Interruption of the adipose conversion of 3T3 cells by biotin deficiency: differentiation without triglyceride accumulation

Abstract

Abstract During the adipose conversion of 3T3 cells in medium supplemented with 10% serum, the accumulation of triglyceride depends upon a small molecular weight component in the serum. When this is removed by exhaustive dialysis of the serum, the cells undergo some changes that are part of the adipose conversion, but very little triglyceride accumulates. After the addition of either a serum dialysate or commercial biotin, cellular lipid begins to accumulate rapidly. The dialyzable factor in the serum has numerous chemical properties of biotin. When cells begin the adipose conversion in the absence of biotin, they are unable to increase their rate of acetate incorporation into triglyceride, but they do undergo the same change in shape to spherical, the same increase in activity of cytoplasmic (NAD-linked) glycerophosphate dehydrogenase (300 fold) and the same increase in lipoprotein lipase (>50 fold) as in the presence of biotin. On the other hand, in the biotin-deficient state the activity of glycerophosphate acyltransferase and of malic enzyme rises to only a small fraction of the control level. After the addition of biotin to the biotin-deficient cultures, the activity of both enzymes increases to the control level within 24–48 hr. These results are consistent with the concept that the glycerophosphate dehydrogenase is a primary enzyme in the adipose conversion: its response (measured as the amount of extractable activity) does not depend upon the rate of cellular fatty acid synthesis. Lipoprotein lipase is similarly independent. On the other hand, both malic enzyme and glycerophosphate acyltransferase may be considered as secondary enzymes in the sense that their response during the adipose conversion is linked to the supply of fatty acid.