Ras associated with diabetes (Rad) is a Ras-related GTPase that promotes cell growth by accelerating cell cycle transitions. Rad knockdown induced cell cycle arrest and premature senescence without additional cellular stress in multiple cancer cell lines, indicating that Rad expression might be critical for the cell cycle in these cells. To investigate the precise function of Rad in this process, we used human Rad as bait in a yeast two-hybrid screening system and sought Rad-interacting proteins. We identified the Grap2 and cyclin D interacting protein (GCIP)/DIP1/CCNDBP1/HHM, a cell cycle-inhibitory molecule, as a binding partner of Rad. Further analyses revealed that Rad binds directly to GCIP in vitro and coimmunoprecipitates with GCIP from cell lysates. Rad translocates GCIP from the nucleus to the cytoplasm, thereby inhibiting the tumor suppressor activity of GCIP, which occurs in the nucleus. Furthermore, in the presence of Rad, GCIP loses its ability to reduce retinoblastoma phosphorylation and inhibit cyclin D1 activity. The function of Rad in transformation is also evidenced by increased telomerase activity and colony formation according to Rad expression level. In vivo tumorigenesis analyses revealed that tumors derived from Rad knockdown cells were significantly smaller than those from control cells (P = 0.0131) and the preestablished tumors are reduced in size after the injection of siRad (P = 0.0064). Therefore, we propose for the first time that Rad may promote carcinogenesis at least in part by inhibiting GCIP-mediated tumor suppression. Cancer Res; 70(11); 4357-65. ©2010 AACR.

Senescence, an inherent tumor suppressive mechanism, is a critical determinant for chemotherapy. In the present study, we show that the monocarboxylate transporter 2 (MCT2) protein was tumor-selectively expressed in human colorectal malignancies and knockdown of MCT2 induces mitochondrial dysfunction, cell-cycle arrest, and senescence without additional cellular stress in colorectal cancer cell lines. Moreover, the reactive oxygen species (ROS) scavenger, N-acetylcysteine, blocked MCT2 knockdown-induced growth arrest and cellular senescence, indicating a pivotal role of ROS in this pathway. Dramatic induction of mitochondrial superoxide generation and decrease in ATP production was observed, indicating that mitochondrial dysfunction is the major mechanism underlying MCT2 knockdown-induced ROS generation. Senescenceassociated DNA damage was also evident from the increase in promyelocytic leukemia bodies, gH2AX foci, and SAHF. Conversely, overexpression of MCT2 prevented doxorubicin-induced ROS accumulation (P ¼ 0.0002) and cell growth inhibition (P ¼ 0.001). MCT2 knockdown suppressed KRAS mutant colorectal tumor growth in vivo. In addition, MCT2 knockdown and cytostatic drug combination further enhanced the antitumor effect. These findings support the use of MCT2 as a promising target for inhibition of colorectal cancer. Mol Cancer Ther; 11(11); 2342-51. Ó2012 AACR.

The constitutive activation of NF-kB is a major event leading to the initiation, development, and progression of cancer. Recently, we showed that the size of preestablished tumors was reduced after the depletion of Kir2.2, an inwardly rectifying potassium channel. To determine the precise mechanism of action of Kir2.2 in the control of tumor growth, we searched for interacting proteins. Notably, NF-kB p65/RelA was identified as a binding partner of Kir2.2 in a yeast two-hybrid analysis. Further analyses revealed that Kir2.2 directly interacted with RelA in vitro and coimmunoprecipitated with RelA from cell lysates. Kir2.2 increased RelA phosphorylation at S536 and facilitated its translocation from the cytoplasm to the nucleus, thereby activating the transcription factor and increasing the expression level of NF-kB targets, including cyclin D1, matrix metalloproteinase (MMP)9, and VEGF. Kir2.2 was overexpressed in human cancer and the expression level was correlated with increased colony formation and tumor growth in mouse tumor models. On the basis of these findings, we propose an unconventional role for Kir2.2 as a constitutive RelA-activating protein, which is likely to contribute to tumor progression in vivo. Cancer Res; 73(3); 1056-62. Ó2012 AACR.