Stem cells, Notch-1 signaling, and oxidative stress: a hellish trio in cancer development and progression within the airways. Is there a role for natural compounds?

Abstract

Notch-1 signaling plays a crucial role in stem cell maintenance and in repair mechanisms in various mucosal surfaces, including airway mucosa. Persistent injury can induce an aberrant activation of Notch-1 signaling in stem cells leading to an increased risk of cancer initiation and progression. Chronic inflammatory respiratory disorders, including chronic obstructive pulmonary disease (COPD) is associated with both overactivation of Notch-1 signaling and increased lung cancer risk. Increased oxidative stress, also due to cigarette smoke, can further contribute to promote cancer initiation and progression by amplifying inflammatory responses, by activating the Notch-1 signaling, and by blocking regulatory mechanisms that inhibit the growth capacity of stem cells. This review offers a comprehensive overview of the effects of aberrant Notch-1 signaling activation in stem cells and of increased oxidative stress in lung cancer. The putative role of natural compounds with antioxidant properties is also described.

Notch-1 signaling activation preserves stem cells and cancer stem cells. Oxidative stress is the result of an imbalance between pro and antioxidant mechanisms. Cigarette smoke increases oxidative stress and affects Notch-1 signaling activation.

Graphical Abstract

Graphical Abstract.

1. Introduction

Tissue stem/progenitor cells are crucial in the homeostasis and tissue repair of various mucosal surfaces following injury, including the airway mucosa [1]. There are several subgroups of stem cells (embryonic stem cells and somatic or adult non-embryonic stem cells) identified by different cellular markers that correlate with various cellular events. Continued activation of repair processes to restore damage due to a persistent inflammatory insult can lead to the transformation of stem cells into cancer stem cells (CSCs) [2]. Responses to stimuli from the surrounding microenvironment, activation of various signaling pathways, epigenetic modifications, and self-renewal processes activate dedifferentiation processes can further contribute to generate undifferentiated cancer cell progenitors.

The Notch pathway plays an important role in many processes that preserve stem cells because it regulates mechanisms involved in angiogenesis, self-renewal, adhesion, and differentiation [1]. These physiological functions of Notch signaling explain why it is one of the most activated pathways in cancer cells participating in all cancer processes from initiation to cancer metastasis [2].

Increased oxidative stress can also contribute to cancer initiation and progression [3]. In this regard, it has been demonstrated that oxidative stress activates several signaling pathways that control processes of self-renewal ability of stem or CSCs. During chronic inflammatory processes, increased reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as peroxynitrites and nitrogen oxides, can generate mutagenic DNA adducts and can fuel processes favoring cancer progression including drug resistance and angiogenesis [3].

The present review illustrates the role of interaction among stem cells, Notch-1, and oxidative stress in the onset and progression of cancer lesions of the airways focusing attention on the associated chronic inflammatory diseases and on the role played by cigarette smoke exposure. The role of natural compounds in preventing cancer development and progression by modulating stem cells, Notch-1, and oxidative stress interactions is also discussed.

2. Stem cells, stem cell niches, and CSCs

Stem cells are classified into mesenchymal stem cells, hematopoietic stem cells, pluripotent or multipotent stem cells, and tissue-specific stem cells. Stem cells maintain tissue homeostasis thanks to their proliferative capacity and their ability to become specialized cell populations. Stem cells and progenitor cells differ in their self-renewal capacity. In this regard, it has been demonstrated that stem cell undergoes asymmetric cell division that leads to the generation of a stem cell with perpetual self-renewal capacity and a progenitor cell that is committed to differentiation and proliferation without self-renewal capacity [4]. Two main kinds of stem cells are recognized in humans: embryonic stem cells and adult stem cells (ASCs). In tissues or organs, ASCs are undifferentiated cells that thanks to their ability of self-renewal and of multipotency can differentiate into all of the major specialized cell types of a tissue or organ. These cells, frequently in a resting state, enter the cell cycle sensing stimuli from their niche when necessary, as during wound healing upon tissue injury. In this regard, cyclin-dependent kinase inhibitors, p53 and p21 axis, epigenetic alterations including DNA methylation and histone alterations, and the Notch signaling pathway can all be able to modify the quiescent state of ASCs promoting the access to a proliferative and metabolic active state [5].

