Diversity of Chromanol and Chromenol Structures and Functions: An Emerging Class of Anti-Inflammatory and Anti-Carcinogenic Agents

Natural chromanols and chromenols comprise a family of molecules with enormous structural diversity and biological activities of pharmacological interest. A recently published systematic review described more than 230 structures that are derived from a chromanol ortpd chromenol core. For many of these compounds structure-activity relationships have been described with mostly anti-inflammatory as well as anti-carcinogenic activities. To extend the knowledge on the biological activity and the therapeutic potential of these promising class of natural compounds, we here present a report on selected chromanols and chromenols based on the availability of data on signaling pathways involved in inflammation, apoptosis, cell proliferation, and carcinogenesis. The chromanol and chromenol derivatives seem to bind or to interfere with several molecular targets and pathways, including 5-lipoxygenase, nuclear receptors, and the nuclear-factor “kappa-light-chain-enhancer” of activated B-cells (NFκB) pathway. Interestingly, available data suggest that the chromanols and chromenols are promiscuitively acting molecules that inhibit enzyme activities, bind to cellular receptors, and modulate mitochondrial function as well as gene expression. It is also noteworthy that the molecular modes of actions by which the chromanols and chromenols exert their effects strongly depend on the concentrations of the compounds. Thereby, low- and high-affinity molecular targets can be classified. This review summarizes the available knowledge on the biological activity of selected chromanols and chromenols which may represent interesting lead structures for the development of therapeutic anti-inflammatory and chemopreventive approaches.


INTRODUCTION
Chromanols and chromenols are collective terms for about 230 structures derived from photosynthetic organisms like plants, algae, cyanobacteria, fungi, corals, sponges, and tunicates (Birringer et al., 2018). Both compound classes are formed by a cyclization of substituted 1,4-benzoquinones. While 6hydroxy-chromanols are derived from a 2-methyl-3,4-dihydro-2H-chromen-6-ol structure, 6-hydroxy-chromenols are derived from 2-methyl-2H-chromen-6-ol ( Figure 1). The respective bicyclic core structure is associated to a side-chain with varying chain length and modifications, resulting in a great diversity of chromanol and chromenol derivates (Birringer et al., 2018). In a systematic review, Birringer and coworkers were the first implying the great potential of these structures by providing a comprehensive overview of the structural diversity and chemical transformation of all 230 chromanols and chromenols known at that time together with their natural source. The aim of the comprehensive review was rather the detailed description of the complexity of this group of compounds than an outline of their biological activity. Based on this systematic review, the intention of our review was to more selectively describe the effects of this class of natural products on signaling pathways involved in inflammation, apoptosis, cell proliferation, and carcinogenesis, and the underlying molecular modes of action for selected chromanols and chromenols. Our review therefore represents a useful and relevant addition to the work of Birringer et al., focusing on the evaluation of selected compounds with known biological activity as possible lead structures for putative therapeutic approaches. Based on the mentioned inclusion criteria, we here focus on tocopherol (TOH) and tocotrienol (T3) structures, sargachromanols, amplexichromanols, and sargachromenols, which show structure-activity relationships with mostly antiinflammatory as well as anti-carcinogenic activities.
Tocopherols and T3s differ in the saturation of the side-chain and form in its entirety the group of vitamin E. Based on the methylation pattern of the chromanol ring system a-, b-, g-, dforms of TOHs and T3s can be distinguished. Oxidative modifications of the terminal side-chain increase antiinflammatory activities. Therefore, hepatic metabolites of vitamin E are supposed to have important physiological activities and will also be included in this review. Sargachromanols (SCA), sargachromenols (SCE), and amplexichromanols (AC) have a tocotrienol-derived backbone implying similar biological activities. Our review focuses in more detail on the current knowledge about the biological activity as well as on potential regulatory pathways and molecular targets of chromanols and chromenols.

