Receptor-mediated endocytosis of low-density lipoprotein by cultured human glomerular cells

Lipoprotein (a) stimulates mitogen activated protein kinase in human mesangial cells

Evidence suggests an important role of elevated serum lipoproteins in the progression of renal glomerulosclerosis. We report here that lipoprotein (a) (Lp(a)) increased phosphorylation and activity of mitogen activated protein kinase (MAPK) in human mesangial cells. When protein kinase C (PKC) was depleted by long-term incubation with the phorbol 12-O-myristate 13-acetate the effect of Lp(a) on MAPK activation was completely inhibited. Forskolin, a stimulator of the adenylyl cyclase, and dibutyryl-cAMP reduced the effect of Lp(a) on MAPK phosphorylation and activation. We conclude that Lp(a) stimulates the MAPK cascade via activation of PKC and that activation of protein kinase A counteracts Lp(a) induced MAPK activation in human mesangial cells.

LDL receptor activity in human leukocyte subtypes: regulation by insulin

OBJECTIVES

LDL receptors of leukocytes play a key role in lipoprotein uptake, immunoregulation and the pathogenesis of atherosclerosis. Numerous studies with different methods of low reliability yielded conflicting results of its regulation in leukocyte subtypes.

DESIGN AND METHODS

LDL receptors of human leukocytes were measured with use of the monoclonal antibody C-7. Specific C-7 binding was detected by FACS analysis using phycoerythrin-anti-mouse-IgG. Parallel incubations with FITC-labelled anti-LEU 4 (CD 3), anti-LEU 12 (CD 19) and anti-MY 4 (CD 14) antibodies were used to distinguish C-7 binding of specific cell types (T-, B-lymphocytes and monocytes).

RESULTS

In contrast to monocytes, T and B-lymphocytes freshly isolated from healthy blood donors had no detectable binding capacity for C-7. After 24 and 48 h incubation of cells in a lipid-free medium, lymphocytes acquired some C-7 binding, albeit still much less than monocytes. Incubation with insulin for 24 h in a concentration of 0.5 microgram/mL led to an increase in C-7 binding for monocytes (up to 180%). Saturation experiments with the ligand suggests an increase in the number of receptors. In contrast the same insulin concentration inhibited C-7 binding of B- and T-lymphocytes by 35%.

CONCLUSIONS

FACS analysis using monoclonal antibodies seems to be a feasible method for the investigation of lipid metabolism in leukocytes. The LDL receptor expression and its regulation by insulin differs in circulating monocytes and lymphocytes.

Oxidized LDL stimulates the expression of TGF-beta and fibronectin in human glomerular epithelial cells

Abnormal lipid accumulation in glomeruli is a recognized early event in the development of glomerulosclerosis. The presence of LDL and scavenger receptors has recently been demonstrated in glomerular cells, including the visceral epithelial cells. To explore the possible molecular mechanisms of lipid-induced glomerular injury, the present investigation was conducted to examine the effects of oxidized LDL (ox-LDL) on the expression of transforming growth factor (TGF)-beta and fibronectin by cultured human glomerular epithelial cells (GEC). Cultured GEC were exposed to human ox-LDL (0 to 100 micrograms/ml) for various time points. Ox-LDL induced a dose- and time-dependent increase in the expression of TGF-beta mRNA. Actinomycin D, a transcriptional inhibitor, but not cycloheximide, a protein synthesis inhibitor, inhibited the response. GEC exposed to ox-LDL also demonstrated elevated levels of fibronectin mRNA. In addition, treatment of GEC with ox-LDL resulted in increased TGF-beta and fibronectin protein expression as detected by immunocytochemistry. Addition of anti-TGF-beta antibody significantly inhibited the increase in fibronectin message level induced by ox-LDL. These data suggest that ox-LDL stimulates matrix protein fibronectin in GEC by a mechanism involving expression of TGF-beta. Thus, accumulation of lipids in human glomerular epithelial cells may contribute to the pathogenesis of glomerulosclerosis through TGF-beta mediated mechanism(s).

