In this pathway, a specific category of proteins, such as lipoxygenase (LOX), cyclooxygenase (COX), and cytochrome P450, which contains peroxidase activity, catalyze LP, where it is also involved in the formation of R-OOH. The proteins that have pseudo-peroxidase activity (cytochrome c) under some conditions can also catalyze LP via binding to cardiolipin [30–32]. LOX specifically catalyzes the oxidation of abundantly found arachidonic (C20:4) and linoleic (C18:2) polyenoic fatty acids by molecular oxygen to form hydroperoxyl groups at distinct carbon positions of acyl chains [33]. Several reports have shown that among COX-1 and COX-2 isoforms, the latter is crucially expressed in tumoral tissues [34–36]. Some contrasting reports revealed the existence of the enzyme overexpression product eicosanoids in breast, lung, and pancreatic cancer. The proinflammatory product prostaglandin E₂ (PGE₂) is found to be produced via COX-2 in mutagenesis, angiogenesis, and cell migration, all linked with cancer. The COX-2 mechanism pathway has been proven using biological cell lines, i.e., human colorectal HT-29 and human prostate carcinoma DU145 cell lines [37, 38]. A crucial interlink is found between the production of PGE₂ along with tumor cell resistance to programmed cell death via the activation of a cascade of P2Y2 P2Y2/Src/p38 [P2Y2 nucleotide receptor, Src is the Src protein-tyrosine kinase, and p38 refers to the p38 mitogen-activated protein kinase (MAPK) signaling pathway]. Through this membrane released arachidonic acid (AA) by PLA₂ overexpression, simultaneously with the overexpression of COX-2, with PGE₂ production [38]. Figure 2 illustrates the conversion of membrane phospholipids into AA, as well as the involvement of COX-1, COX-2, and LOX in tumor growth, and the role of inhibitors in the apoptosis of cancerous cells.

In non-enzymatic reactions, the initiation of radical chain reactions is essential for LP mediated via the Fenton reaction by involvement of transition metals, i.e., iron. Iron is highly reactive within cells and generates •OH by reacting with endogenously produced H₂O₂. The production of hydroxyl and ROO• initiates the lipid production. Similarly, like H₂O₂, PLOOH is also involved in the Fenton reaction, ultimately leading by a series of reactions to the generation of lipid hydroxy radical (PLO•) and lipid peroxyl radical (PLOO•) respectively [39, 40]. If PLOOH is neutralized, LP cannot take place; if not scavenged by antioxidants, propagation of LP takes place to the neighboring PUFA-phospholipids. This propagation leads to the deterioration of the plasma membrane, and LP causes fluidity and leakage in the cell membrane and obstructs membrane-bound enzyme activity [41]. In the absence of an efficient antioxidant defense system, lipid hydroperoxides become unstable and can lead to the production of secondary products, i.e., MDA, 4-HNE, and other reactive aldehydes [42, 43]. These deleterious effects lead to a reduction in cellular processes and aggravate cytotoxicity. Ultimately, uncontrolled cellular growth and lump formation take place, leading to apoptosis [8, 44]. Though, optimum level of LP is substantial for physiological processes and cell signaling, elevated LP causes morbidity of cells via involvement in several pathological conditions like neurodegenerative diseases, cardiovascular diseases, inflammation, and cancers [43, 45–47].

LP results in secondary metabolic products, MDA, also known as MDA, which can be produced via either peroxidation of PUFAs present in the membrane or prostaglandin production [48, 49]. MDA is a highly reactive and more toxic aldehyde, and when accumulated, it leads to membrane permeability and hinders the membrane fluidity of the bilayer lipid membrane. It is very mutagenic and reacts with biomolecules, specifically forming adducts with deoxyadenosine(dA) and deoxyguanosine(dG) of DNA, resulting in the formation of DNA adducts [50–52]. MDA-DNA adduct formation results in strand breaks, cell cycle arrest, and ultimately cell death. This is also linked to the development and progression of carcinogenesis. MDA is significantly used as a biomarker for the identification of LP [53, 54].