Tocopherols and Tocotrienols
Vitamin E, more precisely RRR-a-tocopherol, has been identified in 1922 as a vital factor for fertility in rats (Evans and Bishop, 1922). Vitamin E does naturally occur in various plant-derived foods, such as oils, nuts, germs, seeds as well as vegetables and, in lower amounts, fruits. Thus, vitamin E represents the most widely distributed and abundant chromanol in nature. The term vitamin E comprises different lipophilic molecules that consist of the chromanol ring structure with a covalently bound phytyl-like side-chain. Depending on the saturation of the C-16′ side-chain, these molecules are classified as TOH, T3s (Figure 2), and vitamin E related structures named tocomonoenols and marine-derived TOHs. Tocopherols are characterized by a saturated phytyl side-chain whereas tocomonoenols, marine-derived TOHs and T3 are unsaturated at either the terminal isoprene unit or have three double bonds within the side-chain (Fujisawa et al., 2010;Kruk et al., 2011). Further, the methylation pattern of the chromanol ring determines the classification as a-, b-, g-, and d-forms of TOHs and T3s. Although several similar molecules form the group of vitamin E, only a-TOH seems to have vitamin property in animals and humans. For instance, in rats a-TOH preserves fertility, whereas in humans the deficiency disease ataxia with vitamin E deficiency (AVED) is prevented by a-TOH supplementation (Azzi, 2019).
For a long time, the health-promoting effects of vitamin E were only attributed to its antioxidant properties, but more recent studies revealed additional non-antioxidant functions of vitamin E. It is evident that vitamin E modulates gene expression and enzyme activities and also interferes with signaling cascades (Brigelius-Flohé, 2009;Zingg, 2019). Examples for these regulatory effects are the suppression of inflammatory mediators, reactive oxygen species (ROS) and adhesion molecules, the induction of scavenger receptors as well as the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-кB) (reviewed in Glauert, 2007;Rimbach et al., 2010;Wallert et al., 2014b;Zingg, 2019).
All forms of vitamin E undergo metabolic degradation in the liver. Although the detailed mechanisms remain poorly understood, the principles of the degradation of vitamin E to vitamer-specific physiological metabolites with intact chromanol ring (the nomenclature as a-, b-, gand d-metabolites is used as described for the metabolic precursors in order to distinguish the different forms of vitamin E metabolites) is widely accepted (Figure 3). Thus, enzymatic modifications are restricted to the side-chain (extensively reviewed in (Kluge et al., 2016;Schmölz et al., 2016)). a-Tocopherol is the main form of vitamin E in the human body due to its higher binding affinity to the atocopherol transfer protein (Hosomi et al., 1997). Thus, we will focus on the metabolic conversion of a-TOH in the following. Nevertheless, it should be noted that all forms of vitamin E (TOHs as well as T3s) follow the same metabolic route. However, due to the unsaturated side-chain, the degradation of T3s requires further enzymes such as 2,4 dienoyl-coenzyme A (CoA) reductase and 3,2-enoyl-CoA isomerase, which are also involved in the metabolism of unsaturated fatty acids (Birringer et al., 2002). The initial step of a-TOH modification via w-hydroxylation in the endoplasmic reticulum leads to the formation of the long-chain metabolite (LCM) a-13′hydroxychromanol (a-T-13′-OH). It is supposed that this hydroxylation is catalyzed by cytochrome P450 (CYP)4F2 and CYP3A4 (Parker et al., 2000;Sontag and Parker, 2002). After its transfer from the endoplasmic reticulum to the peroxisome, a-T-13′-OH is converted to a-13′-carboxychromanol (a-T-13′-COOH) via w-oxidation, likely via a two-step mechanism involving alcohol and aldehyde dehydrogenases. a-T-13′-OH and a-T-13′-COOH have been found in human serum (Wallert et al., 2014a;Ciffolilli et al., 2015;Giusepponi et al., 2017), supporting the idea of a more complex physiologic role of vitamin E with physiological relevance of its metabolites for various processes. In healthy humans a-TOH is the most abundant form of vitamin E, occurring in concentrations of about 20-30 µM in serum (Péter et al., 2015). However, supplementation of a-TOH increases a-TOH serum