Multidrug resistance
protein (MRP) is a member of the
ATP-binding cassette superfamily of
transport proteins which has been demonstrated to cause multidrug resistance when transfected into previously sensitive cells. Sixteen eicosomeric
oligonucleotides complementary to different regions along the entire length of the MRP
mRNA reduced MRP
mRNA and
protein levels in
drug-resistant
small cell lung cancer cells that highly overexpress this
protein. In MRP-transfected HeLa cells that express intermediate levels of MRP, one
oligonucleotide, ISIS 7597, targeted to the coding region of the MRP
mRNA, decreased the levels of MRP
mRNA to < 10% of control levels in a concentration-dependent manner. This effect was rapid but transient with a return to control levels of MRP
mRNA 72 hr
after treatment. A double treatment with ISIS 7597 produced a sustained inhibition, resulting in a greater than 90% reduction in MRP
mRNA for 72 hr and a comparable decrease in
protein levels. Increased sensitivity to
doxorubicin was observed under these conditions. Northern blotting analyses using two
DNA probes corresponding to sequences 5' and 3' of the ISIS 7597 target sequence, respectively, revealed the presence of low levels of two smaller sized
RNA fragments as expected from an
RNase H-mediated mechanism of action of the
antisense oligonucleotide. These studies indicate that a specific reduction in MRP expression can be achieved with
antisense oligonucleotides, a finding that has potential implications for the treatment of
drug-resistant
tumors.