Regulatory effects of degraded polysaccharides from Gracilaria Lemaneiformis on glycolipid metabolism via the PPAR-γ signaling pathway

A prolonged high-sugar and high-fat diet predisposes individuals to insulin resistance and dysregulated cholesterol and triglyceride synthesis, thereby disrupting glycolipid metabolism. Gracilaria lemaneiformis is rich in polysaccharides and has physiological that exhibit hypoglycemic, hypolipidemic, and antihypertensive effects. This study aims to investigate the mechanism by which polysaccharides degradation extracted from Gracilaria lemaneiformis (GLP-HV) regulate glycolipid metabolism through the PPAR-γ signaling pathway. This research employed human hepatocellular carcinoma (HepG2) cells and Kunming (KM) mice as experimental models. Biochemical parameters were assessed, lipid accumulation in cells was observed via Oil red O staining, and pancreatic islet morphology and distribution in mice were examined using hematoxylin-eosin (HE) staining. Furthermore, the expression levels of key factors in the PPAR-γ signaling pathway were analyzed using real-time quantitative PCR and Western blot techniques. Spearman correlation analysis was subsequently applied to investigate the relationship between glycolipid metabolism activity and the PPAR-γ signaling pathway. The results showed that free fatty acids (FFA) and lipid peroxides (LPO) levels were (4.17±1.47) μmol/mg prot and (5.49±0.41) ng/mg prot in MODE group, which higher than NC group. LPL activity was significantly higher in the NC group (204.18±13.82) than in the MODE group (161.57±8.74) U/mg prot. GLP-HV significantly reduced FFA and LPO levels, while enhancing the LPL activity. The Oil red O staining result displayed GLP-HV significantly reduced the lipid aggregation state in HepG2 cells. The absorbance values of Oil red O in GLP-HV groups at doses of 40 and 80 μg/mL were (1.29±0.07) and (1.11±0.07), respectively, both of which were significantly lower than that in MODE group (1.76±0.08). Moreover, the content of the levels of total cholesterol (TC), triglycerides (TG), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) were (3.59±0.42), (1.51±0.33), (2.45±0.36) and (6.03±0.32) mmol/L in MODE group, respectively. GLP-HV administration significantly ameliorated dyslipidemia in mice across a dose range of 50-150 mg/(kg·day), markedly reducing serum levels of TC, TG and LDL-C while increasing HDL-C. H.E. staining showed that the number of islet cells in the group with disordered glycolipid metabolism was reduced and their morphology was abnormal. After oral administration of GLP-HV intervention, the structure of the islets in mice showed a recovery trend. These changes markedly ameliorated intracellular lipid accumulation and improved the morphology and distribution of pancreatic islet cells in mice. At the molecular level, polysaccharide degradation from G. lemaneiformis upregulated the genes and proteins expression of PPAR-γ, AMPK, and HSL, while suppressing the expression of SREBP-1c, ACC, and FAS. Correlation analysis revealed that the modulatory activity on glycolipid metabolism exhibited a positive correlation with PPAR-γ, AMPK, and HSL, and a negative correlation with ACC and SREBP-1c. The polysaccharide degradation of G. lemaneiformis significantly ameliorated glycolipid metabolism in both cell and mice models. The underlying regulatory mechanism was mediated via the PPAR-γ signaling pathway. This PPAR-γ/AMPK/SREBP-1c/ACC/FAS/HSL regulatory axis maintained the dynamic homeostasis of systemic glycolipid metabolism.