concentration in humans up to 90 µM (Dieber-Rotheneder   , 1991). Following supplementation, the hepatic metabolism is enhanced to protect the liver from excessive accumulation of a-TOH. Consequently, metabolites of vitamin E are formed and accumulate in turn in human serum. The LCMs a-T-13′-OH and a-T-13′-COOH were found in concentrations of 1-7 nM and 1-10 nM at baseline, respectively, whereas supplementation of a-TOH increased serum concentrations of the LCMs up to 12-32 nM and 3-55 nM, respectively (Wallert et al., 2014a;Ciffolilli et al., 2015;Giusepponi et al., 2017). Recent studies showed that the active metabolites of vitamin E exert effects on lipid metabolism, apoptosis, proliferation, and inflammatory processes as well as xenobiotic metabolism (Wallert et al., 2014a;Jang et al., 2016;Podszun et al., 2017;Schmölz et al., 2017). Finally, a-T-13′-COOH is excreted via bile and feces or is further degraded via several rounds of oxidation to the hydrophilic short-chain metabolite a-carboxyethylhydroxychromanol (CEHC), which is largely excreted via urine (Zhao et al., 2010;Johnson et al., 2012;Jiang, 2014). Another characteristic of the hepatic degradation of vitamin E is that the metabolites are chemically modified. In particular, the LCMs and the short-chain metabolites (SCMs) have been found as sulfated or glucuronidated conjugates in different biological matrices (Galli et al., 2002;Wallert et al., 2014a). Freiser and Jiang (2009) reported that more than 75% of g-CEHC in the plasma of g-T3-supplemented rats occurred in conjugated form. Further, also the LCMs, especially 13′-COOH and 11′-COOH metabolites were found as conjugates. Conjugation (sulfation or glucoronidation) seems to occur in the liver in parallel to the b-oxidation of the side-chain of vitamin E (Freiser and Jiang, 2009). Beside the mentioned LCMs, intermediate-chain metabolites (ICMs) and SCMs that are formed via hepatic degradation of the different vitamin E forms, and vitamin E is also the precursor of quinones, representing another class of vitamin E-derived metabolites that exhibit antioxidant activity. Vitamin E quinones, in particular a-TOH-derived quinones, are formed as byproducts of a-TOH oxidation during peroxidation reactions in in vitro systems (Liebler et al., 1990;Infante, 1999). In addition, these metabolites can also be synthesized by photosynthetic organisms (Liebler et al., 1990). Although the knowledge on this group of tocopherol-derived metabolites is sparse, a-TOH quinone has been described as an essential enzymatic cofactor for fatty acid desaturase (Liebler et al., 1990).
The natural compound d-T3-13′-COOH, also known as dgarcinoic acid or d-tocotrienolic acid, shares structural similarity with the d-T-LCM d-T-13′-COOH, the second LCM originating from the hepatic metabolism of d-TOH. As described previously, hepatic metabolism of tocotrienols follows that of tocopherols. Consequently, d-T3-13′-COOH is formed during the degradation of d-T3. Since the concentration of d-T3 in human plasma is below 1% compared to a-TOH, the physiological relevance of d-T3-13′-COOH in humans is likely low. So far, the detection of d-T3-13′-COOH in human blood is still pending. However, local accumulation of d-T3-13′-COOH in cells or tissues cannot be excluded. d-T3-13′-COOH can be obtained in relatively high amounts and purity from the seeds of Garcinia kola E. Heckel (Bartolini et al., 2019;Wallert et al., 2019), a plant that is used in traditional African ethnomedicine (extensively reviewed in Kluge et al., 2016). This compound can be used as precursor for the semi-synthesis of aand d-LCMs (including a-T-13′-OH, a-T-13′-COOH, d-T-13′-OH, and d-T-13′-COOH) for experimental use in vitro and in mice and is therefore important for vitamin E metabolite research (Maloney and Hecht, 2005;Birringer et al., 2010). Further, d-T3-13′-COOH also appeared to be a potent anti-inflammatory (Wallert et al., 2019) and anti-proliferative agent (Mazzini et al., 2009) and has been shown to act as an inhibitor of DNA polymerase b (Maloney and Hecht, 2005), indicating that d-T3-13′-COOH may disturb base excision repair in tumor cells. A recent preprint of Bartolini et al. described d-T3-13′-COOH as a potent agonist of PXR, which is known to be involved in inflammatory processes (Bartolini et al., 2019).

