Polyamine regulation of porcine reproductive and respiratory syndrome virus infection depends on spermidine-spermine acetyltransferase 1

PEDV infection affects the expression of polyamine-related genes inhibiting viral proliferation

Porcine epidemic diarrhea virus (PEDV) is an alpha-coronavirus that causes epidemic diarrhea in swines. The mortality of PEDV infection in one-week-old piglets is extremely high, which causes a huge significant economic loss to the global pig husbandry and blocks its healthy development. There was a lack of adequate studies to elucidate pathogenic mechanism associated with PEDV infection. In the present study, we detected the expression profiles of polyamine metabolism associated genes in Vero cells infected with PEDV by RT-qPCR. It is shown that PAOX(acetylpolyamine oxidase), SMOX(spermine oxidase), SAT1(spermidine-spermine acetyltransferase 1), ODC1(ornithine decarboxylase 1), DHPS(deoxyhypusine synthase) and EIF5A( eukaryotic initiation factor 5A) were significantly upregulated. Through intervening SAT1 level in PEDV-infected Vero cells, it is identified that overexpression of SAT1 inhibited PEDV replication by reducing polyamine levels. Furthermore, polyamine depletion and upregulation were found to regulate the proliferation of PEDV. PEDV infection in Vero cells did not result in a significant change in the protein level of eIF5A, and in addition, the activated eIF5A did not affect the proliferation of PEDV. Our results provided new insights into the influence of polyamine metabolism on the proliferation of PEDV.

Porcine reproductive and respiratory syndrome virus nsp4 positively regulates cellular cholesterol to inhibit type I interferon production

Cellular cholesterol plays an important role in the life cycles of enveloped viruses. Previous studies by our group and other groups have demonstrated that the depletion of cellular cholesterol by methyl-β-cyclodextrin (MβCD) reduces the proliferation of porcine reproductive and respiratory syndrome virus (PRRSV), a porcine Arterivirus that has been devastating the swine industry worldwide for over two decades. However, how PRRSV infection regulates cholesterol synthesis is not fully understood. In this study, we showed that PRRSV infection upregulated the activity of protein phosphatase 2 (PP2A), which subsequently activated 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme in the cholesterol synthesis pathway, to increase the levels of cellular cholesterol. By screening the PRRSV-encoded proteins, we showed that nsp4 dominated the upregulation of cellular cholesterol, independently of the 3C-like protease activity of nsp4. A mutation analysis showed that domain I (amino acids 1–80) of PRRSV nsp4 interacted with PR65 alpha (PR65α), the structural subunit, and PP2Ac, the catalytic subunit, of PP2A. Importantly, domain I of nsp4 inhibited Sendai virus-induced interferon β production, and this inhibitory effect was eliminated by Lovastatin, an HMGCR inhibitor, indicating that the upregulation of cellular cholesterol by nsp4 is a strategy used by PRRSV to suppress the antiviral innate immunity of its host. Collectively, we here demonstrated the mechanism by which PRRSV regulates cellular cholesterol synthesis and reported a novel strategy by which PRRSV evades its host's antiviral innate immune response.

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PRRSV nsp4 up-regulates cellular cholesterol via the PP2A-HMGCR pathway.

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Nsp4 domain I (amino acids 1–80) interacts with A and C subunits of PP2A.

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Nsp4 domain I inhibits IFN-I production by upregulating cellular cholesterol.

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The HMGCR inhibitor Lovastatin inhibits PRRSV proliferation.