The loss of stem cell number and activity over time can explain organ dysfunction with age. Reduced proliferation rate, morphological, and biochemical changes of tissue stem cells depict a senescent state of these cells. The senescence processes can also be triggered by chronic insult exposure [6]. It is now more evident that alterations in stem cell niches in chronic degenerative diseases are responsible for stem cell aging due to the presence of suppressive stimuli that overcome activation stimuli. Stem cell niches include fibroblasts, endothelial, and immune cells that, releasing multiple factors, affect the stem cell niche microenvironment. Increased expression of signals such as Nox4 in stem cell niches impairs alveolar type 2 (AT2) to alveolar type 1 (AT1) differentiation through elevated ROS levels [7]. Accordingly, antioxidants can be useful in improving regeneration processes of alveoli restoring AT2 to AT1 differentiation processes in chronic diseases with uncontrolled oxidative stress [8]. Furthermore, aberrant activation of repair processes in response to chronic insults can promote mutagenic DNA adducts in stem cells thus favoring tumor initiation and then the consequent development of CSCs [8]. CSCs can be alternately originated by the induction of dedifferentiation processes. In this regard, it is known that all differentiated cells can undergo dedifferentiation processes and, by expressing the appropriate transcription factors, can acquire a pluripotency state [9]. CSCs are a small group of cells with potential multidirectional differentiation capacity since they are capable of self-renewal and can undergo differentiation to generate the phenotypic heterogeneity observed in cancer. Many tumors have highly proliferating cells and slowly proliferating cells inside them [10]. CSCs other than promoting cancer initiation may be responsible for tumor progression, metastasis, resistance to therapy, and subsequent tumor recurrence [10]. Cancer relapse after chemotherapy or radiotherapy is initiated by CSCs since this subpopulation is highly resistant to these treatments [11, 12]. Highly drug-resistant CSC population cells use redox systems to counteract the cytotoxic activities of anticancer agents. The CSCs, in a similar fashion to non-CSCs, are encompassed by a specialized cell microenvironment that sends activating signals to these cells [13, 14]. Collectively all these data support the notion that the main difference between normal and CSCs resides in no control over the cell number by CSCs [3].

3. Notch-1 signaling activation and CSCs

The Notch pathway plays a key role in the development and maintenance of the hematopoietic cell population related to the development, proliferation, and differentiation of stem cells [1]. It is known that the Notch pathway plays a role in the different cellular pathways in the various stages of growth of hematopoietic cells, also involving hematopoietic CSCs in the mechanism of self-regeneration or differentiation of common lymphoid precursors in T or B cells [2].

3.1 Description of the Notch pathway

Four Notch receptors (Notch-1 to Notch-4) and five ligands are expressed in humans: Delta-like 1, 3, 4 (Dll1, Dll3, Dll4), and Jagged 1, 2. It is known that Notch receptors are made up of three domains: extracellular Notch domain (NECD), transmembrane Notch domain (NTM), and intracellular Notch domain (NICD) [15, 16]. Notch signaling activation starts upon interaction with a ligand that induces two proteolytic cleavages. The first trigger of cleavage is catalyzed by ADAM family metalloproteases and leads to the removal of the extracellular domain of Notch. The residual part of the Notch protein undergoes a second cleavage through Notch gamma-secretase generating the activated form of the Notch receptor, NICD. The NICD molecule migrates into the nucleus and combines with other proteins, consequently modulating its main target gene expression, Hes and Hey family [15]. Inside the nucleus, NICD forms a complex with the DNA-bound transcription factor CSL and a Mastermind family coactivator (MAML) [16].

All four Notch receptors can be activated during physiological or pathological conditions. The consequence of Notch-1 activation on cell proliferation and the cellular apoptotic mechanisms can be regulated by modulating the expression of cell cycle regulatory proteins [17]. All four Notch receptors can be activated during physiological or pathological conditions. To our knowledge, the main difference relies on the overactivation of the Notch pathway during pathological conditions. Increased oxidative stress could be considered as one of the main triggers that promotes this overactivation.

