1. Introduction
Cisplatin (cis-diamminedichloroplatinum (II)) was first approved for clinical use by the FDA in 1978 and has continued to be used as a first-line chemotherapeutic agent for the treatment of approximately 50% of solid tumors, including lung, head and neck, breast, testicular, ovarian, prostate, and bladder cancers
[1,2][1][2]. Cisplatin was initially synthesized by Michel Peyrone in 1845 and therefore was initially called Peyrone’s salt. In testing the effects of platinum compounds on
E. coli proliferation, Rosenberg and his group found that cisplatin also has inhibitory effects on sarcoma 180 and L1210 leukemia cells
[3,4][3][4]. Prior to this time, chemotherapeutic drugs in the clinic were all natural or synthetic organic compounds, and cisplatin became the first antitumor candidate containing heavy metal elements. After approximately 15 years of preclinical experimentation, through 1975, clinical trials led by the J.M. Hill laboratory confirmed the antiproliferative activity of cisplatin against multiple solid tumors
[5]. Although cisplatin has a wide range of antitumor activity, its side effects continue to limit its therapeutic use and efficacy. In the clinic, patients treated with cisplatin often experience symptoms of renal tubular necrosis (nephrotoxicity), hearing loss or cochlear damage (ototoxicity), and peripheral sensory neuropathy (neurotoxicity)
[6]. These side effects appear more frequently with increasing doses of the drug. In addition to side effects, patients with solid tumors will frequently develop resistance to cisplatin, forcing physicians to consider other treatment options. As cisplatin resistance is often associated with cross-resistance to other commonly used cytotoxic chemotherapeutic drugs, such as doxorubicin and etoposide, this results in a reduction in treatment options
[7]. There are many factors leading to cisplatin resistance, including alterations in DNA metabolism, epigenetic and transcriptional modifications, activation of drug efflux systems, and subcellular drug localization and translocation
[8].
The mechanisms mediating the antitumor actions of cisplatin have been studied for decades, with DNA being the primary drug target. Once inside the cell, cisplatin undergoes aquation to form [Pt(NH3)2Cl(OH2)]+ and reacts with DNA to form monoadducts, interstrand, intrastrand or DNA–protein cross-links, affecting the DNA double helix structure and nucleosomes of cancer cells
[9,10][9][10]. This leads to replication and transcriptional repression, and DNA double-strand breaks (DSBs), which then initiate DNA repair. Once DNA repair fails or is overwhelmed by excessive DNA damage, cell death is triggered
[11]. An increased capacity to repair DNA is considered as the most significant feature of platinum-resistant cells
[12,13][12][13].
The central downstream event following cisplatin interaction with cellular DNA is apoptosis
[14,15,16][14][15][16]. The intrinsic pathway of cisplatin-induced apoptosis involves the promotion of oxidative stress, whereby cisplatin-treated cells accumulate excessive reactive oxygen species (ROS) (hydroxyl radicals and superoxide). Abnormally accumulated ROS damages mitochondrial respiratory function, leading to mitochondrial dysfunction
[17]. ROS, influencing the pro-apoptotic protein Bax, also cause damage to mitochondrial DNA and a reduction in mitochondrial membrane potential, which promotes mitochondrial destruction. Cytochrome c and caspase 9 are then released by damaged mitochondria and evoke a cascade of caspase cleavage reactions
[18].
The extrinsic pathway of cisplatin-induced apoptosis is mediated via a type-II membrane protein that activates the Fas receptor in conjunction with the Fas ligand, thereby promoting the formation of the apoptosome complex by the Fas-associated death domain and pro-caspase 8. This apoptosome complex activates caspase 3, caspase 6, and caspase 7, ultimately leading to apoptosis
[19]. In addition, cisplatin generally arrests cells in the G1/S or G2 phase of the cell cycle, providing time for repair of damaged DNA prior to DNA synthesis. When cells fail to repair DNA damage at the cell cycle checkpoints, they are forced to re-enter the cycle prematurely, progressing to apoptosis
[20,21][20][21]. As a “gatekeeper”, the activation of p53 also contributes to cisplatin-induced tumor cell apoptosis
[22]. In addition, the p21, MDM2, GADD45
[23], MAPK pathway
[24], and PI3K/Akt pathways
[25], which are related to p53 and cell cycle regulation, have all been shown to be involved in cisplatin-induced apoptosis.
Macroautophagy (which w
e will
be refer
ed to as autophagy) is a critical process in eukaryotic cells whereby superfluous organelles, misfolded proteins, and other cellular debris are cleared, restoring a state of cellular equilibrium
[26]. This process is an evolutionarily conserved process whereby cellular debris or toxic cellular components are engulfed by the autophagosome, a double-layered membrane structure, and transported to acidic lysosomes, where they undergo degradation and recycling
[27]. Autophagy occurs in cells under nutrient-poor conditions, responding to the decline in external energy sources. Therefore, autophagy is generally considered to reflect a survival-promoting function. However, if autophagy is continuously or overly activated, cell death will be triggered. Upon cisplatin treatment, autophagy induction has been detected in both cisplatin-sensitive and cisplatin-resistant cancer cells. In fact, the basal level of autophagy was significantly elevated in cisplatin-resistant cells
[28,29,30,31][28][29][30][31].
Defective apoptosis is one cause of cisplatin resistance, which confers a survival advantage to tumor cells. This defect facilitates the generation of cellular stress-mediated autophagy, which precedes or effectively blocks the apoptotic cascade. A large number of studies have shown that when cisplatin-induced autophagy is inhibited in cancer cells, the manner of cell death switches to apoptosis
[28]. Therefore, taken together, cisplatin-induced autophagy is often considered one of the primary factors thwarting its chemotherapeutic effects. However, the role of autophagy is often far more complex than has been appreciated.
In addition to autophagy and apoptosis, the tumor cell response following cisplatin treatment can include cellular senescence, as in some cases, persistent DNA damage leads to long-term growth arrest
[32]. Although senescence was previously considered an irreversible response after chemotherapy, recent studies from a number of laboratories
, including our own, have shown that tumor cells have the capacity to escape from this therapy-induced senescence
[33].