Microcystin-LR

IC50: 1.7 nM and 0.04 nM for type 1 and type 2A protein phosphatases, respectivley.

Protein phosphatases 1 and 2A, two of the major phosphatases in eukaryotic cells that dephosphorylate serine and threonine residues. Microcystin-LR is a potent inhibitor of type 1 and type 2A protein phosphatases.

In vitro: Microcystin-LR inhibited protein phosphatases 1 (PP1) and 2A (PP2A) with Ki values below 0.1 nM. Protein phosphatase 2B is inhibited 1000-fold less potently, while six other phosphatases and eight protein kinases tested are unaffected. Microcystin-LR inhibited enzymes in a remarkably similar manner to okadaic acid, a potent tumour promoter that is also the toxin responsible for diarrhetic shellfish poisoning [1].

In vivo: After 100 i.p. injections of a sublethal dose (20 μg/kg) of MCLR, neoplastic nodules were observed without the use of an initiator. Multiple neoplastic nodules up to 5 mm in diameter were observed in the liver of mice in both groups, i.e. those injected 100 times i.p. and those injected 100 times with a 2 month withdrawal. In contrast, when 80 μg/kg was orally administered 100 times, characteristic chronic injuries such as fibrous changes and nodule formation were not observed [2].

Clinical trials: Currenlty no clinical data are available.

References:
[1] MacKintosh C, Beattie KA, Klumpp S, Cohen P, Codd GA.  Cyanobacterial microcystin-LR is a potent and specific inhibitor of protein phosphatases 1 and 2A from both mammals and higher plants. FEBS Lett. 1990 May 21;264(2):187-92.
[2] Ito E, Kondo F, Terao K, Harada K.   Neoplastic nodular formation in mouse liver induced by repeated intraperitoneal injections of microcystin-LR. Toxicon. 1997 Sep;35(9):1453-7.

Quantitative analysis of glutathione and cysteine S-conjugates of microcystin-LR in the liver, kidney and muscle of common carp (Cyprinus carpio) in Lake Taihu.[Pubmed:28362311]

J Water Health. 2017 Apr;15(2):300-307.

Tissue distribution of microcystin (MC)-LR-GSH, MC-LR-Cys and MC-LR of omnivorous fish in Lake Taihu was investigated. MC-LR and MC-LR-Cys were detected in liver, kidney and muscle. The concentration of MC-LR in liver and kidney was 0.052 mug g(-1) DW and 0.067 mug g(-1) DW, respectively. MC-LR-Cys appeared to be an important metabolite with average contents of 1.104 mug g(-1) DW and 0.724 mug g(-1) DW in liver and kidney, and the MC-LR-Cys/MC-LR ratio in liver and kidney reaching as high as 21.4 and 10.8. High MC-LR-Cys/MC-LR ratio and a significant correlation between MC-LR-Cys and MC-LR concentration in liver, suggest that liver is more active in detoxification of MC-LR by formation of MC-LR-Cys for omnivorous fish. Furthermore, there might be a balance between the accumulation and depuration/metabolism of MC-LR-Cys in kidney. The MC-LR-Cys can be formed in kidney directly, or transported from liver or other tissues, while the MC-LR-Cys in kidney might be dissociated to MC-LR or excreted. Although MC-LR and its metabolites were scarcely detected in muscle, it is necessary to investigate the distribution of toxic metabolites in edible muscle.

Microcystin-LR Binds Iron, and Iron Promotes Self-Assembly.[Pubmed:28368104]

Environ Sci Technol. 2017 May 2;51(9):4841-4850.

