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Differential basolateral-apical distribution of scavenger receptor, class B, type I in cultured cells and the liver.

ABSTRACT: The high-density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake into the liver, which finally results in cholesterol secretion into the bile. Despite several reports, the distribution of hepatic SR-BI between the sinusoidal and canalicular membranes is still under debate. We present immunohistological data using specific markers showing that the bulk of SR-BI is present in sinusoidal membranes and, to a lesser extent, in canalicular membranes in murine and human liver sections. In addition, SR-BI was detected in preparations of rat liver canalicular membranes. We also compared the in vivo findings to HepG2 cells, a widely used in vitro hepatocyte model. Interestingly, SR-BI was enriched in bile canalicular-like (BC-like) structures in polarized HepG2 cells, which were cultivated either conventionally to form a monolayer or in Matrigel to form three-dimensional structures. Fluorescently labeled HDL was transported into close proximity of BC-like structures, whereas HDL labeled with the fluorescent cholesterol analog BODIPY-cholesterol was clearly detected within these structures. Importantly, similarly to human and mouse liver, SR-BI was localized in basolateral membranes in three-dimensional liver microtissues from primary human liver cells. Our results demonstrate that SR-BI is highly enriched in sinusoidal membranes and is also found in canalicular membranes. There was no significant basolateral-apical redistribution of hepatic SR-BI in fasting and refeeding experiments in mice. Furthermore, in vitro studies in polarized HepG2 cells showed explicit differences as SR-BI was highly enriched in BC-like structures. These structures are, however, functional and accumulated HDL-derived cholesterol. Thus, biological relevant model systems should be employed when investigating SR-BI distribution in vitro.

SUBMITTER: Fruhwurth S

PROVIDER: S-EPMC4241236 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundScavenger receptor class B type I (SR-BI) plays a key role in high density lipoproteins (HDL) metabolism. SR-BI deficiency in mice results in enhanced susceptibility to atherosclerosis with abnormal large, cholesterol enriched, and functional impaired HDL. This study was to characterize the protein markers of dysfunctional HDL in SR-BI deficient (SR-BI-/-) mice and to test if the defective of HDL might be affected by probucol treatment.MethodsShotgun proteomics and 2-D gel electrophoresis were performed to examine the profile of HDL protein and distribution of HDL particles isolated from SR-BI-/- mice. HDL's cell-function, paraoxonase 1 (PON1) and myeloperoxidase activity were assessed. The mice were treated with 1.2 mg/g/day probucol for 6 weeks and the impact on HDL protein markers was analyzed. The differential proteins were quantified by Western blotting.ResultsThe relative amount of protein in SR-BI-/- HDL was decreased by about 25% compared to that in HDL from wild type (WT) mice. Compared to WT HDL, relative protein abundance of representative apoAI and PON1 in SR-BI-/- HDL were significantly reduced, whereas acute-phase protein serum amyloid A (SAA) and apoAIV, proteinase inhibitor proteins α-1-antitrypsin (A1AT) were increased. The distribution of plasma apoAI-containing HDL particles in SR-BI-/- mice was also dramatically altered, although plasma apoAI level was no difference. The protein alterations were accompanied with dysfunction of SR-BI-/- HDL, evidenced by impaired cholesterol homeostasis in macrophages, and reduced anti-oxidative and anti-inflammatory effects. Probucol treatment of SR-BI-/- mice could restored the relative contents of critical proteins including apoAI, PON1, SAA, apoAIV and A1AT on HDL, and improve HDL dysfunction despite decreased HDL-C level.ConclusionSR-BI deficiency leading to dysfunctional HDL is closely related to alteration of HDL protein, suggesting that identification of apoAI, PON1, SAA, apoAIV, and A1AT may serve as the valuable protein markers for diagnosis and therapeutics of dysfunctional HDL-related metabolic diseases.

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Differential basolateral-apical distribution of scavenger receptor, class B, type I in cultured cells and the liver.

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