Cardiolipin in Immune Signaling and Cell Death

Cardiolipin (CL) is a tetra-acylated diphosphatidylglycerol lipid. In physiological conditions, CL presents unsaturated chains and is located in the inner mitochondria membrane (IMM). Dead signals, infection, or disease may change the level of CL saturation and oxidation and cause its translocation to the cytosolic side of the outer mitochondrial membrane (OMM), affecting mitochondrial function and the inflammatory response. In this review, we summarize the emerging proapoptotic, pro-, and anti-inflammatory functions of cytosolic-exposed CL and how they are regulated by CL chain saturation and oxidation. We underline how the unique dimeric phospholipid structure confers peculiar properties on CL in the regulation of cell death and immune system proteins, such as the Nucleotide-binding domain and leucine-rich repeat-containing pyrin protein 3 (NLRP3), caspases (Casp), and Toll-like receptor 4 (TLR4). We also provide an overview of the human diseases in which CL deficiency or modification are implicated and of the use of exogenous unsaturated CL (uCL) as a novel therapeutic approach. Proapoptotic or pathogen signals induce translocation of cardiolipin (CL) from the inner to the outer mitochondrial membrane (OMM).Exposed CL mediates degradation of damaged intracellular mitochondria and phagocytosis of mitochondria from the extracellular milieu.Exposed CL in mitochondria acts as a platform for inflammasome localization and activation.Oxidation of CL and exposure on the OMM induce cytochrome c release and apoptosis.Exogenous uCL inhibits lipopolysaccharide (LPS)-induced proinflammatory signaling via TLR4, whereas saturated CL mimics bacterial LPS and activates TLR4.