巨噬细胞修饰低密度脂蛋白对细胞受体活性的影响及前列腺素E_2的作用

为研究低密度脂蛋白经细胞修饰后对细胞受体的影响及前列腺素E2的作用，利用免疫细胞化学和生物亲和素-酶联免疫吸附法分别检测小鼠腹腔巨噬细胞清道夫受体、小鼠皮肤纤维母细胞低密度脂蛋白受体的结合活性，结果发现修饰组的巨噬细胞胞浆及胞膜有强阳性黄褐色颗粒，而药物组仅中度着色；修饰组低密度脂蛋白与其受体结合量（54±13μg／g细胞蛋白）较对照组（144±μg／g细胞蛋白）显著下降（P＜0．01）；药物组受体结合量（100±11μg／g细胞蛋白）较修饰组明显提高（P＜0．05）。表明细胞修饰低密度脂蛋白能够刺激清道夫受体活性，抑制低密度脂蛋白受体活性；前列腺素E2（20mg／L）可对抗细胞修饰低密度脂蛋白对此二种受体的作用。     

两种细胞因子和表皮生长因子、纤维母细胞生长因子对血管平滑肌细胞清道夫受体活性的影响

为探讨有关的细胞因子和生长因子是否影响平滑肌细胞清道夫受体与氧化型低密度脂蛋白结合活性，以Ｃｕ＾２＋氧化法制备的氧化型低密度脂蛋白免疫豚鼠，制备抗氧化型低密度脂慢白抗体，在培养的小牛主动脉平滑肌细胞中，用细胞酶联免疫吸附检测法测定清道夫受体结合氧化型低密度脂蛋白的活性。结果发现，碱性纤维母细胞生长因子、表皮生长因子、肿瘤坏死因子可提高血管平滑肌细胞表面清道夫受体活性，而粒细胞－巨噬细胞集落刺激因子     

两种细胞因子和表皮生长因子、纤维母细胞生长因子对血管平滑肌细胞清道夫受体活性的影响

为探讨有关的细胞因子和生长因子是否影响平滑肌细胞清道夫受体与氧化型低密度脂蛋白结合活性，以Ｃｕ２＋氧化法制备的氧化型低密度脂蛋白免疫豚鼠，制备抗氧化型低密度脂蛋白抗体，在培养的小牛主　动脉平滑肌细胞中，用细胞酶联免疫吸附检测法测定清道夫受体结合氧化型低密度脂蛋白的活性。结果发现，碱性纤维母细胞生长因子、表皮生长因子、肿瘤坏死因子可提高血管平滑肌细胞表面清道夫受体活性，而粒细胞－巨噬细胞集落刺激因子对清道夫受体活性无影响。此结果提示，某些细胞因子、生长因子对平滑肌细胞清道夫受体活性具有调节作用。     

人红细胞胰岛素受体自身磷酸化的酶联免疫吸附测定

目的建立人红细胞胰岛素受体自身磷酸化测定法。方法应用合成的磷酸酪氨酸钥孔帽贝血蓝蛋白（ＰＹＫＬＨ）免疫家兔得到抗血清，将纯化的抗磷酸酪氨酸抗体作为第一抗体，羊抗兔Ｉｇ酶结合物（辣根过氧化物酶标记）作为第二抗体，建立双抗体夹心酶联免疫吸附测定法。结果批内变异系数为８．４％，批间变异系数为９．０％。２０例健康人的红细胞ＩＲ的自身磷酸化活性，测定值为０．１５０±０．０２２ｎｍｏｌＰＹ／ｍｇ受体蛋白。在体外，胰岛素在１０－６ｍｏｌ／Ｌ时对自身磷酸化达到最大激活。结论用酶免法测定人红细胞胰岛素受体灵敏、特异、简便，为胰岛素作用机制的理论研究、胰岛素受体基因突变的检测和糖尿病及其它胰岛素抵抗综合症的病因及治疗的探讨提供了有效的方法。     

