ALTERED SPHINGOMYELINASE AND CERAMIDE EXPRESSION IN THE SETTING OF ISCHEMIC AND NEPHROTOXIC ACUTE-RENAL-FAILURE

Diverse physical and chemical stimuli can activate sphingomyelinases ( SMases), resulting in sphingomyelin (SM) hydrolysis with ceramide rele ase. Since ceramide can profoundly impact a host of homeostatic mechan isms, the concept of a ''SM (or SMase) signaling pathway'' has emerged . We recently documented that ceramide levels fall abruptly during ren al ischemia, and then rebound to twice normal values during early repe rfusion (30 to 90 min). Therefore, the present study assessed whether these ceramide changes are paralleled, and hence potentially mediated, by comparable changes in SMase activity. Mice were subjected to 45 mi nutes of renal ischemia +/- 30 minutes, 90 minutes, or 24 hours of rep erfusion. Renal cortices (or isolated proximal tubules) were then assa yed for SMase activity (acidic, neutral forms). To characterize whethe r early post-ischemic ceramide increments are a relatively persistent event, ceramide was assayed following a 24-hour reperfusion period. Fi nally, to assess whether the observed perturbations were unique to pos t-ischemic injury, SMase and ceramide were quantified in the setting o f glycerol-induced myohemoglobinuria and anti-glomerular basement memb rane (alpha GBM) antibody-induced acute renal failure (ARF). Ischemia induced abrupt declines (similar to 50%) in both acidic and neutral SM ase activities, and these persisted in an unremitting fashion througho ut 24 hours of reperfusion. Nevertheless, increased ceramide expressio n (2x normal) resulted. Myohemoglobinuria also suppressed acidic/neutr al SMases, and again, ''paradoxical'' ceramide increments were observe d. Finally, alpha GBM nephritis increased ceramide levels, but in this instance, a correlate was increased SMase activity. These results sug gest that: (1) ceramide is an acute renal ''stress reactant,'' increas ing in response to diverse renal insults; (2) this response may occur independently of the classic SM pathway, since the ceramide increments can seemingly be dissociated from increased SMase activity; and (3) g iven the well documented impact of ceramide and the SM(ase) pathway on apoptosis, cell proliferation, differentiation, and tissue inflammati on, the present results have potentially broad ranging implications fo r the induction and evolution of diverse forms of ARF.