Isopentenyladenine as a mediator of mevalonate-regulated DNA replication

The mechanism by which the cholesterol precursor, mevalonate, regulates S-phase DNA replication was examined in synchronized BHK cells. As previously demonstrated by this laboratory, blocking 3-hydroxy-3-methylglutaryl-CoA reductase [mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34] with the competitive inhibitor compactin suppresses DNA synthesis specifically during the S phase of the cell cycle. In the present study, known mevalonate derivatives were examined as possible mediators by which mevalonate controls DNA replication. Of the compounds studied, only isopentenyladenine and its 4'-hydroxylated analogue, zeatin, could substitute for mevalonate in restoring DNA replication in compactin-blocked cells. Moreover, these two derivatives proved to be at least 100 times more active than mevalonate, and both restored DNA relication to normal within 15 min of their being added to the medium. In addition, isopentenyladenine, like mevalonate, stimulated DNA synthesis specifically during the S phase of the cell cycle. Isopentenyladenine also reversed the inhibition of DNA synthesis caused by nalidixic acid, an antibiotic that does not inhibit cholesterol synthesis. These findings indicate that isopentenyladenine or a closely related derivative may mediate the regulatory role of mevalonate in DNA replication and suggest that such isoprenes may act upon DNA replication at a site common to that inhibited by nalidixic acid.