Upregulation of Interferon-γ Binding by Tumor Necrosis Factor and Lymphotoxin: Disparate Potencies of the Cytokines and Modulation of Their Effects by Phorbol Ester

Abstract

Tumor necrosis factor (TNF) and interferon-γ (IFN-γ) are immune-modulating cytokines that exert synergistic cytotoxic effects in several types of tumor cells, including ASPC-1 human pancreatic carcinoma cells. Lymphotoxin (LT), is a cytokine that binds to the TNF receptor and mimicks most of the biological actions of TNF. In the present study, we examined ASPC-1 cells for cytokine-mediated modulation of TNF and IFN-γ receptors. Treatment of ASPC-1 cells with recombinant human IFN-γ (rhIFN-γ) did not significantly alter ^12SI-rhTNF binding. In contrast, treatment with rhTNF led to a dose- and time-dependent increase in ¹²⁵I-rhIFN-γ binding and internalization. Scatchard analysis revealed that rhTNF increased the number of ¹²⁵I-rhIFN-γ binding sites from 11,000 sites/cell to 23,000 sites/cell without altering receptor affinity. Although rhLT also increased ¹²⁵I-rhIFN-γ binding, it was 100-fold less potent than rhTNF. In contrast, rhLT was only 10-fold less potent than rhTNF in displacing ¹²⁵I-rhTNF from its receptor. The phorbol ester 12-0-tetradecanoyl phorbol-13-acetate (TPA) blocked the rhLT- and rhTNF-mediated increase in ¹²⁵I-rhIFN-γ binding and markedly decreased ¹²⁵I-rhTNF binding. These data suggest that both TNF and LT upregulate IFN-γ receptors in ASPC-1 cells, but that LT is much less efficient than TNF. Further, the TPA-induced attenuation of IFN-γ receptor upregulation suggests that protein kinase C activation can regulate the TNF/LT-mediated pathways involved in IFN-γ receptor upregulation.