3.2 Notch, CSCs, and the induction of epithelial–mesenchymal transition processes

The activation of the Notch pathway and thus the expression of its target gene, HES1, plays a fundamental role in the maintenance of CSCs, in a framework similar to what occurs during their embryonic development [18] (Fig. 1). The Notch signaling is also involved in the induction of epithelial–mesenchymal transition (EMT) processes [19]. EMT is a highly coordinated process that, when activated, through key transcription factors such as Snail, Twist, and Zeb, leads to the loss of epithelial cell polarity together with the loss of cell–cell adhesion and the acquisition of mesenchymal cell properties [19]. Although the EMT physiologically concurs with the events finalized to tissue repair, during lung cancer processes, this process can be used by cancer cells to promote metastasis. Stemness is a property associated with EMT processes. CSCs are characterized by increased EMT transcription factors, along with reduced intercellular adhesion protein expression. Notch signaling can promote EMT processes through different mechanisms [20]. Notch signaling activation increases NF-κB thus inducing metalloproteases, TGF-beta, or Twist thus promoting EMT. In addition, Notch can induce the expression of transcription factors including Snail that increase mesenchymal markers, such as vimentin, other than reducing epithelial markers, including E-cadherin [20]. It is also well established that EMT in cancer has been depicted as a dedifferentiation event with a relevant role in the acquisition of stem cell characteristics [21].

Figure 1.

(A) Upon Notch ligand and Notch-1 receptor interaction, ADAM-family metalloproteases catalyze a first cleavage that leads to the removal of the extracellular domain of the Notch-1 receptor. (B) Gamma-secretase mediates a second Notch cleavage that generates the activated form of the Notch receptor, NICD. (C) The NICD migrates into the nucleus, where interacting with other proteins (CSL and MAML), modulates primary target genes, such as the Hes and Hey family. (D) Fully activated Notch signaling and target molecules maintain and preserve stem cell niches and promote EMT and angiogenesis processes. Pharmacological approaches to block the Notch pathway are shown: mAbs against receptor and ligand, gamma-secretase inhibitor (GSI), and Notch transcription complex inhibitor (CB-103).

3.3 Pharmacological targeting

Notch signaling could be targeted by inhibiting the signaling pathway by two major classes of Notch inhibitors that primarily focus on the clinical development of promising agents that either block Notch receptor cleavages such as secretase inhibitors (GSIs) or interfere with the Notch ligand–receptor interaction by monoclonal antibodies directed against Notch ligand or Notch receptor or inhibit the Notch transcription complex, such as CB-103 [22].

Notch signaling is an important mechanism for direct cell–cell communication involved in cell fate determination, stem cell potential, and lineage commitment. The biological function of this pathway is critically context-dependent. The finding that Notch signaling elicits a duality of signals, involved in growth/differentiation control, and cell survival supports the inhibition of this pathway as one of the new approaches for cancer therapy [23, 24]. A therapeutic strategy involving the combined use of traditional therapies and Notch inhibitors may prove more effective in removing stem cells with drug resistance thus leading to effective cancer eradication [2]. This approach of targeting the signaling pathway of CSCs, such as the Notch pathway, represents a promising future direction for the therapeutic strategy to cure cancer. The choice of the most appropriate inhibitor for each patient relies on the best patient stratification according to efficacy and pharmacodynamic biomarkers.

4. Oxidative stress role in Notch-1 signaling activation and CSCs

4.1 Mechanisms for ROS production

Oxidative stress is the result of an excessive production of ROS or RNS and defective antioxidant mechanisms [25].

ROS and RNS are free radicals, i.e. chemical species that have at least one unpaired electron in the outer part that gives them high reactivity. ROS and RNS production is triggered by multiple cellular processes in response to exogenous and endogenous stimuli. ROS have higher reactivity than RNS. The cellular sources of ROS formation are mitochondria, plasma membranes, chloroplasts, peroxisomes, and endoplasmatic reticulum stimuli [26]. In eukaryotic cells, ROS production is mainly related to the biochemical processes associated with mitochondrial cellular respiration for oxidative ATP production [27]. The main ROS derived from molecular oxygen are superoxide and hydrogen peroxide.

During cellular respiration within mitochondria molecular oxygen is reduced to water in the electron transport chain and superoxide radical is produced. Manganese superoxide dismutase then converts the superoxide radical to hydrogen peroxide. ROS production addresses the need for the response or adaptation to bioenergetic changes in the cells.

