Effect Of High-density Lipoprotein On Inflammatory Adipocytes

BackgroundAtherosclerosis (AS), the so called "silent killer", is the leading disease challenging human health and responsible for the morbidity and mortality of population in developed and even partly developing countries. It is well acknowledged that AS presents an inflammatory disease. Obesity, which is associated with increased systemic inflammation, is demonstrated as an independent risk factor for coronary heart disease (CHD). Adipose. tissue has direct connection between obesity and systemic inflammation. Adiposities' function is of vital importance in the development of obesity-associated diseases and the process of vascular injury. Adipoeytes are, at the present, regarded as the crossroads of energy homeostasis, inflammation, and atherosclerosis. Therefore, the studies regarding adiposities' function is accordingly highly interested by people.Adipose tissue, the largest energy reservoir in the body, can preserve redundant energy in the form as triglyceride. However, it has been shown that adipose tissue is not only an energy-regulating organ, but also an important endocrine organ in that it can secret various adipokines and chemokines, many of which might play significant roles in atherogenesis. There is an increase of metabolic diseases during the dysfunction of adipocytes, including obesity, diabetes, hypertension and dyslipidemia and so on, the majority of which can then lead to an increase of AS risk. Interleukin-8 (IL-8) is the most important cytokines produced by different cellular types in response to various stimuli. It is known as one of the super family of chemotatic factors, secreted by a serial of proatherogenic cells such as endothelial cells, peripheral blood mononuclear cells, smooth muscle cells and adipocytes. It is clinically observed that the levels of IL-8 in circulation may be relative with AS. However, it still remains to be researched about the secretion of IL-8 by adipocytes in the state of inflammation.Adipocytes also display an important role in cholesterol metabolism in the body. Because adipose tissue is found as the largest pool of free cholesterol in vivo and has a buffer action for circulatory cholesterol metabolism. Moreover, cholesterol effiux has been proven crucial for the balance of cholesterol metabolism in adipocytes.Scavenger receptor class B type I. (SR-BI), a high density lipoprotein (HDL) receptor, can induce the effiux of free cholesterol from peripherical cells to HDL. Interestingly, SR-BI has been observed highly expressed in adipocytes. But it is still unclear about the function of SR-BI on cholesterol effiux in inflammatory adipocytes.It has been well known about the important role of oxidized low density lipoprotein (ox-LDL) in the development and progress of AS. It is regarded as one of important regulatory pathway for ox-LDL entering monocytes that peroxisome proliferators- activated receptorγ(PPARγ) induces the expression of CD36 at transcriptional level. Our pretrial work has substantiated that adipocytes have the potency to intake ox-LDL, considerably involving PPARγ-CD36 pathway, which shows adipose tissue is possibly an important organ removing ox-LDL in vivo. Therefore, it is very necessary for us to further implore the uptake and degradation of ox-LDL by adipocytes in inflammation since inflammation is regarded as a key factor in the development of AS.High density lipoprotein (HDL) is a heterogeneous protein with the almost moiety of lipid and protein and it is a hot issue on the atheroprotective role of HDL. The .related mechanisms underlying anti-AS by HDL includes cholesterol reverse transport, anti-inflammation, anti-oxidation, anti-thrombosis as well as endothelial protection. It is still unproven that there is a directed effect of HDL on the processes, including the secretion of IL-8, cholesterol effiux and the uptake and degradation of ox-LDL related inflammatory adipocytes, or not. There, it is believed to markedly improve the diagnosis and therapy of cardiovascular disease to recognize related risk factors and more efficient drug of AS, since AS is taken as a disease involving multifactor.ObjectiveWe will illustrate the relationship inflammation, adipocytes and AS for further comprehension of anti-AS by HDL, which maybe help to determine a novel therapeutic target in treatment of obesity-related metabolic diseases, via observing the secretion of IL-8 by adipocytes in inflammation, exploring the effect of HDL on the secretion of IL-8 by adipocytes and its possible mechanism, detecting the effect of HDL on the expression of SR-BI and the cholesterol effiux in adipocytes in inflammation, as well as investigating the effect of HDL on and the uptake and degradation of ox-LDL by inflammatory adipocytes and its involved mechanism.Methods3T3-L1 adipocytes were cultured and induced to differentiation and maturation. After cells were starved for 24 hours with serum-free medium containing 0.2% BSA, HDL in various concentrations (0μg/ml, 10μg/ml, 50μg/ml and 100μg/ml) were added and 6 hours later, 100 ng/ml LPS were added for co-incubation for 6 hours. Then, supernatant and cells were gathered respectively, stored in -70^C for future application. After then, IL-8 in supernatant was detected with ELISA, the mRNA expression of CD36, PPARy and SR-BI in cells was determined via RT-PCR, the effiux rates of 3H-cholesterol in cells were detected by means of liquid scintillation counter, and the uptake and degradation of ox-LDL by adipocytes in individual groups were measured by radioligand assay.Results1. LPS stimulated the secretion of IL-8 and reduced the mRNA expression of PPARγin adipocytes. HDL decreased the levels of IL-8 from inflammatory adipocytes dose-dependently and accordingly up-regulated the mRNA expression of PPARγin these cells.2. LPS decreased cholesterol effiux in stimulating adipocytes while HDL promoted the cholesterol effiux from inflammatory adipocytes in a dose-dependent manner. The mRNA expression of SR-BI declined in adipocytes with stimulated by LPS while HDL up-regulated the mRNA expression of SR-BI.3. ¹²⁵I-ox-LDL was uptaked by adipocytes and its uptake considerably declined with the stimulation of LPS, which was relative to the mRNA expression of PPAR^/and CD36. In contrast, HDL increased the uptake of¹²⁵I-ox-LDL by inflammatory adipocytes via the pathway of PPARγ-CD36.Conclusions1. LPS can induce the inflammatory response of adipocytes via increasing the secretion of IL-8 from adipocytes while HDL can inhibit IL-8 secretion in inflammatory adipocytes through the up-regulation of PPARγrnRNA expression, which maybe one of anti-inflammatory mechanisms by HDL.2. Inflammation can decrease the cholesterol efflux of adipocytes while HDL can promote the cholesterol efflux of inflammatory adipocytes via SR-BI pathway.3. Inflammation can impair the ability of adipocytes' uptaking ox-LDL while HDL can enhance ox-LDL uptake by inflammatory adipocytes through PPARγ-CD36 pathway.