Potent cytotoxicity of the phosphatase inhibitor microcystin LR and microcystin analogues in OATP1B1- and OATP1B3-expressing HeLa cells

Abstract

Microcystins are a family of cyclic peptides that are potent inhibitors of the protein phosphatase families PP1 and PP2A. Only three human proteins are thought to be able to mediate the hepatic uptake of microcystins (the organic anion-transporting polypeptides OATP1B1, OATP1B3, and OATP1A2), and the predominant hepatic expression of these transporters accounts for the liver-specific toxicity of microcystins. A significant obstacle in the study of microcystins as anticancer drugs is the requirement of specific transport proteins for cellular uptake. We report that OATP1B3 mRNA is up-regulated in non -small cell lung cancer tumors in comparison with normal control tissues. This finding led to the exploration of microcystins as potential anticancer agents. We have developed a HeLa cell model with functional OATP1B1 and OATP1B3 activity. Transiently transfected HeLa cells are over 1,000-fold more sensitive to microcystin LR than the vector-transfected control cells, showing that transporter expression imparts marked selectivity for microcystin cytotoxicity. In addition, microcystin analogues showed variable cytotoxicities in the OATP1B1-and OATP1B3-transfected cells, including two analogues with IC 50 values <1 nmol/L. Cytotoxicity of microcystin analogues seems to correlate to the inhibition of PP2A in these cells and induces rapid cell death as seen by chromatin condensation and cell fragmentation. These studies show that microcystin-induced phosphatase inhibition results in potent cytotoxicity when microcystin compounds can gain intracellular access and are a potent novel class of therapeutic agents for tumors expressing these uptake proteins. [Mol Cancer Ther 2007;6(2):587 -98]

Role of Organic Anion-Transporting Polypeptides (OATPs) in Cancer Therapy

“…Since the first report on the decreased OATP1B1 protein levels in HCC cell lines (67), many subsequent investigations have reported similar findings using HCC tissue samples and cell lines (37)(38)(39)(40). However, there is also a report noting no significant reduction of OATP1B1 in HCC tissues compared to nonmalignant liver tissues (41).…”

“…It should be noted that these investigations did not include the control group with no cancer, so the results do not provide evidence on differential expression of OATP1B1 or OATP1B3 in hepatic cancer tissues relative to the nonmalignant liver tissues. Other reports indicate that the expression of OATP1B1 and OATP1B3 is reduced in hepatic cancer tissues compared to nonmalignant liver cells (37)(38)(39)(40)(41).…”

Review and analysis of occurrence, exposure and toxicity of cyanobacteria toxins in food

“…The inhibitory capacity of single MC congeners on PP1 and PP2A in vitro is comparable,with IC 50 values in the nanomolar range (Hoeger et al, 2007;Monks et al, 2007;Fischer et al, 2010;Vesterkvist et al,2012).…”

A review of neurotoxicity of microcystins

“…It is assumed that different MC congener transportations are decisively different by OATPs/Oatps (Feurstein et al 2009;Fischer et al 2010;Monks et al 2007). Most of the MCs accumulating in the brain of rat were MC-RR, and only the trace amount of MC-LR was observed, indicating a specific inhibition mechanism in the BBB of the rats (Wang et al 2008).…”