The expression and significance of P-glycoprotein, lung resistance protein and multidrug resistance-associated protein in gastric cancer

Gastric cancer (GC) is one of the most common malignancies worldwide. Despite noticeable advancements in the prevention, diagnosis and treatment, GC still accounts for over 10% of global cancer mortality, and remains the second most frequent cause of cancer death after lung cancer [1, 2], while in Asia, it is the top killing cancer [3]. Among the estimated 934,000 GC new cases and 700,000 GC deaths in 2002, China alone accounts for almost 42% of the global total, with age-standardized incidence rates of 41.4/100,000 for males and 19.2/100,000 for females [2]. A recent national survey in China shows that GC is the No 3 cancer killer after lung cancer and liver cancer, with 24.71/100,000 death rate [4].

Current major treatment modalities for GC include surgery and chemotherapy/radiotherapy. Curative gastrectomy with proper loco-regional lymph node dissection is the treatment of choice for resectable GC [5]. The effects of chemotherapy for GC are limited because multidrug resistance (MDR) problem in the primary tumor usually leads to treatment failure. There are quite a number of different mechanisms accounting for drug resistance, and MDR protein family plays an essential role. MDR refers to subsequent and cross-over resistance to drug of different categories, after exposure of tumor to a chemotherapeutic agent [6]. Currently, the over expressions of P-glycoprotein (P-gp), Multidrug resistance-associated protein (MRP) and Lung resistnce protein (LRP) are most extensively studied in MDR. Using immunohistochemical technique, this study was to determine the protein expressions of P-gp, LRP and MRP in GC tissues from patients without chemotherapy, and explored their expressions with clinico-pathological factors.

Chemotherapy is an important treatment option in the multi-disciplinary treatment strategy against GC. It has been established that postoperative chemotherapy could help reduce the recurrence and improve the progression-free survival in resectable GC [8–10] and even in metastatic GC [11]. Most patients, however, will ultimately experience relapse and treatment failure usually within 2-3 years after surgery. A major cause for such recurrence is the chemoresistance in GC, which results from several molecular mechanisms. Among these, drug efflux transporters are the most intensively studied molecular families, including ATP-binding-cassette (ABC transporter) [12], which uses ATP to pump drugs out of the target cell and reduce the intracellular drug concentrations leading to drug resistance. Two members of the ABC transporter superfamily including P-gp and MRP play a major role in resistance [13]. Lung resistance protein (LRP) is a member of the vault proteins involved in MDR. LRP has been shown to shuttle anthracyclines out of the nucleus [14].

The expression of P-gp, MRP and LRP are positively correlated with the level of drug resistance. The assessment of MDR proteins over-expression is useful in determining the most appropriate chemotherapy regimen for GC. However, the positive rates of P-gp, MRP and LRP reported in the literature are variable. Alexander et al. [15] found by immunohistochemistry that the positive rates of MRP, LRP and P-gp were 55%, 10% and 0%, respectively. Fan et al. [16] found by reverse transcription polymerase chain reaction (RT-PCR) in 50 GC patients that the mRNA expressions of MRP, LRP, and MDR1 were 12.0%, 10.0% and 10.0%, respectively. More recent studies [17–19] using immunohistochemistry found that the positive rates of MRP and LRP ranged from 39.4% to 88.9%. The positive rates of these three MDR markers in our 59 patients are higher compared with those results, probably due to improved detection technology.

Our study found no significant differences among the expressions of P-gp, MRP and LRP in GC of different pathological types, in agreement with findings by Shi et al [20], who found that the positive rates of P-gp and LRP were 49.2% and 58%, respectively, and such expression was closely related to clinicopathological staging but not related to tumor differentiation. In our study, MRP and LRP expression was not related to tumor invasion depth or lymphatic metastasis. Based on these findings, we propose that innate resistance may exist in those 59 GC patients even without prior chemotherapy.

P-gp confers resistance to cytotoxicity by chemotherapy drugs, cytokine TNF-alpha, and ultraviolet light [21]. Faggad et al. [22] found that MRP1 expression was as an independent negative prognostic factor for overall survival in ovarian cancer. As the patients in our group had mixed postoperative treatment, it is impossible to correlate these findings with clinical outcomes. This is the limitation of the current study, and future work should be done to elaborate on this issue.

The expression of P-gp, MRP and LRP confers different drug resistance profiles [23], including P-gp conferring resistance to doxorubicin, vincristine, vinblastine, actinomycin-D and paclitaxel, MRP conferring resistance to etoposide and epirubicin, and LRP conferring resistance to carboplatin and Melphalan. Our study found these molecules are interrelated, and P-gp is correlated with LRP (r = 0.803), especially for moderately differentiated adenocarcinoma (r = 0.915). The finding suggests that both two resistance mechanisms exist in most patients.

As the resistance mechanisms of P-gp, MRP and LRP are clarified, suggestions are proposed if we can block all the ABC transporters at once [24]? Recent studies revealed some new methods to overcome MDR, such as specific PI3K inhibitors to reduce P-gp [25, 26]. Du [27] showed that RP L6 could regulate MDR in GC cells by suppressing drug-induced apoptosis. Robey [28] reported an initial phase I studies of CBT-1, an orally-administered, bisbenzylisoquinoline plant alkyloid as P-gp inhibitor. CBT-1 at 1 μM completely reversed P-gp-mediated resistance to vinblastine, paclitaxel and depsipeptide.

Although the value of systemic chemotherapy for GC is controversial, several studies have demonstrated that GC could benefited for chemotherapy [29], although MDR remains a major challenge to effective chemotherapy [30]. Combined determination of P-gp, MRP and LRP may help tailor the chemotherapy regimes and predict the outcomes of treatment.

There are high percentages of innate expressions of P-gp, LRP and MRP in GC without prior chemotherapy, which may contribute to the poor response to chemotherapy of GC.