Evaluation of the hCMEC/D3 cell line, a new "in vitro" model of the human blood-brain barrier for transport and gene regulation studies

Brain endothelial capillary cells form the blood-brain barrier (BBB), a highly selective
membrane between the peripheral blood and the central nervous system. The main
functions of the BBB are to protect the brain tissue by preventing the entry of toxic
compounds and to supply it with nutrients in order to assure proper function. Tight
junctions are the key elements for the establishment of a tight barrier and seal the
intercellular gaps against passive diffusion of hydrophilic compounds. A second important
characteristic of the brain capillary endothelial cells are transport proteins that prevent
brain penetration of their substrates by pumping them back in the blood. These
compounds include a series of clinically used drugs. Important drug efflux transporters
located at the BBB are P-glycoprotein (P-gp), the breast cancer resistance protein
(BCRP) and the family of multidrug resistance proteins (MRP).
During drug development, the question of whether a drug candidate reaches the brain
tissue is of great importance. Therefore, models are needed to predict the BBB
permeability of new compounds. In the past, in vitro models have been developed to
address this question. These models include isolated brain capillaries, isolated primary
brain capillary endothelial cells and BBB cell lines of various origins. A major problem
encountered with these cell lines was an insufficient paracellular resistance.
Recently, the hCMEC/D3 cell line was generated by immortalizing primary human brain
endothelial cells. In culture this cell line shows a morphology that closely resembles to
primary cells, forms tight monolayers and expresses BBB markers such as chemokine
receptors, tight junctional molecules and ATP binding cassette (ABC)-transporters.
The aim of this thesis was to evaluate the hCMEC/D3 cell line as an in vitro model of the
human BBB to study 1) permeability properties including para- and transcellular diffusion
as well as active transport 2) the influence of endo- and exogenous factors on the
paracellular permeability and 3) the regulation of breast cancer resistance protein and Pglycoprotein
by pro-inflammatory cytokines.
The first study describes the characterization of the hCMEC/D3 cells as an in vitro model
of the human BBB for permeability studies (section Error! Reference source not
found.). The ability of the cells to allow discrimination between para- and transcellular
diffusion was investigated by measuring the transport of a series of compounds with
different physicochemical properties. A ratio of 2.8 was observed when comparing the
permeabilities of the compounds with the highest and the lowest diffusion rate. The
passive permeability of sucrose could be reduced significantly by replacing fetal calf
serum with human serum. Furthermore, quantitative mRNA expression of the ABCtransporters
P-gp, BCRP, MRP1, MRP2, MRP3, MRP4, MRP5 as well as the human
transferrin receptor (hTfR) was shown. Protein expression of P-gp, BCRP and the hTfR
was detected and functional activity of P-gp, BCRP and the MRPs was investigated in
efflux experiments. Furthermore, bidirectional P-gp transport activity was observed.
In a second project the impact of endo- and exogenous factors on the paracellular
permeability of hCMEC/D3 monolayers was assessed, since it is know that the molecular
assembly of tight junctions depends on the surrounding milieu (section Error! Reference
source not found.). Based on reports in the literature, the cells were incubated with a
variety of compounds that included anti-inflammatory drugs, growth factors and
antioxidants. The effects on the monolayer tightness of hCMEC/D3 were investigated by
measuring the transport of sucrose, a paracellular permeability marker. N-acetylcystein
(NAC), atorvastatin and sodium nitroprusside (SNP) reduced the sucrose permeability
significantly, and slightly increased zonula occludens protein (ZO-1) expression.
Additionally, NAC and SNP reduced the generation of reactive oxygen species (ROS),
which have been reported to disrupt the assembly of tight junctions.
The effect of the pro-inflammatory cytokines IL-1[beta], IL-6 and TNF-[alpha] on the expression and
activity of the ABC-transporters BCRP and P-gp was investigated in the hCMEC/D3 cell
line (section Error! Reference source not found.). IL-1[beta], IL-6 and TNF-[alpha], which are
know to be elevated during various diseases, suppressed significantly BCRP mRNA
expression. In addition, BCRP activity was reduced under the influence of all tested
cytokines, as shown by efflux experiments. P-gp mRNA levels were slightly reduced by
IL-6 but significantly increased after TNF-[alpha] treatment. TNF-[alpha] also increased the protein
expression of P-gp. This in vitro study indicates that expression levels of BCRP and P-gp
at the BBB might be altered during acute or chronic inflammation, resulting in a changed
brain penetration of their substrates.
In an isolated project, the pharmacokinetics and pharmacodynamics of increasing oral
doses of the satiety peptides GLP-1 and PYY3-36 were assessed in healthy male
volunteers. Oral administration of either peptide induced a rapid and dose-dependent
increase in plasma drug concentrations. Oral administration of GLP-1 induced a potent
effect on insulin release and both peptides suppressed ghrelin secretion. In conclusion,
this study showed, for the first time, that satiety peptides such as GLP-1 and PYY3-36
can be orally delivered safely and effectively in humans.