LP of PUFAs produces an array of secondary metabolic products and lipid electrophiles. Among all the lipid byproducts, 4-HNE is one of the most extensively studied lipid oxidative products. Though the biological consequences of 4-HNE are well established, the exact pathway via which its formation takes place is still not clear [55]. Many groups have suggested that 4-HNE is produced from the decomposition of the hydroperoxide of ω-6 PUFAs at the sn-2 position of glycerophospholipids present in cellular lipid membranes. It was found that most of the 4-HNE production is contributed by linoleic acid (LA, 18:2, ω-6) and AA (20:4, ω-6), which are abundantly present in cellular membranes [56]. A significant amount of HNE was also produced from the oxidation of mitochondrial phospholipids and cardiolipin, along with other oxidation products. HNE, linked with biomolecules and forms adducts, thus leading to the deterioration of structural proteins and DNA. The HNE adducts substantially impact key signaling pathways such as the mitogen-activated kinase signaling pathway, Nrf2, activating protein AP-2, and NF-κB, etc. [57]. Microtubule dysfunction and deteriorated synaptic system are also significantly affected by the abnormality in glucose uptake at synapses, crucially intervened by HNE [58].

Established reports also indicate the key role of HNE in controlling cell proliferation and differentiation. It has been demonstrated that cell line K562 derived from human erythroleukemia, cell proliferation is found to be decreased, as well as c-myc oncogene expression is blocked significantly by HNE [59, 60]. Pizzimenti et al. [61] and Rinaldi et al. [62] have reported that HNE is found to be involved in hindering the c-myc expression in U937, ML1 human leukemic cells, and murine erythroleukemic (MEL) cells. In tumor cell line SK-N-BE (neuroblastoma cells), HNE efficiently blocks cell proliferation and elicits apoptosis. In tumor cells that significantly express p53 and its family members (p63, p73), HNE is crucially found to be involved in inducing expression of these cells, thus resulting in inhibiting proliferation [63, 64]. HNE-mediated inhibition of human colon tumor cells through regulation of MAP kinase and PPAR gamma pathways has also been reported [65, 66]. Similarly, inhibition of cell proliferation is also reported in breast cancer cells, i.e., MCF7, by treatment with conjugated LA, leading to the elevation of endogenous levels of HNE [67] and in osteosarcoma cells treated with HNE [68]. Numerous reports reflect the pivotal role of HNE in controlling cell proliferation in tumor cells, whereas it does not affect the normal differentiation and growth of normal cells [69].

Among all the LP products, acrolein is the most reactive peroxidation product of PUFAs. It is also generated from the partial combustion of organic materials or fuel such as coal, petrol, and wood [70]. Apart from this, acrolein can also be present in cigarette smoke and sources like cyclophosphamide bioactivation. Threonine metabolism by myeloperoxidase of activated phagocytes is also involved in acrolein production [71]. It consists of three carbon atoms bonded via a double bond and is present in 40 times more concentration than any other transient reactive oxidant species [72]. Acrolein is also an electrophile and binds to the nucleophilic sites of basic cellular enzymes, DNA, and proteins, specifically to histidyl, lysyl, and cysteinyl residues, along with the N-terminal amino group of proteins. Acrolein also forms adducts with cysteinyl residues of enzymes (which are crucially involved in the catalytic activity of enzymes) and hinders the enzyme activity via interfering with substrate binding, thus leading to enzyme deactivation [73].

Neurofilament aggregation is also found to be induced by acrolein, which plays a vital role in cross-linking, resulting in OS induced production of a large amount of protein carbonylation, leading to neurodegenerative diseases [74]. Feng et al. [75] have established that acrolein is a major contributing agent for cigarette smoke-related lung cancer, playing a malicious role in DNA damage and obstructing DNA repair. Others have demonstrated that acrolein suppresses anticancer drug-induced cytotoxicity via the overexpression of CLDN1. Enhanced CLDN1 expression also upregulates the Nrf2 signaling pathway. Acrolein-mediated silencing of CLDN1 is directly linked to less sensitivity towards anticancer drugs, thus leading to cancer progression [76]. A report by Tsai et al. [77] showed that acrolein-induced DNA damage is found to be significantly higher in colorectal cancer tissues than in normal epithelial cells in colorectal cancer patients. Acrolein plays a crucial role in oncogenic transformation through activation of the RAS/MAPK signaling pathway, thus resulting in colon tumorigenesis [77, 78]. The underlying mechanisms are established by using cDNA microarray analysis with Ingenuity Pathway Analysis in NIH/3T3 Acr-clone#4 cells to establish that acrolein is involved in oncogenic transformation. The Tsai group [77] successfully demonstrated that four genes (Rnd1, Rras2, myc, and PI3Kcb) involved in the RAS/MAPK signaling pathway were upregulated in acrolein-transformed clone #4 (NIH/3T3 Acr-clone #4). These results were further supported by Western blot analysis, along with the finding that acrolein activated the RAS/MAPK signaling pathway and increased c-myc in NIH/3T3 cells and the human normal colon epithelium CCD-841CoN. Surprisingly, acrolein was found to induce cell proliferation, colony formation activity, and cell migration capacity in CCD-841CoN cells. RAS/MAP signaling pathways are crucially activated in the CCD-841CoN Acr clone as well as in human colon cancer cell lines, SW480 and HCT116. The study demonstrated that acrolein prominently mediated and induced oncogenic transformation through activation of the RAS/MAPK pathways [77].

