Tamoxifen reduces P-gp-mediated multidrug resistance via inhibiting the PI3K/Akt signaling pathway in ER-negative human gastric cancer cells

Abstract

Multidrug resistance (MDR), mediated by overexpression of drug efflux transporters such as P-glycoprotein (P-gp), is a major problem limiting successful chemotherapy of gastric cancer. Tamoxifen (TAM), a triphenylethylene nonsteroidal antiestrogen agent, shows broad-spectrum antitumor properties. Emerging studies demonstrated that TAM could significantly reduce the MDR in a variety of human cancers. Here we investigated the effects and possible underlying mechanisms of action of TAM on the reversion of MDR in ER-negative human gastric cancer cells. Our results demonstrated that in MDR phenotype SGC7901/CDDP gastric cancer cells TAM dramatically lowered the IC₅₀ of CDDP, 5-FU and ADM, increased the intracellular Rhodamine123 accumulation and induced G0/G1 phase arrest, while G2/M phase decreased accordingly. Furthermore, at the molecular level, TAM substantially decreased the expression of P-gp, p-Akt and the Akt-regulated downstream effectors such as p-GSK-3β, p-BAD, Bcl-XL and cyclinD1 proteins without affecting the expression of t-Akt, t-GSK-3β, t-BAD proteins in SGC7901/CDDP cells. Thus, our findings demonstrate that TAM reverses P-gp-mediated gastric cancer cell MDR via inhibiting the PI3K/Akt signaling pathway.

Introduction

Gastric cancer (GC) ranks fourth among the most commonly diagnosed tumors and is the second leading cause of cancer-related deaths worldwide [1]. To date, chemotherapy remains the fundamental treatment for GC which undergoes radical operation to improve overall survival and the quality of life of patients. However, the major drawback to the treatment success of chemotherapy is the development of MDR, which refers to tumor cells become insensitive to multiple chemotherapeutic drugs after repeatedly using single chemotherapeutic drug [2]. Recently, several novel factors acting downstream of the initial drug-induced insult have been shown associated with the development of MDR, such as enhanced DNA repair activity, defective apoptosis pathway, or altered metabolism of drugs [3]. But the well-recognized mechanism leading to MDR is the overexpression of MDR1 gene product, P-glycoprotein (P-gp). Thus, circumvention of P-gp-mediated MDR is still the mainstay of MDR-reversion in clinical chemotherapy. Since the 1980s, a wide spectrum of chemosensitizers specific against P-gp, such as verapamil, trifluoperazine and cyclosporine have been examined for their ability to overcome P-gp-mediated MDR. Unfortunately, the clinical application of these compounds showed disappointing results which attributed to pharmacokinetic interactions with anticancer drugs and serious side effects [4]. Therefore, it is of great importance to find acceptable modulators and/or strategies for effective clinical application in human cancers, including GC.

Tamoxifen (TAM), a nonsteroidal triphenylethylene antiestrogen drug, is widely used in adjuvant treatment as well as in the first-line treatment of ER-positive breast cancer. Extensive studies have confirmed that tamoxifen exerts its biological functions in both ER-mediated and non-ER-mediated mechanisms in a variety of human tumors. To date, tamoxifen has been reported to perform its growth inhibitory effects through ER-dependent manner in human breast cancer and colon cancer as well as human hepatocellular carcinoma via ER-independent manner, respectively [5], [6]. Furthermore, precursor studies have verified that TAM can effectively reverse multidrug resistance in colorectal cancer, gastrointestinal cancer and cholangiocarcinoma cells via directly binds and inhibits P-gp [7], [8], [9]. Thus, TAM has a myriad of interesting, beneficial effects in human cancers, especially for tumors with MDR.

The PI3K/Akt signaling pathway might be the best characterized and most illustrations pathway in the transmission of anti-apoptotic signals in cell survival. Akt is a key mediator of PI3K signaling located at an intersection of multiple pathway implicated in cell proliferation, survival, transcription and metabolic processes [10], [11]. In recent years, accumulation evidences suggest that the PI3K/Akt signaling pathway is frequently activated in human gastrointestinal cancers [12]. Furthermore, previously studies have shown that inhibition of PI3K/Akt pathway could effectively reverse P-gp-mediated MDR in human gastric cancer and murine leukemic cell line as well as the activated Akt is associated with the increased MDR to multiple chemotherapeutic drugs (such as cis-platinum, 5-FU and adriamycin) in primary gastric cancer tissues, indicating that Akt may be a novel molecular target for acceptable modulators to restore chemosensitivity of human MDR cancer cells [13], [14], [15].

Therefore, on the basis of prior studies, we hypothesized that TAM may affect the P-gp-mediated MDR of human gastric cancer cells. In the present study, we aimed to verify whether PI3K/Akt is involved in the reversal effects of TAM on ER-negative human gastric cell MDR.