Stress signals activate the SAPK/JNK and p38 MAPK classes of protein kinases, which mediate cellular responses, including steps in apoptosis and the maturation of some cell types. We now show that stress signals initiated by transforming growth factor-␤1 (TGF-␤1) induce G 1 arrest through protein stabilization of the CDK inhibitor p21Cip1 . TGF-␤1 was previously shown to increase p21 protein levels, which in turn mediated G 1 arrest through inactivation of the CDK2-cyclin E complex in HD3 cells (Yan, Z., Kim, G.-Y., Deng, X., and Friedman, E. (2002) J. Biol. Chem. 277, 9870 -9879). We now demonstrate that the increase in p21 abundance is caused by a post-transcriptional, SMAD-independent mechanism. TGF-␤1 activated p38␣ and JNK1, which initiated the phosphorylation of p21. TGF-␤1 treatment increased the half-life of p21 by 3-4-fold. The increase in p21 stability was detected following activation of p38␣ and JNK1, and treatment of cells with the p38 inhibitor SB203580 prevented this increase in p21 stability. p38␣ and JNK1 phosphorylated p21 in vivo, and both p38␣ and JNK1 phosphorylated p21 at Ser 130 in vitro. Peptide mapping demonstrated that both TGF-␤1 and p38␣ induced phosphorylation of p21 at Ser 130 in vivo, and mutation of Ser 130 to alanine rendered p21 less stable than wild-type p21. TGF-␤1 increased the stability of wildtype p21, but not the p21-S130A mutant. These findings demonstrate that SAPKs can mediate cell cycle arrest through post-translational modification of p21.

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CNS remyelination and functional recovery often occur after experimental demyelination in adult rodents. This has been attributed to the ability of mature oligodendrocytes and/or their precursor cells to divide and regenerate in response to signals in demyelinating lesions. To determine whether oligodendrocyte precursor cells exist in the adult human CNS, we have cultured white matter from patients undergoing partial temporal lobe resection for intractable epilepsy. These cultures contained a population of process-bearing cells that expressed antigens recognized by the O4 monoclonal antibody, but these cells did not express galactocerebroside (a marker for oligodendrocytes), glial fibrillary acidic protein (a marker for astrocytes), or vimentin. Selective elimination of O4-positive (O4+) cells by complement-mediated lysis resulted in inhibition of oligodendrocyte development in vitro. Since O4+ cells have an antigenic phenotype reminiscent of the rat adult oligodendrocyte-type 2 astrocyte progenitor and appear to develop into oligodendrocytes rather than type 2 astrocytes with time in culture, we call them "pre-oligodendrocytes." Neither pre-oligodendrocytes nor oligodendrocytes incorporated 3H-thymidine in response to rat astrocyte-conditioned medium, platelet-derived growth factor, insulin-like growth factor (IGF-1), or basic fibroblast growth factor (bFGF). However, IGF-1 increased the relative abundance of oligodendrocytes to pre-oligodendrocytes, while bFGF had the opposite effect. Cells with the antigenic phenotype of pre-oligodendrocytes were also identified in tissue prints of adult human white matter. We propose that, in human demyelinating diseases such as multiple sclerosis, pre-oligodendrocytes may divide and/or migrate in response to signals present in demyelinated lesions and thus facilitate remyelination.

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