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Mitogen-activated protein kinase (MAPK) modulates the barrier function of tight junctions. We identified a putative phosphorylation site for MAPK at around Thr203 (PKPTP) in claudin-1, and determined the biological significance of this site. To this end, using the rat lung endothelial cell line RLE, we generated cells expressing doxycycline (Dox)-inducible wild-type claudin-1 and its mutant with substitution of Thr203 to Ala, and named them RLE:rtTA:CL1 and RLE:rtTA:CL1T203A, respectively. We herein show, by measurement of transendothelial electrical resistance and paracellular flux of mannitol and inulin, that functional tight junctions were reconstituted in both cells by Dox-induced expression of claudin-1. Interestingly, the barrier functions of tight junctions were less developed in RLE:rtTA:CL1T203A cells compared with RLE:rtTA:CL1 cells. Consistently, levels of both detergent-insoluble claudin-1 protein and its threonine-phosphorylation after Dox treatment were low in RLE:rtTA:CL1T203A cells compared to RLE:rtTA:CL1 cells. Furthermore, pretreatment with the MAPK inhibitor PD98059 markedly suppressed the barrier function and amount of detergent-insoluble claudin-1 in Dox-exposed RLE:rtTA:CL1 cells, whereas it marginally influenced those in RLE:rtTA:CL1T203A cells. These findings indicate that Thr203 of claudin-1 is required to enhance the barrier function of claudin-1-based tight junctions, probably via its phosphorylation and subsequent integration into tight junctions.

Cyclic AMP (cAMP) promotes functions of tight junctions in endothelial cells, although its target remains unknown. We showed here that cAMP increased gene expression of claudin-5 and decreased that of claudin-1 in porcine blood-brain-barrier endothelial cells via protein kinase A (PKA)-independent and -dependent pathways, respectively. cAMP also enhanced immunoreactivity of claudin-5 along cell borders and in the cytoplasm, reorganized actin filaments, and altered signals of claudin-5, occludin, ZO-1, and ZO-2 along cell boundaries from zipperlike to linear patterns. In contrast, claudin-1 was detected only in the cytoplasm in a dotlike pattern, and its immunolabeling was reduced by cAMP. Interestingly, 31- and 62-kDa claudin-5 immunoprecipitates in the NP-40-soluble and -insoluble fractions, respectively, were highly phosphorylated on threonine residue(s) upon cAMP treatment. All these changes induced by cAMP, except for claudin-5 expression and its signals in the cytoplasm, were reversed by an inhibitor of PKA, H-89. We also demonstrated that cAMP elevated the barrier function of tight junctions in porcine blood-brain-barrier endothelial cells in PKA-dependent and -independent manners. These findings indicate that both PKA-induced phosphorylation of claudin-5 immunoprecipitates and cAMP-dependent but PKA-independent induction of claudin-5 expression could be involved in promotion of tight-junction function in endothelial cells.

Endothelial Fas ligand (FasL) contributes to the "immune privilege" of tissues such as testis and eye, in which apoptosis is induced in infiltrating Fas-positive activated T cells and results in the inhibition of leukocyte extravasation. In this study, we examined the role of endothelial FasL in controlling cancer cell transmigration using rat lung endothelial (RLE) cell line bearing a doxycycline-inducible hepatocyte nuclear factor (HNF)-4alpha expression system. We showed that a detectable level of FasL was expressed in RLE cells and that this expression was markedly up-regulated and well correlated to the degree of HNF-4alpha expression in a time-dependent manner. When various cancer cells were overlaid on an RLE monolayer sheet, we examined the ability of endothelial FasL to induce massive apoptosis in Fas-expressing cancer cells and found a causal link to inhibition of the transmigration. Finally, we showed that FasL was expressed in capillaries of the rat brain by immunohistochemical staining, suggesting that FasL serves its functions not only in vitro, but also in vivo. These results raise the possibility that HNF-4alpha is involved in regulating cancer cell transmigration by modulating the Fas-FasL system.