HUMAN HIGH-DENSITY-LIPOPROTEINS STIMULATE ENDOTHELIN-1 RELEASE BY CULTURED HUMAN RENAL PROXIMAL TUBULAR CELLS

The vasoconstrictive and mitogenic actions of endothelins have been im plicated in the pathogenesis of progressive renal disease. In the pres ent study, we have assessed whether plasma high density lipoproteins ( HDL), the major filtered urinary lipoprotein in nephrotic states, can influence endothelin-1 (ET-1) production by cultured human renal proxi mal tubular cells. Human HDL was found to stimulate ET-1 secretion up to fourfold in a dose- and time-dependent manner; the effect was great er in subconfluent cultures than in confluent ones. There was little d ifference between the stimulatory effect of HDL(2) and the major HDL s ubclass, HDL(3). Preincubation of the cells with albumin did not aboli sh the HDL effect, while partially- or fully-delipidated HDL(3) largel y reproduced the effect of whole HDL(3). These findings suggest that s timulation of ET-1 secretion was not simply due to protein or lipid re pletion of the cells. Rather, the effect was mediated by HDL apolipopr oteins, although binding to the HDL receptors involved in cellular cho lesterol homeostasis was not required as tetranitromethane-modified HD L(3) was an equally effective agonist of ET-1 release. Apolipoprotein (ape) A-I was indirectly implicated in the process since modified HDL( 3) in which apoA-II largely replaced apoA-I was less potent than HDL(3 ). A one hour exposure of the cells to HDL(3) was sufficient to activa te ET-1 production for the following 12 hours, although maximum activa tion required six hours. The activation process appeared to be indepen dent of both cyclic AMP and protein kinase C (PKC): HDL(3) and phorbol myristate acetate had an additive effect; PKC-depleted cells remained responsive to HDL(3); and the PKC inhibitor, staurosporine, did not r educe HDL(3), stimulation of ET-1 production. We conclude that the apo lipoprotein constituents of HDL(3) enhance ET-1 secretion by cultured renal tubular cells via a novel PKC-independent pathway. Whether this action of human HDL on tubular ET-1 production also occurs in vivo mer its investigation, since it may have pathophysiological significance f or the progressive tubulointerstitial fibrosis associated with chronic proteinuria.