We have established that a preferential export of
pyruvate-generated
citrate occurs from
cholesterol-rich
tumor mitochondria, with both isolated mitochondrial systems as well as with viable
tumor tissue slices (i.e., with whole
tumors cells). Furthermore, we have demonstrated that the more rapid
citrate efflux kinetics (catalyzed by the tricarboxylate exchange carrier) of isolated
tumor mitochondria is completely inhibited upon addition of
1,2,3-benzenetricarboxylate (BTC) and have shown that this inhibition is apparently also obtained in viable
tumor tissue when the inhibitor is added to the tissue incubation. Upon BTC inhibition of
tumor mitochondrial
citrate export in viable
tumor tissue incubations, the incorporation of [14C]
pyruvate into newly synthesized
cholesterol is severely inhibited as well. Among the most interesting conclusions drawn from our results, we catalog the following. The preferential export of
citrate from isolated
tumor mitochondria appears to be coupled, functionally, to a high linear rate of incorporation of 14C from
pyruvate to
cholesterol in viable
tumor tissue slices, simultaneously supporting the postulate of a truncated Krebs cycle and corroborating the well-established deregulated and continuous cholesterogenesis pathway in
tumors, especially
hepatomas. The extent of [14C]
pyruvate flux to newly generated
cholesterol in either
tumor or normal liver tissue is inversely related to the extent of 14CO2 production. Despite the evolution of some CO2 during cholesterogenesis, the predominant portion presumably arises via metabolic processing of
pyruvate-generated
citrate during Krebs cycle-linked respiration. Isolated
tumor mitochondrial systems, as well as viable
tumor tissue incubations, can manifest a reversal in the pattern of enhanced mitochondrial
citrate efflux coupled to increased cholesterogenesis, when BTC is added to the system. This implies that BTC, a hydrophobic but negatively charged moiety at pH 7, can indeed penetrate the plasma membrane of cells. Upon entry into the cell, BTC apparently blocks the
tricarboxylate carrier of
tumor tissue mitochondria, thus forcing the mitochondrial
citrate into Krebs cycle-linked respiration rather than permitting it to serve as the predominant provider of an increased supply of cytosolic
acetyl CoA precursor required for deregulated cholesterogenesis during the development of the
tumor.