DOI: 10.31038/CST.2018333

Abstract

Chemoresistance is one of the most important factors leading to high mortality in ovarian cancer (OC). Overexpression of P-glycoprotein (P-gp) in OC cells may results in resistance to paclitaxel treatment by pumping the drug out of the cells, which in turn decreases the intracellular drug concentration. Additionaly overproduction of midkine (MK) can also affect the development of chemoresistance in OC. Although, the mechanisms of action of P-gp and MK are not the same, overexpression of both proteins in OC may intensify chemoresistance to paclitaxel treatment. Therefore, simultaneously inhibition of P-gp and MK in overcoming chemoresistance to drugs may improve treatment results in OC.

Introduction

Ovarian cancer (OC) is the fourth most common type of gynecological cancers worldwide and has the highest mortality rates among female genital tract malignancies [1–3]. Even patients with same clinical characteristics, such as cancer stage, histological type and grade display different disease progression and treatment results [3–5]. Due to absence of specific symptoms in the early stage, OCs are diagnosed at the advanced stages in two thirds of the patients [6]. The overall 5 year survival rate is still less than 40% despite some advances in the treatment of OC, including the combination of surgery, radiation and chemotherapy. This may be attributed to the late stage diagnosis, poor prognosis and resistance to chemotherapy, which is one of the major problems to controlling malignant tumors [3, 7, 8]. The first-line treatment of OC is cytoreductive surgery followed by adjuvant chemotherapy, including paclitaxel and cisplatin [3, 9–11]. Paclitaxel, administered as monotherapy or in combination with cisplatin, is potentially effective therapeutic regimen in OC. Paclitaxel may be regarded as a mitotic poison and affects the cellular microtubule network. It inhibits chromosome alignment and segregation and then trigger the apoptosis pathway [10, 11].

Initial response rates to chemotherapy vary between 40 and 80% in OCs. However, majority of these patients who respond to chemotherapy at first, eventually have recurrence following the development of chemoresistance. Thus, acquired resistance is the main cause of unsuccessful treatment in OC. The molecular mechanisms behind chemoresistance is multifactorial and involves multiple processes, including drug transport and metabolism, DNA repair and apoptosis. Currently, the factors that affect the development of chemoresistance in OC has not been completely understood [6, 12]. Chemoresistance is usually attributed to the overproduction of P-gp. It has been reported that overexpression of P-gp is the major factor for reduced chemo-sensitivity in a lot of malignancies, including OC [6, 12–14]. It has been demonstrated that the overexpression of P-gp in aggressive OC cells results in the development of resistance to paclitaxel treatment [10, 11, 15]. Although the mechanism of P-gp-induced chemoresistance is not fully known, it is considered to acts essentially as an efflux pump and plays an important role in the exclusion of drugs from tumor cells, resulting in decreased accumulation of chemotherapy drugs within cancer cells [8, 10, 11, 15].

Another important protein, MK, is overexpressed in many cancers, including OC and induces the growth and survival of tumors. On the other hand, overproduction of MK can also affect the development of chemoresistance. The chemoresistance caused by MK is mainly due to its inhibitory action on the apoptosis process.

Our proposal is that both proteins, namely P-gp and MK, may protect tumor cells against chemotherapeutic drugs more effectively by a synergistically way than they do one by one and they could increase chemoresistance [3, 16–19]. Therefore, it can be speculated that inhibition of both proteins may enhance the effectiveness of paclitaxel chemosensitivity in OC.

The role of P-glycoprotein in chemoresistance to paclitaxel /cisplatin in ovarian cancer

ATP-binding cassette transporter B1 (ABCB1), also known as P-gp or multidrug resistance protein 1 (MDR1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types [20]. It is a 170 kD transmembrane glycoprotein and has unusually broad polyspecificity for structurally different substances, including anticancer drugs such as paclitaxel and cisplatin. Most of these substances are hydrophobic, thus, P-gp acts like a ‘’hydrophobic vacuum cleaner’’ [20].

