Loss of cardiolipin in palmitate-treated GL15 glioblastoma cells favors cytochrome c release from mitochondria leading to apoptosis

Unlike oleate and linoleate, palmitate induced mitochondrial apoptosis in GL15 glioblastoma cells. Decrease in membrane potential in a subpopulation of mitochondria of palmitate-treated cells was revealed using the 5,5',6,6'-tetrachloro-1,1',3,3'- tetraethylbenzimidazolylcarbocyanine iodide probe. The diminished ability to reduce a tetrazolium salt indicated an impairment of mitochondrial function. Up to 50% cytochrome c (cyt c) was detached from the inner mitochondrial membrane and released outside mitochondria in palmitate-treated cells, whereas no release was detected after oleate and linoleate treatments. Cyt c release into the cytosol was followed by caspase 3 activation. Released cyt c and caspase 3 activity were not affected by neutral and acid sphingomyelinase inhibitors and by the inhibitor of serine palmitoyltransferase cycloserine, indicating that apoptosis was independent of the ceramide pathway, nor the mitochondrial pro-apoptotic AIF or Bcl-2/Bax factors appeared to be involved in the effect. Utilization of palmitate by GL15 cells altered phospholipid composition. Cardiolipin (CL), the lipid involved in cyt c interaction with the inner mitochondrial membrane, was decreased and highly saturated. This produced an imbalance in hydrophilic/hydrophobic interactions underlying the anchorage of cyt c, by weakening the hydrophobic component and facilitating detachment of the protein and activation of downstream processes. The primary role of CL was explored by supplying GL15 with exogenous CL through a fusion process of CL liposomes with cell plasma membrane. Fused CL moved to mitochondria, as detected by nonylacridine orange probe. Enrichment of mitochondrial membranes with CL prior to palmitate treatment of cells caused decreased cyt c release and caspase 3 activity.