Sargachromanols
Sargachromanols (SCA) comprise a group of chromanols that occur in the brown algae family Sargassaceae ( Figure 4). Their high structural diversity results from various side-chain modifications, leading to their classification from SCA-A to SCA-S. The entirety of sargachromanols has been isolated from Sargassum siliquastrum and has been classified via twodimensional nuclear magnetic resonance experiments (Jang et al., 2005;Im Lee and Seo, 2011). The extensive analysis revealed detailed structural differences between the sargachromanols. For example SCA-C contains a 9′-hydroxyl group with R-configuration in the side-chain, while SCA-F has a methoxy group at C-9′ and a hydroxyl group with Rconfiguration at C-10′ (extensively reviewed in Birringer et al., 2018). SCAs have been reported to exhibit various biological activities, including anti-oxidative (Lim et al., 2019) (SCA-G), anti-osteoclastogenic (Yoon et al., 2012b;Yoon et al., 2013) (SCA-G), anti-inflammatory (Yoon et al., 2012a;Lee et al., 2013;Heo et al., 2014) (SCA-G and SCA-D), as well as antidiabetic (Pak et al., 2015) (SCA-I) ones. To the best of our knowledge, metabolism of sargachromanols in humans or animals has not been investigated.

Amplexichromanols
Amplexichromanols represent a small group of hydroxylated T3 derivatives found in different parts of Garcinia plants. For instance, lipophilic extracts from the bark of Garcinia amplexicaulis were used to isolate g-AC and d-AC ( Figure 5). The chemical structure of g-AC and d-AC are similar to g-T3 and d-T3, respectively, but carry two additional hydroxyl groups at C-13′ and C-14′. In an initial in vitro experiment, d-AC reduced vascular endothelial growth factor induced cell proliferation in low nanomolar concentrations, while g-AC had no effect. This observation probably indicates distinct efficiencies for the different amplexichromanols (Lavaud et al., 2013). However, further experiments revealed strong anti-oxidative potential for both compounds (Lavaud et al., 2015), but nothing is known about the metabolization, systemic distribution, tissue accumulation, or excretion of amplexichromanols so far.

Chromenols
Chromenols consist of a 2-methyl-2H-chromen-6-ol core that is associated with a side-chain with varying chain length and varying chemical modifications, leading to high structural diversity. The multitude of these compounds can be obtained from photosynthetic organisms like plants, algae, cyanobacteria, fungi, corals, sponges, and tunicates (Birringer et al., 2018). As the current knowledge on the biological functions of chromenol structures is sparse, this review will exemplarily focus on the most studied sargachromenols ( Figure 6). Similar to their chromanol counterparts, sargachromenols were named after the brown algae species Sargassum serratifolium, from which they have been isolated first (Kusumi et al., 1979). Just like sargachromanols, sargachromenols comprise a molecule class of high structural diversity due to different side-chain modifications. In the first systematic review on the field of chromanols and chromenols, Birringer and coworkers described 15 sargachromenols, 13 compounds with marine origin (brown algae) and two with marine and plant origin (Birringer et al., 2018). As an example, d-SCE, a structure consisting of a d-chromenol ring system with an unsaturated side-chain containing a carboxy group at C-15′, is widely distributed in algae of the Sargassaceae family but can also be obtained from plants like Iryanthera juruensis. Another interesting sargachromenol is dehydro-d-T3, or Sargol, which is supposed to serve as a biosynthetic precursor for most of the sargachromenols and is occurring in brown algae (Birringer et al., 2018). Brown algae from the Sargassaceae family have been used in traditional Asian medicine as well as in health promoting diets, revealing a variety of biological functions . For example, ethanolic extracts from the Sargassaceae species Myagropsis myagroides, an alga that grows at the coast of East Asia, revealed potent anti-inflammatory activity. After HPLC-based separation, sargachromenols   (mostly d-SCE) have been identified as the most potent antiinflammatory compounds within these extracts, based on their inhibitory effect on nitric oxide (NO) production in lipopolysaccharide (LPS)-treated immortalized murine microglial BV-2 cells . Beside their antiinflammatory activity, anti-carcinogenic (Hur et al., 2008), anti-photoaging , and anti-cholinesterase activities (Choi et al., 2007) have been described for SCEs. Further, sargachromenols isolated from Sargassum macrocarpum mediate nerve-growth-factor-driven neuronal growth in pheochromocytoma of rat adrenal medulla derived PC12D cells (Tsang et al., 2005).

BIOLOGICAL ACTIVITY OF NATURAL CHROMANOLS AND CHROMENOLS
Based on published data, we have chosen signaling pathways that are central for inflammation, apoptosis, cell proliferation, and carcinogenesis ( Figure 7). Respective effects of tocopherol-derived (T) and tocotrienol-derived (T3) chromanol and chromenol structures on nuclear receptors and target enzymes were screened and are discussed in the following.

