Abstract
Azole resistance in
Candida albicans
may be due to several mechanisms. It has been demonstrated that
C. albicans
possesses sequences with a high degree of homology with the human MDR-1 gene
coding for P-glycoprotein (P-gp), belonging to the ATP-binding cassette transporter (ABC) superfamily and responsible for the multidrug resistance (MDR) in tumor cells. On this basis, the expression and
intracellular localization of human P-gp-like molecule in
C. albicans
strains showing different sensitivity to fluconazole were investigated by flow cytometry and immunoelectron microscopy. Post-embedding
immunolabeling revealed that monoclonal antibody (mAb) MM4.17, which recognizes an external epitope of human P-gp, reacted with both fluconazole-sensitive (3153 and CO 23-1) and fluconazole-resistant (AIDS
68 and CO 23-2, isolated from AIDS patient and
in vitro
drug-selected, respectively) strains of
C. albicans
. However, the resistant strains displayed a number of MM4.17-reactive epitopes much
higher than the drug-sensitive ones. The
C. krusei
ATCC 6458 strain, whose resistance is not mediated by the presence of ABC transporters, was not reactive at all with mAb MM4.17. The specificity
of the immunolabeling was confirmed by a competitive inhibition assay performed by using phage clone particles capable of mimicking the MM4.17-reactive epitope. The flow cytometric analysis confirmed a
higher level of intracytoplasmic P-gp expression in azole-resistant strains of
C. albicans
. Both cyclosporin A and verapamil, which are well-known MDR inhibitors, strongly reduced the MICs for fluconazole
and itraconazole of the tested azole-resistant AIDS 68 strain, while they did not influence the MICs of either the sensitive 3153 strain of
C. albicans
or the ATCC 6458 strain of
C. krusei
.
Overall, our data suggest the existence of a P-gp-like drug efflux pump in
C. albicans
that may participate in the mechanisms of azole-resistance of this fungus.