DIRECT INHIBITION OF MITOCHONDRIAL RESPIRATORY-CHAIN COMPLEX-III BY CELL-PERMEABLE CERAMIDE

Ceramide is a lipid second messenger that mediates the effects of tumo r necrosis factor a! and other agents on cell growth and differentiati on, Ceramide is believed to act via activation of protein phosphatase, proline-directed protein kinase, or protein kinase C, Tumor necrosis factor alpha-induced common pathway of apoptosis is associated with am early impairment of mitochondria. Herein, we demonstrate that ceramid e earn directly inhibit mitochondrial respiratory chain function, In i solated mitochondria, a rapid decline of mitochondrial oxidative phosp horylation occurs in the presence of N-acetylsphingosine (C-2-ceramide ), a synthetic cell-permeable ceramide analog. An investigation of the Bite of ceramide action revealed that tbe activity of respiratory cha in complex III is reduced by C-2-ceramide with half-maximum effect al 5-7 PM. in contrast, N-acetylsphinganine (C-2-dihydroceramide), which lacks a functionally critical double bond and is ineffective in cells, did not alter mitochondrial respiration or complex III activity. We s uggest that these in vitro observations may set the stage for identify ing a novel mechanism of regulation of mitochondrial function in vivo.