Identification and characterisation of a multidrug resistance-related protein mRNA in the blue mussel Mytilus edulis

Membrane-associated transport proteins were discovered in the 1970s in mammals and were considered to be expressed in response to chemotherapy during cancer treatment. Prominent members of this class of proteins are multidrug resistance-related or -associated proteins (MRPs). Besides their expression in cancer cells, MRPs are ubiquitously expressed in normal tissues and are active transporters of reduced glutathione, glucuronate and organic anions of toxicological relevance, either conjugated or unconjugated with sulphate. MRPs may also provide aquatic organisms with resistance to chemicals in a polluted environment by binding xenobiotics and excreting them from cells in an energy-dependent manner. The present study investigated expression of MRPs as part of the mulfixenobiotic resistance (MXR) system in the blue mussel Mytilus edulis. We isolated and characterised 2 putative mrp cDNA fragments, mrp1 and mrp2, which showed 50 to 70% homology on the protein level with MRPs of other species. The mrp1 fragment could not be linked with any mRNA in Northern blots of M. edulis tissues, whereas the mrp2 fragment hybridised with an mRNA of approximately 4.6 kb. Mrp2 showed tissue-specific expression patterns. Highest expression was found in digestive gland and gill tissue. Its expression could be induced 2-fold by the model carcinogen 2-acetylaminofluorene (AAF), whereas mrp1 expression was unaffected. The cDNA fragment of the inducible form was then integrated into a multiplex PCR system for analysis of multixenobiotic resistance in the blue mussel, in concert with other detoxification and biotransformation genes.