Stimulation of Proximal Tubular Cell Apoptosis by Albumin- Bound Fatty Acids Mediated by Peroxisome Proliferator Activated Receptor- MUSTAFA ARICI,* RAVINDER CHANA, † ANDREW LEWINGTON,* † JEZ BROWN, AND* NIGEL JOHN BRUNSKILL* † † Department of Cell Physiology and Pharmacology, and *Department of Nephrology, University of Leicester, Faculty of Medicine and Biological Sciences, Leicester, England. 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 pri- mary cultures of human proximal tubule cells. Luciferase re- porter 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. Overex- pression of PPAR in these cells also results in enhanced apoptosis. Both fatty acid–induced PPAR activation and apo- ptosis 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 accompa- nied 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 pur- suit of lipid-lowering strategies in proteinuric renal disease. njb18@le.ac.uk In patients with renal disease, the presence of proteinuria is an adverse prognostic sign such that those individuals with pro- teinuria are more likely to progress to end-stage renal failure than those with absent or very modest proteinuria (1,2). His- topathologic observations reveal a very close correlation be- tween progressive renal functional impairment and tubuloin- terstitial inflammation, scarring, and fibrosis (3). The most prevalent protein in the urine of nephrotic patients is albumin, and several authors have postulated that albumin may have a detrimental effect on tubulointerstitial function when filtered in excess into the proximal tubule. Thus in vivo proteinuria has been associated with proliferation and apoptosis of proximal tubule cells (PTC) and interstitial inflammation (4,5). In vitro albumin stimulates various intracellular signaling pathways in PTC and induces them to produce various chemoattractants (6 –10). Some authors have demonstrated a pro-apoptotic effect of albumin in cultured PTC, and others demonstrate an oppo- site effect (11,12). Although there now seems little doubt that albumin can alter the properties of PTC in a number of ways, whether albumin per se is a major mediator of renal tubulointerstitial disease remains controversial. One common criticism leveled at the albumin-mediated renal damage hypothesis is the example of minimal change disease. Individuals with this condition man- ifest heavy proteinuria, yet, in contrast to those with other nephrotic states, they generally fail to develop tubulointerstitial disease and renal impairment. Urinary albumin in minimal change disease displays a crucial difference to that found in other nephroses being relatively devoid of fatty acids (13). Thus it is likely that albumin-bound fatty acids (FA) may be important mediators of renal tubulointerstitial disease. Al- though large-scale trials of lipid-lowering strategies on the progression of renal impairment in renal disease have not been performed, a recent meta-analysis of several smaller trials suggests a renoprotective effect of lipid reduction equivalent to that observed for angiotensin-converting enzyme (ACE) inhib- itors in proteinuric conditions (14). Circulating albumin is able to bind and act as a carrier for many serum-derived molecules. In particular, albumin pos- sesses five to seven high-affinity binding sites for FA, and these FA are carried with albumin into the proximal tubule (15,16). Long-chain FA are very poorly soluble, and filtered albumin thus has the capacity to present these molecules to PTC in an unregulated manner at concentrations far above that Received March 22, 2002. Accepted September 24, 2002. Correspondence to Dr. Nigel John Brunskill, Department of Cell Physiology and Pharmacology, University of Leicester, Faculty of Medicine and Biolog- ical Sciences, Medical Sciences Building, University Road, PO Box 138, Leicester, LE1 9HN, England. Phone: 44-116-258-8043; Fax: 44-116-252- 5045; E-mail: njb18@le.ac.uk 1046-6673/1401-0017 Journal of the American Society of Nephrology Copyright © 2002 by the American Society of Nephrology DOI: 10.1097/01.ASN.0000042167.66685.EA J Am Soc Nephrol 14: 17–27, 2003