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Department of Physiological Chemistry, University of Groningen, 9713 AV, Groningen, The Netherlands
HepG2 cells are highly differentiated
hepatoma cells that have retained an apical, bile canalicular (BC) plasma membrane polarity. We investigated
the dynamics of two BC-associated sphingolipids, glucosylceramide (GlcCer) and sphingomyelin (SM). For
this, the cells were labeled with fluorescent acyl chainlabeled 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- amino]hexanoic acid (C6-NBD) derivatives of either
GlcCer (C6-NBD-GlcCer) or SM (C6-NBD-SM). The
pool of the fluorescent lipid analogues present in the
basolateral plasma membrane domain was subsequently depleted and the apically located C6-NBD-lipid
was chased at 37°C. By using fluorescence microscopical analysis and a new assay that allows an accurate estimation of the fluorescent lipid pool in the apical membrane, qualitative and quantitative insight was obtained
concerning kinetics, extent and (intra)cellular sites of
the redistribution of apically located C6-NBD-GlcCer
and C6-NBD-SM. It is demonstrated that both lipids
display a preferential localization, C6-NBD-GlcCer in
the apical and C6-NBD-SM in the basolateral area. Such a preference is expressed during transcytosis of
both sphingolipids from the apical to the basolateral
plasma membrane domain, a novel lipid trafficking
route in HepG2 cells. Whereas the vast majority of the
apically derived C6-NBD-SM was rapidly transcytosed
to the basolateral surface, most of the apically internalized C6-NBD-GlcCer was efficiently redirected to the
BC. The redirection of C6-NBD-GlcCer did not involve
trafficking via the Golgi apparatus. Evidence is provided which suggests the involvement of vesicular compartments, located subjacent to the apical plasma membrane. Interestingly, the observed difference in
preferential localization of C6-NBD-GlcCer and C6NBD-SM was perturbed by treatment of the cells with
dibutyryl cAMP, a stable cAMP analogue. While the
preferential apical localization of C6-NBD-GlcCer was
amplified, dibutyryl cAMP-treatment caused apically
retrieved C6-NBD-SM to be processed via a similar
pathway as that of C6-NBD-GlcCer.
The data unambiguously demonstrate that segregation of GlcCer and SM occurs in the reverse transcytotic route, i.e., during apical to basolateral transport,
which results in the preferential localization of GlcCer
and SM in the apical and basolateral region of the cells,
respectively. A role for non-Golgi-related, sub-apical vesicular compartments in the sorting of GlcCer and
SM is proposed.
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