Membrane-associated serine/threonine protein phosphatase in endometrial cancer

MCLR-induced PP2A inhibition and subsequent Rac1 inactivation and hyperphosphorylation of cytoskeleton-associated proteins are involved in cytoskeleton rearrangement in SMMC-7721 human liver cancer cell line

Cyanobacteria-derived toxin microcystin-LR (MCLR) has been widely investigated in its effects on normal cells, there is little information concerning its effects on cancer cells. In the present study, the SMMC-7721 human liver cancer cell line treated with MCLR was used to investigate the change of PP2A, cytoskeleton rearrangement, phosphorylation levels of PP2A substrates that related with cytoskeleton stability and explored underlying mechanisms. Here, we confirmed that MCLR entered into SMMC-7721 cells, bound to PP2A/C subunit and inhibited the activity of PP2A. The upregulation of phosphorylation of the PP2A/C subunit and PP2A regulation protein α4, as well as the change in the association of PP2A/C with α4, were responsible for the decrease in PP2A activity. Another novel finding is that the rearrangement of filamentous actin and microtubules led by MCLR may attribute to the increased phosphorylation of HSP27, VASP and cofilin due to PP2A inhibition. As a result of weakened interactions with PP2A and alterations in its subcellular localization, Rac1 may contribute to the cytoskeletal rearrangement induced by MCLR in SMMC-7721 cells. The current paper presents the first report demonstrating the characteristic of PP2A in MCLR exposed cancer cells, which were more susceptible to MCLR compared with the normal cell lines we previously found, which may be owing to the absence of some type of compensatory mechanisms. The hyperphosphorylation of cytoskeleton-associated proteins and Rac1 inactivation which were induced by inhibition of PP2A are shown to be involved in cytoskeleton rearrangement.

Gonad differential proteins revealed with proteomics in oyster (Saccostrea cucullata) using alga as food contaminated with cadmium

As mercury and lead, cadmium (Cd) is one of the highly toxic metals in both the ocean and land environments, but its toxicological mechanism in organisms including human is still unclear because of the complex toxicological pathways in vivo. Here, the alga Chlorella vulgaris were cultivated at room temperature under the stress of cadmium (1 mg L(-1)) to obtain a toxic food, and then the contaminated food were directly supplied to oyster (Saccostrea cucullata) in seawater. After feeding with C. vulgaris contaminated with Cd (C. vulgaris-Cd), the differential proteins in the oyster gonad (OG) were effectively separated and identified with proteomic approaches. Eleven protein spots were observed to be significantly changed in the OG feeding with C. vulgaris-Cd, which seven spots of these differential proteins were down-regulated while four spots were up-regulated. These altered spots were further excised in gels and identified by a combined technique of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and database searching. A portion of these differential proteins were further proofed by real-time PCR and Western blotting. The results indicate that the major functions of these differential proteins were described as follows: binding, protein translocation, catalysis, regulation of energy metabolism, reproductive function and skeleton structure. These differential proteins in part may effectively provide a few novel biomarkers for the evaluation of Cd pollution level via a food pathway for harming halobios, mammal and human health, and for understanding the complex mechanisms of Cd toxicity in vivo.

Gi protein-mediated translocation of serine/threonine phosphatase to the plasma membrane and apoptosis of ovarian cancer cell in response to gonadotropin-releasing hormone antagonist cetrorelix

