J. Mankertz et al., Expression from the human occludin promoter is affected by tumor necrosis factor alpha and interferon gamma, J CELL SCI, 113(11), 2000, pp. 2085-2090
The 65 kDa protein occludin is a membrane-spanning part of the epithelial t
ight junction, which is the main barrier of the paracellular pathway. The f
unction of occludin as part of tight junctions is still poorly understood a
nd even less is known about the regulatory mechanisms that influence occlud
in gene expression. This study aimed to identify the sequences essential in
cis for genomic regulation of tight junction formation and to investigate
their funcional role in cytokine-dependent tight junction regulation,
Using genome walking cloning of occludin-specific human genomic DNA sequenc
es, a 1853 bD DNA fragment containing the transcription start point of occl
udin cDNA sequences was amplified and sequenced. Subcloning of this fragmen
t in front of the luciferase reporter gene revealed strong expression of en
zymatic activity after transfection of the human intestinal cell line HT-29
/B6. With subsequent deletions of parts of the promoter fragment, its size
was reduced to 280 bp that are necessary and sufficient to mediate promoter
activity. Tumor necrosis factor alpha and another cytokine involved in inf
lammation, interferon gamma, reduced transepithelial resistance in HT-29/B6
cells, which was preceded by a decrease in occludin mRNA expression as rev
ealed by northern blot analysis. Tumor necrosis factor alpha and interferon
gamma diminished occludin promoter activity alone and even synergistically
, suggestinga genomic regulation of alterations of the paracellular barrier
.
In conclusion, proinflammatory cytokines such as tumor necrosis factor alph
a and interferon gamma can downregulate the expression of the transmembrane
tight junction strand protein occludin, paralleling the barrier disturbanc
e detected electrophysiologically. This could be an important mechanism in
gastrointestinal diseases accompanied by barrier defects, for example infla
mmatory bowel diseases.