猪肾上腺皮质细胞膜高密度脂蛋白结合蛋白的研究

为鉴定猪肾上腺皮质细胞膜是否存在高密度脂蛋白结合蛋白，并研究其特性，从新鲜猪肾上腺皮质细胞膜提取高密度脂蛋白结合蛋白，应用亲和层析法将其初步纯化。获得的可溶性蛋白经Ligandblot结合分析和Dotblot法测定，结果发现此膜蛋白与高密度脂蛋白有高度亲和力，用Westernblot法测得该蛋白的分子量为80kDa，亲和层析提取得到的结合蛋白分子量为77kDa。应用生物素-亲和素酶联免疫吸附试验测定该蛋白的结合特性：平衡解高常数（kd）=21．6μg／g，最大结合量（Bmax）=678.2mg／L蛋白。结果提示纯化的猪肾上腺皮质细胞膜高密度脂蛋白结合蛋白具有很高的亲和力，证明亲和层析法条件温和。     

内分泌和遗传疾病

912222 酶联免疫吸附法测定促甲状腺激素受体抗体及临床应用/周肃…//天津医药。-1990,18(11)-648～649 根据酶联免疫吸附测定法原理,利用豚鼠细胞膜蛋白,建立了一种直接测定血清中促甲状腺激素受体抗体(TRAb)的方法,用以检测Graves病患者63例。结果TRAb阴性率为71.5％,而过去检测患者血清中长效甲状腺刺激因子免疫活性,阴性率只达42.5％。认为该法简快准确,可用于临床检验。图2参8(林中昌)     

瘦素对培养大鼠原代肝细胞葡萄糖吸收及肝细胞膜胰岛素受体磷酸化的影响

目的：观察瘦素对培养大鼠原代肝细胞葡萄糖吸收及肝细胞膜胰岛素受体磷酸化的影响,探讨瘦素参与胰岛素抵抗的分子生物学机制。方法：体外培养大鼠肝细胞,根据培养基中加入瘦素的浓度将实验分为空白对照组以及50、100、200、500／μg／L瘦素组,共5组,每组分别培养2h和24h,用全自动生化分析仪检测培养基中剩余的葡萄糖浓度,用ELISA方法测定肝细胞膜磷酸化胰岛素受体β亚基（pY 1158）含量。结果：①培养基剩余葡萄糖检测,瘦素抑制肝细胞对葡萄糖的吸收,且抑制效应随瘦素浓度增加和时间延长而增强（P〈0.01）;②肝细胞膜磷酸化胰岛素受体β亚基（pY1158）水平测定,50／μg／L瘦素组与对照组比较差异无统计学意义（P〉0．05）,100、200、500／2μg／L瘦素组与对照组比较差异有极显著性（P〈0．01）,并随瘦素剂量上升含量进一步下降。结论：瘦素与胰岛素抵抗相关,抑制细胞膜胰岛素受体β亚基（pY1158）磷酸化可能是其参与胰岛素抵抗的机制之一。     

低密度脂蛋白受体抑制剂对低密度脂蛋白和脂蛋白(a)代谢的影响

应用低密度脂蛋白受体抑制剂乳酸肝褐质和受体结合蛋白经刺猬腋下静脉注入，２ｍｉｎ后注射１２５Ⅰ－低密度脂蛋白或１２５Ⅰ－脂蛋白（３），６ｈ后处死，测定血、肝、肾、脾、胆汁和肾上腺的放射活性。实验发现：乳酸肝褐质和受体结合蛋白均能抑制低密度脂蛋白受体活性，使各组织摄取低密度脂蛋白分别降低１５％～８６％以上，但乳酸肝褐质和全体结合蛋白对脂蛋白（ａ）的组织摄取不但无抑制作用，反而能使脂蛋白（ａ）进入组织量增加，激活率分别达４０％～１２６％。实验结果说明：脂蛋白（ａ）虽然与低密度脂蛋白结构相似，含有７０％载脂蛋白Ｂ１００，但由于与载脂蛋白（ａ）以二硫键的形式连接，改变了空间结构，失去了与低密度脂蛋白受体结合的能力，并且推测乳酸肝褐质和受体结合蛋白在抑制低密度脂蛋白受体的同时，可能激活了细胞膜的其它机制，从而利于脂蛋白（ａ）进入细胞内。     

