Occludin as direct target for glucocorticoid-induced improvement of blood-brain barrier properties in a murine in vitro system

Homeostasis of the central nervous system (CNS) microenvironment is essential for its normal function. It is maintained by the blood-brain barrier (BBB) which regulates the transport of molecules from blood into brain and backwards. The integrity of the BBB is compromised in many disorders of the human CNS; therapeutical. strategies for several of these diseases include treatment with glucocorticoids, but the molecular basis of how glucocorticoids regulate BBB permeability is not understood. Here, we report the generation and characterization of a murine immortalized brain (cerebral) capillary endothelial (cEND) cell line which expresses the BBB marker occludin at intercellular tight junctions (TJ). Hydrocortisone at physiological concentrations induced upregulation of occludin, accompanied by a threefold enhancement of transendothelial electrical resistance to values up to 1000 Omega cm(2). Insulin enhanced the glucocorticoid response. At the molecular level, hydrocortisone induces increase of occludin at protein and mRNA levels by activation of the glucocorticoid receptor (GR) and its binding to putative glucocorticoid responsive elements in the occludin promoter. At the same time, insulin potentiated the ligand-dependent GR transactivation via induction of the GR in this in vitro system. This study thus provides insights into the molecular processes of barrier genesis, and may help to elucidate mechanisms of brain pathology at the microvascular level.