Serine/threonine protein phosphatase 2A (PP2A), a crucial enzyme in apoptosis control, has been demonstrated within the plasma membrane as well as in the soluble fraction. This study aimed to examine hormonal translocation of PP2A to the plasma membrane in gonadotropin-releasing hormone (GnRH)-responsive ovarian cancer cells. Apoptosis of ovarian cancer cell lines Caov-3 and SK-Ov-3 was quantified by nuclear morphology after staining with Hoechst 33342 dye. PP2A protein and activity in plasma membrane were assessed by immunohistochemical staining with PP2A-specific antibodies and by the measurement of the dephosphorylation of phosphopeptide highly selective for the PP2A, respectively. Incubation for 48 h with a GnRH antagonist cetrorelix caused parallel increases in the percentage of cells undergoing apoptosis and the membrane-associated PP2A activity; half-maximal effects occurred with 5 nmol/l cetrorelix. PP2A protein was also localised to the plasma membrane when the cell lines were exposed to cetrorelix. Pretreatment of the cells with pertussis toxin, but not cholera toxin, completely inhibited cetrorelix-stimulated apoptotic cell death and PP2A redistribution. These findings demonstrate that translocation of PP2A to plasma membrane is closely coupled to the onset of apoptosis in ovarian cancer cells exposed to GnRH antagonist. These GnRH-induced cellular events may be mediated through pertussis toxin-sensitive Gi protein-linked GnRH receptor.

Deactivation of sphingosine kinase 1 by protein phosphatase 2A

Sphingosine kinase 1 (SK1) is an important regulator of cellular signaling that has been implicated in a broad range of cellular processes. Cell exposure to a wide array of growth factors, cytokines, and other cell agonists can result in a rapid and transient increase in SK activity via an activating phosphorylation. We have previously identified extracellular signal-regulated kinases 1 and 2 (ERK1/2) as the kinases responsible for the phosphorylation of human SK1 at Ser(225), but the corresponding phosphatase targeting this phosphorylation has remained undefined. Here, we provide data to support a role for protein phosphatase 2A (PP2A) in the deactivation of SK1 through dephosphorylation of phospho-Ser(225). The catalytic subunit of PP2A (PP2Ac) was found to interact with SK1 using both GST-pulldown and coimmunoprecipitation analyses. Coexpression of PP2Ac with SK1 resulted in reduced Ser(225) phosphorylation of SK1 in human embryonic kidney (HEK293) cells. In vitro phosphatase assays showed that PP2Ac dephosphorylated both recombinant SK1 and a phosphopeptide based on the phospho-Ser(225) region of SK1. Finally, both basal and tumor necrosis factor-alpha-stimulated cellular SK1 activity were regulated by molecular manipulation of PP2Ac activity. Thus, PP2A appears to function as an endogenous regulator of SK1 phosphorylation.

Expression and function of protein phosphatase PP2A in malignant testicular germ cell tumours

Testicular germ cell tumours (TGCT) represent the most common malignancy in young males. We reported previously that two prototype members of the mitogen-activated protein kinase (MAPK) family, the MAPK ERK kinase (MEK) and extracellular signal-regulated kinase (ERK), are inactive in malignant testicular germ cells and become active after drug stimulation, leading to apoptosis of tumour cells. In this study, we asked whether the protein phosphatase PP2A, a known inhibitor of the MEK-ERK pathway, participates in the proliferation and/or apoptosis of primary TGCT (n = 48) as well as two TGCT cell lines (NTERA and NCCIT). Quantitative RT-PCR, immunohistochemistry, western blot analyses and phosphatase assay indicate that primary TGCT as well as TGCT cell lines express PP2A and that PP2A is active in TGCT cell lines. The inhibition of PP2A by application of two PP2A inhibitors, cantharidic acid (CA) and okadaic acid (OA), results in a significant increase in caspase-3-mediated apoptosis of TGCT cell lines. Thereby, PP2A inhibition was accompanied by phosphorylation and activation of MEK and ERK. Functional assays using the MEK inhibitor PD98059 demonstrated that the phosphorylation of MEK and ERK was required for the induction of caspase-3-mediated apoptosis of malignant germ cells. Thus, our data suggest that inhibition of PP2A mediates its apoptosis-inducing effect on TGCT through activation of the MEK-ERK signalling pathway that leads to caspase-3-mediated apoptosis of tumour cells. In addition our results support previous observations that PP2A exerts an anti-apoptotic effect on malignant tumour cells.