The role of scavenger receptor BI in hepatitis - eTheses Repository ...

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uptake of cholesterol from LDL (303). The membranous environment in which
SR-BI resides appears to determine its activity in respect of selective uptake;
association with lipid rafts favours selective uptake from LDL whereas uptake
from HDL occurs preferentially within disordered regions of the membrane
(258). It is believed that HDL and LDL possess alternative modes of binding to
SR-BI, possibly via distinct domains. However, HDL can compete for the
binding of LDL, whereas LDL only partially inhibits HDL binding, suggesting a
complex ligand-receptor interplay (161).
Although originally thought to be a strictly non-endocytic receptor, it has
become clear that SR-BI is capable of endocytosing both HDL and LDL
independent of selective uptake (231, 287, 300). In the case of HDL, whole
particle uptake is followed by selective sorting of the constituent apoprotein
components within the endosome for resecretion, a process known as retro
endocytosis, whereas the cholesterol component exits the body via biliary
secretion (239, 286, 287, 338). In polarised hepatocytes SR-BI localises to
both the sinusoidal (basal) and canaliculur (apical) surfaces; it is thought to
transcytose between the two in response to cholesterol requirements (53,
123, 287). Although the precise endocytic pathway employed by SR-BI is
unknown, its trafficking is thought to be mediated by factors such as
membrane associated protein 17, which targets SR-BI-HDL complexes to the
sub apical recycling compartment of the endosomal pathway (286). A recent
study suggests that intracellular SR-BI may mediate the trafficking of
cholesterol between organelles (4).