Mirk Kinase is overexpressed in pancreatic ductal adenocarcinoma and mediates increased cell survival

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Mirk/dyrk1B is a member of the dyrk/minibrain family of serine/threonine kinases which mediate the transition from growth to differentiation in lower eukaryotes and mammals. Mirk is expressed at low levels in most human tissues, but is upregulated in skeletal muscle. Serial analysis of Mirk gene expression demonstrated that, similar to Mirk’s elevated expression in skeletal muscle, Mirk mRNA levels were elevated in each of two pancreatic ductal carcinomas, Panc 91 and Panc 96 and in each of two cell lines derived from such cancers, Panc 1 and CAPAN1, while Mirk mRNA was undetectable in short term primary cultures prepared from two samples of normal pancreatic ducts. Mirk protein was detected in each of four pancreatic carcinoma cell lines by western blotting. Affinity-purified antibody to the unique C-terminus of Mirk detected only the Mirk 65/69 kDa Mirk doublet in pancreatic carcinoma cells in a western blot which separated proteins of about 20 kDa to 100 kDa indicating that the antibody would be selective enough to be employed for immunohistochemistry. Mirk protein was detected in 27 of the 31 cases (87%) of pancreatic ductal adenocarcinoma by immunohistochemistry. Normal pancreatic ducts were present in 9 cases, and either no Mirk or very low Mirk levels were observed in 8 of these cases, replicating the results of the SAGE analysis. In one case, Mirk levels were high in the tumor and high focally in the normal appearing ducts, perhaps suggestive of early premalignant changes in the ducts. In contrast, Mirk protein was sometimes observed in the normal pancreatic acini, which are not believed to give rise to pancreatic cancer. Mirk is expressed in the nucleus and perinuclear area in proliferating myoblasts and in NIH3T3 cells where Mirk mediates growth arrest in G0/G1 through stabilization of the CDK inhibitor p27kip1 and the destabilization of the G1 cyclin, cyclin D1. However, Mirk is unlikely to mediate cell cycle arrest in pancreatic cancer cells, as Mirk was found in the perinuclear region, not in the nucleus, in each case of pancreatic ductal adenocarcinoma examined in this study. Depletion of Mirk by RNA interference from colon carcinoma cells and C2C12 myoblasts decreases cell survival under stress conditions, while overexpression of Mirk in colon carcinoma cells leads to increased survival. We speculate that Mirk is upregulated during the evolution of pancreatic ductal adenocarcinomas because Mirk also mediates the survival of these carcinoma cells.