Cell cycle-related signaling pathways modulated by peripheral benzodiazepine receptor ligands in colorectal cancer cells

The peripheral benzodiazepine receptor (PBR) has been implicated in growth control of various tumor models. PBR-specific ligands induced both apoptosis and G0/G1 cell cycle arrest in colorectal cancers. While the molecular mechanisms of PBR ligand-induced apoptosis have been addressed, the signaling pathways leading to cell cycle arrest are still unknown. Using cDNA array technology, we identified the signaling molecules involved in cell cycle arrest induced by the PBR ligands FGIN–1–27 and PK 11195. Differential gene expression was confirmed by semi-quantitative RT-PCR or Western blot analysis of gene products. The PBR-mediated signaling involved the inactivation of ERK1/2, the upregulation of the cyclin-dependent kinase inhibitors p21 Waf1/CIP1 and p27 Kip1 , cdc16, and the cell cycle inhibitors gadd45 and gadd153 as well as the down regulation of the cyclins D1 and B1. The p21-deficient colorectal cancer cell line HCT116 p21 -/- was significantly less sensitive to PBR ligands than the parental HCT116 wild-type cells, demonstrating the functional involvement of p21 Waf1/CIP1 in PBR-mediated G1 arrest. The relevance of the ERK1/2 pathway was shown by using the MEK inhibitor PD98059. This study thus revealed the PBR ligand-triggered signaling pathways leading to cell cycle arrest. Moreover, we showed the functional implication and interaction of differentially expressed gene products and provided a model of signaling pathways involved in PBR ligand-induced G1 arrest. These results form the basis for future PBR-based therapeutic approaches.