Ceramide and mitochondria in ischemia/reperfusion

Abstract

A hallmark of tissue injury in various models of ischemia/reperfusion (IR) is mitochondrial dysfunction and the release of mitochondrial proapoptotic proteins leading to cell death. Although IR-induced mitochondrial injury has been extensively studied and key mitochondrial functions affected by IR are chiefly characterized, the nature of the molecule that causes loss of mitochondrial integrity and function remains obscure. It has become increasingly clear that ceramide, a membrane sphingolipid and a key mediator of cell stress responses, could play a critical role in IR-induced mitochondrial damage. Emerging data point to excessive ceramide accumulation in tissue and, specifically, in mitochondria after IR. Exogenously added to isolated mitochondria, ceramide could mimic some of the mitochondrial dysfunctions occurring in IR. The recent identification and characterization of major enzymes in ceramide synthesis is expected to contribute to the understanding of molecular mechanisms of ceramide involvement in mitochondrial damage in IR. This review will examine the experimental evidence supporting the important role of ceramide in mitochondrial dysfunction in IR to highlight potential targets for pharmacological manipulation of ceramide levels.

Figure 1. Biosynthesis of ceramide and its conversion into other bioactive sphingolipids

De novo ceramide synthesis begins with the conversion of serine and fatty acyl CoA into 3-ketosphinganine by serine palmitoyl transferase (SPT), then 3-ketosphinganine is converted into dihydrosphingosine. Myriocin is a potent inhibitor of SPT activity. (Dihydro) ceramide synthase (LASS/CerS) acylates dihydrosphingosine to form dihydroceramide, which is then reduced to ceramide by dihydroceramide desaturase. Ceramide is also produced by sphingomyelinases (SMases) through sphingomyelin (SM) degradation in sphingomyelinase pathway. Ceramidase converts ceramide into sphingosine, which is phosphorylated by sphingosine kinase (SK) to generate sphingosine-1-phosphate. Ceramide is phosphorylated by ceramide kinase (CK) yielding ceramide-1-phosphate (C1P). In the salvage or recycling pathway, complex sphingolipids are broken down to ceramide by β-glucosidase and then by ceramidase to sphingosine, which is re-acylated to ceramide by LASS/CerS. Fumonisin B1 inhibits LASS/CerS activity.

Figure 2. Ceramide modulates mitochondrial functions through direct and indirect mechanisms

A. Indirect modulation of mitochondrial functions by ceramide occurs through the change in the ratio of pro-apoptotic/anti-apoptotic proteins of Bcl-2 family at the outer mitochondrial membrane. B. Direct modulation of mitochondrial functions by ceramide include a) formation of ceramide channels permeable for cytochrome c in the outer mitochondrial membrane; b) potentiation of mitochondrial permeability transition pore opening (MPTP) in the inner membrane in the presence of Ca²⁺ or Bax (ceramide-induced Ca²⁺ release from the endoplasmic reticulum (ER) can contribute to the processes; c) potentiation of Bax insertion (activation) in to the outer membrane, d) inhibition of the respiratory chain (RC) with a subsequent increase in reactive oxygen species (ROS) formation.