Carboxyl group-modified α-lactalbumin induces TNF-α-mediated apoptosis in leukemia and breast cancer cells through the NOX4/p38 MAPK/PP2A axis

To clarify the mechanism of semicarbazide-modified α-lactalbumin (SEM-LA)-mediated cytotoxicity, we investigated its effect on human U937 leukemia cells and MCF-7 breast cancer cells in the current study. SEM-LA induced apoptosis in U937 cells, which showed increased NOX4 expression, procaspase-8 degradation, and t-Bid production. FADD depletion inhibited SEM-LA-elicited caspase-8 activation, t-Bid production, and cell death, indicating that SEM-LA activated death receptor-mediated apoptosis in U937 cells. SEM-LA stimulated Ca2+-mediated Akt activation, which in turn increased Sp1- and p300-mediated NOX4 transcription. The upregulation of NOX4 expression promoted ROS-mediated p38 MAPK phosphorylation, leading to protein phosphatase 2A (PP2A)-regulated tristetraprolin (TTP) degradation. Remarkably, TTP downregulation increased the stability of TNF-α mRNA, resulting in the upregulation of TNF-α protein expression. Abolishment of Ca2+-NOX4-ROS axis-mediated p38 MAPK activation attenuated SEM-LA-induced TNF-α upregulation and protected U937 cells from SEM-LA-mediated cytotoxicity. The restoration of TTP expression alleviated the effect of TNF-α upregulation and cell death induced by SEM-LA. Altogether, the data in this study demonstrate that SEM-LA activates TNF-α-mediated apoptosis in U937 cells through the NOX4/p38 MAPK/PP2A axis. We think that a similar pathway can also explain the death of MCF-7 human breast cancer cells after SEM-LA treatment.