Geraniol reverses obesity by improving conversion of WAT to BAT in high fat diet induced obese rats by inhibiting HMGCoA reductase

Obesity, driven by sustained energy intake exceeding expenditure, leads to excess triglyceride storage in white adipose tissue (WAT) and is associated with metabolic disorders including CVD and type 2 diabetes (T2DM). Adipose tissue macrophages (ATMs) contribute to insulin resistance via proinflammatory signaling, whereas M2 macrophage activation supports thermogenesis in brown adipose tissue (BAT). HMG-CoA reductase is central to cholesterol biosynthesis; its inhibition (e.g., by statins) can improve insulin sensitivity and influence adipose tissue macrophage polarization but long-term statin use has limitations. Geraniol, an acyclic monoterpene, has reported immunomodulatory, anti-inflammatory, and hypoglycemic effects and has been suggested to inhibit HMG-CoA reductase. The study aimed to evaluate whether geraniol mitigates HFD-induced obesity, improves glucose and lipid metabolism, and promotes WAT-to-BAT conversion, and to explore interaction with HMG-CoA reductase as a potential mechanism.

Background literature highlights: (1) Obesity prevalence and its link to metabolic diseases; (2) WAT as a triglyceride storage depot and BAT as thermogenic tissue, with ATMs influencing insulin resistance (M1-like proinflammatory) and thermogenesis (M2-like); (3) HMG-CoA reductase inhibition reduces cholesterol synthesis and may enhance insulin sensitivity and M2 activation in BAT; (4) Limitations of chronic statin therapy motivate exploration of phytochemicals; (5) Geraniol exhibits anti-inflammatory, immunomodulatory, neuroprotective, antitumor, and hypoglycemic activities, and has been reported to inhibit HMG-CoA reductase and improve glycemic control in diabetic rodent models. This evidence supports investigating geraniol as an anti-obesity and insulin-sensitizing agent that may modulate adipose tissue phenotype.

Animals and ethics: Thirty healthy male Albino Wistar rats (180–200 g) were housed under controlled conditions (23 ± 2 °C; 55 ± 5% humidity; 12 h light/dark). Protocol approved by IAEC, Amity University Lucknow (AUUP/AIP/4.2/2021). Experimental design: Rats randomized into five groups (n = 6/group): Normal control; Negative control (HFD); Geraniol 200 mg/kg; Geraniol 400 mg/kg; Standard (atorvastatin). Obesity induction: High-fat diet (HFD; 31% fat, 12% protein, 46% carbohydrate; 516.5 kcal/100 g feed) for 4 weeks to all except control. Interventions: From week 5 to 8, geraniol groups received geraniol 200 or 400 mg/kg p.o.; standard group received atorvastatin 20 mg/kg p.o. Measurements: Feed intake recorded daily; body weight weekly. Blood glucose measured by GOD-POD method at end of weeks 4 and 8. OGTT performed at end of week 8 after overnight fast; glucose measured at 0, 30, 60, 90, 120 min. Blood and serum: Blood collected via retro-orbital plexus for glucose; tail vein sampling for OGTT. Serum separated (3000 rpm, 10 min, 4 °C) for biochemical assays (glucose, lipid profile: total cholesterol, triglycerides, HDL, LDL). Tissue collection: After 8 weeks, rats sacrificed by cervical dislocation. Adipose depots isolated and weighed: subcutaneous, epididymal, inguinal, mesenteric, triceps, intraperitoneal, and interscapular WAT/BAT. Organs (liver, pancreas, heart) isolated, washed in cold PBS (pH 7.4), blotted, and weighed. Oxidative stress assays: Liver homogenates analyzed for superoxide dismutase (SOD) using riboflavin-methionine-NBT photoreduction assay (absorbance 560 nm) and lipid peroxidation via malondialdehyde (MDA) using thiobarbituric acid-reactive substances assay (absorbance 532 nm). Histopathology: Heart, pancreas, and interscapular adipose tissue fixed (heart in 4% formalin; others in 10% neutral buffered formalin), paraffin-embedded, sectioned, and H&E stained. Microscopy performed using Nikon Eclipse 80i; photomicrographs taken from three random fields per slide. In silico docking: Geraniol structure obtained from PubChem (SDF), converted to PDB via OpenBabel, then to PDBQT. HMG-CoA reductase crystal structure (PDB ID: 1HWK) retrieved. Protein prepared by removing heteroatoms, adding polar hydrogens, Kollman charges, and Gasteiger charges using AutoDock 4.2 and Discovery Studio Visualizer v20.1. Lamarckian genetic algorithm used with population size 150, mutation rate 0.02, crossover rate 0.8, and 2.5 × 10^5 operations. Grid spacing 0.375 Å; binding energies, hydrogen bonds, hydrophobic interactions analyzed. Statistics: Data expressed as mean ± SEM (n = 6). One-way ANOVA with Dunnett’s post hoc for group comparisons; two-way repeated-measures ANOVA with Bonferroni post hoc for OGTT/time-course data. Significance at p < 0.05.