4.2 Correlation between oxidative stress, cancer, and Notch pathway

The increase of ROS in cancer cells is known to play a role in the initiation and progression of cancer. In this regard, it is important to underline that the most significant mutagens in stem cells are potent inducers of oxidative stress. Several signaling pathways enhanced by oxidative stress affect the self-renewal ability of stem or CSCs, suggesting important roles in tumorigenesis processes. The presence of ROS can produce cellular stress and damage that may produce cell death [28]. Oxidative stress can either activate cell survival or apoptosis mechanisms, depending on the severity, and duration of exposure [29, 30]. In this regard, it has been demonstrated that ROS, at low concentrations, induces both normal cell and cancer cell proliferation, as well as cell survival [31, 32] while intermediate concentrations promote cell cycle arrest and cell differentiation [33, 34]. Higher ROS concentrations damage proteins, DNA, and RNA, and result in mutations that can promote both carcinogenesis in normal cells or resistance to drugs in cancer cells [35, 36]. Although ROS can enhance cytotoxicity and apoptosis, most cancer cells can remain viable in the presence of intrinsic oxidative stress, enabling them to avoid apoptosis and become resistant to many chemotherapeutic drugs [37, 38]. Modulating ROS to overcome multidrug resistance in cancer could be an effective strategy to fight cancer growth [3]. The fluctuation of ROS levels can activate Notch signaling via activation of Nuclear factor erythroid 2-related factor 2 (Nrf2) in cells with staminal properties [39]. The increase in ROS levels activates Nrf2 that, other than promoting antioxidant responses, triggers Notch signaling activation to maintain homeostasis balance. Nrf2 silencing downregulates both Notch ligand and receptor expression [39]. Aberrant Nrf2-Notch crosstalk can synergistically cause neoplastic proliferation in lung cancer [40].

5. Stem cells in repair of the airways and initiation of lung cancers

5.1 Stem cells in airways repair

Airway epithelium, constantly exposed to environmental hazards and oxidative stress-mediated injury, can be damaged and should be repaired to maintain homeostasis. Multiple studies indicate that cells with a stem-like behavior are present throughout the airways and play a fundamental role in repair mechanisms in response to airway injury (Fig. 2). Studies of airway repair processes in the trachea have identified two cellular populations that act as stem cells in response to injury. In this district, it is possible to identify a population of tracheal multipotent basal cells that maintain bromodeoxyuridine expression and express keratin-5 (K5) [41] and a population expressing the keratin-14 (K14) marker [42]. Excessive self-renewal, if not prevented, can lead to cancer.

Figure 2.

(A) Airway epithelium, constantly exposed to oxidative stress-mediated injury, can be damaged and should be repaired to maintain homeostasis. (B) In tracheal airway epithelium, stem cells (basal cells) constantly exposed to oxidative stress-mediated injury can lead to squamous cell carcinoma. (C) In the airway epithelium of bronchioles, stem cells (club variant cells) constantly exposed to oxidative stress-mediated injury can lead to small cell carcinoma. (D) In the airway epithelium of alveoli, cells such as (Bronchiolar Club cells) are constantly exposed to oxidative stress-mediated injury can lead to adenocarcinoma.

5.2 Initiation of lung cancers

It has been established that tracheal squamous cell carcinoma originates from K5⁺ stem cells. This finding may be useful to promote further targeted therapy research aimed to identify and validate molecules directed against K5⁺ cells thus eliminating, in a selective way, only the stem cell niche of squamous carcinoma without affecting K14⁺ stem cells that can continue to preserve tissue homeostasis. Importantly, CSCs maintain multipotency, i.e. the ability to give rise to phenotypically different cells with different expression of markers that confer drug resistance. Cells with higher expression of anti-apoptotic markers and drug-resistant proteins survive even after chemotherapy, and this explains why despite the effect of marked reduction of tumor mass after therapy, the onset of metastasis resistant to that treatment is observed after time. The selection of chemoresistant CSCs that continue to proliferate undisturbed after therapy could explain the development of aggressive lung cancers with a high propensity to metastasize [43]. Lung tumors are classified as small-cell lung cancers (SCLC) and non-small cell lung cancers (NSCLC). Lung adenocarcinoma is the most frequent cancer among the NSCLC. Similar to other cancers, lung cancer originates from molecular alterations in airway stem cells induced by mutagenic substances. In this regard, a subtype of lung adenocarcinomas with specific molecular, pathological, and clinical phenotypes maintains the gene expression and molecular characteristics typical of basal cells (BCs) of the human airways [44]. A cluster of lung adenocarcinomas with high expression of BC signature genes has a poorer tumor grade, shorter survival, and higher metastatic potential. These lung adenocarcinomas are characterized by an increased frequency of KRAS mutations and by activation of multiple pathways regulating cell cycle, extracellular matrix organization, and differentiation processes with suppression of ciliated and exocrine bronchiolar cell-related genes [44].