In 1990, one more peroxidation product was identified, which resembles prostaglandin compounds generated from the peroxidation of AA in situ in phospholipids known as F₂-isoprostanes (F₂-IsoPs) [79]. Unlike other LP products, Isoprostanes are prone to detection easily owing to chemical and metabolic stability in a number of biological samples such as urine, plasma, and tissues, respectively [56]. Isoprostanes can be accumulated and aggregated in cell membranes, thus impairing the fluidity of membranes and aiding in the etiology of serious ailments such as neurodegenerative diseases, cancers, etc. F₂-IsoPs elevated level is analyzed by employing gas chromatography/negative ion chemical ionization mass spectrometry for OS level [80–82]. Figure 3 depicts a schematic representation of the LP and the formation of the secondary metabolic product of LP.

Many groups are engaged extensively to prove the involvement of isoprostanes with cancers [83–85]. Ma et al. [86] have reported that the UPLC-MS/MS-based method is an efficient tool to measure the 8-isoprostane plasma concentration and identified as a biomarker for early lung cancer screening [86]. Rasool et al. [87] have established the key role of isoprostanes and matrix metalloproteinase-7 (MMP7) in the development of colorectal cancer in males. More studies are underway to unravel the exact pathway for the link between isoprostanes and carcinogenesis.

Undeniably, Nrf2 plays a substantial role in anti-tumor activity by sustaining redox homeostasis, modulating cell growth, cell differentiation, and anti-inflammatory activities [106]. Reports revealed that the Nrf2 signaling pathway is crucially involved in detoxification of ROS/RNS via modulating drug-metabolizing enzymes, thus leading to the suppression of the OS-mediated cancer development. It is well documented that Nrf2 activation is perceived in several cancers, such as bladder cancer, cervical cancer, breast cancer, colon cancer, glioblastoma, glioma, hepatocellular carcinoma, lung cancer, pancreatic cancer, multiple myeloma, ovarian cancer, and gastric cancer [107, 108].

One piece of evidence has suggested that in the Nrf2 knockout model, Nrf2 facilitated protection in the skin, bladder, and stomach against chemically induced carcinogenesis. In another report, on exposure to benzopyrene, Nrf2-null mice were shown to develop gastric neoplasia [109]. Multiple reports have established that upon exposure to azoxymethane followed by dextran sodium sulfate, intestinal tumors develop significantly more in Nrf2-deficient mice than in the wild-type mice [110, 111]. Reports also indicate that many cancers, such as mammary, skin, and colorectal cancers, have increased substantially with administration of carcinogenic molecules [112]. The exact pathway of Nrf2-mediated preventive mechanism of cells against carcinogenesis is via reducing ROS/RNS levels, leading to the inhibition of lipid peroxide products and DNA damage. Additional experimental evidences bolster the claim that Nrf2 reflects protective behavior towards carcinogenesis. An experiment demonstrated in mice, possessing a single nucleotide polymorphism (SNP) in the promoter location of the murine Nrf2 gene, revealed the reduced expression of Nrf2, posing a threat for hyperoxia-induced lung morbidity [113]. Suzuki et al. [114] and Zhao et al. [115] have reported that owing to this SNP, the mRNA expression level of Nrf2 declined, linking to the elevated vulnerability towards non-small-cell lung cancer (NSCLC). A plethora of evidence has illustrated the preventive impact of the Nrf2 signaling pathway against several cancers [116–118].

Though the preventive effect of Nrf2 is well established, the dark side and detrimental effects of Nrf2 in cancer activation and progression are also well demonstrated by many reports. Modulation of Nrf2 activation plays a crucial role in disease progression. The emergence of the negative effect of Nrf2 in carcinogenesis and the progression of several cancers fueled controversy about the beneficial role of Nrf2 in cancer prevention or tumor suppression. Nrf2 hyperactivation within tumor cells raised questions on its preventive measures as antioxidants, as well as anticancer effect or chemoprevention [119]. Reports are suggestive of an antagonistic effect of overexpression of Nrf2 in the survival of cancer cells, thus leading to vital involvement in cancer progression [120], and it was evident that cancer cells employed Nrf2 as a self-protective mechanism for survival [121].

