Growth arrest and apoptosis in adult T cell leukemia cell lines following IL-2 deprivation.

Adult T-cell leukemia (ATL) is a peripheral T-cell neoplasm caused by human T-cell leukemia virus type-I (HTLV-I). Since ATL cells often require IL-2 for their proliferation and survival, we examined the effect of IL-2 deprivation on the IL-2-dependent ATL cells established from ATL patients. After IL-2 withdrawal, these cells were arrested in the G1 phase and then underwent apoptosis. p27Kip1 was observed to act as a cell cycle inhibitor. A decrease in the amount of Bcl-xL was more distinct than that of Bcl-2, while Bax increased slightly during IL-2 withdrawal. The activation of caspase-3 and the loss of mitochondrial membrane potential were also observed. An overexpression of Bcl-xL protein in the KK1, one of the ATL cell lines, suppressed apoptosis by the 3rd day, however, apoptosis could not be prevented completely. Thereafter, a decrease in Bcl-xL and an activation of caspase-3 were observed even under the overexpression of Bcl-xL. The mitochondrial membrane potential and the intra-cellular levels of reactive oxygen species (ROS) also changed due to IL-2 deprivation. From these results, the IL-2 signals are considered to be essential for the survival of ATL cells, and the interruption of IL-2 signaling might thus be useful as a potentially new treatment for ATL.