Abrogation of microcystin cytotoxicity by MAP kinase inhibitors and N-acetyl cysteine is confounded by OATPIB1 uptake activity inhibition

Sub-Chronic Microcystin-LR Liver Toxicity in Preexisting Diet-Induced Nonalcoholic Steatohepatitis in Rats

Microcystin-LR (MCLR) is a hepatotoxic cyanotoxin reported to cause a phenotype similar to nonalcoholic steatohepatitis (NASH). NASH is a common progressive liver disease that advances in severity due to exogenous stressors such as poor diet and toxicant exposure. Our objective was to determine how sub-chronic MCLR toxicity affects preexisting diet-induced NASH. Sprague-Dawley rats were fed one of three diets for 10 weeks: control, methionine and choline deficient (MCD), or high fat/high cholesterol (HFHC). After six weeks of diet, animals received vehicle, 10 µg/kg, or 30 µg/kg MCLR via intraperitoneal injection every other day for the final 4 weeks. Incidence and severity scoring of histopathology endpoints suggested that MCLR toxicity drove NASH to a less fatty and more fibrotic state. In general, expression of genes involved in de novo lipogenesis and fatty acid esterification were altered in favor of decreased steatosis. The higher MCLR dose increased expression of genes involved in fibrosis and inflammation in the control and HFHC groups. These data suggest MCLR toxicity in the context of preexisting NASH may drive the liver to a more severe phenotype that resembles burnt-out NASH.

Altered cellular metabolism of HepG2 cells caused by microcystin-LR

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.

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.

Selectivity and potency of microcystin congeners against OATP1B1 and OATP1B3 expressing cancer cells

Microcystins are potent phosphatase inhibitors and cellular toxins. They require active transport by OATP1B1 and OATP1B3 transporters for uptake into human cells, and the high expression of these transporters in the liver accounts for their selective hepatic toxicity. Several human tumors have been shown to have high levels of expression of OATP1B3 but not OATP1B1, the main transporter in liver cells. We hypothesized that microcystin variants could be isolated that are transported preferentially by OATP1B3 relative to OATP1B1 to advance as anticancer agents with clinically tolerable hepatic toxicity. Microcystin variants have been isolated and tested for cytotoxicity in cancer cells stably transfected with OATP1B1 and OATP1B3 transporters. Microcystin variants with cytotoxic OATP1B1/OATP1B3 IC₅₀ ratios that ranged between 0.2 and 32 were found, representing a 150-fold range in transporter selectivity. As microcystin structure has a significant impact on transporter selectivity, it is potentially possible to develop analogs with even more pronounced OATP1B3 selectivity and thus enable their development as anticancer drugs.

The effect of breast cancer resistance protein, multidrug resistant protein 1, and organic anion-transporting polypeptide 1B3 on the antitumor efficacy of the lipophilic camptothecin 7-t-butyldimethylsilyl-10-hydroxycamptothecin (AR-67) in vitro

AR-67 (7-t-butyldimethylsilyl-10-hydroxycamptothecin) is a lipophilic camptothecin analog, currently under early stage clinical trials. Transporters are known to have an impact on the disposition of camptothecins and on the response to chemotherapeutics in general due to their expression in tumor tissues. Therefore, we investigated the interplay between the breast cancer resistance protein (BCRP), multidrug resistant protein 1 (MDR1), and organic anion-transporting polypeptide (OATP) 1B1/1B3 transporters and AR-67 and their impact on the toxicity profile of AR-67. Using cell lines expressing the aforementioned transporters, we showed that the lipophilic AR-67 lactone form is a substrate for efflux transporters BCRP and MDR1. Additionally, OATP1B1 and OATP1B3 facilitated the uptake of AR-67 carboxylate in SLCO1B1- and SLCO1B3-transfected cell systems compared with the mock-transfected ones. Notably, both BCRP and MDR1 conferred resistance to AR-67 lactone. Prompted by recent studies showing increased OATP1B3 expression in certain cancer types, we investigated the effect of OATP1B3 expression on cell viability after exposure to AR-67 carboxylate. OATP1B3-expressing cells had increased carboxylate uptake as compared with mock-transfected cells but were not sensitized because the intracellular amount of lactone was 50-fold higher than that of carboxylate and comparable between OATP1B3-expressing and OATP1B3-nonexpressing cells. In conclusion, BCRP- and MDR1-mediated efflux of AR-67 lactone confers resistance to AR-67, but OATP1B3-mediated uptake of the AR-67 carboxylate does not sensitize OATP1B3-expressing tumor cells.

Pro- and anti-inflammatory cytokine expression in carp blood and head kidney leukocytes exposed to cyanotoxin stress--an in vitro study

Cyanotoxins are toxic, secondary metabolites produced by different species of cyanobacteria that are present all over the world in aquatic environments. No data are available about the molecular mechanisms underlying the stress associated with exposure of fish immune cells to low concentrations of cyanotoxins. The purpose of this study was to determine whether the expression of cytokines that underlie immune regulation are changed after incubation of fish leukocytes with pure cyanotoxins: microcystin- LR (MC-LR), anatoxin-a (Antx-a), or an extract containing Antx-a. The study investigated the relative gene expression of four important cytokines, IL-1β, TNF-α, IL-10, and TGF-β, in carp head kidney and blood leukocytes exposed to toxins at concentrations of 0.01 or 0.1 μg/ml for 4 h. The data showed that pure toxins could induce dysregulation of pro-/anti-inflammatory cytokine expression. Expression of cytokine IL-1 β was highly upregulated following Antx-a exposure, whereas MC-LR induced merely moderate reactions. The expression of TNF-α mRNA was significantly suppressed in blood and head kidney cells incubated with toxins at the higher concentration. These results showed that pure toxins dysregulated the expression of pro-inflammatory cytokines IL-1β and TNF-α more promptly than the anti-inflammatory cytokines TGF-β and IL-10. In contrast, the studies demonstrated a clearly downward trend of pro-inflammatory cytokines and an upward trend of anti-inflammatory cytokines in leukocytes exposed to an extract containing defined concentrations of Antx-a. This study suggests that cyanotoxins present in aquatic environments may exert immunotoxic effects by altering the transcription of important mediators of the fish immune system.

Cyanobacterial cyclopeptides as lead compounds to novel targeted cancer drugs

Cyanobacterial cyclopeptides, including microcystins and nodularins, are considered a health hazard to humans due to the possible toxic effects of high consumption. From a pharmacological standpoint, microcystins are stable hydrophilic cyclic heptapeptides with a potential to cause cellular damage following uptake via organic anion-transporting polypeptides (OATP). Their intracellular biological effects involve inhibition of catalytic subunits of protein phosphatase 1 (PP1) and PP2, glutathione depletion and generation of reactive oxygen species (ROS). Interestingly, certain OATPs are prominently expressed in cancers as compared to normal tissues, qualifying MC as potential candidates for cancer drug development. In the era of targeted cancer therapy, cyanotoxins comprise a rich source of natural cytotoxic compounds with a potential to target cancers expressing specific uptake transporters. Moreover, their structure offers opportunities for combinatorial engineering to enhance the therapeutic index and resolve organ-specific toxicity issues. In this article, we revisit cyanobacterial cyclopeptides as potential novel targets for anticancer drugs by summarizing existing biomedical evidence, presenting structure-activity data and discussing developmental perspectives.