Autocrine production of interleukin-6 confers cisplatin and paclitaxel resistance in ovarian cancer cells

Abstract

It has been shown that IL-6 is elevated in the serum and ascites of ovarian cancer patients, and increased IL-6 concentration correlates with poor prognosis and chemoresistance. However, the role of IL-6 expression in the acquisition of the chemoresistance phenotype and the underlining mechanisms of drug resistance in ovarian cancer cells remain unclear. Here we demonstrate that both exogenous (a relatively short period of treatment with recombination IL-6) and endogenous IL-6 (by transfecting with plasmid encoding for sense IL-6) induce cisplatin and paclitaxel resistance in non-IL-6-expressing A2780 cells, while deleting of endogenous IL-6 expression in IL-6-overexpressing SKOV3 cells (by transfecting with plasmid encoding for antisense IL-6) promotes the sensitivity of these cells to anticancer drugs. IL-6-mediated resistance of ovarian cancer cells exhibits decreased proteolytic activation of caspase-3. Meanwhile, the further study demonstrates that the chemoresistance caused by IL-6 is associated with increased expression of both multidrug resistance-related genes (MDR1 and GSTpi) and apoptosis inhibitory proteins (Bcl-2, Bcl-xL and XIAP), as well as activation of Ras/MEK/ERK and PI3K/Akt signaling. Therefore, modulation of IL-6 expression or its related signaling pathway may be a promising strategy of treatment for drug-resistant ovarian cancer.

Introduction

The currently most frequently used therapy for the treatment of ovarian cancer is a combination of carboplatin and paclitaxel. Although up to 80% of patients initially respond well to therapy, the majority of patients suffer recurrent disease [1]. In some cases, patients respond well to repeated treatment with the same chemotherapeutic regimen but they will inevitably succumb to the disease following the eventual emergence of drug resistance. As a consequence, the overall 5-year survival is only 30% [1]. Thus, there is a pressing need to either identify novel therapies for ovarian cancer or to discover drugs which (re)sensitize tumor cells to existing chemotherapy. Several factors have previously been implicated in drug resistance, including genes which regulate drug influx and efflux, drug metabolism, damage repair, and the apoptotic response to drug-induced damage. Indeed, it is possible that numerous resistance mechanisms could contribute to a drug-resistant phenotype and these mechanisms might be coordinately regulated [2].

Interleukin-6 (IL-6) is a pleiotropic cytokine that plays a major role in the response to injury or infection and is involved in the immune response, inflammation, and hematopoiesis [3]. IL-6 is a vital regulator of physiologic functions in diverse organ systems, including the central nervous system, the cardiovascular system, the immune system, the hepatic system, and others [3]. In addition, its dysregulation impacts numerous disease states, including many types of cancer [4]. Several studies have addressed the role of IL-6 in promoting the chemoresistance of multiple myeloma [5], [6], renal cell carcinoma [7], cholangiocarcinoma [8], prostate cancer [9], [10], [11], and breast cancer cells [12].

IL-6 was elevated in the serum and peritoneal fluid from patients with ovarian cancer [13], [14], [15], and high levels of IL-6 in body fluids were associated with poor prognosis and resistance to chemotherapy [16], [17]. It has been demonstrated that ovarian cancer cell lines NOM1 and SKOV cultured with IL-6 have increased chemotactic and/or chemokinetic activity and increased overall invasiveness [18]. Moreover, IL-6 has been reported to be is a potent proangiogenic cytokine in ovarian cancer cells [19]. More recent studies have suggested that IL-6 secreted by ovarian cancer cell lines could be involved in their tumorigenic potential, particularly potentiating their capacity to secrete matrix metalloproteinase-9 [20]. Our previous study demonstrated that IL-6 may contribute to ovarian cancer cell growth partly through the activation of androgen receptor (AR) and estrogen receptor (ER) pathways [21], [22]. In vitro studies with ovarian cancer cell lines show that generation of paclitaxel-resistant sublines is often associated with increased IL-6 mRNA expression and protein secretion using cDNA array technology [23]. However, the role of IL-6 expression in the acquisition of the chemoresistance phenotype and the underlining mechanisms of drug resistance in ovarian cancer cells are not yet fully understood.

IL-6 acts through a hexametric receptor, which contains the ligand-binding IL-6α chain and the common cytokine receptor signal-transducing subunit gp130. The binding of IL-6 to gp130 activates multiple signal transduction pathways such as signal transducers and activators of transcription (JAK/STATs) pathway, Ras/MEK (mitogen-activated protein or extracellular signal-regulated kinase kinase)/ERK (extracellular signal-regulated kinase) pathway, and PI3K (phosphotidylinositol 3 kinase)/Akt pathway [24]. Recently, growing evidence suggests activation of Ras/MEK/ERK [25], [26], [27] and PI3K/Akt [28], [29], [30], [31] signaling pathways play an important role in chemoresistance of ovarian cancer. Therefore, we hypothesized that one potential mechanism that IL-6 induces chemoresistance of ovarian cancer cells by triggering activation of Ras/MEK/ERK and PI3K/Akt signaling.

In this study, we investigated the role of IL-6 expression in modulating cellular sensitivity to chemotherapeutic drugs in ovarian cancer cells. Furthermore, we also explored possible underlying mechanisms involved in drug resistance induced by IL-6. Our data suggest that the autocrine production of IL-6 by ovarian cancer cells promotes resistance of these cells to chemotherapy through decrease of proteolytic activation of caspase-3. The further study demonstrates that IL-6-induced resistance of ovarian cancer cells may be associated with up-regulation of both multidrug resistance-related genes [multidrug resistance gene 1 (MDR1) and glutathione S transferase pi (GSTpi)] and apoptosis inhibitory proteins [Bcl-2, Bcl-xL and X-linked inhibitor of apoptosis (XIAP)], as well as activation of Ras/MEK/ERK and PI3K/Akt signaling.