The microcystin-producing Microcystis aeruginosa PCC 7806 and its close strain, the nonproducing Microcystis aeruginosa PCC 7005, grow similarly in the presence of 17 muM iron. Under severe iron deficient conditions (0.05 muM), the toxigenic strain grows slightly less than in iron-replete conditions, while the nonproducing microcystin strain is not able to grow. Isothermal titration calorimetry performed at cyanobacterial cytosol or meaningful environmental pHs values shows a Microcystin-LR dissociaton constant for Fe(2+) and Fe(3+) of 2.4 muM. Using atomic force microscopy, 40% of Microcystin-LR dimers were observed, and the presence of iron promoted its oligomerization up to six units. Microcystin-LR binds also Mo(6+), Cu(2+), and Mn(2+). Polymeric microcystin binding iron may be related with a toxic cell colony advantage, providing enhanced iron bioavailability and perhaps affecting the structure of the gelatinous sheath. Inside cells, with microcystin implicated in the fitness of the photosynthetic machinery under stress conditions, the toxin would be involved in avoiding metal-dependent Fenton reactions when photooxidation causes disassembly of the iron-rich photosystems. Additionally, it could be hypothesized that polymerization-depolymerization dynamics may be an additional signal that could trigger changes (for example, in the binding of microcystin to proteins).

Altered cellular metabolism of HepG2 cells caused by microcystin-LR.[Pubmed:28336091]

Environ Pollut. 2017 Jun;225:610-619.

This study aimed to evaluate the possible effects of Microcystin-LR (MC-LR) exposure on the metabolism and drug resistance of human hepatocellular carcinoma (HepG2) cells. For this purpose, we first conducted an experiment to make sure that MC-LR could penetrate the HepG2 cell membrane effectively. The transcriptional levels of phase I (such as CYP2E1, CYP3A4, and CYP26B1) and phase II (such as EPHX1, SULTs, and GSTM) enzymes and export pump genes (such as MRP1 and MDR1) were altered by MC-LR-exposure for 24 h, indicating that MC-LR treatment may destabilize the metabolism of HepG2 cells. Further research showed that the CYP inducers omeprazole, ethanol, and rifampicin inhibited cell viability, in particular, ethanol, a CYP2E1 inducer, induced ROS generation, lipid peroxidation, and apoptosis in HepG2 cells treated with MC-LR. The CYP2E1 inhibitor chlormethiazole inhibited ROS generation, mitochondrial membrane potential loss, caspase-3 activity, and cytotoxicity caused by MC-LR. Meanwhile, the results also showed that co-incubation with the ROS scavenger l-ascorbic acid and MC-LR decreased ROS levels and effectively prevented apoptosis. These findings provide an interesting mechanistic explanation of cellular metabolism associated with MC-LR, i.e., MC-LR-exposure exerted toxicity on HepG2 cells and induced apoptosis of HepG2 cells via promoting CYP2E1 expression and inducing excessive ROS in HepG2 cells.

Photorelease of microcystin-LR from resuspended sediments.[Pubmed:28366384]

Harmful Algae. 2017 Mar;63:1-6.

A series of ten photolysis experiments was conducted with sediments exposed to Microcystis sp. blooms to determine if sunlight is capable of mobilizing the biotoxin Microcystin-LR (MC-LR) into the water column. There was a net photorelease of MC-LR in irradiated suspensions in all cases relative to dark controls, ranging from 0.4 to 192mugL(-1)g(-1) into the dissolved phase. This should be viewed as a minimum estimate of photorelease due to concurrent photodegradation of dissolved toxin. Dissolved MC-LR concentrations in a sediment suspension increased linearly in the aqueous phase during a six-hour irradiation with simulated sunlight suggesting that longer exposure times produce greater quantities of MC-LR. There was a significant positive correlation between photorelease of toxin and percent organic carbon of the resuspended material, implying that organic-rich sediments yield the greatest photorelease of MC-LR upon exposure to full spectrum sunlight. Samples exposed to photosynthetically active radiation (400nm-700nm) were responsible for less than 2% of the photorelease compared to full spectrum exposures. Model calculations indicate that photochemical processing of bloom impacted sediments could be responsible for as much as 100% of the average standing stock of MC-LR in a freshwater pond located in southeastern North Carolina, where surface water concentrations were also measured. Mass spectrometric analysis revealed a new peak in light exposed flasks that appears to be a photo-induced isomerized product of MC-LR. Photoproduction from resuspended sediments therefore represents a significant but previously unrecognized source of highly toxic MC-LR and photoproducts of unknown toxicity and fate to aquatic ecosystems.