When
citrate export from mitochondria is blocked with
1,2,3-benzenetricarboxylate (BTC) during the G1/S phase of the cell cycle, both
DNA synthesis and cell growth are dramatically inhibited in
suspension-grown 70Z/3 murine
lymphoma cell cultures sustained under otherwise optimal conditions. Synchronized (G0/G1 or G1/S) and unsynchronized cultures are susceptible to this phenomenon. BTC prevents two requirements from being met. (1) It deprives the cytosol of the
acetyl CoA necessary for operation of the cholesterogenesis pathway, thereby depleting the supply of
mevalonate (MVA) implicated as a requirement for triggering
DNA synthesis. (2) It behaves as a nonmetabolizable divalent
cation chelator, reducing the availability of Ca2+ and Mg2+, which, in whole cells are both required for
DNA synthesis. Such inhibitions are reversible. In whole cells, removal of the inhibitor yields rapid and complete recovery of
DNA synthesis. During the prolonged presence of BTC, the addition of MVA plus the Ca2+
ionophore A23187 allows partial recovery of
DNA synthesis. In isolated,
DNA synthesizing nuclei, on the other hand, the slight inhibition of
DNA synthesis by BTC is reversed merely by addition of Mg2+. We conclude that the uninterrupted production of
citrate-derived MVA via the mitochondria, at the G1/S boundary of the cell cycle (i.e., subsequent to peak
cholesterol synthesis), is mandatory for initiating the duplication of the cell genome. Consequently, by its mitochondrial site of action, BTC can severely limit the otherwise continuous supply of MVA during late G1, which in turn, prevents entry into the S phase, and thereby cell proliferation.