P-gp leads to chemoresistance by pumping drugs out of the cells and decreases the intracellular drug concentration [9]. P-gp is also associated with a more progressed malignant phenotype in carcinogenesis. The function of P-gp in relation to cellular differentiation may be pleiotropic, depending on the origins from which the cancer arises [8]. P-gp is localized in the membrane of epithelial cells in the intestine, liver, proximal tubule of the kidney and in the capillary endothelial cells. It functions as a blood–brain barrier, blood–placenta barrier and blood-testis barrier and protects them from toxic xenobiotics [20]. This transporter may affect the pharmacological treatment of numerous diseases by changing drug pharmacokinetics and inhibiting accumulation of anticancer drugs in cancer cells. Cancer cells of some tissues also produce very large amount of P-gp, which lead to chemoresistance by transfering chemotherapeutic agents out of cancer cells. Additionally, increased intestinal expression of P-gp can inhibit the absorption of orally administered drugs, promotes their biliary and renal elimination and as a result, decreases plasma concentrations of these drugs, which causes unsuccesful treatment [6, 19, 20].

Fojo et al. have reported that the MDR1 gene is overexpressed in many cancers arising from some tissues in which the MDR1 gene is expressed at high levels. Most of these cancers are resistant to chemotherapy, and the MDR1 gene plays an important role in intrinsic and acquired chemoresistance [8, 21]. Approximately 40% of OCs after chemotherapy produce P-gp at high level, suggesting chemoresistance in OCs may be most likely acquired [8, 22]. However, some OC cases before chemotherapy are intrinsically multidrug resistant, which can be determined by MDR1 gene expression, and this phenotype should be taken into account for effective chemotherapy of ovarian epithelial carcinomas [8]. It has been revealed that the overexpression of P-gp in aggressive OC cells is associated with the development of resistance to paclitaxel treatment [10, 11, 15]. In contrast, downregulation of P-gp increases the effectiveness of certain chemotherapeutic agents. For example, myricetin (a dietary-flavonoid) enhances the chemotherapeutic potential of paclitaxel in OC cells by downregulating P-gp and inhibits the migratory properties of OC cells [10]. Alike, microRNAs (miRNA), which are endogenous, noncoding RNAs may regulate the ABCB1 gene. Recently, Sun et al. have demonstrated that miR-186 overexpression may sensitize OC cells to paclitaxel and cisplatin by downregulating P-gp in the OC cell lines [9]. Another study has demonstrated that miR-21 may regulate the production of MDR1/P-gp, by targeting hypoxia-inducible factor-1α (HIF-1α, ) which influences the development of drug resistance in paclitaxel-resistant OC A2780/taxol cell lines. Furthermore, the inhibition of miR-21 may sensitize A2780/taxol cells to paclitaxel [12]. Aditionally, upregulation of miR-27a expression results in inhibition of P-gp expression and decreases paclitaxel-resistance in OC cell line [15].

As the expression of P-gp in cancer cells usually results in multidrug resistance (MDR) to chemotherapeutic drugs, which is the main cause of chemotherapy failure in cancer treatment, it is important to develop new treatment strategies, which target P-gp [11]. Some MDR reversal agents that inhibit the drug efflux activity of P-gp could increase the intracellular drug levels [11]. It has been demonstrated that MDR1 expression levels after promethazine (an antihistaminic agent) administration is significantly reduced and verapamil (a calcium channel antagonist) leads to a significant decrease in MDR1 mRNA levels and downregulates P-gp activity [23].

