ALTERED CERAMIDE AND SPHINGOSINE EXPRESSION DURING THE INDUCTION-PHASE OF ISCHEMIC ACUTE-RENAL-FAILURE

Recent evidence indicates that a ''sphingomyelin signaling pathway'' e xists: in response to heterogeneous influences, sphingomyelin is hydro lyzed, liberating ceramide, and subsequently its sphingoid base, sphin gosine. Ceramide and sphingosine can influence diverse cellular proces ses, including cell differentiation, proliferation. protein traffickin g, and apoptosis. Each of these processes have important implications for post-ischemic acute renal failure (ARF). However, sphingosine and ceramide expression during the induction of ischemic/reperfusion injur y have not been previously assessed. To this end, CD-1 mice were subje cted to 45 minutes of unilateral renal ischemia +/- reperfusion, follo wed by cortical sphingosine, ceramide, and sphingomyelin assessments. Contralateral kidneys served as controls. Ischemia caused similar to 5 0% sphingosine and ceramide decrements. During reperfusion, sphingosin e rebounded to normal values. Conversely, ceramide rose to, and was ma intained at, supranormal levels (similar to 175% of controls). Subsequ ent studies performed with hypoxic or oxygenated isolated proximal tub ules suggested that these changes: (1) had a multifactorial basis; (2) were partially simulated by enhanced PLA, activity; (3) and were diss ociated from alterations in net sphingomyelin content. To assess the p otential pathogenic relevance of the documented ceramide increments, c ultured human proximal tubule (HK-2) cells were subjected to ATP deple tion/Ca2+ ionophore or PLA(2)-induced attack with or without exogenous C2 ceramide loading. Ceramide worsened both forms of injury without e xerting an independent lethal effect. Conversely, ceramide markedly at tenuated arachidonic acid cytotoxicity. This occurred without any decr ease in arachidonate uptake, suggesting a direct cytoprotective effect . In conclusion: (1) sphingosine and ceramide fluxes are hallmarks of early ischemic/reperfusion injury; (2) these changes occur via diverge nt metabolic pathways; and (3) that ceramide increments can affect div ergent injury pathways, and that sphingosine and ceramide have potent cell signaling effects, suggest that the currently documented sphingos ine/ceramide fluxes could have important implications for the inductio n phase and evolution of post-ischemic ARF.