Low density lipoprotein enhances the cellular action of arginine vasopressin in rat glomerular mesangial cells in culture

The present study was undertaken to determine whether low density lipoprotein (LDL) modulates the cellular action of arginine vasopressin (AVP) in rat glomerular mesangial cells in culture. AVP increased cellular free calcium ([Ca2+]i) in a dose-dependent manner. When cells were preincubated for 24 h with 10 microgram/ml LDL, the 1 x 10(-7) M AVP-mobilized [Ca2+]i was 874 nM, a value significantly greater than that of 375 nM in the intact cells. AVP caused a biphasic change in cellular pH (pHi), namely, an early acidification followed by a sustained alkalinization, and the change in pHi produced by AVP was also enhanced by LDL. AVP stimulated a 2.2-fold increase in [3H]thymidine incorporation, an effect significantly greater in the presence of 10 micrograms/ml LDL. Furthermore, 1 x 10(-7) M AVP significantly activated mitogen-activated protein kinase from 14.0 to 24.5 pmol/mg protein. Such an activation was significantly enhanced by the LDL pretreatment. Both [3H]thymide incorporation and mitogen-activated protein kinase were not altered by 10 micrograms/ml LDL. [3H]AVP receptor binding was not affected by the LDL pretreatment. 1 x 10(-7) M AVP increased inositol trisphosphate production by 1.9-fold, an effect significantly greater in the presence of LDL. These results indicate that LDL enhances the cellular action of AVP and the AVP-stimulated cellular proliferation in glomerular mesangial cells. A site of action of LDL is the hydrolysis of phosphatidylinositol.

The role of lipids in nephrosclerosis and glomerulosclerosis

Hyperlipidemia and lipoprotein abnormalities are often encountered in patients with nephrotic syndrome or chronic renal disease and also in those undergoing haemodialysis and with renal transplant. Even though the significance of lipid deposition in renal tissue and the role of lipoproteins in the pathogenesis of renal disease in man is unclear, experimental and clinical data indicate a possible damaging effect of a disturbed lipid metabolism on the kidney. In humans, glomerular lipid deposition is observed in genetic diseases such as Fabry's disease, lecithin:cholesterol acyltransferase activity (LCAT) deficiency and arteriohepatic dysplasia, and in diseases with acquired disturbance of lipid metabolism such as nephrotic syndrome and cholestatic liver disease. Studies on animals with lupus nephritis, aminonucleoside nephrosis, reduced renal mass, diabetes mellitus or systemic hypertension have shown that cholesterol can increase the incidence of glomerulosclerosis. As most of these studies have been performed in the rat, which has a different lipoprotein profile to that of man, these results should be carefully interpreted with regard to their relevance for humans. In vitro cell culture studies on human glomerular cells have given some preliminary insights into the cellular mechanisms of lipid induced glomerular damage. Apo E-containing lipoproteins, which are pathologically elevated in many renal diseases, are avidly taken up by human mesangial cells. These cells seem to play a central role in the initiation of glomerulosclerosis by inducing proliferation and production of excess extracellular matrix. Lipoproteins are able to stimulate DNA synthesis in these cells, and increase the synthesis of mitogens and extracellular matrix protein. The pathogenic role of oxidized lipoproteins has not yet been defined. Human mesangial cells do not seem to take up these modified lipoproteins. However, macrophages infiltrate glomeruli and may constitute the stimulus for the generation of minimally modified lipoproteins and their cellular uptake. The data from animal experiments suggest that treatment that corrects hyperlipidemia may have an ameliorative effect on renal function. Thus, there are strong indications that lipoproteins may play a critical role in mediating the development of glomerulosclerosis.

