Down regulation of Hmgcr in response to Influenza A infection is independent of the IFN response in human cells

Previous studies have demonstrated that the product of the Interferon stimulated gene Ch25h, 25-Hydroxycholestrol, provides an immediate and rapid mechanism for down-regulating sterol biosynthesis, through the inhibition of Hmgcr gene expression and proteolytic degradation of HMGCR protein. Further studies provide evidence that inhibition of the sterol biosynthesis pathway by 25-HC has broad antiviral effects. In this study, Influenza A virus (IAV) replication was inhibited in cells treated with Fluvastatin or where Hmgcr expression was inhibited with an siRNA. Treatment of A549 cells with 25-HC however, resulted in a 2-fold enhancement of IAV replication despite the fact that 25-HC promotes the proteolytic degradation of HMGCR. A549 cells infected with IAV revealed a rapid loss of HMGCR protein, a reduction in Hmgcr gene expression as well as an increase in the expression of Ch25h, that were all independent of the IFN pathway. Infection of both wild-type and Ch25h-/- murine BMDMs with IAV also revealed a rapid loss of HMGCR abundance indicating that 25-HC independent mechanisms exist for promoting proteolytic degradation of HMGCR. These data for the human A549 cell line contrast with the induction of Ch25h and subsequent loss of HMGCR in murine cells that has been shown to be dependent on IFN signalling.\n\nImportanceCholesterol, a lipid, is mainly produced by the liver or obtained from everyday foods. It is an essential element of the structure of cells and is vital for maintaining the normal function of human body. In cells, cholesterol can be oxidised to oxysterols by biological catalysts, known as enzymes. Certain oxysterols have recently emerged as important elements in the immune response to micro-organisms. This project studied a key enzyme, which is a component of the cholesterol metabolism, known as 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR). A set of experiments were designed and performed to study the behaviour of HMGCR following viral infections. This study lays the foundation for re-building a map of cholesterol metabolism and the immune response to viral infection. It will be useful for understanding the importance of cholesterol metabolism in infection and exploring novel antiviral strategies.