Polyamine metabolism and virus replication.

Abstract

It has become apparent during the past two decades that the vital functions of eukaryotic cells are intimately associated with the synthesis of the polyamines, spermidine and spermine (Pegg & McCann, 1982). Many studies using either whole animals or cell cultures have shown marked increases in ornithine decarboxylase (EC 4.1.1.17) in growing organs and tissues which leads to the formation of putrescine, a key step in the eukaryotic pathway of polyamine biosynthesis. Spermidine is formed from putrescine by the addition of an aminopropyl group derived from decarboxylated S-adenosylmethionine and the transfer of a second aminopropyl group converts spermidine to spermine. These reactions are dependent on another important enzyme of polyamine metabolism, S-adenosylmethionine decarboxylase (EC 4.1.1.50). Cellular levels of putrescine, spermidine and spermine increase sequentially during the cell cycle with biphasic peaks, firstly during the early Sphase of DNA synthesis and, later, before cell division. The essential nature of such increases for the maintenance of these cellular events has been demonstrated convincingly with the recent availability of specific inhibitors of polyamine biosynthesis. Both DFMO, a catalytically activated inhibitor of ornithine decarboxylase, and MGBG, a reversible inhibitor of S-adenosylmethionine decarboxylase, can inhibit cellular DNA replication and, ultimately, block cell division. These inhibitors have also been used to show that polyamines can play a further r6le in the differentiation of mammalian cells but, as with cell mitosis, the physiological significance of spermidine and spermine is poorly understood in molecular terms. Viruses are obligate intracellular parasites consisting of nucleic acid enclosed within a protective protein coat. Since virus particles are metabolically inert they are absolutely dependent on the host cell for their replication. Thus, the production of virus-specific nucleic acid and protein directed by the infective genome should be subject to the same physiological constraints on macromolecular synthesis found in uninfected cells. By the application of appropriate biochemical techniques it should be possible to correlate molecular events in the virus growth cycle with quantitative and/or qualitative changes in polyamine metabolism in the infected cells. Although further extrapolations must be made cautiously, such information may point also to the molecular rdes of polyamines in cellular biology. This proposition has been investigated by using vaccinia virus replication as a model system. Ornithine decarboxylase activity in HeLa cells increases markedly after infection with vaccinia virus to reach a peak at about 5 h post-infection before returning to undetectable levels by 9 h post-infection (Hodgson & Williamson, 1975). This temporal pattern of enzyme induction is remarkable for two reasons: host protein synthesis in HeLa cells is inhibited rapidly after vaccinia infection, reaching minimum levels by 5 h post-infection, and ornithine decarboxylase in mammalian cells has a very short half-life. It is possible, therefore, that ornithine decarboxylase activity in vaccinia-infected HeLa cells is determined by a virusspecified enzyme. The genome of vaccinia virus is known to encode a number of enzymes required for virus DNA