Common tumors from each representative airway zone have been associated with stem cell niches differentiated by cellular expression of surface markers. The lung CSCs can be identified by markers that mainly include surface biomarkers such as CD133, CD44, and ATP-binding cassette subfamily G member 2 [45]. CD133⁺ cells isolated from fresh lung tumor samples can grow indefinitely in culture and this property is normally found in cell lines transformed or immortalized by viral transfections [46]. CD133⁺ cells can proliferate into phenotypically diverse types of tumors. Lung cancer patients with increased numbers of CD133⁺ cells show a worse prognosis than those with reduced numbers of this type of cells. In fact, CD133 lung CSCs exhibit increased resistance to cisplatin-based therapies and this finding should be considered for the choice of chemotherapeutic regimens in lung cancer patients [47]. With regard to the CD44 marker, it has been demonstrated that overexpression of this marker in lung cancer patients leads to poor survival due to increased metastatic potential confirmed by high CD44 expression in metastatic lymph nodes [48]. ATP-binding cassette subfamily G member 2 favors chemotherapy resistance in lung cancer. In this regard, it has been demonstrated that ATP Binding Cassette Subfamily B Member 1 (ABCB1) inhibitor restores the etopoposide activity inducing lung cancer cell apoptosis [49].

All these findings together confirm that lung CSCs have an important impact on cancer prognosis, strongly suggests that the specific targeting of lung CSCs or the pathways that are able to confer stem cell-like properties to lung cancer cells are effective therapeutic strategies possible in the future [50].

6. Role of cigarette smoke, stem cells, oxidative stress, and Notch-1 signaling pathway in the relationship between chronic obstructive pulmonary disease and lung cancer

Cigarette smoking promotes repeated damage/repair cycles that can compromise the integrity of lung tissues by exhausting the repair potential of stem cells [51]. Furthermore, cigarette smoke contains carcinogenic substances and it is known to promote the onset and progression of cancer. One of the mechanisms through which it promotes cancer progression is the activation of the Notch pathway. Dysregulation of this pathway causes a wide range of respiratory diseases including chronic obstructive pulmonary disease (COPD) and lung cancer [52].

Activation of Notch-1 signaling is promoted by hypoxia-induced microRNAs (miRNAs) and may play a role in triggering lung cancer progression by acting in the regulation of EMT and contributing to aggressive tumor growth, invasion as well as metastasis, and cancer recurrence [24]. In this regard, nutrients and oxygen deprivation induce stromal fibroblasts to release IL-6 and growth factors thus activating a paracrine loop that in turn drives both cell dedifferentiation and tumor formation/progression [53]. Hypoxia also plays a role in the induction of CD133 expression, which has been used as a stem cell biomarker [54]. In patients with COPD, hypoxia may be one of the fundamental biological phenomena that can contribute to the development and aggressiveness of lung cancer [55].

6.1 Impact of cigarette smoke on Notch signaling in non-tumoral cells

Notch-1 is a fundamental signaling system involved in cellular machinery that relies on direct cell–cell contact [56]. In bronchial epithelial cells, activation of Notch-1 by Jag-1 increases Ki-67 and cell proliferation [57]. Exposure to cigarette smoke extract (CSE) in non-tumor bronchial epithelial cells acts negatively on Notch-1 signaling, reducing both the expression of the Notch-1 ligand, Jag-1, and the translocation of the Notch-1 activated form to the nuclear region [57]. Furthermore, although CSE increases the expression of PCNA, Ki-67, and p21, reducing cell proliferation and repair processes are observed.

It has also been demonstrated that CSE exposure promotes a persistent increase in ROS and reduces antioxidant responses [58]. In this way, it could compromise the physiological repair process of the airway epithelium. In this regard, acute exposure to arsenite increases the oxidative stress mechanism and inhibits the proliferation of keratinocyte cells through upregulation of the p21 oncogene and through downregulation of the Notch-1 molecule [59].

The harmful effects of CSE on airway cellular homeostasis could also be due to the regulation of alarmins, such as IL-33. IL-33 belongs to the IL-1 superfamily of cytokines and is produced by endothelial and epithelial cells. IL-33 can translocate into the nucleus and can act as a transcription factor regulating the expression of some important target genes [60]. IL-33 can also be released in the extracellular district and therefore, after binding with its receptor expressed on immune cells, plays a role in the activation of allergic inflammation inducing the release of the cytokines IL-5 and IL-13 [61] and of mucus from the metaplastic epithelium [62]. IL-33 is known to contribute to cellular homeostasis in the airways in response to injury [63] and induces a significant decrease in the expression of the Jag1 molecule [64]. A previous study demonstrates that although CSE increases the presence of IL-33 at the intracellular level, it reduces its release [65].

