## 4 The contribution of fatty acid and cholesterol biosynthesis to HCV RNA replication  
Microarray studies in chimpanzees have indicated that both acute and chronic HCV infection correlates with changes in the expression patterns of genes, which have either a direct role in lipid metabolism or regulate the fatty acid and cholesterol biosynthesis pathways [74-76] suggesting a link between viremia and lipid metabolism. Direct evidence from tissue culture models that lipid biosynthesis contributes to replication of the HCV genome emerged from analysis of changes in viral RNA mediated by modulators and inhibitors of fatty acid and cholesterol metabolism [76-78]. In the cholesterol biosynthesis pathway, there has been particular focus on statins, since these compounds are potent inhibitors of HMG-CoA reductase and are widely used as a therapeutic to lower blood cholesterol levels. Early studies established that lovastatin reduced HCV RNA levels by more than 20-fold in cells harboring viral replicons [77, 78]. Subsequent comparative analysis of a range of statins demonstrated that mevastatin, simvastatin and atorvastatin were more potent inhibitors of HCV RNAreplication than lovastatin [79, 80]; fluvastatin had either equivalent [79] or greater potency [80] compared to lovastatin. Interestingly, in neither of these studies did pravastatin have any effect on viral RNA levels, suggesting that inhibition of HCV RNA synthesis may not result solely from effects of statins on HMG-CoA reductase [80]. Combining statins with either IFN-\(\alpha\) or anti-HCV compounds enhances reduction in HCV RNA levels and can delay the emergence of resistance mutants in tissue culture cells [79, 80]. These data provide promise that statins could be combined with the current standard treatment regimens to improve response to therapy. However, early clinical trials have given equivocal results. Monotherapy with fluvastatin has been reported to give moderate and short-lived reductions in viral load [81] while statins apparently improve ALT levels [82] and, in combination with IFN-\(\alpha\) and ribavirin, enhance the rapid virological response [83]. However, other studies report no improvement in sustained clearance of virus following use of statins as an adjunct to current treatment regimens [83-85].  
Interestingly, the inhibitory effect of statins on RNA replication in tissue culture cells can be alleviated by addition of geranylgeraniol [78] and inducing the accumulation of geranylgeranyl intermediates gives higher levels of viral RNA synthesis [77]. These results led to identification of FBL2, which is modified by geranylgeranylation (see Section 2.1), as a cellular factor that binds to HCV NS5A, thereby promoting HCV RNA synthesis through a mechanism that is, as yet, unknown [86].  
Compared to components engaged in cholesterol metabolism, the role of the fatty acid biosynthesis pathway in HCV RNA replication has received somewhat less attention. Inhibition of acetyl-CoA carboxylase, which irreversibly converts acetyl CoA to malonyl CoA, gives a three-fold decrease in viral RNA replication [77]. Treatment of cells harboring the HCV replicon with cerulenin, a fatty acid synthase inhibitor, also yields a modest lowering of HCV RNA levels [76]. In agreement with a contribution of fatty acid biosynthesis to RNA replication, the abundance of viral RNA increased upon addition of saturated and monounsaturated fatty acids to cells [77]. By contrast, polyunsaturated fatty acids (PUFAs; e.g., arachidonic, eicosapentaenoic and docosahexaenoicenoic acids) lowered HCV genome synthesis [77, 87]. PUFAs can suppress lipogenesis in the liver however impairment of HCV RNA replication does not apparently operate by this mechanism [77] but rather through production of lipid peroxides from exogenously administered PUFAs [88].