Metabolism of farnesyl diphosphate in tobacco BY-2 cells treated with squalestatin

Plant isoprenoids represent a large group of compounds with a wide range of physiological functions. In the cytosol, isoprenoids are synthesized via t he classical acetate/mevalonate pathway. In this pathway, farnesyl diphosph ate (FPP) occupies a central position, from which isoprene units are dispat ched to the different classes of isoprenoids, with sterols as the major end products. The present work deals with effects of squalestatin (SQ) on the metabolism of FPP in proliferating and synchronized cultured tobacco cv. Br ight Yellow-2 cells. SQ is a potent inhibitor of squalene synthase (SQS), t he first committed enzyme in the sterol pathway. At nanamolar concentration s, SQ severely impaired cell growth and sterol biosynthesis, as attested by the rapid decrease in SQS activity. At the same time, it triggered a sever al-fold increase in both the enzymic activity and mRNA levels of 3-hydroxy- 3-methylglutaryl CoA reductase. When SQ was added to cells synchronized by aphidicolin treatment, it was found to block the cell cycle at the end of G (1) phase, but no cell death was induced. Tobacco cells were also fed exoge nous tritiated trans-trans farnesol, the allylic alcohol derived from FPP, in the presence and absence of SQ. Evidence is presented that this compound was incorporated into sterols and ubiquinone Q(10). In the presence of SQ, the sterol pathway was inhibited, but no increase in the radioactivity of ubiquinone was observed, suggesting that this metabolic channel was already saturated under normal conditions.