No abstract

The phosphorylation of cyclin D1 at threonine 286 by glycogen synthase kinase 3␤ (GSK3␤) has been shown to be required for the ubiquitination and nuclear export of cyclin D1 and its subsequent degradation in the proteasome. The mutation of the nearby residue, threonine 288, to nonphosphorylatable alanine has also been shown to reduce the ubiquitination of cyclin D1, suggesting that phosphorylation at threonine 288 may also lead to degradation of cyclin D1. We now demonstrate that the G 0 /G 1 -active arginine-directed protein kinase Mirk/dyrk1B binds to cyclin D1 and phosphorylates cyclin D1 at threonine 288 in vivo and that the cyclin D1-T288A construct is more stable than wild-type cyclin Cell cycle progression in eukaryotic cells is mediated by cyclin-dependent kinases (CDKs). 1 The D-type cyclins, D1, D2, and D3, increase in nuclear abundance in G 1 in response to mitogens, facilitate the import of CDK4 into the nucleus (1), and assemble combinatorially with CDK4 or CDK6 into complexes that phosphorylate the retinoblastoma protein, releasing factors needed for the progression into S phase. Cyclin D1 is translocated into the cytoplasm during S phase where it is destroyed by the proteasome following phosphorylation at threonine 286 by GSK3␤ (2, 3). Mutant cyclin D1-T286A, which cannot be phosphorylated by GSK3␤, is stabilized in the nucleus and is capable of transforming murine fibroblasts, whereas overexpression of wild-type cyclin D1 cannot act alone to transform such cells (4). A cyclin D1 isoform derived by alternative splicing was shown to lack threonine 286, enabling this cyclin D1 isoform to remain nuclear throughout the cell cycle, remain highly expressed, and function to facilitate transformation of NIH3T3 cells (5). This cyclin D1 splice variant was also found in tumor-derived cells and primary human esophageal tumors (5). Overexpression of cyclin D1 occurs in several cancers including breast, pancreatic, and esophageal (6), suggesting that either increased transcription, transcription of stable splice variants, or dysregulation of cyclin D1 turnover may frequently occur in cancer.In this study, we have studied the interaction of the ubiquitously expressed protein kinase Mirk/dyrk1B with cyclin D1. Mirk/dyrk1B is an arginine-directed serine/threonine kinase (7), which functions as a transcriptional co-activator and is activated through the stress-activated mitogen-activated protein kinase kinase MKK3 (8). We have shown recently that Mirk stabilizes the CDK inhibitor p27kip1 in the G 0 phase of the cell cycle in NIH3T3 fibroblasts, whereas depletion of Mirk by RNA interference increases cell cycling as measured by increased PCNA expression (9). Mirk expression is decreased by mitogen activation of the MEK-ERK pathway during G 1 (10), restricting Mirk function primarily to G 0 and early G 1 . We now confirm that transient overexpression of Mirk in nontransformed Mv1Lu lung epithelial cells increases the length of G 0 /G 1 by FACS analysis and that Mirk targets the G 1 cell cycle regulator, cyclin D1, t...

The Rho family of small GTPases regulates numerous signaling pathways that control the organization of the cytoskeleton, transcription factor activity, and many aspects of the differentiation of skeletal myoblasts. We now demonstrate that the kinase Mirk (minibrain-related kinase)/dyrk1B is induced by members of the Rhofamily in myoblasts and that Mirk is active in skeletal muscle differentiation. Mirk is an arginine-directed serine/threonine kinase which is expressed at elevated levels in skeletal muscle compared with other normal tissues. A Mirk promoter construct was activated when C2C12 myoblasts were switched from growth to differentiation medium and was also activated by the Rho family members RhoA, Cdc42, and to a lesser degree

The kinase Mirk/dyrk1B mediated the clonogenic growth of pancreatic cancer cells in earlier studies. It is now shown that Mirk levels increased 7-fold in SU86.86 pancreatic cancer cells when over a third of the cells were accumulated in a quiescent G 0 state, defined by Hoechst/Pyronin Y staining. Depletion of Mirk by a doxycycline-inducible short hairpin RNA increased the G 0 fraction to f50%, suggesting that Mirk provided some function in G 0 . Mirk reduced the levels of reactive oxygen species (ROS) in quiescent cultures of SU86.86 cells and of Panc1 cells by increasing transcription of the antioxidant genes ferroxidase, superoxide dismutase (SOD)2, and SOD3. These genes were functional antioxidant genes in pancreatic cancer cells because ectopic expression of SOD2 and ferroxidase in Mirk-depleted cells lowered ROS levels. Quiescent pancreatic cancer cells quickly lost viability when depleted of Mirk because of elevated ROS levels, exhibiting up to 4-fold less colony-forming activity and 4-fold less capability for dye exclusion. As a result, reduction of ROS by N-acetyl cysteine led to more viable cells. Mirk also destabilizated cyclin D1 and D3 in quiescent cells. Thus, quiescent pancreatic cancer cells depleted of Mirk became less viable because they were damaged by ROS, and had increased levels of G 1 cyclins to prime cells to escape quiescence. [Cancer Res 2009;69(8):3317-24]

The kinase Mirk/dyrk1B is essential for the differentiation of C2C12 myoblasts. Mirk reinforces the G 0 /G 1 arrest state in which differentiation occurs by directly phosphorylating and stabilizing p27Kip1 and destabilizing cyclin D1. We now demonstrate that Mirk is antiapoptotic in myoblasts. Knockdown of endogenous Mirk by RNA interference activated caspase 3 and decreased myoblast survival by 75%, whereas transient overexpression of Mirk increased cell survival. Mirk exerts its anti-apoptotic effects during muscle differentiation at least in part through effects on the cell cycle inhibitor and pro-survival molecule p21Cip1 . Overexpression and RNA interference experiments demonstrated that Mirk phosphorylates p21 within its nuclear localization domain at Ser-153 causing a portion of the typically nuclear p21 to localize in the cytoplasm. Phosphomimetic GFP-p21-S153D was pancellular in both cycling C2C12 myoblasts and NIH3T3 cells. Endogenous Mirk in myotubes and overexpressed Mirk in NIH3T3 cells were able to cause the pancellular localization of wild-type GFPp21 but not the nonphosphorylatable mutant GFP-p21-S153A. Translocation to the cytoplasm enables p21 to block apoptosis through inhibitory interaction with pro-apoptotic molecules. Phosphomimetic p21-S153D was more effective than wild-type p21 in blocking the activation of caspase 3. Transient expression of p21-S153D also increased myoblast viability in colony forming assays, whereas the p21-S153A mutant had no effect. This Mirk-dependent change in p21 intracellular localization is a natural part of myoblast differentiation. Endogenous p21 localized exclusively to the nuclei of proliferating myoblasts but was also found in the cytoplasm of post-mitotic multinucleated myotubes and adult human skeletal myofibers.