Abstract. In nephrotic syndrome, large quantities of albumin enter the kidney tubule. This albumin carries with it a heavy load of fatty acids to which the proximal tubule cells are exposed at high concentration. It is postulated that exposure to fatty acids in this way is injurious to proximal tubule cells. This study has examined the ability of fatty acids to interact with peroxisome proliferator-activated receptors (PPAR) in primary cultures of human proximal tubule cells. Luciferase reporter assays in transiently transfected human proximal tubule cells were used to show that albumin bound fatty acids and other agonists activate PPAR␥ in a dose-dependent manner. One of the consequences of this activation is apoptosis of the cells as determined by changes in cell morphology, evidence of PARP cleavage, and appearance of DNA laddering. Overexpression of PPAR␥ in these cells also results in enhanced apoptosis. Both fatty acid-induced PPAR activation and apoptosis in these cells can be blocked by PPAR response element decoy oligonucleotides. Activation of PPAR␥ by the specific agonist PGJ 2 is associated with inhibition of cell proliferation, whereas activation by albumin bound fatty acids is accompanied by increased proliferation. However, the net balance of apoptosis/proliferation favors deletion of cells. These results implicate albumin-bound fatty acids as important mediators of tubular injury in nephrosis and provide fresh impetus for pursuit of lipid-lowering strategies in proteinuric renal disease.

Peroxisome proliferator-activated receptor ␥ (PPAR␥) has key roles in the regulation of adipogenesis, inflammation, and lipid and glucose metabolism. C-peptide is believed to be inert and without appreciable biological functions. Recent studies suggest that C-peptide possesses multiple functions. The present study investigated the effects of insulin and C-peptide on PPAR␥ transcriptional activity in opossum kidney proximal tubular cells. Both insulin and C-peptide induced a concentration-dependent stimulation of PPAR␥ transcriptional activity. Both agents substantially augmented thiazolidinedione-stimulated PPAR␥ transcriptional activity. Neither insulin nor C-peptide had any effect on the expression levels of PPAR␥. GW9662, a PPAR␥ antagonist, blocked PPAR␥ activation by thiazolidinediones but had no effect on either insulin-or C-peptide-stimulated PPAR␥ transcriptional activity. Co-transfection of opossum kidney cells with dominant negative mitogen-activated protein kinase kinase significantly depressed basal PPAR␥ transcriptional activity but had no effect on that induced by either insulin or C-peptide. Both insulin-and C-peptide-stimulated PPAR␥ transcriptional activity were attenuated by wortmannin and by expression of a dominant negative phosphatidylinositol (PI) 3-kinase p85 regulatory subunit. In addition PI 3-kinase-dependent phosphorylation of PPAR␥ was observed after stimulation by C-peptide or insulin. C-peptide effects but not insulin on PPAR␥ transcriptional activity were abolished by pertussis toxin pretreatment. Finally both C-peptide and insulin positively control the expression of the PPAR␥-regulated CD36 scavenger receptor in human THP-1 monocytes. We concluded that insulin and C-peptide can stimulate PPAR␥ activity in a ligand-independent fashion and that this effect is mediated by PI 3-kinase. These results support a new and potentially important physiological role for C-peptide in regulation of PPAR␥-related cell functions.

PPAR gamma agonists exert differential effects in proximal tubule cells with thiazolidinediones behaving as partial agonists.

Baines RJ, Chana RS, Hall M, Febbraio M, Kennedy D, Brunskill NJ. CD36 mediates proximal tubular binding and uptake of albumin and is upregulated in proteinuric nephropathies. Am J Physiol

Low density lipoprotein (LDL) deposition and local oxidation play a key role in the pathogenesis of atherosclerosis and may likewise contribute to glomerular injury. These studies were designed to determine whether cultured human mesangial cells oxidize homologous LDL and to compare the effects of unmodified and oxidized lipoprotein on cell proliferation, viability and eicosanoid production. Cell-mediated lipoprotein oxidation was demonstrated and could be suppressed by oxygen free radical scavengers and inhibitors of arachidonic acid metabolism. When incubated with cells, oxidized LDL (Ox-LDL) at concentrations up to and including 100 micrograms/ml reduced 3H-thymidine incorporation without causing cytotoxicity as assessed by lactate dehydrogenase release. Under the same conditions there was a concentration-dependent increase in the synthesis of prostaglandins E2,6-keto-PGF1 alpha and thromboxane B2. In contrast, unmodified LDL enhanced DNA synthesis at concentrations less than 40 micrograms/ml and had little effect on eicosanoid production. These results demonstrate that exogenous oxidized LDL inhibits mesangial cell proliferation and increases eicosanoid synthesis. Unmodified lipoprotein can be directly oxidized by these cells through mechanisms that involve generation of oxygen free radicals.