The MST1 kinase was recently identified as playing an essential role in the promotion of lymphocyte polarization and adhesion stimulated by chemokines and TCR signaling. However, the physiological relevance of the Mst1 pathway in thymocyte development is not completely understood. In this study, we analyzed the effect of Mst1 disruption on thymocyte development and migration. Mst1-deficient (Mst1−/−) mice displayed an accumulation of mature thymocytes in the thymus, a dramatic reduction of lymphocytes in blood and peripheral lymphoid tissues, and a decrease of homing ability to peripheral lymph nodes. Mst1−/− thymocytes were impaired in chemotactic response to chemokines, such as CCL19, but not to sphingosine-1-phosphate. Further analyses of Mst1−/− mice revealed a severe impairment in the egress of mature T cells from the thymus. T lineage-specific knockout of the Mst1 gene demonstrates a cell-intrinsic role for Mst1 in regulating T cell development. Our study indicates that Mst1 is crucial in controlling lymphocyte chemotaxis and thymocyte emigration.

Foxp3 expression and regulatory T cell (Treg) development are critical for maintaining dominant tolerance and preventing autoimmune diseases. Human MST1 deficiency causes a novel primary immunodeficiency syndrome accompanied by autoimmune manifestations. However, the mechanism by which Mst1 controls immune regulation is unknown. In this article, we report that Mst1 regulates Foxp3 expression and Treg development/function and inhibits autoimmunity through modulating Foxo1 and Foxo3 (Foxo1/3) stability. We have found that Mst1 deficiency impairs Foxp3 expression and Treg development and function in mice. Mechanistic studies reveal that Mst1 enhances Foxo1/3 stability directly by phosphorylating Foxo1/3 and indirectly by attenuating TCR-induced Akt activation in peripheral T cells. Our studies have also shown that Mst1 deficiency does not affect Foxo1/3 cellular localization in CD4 T cells. In addition, we show that Mst1−/− mice are prone to autoimmune disease, and mutant phenotypes, such as overactivation of naive T cells, splenomegaly, and autoimmune pathological changes, are suppressed in Mst1−/− bone marrow chimera by cotransplanted wt Tregs. Finally, we demonstrate that Mst1 and Mst2 play a partially redundant role in Treg development and autoimmunity. Our findings not only identify Mst kinases as the long-searched-for factors that simultaneously activate Foxo1/3 and inhibit TCR-stimulated Akt downstream of TCR signaling to promote Foxp3 expression and Treg development, but also shed new light on understanding and designing better therapeutic strategies for MST1 deficiency–mediated human immunodeficiency syndrome.

PHLPP1 and PHLPP2 phosphatases exert their tumor-suppressing functions by dephosphorylation and inactivation of Akt in several breast cancer and glioblastoma cells. However, Akt, or other known targets of PHLPPs that include PKC and ERK, may not fully elucidate the physiological role of the multifunctional phosphatases, especially their powerful apoptosis induction function. Here, we show that PHLPPs induce apoptosis in cancer cells independent of the known targets of PHLPPs. We identified Mst1 as a binding partner that interacts with PHLPPs both in vivo and in vitro. PHLPPs dephosphorylate Mst1 on the T387 inhibitory site, which activate Mst1 and its downstream effectors p38 and JNK to induce apoptosis. The same T387 site can be phosphorylated by Akt. Thus, PHLPP, Akt, and Mst1 constitute an autoinhibitory triangle that controls the fine balance of apoptosis and proliferation that is cell type and context dependent.

Mammalian Sterile 20-like kinase 1 (MST1) protein kinase plays an important role in the apoptosis induced by a variety of stresses. The MST1 is a serine/threonine kinase that is activated upon apoptotic stimulation, which in turn activates its downstream targets, JNK/p38, histone H2B and FOXO. It has been reported that overexpression of MST1 initiates apoptosis by activating p53. However, the molecular mechanisms underlying MST1-p53 signaling during apoptosis are unclear. Here, we report that MST1 promotes genotoxic agent-induced apoptosis in a p53-dependent manner. We found that MST1 increases p53 acetylation and transactivation by inhibiting the deacetylation of Sirtuin 1 (Sirt1) and its interaction with p53 and that Sirt1 can be phosphorylated by MST1 leading to the inhibition of Sirt1 activity. Collectively, these findings define a novel regulatory mechanism involving the phosphorylation of Sirt1 by MST1 kinase which leads to p53 activation, with implications for our understanding of signaling mechanisms during DNA damageinduced apoptosis.The protein kinase mammalian Sterile 20-like kinase 1 (MST1) 4 contains a Ste20-related kinase catalytic domain in the amino-terminal segment followed by a regulatory domain at the COOH terminus (1). It has been implicated in diverse biological functions, including cell proliferation, differentiation, morphogenesis, and cytoskeletal rearrangements. Previous studies have indicated that the noncatalytic tail of MST1 is cleaved by caspase-3 in response to a number of apoptotic stimuli such as death receptor triggering by CD95/FasL, staurosporine or ceramide, and heat shock and arsenite. The amino-terminal fragment of cleaved MST1 translocates into the nucleus where it contributes to chromatin condensation and then apoptosis (2-4). Furthermore, when MST1 is overexpressed, cleavage and subsequent induced apoptosis can also be observed (5). It has been reported that the two major cleavage sites are Asp-326 and Asp-349, and kinase activation, nuclear translocation, and the ability of MST1 to induce cell death are apparently attenuated by mutating these cleavage sites (4, 6). Recently threonine 183 has been identified as a crucial phosphoactivation site in subdomain VIII of MST1, and it is essential for kinase activation. Autophosphorylation of threonine 183 within the MST1 kinase domain is required for its activation (7). Hippo, a mammalian homolog of MST1/2 in Drosophila, has been extensively shown to restrain cell growth and proliferation through inhibition of the transcription and/or degradation of cyclin E and Drosophila Inhibitor of Apoptosis proteins (8, 9) or the phosphorylation and inhibition of Yorkie (10). In mammals, it has been shown that MST1 can activate c-Jun amino-terminal kinase (JNK) and p38 MAPK kinase signaling pathways through MKK4/MKK7 and MKK3/MKK6, respectively (3). Recently, it has been suggested that JNK is essential and sufficient for MST1 activation and MST1-mediated apoptosis via phosphorylation of serine 82 in MST1 (11,12). In addition, MST1-induced c...

Overexpression of HIF-1alpha is predictive of a poor outcome and might be a novel therapeutic target in human osteosarcoma.