Oxidative stress can induce apoptosis through activation of MstI, subsequent phosphorylation of FOXO and nuclear translocation. MstI is a common component of apoptosis initiated by various stresses. MstI kinase activation requires autophosphorylation and proteolytic degradation by caspases. FOXO2 (forkhead box O; forkhead members of the O class) are transcription factors. FOXO factors mediate cell death by regulating numerous apoptotic genes transcription (1-3). FOXO factors are insulin-sensitive transcription factors with a variety of downstream targets and interacting partners. Insulinmediated inhibition of FOXO factors is predominantly regulated through a shuttling mechanism that distributes FOXO localization to the cytoplasm, thereby terminating its transcriptional function (4). FOXO factors contain three highly conserved putative Akt recognition motifs (RXRXX(S/T), where X denotes any residue), two at the N and C termini, respectively, and one located in the forkhead domain. All FOXO proteins require Akt phosphorylation in the N terminus and in the forkhead domain to translocate from the nucleus to the cytoplasm (5). The two phosphorylated residues are essential components for translocation, as they influence the NLS (nuclear localization sequence) function and the association with 14-3-3 proteins (2). Recently, Bonni and co-workers demonstrated that MstI mediates oxidative stress-induced neuronal apoptosis through FOXO factors by phosphorylating FOXO3 on Ser 207 . This phosphorylation triggers its nuclear translocation and disrupting the association between 14-3-3 and FOXO in the cytoplasm (6).MstI belongs to class II GC (protein Ser/Thr) kinase (7), which contains 487 residues and predominantly resides in the cytoplasm. MstI consists of an N-terminal catalytic domain in the Ste20 class, followed by a non-catalytic tail comprising of an autoinhibitory domain and a coiled-coil domain that mediates dimerization (8). It has been suggested that, physiologically, MstI exists as an autoinhibitory homodimer that is activated after post-translational modification such as phosphorylation and/or cleavage. Although caspase-mediated cleavage removes the C-terminal regulatory domain, which is associated with an increase in MstI activity, there is evidence that caspase-mediated cleavage alone cannot activate MstI and both phosphorylation and proteolysis are necessary to activate fully this enzyme (9 -11). Indeed, Lee et al. (12) (9,11). Of these, Thr 183 and Thr 187 appear to be essential for kinase activity (11). It has been proposed before that phosphorylation of MstI on Thr 183 and possibly Thr 187 is induced by an existing active MstI (11). Overexpression of MstI alone is sufficient to initiate apoptosis in various cells, which involves activation of SAPK (stress-activated protein kinase)/JNK (13), p53 (14), FOXO (6). MstI cycles rapidly and continuously through the nucleus (6, 15), and associates with DAP-4 (death-associated protein-4) in the nucleus (14), where the catalytic fragment generated during apoptosi...

Serine/arginine (SR) protein-specific kinase (SRPK), a family of cell cycle-regulated protein kinases, phosphorylate SR domain-containing proteins in nuclear speckles and mediate the pre-mRNA splicing. However, the physiologic roles of this event in cell cycle are incompletely understood. Here, we show that SRPK2 binds and phosphorylates acinus, an SR protein essential for RNA splicing, and redistributes it from the nuclear speckles to the nucleoplasm, resulting in cyclin A1 but not A2 up-regulation. Acinus S422D, an SRPK2 phosphorylation mimetic, enhances cyclin A1 transcription, whereas acinus S422A, an unphosphorylatable mutant, blocks the stimulatory effect of SRPK2. Ablation of acinus or SRPK2 abrogates cyclin A1 expression in leukemia cells and arrest cells at G 1 phase. Overexpression of acinus or SRPK2 increases leukemia cell proliferation. Furthermore, both SRPK2 and acinus are overexpressed in some human acute myelogenous leukemia patients and correlate with elevated cyclin A1 expression levels, fitting with the oncogenic activity of cyclin A1 in leukemia. Thus, our findings establish a molecular mechanism by which SR splicing machinery regulates cell cycle and contributes to leukemia tumorigenesis. [Cancer Res 2008;68(12):4559-70]

Molecular biological studies con¢rmed that two glutamate dehydrogenase isozymes (hGDH1 and hGDH2) of distinct genetic origin are expressed in human tissues. hGDH1 is heat-stable and expressed widely, whereas hGDH2 is heat-labile and speci¢c for neural and testicular tissues. A selective de¢-ciency of hGDH2 has been reported in patients with spinocerebellar ataxia. We have identi¢ed an amino acid residue involved in the di¡erent thermal stability of human GDH isozymes. At 45 ‡C (pH 7.0), heat inactivation proceeded faster for hGDH2 (half life = 45 min) than for hGDH1 (half-life = 310 min) in the absence of allosteric regulators. Both hGDH1 and hGDH2, however, showed much slower heat inactivation processes in the presence of 1 mM ADP or 3 mM L-Leu. Virtually most of the enzyme activity remained up to 100 min at 45 ‡C after treatment with ADP and L-Leu in combination. In contrast to ADP and L-Leu, the thermal stabilities of the hGDH isozymes were not a¡ected by addition of substrates or coenzymes. In human GDH isozymes, the 443 site is Arg in hGDH1 and Ser in hGDH2. Replacement of Ser by Arg at the 443 site by cassette mutagenesis abolished the heat lability of hGDH2 with a similar half-life of hGDH1. The mutagenesis at several other sites (L415M, A456G, and H470R) having di¡erences in amino acid sequence between the two GDH isozymes did not show any change in the thermal stability. These results suggest that the Ser443 residue plays an important role in the di¡erent thermal stability of human GDH isozymes.