Constitutive phosphatidylinositol 3-kinase (PI3K)-AKT activation has a causal role in adult T-cell leukaemia-lymphoma (ATLL) and other cancers. ATLL cells do not harbour genetic alterations in PTEN and PI3KCA but express high levels of PTEN that is highly phosphorylated at its C-terminal tail. Here we report a mechanism for the N-myc downstream-regulated gene 2 (NDRG2)-dependent regulation of PTEN phosphatase activity via the dephosphorylation of PTEN at the Ser380, Thr382 and Thr383 cluster within the C-terminal tail. We show that NDRG2 is a PTEN-binding protein that recruits protein phosphatase 2A (PP2A) to PTEN. The expression of NDRG2 is frequently downregulated in ATLL, resulting in enhanced phosphorylation of PTEN at the Ser380/Thr382/Thr383 cluster and enhanced activation of the PI3K-AKT pathway. Given the high incidence of T-cell lymphoma and other cancers in NDRG2-deficient mice, PI3K-AKT activation via enhanced PTEN phosphorylation may be critical for the development of cancer.

Follistatin is an activin-binding protein that prevents activin from binding to its receptors and neutralizes its activity. Follistatin also binds bone morphogenetic proteins (BMPs). In this study, we report the identification of a novel follistatin-like protein from mouse. The mouse cDNA encodes a 256-residue precursor and most likely a mouse homologue of human FLRG, which was found at the breakpoint of the chromosomal rearrangement in a B-cell line. Whereas follistatin has three follistatin domains, which are presumed to be growth factor binding motifs, FLRG possesses only two follistatin domains. Northern blotting revealed that mRNAs for FLRG were abundantly expressed in heart, lung, kidney, and testis in mouse. The recombinant mouse FLRG proteins were found to have binding activity for both activin and bone morphogenetic protein-2. Like follistatin, FLRG has higher affinity for activin than for BMP-2. The FLRG protein inhibited activin-induced and BMP-2-induced transcriptional responses in a dose-dependent manner. Glutathione S-transferase fusion proteins encoding various regions of FLRG were produced and studied. Ligand blotting using 125 I-activin revealed that the COOHterminal region containing the second follistatin domain was able to bind activin. Our finding implies that cellular signaling by activin and BMPs is tightly regulated by multiple members of the follistatin family.

Acute myeloid leukemia with high ecotropic viral integration site-1 expression (EVI1(high) AML) is classified as a refractory type of leukemia with a poor prognosis. To provide new insights into the prevention and treatment of this disease, we identified the high expression of EVI1-regulated G protein-coupled receptor 56 (GPR56), and the association of high cell adhesion and antiapoptotic activities in EVI1(high) AML cells. Knockdown of GPR56 expression decreased the cellular adhesion ability through inactivation of RhoA signaling, resulting in a reduction of cellular growth rates and enhanced apoptosis. Moreover, in Gpr56(-/-) mice, the number of hematopoietic stem cells (HSCs) was significantly decreased in the bone marrow (BM) and, conversely, was increased in the spleen, liver and peripheral blood. The number of Gpr56(-/-) HSC progenitors in the G0/G1-phase was significantly reduced and was associated with impaired cellular adhesion. Finally, the loss of GPR56 function resulted in a reduction of the in vivo repopulating ability of the HSCs. In conclusion, GPR56 may represent an important GPCR for the maintenance of HSCs by acting as a co-ordinator of interactions with the BM osteosteal niche; furthermore, this receptor has the potential to become a novel molecular target in EVI1(high) leukemia.

We have identified a mouse PDZ protein that interacts with the activin type IIA receptor (ActRIIA), which we named activin receptor-interacting protein 1 (ARIP1). By using yeast two-hybrid screening, we isolated a cDNA clone of ARIP1 from a mouse brain cDNA library. We detected two forms of ARIP1, ARIP1-long and ARIP1-short, which may be produced by alternative splicing. ARIP1-long had one guanylate kinase domain in the NH 2 -terminal region, followed by two WW domains and five PDZ domains (PDZ1-5). ARIP1-short had a deletion in the NH 2 -terminal region and lacked the guanylate kinase domain. Both forms interacted with ActRIIA through PDZ5. The COOH-terminal residues of ActRIIA (ESSL) agree with a PDZ-binding consensus motif, and ARIP1 recognized the consensus sequence. ARIP1 interacts specifically with ActRIIA among the receptors for the transforming growth factor ␤ family. Interestingly, ARIP1 also interacted with Smad3, which is an activin/ transforming growth factor ␤ intracellular signaling molecule. The mRNA of ARIP1 was more abundant in the brain than in other tissues. Overexpression of ARIP1 controls activin-induced and Smad3-induced transcription in activin-responsive cell lines. These findings suggest that ARIP1 has a significant role in assembling activin signaling molecules at specific subcellular sites and in regulating signal transduction in neuronal cells.

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