Differential distribution of cAMP-dependent protein kinase isoforms in various tissues of the bivalve mollusc Mytilus galloprovincialis

Insights into the regulation of the MXR response in haemocytes of the Mediterranean mussel (Mytilus galloprovincialis)

This study investigated functional and transcriptional modulation of the Multixenobiotic resistance (MXR) system as a cytoprotective mechanism contributing to the physiological chemoresistance of haemocytes in the Mediterranean mussel. Basal transport activity was assessed using the model substrate rhodamine 123 and specific inhibitors for the MXR-related transporters P-glycoprotein (ABCB mRNA) and Multidrug resistance-related protein (ABCC mRNA). Results showed that MXR activity in mussel haemocytes was mainly supported by the Mrp-mediated efflux. In agreement, ABCC was expressed at higher levels than ABCB. Activation of the cyclic-AMP (cAMP) dependent protein kinase A (PKA) resulted in increased rhodamine efflux, which was counteracted by the selective PKA inhibitor H89. Although serotonin, a physiological modulator of cAMP/PKA signaling and ABCB transcription in haemocytes, did not affect basal MXR transport, the environmental pharmaceuticals fluoxetine, propranolol, and carbamazepine, which interact in different ways with the adrenergic and serotoninergic pathways, were showed to act as modulators and substrates of MXR-related transporters and to affect cell viability. While the increased MXR activity may have lowered the cytotoxic effects of propranolol and carbamazepine, the lack of MXR efflux induction by fluoxetine may play a role in the observed cytotoxicity of the compound.

Cyclic-AMP mediated regulation of ABCB mRNA expression in mussel haemocytes

Background

The multixenobiotic resistance system (MXR) allows aquatic organisms to cope with their habitat despite high pollution levels by over-expressing membrane and intracellular transporters, including the P-glycoprotein (Pgp). In mammals transcription of the ABCB1 gene encoding Pgp is under cAMP/PKA-mediated regulation; whether this is true in mollusks is not fully clarified.

Methodology/Principal Findings

cAMP/PKA regulation and ABCB mRNA expression were assessed in haemocytes from Mediterranean mussels (Mytilus galloprovincialis) exposed in vivo for 1 week to 0.3 ng/L fluoxetine (FX) alone or in combination with 0.3 ng/L propranolol (PROP). FX significantly decreased cAMP levels and PKA activity, and induced ABCB mRNA down-regulation. FX effects were abolished in the presence of PROP. In vitro experiments using haemocytes treated with physiological agonists (noradrenaline and serotonin) and pharmacological modulators (PROP, forskolin, dbcAMP, and H89) of the cAMP/PKA system were performed to obtain clear evidence about the involvement of the signaling pathway in the transcriptional regulation of ABCB. Serotonin (5-HT) decreased cAMP levels, PKA activity and ABCB mRNA expression but increased the mRNA levels for a putative 5-HT₁ receptor. Interestingly, 5-HT₁ was also over-expressed after in vivo exposures to FX. 5-HT effects were counteracted by PROP. Forskolin and dbcAMP increased PKA activity as well as ABCB mRNA expression; the latter effect was abolished in the presence of the PKA inhibitor H89.

Conclusions

This study provides the first direct evidence for the cAMP/PKA-mediated regulation of ABCB transcription in mussels.

Interactive effects of nickel and chlorpyrifos on Mediterranean mussel cAMP-mediated cell signaling and MXR-related gene expressions

The aim of this study was to infer putative interactive effects of a binary mixture between nickel (Ni) and chlorpyrifos (CHP) on mussel cell signaling, and also to unravel downstream effects on transcriptional regulation mediating cytoprotective responses. Mussels were exposed for 4 days to Ni (0.77 mg/L), CHP (4.5 mg/L), or the mixture Ni/CHP (0.135 mg/L Ni and 0.61 mg/L CHP). Cyclic AMP content and PKA activity in gills were evaluated as biological endpoints related to cell signaling. Expression of the MgPgp (ABCB1) and MgMvp genes was also assessed as involved in the mussel MXR mechanism. Levels of cAMP and PKA activities were not modified in mussels exposed to Ni or CHP, whereas they significantly increased in organisms exposed to the mixture. Similar responses were also detected for MgPgp expression, which is thought to be under cAMP/PKA-mediated regulation. Expression of MgMvp was unaffected by CHP or Ni/CHP exposure, and increased by Ni. The differential regulation of MgPgp and MgMvp expressions could be ascribed to the different intracellular localization and function of the two transporters. On the whole, present data indicated that Ni and CHP elicited interactive effects on mussel physiology, both at the signal transduction and at the gene expression levels.