J Biol Chem, Vol. 273, Issue 33, 20929-20934, August 14, 1998

Fatty Acid Cycling in Human Hepatoma Cells and the Effects of Troglitazone

W-N. Paul Lee, Shu Lim, Sara Bassilian, E. Anne Bergner, and John Edmond^¶

From the  Harbor-UCLA Medical Center, Torrance, California 90502 and ^¶ UCLA School of Medicine, Los Angeles, California 90024

Fatty acid cycling by chain shortening/elongation in the peroxisomes is an important source of fatty acids for membrane lipid synthesis. Its role in the homeostasis of nonessential fatty acids is poorly understood. We report here a study on the cycling of saturated fatty acids and the effects of troglitazone in HepG2 cells in culture using [U-¹³C]stearate or [U-¹³C]oleate and mass isotopomer analysis. HepG2 cells were grown in the presence of 0.7 mmol/liter [U-¹³C]stearate or [U-¹³C]oleate, and in the presence and absence of 50 µM troglitazone for 72 h. Fatty acids extracted from cell pellets after saponification were analyzed by gas chromatography/mass spectrometry. Peroxisomal -oxidation of uniformly ¹³C-labeled stearate (C18:0) and oleate (C18:1) resulted in chain shortening and produced uniformly labeled palmitate (C16:0) and palmitoleate (C16:1). In untreated cells, 16% of C16:0 was derived from C18:0 and 26% of C16:1 from C18:1 by chain shortening. Such contributions were significantly increased by troglitazone to 23.6 and 36.6%, respectively (p < 0.001). Desaturation of stearate contributed 67% of the oleate, while reduction of oleate contributed little to stearate (2%). The desaturation of C18:0 to C18:1 was not affected by troglitazone. Our results demonstrated a high degree of recycling of C18:0 and C18:1 to C16:0 and C16:1 through chain shortening and desaturation. Chain shortening was accompanied by chain elongation in the synthesis of other long chain fatty acids. Troglitazone specifically increased recycling by peroxisomal -oxidation of C18 to C16 fatty acids, and the interconversion of long chain fatty acids was associated with reduced de novo lipogenesis.