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 lM. The inhibition of intracellular PP2A in cultured cells is obviously observed with 20 lM 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. (Cancer Sci 2010; 101: 743-750) T he phosphorylation of intracellular protein plays a central role in the regulation of eukaryotic cell function and metabolism. Protein phosphatase (PP)2A is a ubiquitous serine ⁄ threonine phosphatase in eukaryotic cells, which regulates the phosphorylation state and function of a broad range of proteins.(1,2) We previously isolated a PP2A-specific inhibitor, cytostatin, from an actinomycete culture.(3-5) Its analog, fostriecin, and other PP2A-specific inhibitors have also been reported since mid 1990s.(6-11) These new inhibitors have higher specificities to PP2A than okadaic acid, a previously identified inhibitor (PP1 IC 50 ⁄ PP2A IC 50 : fostriecin, 10 4

Delimitation and characterisation of <I>Talaromyces purpurogenus </I> and related species

“…This group is also biotechnologically important, because of their production of enzymes (Carvallo et al 2003, Jeya et al 2010) and extrolites. For example, the mycotoxin rubratoxin A & B produced by T. purpurogenus has been shown to act as cancer metastasis suppressors (Wada et al 2010) and spiculisporic acid can be used as a detergent (Ishigami et al 2000). From a biotechnological point of view we would recommend using T. ruber for enzyme production, because T. purpurogenus produces four types of mycotoxins and T. amestolkiae and T. stollii are potentially pathogenic to immuno-compromised persons.…”

“…Three teens drinking homemade rhubarb wine, which had a high level of rubratoxin B became critically ill, with one requiring immediate liver transplant. Even though rubratoxin B has negative health effects, it has potential as an anti-tumor agent (Wang et al 2007, Wada et al 2010). Penicillium crateriforme has also been reported to produce the mouse mycotoxin spiculisporic acid (Oxford & Raistrick 1934, Fujimoto et al 1988).…”

Heterologous Production of Fungal Maleidrides Reveals the Cryptic Cyclization Involved in their Biosynthesis

“…The structures of its isomers glaucanic acid ( 2 )3 and heveadride ( 3 )5 were also solved at that time. More recent examples include phomoidride A ( 4 , an inhibitor of squalene synthase and Ras farnesyl transferase),6 rubratoxins such as 5 (recently shown to suppress cancer metastasis),7 cornexistin ( 6 , which shows potent and selective herbicidal activity),8 and zopfiellin ( 7 ) 9. Heptadrides are also known, for example, agnestadride A ( 8 ), which was isolated along with byssochlamic acid ( 1 ) from Byssochlamys fulva 10…”

Force-inhibiting effect of Ser/Thr protein phosphatase 2A inhibitors on bovine ciliary muscle

“…This could be explained by a different potency to PP1. Okadaic acid would inhibit PP1 with a K i of 147 nmol/l (19), while fostriecin (24) and rubratoxin A (25) would not inhibit PP1 at the concentrations we tested in the present study. In the previous studies, potent PP1 inhibitors, such as calyculin A and tautomycin, induced Ca 2+ -independent contractions in various smooth muscle preparations (28, 29).…”

“…To confirm that those inhibitory effects of okadaic acid were due to specific inhibition of PP2A, we examined other selective PP2A inhibitors, fostriecin (IC 50 = 3.2 nmol/l for PP2A and 131 μmol/l for PP1 (24)) and rubratoxin A (K i = 28.7 nmol/l for PP2A (25)). Fostriecin at a lower concentration (3 μmol/l) completely inhibited ionomycin-induced contraction in BCM (2.0 ± 1.6%, n = 6, P < 0.01, Fig.…”