Mirk/Dyrk1b Kinase Inactivation Increases Gemcitabine Efficacy Towards Quiescent Pancreatic Cancer Cells

A major problem in the treatment of cancer is the presence of residual quiescent cancer cells that are relatively insensitive to most chemotherapeutic drugs and radiation. Such cancer cells are believed to cause tumor recurrence when they re-enter the cell cycle under favorable conditions of the microenvironment. Gemcitabine is the drug most often used for treatment of pancreatic cancer, but the response rate is less than 20%. Since stratagems to increase delivery of gemcitabine by inhibiting stromal hedgehog signaling have shown some success, identifying methods to increase gemcitabine efficacy is timely. The serine/threonine kinase Mirk/dyrk1B is expressed at very low levels in most normal tissues, but is overexpressed or amplified in most pancreatic cancers. Mirk maintains pancreatic cancer cells in a viable quiescent state through its transcriptional co-activator functions for a set of antioxidant proteins that decrease intracellular levels of reactive oxygen species (Cancer Res 2009;69:3317-24). We now show that Panc1 and SU86.86 pancreatic cancer cells undergo a reversible quiescent arrest in G0 through activation of Chk2 and autophagy to reduce ribosomal RNA levels, with little apoptosis or senescence. Depletion of Mirk/dyrk1B by either induced shRNA or transfected RNAi duplexes or inhibition of Mirk9s kinase activity by a small molecule inhibitor increased DNA damage in quiescent pancreatic cancer cells, either grown attached to petri dishes or in small clumps in suspension as a model for ascites. DNA damage was detected by increased phosphorylation of the histone protein H2AX and by an increase in S phase checkpoints after the quiescent cells re-entered cycle in response to serum mitogens. Pharmacological inhibition of Mirk kinase induced a dose-dependent cleavage of the apoptotic marker proteins PARP and caspase 3, reduced Akt activation, and led to cleavage of the autophagic marker protein LC3. Pretreatment with the Mirk kinase inhibitor increased tumor cell kill 5-fold by short exposures to marginally toxic gemcitabine concentrations. In prior studies Mirk depletion had no detectable effect on the viability of normal diploid fibroblasts (Cancer Res 2007;67:7247-55), while other investigators have shown that embryonic knockout was not lethal (Leder et al, Biochem J 2003;372:881-8). Thus the small molecule Mirk/dyrk1B kinase inhibitor sensitized pancreatic cancer cells to low gemcitabine concentrations, while knockout or depletion of Mirk/dyrk1B had no detectable effect on normal tissue. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 649. doi:10.1158/1538-7445.AM2011-649