The peripheral benzodiazepine receptor (PBR) has been implicated in growth control of various tumour models. Although colorectal cancers were found to overexpress PBR, the functional role of PBR in colorectal cancer growth has not been addressed to date. Using primary cell cultures of human colorectal cancers and the human colorectal carcinoma cell lines HT29, LS174T, and Colo320 DM we studied the involvement of PBR in the growth control and apoptosis of colorectal cancers. Both mRNA and protein expression of PBR were detected by RT-PCR and flow cytometry. Using confocal laser scanning microscopy and immunohistochemistry the PBR was localized in the mitochondria. The specific PBR ligands FGIN-1-27, PK 11195, or Ro5-4864 inhibited cell proliferation dose-dependently. FGIN-1-27 decreased the mitochondrial membrane potential, which indicates an early event in apoptosis. Furthermore, FGIN-1-27, PK 11195 or Ro5-4864 increased caspase-3 activity. In addition to their apoptosis-inducing effects, PBR ligands induced cell cycle arrest in the G 1/G 0-phase. Thus, our data demonstrate a functional involvement of PBR in colorectal cancer growth and qualify the PBR as a possible target for innovative therapeutic approaches in colorectal cancer. © 2001 Cancer Research Campaign http://www.bjcancer.com

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Gastrointestinal neuroendocrine (NE) cells synthesize, store and secrete γ‐aminobutyric acid (GABA). Recently, an autocrine‐paracrine function of GABA has been proposed for secretion from NE cells. To search for functional GABAA receptors in NE gut cells, we performed whole‐cell and perforated‐patch‐clamp studies in the intestinal cholecystokinin (CCK)‐secreting NE cell line STC‐1. Application of GABA evoked currents in STC‐1 cells. These effects were mimicked by muscimol, an agonist of GABAA receptors, and blocked by picrotoxin or bicuculline, antagonists of GABAA receptors. The GABA‐ or muscimol‐activated currents reversed near 0 mV, which under the recording conditions used was consistent with the activation of the GABAA receptor‐Cl− channel complex. In contrast to the effect on most neurons, GABA as well as muscimol led to a (reversible) depolarization of the membrane potential of STC‐1 cells. Membrane depolarization in turn activated voltage‐gated Ca2+ channels and increased intracellular Ca2+ concentrations in STC‐1 cells. In accordance with the observed membrane depolarization and activation of voltage‐gated Ca2+ channels, both GABA and muscimol stimulated Ca2+‐dependent CCK release. In contrast, bicuculline inhibited the GABA‐induced secretion of CCK. Using the reverse transcription‐polymerase chain reaction (RT‐PCR), mRNA of the GABAA receptor subunits α2, α3, α5, β1, β3 and δ could be detected in STC‐1 cells. In summary, we have shown that the CCK‐secreting gut NE cell line STC‐1 expresses functional GABAA receptors and that GABA stimulates CCK release. Thus, GABA is involved in the fine tuning of CCK secretion from the gut NE cell line STC‐1.