Inflammation
Inflammation is essential for wound healing as well as defense and clearance of pathogens (Kunnumakkara et al., 2018). However, excessive and persistent inflammation is a driving force for many chronic diseases. In addition to obvious inflammatory diseases such as rheumatoid arthritis, it is well accepted that cancer, Alzheimer's disease, and metabolic syndrome-related diseases like atherosclerosis, non-alcoholic fat liver disease, and diabetes mellitus type 2 are triggered by chronic low-grade inflammation (Kunnumakkara et al., 2018). As systemic inflammation is a complex process, this review refers only to inflammatory pathways that have been studied for chromanol and/or chromenol structures. Key regulatory factors and mediators of inflammatory processes in this context are receptors that sense proinflammatory stimuli, e.g. the toll-like receptors (TLRs), intracellular signaling molecules, like mitogenactivated protein kinases (MAPKs), and transcription factors, such as NF-kB or nuclear factor erythroid 2-related factor 2 (Nrf2). Further, enzymes that produce pro-inflammatory mediators such as prostaglandins (PGs) and leukotrienes (LTs) play a central role during the coordinated orchestra of the inflammatory process. This includes cyclooxygenases (COX) and lipoxygenases (LO). Other key players of inflammation are cytokines which are secreted by various cells and affect the interaction and communication between the different types of cells involved in inflammation (Aggarwal, 2009;Kunnumakkara et al., 2018). Important pro-inflammatory cytokines are interleukin (IL)-1b, IL-6, and IL-8 as well as tumor necrosis factor-a (TNF-a). Another important signaling molecule in inflammatory processes is nitric oxide (Aggarwal, 2009). In the following, chromanol and chromenol structures regulating the expression of key pro-inflammatory enzymes and the respective formation of signaling molecules are outlined.

Chromanols
A detailed overview on the biological activities of chromanols linked to inflammation is provided in Table 1.

Tocopherols and Tocotrienols
Data available for TOHs and T3s correlate with their abundance in humans. Therefore, aand g-TOH as well as their respective T3 forms were mostly investigated so far. a-Tocopherol is regarded as the only form within the group of vitamin E that has been shown to mediate actual vitamin E function (Azzi, 2019). Further, a-TOH is considered as the most abundant vitamin E form in human nutrition, followed by g-TOH.
Relevance of T3s as anti-inflammatory compounds has just recently come to fore of research and will be presented in the following sections.
Tocotrienols. Recent publications reported a more pronounced anti-inflammatory capacity of T3s compared to TOHs, with g-T3 and a-T3 showing the strongest effects. a-, d-, and g-T3 significantly decreased LPS-mediated formation of nitric oxide (by 11%, 31%, 19%, respectively) and PGE 2 (by 30%, 55%, 20%, respectively) in RAW264.7 macrophages treated with 23.5 µM of the respective compound (Yam et al., 2009) as well as bone marrow-derived macrophages (BMDMs) using 1 µM of g-T3 (Kim et al., 2018). Expression of COX-2 mRNA was inhibited by a-, d-, and g-T3, whereas protein expression remained unchanged (Jiang et al., 2008;Yam et al., 2009;Kim et al., 2018). In addition, cytokine-driven inflammation is also dampened by a-, d-, and g-T3, which reduced the release of IL-6 and TNF-a in LPS-stimulated RAW264.7 cells. However, g-T3 reduced expression of IL-6 and TNF-a mRNA as well as the secretion of IL-6, but not of TNF-a in this cell model (Yam et al., 2009). Furthermore, first reports suggest inhibitory effects of g-T3 on the NLR family pyrin domain containing 3 (NLRP3) inflammasome. In brief, 1 µM g-T3 suppressed mRNA expression of pro-IL-1b and -18 as well as respective formation of active IL-1b and -18. This has been observed in LPS/nigericin-as well as LPS/palmitate-stimulated BMDMs and db/db mice fed with a diet containing 0.1% g-T3 for eight weeks (Kim et al., 2016;Kim et al., 2018).

