Study On The Effect Of LJP61A On Atherosclerotic LDLr^-/- Mice Induced By HFD Via Ameliorating Insulin Resistance

As a major bioactive substance of Laminaria japonica, a well-known marine vegetable, the polysaccharides have the ability of anti-atherosclerosis. The overproduction of very low density lipoprotein (VLDL) is an important cause for the initiation and development of atherosclerosis, which is highly associated with insulin signaling. The aim of the current study is to uncover the underlying mechanism of a known structure Laminaria japonica polysaccharide (LJP61A) against atherosclerosis via ameliorating insulin resistance in HFD-fed LDLr-/- mice. The main results were obtained as follows:(1) LJP61A ameliorates systemic insulin resistance and hepatic steatosis in HFD-induced atherosclerotic LDLr-/- mice:The effects of LJP61A on systemic insulin resistance and hepatic steatosis were researched using an atherosclerotic model of LDLr-/-mice induced by high fat diet (HFD). When the dosage of LJP61A reached 200 mg/kg/day, the plasma glucose, plasma insulin and HOMA-IR were significantly decreased by 35.6%, 36.6% and 59.3% compared to those of model group, respectively. The administration of LJP16A was also observed to inhibit HFD-induced fat accumulation in hepatocellular vacuoles by hematoxylin and eosin (H&E) staining.(2) LJP61A inhibited the overproduction of VLDL:After oral administration of LJP61A at 200 mg/kg/day, plasma levels of VLDL apoB, VLDL TG and VLDL CE were observed to be decreased by 47.1%,65.1% and 46.2% compared to those of HFD group, respectively, indicating that LJP61A inhibited the VLDL overproduction induced by HFD. According to the western-blot analysis, both PI3K-AKT-mTORC1 and PI3K-AKT-Foxol pathways meditated by IRS1 and IRS2 were modulated by LJP61A. Furthermore, the mRNA expressions of SREBP-1c, ACC1, FAS, Sortilin, apoB, apoCIII and MTP, downstream genes of mTORCl and Foxol, were regulated by LJP61A. The results indicated that LJP61A may affect VLDL secretion through the modulation of insulin signaling.(3) LJP61A inhibited TG synthesis and apoB secretion in insulin-resistant HepG2 cells:HepG2 cells under high glucose plus insulin conditions were applied to mimick hepatic insulin resistance in vivo. The study found that LJP61A inhibited the activation of mTORC1 and Foxo1, leading to the modulation of mRNA expressions of mTORC1 and Foxol downstream genes involved in VLDL assembly and secretion. Meanwhile, IRS-PI3K-AKT signaling, the upstream signaling of mTORC1 and Foxo1, were also observed to be regulated by LJP61A. To verify whether the effect of LJP61A on the expression of mTORC1 and Foxo1 is the result of the regulation of insulin signaling, we used mTOR inhibitor (Rapamycin) and Foxo1 inhibitor (AS 1842856) were employed to treat HepG2 cells. We found the regulation of mRNA expressions of SREBP-lc, ACC1, FAS and Sortilin through PDK-AKT-mTORC1 pathway, leading to the suppression of TG synthesis and stimulation of apoB degradation. Moreover, LJP61A decreased the expression of nuclear Foxo1 and mRNA expression of apoCIII and MTP via ameliorating the impaired insulin signaling, resulting in the regulation of apoB lipidation.In conclusion, LJP61A ameliorated insulin resistance and inhibited VLDL overproduction via the regulation of insulin signaling, leading to the inhibiton of atherosclerosis.