Interaction of anthracyclines with plasma membranes from tumour cells: implications on drug resistance

Disruption of cellular signaling pathways by daunomycin through destabilization of nonlamellar membrane structures

Albeit anthracyclines are widely used in the treatment of solid tumors and leukemias, their mechanism of action has not been elucidated. The present study gives relevant information about the role of nonlamellar membrane structures in signaling pathways, which could explain how anthracyclines can exert their cytocidal action without entering the cell [Tritton, T. R. & Yee, G. (1982) Science 217, 248-250]. The anthracycline daunomycin reduced the formation of the nonlamellar hexagonal (HII) phase (i.e., the hexagonal phase propensity), stabilizing the bilayer structure of the plasma membrane by a direct interaction with membrane phospholipids. As a consequence, various cellular events involved in signal transduction, such as membrane fusion and membrane association of peripheral proteins [e.g., guanine nucleotide-binding regulatory proteins (G proteins and protein kinase C-alpha beta)], where nonlamellar structures (negative intrinsic monolayer curvature strain) are required, were altered by the presence of daunomycin. Functionally, daunomycin also impaired the expression of the high-affinity state of a G protein-coupled receptor (ternary complex for the alpha 2-adrenergic receptor) due to G-protein dissociation from the plasma membrane. In vivo, daunomycin also decreased the levels of membrane-associated G proteins and protein kinase C-alpha beta in the heart. The occurrence of such nonlamellar structures favors the association of these peripheral proteins with the plasma membrane and prevents daunomycin-induced dissociation. These results reveal an important role of the lipid component of the cell membrane in signal transduction and its alteration by anthracyclines.

Cytosolic free Ca2+ in daunorubicin and vincristine resistant Ehrlich ascites tumor cells. Drug accumulation is independent of intracellular Ca2+ changes

The possible role of intracellular calcium on daunorubicin (DNR) accumulation in wild-type (EHR2) and multi-drug resistant (MDR) Ehrlich ascites tumor cell subline was investigated. DNR accumulation was not enhanced either by increasing the concentration of cellular calcium with the calcium ionophore ionomycin nor by chelating the cytosolic free Ca2+ by the membrane permeable Ca2(+)-buffering agents BAPTA or MAPTAM. No effect was observed in the presence of extremely low extracellular calcium concentration that prevent transmembrane calcium influx or when the cells were calcium depleted using EGTA and ionomycin. Using the fluorescent Ca2+ indicator fura-2 it is further shown that both drug-resistant daunorubicin (EHR2/DNR+) and vincristine (EHR/VCR+) sublines had lower (50-80 nM) concentration of cytosolic free calcium ([Ca2+]i) compared to their corresponding wild-type parenteral tumors (140-180 nM). In calcium free medium, however, no significant difference was found, all cell lines having a [Ca2+]i of 60-80 nM. Furthermore, the total amount of Ca2+ released to the cytosol with 10 microM ionomycin and 5 mM EGTA was 3-4-fold higher in EHR2 than in EHR2/DNR+ or EHR2/VCR+. Mobilization of Ca2+ with 1 microM ionomycin was almost identical in the presence and absence of Ca2+ in the extracellular medium in EHR2 as well as in EHR2/DNR+ suggesting that the increase in [Ca2+]i is mainly due to discharge of Ca2+ from intracellular stores. Furthermore, the total cell calcium [Ca2+]t concentration was slightly higher in EHR2/DNR+ and EHR2/VCR+ cells compared to EHR2. Incubation of the cells with the Ca2(+)-channel blocker verapamil or the intracellular Ca2(+)-antagonist TMB-8 causes depression of the Ca2(+)-response in terms of rise in [Ca2+]i caused by ionomycin. Sorcin, a major calcium-binding protein (Mr 22 kDa), is shown to be overproduced in EHR2/DNR+ cells. The overproduction of this protein in resistant cells may be related to the difference in the intracellular calcium observed in this study. Thus, though handling of Ca2+ is different in wild-type and MDR cell lines, our data suggest that calcium is not involved directly in drug transport processes and the level of Ca2+ per se have no influence on drug accumulation.