Proteolytic degradation of HDL1 on the fat cell surface is nutritionally regulated

Abstract

The importance of plasma HDL apolipoprotein concentration as a predictor of atherosclerotic risk is well recognized, yet the processes of HDL modification and degradation in various cells are not clearly understood. We examined the characteristics of HDL1 apolipoprotein degradation and cellular uptake by rat adipocytes and determined the effects of fasting on these processes. Epididymal and perirenal adipocytes were isolated from male Wistar rats (310 ± 4 g) fed ad libidum and incubated with 5 μg of rat 125I-labeled HDL, (d: 1.07–1.10 g/mL) mL−1 for 2 h at 37 °C. Cellular uptake of HDL1 was calculated as the trichloroacetic acid precipitable radioactivity associated with adipocytes following incubation. Intracellular and medium degradation of HDL1 were determined as trichloroacetic acid soluble 125I counts associated with cells and measured in the postincubation medium, respectively. Fifty to sixty percent of cellular uptake and degradation of HDL1 was inhibited by the addition of 25-fold excess unlabeled HDL. HDL1 degradation measured in the medium was 10- to 12-fold greater than cellular uptake of HDL1 apolipoproteins. Intracellular degradation of HDL1 was negligible. The presence of EDTA in the incubation medium reduced HDL1 degradation measured in the medium, but enhanced HDL1 cellular uptake. Conditioned medium separated from cells after 2 h of incubation at 37 °C in the absence of HDL and subsequently incubated with 125I-labeled HDL1 for an additional 2 h at 37 °C, degraded less than 5% of HDL compared with degradation in the presence of cells. These results suggest that rat adipocytes degrade, or modify, HDL1 particles, possibly by interactions with cell surface proteases. Compared with the fed state, fasting rats for 24 h significantly decreased cell-dependent, surface degradation of HDL1 by perirenal adipocytes and, to a lesser extent, HDL1 degradation by epididymal cells. In contrast to HDL1 degradation, cellular uptake of HDL1 increased as a result of fasting, but this increment was not statistically significant. Therefore, the cellular uptake and surface degradation of HDL1 by adipocytes appear to be dissociated processes. The significant decrease in HDL1 degradation as a result of fasting indicates that this process, presumably mediated by a lipoprotein protease, is nutritionally regulated.Key words: HDL metabolism, lipoprotein protease, adipocyte HDL receptor, lipoprotein degradation.