Tumor necrosis factor alpha increases tyrosine phosphorylation of a 23-kDa nuclear protein in U937 cells through ceramide signaling pathway.

In many tumor cell lines, tumor necrosis factor alpha (TNF alpha) causes profound alterations in nucleus including chromatin condensation and DNA fragmentation. However the mechanism is largely unknown. Here we demonstrate that TNF alpha treatment of U937 cells increased tyrosine phosphorylation of a 23-kDa nuclear protein (P23) maximally by 13-fold, which occurred after 2.5 h treatment concomitantly with occurrence of DNA fragmentation. Tyrosine kinase inhibitor, herbimycin A, inhibited either tyrosine phosphorylation of P23 or DNA fragmentation, suggesting that the P23 phosphoprotein may be involved in the TNF alpha-induced changes in nucleus. Increase of intracellular Cer level by hydrolysis of cell surface SM with bacterial sphingomyelinase or addition of either C2-Cer or natural Cer also resulted in increase of P23 tyrosine phosphorylation, which was detectable at 1.5 h treatment and reached maximum after 2 h treatment, indicating that P23 phosphoprotein may be a downstream effector of Cer signaling pathway. Our data provide further evidence that Cer signaling pathway may mediate signal transduction of TNF alpha.