Activation of p38 mitogen-activated protein kinase and c-Jun NH2-terminal kinase by double-stranded RNA and encephalomyocarditis virus: Involvement of RNase L, protein kinase R, and alternative pathways

Double-stranded RNA (dsRNA) accumulates in virus-infected mammalian cells a nd signals the activation of host defense pathways of the interferon system , We describe here a novel form of dsRNA-triggered signaling that leads to the stimulation of the p38 mitogen-activated protein kinase (p38 MAPK) and the c-Jun NH,terminal kinase (JNK) and of their respective activators MKK3/ 6 and SEK1/MKK4. The dsRNA-dependent signaling to p38 MAPK was largely inta ct in cells lacking both RNase L and the dsRNA-activated protein kinase (PK R), i.e., the two best-characterized mediators of dsRNA-triggered antiviral responses. In contrast, activation of both MKK4 and JNK by dsRNA was great ly reduced in cells lacking RNase L (or lacking both RNase L and PKR) but w as restored in these cells when introduction of dsRNA was followed by inhib ition of ongoing protein synthesis or transcription. These results are cons istent with the notion that the role of RNase L and PKR in the activation o f MKK4 and JNK is the elimination, via inhibition of protein synthesis, of a labile negative regulator(s) of the signaling to JNK acting upstream of S EK1/MKK4, In the course of these studies, we identified a long-sought site of RNase L-mediated cleavage in the 28S rRNA, which could cause inhibition of translation, thus allowing the activation of JNK by dsRNA, We propose th at p38 MAPK is a general participant in dsRNA-triggered cellular responses, whereas the activation of JNK might be restricted to cells with reduced ra tes of protein synthesis. Our studies demonstrate the existence of alternat ive (RNase L- and PKR-independent) dsRNA-triggered signaling pathways that lead to the stimulation of stress-activated MAPKs. Activation of p38 MAPK ( but not of JNK) was demonstrated in mouse fibroblasts in response to infect ion with encephalomyocarditis virus (ECMV), a picornavirus that replicates through a dsRNA intermediate. Fibroblasts infected with EMCV (or treated wi th dsRNA) produced interleukin-6, an inflammatory and pyrogenic cytokine, i n a p38 MAPK-dependent fashion. These findings suggest that stress-activate d MAPKs participate in mediating inflammatory and febrile responses to vira l infections.