Beta-migrating very low density lipoproteins induce foam cell formation in mouse mesangial cells

Hypercaloric Diet Promotes Metabolic Disorders and Impaired Kidney Function

Poor dietary habits such as overconsumption of hypercaloric diets characterized by a high content of fructose and fat are related to metabolic abnormalities development such as obesity, diabetes, and dyslipidemia. Accumulating evidence supports the hypothesis that if energy intake gradually exceeds the body's ability to store fat in adipose tissue, the prolonged metabolic imbalance of circulating lipids from endogenous and exogenous sources leads to ectopic fat distribution in the peripheral organs, especially in the heart, liver, and kidney. The kidney is easily affected by dyslipidemia, which induces lipid accumulation and reflects an imbalance between fatty acid supply and fatty acid utilization. This derives from tissue lipotoxicity, oxidative stress, fibrosis, and inflammation, resulting in structural and functional changes that lead to glomerular and tubule-interstitial damage. Some authors indicate that a lipid-lowering pharmacological approach combined with a substantial lifestyle change should be considered to treat chronic kidney disease (CKD). Also, the new therapeutic target identification and the development of new drugs targeting metabolic pathways involved with kidney lipotoxicity could constitute an additional alternative to combat the complex mechanisms involved in impaired kidney function. In this review article, we first provide the pathophysiological evidence regarding the impact of hypercaloric diets, such as high-fat diets and high-fructose diets, on the development of metabolic disorders associated with impaired renal function and the molecular mechanisms underlying tissue lipid deposition. In addition, we present the current progress regarding translational strategies to prevent and/or treat kidney injury related to the consumption of hypercaloric diets.

The effect of cholesterol overload on mouse kidney and kidney-derived cells

Introduction: Dyslipidemia is one of the onset and risk factors of chronic kidney disease and renal function drop is seen in lipoprotein abnormal animal models. However, the detailed molecular mechanism of renal lipotoxicity has not been clarified. Therefore, the present study aimed to investigate the influence of cholesterol overload using mouse kidney tissue and kidney-derived cultured cells.

Methods: C57BL/6 mice were fed normal diet (ND) or 1.25% cholesterol-containing high-cholesterol diet (HCD) for 11 weeks, and we used megalin as a proximal tubule marker for immunohistology. We added beta-very low density lipoprotein (βVLDL) to kidney-derived cells and examined the effect of cholesterol overload on megalin protein and mRNA expression level, cell proliferation and cholesterol content in cells.

Results: In the kidney of HCD mice, the gap between glomerulus and the surrounding Bowman’s capsule decreased and the expression level of megalin decreased. After βVLDL treatment to the cells, the protein expression and mRNA expression level of megalin decreased and cell proliferation was restrained. We also observed an increase in cholesterol accumulation in the cell and free cholesterol/phospholipid ratios increased.

Conclusions: These findings suggest that the increased cholesterol load on kidney contribute to the decrease of megalin and the overloaded cholesterol is taken into the renal tubule epithelial cells, causing suppression on cell proliferation, which may be the cause of kidney damage.

Very low-density lipoprotein stimulates the expression of monocyte chemoattractant protein-1 in mesangial cells

BACKGROUND

Elevated plasma levels of very low-density lipoprotein (VLDL) are associated with an increased risk for focal glomerulosclerosis, which is analogous to atherosclerosis. One feature of focal glomerulosclerosis is the presence of foam cells derived from the infiltration of circulating monocytes. Mesangial cells are able to express monocyte chemoattractant protein-1 (MCP-1). In this study, the ability of VLDL to stimulate MCP-1 expression in mesangial cells and consequent monocyte adhesion was investigated.

METHODS

For adhesion studies, mesangial cells isolated from Sprague-Dawley rats were treated with VLDL for six hours, followed by a one-hour incubation with Tamm-Horsfall protein-1 (THP-1) cells. Mesangial MCP-1 mRNA levels were determined by reverse transcription-polymerase chain reaction. MCP-1 protein was determined by solid-phase immunoassay.

RESULTS

VLDL (100 to 300 microg/mL) significantly enhanced the expression and secretion of MCP-1 (54 to 285 ng/well) in mesangial cells. Such an effect was accompanied by the increased adhesion of monocytes to mesangial cells and later the formation of foam cells from monocytes after ingesting excessive amounts of VLDL lipids. VLDL-induced MCP-1 expression and monocyte adhesion were blocked by a protein kinase C inhibitor (staurosporine), as well as a calcium channel blocker (diltiazem).

CONCLUSIONS

Our results demonstrate that elevated levels of VLDL, through the action of MCP-1, may contribute to the infiltration of monocytes into the mesangium and subsequent foam cell formation. Hence, VLDLs may play a role in the pathogenesis of focal glomerulosclerosis. One of the mechanisms of such effect may be mediated through the calcium-dependent protein kinase C pathway.

