INTERLEUKIN-2 INDUCES A TRANSIENT DOWN-REGULATION OF PROTEIN PHOSPHATASE-1 AND PHOSPHATASE-2A ACTIVITY IN HUMAN T-CELLS

Stimulation of human CD4(+) T cell lines with interleukin 2 (IL-2) ind uces tyrosine, serine and threonine phosphorylation of a series of pro teins involved in the IL-2 receptor (IL-2R) signaling pathway. Here, w e examined whether IL-2 induces changes in the activity of protein ser ine/threonine phosphatases in antigen specific, CD4(+) human T cell li nes. Using inhibitors of protein phosphatases 1 (PPI), PP2A, and PP2B, we provide evidence, that IL-2 induces a downregulation of PP activit y in the cytoplasmic/membrane fraction. Thus, IL-2R ligation for 30 mi n triggers a 16 percent decrease in total PP2A activity (p<0.00005, n= 17) and a seven percent decrease in PP1 activity (p<0.00005, n=17). Cy tokine-induced downregulation of PP2A activity reaches a maximum 60 mi n after IL-2R ligation, and returns to baseline levels within two hour s. Downregulation of PP1 activity reaches a maximum after 30 min and i s largely reversed one hour after IL-2 stimulation. As determined from immunoblotting experiments using a specific anti-PP1 or anti-PP2A ant ibody, the amount of PP1 and PP2A recovered from cytosolic/membrane fr action remains unchanged after IL-2 treatment suggesting that the drop in PP1/PP2A activity might be due to a regulatory change rather than to a change in the amount of PP1 and PP2A. In conclusion, we provide e vidence, for the first time, that IL-2 induces a transient downregulat ion of PP2A activity in T cells. In addition, our findings indicate th at cytoplasmic PP1 activity is transiently downregulated following IL- 2R ligation in antigen-specific, human CD4(+) T cells.