CD45 Defines Signaling Thresholds Critical for Proliferation and Apoptosis in Myeloma Cells.

Expression of the common leukocyte antigen, CD45 is quite variable in human myeloma cells and cell lines, such as U266, and CD45⁺ U266 proliferates in response to a growth factor, interleukin-6 (IL-6). IL-6 as a mitogenic stimulus, activated a src family kinase, Lyn and phospholipase C (PLC)-g2 followed by Ca²⁺ influxes and protein kinase C (PKC) activation in CD45⁺ U266. Here we show that compared with CD45⁻ U266, CD45⁺ U266 was more sensitive to various apoptotic stimuli, such as oxidative stress and endoplasmic reticulum (ER)-stress. Both release of apoptosis-inducing factor or cytochrome c and the activation of caspase-9 and caspase-3 were enhanced in CD45⁺ U266 more than in CD45⁻ U266. Increased susceptibility to the stress-induced apoptosis was also observed in other CD45-expressing myeloma cell lines and the CD45-transfected U266. Reactive oxygen species (ROS) and calcium ion (Ca²⁺) seem to be involved in the susceptibility to apoptosis of CD45⁺ U266 because either an anti-oxidant, DTT or a Ca²⁺ chelating agent, BAPTA blocked the apoptosis. As the oxidative stress-induced Ca²⁺ influxes and apoptosis of CD45⁺ U266 were blocked by a src family kinase inhibitor, PP2, CD45 phosphatase regulating the src family kinase activity plays an important role in the augmented apoptosis in CD45⁺ U266 by oxidative stress. The intracellular accumulation of ROS may activate src associated with CD45, and the activated src kinases accelerate the Ca²⁺ influxes followed by calicineurin activation, mitochondrial translocation of Bad, and inhibition of Bcl-2 function. These results indicate that the CD45-expression renders myeloma cells competent with not only mitogenic but also apoptotic stimuli, resulting in either proliferation or apoptosis of CD45⁺ myeloma cells dependently upon the stimuli. Furthermore, voltage-dependent anion channel (VDAC)-1 was identified as a gene highly expressed in CD45⁺ U266 by cDNA subtraction. The increased expression of VDAC-1 seemed to augment the sensitivity to the ER-stress because the VDAC-1-transfected CD45⁻ U266 was confirmed to be susceptible to the thapsigargin-induced apoptosis. Thus, CD45 may define the signaling thresholds that are critical for the IL-6-induced proliferation and the oxidative stress-induced apoptosis of myeloma cells, and the CD45 expression accompanied by the increased VDAC-1 expression sensitizes myeloma cells to the various extracellular stimuli triggering apoptosis via the mitochondrial pathways.