Rubratoxin A specifically and potently inhibits protein phosphatase 2A and suppresses cancer metastasis.
ABSTRACT Although cytostatin analog protein phosphatase 2A (PP2A)-specific inhibitors are promising candidates of a new type of anticancer drug, their development has been hindered because of their liability. To find new classes of PP2A-specific inhibitors, we conducted a screening with microbial metabolites and found that rubratoxin A, a classical mycotoxin, is a highly specific and potent inhibitor of the enzyme. While rubratoxin A inhibits PP2A at Ki = 28.7 nm, it hardly inhibited any other phosphatases examined. Rubratoxin B, a close analog, also specifically but weakly inhibits PP2A at Ki = 3.1 microM. The inhibition of intracellular PP2A in cultured cells is obviously observed with 20 microM rubratoxin A treatment for 3 h, inducing the overphosphorylation in PP2A substrate proteins. Although rubratoxins and cytostatin differ in the apparent structures, these compounds share similarities in the structures in detail and PP2A-binding manners. Rubratoxin A showed higher suppression of tumor metastasis and reduction of the primary tumor volume than cytostatin in mouse experiments. As a successor of cytostatin analogs, rubratoxin A should be a good compound leading to the development of antitumor drugs targeting PP2A.
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ABSTRACT: It has been known for over a decade that inhibition of protein phosphatase 1 (PP1) activity prevents entry into M phase, but the relevant substrate has not been identified. We report here that PP1 is required for dephosphorylation of the Cdc2-directed phosphatase Cdc25 at Ser287 (of Xenopus Cdc25; Ser216 of human Cdc25C), a site that suppresses Cdc25 during interphase. Moreover, PP1 recognizes Cdc25 directly by interacting with a PP1-binding motif in the Cdc25 N-terminus. We have also found that 14-3-3 binding to phospho-Ser287 protects Cdc25 from premature dephosphorylation. Upon entry into M phase, 14-3-3 removal from Cdc25 precedes Ser287 dephosphorylation, suggesting the existence of a phosphatase- independent pathway for 14-3-3 removal from Cdc25. We show here that this dissociation of 14-3-3 from Cdc25 requires the activity of the cyclin-dependent kinase Cdk2, providing a molecular explanation for the previously reported requirement for Cdk2 in promoting mitotic entry. Collectively, our data clarify several steps important for Cdc25 activation and provide new insight into the role of PP1 in Cdc2 activation and mitotic entry.The EMBO Journal 12/2003; 22(21):5734-45. · 9.82 Impact Factor
- Journal of the Chemical Society 01/1963;
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ABSTRACT: Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.Journal of Clinical Investigation 12/2005; 115(11):3193-204. · 12.81 Impact Factor