependent manner. Interestingly, in both process 10 µM of delphinidin reduced intracellular triglyceride accumulation more than 20 µM. Moreover, 10 µM reduced triglyceride levels in the maturation process (67%) instead of the early differentiation process (46%). A possible reason for this result could be the delayed effect of delphinidin treatment on cell cycle progression. The lipid-lowering effect of delphinidin is largely attributed to the early stage of adipogenesis, but along with this study, the positive effect of delphinidin appears in the intermediate and late adipogenesis phases.
4.3. Metabolomics
This study is the first to show the effect of delphinidin treatment on metabolic pathways in 3T3-L1 cells. During the differentiation process, the metabolite significantly increased with delphinidin was glyceraldehyde, a triose monosaccharide intermediate in the metabolism of fructose [28]. Glyceraldehyde is highly reactive and can modify and cross-link proteins, which are cytotoxic and inhibit intracellular glutathione levels [29]. The study suggests that delphinidin might have an effect on fructose metabolism only in lower concentrations.
During the maturation, the mannose metabolite significantly changed. It belongs to the class of organic compounds known as hexose phosphates [30]. Mannose is involved in a number of enzymatic reactions, especially it can be converted into fructose 6-phosphate through its interaction with the enzyme mannose-6-phosphate isomerase. Mannose inhibits adipogenic differentiation and regulates function for glucose metabolism [31]. Our results suggest that 5 µM delphinidin treated cells could be more effective. Surprisingly, the higher levels of delphinidin show lower levels of mannose. Therefore, delphinidin could be positively reduce adipogenic differentiation at a lower dose. To further explore the role of delphinidin with metabolites, it is necessary more studies, where metabolomic analyses were carried out using both GC-MS and liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-qTOF-MS) methods.
Plant-based bioactive compounds show potential in anti-obesity effects, however research on delphinidin is limited. Our study explored delphinidin's effects on 3T3-L1 preadipocytes, indicating its potential to contribute to browning by increasing UCP1 and PRDM16 expression and reducing triglyceride levels during maturation. However, we did not observe any effects of delphinidin on the key expressions of beige specific genes (PPARγ, PGC1 α and CIDEA). Additionally, the impact on metabolites such as glyceraldehyde, alanine, porphine, mannose and p cresol remain uncertain. While our foodomics approach offers valuable insights into the potential of delphinidin as a dietary supplement for obesity management, further in-depth in vitro and in vivo studies are essential to fully elucidate its browning effects and therapeutic viability.