Effectors of fatty acid synthesis in hepatoma tissue culture cells

An investigation was undertaken to better understand the process of fatty acid synthesis in hepatoma tissue culture (HTC) cells. By comparing the findings to the normal liver some of the differences between normal and cancer tissue were defined. Incubation of the HTC cells in a buffered salt-defatted albumin medium showed that fatty acid synthesis was dependent upon the addition of substrate. The order of stimulation was glucose + pyruvate ∼- glucose + alanine ∼- glucose + lactate ∼- pyruvate > glucose > alanine ⪢ no additions. Fatty acid synthesis in HTC cells was decreased by oleate. In these respects HTC cells are similar to the liver; however, in contrast to the normal liver, N6, O2-dibutyryl cyclic adenosine 3′,5′-monophosphate (dibutyryl-cAMP) did not inhibit glycolysis or fatty acid synthesis. The cytoplasmic redox potential, as reflected by the lactate to pyruvate ratio, was found to be elevated compared to normal liver but unchanged by the addition of dibutyryl cAMP. Since higher rates of fatty acid synthesis are associated with lower lactate-to-pyruvate ratios in normal liver, it was expected that by decreasing the lactate-to-pyruvate ratio in HTC cells the rate of fatty acid synthesis would increase. One way to lower the lactate to pyruvate ratio is to increase the activity of the malate-aspartate shuttle. Stimulators of the hepatic malate-aspartate shuttle in normal liver (ammonium ion, glutamine, and lysine) had mixed effects on the redox state and fatty acid synthesis in HTC cells. Both ammonium ion and glutamine decreased the redox potential and increased the rate of fatty acid synthesis. Lysine was without effect on either process. Since NH4Cl and glutamine stimulate the movement of reducing equivalents into the mitochondria and decrease the redox potential, then the stimulation of fatty acid synthesis by NH4Cl and glutamine may be due to an increase in the movement of reducing equivalents into the mitochondria. However, if the shuttle were rate determining for fatty acid synthesis the rate from added lactate would be the same as from glucose alone but would be lower than from pyruvate which does not require the movement of reducing equivalents. This was not the case. Lactate and pyruvate gave comparable rates which were higher than glucose alone. Other possible sites of stimulation were investigated. The possibility that NH4+ and glutamine stimulated fatty acid synthesis by activating pyruvate dehydrogenase was excluded by finding that dichloroacetate, an activator of pyruvate dehydrogenase, did not stimulate fatty acid synthesis when glucose was added. Stimulation by NH4+ and glutamine at steps beyond pyruvate dehydrogenase was ruled out by the observation that NH4+ caused no stimulation from added pyruvate. NH4+ and glutamine did not alter the pentose phosphate pathway as determined by 14CO2 production from [1-14C]- or [6-14C]glucose. Ammonium ion and glutamine increased glucose consumption and increased lactate and pyruvate accumulation. The increased glycolysis in HTC cells appears to be the explanation for the stimulation of fatty acid synthesis by NH4+ and glutamine, even though glycolysis is much more rapid than fatty acid synthesis in these cells. The following observations support this conclusion. First, the percentage increase in glycolysis caused by NH4+ or glutamine is closely matched by the percentage increase in fatty acid synthesis. Second, the malate-aspartate shuttle, the pentose phosphate pathway, and the steps past pyruvate are not limiting in the absence of NH4+ or glutamine.