A systems framework for investigating the roles of multiple transporters and their impact on drug resistance.

Efflux transporters are a fundamental component of both prokaryotic and eukaryotic cells, play a crucial role in maintaining cellular homeostasis, and represent a key bridge between single cell and population levels. From a biomedical perspective, they play a crucial role in drug resistance (and especially multi-drug resistance, MDR) in a range of systems spanning bacteria and human cancer cells. Typically, multiple efflux transporters are present in these cells, and the efflux transporters transport a range of substrates (with partially overlapping substrates between transporters). Furthermore, in the context of drug resistance, the levels of transporters may be elevated either due to extra or intracellular factors (feedforward regulation) or due to the drug itself (feedback regulation). As a consequence, there is a real need for a transparent systems-level understanding of the collective functioning of a set of transporters and their response to one or more drugs. We develop a systems framework for this purpose and examine the functioning of sets of transporters, their interplay with one or more drugs and their regulation (both feedforward and feedback). Using computational and analytical work, we obtain transparent insights into the systems level functioning of a set of transporters arising from the interplay between the multiplicity of drugs and transporters, different drug-transporter interaction parameters, sequestration and feedback and feedforward regulation. These insights transparently arising from the most basic consideration of a multiplicity of transporters have broad relevance in natural biology, biomedical engineering and synthetic biology. Insight, Innovation, Integration: Innovation: creating a structured systems framework for evaluating the impact of multiple transporters on drug efflux and drug resistance. Systematic analysis allows us to evaluate the effect of multiple transporters on one/more drugs, and dissect associated resistance mechanisms. Integration allows for elucidation of key cause-and-effect relationships and a transparent systems-level understanding of the collective functioning of transporters and their impact on resistance, revealing the interplay of key underlying factors. Systems-level insights include the essentially different behaviour of transporters as part of a group; unintuitive effects of influx; effects of elevated transporter-levels by feedforward and drug-induced mechanisms. Relevance: a systems understanding of efflux, their role in MDR, providing a framework/platform for use in designing treatment, and in synthetic biology design.