Expression of the early lymphocyte activation antigen CD69, a C-type lectin, is regulated by mRNA degradation associated with AU-rich sequence motifs.
CD69 is the earliest inducible cell surface glycoprotein acquired during lymphoid activation both in vitro and in vivo under physiological conditions and inflammation. This molecule is involved in lymphocyte proliferation, and functions as a signal-transmitting receptor in T and B lymphocytes, NK cells and platelets. Molecular cloning of CD69 cDNA revealed that this antigen is a type-II integral membrane protein with a C-type lectin domain in the extracellular region. The expression time course of CD69 mRNA has previously been reported to be transient, peaking around 3 h after induction in T lymphocytes, and declining to nearly resting levels by 8 h. We describe herein studies on the stability of the CD69 mRNA in phorbol ester-activated T lymphocytes. The level of CD69 mRNA in these cells declined rapidly with a half-life of less than 60 min. This finding is consistent with the presence of several AU-rich sequence motifs in the 3' untranslated region (3'UTR), which have been implicated in the selective destabilization of short-lived mRNA of mammalian cytokines, and proto-oncogenes. We have therefore introduced a fragment of the 3'UTR of the human CD69 cDNA, which contains the AU-rich sequence motifs, into the 3'UTR of the rabbit beta-globin gene. This inserted sequence causes the otherwise stable beta-globin mRNA to become unstable in vivo. A similar destabilizing effect is observed when the 51-nucleotide AU sequence from the mRNA of the human cytokine granulocyte/macrophage colony-stimulating factor is used as a positive control. Furthermore, the introduction of 194-bp fragment from the CD69 3'UTR containing most of the AU-rich motifs was sufficient to induce the destabilizing effect. We propose that the selective degradation pathway involved in the regulation of the expression of cytokines and proto-oncogenes is implicated in the rapid degradation of CD69 mRNA in activated T lymphocytes. This pathway may constitute a general mechanism to regulate the expression of inducible molecules involved in inflammatory processes.