Metabolites of Tocopherols and Tocotrienols
We here present a report on selected structures formed during hepatic catabolism of vitamin E, for which data on the biological activity was available. Metabolites formed during physiological hepatic metabolism of vitamin E are highly potent antiinflammatory compounds with different efficiencies, depending on their methylation pattern (Azzi, 2019) and the number of isoprene units forming the side-chain (Schmölz et al., 2017). Metabolism of non-a-TOH forms of vitamin E is more pronounced, resulting from the lower affinities of these molecules to the a-tocopherol transfer protein. However, ametabolites revealed significant anti-inflammatory properties. The most widely studied metabolites are the LCMs a-T-13′-OH and -COOH and the short-chain metabolites aand g-3′-T-COOH, likely due to their presence in plasma, feces, and urine, respectively, which may account for their physiological relevance (Jiang et al., 2007).
Degradation of the LCMs of different vitamin E forms results in formation of respective ICMs that are further processed to SCMs. These metabolic end-products do not accumulate in plasma or tissues and their physiological relevance is therefore considered as less important. Hence, data on these metabolites are scarce. To date, anti-inflammatory effects, i.e. the inhibition of COX-2 activity (IC 50 6 µM), by d-9′-T-COOH have been reported in human lung adenocarcinoma A549 cells (Jiang et al., 2008).

Sargachromanols
The sargachromanol forms D, E, and G isolated from Sargassum siliquastrum also exert anti-inflammatory effects in LPSstimulated RAW264.7 macrophages in a concentrationdependent manner. Sargachromanol forms D, E, and G inhibited expression of iNOS protein to 30-50% with concentrations of 15, 12.5, and 20 µM, respectively. In contrast, inhibitory effects on the formation of the respective signaling molecule nitric varies compound-dependent between 10 and 90% , with SCA E being the most effective (Yoon et al., 2012a;Lee et al., 2013;Heo et al., 2014). Within the inflammatory eicosanoid pathway, expression of COX-2 was inhibited by 15% by SCA D and G and up to 90% by SCA E. The IC 50 for the formation of COX-2-derived PGE 2 was 15 µM (SCA D [Heo et al., 2014]), 12.5 µM (SCA E ), and 20 µM (SCA G [Yoon et al., 2012a]), respectively. The LPSinduced production of TNF-a, IL-6 and IL-1b was effectively blocked by SCA D (IC 50 >60, >20-25, and 40 µM, respectively [Heo et al., 2014]), E (IC 50 >25 µM, not investigated and >15 µM, respectively ), and G (IC 50 40, 20, and 20 µM, respectively [Yoon et al., 2012a]). The total inflammatory capacity, as determined by the expression of iNOS and COX-2, the production of their respective signaling molecules, nitric oxide and PGE 2 , as well as the production of cytokines leads to the following estimation of compound effectiveness: SCA E > D > G.

Amplexichromanols
Amplexichromanols can be distinguished as a-, b-, g-, d-forms. d-Amplexichromanols have been shown to inhibit the secretion of TNF-a (IC 50 <10 µM) and IL-1b (IC 50 10 µM) in LPSstimulated monocytes (Richomme et al., 2017). To the best of our knowledge, there are no reports on anti-inflammatory effects of the other forms of AC.

Chromenols
Compared to the complex group of structures comprising the chromanol family, chromenol structures are less ubiquitous. Sargachromenol is described here as a representative of the chromenols with anti-inflammatory effects. An ethanolic extract of Myagropsis myagroides inhibited nitric oxide-, eicosanoid-, and cytokine-mediated pathways and the inflammatory response (Table 2), with sargachromenol being the lead compound in the extract . Further studies using isolated sargachromenol from different sources confirmed the results obtained by Kim et al. For instance, sargachromanol isolated from the marine brown alga Sargassum serratifolium inhibited peroxinitrite anion-mediated albumin nitration with an IC 50 of 5 µM (Ali et al., 2017). Furthermore, the COX-2 pathway was inhibited using 50 µM and 100 ppm sargachromenol isolated from Sargassum micracanthum (Yang et al., 2013) and Iryanthera juruensis seeds (Silva et al., 2007), respectively. Here, the effect sizes of 70 and 84% found by Yang et al. and Silva et al., respectively, are comparable with respect to the inhibition of the expression of COX-2 protein. For the respective signaling molecule PGE 2 an IC 50 value of 30 µM was defined (Yang et al., 2013). In addition, inhibitory effects were observed for the expression of iNOS protein (95%) and the formation of nitric oxide (IC 50 82 µM) (Yang et al., 2013).

