Lipid metabolism in bone and bone cells I. The in vitro incorporation of [¹⁴C]glycerol and [¹⁴C]glucose into lipids of bone and bone cell cultures

1. 1. To study lipid synthesis in mineralizing tissues, calvaria of new-born rats were incubated in buffer containing radioactive glycerol or glucose. The effects of non-labeled ATP, CTP, and glucose on lipid synthesis were investigated. Lipid synthesis by bone cells grown in primary cell culture was also studied. 2. 2. Calvaria and bone cells were found to contain cholesterol, cholesterol esters, mono-, di- and triglycerides, free fatty acids, and various phospholipids. The phospholipids included lecithin, lysolecithin, phosphatidyl ethanolamine, lysophosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, cardiolipin, sphingomyelin, and various unidentified components. Appreciable quantities of phosphatidyl serine were not extractable from calvaria until after demineralization with EDTA. 3. 3. The incubation of calvaria or bone cell cultures with [¹⁴C]glycerol and [¹⁴C]glucose gave rise to lipids labeled in the glycerol moiety of the various glycerides. Triglycerides and lecithin were the two most heavily labeled lipids. Phosphatidyl serine, cholesterol, sphingomyelin, and fatty acids were not labeled with these two labeled precursors. ATP generally stimulated neutral lipid synthesis but retarded phosphohpid synthesis from [¹⁴C]glycerol in a medium containing a low level of unlabeled glucose. An elevated glucose level generally decreased labeling of lipids from [¹⁴C]glycerol, probably by increasing intracellular glycerol phosphate concentrations. At higher levels of ATP and non-radioactive glucose, varied results were obtained which suggest that critical levels of both substances are required for optimal lipogenesis. CTP stimulated the synthesis of diglycerides, triglycerides, and lecithin, but inhibited the labeling of phosphatidyl ethanolamine. The synthesis of lipid from [¹⁴C]glucose was generally inhibited by ATP, but the simple effect of ATP on phosphofructokinase does not totally account for this inhibition.