6.2 Impact of cigarette smoke on Notch signaling in cancer cells

The Notch-1 activation pathway is known to be involved in lung cancer by promoting CSCs maintenance [66]. A histological study on surgical specimens from patients with lung adenocarcinoma has been demonstrated that the lung parenchyma of smokers has an increased expression of Notch-1 but not of CD133 and Jag-1 compared to the lung parenchyma of non-smokers [67]. In a previous study, increased expression of Notch-3 is demonstrated in smokers with lung adenocarcinoma [68]. Furthermore, lung adenocarcinoma cells show constitutive basal activation of Notch-1 signaling as they constitutively express both Notch-1 and Jag-1 and also nuclear NICD. In this regard, the A549 cell line (lung adenocarcinoma) exposed to CSE showed an increase in the expression of the Notch-1 and Hes-1 genes as well as the protein localized in the nuclei without modified Jag-1 gene expression [67].

These results further confirm that the regulation of the Notch pathway can also occur at the transcriptional level, through the activation of molecules, such as Hes-1, that play a role as downstream effectors of the pathway [69]. It has been demonstrated in chronic lymphocytic leukemia that Hes-1 binds to the PTEN promoter region reducing its expression and in turn promoting cell proliferation and inhibiting cell apoptosis [70].

Resistance to the cellular apoptosis mechanism as well as increased cell proliferation are both fundamental events in the progression of cancer. In previous studies, it has been demonstrated that exposure to CSE in A549 induces both the expression of Ki-67 and survivin and that DAPT, a Notch pathway inhibitor, counteracts the CSE effect inducing a downregulation of Ki-67 and survivin [67]. Furthermore, CSE increases the expression of survivin in adenocarcinoma cells also reducing the expression of FoxO3 [71, 72], a transcription factor that negatively controls survivin gene expression [73]. Another study confirms that Notch signaling also promotes cancer progression in other cancer types such as breast cancer by inducing tumor cell survival, proliferation, and apoptotic resistance through increased survivin expression [74]. Survivin promotes cancer progression by exercising a dual role of apoptosis inhibition and mitotic effector, thus protecting tumor cells also from apoptosis induced by drug treatment [75]. Ki-67 promotes cell proliferation and the evaluation of its expression is used as a marker for estimating the degree of histological malignancy and for the prognosis of NSCLC [76]. In fact, it has been demonstrated that in smokers affected by lung adenocarcinoma, the expression of survivin and Ki-67 is increased especially in the initial stages [77]. Another study demonstrates that an increase in survivin expression can also be found in the proximal airways of smoking subjects not affected by lung cancer and this is also confirmed in vitro in bronchial epithelial cells exposed to CSE [78]. Furthermore, it has been observed that nicotine present in cigarette smoke plays a role in increasing survivin in human NSCLC cell lines [79].

All these events are involved in mechanisms promoting cancer development and progression. Indeed, blocking Notch-1 signaling through the inhibitor DAPT or a specific siRNA has been shown to reduce the growth potential of lung CSCs [80].

Taken together, all of this evidence supports a novel mechanism by which exposure to cigarette smoke could promote tumorigenesis by impairing repair processes within the airways due to alterations in redox balance and signaling activation. Nocth-1 and downstream molecular events (Fig. 3).

Figure 3.

(A) Cigarette smoke, induces oxidative stress, affects the self-renewal ability of stem cells impairing tissue repair, and promotes mutagenic events and dedifferentiation processes leading to cancer stem cells. (B) Notch-1 pathway activation affects cell proliferation, survival, and angiogenesis suggesting important roles in the tumorigenesis process. (D) Notch-1 pathway activation confers metastatic properties and resistance to therapy contributing to the progression of cancer including cancer metastasis.

