Abstract
Chloroquine (CQ) and other quinoline-containing antimalarials are important drugs with many therapeutic benefits as well as adverse effects. However, the molecular targets underlying most such effects are largely unknown. By taking a novel functional genomics strategy, which employs a unique combination of genome-wide drug-gene synthetic lethality (DGSL), gene-gene synthetic lethality (GGSL), and dosage suppression (DS) screens in the model organism Saccharomyces cerevisiae and is thus termed SL/DS for simplicity, we found that CQ inhibits the thiamine transporters Thi7, Nrt1, and Thi72 in yeast. We first discovered a thi3Δ mutant as hypersensitive to CQ using a genome-wide DGSL analysis. Using genome-wide GGSL and DS screens, we then found that a thi7Δ mutation confers severe growth defect in the thi3Δ mutant and that THI7 overexpression suppresses CQ-hypersensitivity of this mutant. We subsequently showed that CQ inhibits the functions of Thi7 and its homologues Nrt1 and Thi72. In particular, the transporter activity of wild-type Thi7 but not a CQ-resistant mutant (Thi7T287N) was completely inhibited by the drug. Similar effects were also observed with other quinoline-containing antimalarials. In addition, CQ completely inhibited a human thiamine transporter (SLC19A3) expressed in yeast and significantly inhibited thiamine uptake in cultured human cell lines. Therefore, inhibition of thiamine uptake is a conserved mechanism of action of CQ. This study also demonstrated SL/DS as a uniquely effective methodology for discovering drug targets.
Highlights
Chloroquine (CQ) and other quinoline-containing compounds have been major antimalarial drugs for many decades
We demonstrated that CQ completely inactivates a human thiamine transporter (SLC19A3) [14,15] expressed in yeast cells and significantly inhibited thiamine uptake in HeLa and HT1080 cells, suggesting that such a mechanism(s) of action (MOA) is conserved across species
By using a novel SL/dosage suppression (DS) methodology in the model organism yeast, we discovered that the antimalarial drug CQ inhibits thiamine transporters and causes thiamine deficiency and growth defects
Summary
Chloroquine (CQ) and other quinoline-containing compounds have been major antimalarial drugs for many decades They are effective treatments for systematic lupus erythematosus, rheumatoid arthritis, and many other rheumatic and skin diseases [1]. By centering on the thi3D mutation, we performed genome-wide gene-gene synthetic lethality (GGSL) and dosage suppression (DS) screens and discovered the high affinity thiamine transporter Thi7 [12] as a candidate target of CQ. This unique combination of DGSL, GGSL, and DS screens was termed SL/DS. We showed that CQ likely inhibits the low affinity thiamine transporters Nrt and Thi72 [13] in yeast This MOA is shared by other quinoline-containing antimalarials. This study demonstrated that SL/DS is an effective strategy for drug target identification, especially for discovering non-essential genes as drug targets
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