Endocytosis of apolipoprotein A-V by members of the low density lipoprotein receptor and the VPS10p domain receptor families.
Apolipoprotein A-V (apoA-V) is present in low amounts in plasma and has been found to modulate triacylglycerol levels in humans and in animal models. ApoA-V displays affinity for members of the low density lipoprotein receptor (LDL-R) gene family, known as the classical lipoprotein receptors, including LRP1 and SorLA/LR11. In addition to LDL-A binding repeats, the mosaic receptor SorLA/LR11 also possesses a Vps10p domain. Here we show that apoA-V also binds to sortilin, a receptor from the Vsp10p domain gene family that lacks LDL-A repeats. Binding of apoA-V to sortilin was competed by neurotensin, a ligand that binds specifically to the Vps10p domain. To investigate the biological fate of receptor-bound apoA-V, binding experiments were conducted with cultured human embryonic kidney cells transfected with either SorLA/LR11 or sortilin. Compared with nontransfected cells, apoA-V binding to SorLA/LR11- and sortilin-expressing cells was markedly enhanced. Internalization experiments, live imaging studies, and fluorescence resonance energy transfer analyses demonstrated that labeled apoA-V was rapidly internalized, co-localized with receptors in early endosomes, and followed the receptors through endosomes to the trans-Golgi network. The observed decrease of fluorescence signal intensity as a function of time during live imaging experiments suggested ligand uncoupling in endosomes with subsequent delivery to lysosomes for degradation. This interpretation was supported by experiments with (125)I-labeled apoA-V, demonstrating clear differences in degradation between transfected and nontransfected cells. We conclude that apoA-V binds to receptors possessing LDL-A repeats and Vsp10p domains and that apoA-V is internalized into cells via these receptors. This could be a mechanism by which apoA-V modulates lipoprotein metabolism in vivo.
Nilsson
Stefan K
SK
Department of Medical Biosciences/Physiological Chemistry, Umeå University, SE90187 Umeå, Sweden.
Christensen
Stine
S
Raarup
Merete K
MK
Ryan
Robert O
RO
Nielsen
Morten S
MS
Olivecrona
Gunilla
G
D033942:Adaptor Proteins, Vesicular Transport; D000818:Animals; D000072040:Apolipoprotein A-V; D001054:Apolipoproteins A / Q000737:chemistry*; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004705:Endocytosis; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000196:embryology*; D026502:LDL-Receptor Related Proteins / Q000737:chemistry*; D008562:Membrane Glycoproteins / Q000737:chemistry; D026901:Membrane Transport Proteins / Q000737:chemistry*; D009419:Nerve Tissue Proteins / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000737:chemistry*; D011973:Receptors, LDL / Q000737:chemistry*; D000099183:Sortilin
D033942:Adaptor Proteins, Vesicular Transport; D000072040:Apolipoprotein A-V; D001054:Apolipoproteins A; D026502:LDL-Receptor Related Proteins; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D011956:Receptors, Cell Surface; D011973:Receptors, LDL; D000099183:Sortilin; C437704:APOA5 protein, human; C102582:SORL1 protein, human