Induction of the Triglyceride Pathway Enzymes and of Lipolytic Enzymes during Differentiation in a 'Preadipocyte' Cell Line

The transition of 3T3-L1 fibroblasts to adipocytes is accompanied by a rise in the activity level of the key enzymes of the fatty acid biosynthetic pathway (ATP-citrate lyase and fatty acid synthetase) and also by a more important rise in the activity level of the glycerolphosphate acyltransferase and of the different enzymes of the triglyceride pathway, while the fatty acid uptake by the cells remains at a constant level. This rise is accompanied by a rise in the activity level of the lipolytic enzymes active against triolein among which the lipoprotein-lipase activity. This phenomenon can be prevented by cycloheximide and is not significant when the cells are maintained at the resting state in the absence of insulin. When present both at the growing state and at the resting state, bromodeoxyuridine selectively affects the activity level of the characteristic enzymes of the triglyceride pathway and does not affect the activity level of glucose-6-phosphate and lactate dehydrogenases. A net increase in the triglyceride content of the 3T3-L1 cells is observed during differentiation; it is corroborated by a significant shift towards triglyceride formation when the incorporation of sn14Cglycerol-3-phosphate into lipids by cell-free homogenate is studied. Our results indicate on the one hand that the level of phosphatidate phosphohydrolase better correlates with the triglyceride content of the cell than any other lipogenic enzyme; they indicate on the other hand that diglyceride:acyl-CoA acyltransferase is a sensitive ‘indicator’ of 3T3-L1 fibroblasts differentiation into adipocytes.