Drug Sequestration in Lysosomes as One of the Mechanisms of Chemoresistance of Cancer Cells and the Possibilities of Its Inhibition

Abstract

Resistance to chemotherapeutics and targeted drugs is one of the main problems in successful cancer therapy. Various mechanisms have been identified to contribute to drug resistance. One of those mechanisms is lysosome-mediated drug resistance. Lysosomes have been shown to trap certain hydrophobic weak base chemotherapeutics, as well as some tyrosine kinase inhibitors, thereby being sequestered away from their intracellular target site. Lysosomal sequestration is in most cases followed by the release of their content from the cell by exocytosis. Lysosomal accumulation of anticancer drugs is caused mainly by ion-trapping, but active transport of certain drugs into lysosomes was also described. Lysosomal low pH, which is necessary for ion-trapping is achieved by the activity of the V-ATPase. This sequestration can be successfully inhibited by lysosomotropic agents and V-ATPase inhibitors in experimental conditions. Clinical trials have been performed only with lysosomotropic drug chloroquine and their results were less successful. The aim of this review is to give an overview of lysosomal sequestration and expression of acidifying enzymes as yet not well known mechanism of cancer cell chemoresistance and about possibilities how to overcome this form of resistance.

Keywords: V-ATPase; V-ATPase inhibitors; chemoresistance of cancer cells; lysosomal sequestration; lysosomotropic agents; metallothioneins.

Figure 1

The roles of lysosomes and V-ATPase in cancer cells. (1) Protons produced by glycolysis (Warburg’s effect) are transported across the membrane from the cell by V-ATPase which prevents intracellular acidosis. (2) Proteases are activated in lysosomes and secretory vesicles and transported from the cell by exocytosis. Low extracellular pH in cancer microenvironment is optimal for their activity. Proteases are important for cancer cells invasiveness and angiogenesis. (3) Lysosomes and V-ATPase are important for receptor recycling. (4) Lysosomes are important for influx and efflux of different drugs. (5) Lysosomes and V-ATPase process signal molecules that regulate signal pathway. SM—signal molecule. The structures used in figure are vacuolar ATPase [Protein Data Bank (PDB) entry 5vox] and human cathepsin K (PDB entry 5J94).

Figure 2

Detection of cellular viability by morphology-original magnification 100× (a) and Alamar Blue- (b) both in UKF-NB-4 sensitive and cisplatin-resistant UKF-NB-4^CDDP neuroblastoma cells treated with 100 nM bafilomycin A, 20 μM cisplatin, or a combination of both for 24 h. * p < 0.05 ** p < 0.01 CTRL- control, BAF- bafilomycin A, CDDP- cisplatin.

Figure 3

Confocal microscope images demonstrate co-localization (yellow) of ellipticine (green) and LysoTracker (red), (marker of the acidic lysosomal compartment) in UKF-NB-4 cells. Ellipticine is present (sequestrated) in lysosomes. Cells were incubated with ellipticine with or without bafilomycin A (BafA) Pretreatment of the UKF-NB-4 cells with bafilomycin A prior to ellipticine decreased amounts of formed vacuoles. Nuclei were stained with Hoechst 33342 (Hoechst). Yellow arrows- co-localization of ellipticine a LysoTracker. Original magnification 1000×. Photo M. Belhajova, J. Hrabeta.