Journal of Lipid Research, Vol. 20, 974-985, November 1979
Copyright © 1979 by Lipid Research, Inc.

Acute control of fatty acid synthesis by cyclic AMP in the chick liver cell: possible site of inhibition of citrate formation

Steven D. Clarke , Paul A. Watkins , and M. Daniel Lane

Department of Physiological Chemistry, Johns Hopkins University School of Medicine and Nutrition Program, School of Hygiene and Public Health, Baltimore, MD 21205

Glucagon and N,⁶O²-dibutyryl cyclic adenosine 3',5'-cyclic monophosphate (Bt₂cAMP) inhibit fatty acid synthesis from acetate by more than 90% and prevent citrate formation in chick hepatocytes metabolizing glucose. With substrates that enter glycolysis at or below triose-phosphates, e.g., fructose, lactate, or pyruvate, Bt₂cAMP has no effect on the citrate level and its inhibitory effect on fatty acid synthesis is substantially reversed. Because acetyl-CoA carboxylase requires a tricarboxylic acid activator for activity, it is proposed that regulation of fatty acid synthesis by Bt₂cAMP is due, in part, to changes in the citrate level. Reduced citrate formation appears to result from a cAMP-induced inhibition of glycolysis. Bt₂cAMP inhibits ¹⁴CO₂ production from [1-¹⁴C]-, [6-¹⁴C]-, and [U-¹⁴C]glucose and has little effect on ¹⁴CO₂ formation from [1-¹⁴C]- or [2-¹⁴C]pyruvate or from [1-¹⁴C]fructose. [¹⁴C]Lactate formation from glucose is depressed 50% by Bt₂cAMP. In the presence of an inhibitor of mitochondrial pyruvate transport lactate accumulation is enhanced, but continues to be lowered 50% by Bt₂cAMP. The activity of phosphofructokinase is greatly decreased in Bt₂cAMP-treated cells while the activities of pyruvate kinase and acetyl-CoA carboxylase are unaffected. It appears that decreased glycolytic flux and decreased citrate formation result from depressed phosphofructokinase activity. Fatty acid synthesis from [¹⁴C]acetate is partially inhibited by Bt₂cAMP in the presence of fructose, lactate, and pyruvate despite a high citrate level. Incorporation of [¹⁴C]fructose, [¹⁴C]pyruvate, or [¹⁴C]lactate into fatty acids is similarly depressed by Bt₂cAMP. Synthesis of cholesterol from [¹⁴C]acetate or [2-¹⁴C]pyruvate is unaffected by Bt₂cAMP. These results implicate a second site of inhibition of fatty acid synthesis by Bt₂cAMP that involves the utilization, but not the production, of cytoplasmic acetyl-CoA.—Clarke, S. D., P. A. Watkins, and M. D. Lane. Acute control of fatty acid synthesis by cyclic AMP in the chick liver cell: possible site of inhibition of citrate formation.

Supplementary key words hepatocytes • phosphofructokinase

Submitted on February 15, 1979
Accepted on July 3, 1979

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