Lipid Metabolism in Cultured Cells: XI. UTILIZATION OF SERUM TRIGLYCERIDES

Abstract

Utilization of serum triglycerides by cells in tissue culture has been studied in order to determine their importance as a source of cell lipids and to obtain information on the cellular mechanisms of triglyceride uptake. L strain mouse fibroblasts incorporated serum triglycerides from the growth medium, but the rates of uptake were up to 10-fold less than that of free fatty acids or monoglycerides under the same culture conditions. When cells were grown under conditions of limited supply of serum lipids, however, only about 7% of the cell lipid came from serum free fatty acids and up to 28% was provided by the serum triglycerides. The major portion of the cell lipid under these conditions was derived by de novo synthesis from glucose. Hydrolysis of exogenous triglyceride by lipases in the serum-supplemented tissue culture medium was measured and found to be inadequate to account for the rate of triglyceride utilization observed in these studies. In addition, heat treatment of the serum to inactivate lipase did not significantly affect triglyceride uptake. No excretion of lipase activity by the cells was detected. Incorporation of triglyceride labeled in the glycerol portion with 3H and the fatty acid portion with 14C was measured from untreated, heat-inactivated, or delipidized serum using both monolayers and growing cultures. The 3H:14C ratio in the isolated cellular lipids averaged over 70% of that in the original triglyceride under all conditions of culture. Furthermore, incorporation of the [3H]glycerol portion of the molecule was not diluted out by addition to the growth medium of a large excess of nonradioactive glycerol or a nonhydrolyzable monoglyceride analog. In short term uptake experiments most of the radioactivity in cell lipids was found in the triglyceride fraction, with only minor amounts in the free fatty acids. In long term growth experiments the incorporated triglyceride was converted extensively (70 to 80%) to cellular phospholipid with maintenance of the 3H:14C ratio. The results indicate that L cells in tissue culture can utilize serum triglycerides and suggest that the predominant mechanism of uptake involves the intact molecule and does not require prior hydrolysis.