Study of lipoprotein lipase content in Ob17 preadipocytes during adipose conversion. Immunofluorescent localization of the enzyme.

The development of lipoprotein lipase has been examined during adipose conversion of preadipocyte Ob17 cells. These cells have been previously shown to differentiate in clusters of fat cells. The lipoprotein lipase activity, which increases 20-50-fold during adipose conversion, is fully inhibited by anti-lipoprotein lipase y-globulins (directed against the rat heart enzyme) and is heparin-releasable (ECao 0.15 pg-ml-’ of heparin). Monoacylglycerol lipase activity, already shown to increase in parallel to lipoprotein lipase activity, is not inhibited by anti-lipoprotein lipase antibodies and thus represents a different molecular entity. Identical curves of immunotitration are obtained for the lipoprotein lipase activity present in cell homogenates and for the heparin-releasable activity, indicating no significant difference in antigenicity between the different activities. Studies by indirect immunofluorescence reveal the absence of lipoprotein lipase in early confluent cells. Immediately thereafter (3-6 days post confluence) adipose conversion is accompanied by the appearance of the enzyme in cells present in developing fat clusters. A double labeling procedure using fluorescein and rhodamine immunoglobulin conjugates allows a topographical distinction between cell surface and intracellular lipoprotein lipase. Lipoprotein lipase activities are enhanced in confluent cells after chronic exposure to physiological concentrations of insulin and triiodothyronine. Immunotitration experiments, performed at a constant number of enzyme units under each condition, lead to the conclusion that changes in activity correspond to parallel changes in enzyme levels. The results show a direct modulation by both hormones of the enzyme cell content of adipose cells in culture. Immunofluorescence detection of lipoprotein lipase could be a useful tool for in vivo studies on the cellularity of rat and mouse adipose tissue during development.