醛类修饰低密度脂蛋白抗原结构及其生物学意义的研究

为研究丙二醇和4-羟基土烯醛同时修饰低密度脂蛋白后引起其结构和功能的变化，先用这两种醛类同时修饰低密度脂蛋白，再用琼脂糖凝胶电泳测定了其相对迁移率，然后制成特异性抗体，用放射性标记法测定了这种抗体对细胞降解醛类修饰低密度脂蛋白的影响。结果发现这种修饰脂蛋白的抗原性主要取决于丙二醇的修饰（约70％），与氧化型伙密度脂蛋白有一定程度的类同（约21％），但不同于乙酸化价密度脂蛋白。THP-1细胞降解125I-醛类修饰低密度脂蛋白的能力与125I-氧化型低密度脂蛋白相似，分别为243±53μg／g细胞蛋白和233±35μg／g细胞蛋白，但不如125I-未修饰低密度脂蛋白（441±16μg／g细胞蛋白）；乙酰化低密度脂蛋白能竞争性抑制125I-醛类修饰低密度脂蛋白的降解，而低密度脂蛋白则不能。在小鼠腹腔巨噬细胞，对125I-醛类修饰低密度脂蛋白的降解值约为125I-低密度脂蛋白的10倍，当加入不同浓度针对醛类修饰低密度脂蛋白的抗体后，发现细胞的降解值为原来的18％～223％，提示这种观醛修饰低密度脂蛋白可能主要通过细胞膜清道夫受体途径为细胞所摄取。     

薏苡仁多糖对实验性2型糖尿病大鼠胰岛素抵抗的影响

目的 观察薏苡仁多糖对实验性2型糖尿病大鼠胰岛素抵抗的影响。方法 用小剂量链脲佐菌素（25mg/kg,iv)加高热量饲料（热卡：20.083J/g)建立实验性2型糖尿病大鼠模型，然后用薏苡仁多糖分三个剂量组（25、50、100mg/kg，ip)给药治疗2周，并测定葡萄糖耐量，血浆胰岛素以及肝糖原、肌糖原、肝细胞膜胰岛素受体结合率和肝葡萄糖激酶活性。结果 薏苡仁多糖能改善实验性2型糖尿病大鼠糖耐量异常，增加肝糖原量和肝葡萄糖激酶活性，且呈现一定的量效关系。但对血浆胰岛素水平及胰岛素受体最大结合率和受体最大结合容量均无影响。结论 薏苡仁多糖能够改善实验性2型糖尿病大鼠胰岛素抵抗，这可能与其调节糖代谢酶的活性有关。     

Ability of the LDL receptor from several animal species to recognize the human apo B binding domain: studies with LDL from familial defective apo B-100.

To verify whether the LDL receptors from different animal species recognize the binding domain of human apo B-100 we studied the interaction of LDL from control and familial binding defective apo B-100 (FDB) with cultured cells. Human, monkey, bovine, guinea pig and rabbit LDL receptors distinguish between normal and binding defective LDL with a displacement ratio (defective/normal) of 3.3, 2.6, 3.4, 3.1 and 2.0, respectively. Guinea pig and rabbit receptors, however, showed affinities 2-3-fold lower than the human receptor. Hamster, rat and mouse cells failed to differentiate between normal and FDB LDL with a ratio of 1.2, 0.8, and 1.4; the apparent affinities were 4-8 times lower than that of the human receptor. The data from the latter species suggest that the LDL receptor recognizes an area of human apo B different from the human receptor binding domain. The ability of antibody Mb47 to inhibit the binding of human LDL to human, rabbit and guinea pig but not to mouse cells further stresses this concept. Moreover, in 17 alpha-ethinyl estradiol-treated rats the rate of disappearance from plasma of FDB and control 125I-labelled LDL was identical, thus confirming the in vitro observations. These data suggest that the binding domain of the LDL receptor is functionally conserved in man, monkey, cow, rabbit and guinea pig, but is quite distinct in rat, mouse and hamster.     

