Apoptosis or programmed cell death is an active form of cell death which is essential for tissue homeostasis. Many proteins are involved in the molecular signal transduction of apoptosis. The caspase enzymes, a family of specific cysteine proteases, play a central role in cell death machinery. In this review, we mainly discuss the current understanding of several pathways to activate caspases and some key proteins related to these pathways.

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The aim of the study was to investigate the sensitivity of AHH-1 human lymphoblastoid cells to radiation and its relevance to intracellular events, specifically alteration in cellular energy-producing systems. AHH-1 human lymphoblastoid cells were irradiated with 6 Gy of gamma radiation, and then were collected at the indicated time points. Parallel studies were conducted to assess the effects of radiation on the cell proliferation and apoptotic index. Mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) production were monitored. A marked decrease of cell viability was observed as early as 12 h postirradiation and fraction of apoptotic cells was highest at 24 h. Intracellular ROS generation measured with 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) appeared to be highest as early as 30 min postirradiation and resumed to normal level at 6 h. Unexpectedly, the fluorescence intensity of Rhodamine 123 for measuring MMP did not change during the first 3h after radiation and exhibited an aberrant increase at 6 h. The results suggest that AHH-1 cells are sensitive to radiation-induced apoptosis and ROS generation is an early phase in the apoptosis process. Moreover, the results might cast doubts on those studies using Rhodamine 123 which hypothesized that the fall in MMP is one of the early events of apoptosis.

Bcl-xL belongs to a family of proteins which inhibit apoptosis in a number of stimuli including ionizing radiation. To better understand the effects and mechanisms of Bcl-xL on the apoptosis of lymphocytes and provide experimental basis to treat immune injury induced by radiation, we used normal human lymphoblastoid AHH-1 cells that were engineered to overexpress Bcl-xL proteins. Our results showed that overexpressed Bcl-xL reduced time-dependent increase of apoptosis induced by ionizing radiation. Reactive oxygen species (ROS) generation and Bax protein expression in the transfected AHH1-Bcl-xL cells were also lower compared to parental AHH-1 cells. Unexpectedly, the fluorescence intensity of Rhodomine 123 (Rh 123) for measuring mitochondrial membrane potential (MMP) did not change at all detected time points. These results possess a vital significance for insights into a new strategy for gene therapy of radiation-induced immune injury.

These observations suggest that the changes of lymphocyte subpopulations might play a role in susceptibility to inflammation of Smad3(-/-) mice.