Background information. TSPO (translocator protein), known previously as PBR (peripheral-type benzodiazepine receptor), is a 18 kDa protein expressed in the mitochondrial membrane of a variety of tissues. TSPO has been reported to be over-expressed in human colorectal tumours and cancer cell lines, but its function is not well characterized. Results. We investigated the expression and function of TSPO in the human colon cancer cells HT-29. lmmunohistochemical studies revealed that TSPO is localized in mitochondria, and its endogenous ligand, the polypeptide diazepam-binding inhibitor, in the cytosol. Radioligand binding studies using the specific high-affinity drug ligand [H-3]PK 11195 and membrane fraction demonstrated saturable binding, with K-d and B-max values of 13.5 +/- 1.5 nM and 10.1 +/- 1.0 pmol/mg respectively. PK 11195 induced a rapid and transient dose-dependent rise in intracellular [Ca2+], which was unaffected by extracellular Ca2+, but was blocked by the PTP (permeability transition pore) inhibitor, cyclosporin A, and by the TSPO partial agonist, flunitrazeparn. Using HT-29 clone 19A cell line, which forms cell monolayers, we demonstrated that TSPO ligand stimulated a Ca2+-dependent transepithelial Cl- secretion. This secretion was inhibited: (i) after removal of extracellular Cl-; (ii) by apical addition of the Cl- channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate]; and (iii) by basolateral addition of the Na+-K+-2Cl(-) co-transporter inhibitor bumetanide. Furthermore, the intracellular Ca2+ chelator BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)] and cyclosporin A abolished the rise in PK 11195-induced Cl- secretion. Conclusions. These findings indicate that TSPO is located in mitochondrial membranes of HT-29 and reveal that its activation induces a rise in cytosolic Ca2+, leading to the stimulation of Cl- secretion.
