Khabar, K, Siddiqui, Y., al-Zoghaibi, F., al-Haj, L., Dhalla, M., Zhou, A., Dong, B., Whitgmore, M., Paranjape, J., Al-Ahdal, M., AL-Mohanna, F., Williams, B. and R. Silverman. 2003. RNase L mediates transient control of the Interferon response through modulation of the double-stranded RNA-dependent Protein Kinase PKR. Journal of Biological Chemistry 278, 22, 20124-20132.
This is another paper that may have important implications for CFS patients as the mechanism it describes could perhaps, at least according to my pitiful understanding, explain the ongoing PKR upregulation seen in CFS. Since PKR is involved in apoptosis and iNOS activation (and therefore nitric oxide levels) and other factors, PKR upregulation may play an important role in CFS pathology. The real kicker is that the putative PKR regulator is none other than RNase L. PKR and RNase L form two of the three branches of the interferon immune response.
In this study PKR activity was monitored in cells with (RNase L +/+) and without RNase L (RNase L -/-). PKR activity was two times higher and remained higher for 24 hours longer in cells without RNase L than in cells with RNase L. Further measures indicated that the cause of the increased and extended PKR activity was RNase L’s degradation of PKR’s mRNA. (Not a nice thing to do!).The half-life of PKR mRNA in RNase L- cells was 6.7 hours vs .8 hours (!) in RNase L+ cells. Since the amount of ‘global’ RNA was unaffected by the presence of RNase L, it appears that RNase L was selectively targeting PKR mRNA.
This seemed, on the face of it, to be a strange finding. PKR and 2-5OAS are the only dsRNA activated enzymes found. Both have been demonstrated to play an important role in viral protection and apoptosis. PKR appears, however, to be activated by more factors than RNase L’. PKR responds to a wide variety of situations including, among others, growth factor deprivation, oxidative stress, TNFa activity, and the presence of bacterial lipopolysaccharides (LPS), and viral dsRNA.
Bizarrely, given RNase L’s role in the antiviral response, RNase L-/- cells with high levels of viral infection displayed significantly greater antiviral activity than RNase L+/+ cells. Not only, therefore, does RNase L attenuate PKR activity but under certain conditions it can also inhibit the antiviral activity in the cell (??). That this phenomena was reversed in IFN induced cells suggests that IFN’s induce the activity of antiviral components that are downstream of RNase L. This suggests that if RNase L is induced by other than the IFN response it will target, i.e. destroy RNA that may be beneficial to the immune response?
The authors suggest that RNase L destroys PKR mRNA in order to guard against chronic immune activation and an upregulation of potentially damaging cellular stress pathways by PKR. PKR activity is, after all, not totally blocked by RNase L – it remains elevated for 12 hours even in presence of RNase L – its activity is only shortened. In the authors scenario PKR activity remains high during times of acute infection and then is moderated afterwards. It is possible, as well, that RNase L’s regulation of PKR mRNA is operative during the cellular growth and proliferation activities that both enzymes engage in. One wonders as well – given PKR’s involvement during periods of oxidative stress, growth factor deprivation, etc. if RNase L does not step in and shut down PKR is other situations as well.
It turns out that 2-5OAS induction is not wholly dependent on the IFN signaling pathway. Can RNase L be induced without IFN outside of the laboratory? A recent finding there was no relationship between IFN-a levels and 2-5OAS activity in CFS patients or controls suggests that substantial amounts of 2-5OAS activity may occur in CFS outside the IFN system. Does this suggest RNase L activation contributes to the immune dysfunction in CFS? Could it be that IFN shut down during a chronic disease process triggers anti-antiviral RNase L activity?
If RNase L plays an important role in PKR regulation then the RNase L deregulation seen in CFS could result in chronically high levels of PKR activity and thus chronically high rates of apoptosis and levels of NO. RNase L fragmentation would then be the source of PKR upregulation!)