Researchers from Karolinska Institutet have discovered how platinum chemotherapy can enhance the treatment resistance of ovarian cancer cells, by progressively changing the cancer cell-intrinsic adhesion signaling and cell-surrounding microenvironment.
Platinum chemotherapy is standard treatment in ovarian cancers, but treatment resistance commonly develops. The extracellular matrix (ECM)-derived biochemical and mechanical cues in the tumor microenvironment are known to contribute to the ability of cancer cells to metastasize and resist treatment.
However, how the dynamic communication between the cancer cells and the ECM is affected by, or influences the disease progression and chemotherapy, have remained elusive.
A new study led by Kaisa Lehti, researcher at the Department of Microbiology, Tumor and Cell Biology at KI, and published in Nature Communications, shows that the ECM microenvironment is modulated in metastasis and following chemotherapy. Changes in the ECM proteins variably altered the cell death response of the tumor cells.
“Particularly in the most aggressive solid tumor tissues, cancer cells are surrounded by a prominent fibrotic network of proteins like collagens, known as the extracellular matrix (ECM) and also defined as the matrisome when considered with various associated factors including cytokines and chemokines.
The ECM/matrisome is produced largely by stromal cells, but sensed and remodeled collectively by the cancer cells and the cells of the fibrotic tumor stroma. In tumor cells, specific ECM signaling in stiff microenvironment critically increased their resistance against platinum-mediated, apoptosis-inducing DNA damage,” Kaisa Lehti explains.
The development of drug resistance represents one of the major obstacles to the cure of cancer. In fact, in many instances, tumors that are initially responsive to a given drug will relapse with a tumor no longer sensitive to the initial drug. Several mechanisms contribute to the development of drug resistance, including tumor heterogeneity, reduced drug concentration to the target, alteration in drug target structure, increased repair of the lesions induced. Depending on the drug and tumor under investigation, one or more mechanisms can take place. A better understanding of the different mechanisms of resistance along with the development of suitable models to find ways to overcome resistance is mandatory to improve the outcome for cancer patients.
Platinum containing drugs (cisplatin, carboplatin, and oxaliplatin) are very active agents widely used for the treatment of different malignancies (i.e., testicular germ line, ovarian, lung, head and neck, colon). Specifically, platinum-taxanes doublet represents the gold standard treatment (in both adjuvant and neo-adjuvant setting) in epithelial ovarian cancer (EOC). Epithelial ovarian cancer is one of the most chemo-responsive tumors responding well to both cisplatin and carboplatin, although almost invariably after an initial response, patients will relapse with platinum resistant tumors.
Resistance to platinum-containing drugs has been associated to several mechanisms, including alteration in drug efflux (through for example the modulation of the copper transporter CTR-1 expression) [1,2,3,4,5], alteration in intracellular proteins able to bind and sequester Pt (as for example GSH) [6,7,8,9], and altered expression of pro-survival or anti-survival proteins [10,11,12,13].
Considering that the main target of platinum drugs is DNA, the sensitivity/resistance to these drugs is affected/modulated by the ability of cells to recognize and repair the DNA drug induced damages. Specifically, there is strong preclinical evidence suggesting how the presence or absence of a specific DNA repair pathway (due to mutations, deletion or epigenetic changes of genes involved in DNA repair) is associated with sensitivity/resistance to platinum drugs [14,15,16,17,18].
We will here focus on DNA repair pathways as possible mechanism of resistance to a platinum-based therapy in ovarian cancer. In addition, the importance of predictive biomarkers of response to platinum will be described with the aim to possibly identify patients that will not respond to therapy and could eventually be re-directed to alterative therapeutic strategies.
