The effects of interferon-alpha (IFN-alpha) on the interleukin-6 (IL-6) receptor in a multiple myeloma cell line, U266, have been examined. IFN-alpha inhibits [3H]thymidine incorporation in U266 cells in a time- and dose-dependent manner. Furthermore, IFN-alpha inhibits the ability of IL-6 to induce increases in [3H]thymidine incorporation. While IFN-alpha suppresses the ability of 125I-IL-6 to bind to the IL-6 receptor on U266 cells, this effect is not due to competition of IFN-alpha with IL-6 for the IL-6 receptor. Although IFN-alpha induces IL-6 synthesis in the U266 cell, inhibition of IL-6 binding occurs when IL-6 synthesis is minimal. Furthermore, after pretreatment of U266 cells with neutralizing anti-IL-6 antibodies, IFN-alpha still inhibits 125I-IL-6 binding. These data suggest that IFN-alpha inhibition of 125I-IL-6 binding does not involve IL-6 synthesis. IFN-alpha reduces 125I-IL-6 binding without affecting its affinity, suggesting that IFN-alpha inhibits IL-6 receptor expression. Although pretreatment with cycloheximide inhibits 125I-IL-6 binding, IFN-alpha does not cause a selective decrease in the levels of gp130 or IL-6 receptor mRNA at times when 125I-IL-6 binding is inhibited. These observations indicate that IFN-alpha lowers IL-6 receptor density on U266 cells by mechanisms other than competitive binding or lowering IL-6 receptor mRNA production. Receptor down-regulation may be a mechanism of IFN-alpha-induced inhibition of growth in U266 cells.

Mammalian liver possesses a unique isozyme of phosphate-activated glutaminase that is subject to long-term regulation. In rats during starvation or after consumption of diets containing high amounts of protein (60%), hepatic glutaminase activity was 100% higher than in rats fed a 20% protein diet. Conversely, rats fed low protein diets (0 and 5%) had lower hepatic glutaminase activity when compared with rats fed the 20% protein diet. Differences in activity with different dietary protein levels were not due to differences in the amount of food consumed. The relative abundance of mRNA encoding hepatic glutaminase was lower in rats fed 0% protein and higher in those starved or fed 60% protein diet when compared with rats fed the 20% protein diet. The mRNA elongation assay in hepatic nuclei isolated from these animals demonstrated that the rate of transcription of the glutaminase gene was also different in rats starved or fed different levels of dietary protein. Overall, the results indicate that differences in hepatic glutaminase activity in rats starved or fed different levels of protein are mainly due to differences in the rate of transcription of the gene. In this way the regulation of hepatic glutaminase expression is similar to that seen for other enzymes involved in hepatic amino acid catabolism but differs markedly from that of renal glutaminase, in which changes in transcription rate are not observed and alterations of mRNA turnover are the principle mechanism of long-term regulation.

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