br Conflict of interest br Introduction Ovarian cancer

Conflict of interest
Introduction Ovarian cancer is the most lethal gynecological malignancy, and ranks as the fifth leading cause of cancer death among women. The American Cancer Society estimates that about 22,280 new ovarian cancer cases and 14,240 deaths will occur in the United States in 2016 [1]. Most ovarian cancer patients are asymptomatic and thus approximately 60% of patients have advanced stage III and IV at diagnosis. Despite ongoing advances in treatment therapies, the overall 5-year survival rate of ovarian cancer patients increased slightly from 36% in 1975 to 46% in 2011 [1,2]. The current standard treatment for advanced ovarian cancer includes primary cytoreductive surgery followed by adjuvant combination chemotherapy with paclitaxel and platinum [3,4]. Over 80% of tumors are initially sensitive to standard chemotherapy, but acquire broad cross resistance (multidrug resistance, MDR) that can include a variety of structurally and functionally unrelated chemotherapeutic agents [5]. MDR severely limits the ultimate success of chemotherapy treatment, leading to patient relapse or even death due to drug resistant/metastatic disease [6]. Mechanisms of MDR in ovarian cancer have been associated with several characteristics including: overexpression of a plasma membrane glycoprotein (Pgp), changes in specific proteins targeted by chemotherapy drugs, and alterations in the apoptotic threshold [[7], [8], [9]]. However, the evidence linking these mechanisms to acquired drug resistance in tumors in clinically relevant MDR is lacking. More recent studies have revealed an association between drug resistance and autophagy [10,11]. Autophagy is a cellular mechanism that targets unnecessary or dysfunctional cellular organelles and proteins for degradation in order to overcome stress [12]. This dynamic process includes several key steps: induction, vesicle nucleation and elongation as well as the formation of autophagosomes and autolysosomes [13,14]. Autophagy pathways are activated as a protective mechanism to mediate the acquired MDR phenotype of some cancer Atorvastatin Calcium during chemotherapy [10,15]. Moreover, increasing evidence suggests that the inhibition of autophagy can augment cytotoxicity and restore chemosensitivity in combination with conventional anticancer drugs [16,17]. Autophagy inhibitors can be divided into two categories: upstream inhibitors and downstream inhibitors. 3-MA and Wortmannin belong to the upstream autophagy inhibitors, which block the formation of autophagosomes by suppressing class III phosphatidylinositol 3-kinase (PI3k). Chloroquine (CQ) and its derivative hydroxychloroquine (HCQ) are the downstream autophagy inhibitors, which suppress autophagosome and lysosome fusion, which form autophagolysosomes that degrade the autophagic cargo inside of autolysosomes [18]. A series of autophagy related proteins, particularly microtubule-associated protein 1 light chain 3 (LC3) and Beclin-1, have been used as indicators of autophagy [19,20]. To date, only LC3 is known to exist on the membrane of autophagosomes, so the amount of LC3 usually correlates well with the extent of autophagosome formation [21]. It should be noted that the accumulation of autophagosomes may represent either autophagy induction or a block of autophagosomal maturation [19]. Therefore, the numbers of autophagosomes are insufficient to comprehensively assess the autophagic activity and other approaches that are required to distinguish between induction or suppression of autophagy. Several specific substrates are efficiently degraded by autophagy, of which the well-characterized receptor is p62 (also known as SQSTM1/sequestome 1) [19]. p62 is a vital indicator of autophagic flux, which selectively incorporates into autophagosomes by directly binding to the LC3 on autophagic membranes for subsequent degradation in autolysosomes [22,23]. The term “autophagic flux” refers to the dynamic process of autophagy activity including: autophagosome synthesis, transportation of autophagic substrates to the lysosome, and subsequent degradation inside of the lysosome. As an indicator of autophagy, autophagic flux is more reliable than the number of autophagosomes [19]. Inhibition of autophagy correlates with increased levels of p62 [24]. Therefore, the total cellular expression level of p62 inversely reflects autophagic activity. However, the role of p62 in paclitaxel resistance in ovarian cancer has not been determined. In the present study, we evaluated the expression of p62 in ovarian cancer tissues and cell lines. Our findings showed that p62 was significantly decreased in metastatic and recurrent ovarian cancer tissues and drug resistance cell lines. These results suggest that autophagy plays a key role in the emergence of drug resistance, p62 is a potential predictor of MDR in ovarian cancer. Our study suggests that the targeting of the autophagy pathway can be a potential strategy to reverse drug resistance.