CASEIN KINASE-1 PHOSPHORYLATES THE P75 TUMOR-NECROSIS-FACTOR RECEPTORAND NEGATIVELY REGULATES TUMOR-NECROSIS-FACTOR SIGNALING FOR APOPTOSIS

Cellular responses initiated by tumor necrosis factor (TNF) are mediat ed by two different cell surface receptors with respective molecular m asses of 55 kDa (p55) and 75 kDa (p75), p55 is functional in almost ev ery cell type and can independently transmit most biological activitie s of TNF. In contrast, TNF signaling via p75 seems so far largely rest ricted to cells of lymphoid origin, where it can induce proliferation, cytokine production, and/or apoptosis. The mechanisms that regulate T NF receptor activity are largely unknown. Here we report that the p75 of unstimulated p75-responsive PC60 T cells is phosphorylated on serin e by a kinase activity present in p75 immune complexes. Several lines of evidence indicate that the latter kinase is casein kinase-1 (CK-1). Previous results have shown that the p75 TNF receptor is constitutive ly phosphorylated in vivo. Our data show that the latter in vivo phosp horylation is also at least partially due to CK-1. Pretreatment of cel ls with TNF had no detectable effect on p75 phosphorylation in vitro o r in vivo. However, a specific CK-1 inhibitor potentiated TNF-induced apoptosis mediated by p75, suggesting an inhibitory role for phosphory lation by CK-1. Although in vivo p75 phosphorylation could be seen in both p75-unresponsive and p75-responsive cell lines, in vitro p75 phos phorylation in p75 coimmunoprecipitates could not be observed in cell lines that were biologically unresponsive to p75 stimulation. The latt er observation further indicates a regulatory role for p75 phosphoryla tion in p75-mediated signaling. Taken together, our data demonstrate t hat the p75 TNF receptor is phosphorylated and associated with CK-1, w hich negatively regulates p75-mediated TNF signaling.