Clinical Presentation and Management of Ovarian Cancer
Epithelial ovarian cancer represents one of the major causes of cancer death among women and certainly the most lethal gynecological cancer . Epithelial ovarian cancer tumor is generally diagnosed at late stage (Federation of Gynecologists and Obstetricians (FIGO) stage III/IV) when the tumor is disseminated throughout the peritoneal cavity, limiting the potential benefit of debulking surgery. In fact, ovarian tumors present with metastases that infiltrate the omentum, peritoneum, diaphragm, and the Glissonian capsule and are associated with abdominal effusion (ascites).
Epithelial ovarian cancer is not a single disease but comprises different histological entities (e.g., high-grade serous, endometrioid, mucinous, clear cell and low grade serous) with different biological, molecular, and clinical characteristics. Yet to be defined is also the cell of origin of these different entities, as both fallopian tube and ovary has been advocated as site of tumor development [20,21]. In spite of the evidence suggesting that the different types of EOC represent unique entities, they are still treated with a similar approach consisting of a de-bulking surgery followed by adjuvant chemotherapy.
The latter comprises a platinum containing (mainly cisplatin and carboplatin)-taxanes doublet, which has not changed over the last three decades, with the aim to eliminate all the remaining micro-metastases. Platinum agents were in fact introduced in the late 1970s when different clinical studies reported that cisplatin caused a double in overall response rates and in complete responses as compared to non-platinum containing schedules [22,23].
Nevertheless, even if 80% of EOC patients respond initially to this first-line therapy, almost 80% of those responding cases will recur with resistant disease and the choice of second line therapy remains empirical, mainly based on the time lag between the end of chemotherapy and first relapse, defined as platinum-free interval (PFI) .
In the last decade, the molecular characterization of EOC has revealed that more than 50% of high-grade serous ovarian cancers (HGSOCs), representing 80% of EOCs, have a defect in homologous recombination (HR) repair, underlying the extreme sensitivity of this tumor type to platinum drugs . Many studies, including TGCA (The Cancer Genomic Atlas) , have demonstrated that HGSOC is characterized by a defect in HR by genetic and epigenetic alterations in the genes involved in the pathway, including BRCA 1 and 2 (for a recent review see Reference ).
Not only does this underlying defect support the extreme sensitivity to platinum agents, but it also allows the clinical implementation of targeted therapy with Poly (ADP-ribose) polymerase inhibitors (PARPi). In fact, this class of agents has been clearly shown in preclinical studies to be extremely active in cellular systems deficient in HR repair by a synthetic lethality basis and such activity has been validated in clinical trials in BRCA1/2 mutation carriers with ovarian carcinomas .
The PARPi olaparib, rucaparib, and niraparib have been recently approved in the US and Europe for clinical use against platinum-sensitive ovarian cancers, as they were shown to increase the progression free survival; however, no data on overall survival (OS) are available yet [28,29,30]. Recent data suggest that these compounds are also active in wild type BRCA1/2 tumors .
DNA Repair Systems
Combinations containing platinum-based drugs, mainly cisplatin and carboplatin, represent the first line treatment for EOC. The mechanism of action of both cisplatin and carboplatin involves their interaction with DNA and the formation of mono-adducts, mainly covalently interacting with N7 of guanine. This mono-adduct then evolves, through a second covalent binding, to a DNA crosslink, which can be on the same DNA strand (i.e., intrastrand crosslinks representing the most abundant platinum adducts) or on the opposite strand (i.e., interstrand crosslinks which are responsible for the drug antitumor activity).
The major differences between cisplatin and carboplatin are in the kinetics of mono-adduct and mono-adduct to cross-links formation, due to the different aquation rates and steric hindrance [32,33]. If these lesions are not repaired (either in the form of DNA mono-adduct or in the form of DNA crosslink) the consequence is a block of DNA synthesis and transcription; in addition, the replication fork delay can progress and completely destabilize DNA synthesis with activation of a replicative stress response. In more recent years, the molecular definition of these processes and of the proteins involved have defined their role also in chemo-resistance, including to platinum agents .