Acetyl-low density lipoprotein receptors on rat mesangial cells

To elucidate whether mesangial cells have any scavenger functions for modified lipoproteins, surface binding and cholesteryl ester (CE) formation by acetyl-low density lipoproteins (acetyl-LDL) have been studied in cultured rat renal mesangial cells. Specific binding kinetics for acetyl-LDL were observed with Kd = 28.3 micrograms/ml and Bmax = 1.1 ng/micrograms cell protein at 0 degrees C. The fluorescence microscopic finding demonstrated the enhanced uptake of DiI-acetyl-LDL in mesangial cells. Incorporation of [14C]oleate into CE was enhanced to 6-fold by loading 30 micrograms/ml of acetyl-LDL on 10 micrograms/ml of [14C]oleate-bovine serum albumin conjugate as compared with the control without lipoproteins (P < 0.05). The CE formation was completely inhibited by chloroquine. The light microscopic finding demonstrated the increased CE deposition by acetyl-LDL, resulting in foam cell formation. These results indicate biochemically and morphologically that the mesangial cells take up acetyl-LDL by receptor-mediated endocytosis, and that cholesterols in acetyl-LDL are converted to CE, resulting in an increased cellular cholesterol content. In conclusion, mesangial cells may have a scavenger function similar to macrophages.

Lovastatin retards the progression of established glomerular disease in obese Zucker rats

Considerable experimental evidence indicates that hyperlipidemia can induce glomerular injury. The importance of lipids in the progression of established glomerular disease has not been established and is of clinical relevance because of the frequent association of lipid abnormalities with human renal disease. In the present study, 26-week-old hyperlipidemic obese Zucker rats (OZRs) with established nephropathy were treated for a period of 18 weeks with daily injections of the cholesterol synthesis inhibitor lovastatin (4 mg/kg). Compared with control OZRs treated with vehicle, lovastatin-treated OZRs had significantly (P < 0.05) lower serum cholesterol and triglyceride levels throughout the treatment period. Blood pressure and urine albumin excretion in lovastatin-treated OZRs were reduced over the first 12 weeks of therapy, but increased toward the levels in the control OZRs at the end of the protocol. After 18 weeks of therapy, the incidence of glomerulosclerosis in lovastatin-treated OZRs (23.2% +/- 5.8%) was approximately half of that in vehicle-treated OZRs (44.6% +/- 7.7%) (P < 0.05). The reduction in glomerular injury in lovastatin-treated OZRs was not associated with changes in either glomerular area or glomerular macrophage content. In separate experiments, mesangial cells were cultured from glomeruli isolated from 26-week-old proteinuric OZRs. Lovastatin (5 to 40 mumol/L) caused a significant dose-dependent inhibition of serum-stimulated mesangial cell DNA synthesis. The inhibitory effects of lovastatin were completely prevented in the presence of exogenous mevalonate (100 mumol/L). Thus, lovastatin retarded the progression of established glomerular disease in OZRs.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular mechanisms of lipid injury in the glomerulus

