TNF translationally modulates the expression of G1 protein alpha(i2) subunits in human polymorphonuclear leukocytes.

Abstract

Priming of polymorphonuclear leukocyte responses to chemoattractants by TNF plays an important role in host defenses and inflammatory responses. TNF-induced priming is associated with an 80% increase in the membrane density of G alpha(i2) protein that is coupled to chemoattractant receptors. The present study examines the hypothesis that TNF stimulates increased synthesis of alpha(i2). Within 10 min of addition, TNF stimulated a significant increase in total cellular G alpha(i2), as determined by pertussis toxin-catalyzed ADP ribosylation, which was blocked by the translation inhibitor cycloheximide. Immunoprecipitation of biosynthetically labeled alpha(i2) showed that TNF increased alpha(i2) synthesis by about 20% at 10 min. Nuclear run-ons showed no change in alpha(i2) mRNA synthesis in TNF-treated cells; however, steady state alpha(i2) mRNA levels were reduced following a 10-min exposure to TNF. Pretreatment with cycloheximide prevented the TNF-induced reduction in steady state alpha(i2) mRNA levels. Therefore, TNF stimulates alpha(i2) protein synthesis and mRNA degradation in the same time frame as priming. The increased alpha(i2) synthesis results from increased translation, not transcription, of alpha(i2) mRNA. Simultaneous G alpha(i2) protein synthesis and mRNA degradation provide a mechanism by which TNF priming is associated with a rapid, self-limiting increase in G protein expression.