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Measurement of P-glycoprotein Function
1Department of Medical Oncology, Free University Hospital, Amsterdam, The Netherlands.
Abstract:
The MDR1-encoded P-glycoprotein (Pgp), when overexpressed in tumor cells, confers resistance to many clinically important classes of anticancer drugs. This phenomenon is called multidrug resistance (MDR). The finding that this gene was expressed in many types of human cancers has stimulated many studies into the relevance of this protein for clinical chemotherapy resistance (1). Pgp is a protein that causes a net transport of substrate drug molecules over the plasma membrane of the cell, resulting in a lowered free cytosolic drug concentration. Therefore, the drug target(s) "feel" a lower drug concentration, resulting in less drug-induced damage and cell-kill. The measurement of the active (adenosine triphosphate [ATP]-dependent) drug transport or efflux function of Pgp is, therefore, a theoretically elegant way to quantify the number of active or "functional" Pgp molecules per cell (2,3). Such assays are called functional assays in this chapter. One disadvantage is, however, that without additional data no unambiguous evidence of the molecular nature of the transport protein is obtained, the advantage is that the relevant biological feature is measured. The latter cannot be derived easily from the mRNA or protein expression levels, because a number of factors may influence the net effect of transporter proteins, such as other membrane properties specific to certain cells (3,4). In practice, important criteria for the usefulness of any MDR assay are its specificity, sensitivity, and reproducibility.
Insights
Multidrug resistance (MDR) in cancer, driven by P-glycoprotein (Pgp), reduces chemotherapy effectiveness. Functional assays measuring Pgp
Area of Science:
- Molecular Biology
- Cancer Research
- Pharmacology
Background:
- Overexpression of the MDR1 gene-encoded P-glycoprotein (Pgp) in tumor cells leads to multidrug resistance (MDR).
- Pgp confers resistance to numerous anticancer drugs by actively transporting them out of cells, lowering intracellular drug concentrations.
- This phenomenon significantly impacts the efficacy of clinical chemotherapy.
Purpose of the Study:
- To explore the relevance of P-glycoprotein (Pgp) in clinical chemotherapy resistance.
- To evaluate functional assays as a method for quantifying active Pgp molecules.
- To discuss the advantages and disadvantages of functional assays for MDR assessment.
Main Methods:
- Measurement of active, adenosine triphosphate (ATP)-dependent drug transport or efflux function of Pgp.
- These are referred to as functional assays.
- Comparison of functional assays with mRNA and protein expression levels.
Main Results:
- Functional assays directly measure the biological activity of Pgp, reflecting its impact on drug resistance.
- Expression levels (mRNA or protein) do not always correlate with functional Pgp activity due to other cellular factors.
- Specificity, sensitivity, and reproducibility are crucial criteria for the utility of MDR assays.
Conclusions:
- Functional assays offer a direct measure of Pgp's drug efflux capability, crucial for understanding MDR.
- While expression levels are informative, they may not fully represent the functional impact of Pgp.
- The choice of assay for MDR should prioritize specificity, sensitivity, and reproducibility for clinical relevance.
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