ABSTRACT
Several lines of evidence including reverse transcription polymerase chain reaction, immunoreactivity and their ability to efflux rhodamine 123 have implied the existence of P-glycoprotein in natural killer (NK) cells. It has been a natural tendency to assume that NK-cell P-glycoprotein is identical to the P-glycoprotein of multidrug resistant (MDR) cell lines, however, the present study uncovered major differences. Functionally, NK cells demonstrated a restricted substrate profile, being unable to transport daunorubicin and calcein acetoxymethylester while efficiently transporting other P-glycoprotein substrates. Furthermore, physical differences in NK-cell P-glycoprotein were established by differential reactivity with P-glycoprotein antibodies. NK cells demonstrated strong reactivity with C494 and JSB-1, but did not react appreciably with C219. In addition, NK cells were unable to bind to the antibody MM4.17 unless they had been fixed and permeabilized, yet this antibody normally recognizes an extracellular epitope of P-glycoprotein. These differences culminated in the demonstration using Western analysis that NK cells did not express detectable levels of 170 kDa P-glycoprotein. Instead, NK cells expressed small-molecular-weight 'mini P-glycoprotein' products, of approximately 70 and 80 kDa. Collectively, these data indicate that the predominant P-glycoprotein species of NK cells are novel mini P-glycoproteins and not the classic P-glycoprotein of MDR models.
Subject(s)
ATP Binding Cassette Transporter, Subfamily B/analysis , Killer Cells, Natural/metabolism , ATP Binding Cassette Transporter, Subfamily B/chemistry , ATP Binding Cassette Transporter, Subfamily B/immunology , Antibodies, Monoclonal/metabolism , Antineoplastic Agents, Phytogenic/pharmacology , Biological Transport , Calcium Channel Blockers/pharmacology , Cell Line , Cell Membrane/chemistry , Cytoplasm/chemistry , Drug Resistance, Multiple , Flow Cytometry , Fluorescent Dyes/metabolism , Humans , Killer Cells, Natural/immunology , Molecular Weight , Verapamil/pharmacology , Vinblastine/pharmacologyABSTRACT
The discovery of P-glycoprotein has revealed a fundamental mechanism by which cancer cells evade chemotherapy and this principle has proven relevant to general cellular defence mechanisms in normal physiology. To date this knowledge has promised to improve current cancer chemotherapy through the manipulation of drug combinations according to the P-glycoprotein status of the tumor. Furthermore, the discovery of inhibitors of the protein may provide new therapeutic tools in the treatment of multidrug resistant neoplasia, provided the benefits are deemed greater than the potential detrimental side effects. When looking towards future therapies, however, we must also consider additional mechanisms which undoubtedly contribute to clinical drug resistance. Complete elucidation of this complex cellular defence network will hopefully translate into therapeutic opportunities to circumvent all mechanisms of multidrug resistance, thus positively impacting on patient survival.