Deficiency of low density lipoprotein receptors in liver and adrenal gland of the WHHL rabbit, an animal model of familial hypercholesterolemia.

The WHHL (Watanabe heritable hyperlipidemic) rabbit has been proposed as an animal model for human familial hypercholesterolemia. Homozygous WHHL rabbits have marked increases in the plasma level of low density lipoprotein (LDL), removal of LDL from their plasma is delayed, and LDL receptors are absent from their cultured fibroblasts [Tanzawa, K., Shimada, Y., Kuroda, M., Tsujita, Y., Arai, M. & Watanabe, Y. (1980) FEBS Lett. 118, 81--84]. We here report that membranes from the liver and adrenal gland of WHHL rabbits lack high-affinity LDL receptors. In normal rabbit membranes, binding of LDL to this receptor required calcium and is inhibited by EDTA. The LDL receptor binds rabbit 125I-labeled beta-migrating very low density lipoprotein (beta-VLDL), which contains apoproteins B and E, as well as rabbit 125I-labeled LDL, which contains only apoprotein B. It does not bind high density lipoprotein or methyl-LDL. All of these properties are identical with those of the LDL receptor of cultured fibroblasts. We conclude that a deficiency of hepatic and adrenal LDL receptors contributes to the hypercholesterolemia of the WHHL rabbits.     

Low-density lipoprotein (LDL) binds to a G-protein coupled receptor in human platelets. Evidence that the proaggregatory effect induced by LDL is modulated by down-regulation of binding sites and desensitization of its mediated signaling

We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein. LDL stimulation induced cytosolic [Ca2+]i mobilization, increase in inositol 1,4,5-triphosphate (IP3) formation and a rapid cytosol-to-membrane translocation of protein kinase C (PKC) enzymatic activity. Pertussis toxin inhibited all the stimulatory effects, whereas cholera toxin had no effect. Using ligand-binding assays, we demonstrated that exposing platelet LDL receptors to high concentrations of LDL (1.5 g/l) caused a rapid down-regulation and desensitization, as shown by the reduction in the Bmax, intracellular [Ca2+]i mobilization and IP3 formation to 65, 73 and 63%, respectively. The inhibitory effects were reversible and dose and time dependent. Furthermore, VLDL (0.2 g/l) and IDL (0.07 g/l) induced similar desensitization effects. However, HDL3 (up to 1.5 g/l), chylomicrons (up to 0.5 g/l) and cyclohexandione-modified LDL (which does not bind to platelets) had no significant effects. Protein kinase C inhibitors (150 nmol/l staurosporine, 100 micromol/l H-7, and 10 nmol/l bisindolylmaleimide) inhibited desensitization to 71%, on average. Sequestration blocking agents (0.30 g/l, concanavalin A) had no significant effect if phosphorylation was operative. However, there was a complete blockade with the concurrent inhibition of both pathways. In contrast, cAMP-dependent protein kinase inhibitors (PKI, 1 micromol/l) or beta2-adrenergic receptor kinase inhibitors (100 nmol/l, heparin), had no effect. Overall results indicate that LDL binds to a pertussis sensitive G-protein coupled receptor and that high levels of lipoproteins down-regulate the number of receptors and desensitize its mediated response by a mechanism that involves PKC-phosphorylation and sequestration of binding sites. This new regulatory mechanism may have implications for the thrombogenicity in hyperlipidemia and for effects of lipid lowering therapy.     