Hyperlipidemias may play a role in the progression of various renal diseases, including diabetes mellitus. We therefore examined the characteristics of low-density lipoprotein (LDL) binding and uptake in cultured rat mesangial cells. Mesangial cells bound and took up LDL in a manner consistent with specific receptor mediation. Furthermore, exposure of mesangial cells to LDL enhanced intracellular cholesteryl esterification and decreased de novo cholesterol synthesis. Mesangial cells expressed mRNA for LDL receptor and their expression was downregulated after preloading of cells with LDL. These results are consistent with regulation of cholesterol uptake and metabolism by a specific LDL receptor mechanism. During diabetes the apolipoprotein B of LDL undergoes nonenzymatic glycation, which may alter its affinity for the LDL receptor. Glycation of LDL reduced its affinity for binding to the receptor sites and decreased its uptake by mesangial cells. Thus, during diabetes less LDL may be taken up and more remain extracellularly, where it can be trapped in the matrix. Oxidation of LDL bound to extracellular matrix is believed to be a major factor in the pathobiology of hyperlipidemias. Specific scavenger receptors for oxidized LDL have been described and cloned. We therefore examined whether rat mesangial cells bound and took up oxidized LDL. We demonstrated low-affinity but high-capacity binding sites for oxidized LDL on mesangial cells. In contrast to LDL, which supported mesangial cell proliferation, oxidized LDL was cytotoxic for the cells and resulted in stimulation of mesangial cell prostaglandin E2 production. Trapping of LDL in the extracellular matrix is considered an initial event in LDL-induced vascular pathology. We therefore evaluated binding of LDL and modified LDL to extracellular matrix produced by cultured mesangial cells. Mesangial matrix had a high capacity to bind LDL and modified LDL (glycated or oxidized) in a nonsaturable manner. These results obtained with cultured mesangial cells and their matrix allow the formulation of a working hypothesis. Under normal eulipemic conditions mesangial cells handle LDL in a regulated manner. During hyperlipidemia or expansion of extracellular matrix LDL accumulates in the matrix. There LDL would be subject to oxidative modifications, especially under conditions of mesangial cell stress, such as inflammatory, mechanical, or ischemic injury. Part of the oxidized LDL could be taken up by scavenger receptors on mesangial cells and monocyte-macrophages, resulting in foam cell formation. Excess oxidized LDL, and specifically the lipid peroxides and lysolipids of oxidized LDL, would act as cytotoxic agents on mesangial, epithelial, and endothelial cells, thereby contributing to a vicious cycle of cell damage and sclerosis.

Localization of apolipoprotein(a) and B-100 in various renal diseases

Recently it has become clear that abnormalities of lipid metabolism play a large role in the progression of renal diseases. To investigate the relationship between lipids and kidney tissue, we employed an immunofluorescent technique to determine the localization pattern of apolipoprotein(a) [apo(a)], apoB-100, and low-density lipoprotein receptor in the glomeruli, and analyzed the relationship between their presence and the clinical and histological findings of a total 92 patients with glomerular diseases. Immunostaining showed co-localization of apo(a) and apoB-100 in glomeruli. The patients were divided into three groups, as follows: both apo(a) and apoB-100 positive (Group 1; 38 cases), apo(a) positive only (Group 2; 19 cases) and apo(a) negative (Group 3; 35 cases). Group 1 had more severe proteinuria, higher levels of lipoprotein(a) [Lp(a)], and lower total protein levels than Group 3. Group 1 had a higher prevalence of glomerulosclerosis and interstitial changes than Group 3. Group 2 had more severe proteinuria and a higher prevalence of glomerulosclerosis than Group 3. Although apo(a) and apoB-100 are almost absent in normal controls, these apoproteins [and presumably lipoproteins Lp(a)] are present in the glomeruli of patients with glomerular diseases. The data support the view that these apoproteins play a significant role in progressive renal diseases.

LDL stimulates mesangial fibronectin production and chemoattractant expression

Hyperlipidemia has been associated with glomerulosclerosis and a glomerular monocyte infiltrate in models of progressive renal insufficiency. The pathogenesis of hyperlipidemia-induced renal injury remains unknown. We postulated that the effect of hyperlipidemia may be mediated through LDL-induced activation of mesangial cells, which have recently been shown to possess LDL receptors. To test this hypothesis, cultured human mesangial cells were co-incubated with human LDL. Monolayers treated with LDL demonstrated a greater level of tissue culture supernatant fibronectin than control mesangial cells. This correlated with enhanced expression of fibronectin mRNA in LDL-treated mesangial cells. Additionally, LDL conditioning of mesangial cells caused a dose- and time-dependent increase in the expression of monocyte chemoattractant protein-1 mRNA, a monocyte specific chemotactic factor, as well as an increase in the monocyte chemotactic activity of mesangial supernatants. Thus, the deleterious effects of hyperlipidemia on the kidney may be mediated by the mesangial cell through an increase in production of mesangial matrix and recruitment of inflammatory cells to the glomerulus.