Effect of microcystin-LR on protein phosphatase 2A and its function in human amniotic epithelial cells

Abstract

Due to their toxicity, the increased distribution of microcystins (MCs) has become an important worldwide problem. MCs have been recognized as inhibitors of protein phosphatase 2A (PP2A) through their binding to the PP2A catalytic subunit. However, the exact mechanism of MC toxicity has not been elucidated, especially concerning the cellular response and its autoregulation. To further dissect the role of PP2A in MC-induced toxicity, the present study was undertaken to determine the response of PP2A in human amniotic epithelial (FL) cells treated with microcystin-LR (MCLR), one of the MC congeners. The results show that a low-dose treatment of MCLR in FL cells for 6 h induced an increase in PP2A activity, and a high-dose treatment of MCLR for 24 h decreased the activity of PP2A, as expected. The increased mRNA and protein levels of the PP2A C subunit may explain the increased activity of PP2A. Furthermore, MCLR altered microtubule post-translational modifications through PP2A. These results further clarify the underlying mechanism how MCLR affects PP2A and may be helpful for elucidating the complex toxicity of MCLR.

A review of reproductive toxicity of microcystins

“…MCs were found to interact with PP catalytic subunits (PP1c and PP2Ac) by a two-step mechanism involving rapid binding and inactivation of the catalytic subunits, followed by a slower covalent interaction (within hours) [93]. Recent reports suggest that MCs modulate PPs activity not only by the direct inhibition of enzyme activity, but also through the regulation of protein expression [16][17][18][19][94][95][96][97]. PP1 and PP2A inhibition induces the disruption of the dynamic equilibrium of protein phosphorylation/dephosphorylation, leading to the damage of numerous cellular processes such as cytoskeleton organization, Wnt, Akt, p38, c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2) of mitogen-activated protein kinase (MAPK) signaling pathways, metabolism and cell cycle [20,[98][99][100][101] (Fig.…”

“…The toxicity of MC-LR and MC-RR on human amnion epithelial FL cells are also documented [94,96,[145][146][147]. However, neurobehavioral impairments were observed in adulthood (PD60) but not in the childhood (PD28) of rat offspring, indicating a potential long-term adverse effect of MC-LR on rat offspring [142].…”

Mechanisms of Microcystin-induced Cytotoxicity and Apoptosis

“…Several studies indicated that MCs modulate PPs activity not only by the direct inhibition of enzyme activity, but also through the regulation of protein expression [45,46]. PP1/2A inhibition induces the imbalance of protein phosphorylation/dephosphorylation, leading to the dysregulation of numerous signaling pathways such as NF-κB, p38, c-Jun N-terminal kinase (JNK), extra-cellular signal-regulated kinase 1/2 (ERK1/2) of mitogen-activated protein kinase (MAPK) [47,48].…”

“…Recently, several studies demonstrated that low concentrations of MC-LR stimulate, rather than inhibit, PP2A activity, challenging the longstanding role of MCs as PP2A inhibitor [46,51,52]. The increased mRNA and protein levels of the PP2A C subunit may explain the increased activity of PP2A [46].…”

Tumor Suppressor Lzap Suppresses Wnt/β-Catenin Signaling to Promote Zebrafish Embryonic Ventral Cell Fates via the Suppression of Inhibitory Phosphorylation of Glycogen Synthase Kinase 3

“…In brief, 1 g of protein extracts of sphere-stage embryos were used for each of protein phosphatases (PP) PP2A, PP2B, and PP2C activity assays. Embryos were homogenized in the buffers specific for PP2A, PP2B, and PP2C (33)(34)(35). After centrifugation, the supernatant was further passed through a Sephadex G-25 resin column to remove free phosphates.…”