Minoru Hojo

BACKGROUND: Malignancy is a dreaded complication of organ transplantation. Immunosuppressive drug therapy-induced impairment of the organ graft recipient's immune surveillance is considered to be the mechanism for the heightened incidence and metastatic progression. We identified a cell-autonomous and host-immunity independent mechanism for cyclosporine-associated tumor progression. In this study, we investigated the effect of rapamycin on tumor progression, in the presence and absence of cyclosporine. METHODS: A spontaneously arising renal adenocarcinoma (renal cancer) of BALB/c origin was used as the model tumor. The effect of rapamycin on renal cancer cell phenotype, molecules (E-cadherin, p27 kip1, cyclin D1) implicated in tumor progression, and the effect of rapamycin on in vivo tumor progression were explored in BALB/c mice and in T-cell, B-cell, and natural killer (NK) cell-deficient severe combined immune deficiency (SCID)-beige mice. In the SCID-beige mice, T24 human bladder transitional cell carcinoma also was used as the tumor inoculum. RESULTS: Rapamycin conditioning of renal cancer cells upregulated E-cadherin expression and induced phenotypic transition from invasive spindle, or dome-shaped cells, with exploratory pseudopodia to noninvasive cuboidal cells that formed cell-to-cell adhesions. Rapamycin increased p27 kip1, reduced cyclinD1, and arrested the growth of renal cancer cells in G1/S phase. In vivo, rapamycin prevented tumor growth and metastatic progression in syngeneic BALB/c or SCID-beige mice, and in BALB/c or SCID-beige mice treated with cyclosporine. Rapamycin treatment alone, or with cyclosporine, prolonged the survival of mice inoculated with renal cancer cells or T24 human bladder cancer cells. CONCLUSIONS: Our findings, in addition to unlinking mechanisms of immunosuppression from that of tumor progression, suggest that rapamycin may be of value for the management of posttransplant malignancy.

Interactions between mu-opioid receptor (muOR) and cannabinoid CB1 receptor (CB1R) were examined by morphological and electrophysiological methods. In baby hamster kidney (BHK) cells coexpressing muOR fused to the yellow fluorescent protein Venus and CB1R fused to the cyan fluorescent protein Cerulean, both colors were detected on the cell surface; and fluorescence resonance energy transfer (FRET) analysis revealed that muOR and CB1R formed a heterodimer. Coimmunoprecipitation and Western blotting analyses also confirmed the heterodimers of muOR and CB1R. [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) or CP55,940 elicited K+ currents in Xenopus oocytes expressing muOR or CB1R together with G protein activated-inwardly rectifying K+ channels (GIRKs), respectively. In oocytes coexpressing both receptors, either of which was fused to the chimeric Galpha protein Gqi5 that activates the phospholipase C pathway, both DAMGO and CP55,940 elicited Ca2+-activated Cl(-) currents, indicating that each agonist can induce responses through Gqi5 fused to either its own receptor or the other. Experiments with endogenous Gi/o protein inactivation by pertussis toxin (PTX) supported the functional heterodimerization of muOR/CB1R through PTX-insensitive Gqi5(m) fused to each receptor. Thus, muOR and CB1R form a heterodimer and transmit a signal through a common G protein. Our electrophysiological method could be useful for determination of signals mediated through heterodimerized G protein-coupled receptors.

We investigated the role of G protein coupled-receptor kinases (GRKs) in the desensitization of GABA(B) receptor-mediated signaling using Xenopus oocytes and baby hamster kidney (BHK) cells. Baclofen elicited inward K(+) currents in oocytes coexpressing heterodimeric GABA(B) receptor, GABA(B1a) subunit (GB(1a)R) and GABA(B2) subunit (GB(2)R), together with G protein-activated inwardly rectifying K(+) channels (GIRKs), in a concentration-dependent manner. Repetitive application of baclofen to oocytes coexpressing GABA(B)R and GIRKs did not change peak K(+) currents in the first and second responses, but the latter responses were significantly attenuated by coexpression of either GRK4 or GRK5 with attenuation efficacy of GRK4 > GRK5. Coexpression of other GRKs including GRK2, GRK3, and GRK6 had no effect on GABA(B) receptor-mediated desensitization processes. In BHK cells coexpressing GRK4 fused to Venus (brighter variant of yellow fluorescent protein, GRK4-Venus) with GB(1a)R and GB(2)R, GRK4-Venus was expressed in the cytosol but was translocated to the plasma membranes by GABA(B)R activation. In BHK cells coexpressing GRK4 fused to Cerulean (brighter variant of cyan fluorescent protein, GRK4-Cerulean) with GB(1a)R and GB(2)R-Venus, fluorescence resonance energy transfer (FRET) analysis demonstrated that GRK4-Cerulean formed a protein complex with GB(2)R-Venus. Immunoprecipitation and Western blot analysis confirmed GB(2)R-GRK4 complex formation. GRK5 also formed a complex with GB(2)R on the plasma membranes as determined by FRET and Western blotting but not GRK2, GRK3, and GRK6. Our results indicate that GRK4 and GRK5 desensitize GABA(B) receptor-mediated responses by forming protein complexes with GB(2)R subunit of GABA(B)R at the plasma membranes.