Adaptation of retrovirus producer cells to serum deprivation: Implications in lipid biosynthesis and vector production. 
The manufacture of enveloped virus, particularly retroviral (RV) and lentiviral (LV) vectors, faces the challenge of low titers that are aggravated under serum deprivation culture conditions.  Also, the scarce knowledge on the biochemical pathways related with virus production is still limiting the design of rational strategies for improved production yields.  This work describes the adaptation to serum deprivation of two human RV packaging cell lines, 293 FLEX and Te Fly and its effects on lipid biosynthetic pathways and infectious vector production.  Total lipid content as well as cellular cholesterol were quantified and lipid biosynthesis was assessed by (13)C-NMR spectroscopy; changes in gene expression of lipid biosynthetic enzymes were also evaluated.  The effects of adaptation to serum deprivation in lipid biosynthesis were cell line specific and directly correlated with infectious virus titers: 293 FLEX cells faced severe lipid starvation-up to 50% reduction in total lipid content-along with a 68-fold reduction in infectious vector titers; contrarily, Te Fly cells were able to maintain identical levels of total lipid content by rising de novo lipid biosynthesis, particularly for cholesterol-50-fold increase-with the consequent recovery of infectious vector productivities.  Gene expression analysis of lipid biosynthetic enzymes further confirmed cholesterol production pathway to be prominently up-regulated under serum deprivation conditions for Te Fly cells, providing a genotype-phenotype validation for enhanced cholesterol synthesis.  These results highlight lipid metabolism dynamics and the ability to activate lipid biosynthesis under serum deprivation as an important feature for high retroviral titers.  Mechanisms underlying virus production and its relationship with lipid biosynthesis, with special focus on cholesterol, are discussed as potential targets for cellular metabolic engineering.