A Protective Role for the Human SMG-1 Kinase against Tumor Necrosis Factor-α-induced Apoptosis

The human suppressor of morphogenesis in genitalia-1 (hSMG-1) protein kinase plays dual roles in mRNA surveillance and genotoxic stress response pathways in human cells. Here, we report that small interfering RNA-mediated depletion of hSMG-1, but not ATM, ATR, hUpf1, or hUpf2, in human U2OS osteosarcoma cells markedly increases the magnitude and accelerates the rate of apoptosis induced by tumor necrosis factor-α (TNFα) stimulation. The increase in TNFα-mediated cell killing observed in hSMG-1-depleted cells is not related to the suppression of nonsense-mediated mRNA decay or to the inhibition of TNFα-induced NF-κB activation. Rather, we observed that loss of hSMG-1 accelerates the degradation of the long form of the FLICE-inhibitory protein (FLIPL), an inhibitor of death-inducing signaling complex-mediated caspase-8 activation, in TNFα-treated cells. These results suggest that hSMG-1 plays an important role in cell survival during TNFα-induced stress.