7. Effects of natural compounds

Natural compounds have been widely recognized to have the potential for the prevention of cancer progression and as complementary or standalone treatments for cancer patients [81]. Dietary factors may modulate the impact of adverse environmental exposures or genetic predisposition within the lung. Changes in diet over the past few decades, with decreased consumption of fruits, vegetables, whole grains, and fish, and increased consumption of processed and refined foods have a negative impact on human health increasing disability due to chronic diseases such as COPD or increasing lethality due to cancer onset and progression. It has now emerging evidence that dietary patterns rather than individual nutrients exert a protective effect suggesting that synergistic interactions between multiple nutrients contained in dietary patterns are responsible for the observed benefits [82]. Targeting oxidative stress with antioxidants or boosting endogenous levels of antioxidants can be an effective strategy in the management of COPD and in preventing cancer onset and progression. Adequate fruit and vegetable consumption is associated with lower markers of oxidative stress and inflammation, and higher levels of antioxidant markers [83].

Polyphenols, a class of natural compounds largely present in fruits and vegetables, have been shown to act as anti-inflammatory agents. Phenolic acids, flavonoids, and stilbenes (resveratrol), all belonging to polyphenols, have been reportedly associated with the prevention of chronic diseases and cancer [84]. The most relevant mechanisms by which polyphenols are beneficial are related to the modulation of the Nrf2 pathway [81]. As previously mentioned aberrant Nrf2-Notch crosstalk exerts a relevant role in lung cancer [40] and accordingly several experimental studies demonstrate that polyphenol-mediated cancer cell death is linked to Nrf2 downregulation [81]. Luteolin, a flavonoid suppresses Nrf2 activity by increasing Nrf2 mRNA turnover and sensitizes NSCLC A549 cells to therapeutic drugs. Resveratrol reduces the migratory and invasive abilities of A549 by inhibiting heme oxygenase (HO). Apigenin, a natural dietary flavonoid, is an effective small molecule inhibitor against Nrf2 and has displayed anticancer activity in various cancers [85]. It has been demonstrated that apigenin in A549 increases the production of ROS, with augmented cell death and apoptosis [86]. The toxic effects of apigenin on lung cancer cells are maintained also in the presence of pro-survival events such as leptin pathway activation or exposure to pleural fluids from patients with lung cancer [86]. Moreover, a systematic review and meta-analysis performed on 8799 cases of lung cancer and 17 072 non-tumoral cases demonstrates that the dietary patterns containing fish intake are associated with a decreased risk of lung cancer [87]. In this regard, the protective effects of fish can be associated with the high content of omega-3 that are potent modulators of oxidative stress [88].

Omega 3 metabolites are substrates for oxidants inside the cell into cell membranes and promote the formation of nonenzymatic lipid peroxidation products whose biological activity is context and dose-dependent. In fact, they have also been reported to be cytoprotective and anti-inflammatory in macrophages [89] but are highly toxic in cancer cells [90]. The levels of these toxic compounds are higher in cancer cells than in normal cells since tumor cells contain higher levels of ROS compared to normal cells, principally due to their accelerated metabolism needed to maintain their high proliferation rate. It has been reported that electrophilic keto-derivative of the omega-3 fatty acid docosahexaenoic acid (DHA), endogenously generated by cyclooxygenase-2 and cellular dehydrogenase, promotes anti-lung cancer effects. This electrophilic keto-derivative of the omega-3 s, alone and more when added to gemcitabine, reduces proliferation and increases cell apoptosis on a panel of five histologically different human NSCLC cell lines [91].

8. Conclusions

This review offers a comprehensive view of the effects of oxidative stress in Notch-1 signaling activation in CSCs and adult stem cells (ASCs) in order to better understand both initiation and progression processes in lung cancer. Several mechanisms have been elucidated and potential new therapeutic targets have been identified to contrast cancer growth. Moreover, in COPD patients, targeting oxidative stress with natural compounds can be an easy way to stunt tumor onset, growth, and progression. The role exerted by the complex relationship among Notch, cigarette smoke, and stemness in lung cancer biology deserves further investigation.

Author contributions

Giuseppina Chiappara contributed to the conceptualization and wrote the original draft of the review, Serena Di Vincenzo and Caterina Cascio reviewed the manuscript and Elisabetta Pace is responsible for conceptualization, supervision, and for the final revision of the manuscript. All authors have read, reviewed, and approved the final manuscript.

Conflict of interest statement: All the authors declare that there is no conflict of interest regarding the publication of this paper.

Funding

This work was supported by the Italian National Research Council (FOE 2022-DSB.AD006.361.028) and by the project “One Health Basic and Translational Research Actions Addressing Unmet Needs on Emerging Infectious Diseases (IN-FACT) Proj n. PE00000007 CUP B53C20040570005.

Data availability

Data is contained within the article.