The lipoprotein lipase of white adipose tissue. Changes in the adipocyte cell-surface content of enzyme in response to extracellular effectors in vitro

Abstract

1. An indirect labelled-second-antibody cellular immunoassay for adipocyte surface lipoprotein lipase was used to assess the changes that occurred during -the incubation of cells in the presence and absence of effectors. 2. In the absence of any specific effectors, the amount of immunodetectable lipoprotein lipase present at the surface of adipocytes remained constant throughout the 4 h incubation period at 37 'C. Under such conditions total cellular enzyme activity also remained constant, with no activity appearing in the medium. 3. In the presence of heparin, cell-surface immunodetectable lipoprotein lipase increased by up to 20%, whereas in the presence of cycloheximide they decreased by up to 60%. Thus the obvious turnover of enzyme from this cell-surface site was found to be relatively rapid and dependent for its replenishment, at least in part, on protein synthesis. 4. In the presence of insulin alone, a substantial increase in cell-surface lipoprotein lipase protein occurred, only part of which was dependent on protein synthesis. The total cellular activity of lipoprotein lipase was unaffected by the presence of insulin. The insulin-dependent increase in cell-surface enzyme was potentiated somewhat in the presence of dexamethasone, which was not shown to exert any independent effect. 5. Glucagon, adrenaline and theophylline all produced a significant decline in the cell-surface immunodetectable lipoprotein lipase, which in the case examined (adrenaline) was partially additive with regard to the independent effect of cycloheximide. 6. Cell-surface immunodetectable lipoprotein lipase amounts were decreased significantly when cells were incubated in the presence of either colchicine or tunicamycin. 7. The concerted way in which cell-surface lipoprotein lipase altered during the incubations of adipocytes in the presence of effectors suggested that the translocation of enzyme to and from this cellular site was dependent on hormonal action and the integrity of intracellular protein-transport mechanisms.

Regulation of the synthesis, processing and translocation of lipoprotein lipase

“…Heparin displaces LPL bound to cellular heparan sulphate proteoglycan binding sites [3], forming soluble heparin-LPL complexes in the medium. The presence of LPL on the cell surface of adipocytes has been demonstrated directly by immunodetection [105]. The observation that a substantial (40 %) fraction of cellular LPL activity in 3T3-L1 adipocytes was released into the medium by a 10 min incubation with heparin at 4°C [35] is also consistent with LPL being bound to heparan sulphate proteoglycans on the cell surface.…”

Stimulation of lipoprotein lipase synthesis by refeeding, insulin and dexamethasone

“…Evidence was produced for insulin action on transcription [7], synthesis [5][6][7] and secretion [8] of lipoprotein lipase. Dexamethasone was found to potentiate the effect of insulin on both the activity and synthesis of lipoprotein lipase in adipose tissue [4,9] and on cell-surface content of the enzyme in isolated adipocytes [10].…”

Role of adipocyte-derived lipoprotein lipase in adipocyte hypertrophy

“…An essential element necessary for this mechanism to be effective is adipocyte-derived LPL activity [ 25 - 27 ]. Studies examining the effects of LPL gene ablation in mice have attempted to define the role of adipocyte-derived LPL in fat storage function of adipose tissue [ 28 ].…”