Uptake and metabolism of lactosylceramide on low density lipoproteins in cultured proximal tubular cells from normal and familial hypercholesterolemic homozygotes

The metabolism of low density lipoproteins (LDL), and LDL modified by reductive methylation (M-LDL) of lysine residues, was studied in proximal tubular (PT) cells both from normal human kidney and from urine of patients with homozygous (LDL receptor-negative) familial hypercholesterolemia (FH). LDL and M-LDL was labeled either in the protein moiety with ¹²⁵I or in the lactosylceramide moiety with ³H. The binding and degradation of ¹²⁵I-LDL in normal cells was saturable and displaced by unlabeled LDL but not by M-LDL. The uptake of [³H]lactosylceramide (LacCer) low density lipoprotein in normal renal cells was saturable, and time and temperature-dependent. Exogenously derived [³H]LacCer on LDL was rapidly taken up and catabolized to monoglycosylceramide, or it was utilized for the endogenous synthesis of globotriaosylceramide (trihexosylceramide) and globotetraosylceramide (tetraglycosylceramide). [³H]LacCer M-LDL was taken up less avidly and metabolized less extensively than [³H]LacCer-LDL in normal cells. In homozygous FH renal cells the binding of ¹²⁵I-LDL was not saturable and not displaced by unlabeled LDL. ¹²⁵I-LDL degradation did not occur in FH cells. The homozygous FH PT cells took up a 2-fold greater amount of exogenously derived [³H]LacCer on LDL than normal cells. Yet, most of the [³H]LacCer taken up by FH PT cells accumulated at LacCer, and only small amounts were metabolized to monoglycosylceramide, globotriaosylceramide (trihexosylceramide), or globotetraosylceramide (tetraglycosylceramide). When normal and FH PT cells were preincubated with LDL (0-100 μg/ml medium), there was a 5-fold increase in cellular LacCer levels in FH cells at saturating levels of LDL, whereas there was about a 50% decrease in LacCer levels in normal cells. While the high affinity binding of LDL was not essential for the delivery of LacCer to cells, the data support the conclusion that LDL binding to the LDL receptor facilitates further LacCer processing and metabolism in normal renal cells. We speculate that [³H]LacCer is taken up by FH homozygous cells via a LDL receptor-independent mechanism and accumulates in the cells without significant metabolism. LacCer taken up by this mechanism contributes to the storage of LacCer in FH PT cells.