The effect of tumor-promoting phorbol ester treatment on the binding of interleukin-1β (IL-1β) to specific cell surface receptors was investigated. A 1 h exposure of Raji human B lymphoma cells with the protein kinase C-activating phorbol ester, phorbol dibutyrate (PDBu), reduced IL-1β binding by up to 90% of control cells. This effect was dose-dependent and was not observed with 4-α-phorbol, an inactive tumor promoter. Analysis of 125I-labeled IL-1β binding to intact cells revealed that PDBu caused a 91% decrease in high-affinity cell-surface receptor number without an effect on receptor affinity. The phorbol ester response was rapid (30 min), observed both at 4 and 37°C, and was preceded by the rapid translocation (t ⋘ 6 min) of protein kinase C (PKC) from the cytosol to the cell membrane. The PDBu-induced decrease in IL-1β receptor number was inhibited by prior incubation of cells for 30 min with the PKC inhibitor 1-(5-Isoquinoline sulfonyl)-2-methylpiperazine (H7). The decrease in receptor binding was not due to enhanced IL-1β receptor internalization or shedding into the extracellular medium, since a similar effect was observed with solubilized IL-1β receptor. The most likely explanation for the phorbol ester effect appears to be cell surface inactivation of IL-1 receptors. These data suggest that modulation of PKC activity could play a role in the regulation of the IL-1β receptor. © 1990.
