Megalin acts in concert with cubilin to mediate endocytosis of high density lipoproteins

Cubilin has recently been shown to function as an endocytic receptor for hi gh density lipoproteins (HDL). The lack of apparent transmembrane and cytop lasmic domains in cubilin raises questions as to the means by which it can mediate endocytosis. Since cubilin has been reported to bind the endocytic receptor megalin, we explored the possibility that megalin acts in conjunct ion with cubilin to mediate HDL endocytosis. While megalin did not bind to HDL, delipidated HDL, or apoA-I, it was found to copurify with cubilin isol ated by HDL-Sepharose affinity chromatography. Cubilin and megalin exhibite d coincident patterns of mRNA expression in mouse tissues including the kid ney, ileum, thymus, placenta, and yolk sac endoderm. The expression of both receptors in yolk sac endoderm-like cells was inducible by retinoic acid t reatment but not by conditions of sterol depletion. Suppression of megalin activity or expression by treatment with either megalin antibodies or megal in antisense oligodeoxynucleotides resulted in inhibition of cubilin-mediat ed endocytosis of HDL. Furthermore, megalin antisense oligodeoxynucleotide treatment resulted in reduced cell surface expression of cubilin. These dat a demonstrate that megalin acts together with cubilin to mediate HDL endocy tosis and further suggest that megalin may play a role in the intracellular trafficking of cubilin.