Mitochondrial ultracondensation, but not swelling, is involved in TNFα-induced apoptosis in human T-lymphoblastic leukaemic cells

Abstract

Mitochondrial permeability transition (PT) pore opening and mitochondrial swelling have been reported in association with apoptosis. Conformational alterations of mitochondria induced by tumour necrosis factor-α (TNFα), and the association with TNFα-induced apoptosis, were, therefore, studied in the human acute T-lymphoblastic leukaemia (T-ALL) cell line, CCRF-CEM and its vinblastine-resistant CEM/VLB 100 cell line by transmission electron microscopy (TEM). The CEM/VLB 100 cell line possessed more condensed (C phase) mitochondria in the resting state compared with its parental cell line, consistent with increased activity of the mitochondrial electron transport chain (ETC). Following exposure to TNFα, conformational alterations of mitochondria occurred in both apoptotic and non-apoptotic cells. Orthodox (O phase) mitochondria in non-apoptotic cells underwent C-phase, transitional O-phase and slightly swollen (S-phase) conformational changes. TNFα-induced mitochondrial swelling was a late event and was found to a far lesser extent than mitochondrial condensation. No swollen mitochondria were observed in apoptotic cells. Ultracondensed (UC phase) mitochondria were observed in cells undergoing both TNFα-induced and spontaneous apoptosis and were seen when TNFα-induced apoptosis was inhibited by 3-methyladenine (3MA). The structural integrity of UC phase mitochondria persisted through the apoptotic process. We conclude that TNFα-induced mitochondrial swelling and apoptosis are separate events. Mitochondrial ultracondensation is associated with the processes signalling apoptosis and is not a result of TNFα-induced apoptotic shrinkage.