Intracellular processing of endocytosed triglyceride-rich lipoproteins comprises both recycling and degradation

The current study was performed to investigate the intracellular fate of triglyceride-rich lipoproteins. Triglyceride-rich lipoproteins are responsible for the delivery of lipids to various tissues, however, their intracellular pathway has not yet been elucidated. Here radiolabeled triglyceride-rich lipoproteins, associated with lipoprotein lipase, were used for the quantitative evaluation of the intracellular metabolism. Pulse chase experiments showed that after 90 minutes approximately 60% of the labeled protein, mainly apoproteins E and C, was released intact into the medium, where it re-associates with lipoproteins. Apoprotein B, in contrast, was degraded, following the same pathway as the apoprotein B from low density lipoproteins. In kinetic experiments uptake and intracellular fate of triglyceride-rich lipoproteins was compared to that of transferrin and low density lipoproteins. These experiments revealed that apoproteins were retained inside the cell much longer than transferrin, and unlike low density lipoproteins were not degraded. Using immunofluorescence it was shown that apoprotein E and lipoprotein lipase follow a distinct route from the sorting compartment to the surface, which is clearly distinguishable from the perinuclear transferrin recycling compartment. In contrast, the fluorescence labeled lipids were delivered to lysosomal compartments. The data presented here show that surface proteins of triglyceride-rich lipoproteins, such as apoproteins E and C and lipoprotein lipase follow a recycling pathway, whereas lipids and high molecular mass core proteins are degraded.