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The role of neuroendocrine peptide calcitonin (CT) and its receptor (CTR) in epithelial cancer progression is an emerging concept with great clinical potential. Expression of CT and CTR is frequently elevated in prostate cancers (PCs) and activation of CT-CTR axis in non-invasive PC cells induces an invasive phenotype. Here we show by yeast-two hybrid screens that CTR associates with the tight junction protein Zonula Occludens-1 (ZO-1) via the interaction between the type 1 PDZ motif at the carboxy-terminus of CTR and the PDZ3 domain of ZO-1. Mutation of either the CTR C-PDZ-binding motif or the ZO-1-PDZ3 domain did not affect binding of CTR with its ligand or G-protein-mediated signaling but abrogated destabilizing actions of CT on tight junctions and formation of distant metastases by orthotopically implanted PC cells in nude mice, indicating that these PDZ domain interactions were pathologically relevant. Further, we observed CTR-ZO-1 interactions in PC specimens by proximity ligation immunohistochemistry, and identified that the number of interactions in metastatic PC specimens was several-fold larger than in non-metastatic PC. Our results for the first time demonstrate a mechanism by which PDZ-mediated interaction between CTR and ZO1 is required for CT-stimulated metastasis of prostate cancer. Since many receptors contain PDZ-binding motifs, this would suggest that PDZ-binding motif-adaptor protein interactions constitute a common mechanism for cancer metastasis.

Enhanced ceramide glycosylation catalyzed by glucosylceramide synthase (GCS) limits therapeutic efficiencies of antineoplastic agents including doxorubicin in drug-resistant cancer cells. Aimed to determine the role of GCS in tumor response to chemotherapy, a new mixed-backbone oligonucleotide (MBO-asGCS) with higher stability and efficiency has been generated to silence human GCS gene. MBO-asGCS was taken up efficiently in both drug-sensitive and drug-resistant cells, but it selectively suppressed GCS overexpression, and sensitized drug-resistant cells. MBO-asGCS increased doxorubicin sensitivity by 83-fold in human NCI/ADR-RES, and 43-fold in murine EMT6/AR1 breast cancer cells, respectively. In tumor-bearing mice, MBO-asGCS treatment dramatically inhibited the growth of multidrug-resistant NCI/ADR-RE tumors, decreasing tumor volume to 37%, as compared with scrambled control. Furthermore, MBO-asGCS sensitized multidrug-resistant tumors to chemotherapy, increasing doxorubicin efficiency greater than 2-fold. The sensitization effects of MBO-asGCS relied on the decreases of gene expression and enzyme activity of GCS, and on the increases of C(18)-ceramide and of caspase-executed apoptosis. MBO-asGCS was accumulation in tumor xenografts was greater in other tissues, excepting liver and kidneys; but MBO-asGCS did not exert significant toxic effects on liver and kidneys. This study, for the first time in vivo, has demonstrated that GCS is a promising therapeutic target for cancer drug resistance, and MBO-asGCS has the potential to be developed as an antineoplastic agent.