Enhancement of thyroxine entry into low density lipoprotein (LDL) receptor-competent fibroblasts by LDL: an additional mode of entry of thyroxine into cells

Having demonstrated that plasma low density lipoproteins (LDL) bind T4 through a specific interaction with their sole apolipoprotein, apoB-100, we tested the hypothesis that cells could internalize the LDL-T4 complex via cell surface LDL receptors. These receptors are down-regulated by cholesterol loading and up-regulated by cholesterol deficiency. We, therefore, studied the uptake of [125I]T4 or [125I]T3 by human skin fibroblasts grown in 10% lipoprotein-deficient serum in the absence or presence of LDL. At concentrations of LDL (12.5 and 22 micrograms protein/ml) that gave significant binding of T4 but did not exceed LDL receptor capacity, both the initial rate of saturable T4 uptake and the uptake at equilibrium increased by 27-63%. No significant increase occurred at a LDL concentration of 1.6 micrograms protein/ml (less than 3% occupancy), whereas there was a 20 to 31% reduction at 125 micrograms/ml (approximately 5 times the saturation dose for the LDL receptor). These changes were confirmed with several different LDL preparations and were mimicked by isolated apoB-100 and apoE, the sole ligands for the LDL receptors (apoB/E receptors). T4 uptake did not increase in normal fibroblasts with down-regulated LDL receptors or in LDL receptor-deficient fibroblasts from a patient with familial hypercholesterolemia. In the latter cell line the uptake of T4 (and T3) in the absence of LDL was indistinguishable from that of normal fibroblasts. T3 uptake in normal fibroblasts was not enhanced by LDL. The specificity of the LDL effect was shown by the finding that T4-binding globulin, prealbumin, or serum albumin, at concentrations giving 10-90% T4 bound, failed to increase T4 uptake. Instead, each of these major thyroid hormone-binding plasma proteins caused a dose-dependent decrease in T4 entry. It is concluded that at least two modes of entry into fibroblasts are available for T4. The first is the cell surface thyroid hormone-binding sites, which recognize the free hormone and are present in both normal and LDL receptor-negative fibroblasts. The second, and additional, mode of entry is via the LDL receptors, which recognize the T4-LDL complex, are absent in LDL receptor-negative fibroblasts, are reduced in down-regulated fibroblasts, and are unavailable for T3, owing to the low affinity of T3 for LDL.     

Reduction of Plasma Cholesterol Levels and Induction of Hepatic LDL Receptor by Cerivastatin Sodium (CAS 143201-11-0, BAY w 6228), a New Inhibitor of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase, in Dogs

The effects of cerivastatin sodium (BAY w 6228), a new type of inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, on plasma cholesterol concentrations and the induction of hepatic LDL receptors were investigated with beagle dogs and Hep G2 cells. Oral administration of cerivastatin (0.01, 0.03, and 0.1 mg/kg per day) for 3 weeks reduced plasma total and very low-density lipoprotein plus low-density lipoprotein (VLDL + LDL) cholesterol concentrations and increased hepatic LDL receptor binding activity in dogs. Scatchard plot analysis revealed a 1.9-fold increase in the maximum binding capacity of hepatic LDL receptors in cerivastatin-treated animals. Similar results were obtained by administration of pravastatin (1.0 and 5.0 mg/kg/day) for 3 weeks. Binding activity of the LDL receptor, as well as receptor mRNA and protein concentrations, were increased in a dose-dependent manner (0.01–1.0 μM) by exposure of Hep G2 cells to cerivastatin. The results suggest that cerivastatin reduces plasma cholesterol concentrations by increasing hepatic LDL receptor expression. The mechanism of lowering cholesterol concentration by cerivastatin was the same as with the other previously examined HMG-CoA reductase inhibitors, but the effects with cerivastatin were apparent at doses much lower than the effective doses of the other drugs. Cerivastatin, therefore, shows potential for clinical use as a potent and efficacious plasma cholesterol-lowering drug. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of reagent and cell-generated hydrogen peroxide on the properties of low density lipoprotein.