As a general rule, the presence of DNA lesions activates a DNA damage response (DDR) with the activation of cellular pathways leading to a slow down or interruption of DNA synthesis, a block of the cell cycle, and activation of repair pathways . These pathways are part of the DDR and have a key role in the maintenance of genome integrity [36,37]. Genomic instability has been reported to be a hallmark of cancer [38,39]. Indeed, defects in DDR are associated with an increased risk of developing cancer during life time. Briefly, the first step of DDR consists in the recognition of the damage by “sensors” proteins that identify DNA structures induced by DNA damaging agents and replication stress.
The master sensors are ATM (ataxia-telangiectasia mutated), ATR (ATM- and Rad3-Related), and DNA-PKs (DNA-dependent protein kinase). These are large serine/threonine kinases members of the phosphatidylinositol-3-kinase-like kinase family (PIKKs) and orchestrate a large network of cellular processes to maintain genomic integrity with distinct specificities and functions [40,41]. The ATM kinase is primarily activated by double-stranded DNA breaks (DSBs) and it leads to the phosphorylation of a number of substrates, such as BRCA1, CHK2, and p53, mediating DNA repair, cell-cycle arrest, and apoptosis.
The ATR kinase, essential for the survival of proliferating cells because it monitors replication fork progression, responds to a broad spectrum of DNA damages, including single-stranded DNA breaks (SSBs) and a variety of DNA lesions that interfere with replication . It phosphorylates substrates such as BRCA1, CHK1, p53, and RAD17 that, in turn, mediate inhibition of DNA replication and promote DNA repair. Finally, DNA-PKs regulate a smaller number of targets and play a role primarily in non-homologous end joining (NHEJ) [43,44].
There are six main different DNA repair pathways: mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER), homologous recombination (HR), non-homologous end joining (NHEJ), and Fanconi Anemia (FA) (reviewed by Curtin ). The coordinated interplay among these different pathways results in repair and cell survival with no fixed DNA damage, low levels of repair, and accumulation of DNA damage that cells can tolerate but might predispose to the development of several pathologies, including tumors, or activation of cell death when DNA damage is too high to be repaired.
Alteration (upregulation and downregulation) in these pathways contribute to the sensitivity and resistance to platinum agents (Figure 1). The detailed description of these pathways is beyond the scope of this manuscript [45,46], but it is worth saying that even if each DNA repair pathway is generally activated by specific DNA lesions, some redundancy among the different pathways exists. The lack of a specific repair can be associated with the upregulation of the other.
Even if cells with a specific DNA repair deficiency will try to repair the DNA damage with the remaining DNA repair pathways, most of these cells will be, by definition, extremely sensitive to agents causing DNA damage repaired by that specific repair pathway or to those agents in synthetic lethality with that specific defect .
Almost all the major DNA repair mechanisms can participate in the removal of cisplatin adducts. In particular, several evidences support a pivotal role of NER, MMR, BER, HR, NHEJ, and FA pathways (Table 1). While the intra-strand crosslinks cause a distortion of the DNA double helix and activate mainly the NER pathway, the inter-strand cross-links rely on the coordinated interaction among translational synthesis, HR, NER, and FA pathways .
Briefly, NER is a highly conserved and versatile pathway used to remove “bulky lesions” (i.e., the ones caused by UV light), which distort the DNA double helix and has been involved in the repair of intra- and inter-strand crosslinks (ICLs), including the ones caused by platinum drugs. Two major NER pathways can be recognized: the transcription coupled repair (TCR) and the global genome repair (GGR); the former is a highly specific and efficient system that detects and removes the DNA damage that blocks the progression of RNA polymerase II in actively transcribed genes, while the latter is a slow process that inspects the entire genome.
