The multixenobiotic defense mechanism in mussels is induced by substrates and non-substrates: Implications for a general stress response

A multixenobiotic defense mechanism similar to the p-glycoprotein multidrug transporter has been identified and characterized in several marine invertebrates. Its suggested role is protection from xenobiotics, both natural and anthropogenic. To assess whether MXR functions as a defense mechanism against environmental toxins, studies were undertaken to determine the inducibility of the transporter by environmental toxins. Both MXR substrates (including the pesticide chlorthal) and non-substrates (including Arochlor 1254) appeared to induce MXR activity and protein titer in the gill tissues of the mussel Mytilus californianus. Mussels exposed for 3 days to subacute concentrations of these compounds showed increased ability to extrude a dye substrate from the cells and exhibited a stronger band at 170 kDa on a Western blot relative to animals exposed to clean sea-water. These results suggest that in mussels, MXR induction may be part of a general cellular stress response to organic toxins.