Modulation of coronavirus‐mediated cell fusion by homeostatic control of cholesterol and fatty acid metabolism

Cervin, Marguerite; Anderson, Robert

Cellular susceptibility to fusion mediated by murine coronavirus (mouse hepatitis virus, MHV strain A59) was separated into lipid‐dependent and lipid‐independent mechanisms with the use of subclones and selected mutants of mouse L‐2 fibroblasts. Fusion‐resistant L‐2 cell mutants had similar cholesterol and fatty acid composition as did their fusion‐susceptible parent subclone, and were presumably deficient in a genetically mutable non‐lipid, host cell factor (e.g., fusion protein receptor). On the other hand, cellular sensitivity to virus fusion, which is known to be influenced by cell cholesterol content [Daya et al., 1988], was shown further to be modulated by homeostatic alterations in fatty acid metabolism. Cholesterol supplementation of mouse L‐2 fibroblasts or of peritoneal macrophages from MHV‐susceptible mice elevated susceptibility to viral fusion. Increased fusion susceptibility occurred in cholesterol‐supplemented L‐2 cells in the absence of any detectable alterations i n host cell fatty acid composition, thus demonstrating fusion enhancement by cholesterol alone. L‐2 cells cloned by limiting dilution in normal (not cholesterol‐supplemented) medium were found to be heterogeneous i n cholesterol content. Interestingly, high cholesterol‐containing subclones had increased levels of C‐18:0, C‐18:2, C‐20:4, and C‐22:6 and markedly reduced levels of C‐18:l fatty acids when compared to low cholesterol‐containing subclones. High cholesterol‐containing subclones did not show enhanced susceptibility to viral fusion, suggesting that homeostatic alteration of fatty acid metabolism compensated for the increased cholesterol levels and countered the normally fusion‐enhancing effect of cholesterol alone. Since these observations have potentially important consequences regarding the effects of dietary cholesterol on the severity of virus infection, we examined liver titres and pathology of normal and hypercholesterolemic mice infected with MHV. Hypercholesterolemia had no significant effect on virus replication or on liver pathology in two MHV‐ sensitive strains (Balb/c and AIJ) or in one MHV‐resistant (SJLIJ) of mice. Lipid analyses of the livers from normal and hypercholesterolemic mice showed evidence of two homeostatic mechanisms (cholesterol esterification and alteration of fatty acid composition) which likely counteracted the normally exacerbating effect of cholesterol on MHV cytopathology.