Multidrug resistance in small cell lung cancer: Expression of P-glycoprotein, multidrug resistance protein 1 and lung resistance protein in chemo-naive patients and in relapsed disease

Summary

The aim of this study was to investigate the expression of multidrug resistance-associated proteins in metastatic small cell lung cancer (SCLC) cells correlated to cisplatin/etoposide chemotherapy response and the level of those proteins in relapsed disease. Samples were obtained by transbronchial fine needle aspiration biopsy (TBNA) of enlarged mediastinal lymph nodes in 17 patients. After cytological confirmation of SCLC, cells were stained by a panel of mAbs against internal epitopes of P-gp (JSB-1), MRP1 (MRPr1), LRP (LRP-56) and cytokeratin (MNF116) and analyzed by flow cytometry. We observed a significant negative correlation for better response rate to chemotherapy with individual expression of P-gp (r = −0.93, P < 0.0001; Pearson correlation) and MRP1 (r = −0.78, P = 0.0002; Pearson correlation) in chemo-naive SCLC cells and a non-significant correlation for LRP expression. P-gp and MRP1 expression was markedly increased in metastatic cells in four out of five patients with relapsed disease (4–12 months after starting chemotherapy), in comparison to their chemo-naive values. In conclusion, the results suggest that P-gp and MRP1 might be associated with SCLC cell survival during metastasis and chemotherapy, and that overexpression of those transporters in relapsed disease could assist short-term chemotherapy efficiency.

Introduction

Drug resistance is one of the most important causes of unsuccessful lung cancer chemotherapy. Some tumours are initially resistant and never respond to cytostatic drug treatment; whereas others become resistant after a good initial response [1]. Multidrug resistance is cross-resistance to some structurally and functionally unrelated naturally derived drugs, such as epipodophyllotoxins, vinka alcaloids, antracyclines, taxanes, colchicines and others, and is mostly caused by overexpression of P-gp and MRP [2], [3].

In small cell lung cancer (SCLC), resistance is usually associated with the emergence of drug-resistant cell clones during chemotherapy [4]. Studies of cell culture systems have established that tumour cells exposed to only a single drug expressed cross-resistance to a broad range of structurally and functionally dissimilar drugs. While various mechanisms may contribute to chemotherapy resistance, the primary ones are the so-called “pump” and “non-pump” forms of resistance [5], [6]. The basic mechanism of non-pump resistance is activation of cellular antiapoptotic defence. Pump resistance or transport-mediated resistance is due to the decreased concentration of the active drug in target cells because of decreased drug uptake or increased drug efflux across tumour cell membranes, or to increased drug efflux through cytoplasmic vesicles. Certain proteins are able to transport toxic materials and drugs across cellular membranes, against a concentration gradient, decreasing their intracellular concentration [7]. A large subclass of these proteins is ATP-binding cassette (ABC) proteins, which are expressed in both normal and malignant cells. The main ABC transporters are P-gp and MRP1. The 170 kDa MDR1 P glycoprotein (P-gp) was the first human ABC transporter to be identified, and therefore most of our knowledge regarding these transporters is based on studies of P-gp. The 190 kDa multidrug resistance protein (MRP1) is probably involved in an antioxidant defence mechanism. Most drug-resistant lung cancers overexpress both P-glycoprotein and MRP1 [1], [8], [9].

While the lung resistance protein (LRP) is not an ABC transporter, it is a major vault protein, and is found in the cytoplasm and nuclear membrane. LRP is responsible for the uptake of drugs in cytoplasmic vesicles that are then probably extruded from the cell by exocytosis. Some investigators have reported that overexpression of LRP correlates with resistance to cisplatin [10].

The aim of our study was to investigate the expression levels of P-gp, MRP1 and LRP at diagnosis in chemo-naive patients and to determine if any correlation exists between expression levels and the overall response to chemotherapy. We also investigated the levels of those proteins in patients with relapsed disease.