The initial steps involve the recognition of DNA damage by proteins (XPA, XPC, RPA) following by the unwinding of DNA by the helicase XPD and XPB. The DNA around the damaged site is then cleaved by the XPG 3′ and XPF–ERCC1 5′ nucleases. A DNA polymerase re-synthetizes the DNA sequence on the complementary strand [49,50]. Homologous recombination is an error free mechanism for the repair of DSBs or stalled replication forks occurring in S and G2 phases of the cell cycle, caused by IR, ROS, and some antineoplastic drugs, including platinum-based drugs.
Homologous recombination uses the sister chromatid as a template to repair the DSB. When the break is recognized, BRCA1 mediates the recruitment of the MRN complex. The ATM kinase activates MRN components by phosphorylation and the complex resects the 3′ strand ends on either side of the DSB, the single strand DNA is unwound, and the 3′ strand from the damaged chromosome invades into the sister chromosome with the involvement of BRCA2 and RAD51.
The damaged 3′ end is then extended using complimentary sequence by a DNA polymerase [51,52]. The FA pathway is particularly important for the removal of intra-strand cross-link. This pathway promotes the monoubititination of the FA complementation group D2 (FANCD2)–FA complementation group I (FANCI) heterodimer, activating the DNA damage response. The FA pathways not only ensures an efficient DNA damage repair through homologous recombination, but also coordinates DNA replication and fine-tunes mitotic checkpoints for an error-free chromosome segregation. Loss of the FA pathway by mutation in FA genes renders cells hypersensitive to DNA inter-strand crosslinking agents, including platinum drugs .
Major pathways involved in the repair of platinum DNA damage.
|Platinum DNA Damage||Repair Pathway||Specific Genes Implicated in Platinum Resistance|
|Mono-adduct||Base excision repair (BER)||OGG1, PARP1|
|Intra-strand cross-link||Nucleotide excision repair (NER)|
Mismatch repair (MMR)
Tolerance pathway (translesion synthesis)
|ERCC1, XPF, XPD, XPG|
MLH1, MSH2, MSH6, PMS2
Polymerase (pol) η, ζ Rev1
Famconi Anemia (FA)
Homologous recombination (HR)
|ERCC1, XPF, XPG, XPD|
FA core complex genes, FANCD2
BRCA1, BRCA2, RAD51, RAD52, XRCC2, XRCC3, RPA
Non-homologous end joining (NHEJ)
|BRCA1, BRCA2, RAD51, RAD52, XRCC2, XRCC3, RPA|
XRCC4, XRCC5, KU70, DNA-PKcs
To better understand the role of these DNA repair pathways in the mechanism of action of platinum drugs, we will briefly summarize their importance in cellular response to these agents. Detailed in vitro studies, using isogenic systems, indicated that NER defects are associated with an extreme sensitivity (more than a 100-fold increase as regards parental wild-type cells) to cisplatin. In particular, two components of the NER, excision repair cross-complementing 1 (ERCC1) and Xeroderma Pigmentosum, Complementation Group F (XPF), are those mostly influencing platinum-DNA adducts removal .
In contrast to NER, defects of MMR are associated with resistance to both cisplatin and carboplatin [54,55,56], likely due to the ability of MMR proteins to detect the lesions and to activate apoptotic processes [57,58]. For BER, little evidence linking this repair system to platinum containing drugs sensitivity has been reported [59,60,61].
Homologous recombination and FA have also been shown to be important for the cellular response to platinum agents as cells deficient in these pathways are extremely sensitivity to them [62,63]. Finally, other proteins and pathways participating in the DNA damage response have been involved in cisplatin/carboplatin activity, such as ATR and ATM proteins (DNA damage sensors) and checkpoint proteins CHK1 and CHK2 [64,65]. Recently, the role of CDK12 in master regulation of DNA repair gene transcription, as outlined in a recent review , and in modulating sensitivity to PARP inhibitors and platinum agents has been displayed.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6357127/
More information: Elina A. Pietilä et al, Co-evolution of matrisome and adaptive adhesion dynamics drives ovarian cancer chemoresistance, Nature Communications (2021). DOI: 10.1038/s41467-021-24009-8