Multidrug resistance proteins which catalyse the detoxification of xenobiotics and excretion of metabolites (i.a. in the liver and in the kidney) are very often controlled at the transcriptional level by interaction of exogenous compounds or hormones with nuclear receptors. Although glucocorticoid hormones have also been implicated in the control of expression of proteins from the MRP family, data on their influence on the level of individual proteins from this group in various cell types is far from unequivocal. Since synthetic glucocorticoids have found extensive use as anti-inflammatory drugs, also in the inhaled form in the treatment of asthma, lung cancer is one of the kinds of cancer potentially most prone to transcriptional induction of multidrug resistance proteins by these steroids. MRP3 is the major basolateral active anionic conjugate transporter in hepatocytes and is also expressed at high levels in cells derived from other lineages, including cancer cells where its upregulation may play a significant role in the emergence of clinical multidrug resistance. While it has been found that dexamethasone does not influence mrp3 mRNA levels in cultured rat hepatocytes (Luttringer et al., 2003), no systematic study was performed in human lung-derived cells. Therefore, we embarked on a study of MRP3 regulation by the glucocorticoids most commonly used in clinical practice (dexamethasone, hydrocortisone, prednisone) in a non-small-cell lung cancer cell line A549 which is commonly used as a model of airway epithelium in studies on glucocorticoid action (e.g. Croxtall et al., 2002).

A549 cells, grown in standard conditions, were challenged with glucocorticoids at physiologically and therapeutically relevant concentrations for 24 h and changes in MRP3 expression were followed on three levels: promoter regulation (luciferase reporter construct with a -1958/-13 fragment of the MRP3 gene), mRNA level (semi-quantitative RT-PCR) and activity (cellular transport of the model substrate calcein). We found that the basal expression of MRP3 in A549 cells is relatively high (higher than for MRP1 or MRP2). Dexamethasone and hydrocortisone in the submicromolar concentration range caused a two-fold induction of transcriptional activity at the MRP3 promoter construct, even though its sequence shows no obvious consensus glucocorticoid response elements. Prednisone was much less effective, causing a 1.25-fold induction. A similar induction pattern was observed with regard to mRNA level with no significant induction by prednisone and a two- to threefold increase of MRP3 message after treatment with dexamethasone or hydrocortisone. With regard to organic anion transport activity, all investigated glucocorticoids caused a modest stimulation of this process, but prednisone was more effective than hydrocortisone or dexamethasone at a concentration of 200 nM (135% vs. 115% of control activity), while the relation was reversed at a concentration of 1 µM (120% vs. 140%) or higher.

We conclude that glucocorticoids used in clinical practice have the ability to transcriptionally upregulate human MRP3 gene expression in lung-derived cells where this protein is a major component of the organic anion extrusion system. This phenomenon has to be taken into account when designing treatments for lung cancer and is especially important for patients who have cancer and at the same time are treated with glucocorticoids against inflammatory symptoms (e.g. in asthma). The concentrations at which induction was observed and the ostensible selectivity with regard to molecular species of glucocorticoid seem to point to a mechanism of induction different than simple transactivation by the activated glucocorticoid receptor (GR) and involving secondary activation by gene products induced by the primary glucocorticoid response or signalling cross-talk. Significant induction of transport activity by low concentrations of prednisone, on the other hand, may be explained by GR-mediated induction of a different multidrug resistance gene, e.g. for MRP2 (which has a canonical GRE in its promoter).

Croxtall, JD, van Hal, PT, Choudhury, Q, Gilroy, DW, Flower, RJ (2002) Different glucocorticoids
vary in their genomic and non-genomic mechanism of action in A549 cells. Br J Pharmacol. 135(2):511-9.
Luttringer, O, Theil, FP, Lave, T, Wernli-Kuratli, K, Guentert, TW & de Saizieu, A (2002) Influence of isolation procedure, extracellular matrix and dexamethasone on the regulation of membrane transporters gene expression in rat hepatocytes. Biochem Pharmacol. 64(11):1637-50.