To functionally classify AU-rich elements (AREs) from six
different cytokine mRNAs, we made use of two previously described
HT1080-derived cellular mutants (slowA, slowC) that lack a function
required for the rapid degradation of interleukin-3 (IL-3) mRNA.
Here we show that the defect is specific for ARE-containing
mRNAs, whereas nonsense-mediated decay is intact. Degradation
of β-globin reporter transcripts mediated by the AREs of
IL-3, GM-CSF, and TNFα, as well as by the structurally
different and less potent AREs of IL-2 and IL-6, is impaired
in both mutants. All these reporter transcripts are also sensitive
to decay induced by ectopic expression of the RNA-binding protein
tristetraprolin in the slowC background. Thus, we concluded
that the mutants slowA and slowC define a common mRNA degradation
pathway that targets cytokine AREs. In NIH3T3 cells, this decay
pathway becomes incapacitated by upstream signaling from p38
MAP- or PI3-kinases, which independently stabilize cytokine
ARE-containing transcripts. In contrast, c-fos
ARE-directed mRNA degradation proceeds through a different pathway
not affected by these kinases.