The 2-5A system in viral infection and apoptosis

The 2-5A system is an established endogenous antiviral pathway. Interferon treatment of cells leads to an increase in basal, but latent, levels of 2-5 A-dependent RNase (RNase L) and the family of 2'-5' oligoadenylate sythetas es (OAS). Double-stranded RNA, thought to be derived from viral replication intermediates, activates GAS. Activated OBS converts ATP into unusual shor t 2'-5' linked oligoadenylates called 2-5A [ppp5'(A2'p5')2A]. The 2-5A bind s to and activates RNase L which cleaves single stranded RNA with moderate specificity for sites 3' of UpUp and UpAp sequences, and thus leads to degr adation of cellular rRNA. During apoptosis, generalized cellular RNA degradation, distinct from the d ifferential expression of mRNA species that may regulate specific gene expr ession during apoptosis, has been observed. The mechanism of RNA breakdown during apoptosis has been commonly considered a non-specific event that ref lects the generalized shut down of translation and homeostatic regulation d uring cell death. Due to the similar RNA degradation that occurs during bot h apoptosis and viral infection we investigated the potential role of RNase L in apoptosis. To investigate whether RNase L activity could lead to apoptosis, NIH3T3 cel ls were transfected with a iac-inducible vector containing the human RNase L gene. Treatment of these cells with isopropylthiogalactoside (IPTG) cause d loss of cell viability that was confirmed as an apoptotic cell death by m orphological and biochemical criteria. Similarly, specific allosteric activ ation of endogenous RNase L by introduction of 2-5A directly into L929 cell s also induced apoptosis. In L929 cells poly(I).poly(C) treatment in combin ation with interferon caused an increase in apoptosis whereas neither inter feron or double stranded RNA alone altered cell viability. Therefore, incre ased expression or activation of RNase L causes apoptosis. Inhibition of RNase L, specifically with a dominant negative mutant, suppre ssed poly (I)Ypoly(C)-induced apoptosis in interferon-primed fibroblasts. P oliovirus, a picornovirus with a single-stranded RNA genome, causes apoptos is of HeLa cells. Expression of the dominant negative inhibitor of RNase L in HeLa prevented virus induced apoptosis and maintained cell viability. Th us, reduction or inhibition of RNase L activity prevents apoptosis. Both apoptosis and the 2-5A system can provide defense against viral infect ion in multicellular organisms by preventing production and therefore sprea d of progeny virus. RNase L appears to function in both mechanisms, therefo re, initiation of apoptosis may be one mechanism for the antiviral activity of the 2-5A system. (C) 1998 Elsevier, Paris.