- Feed intake: No significant differences among groups over 8 weeks. - Body weight: HFD (negative control) significantly increased body weight vs control (p < 0.01); geraniol (200 and 400 mg/kg) and standard treatment significantly reduced body weight vs negative control (p < 0.01). - Glycemia: After 4 weeks HFD, fasting glucose increased in HFD groups vs control; after treatment (week 8) geraniol significantly reduced glucose vs negative control (p < 0.01). OGTT: Negative control showed elevated glucose at 0–120 min; geraniol significantly improved glucose tolerance (p < 0.01 vs negative control). - Lipid profile (serum; Table 1): Negative control vs control: total cholesterol 73.97 ± 1.34 vs 32.98 ± 1.24 mg/dl; triglycerides 116.18 ± 2.68 vs 67.68 ± 0.66 mg/dl; HDL 20.20 ± 1.39 vs 42.40 ± 2.69 mg/dl; LDL 123.41 ± 4.46 vs 60.84 ± 2.69 mg/dl (all p < 0.01). Geraniol 400 mg/kg attenuated dyslipidemia: cholesterol 43.88 ± 1.65 mg/dl; triglycerides 81.01 ± 3.29 mg/dl; HDL 36.20 ± 1.16 mg/dl; LDL 71.36 ± 2.47 mg/dl (p < 0.01 vs negative control). - Adipose depots and organ weights: HFD increased weights of triceps, inguinal, subcutaneous, intraperitoneal, interscapular fat depots and organ weights (liver, pancreas, heart) vs control (p < 0.01). Geraniol significantly reduced these weights vs negative control (p < 0.01). - Oxidative stress (liver): HFD increased MDA and decreased SOD vs control (p < 0.01); geraniol reversed these alterations (p < 0.01 vs negative control). - Histopathology: Heart—negative control showed hyaline degenerative changes and intramuscular fat; geraniol improved cardiac histoarchitecture. Pancreas—negative control exhibited degeneration of islets and acinar cells with intratubular fat; geraniol attenuated pathology. Adipose tissue—negative control showed enlarged lipid vacuoles (WAT predominance); geraniol reduced vacuole size and increased BAT features in interscapular adipose tissue. - In silico docking: Geraniol bound HMG-CoA reductase (PDB 1HWK) with binding energy −5.13 kcal/mol, hydrogen bonds with GLN770 (2.031 Å) and ASP767 (2.169 Å); reported van der Waals energy −6.54 and inhibition constant 174.35.

The study demonstrates that geraniol mitigates HFD-induced obesity and metabolic derangements, improving body weight, glycemic control, and dyslipidemia. The histological shift toward increased BAT features and reduced WAT vacuolation in interscapular adipose tissue, alongside reductions in diverse adipose depots and organ weights, supports a WAT-to-BAT conversion and enhanced energy expenditure. Improvements in hepatic oxidative stress markers (decreased MDA, increased SOD) suggest attenuation of oxidative and inflammatory pathways implicated in obesity-related mitochondrial dysfunction and reduced UCP1-mediated thermogenesis. In silico findings that geraniol binds HMG-CoA reductase provide a plausible mechanistic link to improved lipid handling and insulin sensitivity, aligning with literature that HMG-CoA reductase inhibition can favor M2 macrophage activation and BAT thermogenesis. Collectively, these results address the hypothesis that geraniol counters HFD-induced obesity by promoting BAT activity and improving metabolic profiles, potentially via HMG-CoA reductase interaction and downstream anti-oxidative/anti-inflammatory effects.

Geraniol improved insulin sensitivity and reduced obesity in HFD-induced obese rats, evidenced by lower body weight, improved glucose tolerance, corrected dyslipidemia, reduced adipose depots and organ weights, and favorable histological changes including increased BAT characteristics. Mechanistically, geraniol likely promotes WAT-to-BAT conversion and modulates oxidative/inflammatory pathways, with supportive in silico evidence of interaction with HMG-CoA reductase. Future work should validate these mechanisms in vivo, delineate signaling pathways (e.g., UCP1, macrophage polarization), clarify dose-response and pharmacokinetics, and assess translational potential in other models and sexes.