New approaches to assessing P-glycoprotein expression and functional activity

In this review, we describe a new phenomenon (referred to as "UIC2 shift") related to conformational changes in the epitope structure of human P-glycoprotein (Pgp), an ATP-dependent membrane efflux pump encoded by the MDR1 gene. UIC2 shift is defined as the increase in immunoreactivity of a functional monoclonal antibody, UIC2, with the extracellular Pgp epitopes in the presence of Pgp transport substrates under physiological conditions. We propose a model in which Pgp can be presented on the cell membrane in two different conformations. The first conformation, recognized by UIC2 in the absence of Pgp substrates, is characterized by the bound ATP and absence of Pgp efflux function. The second conformation (associated with UIC2 shift) can be detected by UIC2 in the presence of Pgp substrates and is characterized by unbound ATP and active Pgp effux function. The described phenomenon was used to design a series of functional in vitro assays aimed at improving the detection of MDR] expression and function and screening for new Pgp modulators. Specific examples of UIC2 shift-based flow cytometry tests and drug screening platfoms are described to demonstrate the utility of this assay in clinical studies and biotechnology applications.