Sequence of Metabolic Changes during X-Ray-Induced Apoptosis

A cell population undergoing apoptosis usually contains varying proportions of cells in the diverse stages of the process, from very early continuously through to secondary necrosis. This heterogeneity acts as a confounding factor in metabolic studies if a general population is investigated. Using fluorescent probes and multiparameter flow cytometry, we report on metabolic changes occurring during X-ray-induced apoptosis in human peripheral blood lymphocytes and relate the observed alterations to cells at various phases of the process assessed by monitoring the progressive loss of selective plasma membrane permeability. Data show that alterations of mitochondria cardiolipin and a reduction of plasma membrane potential are rather early events as they commence in cells which still possess an impermeable plasma membrane. Conversely, mitochondrial transmembrane potential is impaired only when plasma membrane permeability starts to be altered, that is, in relatively later apoptotic cells, thereby reflecting the complexity of mitochondria demise during apoptosis. The prooxidant/antioxidant balance is altered in cells in early apoptosis with a correlated increase of prooxidants and depletion of thiols, the latter indicative for the progressive impairment of this detoxifying mechanism. The imbalance in prooxidant/antioxidant remained evident through apoptosis suggesting that oxidative damage starts early and then continues, eventually leading to cellular disruption. Assessing cell transit through the apoptotic process and coupling the observed metabolic changes to selected stages of the process enables one to improve the understanding of the temporary sequence of biochemical phenomena occurring in a given model.