Biochemical analysis and electron microscopic study on intracellular virions in NIH/3T3 mouse cells chronically infected with Moloney murine leukaemia virus: effect of interferon

Combined treatment with Pseudomonas aeruginosa toxin and interferon on mouse and human cells

The combined biological effect of Pseudomonas toxin and beta-interferon on mammalian cells was studied on two cell lines. The first was a virus-producing clone derived from NIH/3T3 mouse fibroblasts transformed by Moloney murine sarcoma virus. The second was a clone derived from human amnion cells. The parameters examined were either retrovirus release from the mouse cells or the rate of protein synthesis in both cell lines. When applied together with Pseudomonas toxin, interferon inhibits virus release even at a Pseudomonas toxin concentration that by itself does not exhibit any biological effect on NIH/3T3 cells. This enhancement phenomenon is both Pseudomonas toxin and interferon dose-dependent. Likewise, the combined treatment of either mouse or human cells with Pseudomonas toxin and the appropriate species-specific interferon, inhibits protein synthesis to a much greater extent than either of these agents alone. The kinetics of the inhibition of virus release is different from that seen with protein synthesis indicating that the enhancement phenomenon observed on virus release is not a result of the inhibition of total cellular protein synthesis. Interferon potentiates the effect of Pseudomonas toxin in a species-specific manner, thus suggesting that this process does not occur at the level of cell receptors but is a consequence of a subsequent intracellular event. It is concluded that the enhancement phenomenon does not reflect a direct interaction between interferon and Pseudomonas toxin, since Pseudomonas incubated together with interferon retained its normal biological activity as indicated by the ability of the toxin molecule to transfer the adenine diphosphoribose (ADP-ribose) moiety of nicotinamide-adenine dinucleotide (NAD) onto elongation factor 2 (EF-2).

Accumulation and breakdown of RNA-deficient intracellular virus particles in interferon-treated NIH 3T3 cells chronically producing Moloney murine leukemia virus

Interferon treatment of NIH 3T3 cells chronically infected with Moloney murine leukemia virus inhibited about 95% of virus release. This inhibition was accompanied by a three- to twofold accumulation of intracellular virions. However, this accumulation could be demonstrated only be exogenous reverse transcriptase reaction assay or radioactive labeling of the assembled viral proteins. It could not be shown by the endogenous reverse transcriptase reaction assay, which depended on endogenous viral RNA, or by labeling the encapsidated viral RNA. It was therefore evident that most of the intracellular virions accumulated in interferon-treated cells were RNA deficient. Hybridization analysis revealed that these virions were deficient of genomic viral RNA, whereas size analysis by gel electrophoresis suggested that the deficiency of 4S RNA normally packaged in Moloney murine leukemia virus was even stronger. Our data also suggested that this RNA deficiency was not due to degradation of the encapsidated RNA, but more likely to a defect in virus assembly. RNA-lacking intracellular virions were unstable; they were found to collapse before being released.

Effect of interferon on assembly of intracellular Moloney murine leukemia virus particles in chronically infected 3T3/NIH cells

The effect of interferon (IFN) on virus release and on assembly of intracellular virions in 3T3/NIH cells chronically infected with Moloney murine leukemia virus was studied by short labeling with 3H-leucine (viral proteins), 3H-glucosamine (viral glycoproteins) and 3H-uridine (vital RNA). With all of these labels, IFN pretreatment was found to strongly inhibit extracellular virus release. No difference was found between the extent of labeling of viral proteins and glycoproteins of intracellular virions. Incorporation of 3H-uridine into intracellular virions was strongly reduced by the IFN pretreatment. Since it is rather unlikely that encapsidated RNA is significantly degraded during the relatively short time of label incorporation, this finding suggests that IFN interferes with packaging of viral RNA. The effect of IFN on virus release and on 3H-uridine labeling of intracellular virion were found to develop with the same kinetics, though the maximal inhibition of virus release was stronger.