Methisoprinol enhancement of nucleocytoplasmic transport of putative messenger RNA in rat liver. its relation to host defense against virus infection.

Abstract The experimental antiviral drug, methisoprinol, has been reported to increase the rate of rapid entry of RNA precursors into polyribosomes in diverse tissues. To further the understanding of its mechanism of action, in this paper methisoprinol was examined for effects on the rapidly labeled RNA metabolism of rat liver, in studies that distinguish between drug effect on RNA synthesis and on nucleocytoplasmic RNA transport. When ingested orally at a level of 500 mg/kg/24 hr, methisoprinol was found not to affect the incorporation of orotic acid[ 14 C] into cytoplasmic UMP, UDP or UTP. and not to affect the rate of labeling or the pool size of nuclear UTP. At this dose, methisoprinol was found not to affect the rate of labeling of extracted nuclear RNA, while at a dose four times greater methisoprinol depressed the labeling of nuclear RNA by 32 per cent. In contrast, methisoprinol was observed to markedly increase the early rate of incorporation of radioactive precursor into liver polysomal RNA at times between 15 min and 3 hr at both levels of drug ingestion, while not increasing the incorporation that occurs between 6 hr and 9 hr. Further, in the membrane-poor polyribosomal RNA fraction, methisoprinol treatment increased the proportion of RNA with sedimentation coefficient between 6S and 18S and increased the incorporated radioactivity in the 4-10S region, relative to standard incorporated nuclear counts. These changes were strongly suggestive of an increased presence of informosomal RNAs and of an increase in nucleocytoplasmic transport of rapidly labeled RNA. Also in the membrane-poor polyribosomal RNA fraction, drug treatment was found to increase the amount of polyadenylic acid (poly A)-bearing RNA by 50 per cent and to increase the amount of poly A attached to polyribosomal RNA by a far greater amount. Since poly A is found only as part of putative messenger RNA, these data permit us to conclude that methisoprinol increases the degree of nucleocytoplasmic transport of host messenger RNA (mRNA) in association with an increase in the poly A content of this mRNA, the latter change being especially evident in the membrane-poor polysome fraction. In other studies, methisoprinol and cordycepin (3-deoxyadenosine), an inhibitor of poly A synthesis, were examined for effects on the development of cytopathology in herpesvirus-infected WI-38 cell monolayers, when added 48 hr after infection. When added alone, both methisoprinol and cordycepin inhibited the development of cytopathology during 100hr; however, when added together, the same dosages provoked a mutual antagonism and no antiviral effect was observed. This observation suggests that methisoprinol's direct antiviral effect, as seen in tissue culture, is associated with an enhanced rate of poly A synthesis in the nucleus. Poly A added to the medium external to the cells also inhibited the antiviral effect of cordycepin, but did not exert an antiviral effect by itself. It is noted that lymphocyte-dependent methisoprinol effects on immune responses and virus infection have been observed; thus, methisoprinol may also exert indirect antiviral effects via the immune system. Since poly A is known to actuate nucleocytoplasmic mRNA transport and to contribute to the stability and activity of mRNAs in polysomes in diverse tissues, it is suggested that methisoprinol may enhance the synthesis of certain antiviral principles in both local tissues and lymphocytes through a poly A-related mechanism.