Nrf2 actively participates in cancer cell proliferation and invasion, along with suppression of the antiapoptotic activity of cells via the overstimulation of anti-apoptotic protein Bcl-2. Nrf2 hyperactivation and increased levels of Nrf2 were envisaged in several tumors and malignancies, such as bladder, ovarian, pancreatic, prostate, lung, breast, and endometrium, respectively [122]. Nrf2 activation induced by radiotherapy and chemotherapy resistance is also significantly observed, reflecting its malicious role in cancer progression. Numerous reports documented the deleterious role of Nrf2 in cell proliferation, cancer progression, and suppressing cell death [123–126].

To prevent cancer and carcinogenesis, several natural (polyphenols, carotenoids, flavonoids, isothiocyanates, vitamins, etc.) and synthetic compounds have been identified and studied as Nrf2 activators for modulating its expression and signaling. Compounds typically belonging to phenolic and sulfur-containing groups are found to be promising chemoprotective agents. Curcumin and resveratrol (RSV) are the most extensively studied natural compounds, crucially found to inhibit many types of cancer cell proliferation [152–154]. It was reported by a group that curcumin and RSV were found to be involved in the inhibition of cell growth, which was affected by dosage [155]. The IC₅₀ of DLD-1 cell lines is observed 71.8 μM (20.5 μM curcumin + 51.3 μM RSV), while the IC₅₀ of CaCo-2 cell line was 66.21 μM (18.9 μM curcumin + 47.3 μM RSV) [156]. It has also been reported that curcumin significantly facilitates apoptosis in various cancer cell types in the human colon [157, 158]. Bisht and Maitra [159] have established in a nude mouse xenograft model that curcumin is found to be crucially linked to the obstruction of in vivo growth suppression in head and neck squamous cell carcinoma. Along with these, liposomal formulations of curcumin have been extensively studied in various cancers, including pancreatic, colorectal, and prostate [160–163]. Kunnumakkara et al. [164] have demonstrated that cell proliferation and metastasis are inhibited by curcumin in breast cancer cells via downregulating the Fen1 expression by activating and modulating the Nrf2 cell signaling. Many dietary vitamins, such as ascorbic acid (vitamin C), retinoic acid (vitamin A), flavonoids, phytochemicals, including cryptotanshinone, polyphenols like epigallocatechin gallate (EGCG), theaflavin, luteolin, isothiocyanates, etc., demonstrated a promising role as Nrf2 activators in chemoprevention. Among all, tea polyphenols emerged as an extensively studied natural phytochemical with enormous health benefits. It is bestowed with chemoprotective and therapeutic virtues against carcinogenesis due to tea polyphenols’ redox modulating properties [165, 166]. Bioavailability of tea polyphenols and their concentrations are also determining factors in therapeutics against tumorigenesis. Tea polyphenols modulate Nrf2 via many cross-regulators, thus crucially linking to play a vital role in preventing and reducing cancer risk across major cancer types such as ovarian cancer, breast cancer, colorectal cancer, urinary bladder cancer, prostate cancer, and stomach cancer [167, 168]. A schematic representation of Nrf2 activators is summarized in Figure 7.

