The oncoprotein I-2PP2A/SET negatively regulates the MEK/ERK pathway and cell proliferation

I-2 PP2A /SET, the translocation breakpoint-encoded protein expressed in acute undifferentiated leukemia, was identified as an inhibitor of protein phosphatase 2A (PP2A). Induction of exogenous I-2 PP2A /SET at a ratio of 1:1 to the endogenous protein resulted in suppression of cell proliferation. In contrast, siRNA-mediated depletion of I-2 PP2A /SET resulted in enhanced cell proliferation. Depletion of I-2 PP2A /SET was accompanied with a decrease in the number of cells in G1 and an increase in cells in S phase. To examine the mode of action by which I-2 PP2A /SET suppresses cell proliferation, we determined the effect of over-expressed I-2 PP2A /SET on ERK activation. I-2 PP2A /SET suppressed activation of ERK following EGF stimulation but did not affect activation levels of stress kinases, JNK and p38. By contrast, knocking down I-2 PP2A /SET by siRNA resulted in enhancement of ERK and MEK activations, suggesting that I-2 PP2A /SET negatively regulates MEK/ERK. These data suggest that I-2 PP2A /SET negatively regulates cell growth by inhibiting the G1/S transition and inhibiting the MEK/ERK pathway stimulated by external stimuli. These data demonstrate that I-2 PP2A /SET potentially functions as a tumor suppressor.