PKA-mediated transcriptional regulation of the Mediterranean mussel ABCB1 gene involved in the Multixenobiotic Resistance system

Marine mussels display a high capability of adaptation to contaminated environments, due to the activation of different cytoprotective responses. The Multixenobiotic Resistance system (MXR) represents a broadly distributed physiological mechanism acting as a first-tier defense against xenobiotics. Scarce information is available about the regulatory features of the MXR-related proteins in bivalves, including the P-glycoprotein (Pgp), which is the most studied under an environmental context. Investigations on mammals showed that expression of the ABCB1 gene encoding Pgp is under a cAMP/PKA-mediated regulation. In this study haemocytes from the mussel Mytilus galloprovincialis were selected as a cell model to assess whether a similar evolutionary-conserved regulatory framework exists also in bivalves. The experimental data clearly showed that PKA is directly involved in the regulation of ABCB1 gene transcription. Indeed dB-cAMP, the specific PKA activator, increased the enzyme activity and simultaneously up-regulated the mussel ABCB1 gene expression, while H89, the specific PKA inhibitor, counteracted the stimulatory effects of dB-cAMP on both endpoints. Moreover, serotonin, a physiological agonist acting on mussel haemocytes, induced the concerted down-regulation of both PKA activity and ABCB1 gene expression, while noradrenaline was ineffective. The serotonin effect was abolished by propranolol, which proved able to block 5HT receptors. These data provide the first direct demonstration that the mussel ABCB1 gene is under a PKA transcriptional regulation, and also that physiological modulators of the cAMP/PKA system can directly affect the ability of mussels to cope with environmental pollutants.