The role of midkine in chemoresistance to paclitaxel /cisplatin in ovarian cancer

Midkine (MK), a heparin-binding growth factor, was firstly found to be the product of a retinoic acid-responsive gene during embryogenesis [24, 25]. Despite its high expression during embryogenesis, MK is downregulated to neglible levels in healthy adults and only re-expressed in some pathological processes [16, 25, 26]. MK promotes many cellular functions including survival, growth, migration, reproduction and repair, and gene expression while inhibiting apoptosis [27]. Due to its multiple functions, MK has significant impact on the pathogenesis of neurological, cardiovascular and inflammatory diseases and malignancies [19, 25]. It induces several signal transduction pathways including phosphoinositide 3-kinase (PI3K) and extracellular signal-regulated kinase (ERK), therefore participates in the regulation of diverse biological processes. Recent studies showed that MK expression is influenced by hypoxia, growth factors, and cytokines through a nuclear factor-κB (NF-κB) dependent pathway. The precise regulatory mechanisms behind MK expression is not fully understood [25, 28, 29]. MK plays significant roles as a growth factor during carcinogenesis, such as transformation, fibrinolysis, cell invasiveness, cell survival, anti-apoptosis, and angiogenesis processes [24, 27, 29–33].

It has been shown that MK is overexpressed in various human malignancies, including oral, lung, thyroid, bladder, prostate, cervical and OCs [18, 25, 35–37]. MK is also a plasma-secreted protein, and its levels in blood may increase in patients with malignant diseases [25]. Nakanish et al. have demonstrated that the expression of MK in germ cell ovarian tumors is significantly lower than in epithelial ovarian tumors, and expression in malignant epithelial tumors is significantly higher than in benign ones [18]. MK not only induces carcinogenesis but also contributes to chemoresistance [34]. It is considered that MK-induced chemoresistance is mainly due to inhibitory impact on apoptosis mediated by the Janus-activated kinases (JAKs) and STAT1 by activating the Akt-mediated survival pathway and senescence of tumor cells [19, 31]. On the other hand, it appears that some of the mechanisms of its chemoresistance actions are partially similar to those of P-gp [19].

MK, has been verified overexpressed in many cancers, including OC. It has been shown that MK is increased in the serum of patients with epithelial OC. MK may also be an indicator of the response to paclitaxel and/or cisplatin in the clinical treatment of OC [3, 16–18]. Zhang et al. have demonstrated that cancer-associated fibroblasts (CAFs) in the tumour microenvironment (TME) may lead to the high level of MK in tumours and that CAF-derived MK can induce cisplatin resistance via inhibition of the cell apoptosis in the TME by increasing production of lncRNA ANRIL. CAF-derived MK increases lncRNA ANRIL expression in tumour cells and thus promoting the up-regulation of ABC family proteins, multidrug resistance-associated protein 1 (MRP1) and ABCC2, which ultimately cause resistance to cisplatin. These findings related to the source of MK in tumour tissues, may serve as a novel therapeutic approach for cancer [34]. Further evidence is that a novel midkine inhibitor (iMK) has antitumor effect against oral squamous cell carcinoma and it has been demonstrated that iMK inhibits the expression of MK and suggested that iMK can be effectively used for the treatment of oral squamous cell carcinoma [19, 25, 38].

On the contrary, Wu et al. have suggested that the MK expression has a positive correlation with the predicted survival time and chemosensitivity of OC to paclitaxel/cisplatin. This study proposed that MK could down-regulate the expression of multidrug resistance-associated protein 3 (MRP3), and in turn increases the cytotoxicity of paclitaxel and/or cisplatin [3]. Despite this contrary opinion, it is generally considered that MK increases chemoresistance and decreases effective treatment during chemotherapy. On the other hand, due to its biological significance in carcinogenesis, it is suggested that MK can be regarded as a candidate molecular target for therapy against human carcinomas [25].

Conclusion

Chemoresistance is one of the important factors leading to high mortality in OC. At present paclitaxel and cisplatin are the most used drugs to treat OC. However, numerous patients with OC frequently relapse following the development of chemoresistance to chemotherapeutic agents, including paclitaxel and cisplatin. Overexpression of P-gp and MK have important impacts on chemoresistance in many cancer types, including OCs. Therefore, inhibition of both P-gp and MK may overcome chemoresistance in OCs. However, whether they act synergistically or in contrary remains unclear and further investigations are needed to clarify the interplay of these proteins in cancer cells and in the treatment of malignancies.

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