Drug resistance is a major hurdle to the success of chemotherapy. The permeability glycoprotein (P-gp) is an
important factor dictating drug access to the cells, as it controls the efflux of chemotherapeutic agents against the concentration
gradient. Pmd1, a P-gp-like protein, was recently isolated as a doxorubicin resistance gene in fission yeast. Although
the null mutant of pmd1 (Δpmd1) exhibited sensitivity to doxorubicin, it showed an unexpectedly high resistance
to the drug at relatively high concentrations. The data presented here suggest that this is due to the presence of cooperative
processes that can complement and counteract drug cytotoxicity in the absence of Pmd1. One such factor, Rav1, is an essential
factor in controlling the assembly of the pH-regulating transporter vacuolar-ATPase (V-ATPase) in fission yeast.
The simultaneous disruption of Pmd1 and Rav1 resulted in a prominent accumulation of doxorubicin in the cytoplasm of
cells, accompanied by a decline in cell viability. With concurrent treatment of pharmacological inhibitors in human cervical
cancer cells, P-gp and V-ATPase were further shown to act synergistically to sensitize cells to doxorubicin also in the
human cells. Furthermore, a novel Cornichon-like protein SPAC2C4.05 (herein named as Cor1) was demonstrated for the
first time to be involved in the interaction with P-gp and V-ATPase to counteract doxorubicin-dependent cytotoxicity.
Therefore this study identified a molecular cooperation between multiple membrane transporter proteins that confers
chemoresistance to cells against the chemical insult of doxorubicin. Interestingly, this network exhibited differential effects
to doxorubicin as compared with its close epimeric analog epirubicin, suggestive of the intricacy of the drug response
regulated by this synergistic interaction. A model is discussed on how the versatility of this network can differentiate
closely related chemical drug structures yet allow for the robustness to counteract a vast range of drugs.