Likewise, sulforaphane (SFN), an isothiocyanate prevalently found in cruciferous vegetables such as broccoli, is the most extensively studied natural compound crucially involved in modulating Nrf2-Keap1 signaling, thus leading to the reduction in cancer incidents [169]. The chemoprotective properties of broccoli sprouts are also studied by performing a phase II clinical study done in China, that SFN present in broccoli suppresses alpha-toxin-DNA adducts, thus resulting in protection against hepatocellular carcinoma [170]. SFN crucially aids in directly modifying Keap1, resulting in Nrf2 release and modulating nuclear translocation. Other signaling pathways which linked to the phosphorylation of Nrf2 are also activated by SFN, which leads to the Nrf2 activation [171]. RTA405, commonly known as oleanane triterpenoid, is shown to modulate and activate Nrf2 via binding to Keap1, abrogate it to facilitate Nrf2 degradation. The effect of RTA405 on Nrf2 activation via mere deletion of Keap1 has been demonstrated, and the findings indicate that the mechanism is different, which distinctly affects cancer proliferation and carcinogenesis [172]. Allicin (dillyl thiocyanate), specifically found in crushed garlic, was primarily identified earlier as antimicrobial, anti-inflammatory, and possessing other biological activities [173, 174]. Later, other studies depicted its anticancer activity and showed that it induced apoptosis via elevated translocation and promoter binding activity of Nrf2. It has been established that Nrf2-mediated allicin induced apoptosis of colon cancer cells, thus playing a vital role in checking the cancer cell growth [175–177]. A dietary polyphenol known as RSV (5-[(E)-2-(4-hydroxyphenyl)ethenyl]benzene-1,3-diol), abundantly found in white hellebore, blueberry, peanut, and grapes, with structural similarity to mammalian estrogen 17ꞵ-estradiol (E2) [117]. RSV plays a key role in the reduction of breast cancer, along with having anti-inflammatory, anti-metastatic, anti-angiogenic, and pro-apoptotic activities. RSV is found to be predominantly involved in modulating antioxidant bioactivities by regulating antioxidant gene expression via the Keap1/Nrf2 pathway and SIRT1 [178]. A study revealed that estrogen-induced breast carcinogenesis in the E2-treated rat model and MCF-10A cells is substantially obstructed by RSV [179]. RSV-mediated Nrf2 activation and its nuclear accumulation pose inhibition of carcinogenesis in E2-incubated MCF-10A cells and 7,12-dimethyl-benz(a)anthracene (DMBA)-induced mammary tumor growth in rats, along with reduced inflammatory response and downregulation of many prognostic breast cancer markers such as cyclin D1 (CCND), cytokeratin 19 (CK19), etc. [180]. A study demonstrated that synthetic 5α-reductase inhibitors such as finasteride and durasteride gained potential attention as possible prostate chemopreventive agents. Two clinical trials showed that finasteride and durasteride crucially involve and suppress the occurrence of prostate cancer formation in men [181, 182]. However, in contrast, these studies also revealed the occurrence of aggressive prostate tumors (Gleason scores 7–10) enhanced after the administration of finasteride or durasterideto the patients [183]. These studies revealed the positive side of Nrf2 activation as well as their side effects via many natural and synthetic modulators in preventing cancer cell proliferation as well as in therapeutics.

Literature is rich in reports elucidating the beneficial effects of Nrf2 activation in chemoprevention; however, numerous reports also reflect the harmful role of Nrf2 in chemoresistance [184]. Wang et al. [185] have studied the role of Nrf2 in tissue samples of lung cancer in patients from stage I to stage III. The group demonstrated that overexpression of Nrf2 elevates chemoresistance, whereas downregulation of Nrf2 reduces cancer cell proliferation, with increased sensitivity towards chemotherapeutic drugs like cisplatin, doxorubicin, etc. Therefore, they suggested that to enhance the efficacy and bioavailability of anticancer drugs to treat patients, specific Nrf2 inhibitors should be identified and designed [185]. Figure 8 depicts a few natural and synthetic Nrf2 inhibitors.

Several natural compounds have been identified to possess Nrf2 inhibitory properties. Brusatol, a natural quassinoid, extracted from Brucea javanica, is crucially involved in Nrf2 inhibition by hindering the transcriptional activity of Nrf2, facilitating the elevation in sensitivity of chemotherapy in tumor and cancer cell lines. In contrast, brusatol has also exhibited morbid effects on protein translation, affecting many short-lived beneficial proteins along with Nrf2 [186, 187]. In contrast, brusatol was found to cause side effects such as hypotension, nausea and vomiting in clinical studies [188]. This is very obvious and can be explained as Nrf2 is widely expressed in normal cells, and using Nrf2 as a target for tumor therapy may have some side effects also. The serious challenges of using brusatol for personalized cancer medicine include unclear mechanism of action/pathways, non-specific targets, toxicity, and side effects like nausea, hypotension, etc. Further research is required to explore patient specific development of analogues to improve the therapeutic profile and clinical translation [189]. Few natural flavonoids like luteolin have also been found to be linked to inhibiting Nrf2 transcription, thus aiding in sensitizing the cancer cells towards anticancer drugs [190, 191]. Moreover, studies have also indicated its significant role in Nrf2 activation, raising concerns regarding the exact role of Nrf2 in carcinogenesis. Another compound, ochratoxin A, inhibits Nrf2 at distinct phases, such as Nrf2 translocation into the nucleus, Nrf2 transcription, along Nrf2-DNA binding interference, respectively [192]. Synthetic compounds, i.e., dexamethasone and clobetasol propionate, are also shown to inhibit either obstructing Nrf2 transcription or preventing Nrf2 translocation. Retinoic acid also inhibits Nrf2 via binding to the Neh7 domain of Nrf2, thus resulting in the hindrance of Nrf2 transcription [142]. In addition, numerous natural and synthetic compounds have been identified that impede the expression of Nrf2, consequently leading to cell differentiation and cell proliferation [107].

However, the role of Nrf2 activation and inhibition is controversial in carcinogenesis; a comprehensive investigation is thus required to gain more insight about the exact signaling pathways involved in carcinogenesis.