The methyl ester of okadaic acid is more potent than okadaic acid in disrupting the actin cytoskeleton and metabolism of primary cultured hepatocytes.
OBJECTIVE
Okadaic acid (OA) and microcystins (MCs) are structurally different toxins with the same mechanism of action, inhibition of serine/threonine protein phosphatases (PPs). Methyl okadaate (MeOk), a methyl ester derivative of OA, was considered almost inactive due to its weak inhibition of PP1 and PP2A. Here, we have investigated the activity and potency of MeOk in hepatic cells in comparison with that of OA and MCs.


METHODS
We tested the effects of MeOK, OA and microcystin-leucine and arginine (MC-LR) on the metabolic rate, the actin cytoskeleton and glucose uptake in a rat hepatocyte cell line (Clone 9) and in primary cultured rat hepatocytes. PP2A was assayed to compare OA and MeOk activity.


RESULTS
MeOk disrupted the actin cytoskeleton and depressed the metabolic rate of both types of rat hepatocytes, being six-fold less potent than OA in Clone 9 cells but nearly six-fold more potent in primary cultured hepatocytes. However, unlike OA, MeOk did not change glucose uptake in these cells, suggesting a weak inhibition of PP2A, as confirmed in direct assays of PP2A activity.


CONCLUSIONS
Although MeOk was originally described as a weakly bioactive molecule, it clearly depressed the metabolic rate and disrupted the cytoskeleton in primary and immortalized rat hepatocytes. Furthermore, MeOk affected primary hepatocytes at much lower concentrations than those affecting immortalized cells. These effects were unrelated to PP2A inhibition. Our results suggest the risk to public health from MeOk in foodstuffs should be re-evaluated.
Espiña
Begoña
B
Departamento de Farmacologia, Facultad de Veterinaria, Universidad de Santiago de Compostela, 27002 Lugo, Spain.
Louzao
M Carmen
MC
Cagide
Eva
E
Alfonso
Amparo
A
Vieytes
Mercedes R
MR
Yasumoto
Takeshi
T
Botana
Luis M
LM
D000199:Actins / Q000187:drug effects* / Q000648:ultrastructure; D000818:Animals; D002478:Cells, Cultured; D003599:Cytoskeleton / Q000187:drug effects* / Q000648:ultrastructure; D004988:Ethers, Cyclic / Q000494:pharmacology*; D005947:Glucose / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D008387:Marine Toxins; D052998:Microcystins / Q000494:pharmacology; D019319:Okadaic Acid / Q000494:pharmacology*; D054648:Protein Phosphatase 2 / Q000037:antagonists & inhibitors*; D051381:Rats
D000199:Actins; D004988:Ethers, Cyclic; D008387:Marine Toxins; D052998:Microcystins; D019319:Okadaic Acid; C066239:methyl okadaate; D054648:Protein Phosphatase 2; C057862:cyanoginosin LR; D005947:Glucose