Low density lipoprotein (LDL) isolated from human plasma anticoagulated with EDTA (EDTA/LDL) was 4-fold more resistant to oxidation by reagent H2O2, as assayed by the thiobarbituric acid (TBA) assay, than LDL prepared from plasma anticoagulated with citrate (CDP/LDL). The LDLs required 1-3 mM H2O2 for maximal oxidation by this assay, and ED50S were 1.7 X 10(-3) M for EDTA/LDL and 4.5 X 10(-4) M for CDP/LDL. Oxidation was enhanced 2.3-fold by Cu2+ ions. Rabbit endothelial cell line monolayers released two orders of magnitude less H2O2 than was required to oxidize LDL and failed to induce TBA reactivity in either EDTA/LDL or CDP/LDL after a 24-hr coincubation. However, this LDL was subsequently degraded by mouse macrophages more rapidly than untreated LDL. Freshly isolated human monocytes (2 X 10(6) cells per ml), with or without phorbol myristate acetate (100 ng/ml) to trigger the respiratory burst, did not oxidize LDL in the TBA assay, despite producing large amounts of reactive oxygen intermediates. EDTA/LDL, CDP/LDL, and acetoacetylated LDL failed to trigger H2O2 release from human monocytes or macrophages. These results separate oxidation of LDL as measured by TBA assay from the modification of LDL by rabbit aortic endothelial cell line that leads to its subsequent enhanced degradation by macrophages.     

Glycation of plasma low density lipoproteins increases interaction with arterial proteoglycans

The increased susceptibility to atherosclerosis of diabetic individuals, may result from diabetes-associated modification in plasma low density lipoproteins (LDL) which enhance their interaction with arterial extracellular matrix proteoglycans. Using a nonhuman primate model for human diabetes, studies were conducted to examine diabetes-induced changes in LDL. Plasma LDL were isolated from control (n = 4) and streptozotocin-induced diabetic (n = 3) cynomolgus macaques by differential ultracentrifugation. An in vitro binding assay was used to measure LDL interaction with arterial proteoglycans. Significantly more diabetic LDL bound to proteoglycans than control LDL (12.9+/-0.7 microg LDL cholesterol/microg proteoglycan versus 8.9+/-0.5 microg LDL cholesterol/microg proteoglycan (mean +/- S.E.M.), P < 0.005). Glycation of LDL, determined by fructosamine content, was significantly enhanced in diabetic versus control animals (37+/-3.1 versus 20+/-1.5 micromol/l (mean +/- S.E.M.) P < 0.005). The correlation coefficient between fructosamine content of LDL and its binding to arterial proteoglycans was 0.95. No LDL compositional variables other than glycation correlated with proteoglycan binding. Removal of the glycated portion of LDL from diabetic animals returned LDL proteoglycan binding to normal. These data demonstrate that the diabetes induced glycation of LDL increases its proteoglycan binding properties: thus, a critical mechanism in atherosclerosis, enhanced LDL interaction with arterial proteoglycans, may be accelerated by the diabetic state.     

Hepatic uptake of chylomicron remnants in WHHL rabbits: a mechanism genetically distinct from the low density lipoprotein receptor.