Serotonin enhances the production of type IV collagen by human mesangial cells

BACKGROUND

The plasma concentration of 5-hydroxytryptamine (5-HT) in diabetic patients is higher than that in normal subjects. Since recent reports have demonstrated the presence of 5-HT2A receptor in glomerular mesangial cells, it is possible that 5-HT may be involved in the development of diabetic nephropathy through the 5-HT2A receptor in mesangial cells. Because expansion of the glomerular mesangial lesion is a characteristic feature of diabetic nephropathy, we examined the effect of 5-HT on the production of type IV collagen by human mesangial cells.

METHODS

Human mesangial cells were incubated with 5-HT with or without 5-HT receptor antagonists, protein kinase C (PKC) inhibitor or transforming growth factor-beta (TGF-beta) antibody. Type IV collagen mRNA and protein concentration in medium were measured by Northern blot analysis and enzyme-linked immunosorbent assay (ELISA), respectively. TGF-beta mRNA and bioactivity in the medium were measured by Northern blot analysis and bioassay using mink lung epithelial cells, respectively.

RESULTS

5-HT stimulated the production of type IV collagen by human mesangial cells, which was inhibited by ketanserin and sarpogrelate hydrochloride, 5-HT2A receptor antagonists, but not by ondansetron, a 5-HT3 receptor antagonist. 5-HT increased the bioactivities of both active and total TGF-beta. However, the 5-HT-enhanced production of type IV collagen was completely inhibited by an anti-TGF-beta antibody. Furthermore, a PKC inhibitor, calphostin C, inhibited the 5-HT-induced increase in type IV collagen secretion, and the activity of membrane PKC was increased by 5-HT. Phorbol ester activated type IV collagen production as well as active and total TGF-beta. Calphostin C completely inhibited the 5-HT-enhanced activity of active TGF-beta, but did not inhibit exogenous TGF-beta-induced increase in type IV collagen secretion.

CONCLUSIONS

Our results suggest that 5-HT-enhanced production of type IV collagen by human mesangial cells is mediated by activation of PKC and subsequent increase in active TGF-beta activity.

Human beta-migrating very low density lipoprotein induces foam cell formation in human mesangial cells

To elucidate the mechanism of foam cell formation in the mesangial region of a kidney observed in a familial type III hyperlipoproteinemic patient presenting with diabetes mellitus and nephrotic syndrome, we have examined, in the present study, the effect of human beta-VLDL (apo E2/E2) on foam cell formation in human mesangial cells, since an increase in beta-VLDL is a characteristic feature of this patient. Human beta-VLDL (apo E2/E2) induced foam cell formation in human mesangial cells. The binding of [125I]LDL to human mesangial cells was inhibited completely by both LDL and beta-VLDL. On the other hand, the binding of [125I]beta-VLDL was completely inhibited by beta-VLDL, but partially by LDL. The LDL receptor, but not the VLDL receptor was down-regulated by accumulation of cholesteryl esters. These results suggest that human beta-VLDL (apo E2/E2)-induced foam cell formation in mesangial cells is mediated through both the LDL receptor pathway and the beta-VLDL specific pathway, in which the VLDL receptor is one of the candidates.

Uptake and metabolism of lipoprotein-X in mesangial cells

Progressive glomerulosclerosis is a major complication in patients with familial lecithin:cholesterol acyltransferase (LCAT) deficiency. The lack of LCAT activity results in the accumulation of an abnormal lipoprotein, lipoprotein-X (Lp-X), in the plasma of these patients. Lipoprotein-X contains high levels of unesterified cholesterol and phosphatidylcholine. Lp-X may play a role in the accumulation of lipids in the kidney, which in turn may lead to glomerulosclerosis. The objective of this study is to examine the uptake and metabolism of Lp-X by rat mesangial cells. Our results suggest that Lp-X is taken up by mesangial cells and that the lipids in Lp-X are metabolized. Lysosomes containing unesterified cholesterol and phosphatidylcholine, in a molar ratio similar to Lp-X, were synthesized to investigate the roles individual apolipoproteins (apo CI, II, III and E) play in the uptake of Lp-X. Both apo CI and CIII inhibited its uptake while apo CII (1.5 fold) and E (4 fold) stimulated the uptake of Lp-X. Very low density lipoprotein (VLDL) and low density lipoprotein (LDL) inhibited Lp-X uptake by mesangial cells. However, at higher concentrations of high density lipoprotein (HDL), the uptake of Lp-X was stimulated. Proteoglycans have an important role in regulating the uptake of Lp-X, while cytoskeleton-dependent phagocytosis and the scavenger receptor do not appear to be involved.