Homozygous Watanabe hereditary hyperlipidemic (WHHL) rabbits have a near-complete deficiency of low density lipoprotein (LDL) receptors in liver and other tissues. As a result, these rabbits clear LDL from plasma at an abnormally slow rate. In the current studies we show that WHHL rabbits clear chylomicrons from plasma at a normal rate. Chylomicrons are cleared by a two-step process: (i) hydrolysis of triglycerides in extrahepatic tissues to yield cholesteryl ester-rich remnant particles and (ii) rapid uptake of the remnants by liver. Normal and WHHL rabbits were given intravenous injections of rat chylomicrons labeled either in the lipid portion with [3H]cholesterol and [14C]palmitate or in the protein portion with [125]iodine. All radiolabeled components were removed from plasma at comparable rates in normal and WHHL rabbits. Comparable amounts of radioactivity accumulated in livers of animals from both genotypes. In vitro assays showed that liver membranes from WHHL rabbits were markedly deficient in the binding of 125I-labeled chylomicron remnants as well as 125I-labeled LDL, implying that chylomicron remnants can bind to the hepatic LDL receptor. We conclude that the rabbit liver normally has at least two genetically distinct lipoprotein uptake mechanisms, both of which recognize chylomicron remnants: (i) the LDL receptor and (ii) a specific chylomicron remnant uptake mechanism that is not measured adequately by current in vitro membrane binding assays. WHHL rabbits possess a normal chylomicron remnant uptake mechanism that allows them to clear chylomicrons from plasma at a rapid rate despite their genetic deficiency of LDL receptors.     

The angiotensin II receptor of rabbit liver: characterization in isolated hepatocytes and effect of GTP

A homologous population of specific angiotensin II receptors is present on the cell surface of isolated rabbit hepatocytes. The binding characteristics of [3H]angiotensin II to the cells were: association rate constant (K+1) 0.08 l/nmol per min and dissociation rate constant (K-1) 1.9/min, yielding a dissociation constant (Kd) of 24 nmol/l. A very similar Kd (32 nmol/l) has been derived from saturation binding data which indicate a maximal binding capacity of about 200,000 sites/cell. Analysis of the association binding data to purified liver plasma membranes indicated a Kd of 6 nmol/l in the absence and 30 nmol/l in the presence of GTP. Dissociation was clearly dependent upon the presence of the nucleotide, which shifted the K-1 from 0.12/min to 0.42/min. The studied binding sites are very likely to be involved in the glycogenolytic action of angiotensin II, since a highly significant correlation was established between the biological activity (activation of glycogen phosphorylase) and the binding affinity of a series of agonistic analogues. The reported characteristics of the rabbit hepatic angiotensin II receptors show much similarity with those of rat liver.     

Membrane receptors for very low density lipoprotein (VLDL) inhibitor of lymphocyte proliferation

Physiologic concentrations of human plasma very low density lipoproteins inhibit the DNA synthesis of lymphocytes stimulated by allogeneic cells or lectins. In this report we have compared the effects of isolated lipoproteins [very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL)] and lipoprotein-depleted plasma (LDP) on DNA synthesis by phytohemagglutinin-stimulated human lymphocytes. The relative potency for the inhibition of lymphocyte proliferation was VLDL greater than LDL greater than HDL greater than LDP. Fifty percent inhibition of DNA synthesis was observed at a VLDL protein concentration of 1.5--2.0 microgram/ml. We have further demonstrated the presence of specific receptors for VLDL on human lymphocytes. Native VLDL was more effective than LDL in competing for 125I-VLDL binding sites. Subsequent to binding to lymphocytes, 125I-VLDL was internalized and degraded to acid- soluble products. Based on a Scatchard analysis of VLDL binding at 4 degrees C, the number of VLDL receptors per lymphocyte was estimated at 28,000 +/- 1300. Based on an estimated mean binding affinity for the VLDL receptor complex at half saturation of approximately 8.8 X 10(7) liter/mole, it is estimated that 91% of lymphocyte VLDL receptors are occupied at physiologic VLDL concentrations in blood. Although the immune regulatory role of plasma lipoproteins is uncertain, we suggest tha VLDL and LDL-In may maintain circulating blood lymphocytes in a nonproliferative state via their respective cell receptor mechanisms.