Carcinogenesis
For the evaluation of anti-carcinogenic effects of chromanol and chromenol structures, key apoptotic pathways, such as cleavage of poly-[ADP-ribose]-polymerase 1 (PARP-1), caspases 3, 7, 8, and 9 as well as anti-proliferative and cytotoxic properties on cancer cell lines and further markers of carcinogenesis marker in mice were evaluated (Figure 7). In addition, large-scaled human trials investigating preventive and therapeutic effects of some tested compounds will be discussed in the following chapter.

Chromanols
A detailed overview on the biological activities of chromanols linked to carcinogenesis is provided in Table 3.
Despite of the promising results outlined above, it should be noticed that several human trials failed to confirm preventive effects of vitamin E, in particular a-TOH, against cancer. The Alpha-Tocopherol Beta-Carotene (ATBC) Cancer Prevention Study examined whether a daily supplementation of 50 mg a-TOH and/or 20 mg b-carotene could prevent lung cancer in male smokers (Virtamo et al., 2014). However, after five to eight years of supplementation of either a-TOH or b-carotene or the combination of both failed to prevent lung cancer (Virtamo et al., 2014). In addition, other human intervention trails revealed disappointing results, with the Selenium and Vitamin E Cancer Prevention Trial (SELECT) representing a very interesting one. The aim of the SELECT study was to investigate the preventive potential of a-TOH and/or selenium on prostate cancer. In the SELECT trial, healthy men received a daily dose of either 400 IU all-rac-a-tocopheryl acetate or 200 mg selenium or a combination of both for an average of 5.5 years (Lippman et al., 2009). Supplementation with both compounds failed to prevent prostate cancer development. Surprisingly, daily supplementation with all-rac-a-tocopheryl acetate was slightly, but not significantly, associated with an increased overall risk for prostate cancer (Lippman et al., 2009). Next, in the 7 to 12 years follow-up the subjects who had received a daily dose of 400 IU all-rac-a-tocopheryl acetate showed a significantly enhanced risk for prostate cancer (Klein et al., 2011). This result indicates that a dietary supplementation with high doses of this vitamin E derivate could result in an increased risk for cancer.

Sargachromanols
The group of sargachromanols may serve as anti-carcinogenic agents that suppress cell proliferation as reported for SCA E in HL-60 leukemia cells accompanied by cleavage of PARP-1 as well as caspases 3 and 9 (Heo et al., 2011). However, confirmatory data are pending.

Amplexichromanols
To date, a-AC has been studied only in HepaRG cells, without effects on viability up to concentrations of 10 µM (Richomme et al., 2017). Therefore, studies on anti-carcinogenic effects of amplexichromanols are still on demand.

Chromenols
Within the group of chromenols, d-sargachromenol is the beststudied one. Previous studies revealed an induction of the cleavage of PARP-1 and caspases along with the induction of apoptosis and reduced cell viability in human skin keratinocyte (HaCaT) cells (Hur et al., 2008). Data obtained from cancer cell lines is still lacking.

INTERFERENCE WITH MOLECULAR TARGETS AND KEY PROTEINS CONNECTING INFLAMMATION AND CARCINOGENESIS
Many signaling molecules involved in inflammatory processes play in parallel also key roles in carcinogenesis. We here exemplarily focus on the interaction of selected chromanols and chromenols with the molecular crosstalk of NF-kB (Jurjus et al., 2016), lipoxygenases (Rådmark et al., 2015;Roos et al., 2016;Merchant et al., 2018), MAPK (Gkouveris and Nikitakis, 2017;Jiménez-Martínez et al., 2019), and the inflammasome (Moossavi et al., 2018;Swanson et al., 2019) due to their accepted involvement in both, inflammation and cancer (Figure 7). However, due to the sparse knowledge about their connection to chromanols and chromenols, further topics, like the interaction of tumor and immune cells, adhesion proteins, structure and regulation of tumor microenvironments, mechanisms for programed cell death as well as other prominent signaling pathways (PI3K/Akt/mTOR; PKC; STAT; Wnt/b-catenin), were not considered in this review.

Chromanols
A detailed overview on the interference of chromanols with molecular targets and key enzymes connecting inflammation and carcinogenesis is provided in Table 4.
FIGURE 8 | Heatmap illustrating the effectiveness of chromanol and chromenol structures on selected targets. If not indicated otherwise, the plotted effects represent inhibitory effects of the respective compound on distinct parameters; induced parameters are marked with an arrow (↑). The color coding of the presented heat map ranges from high-affinity targets and parameters (effect with <1 µM) presented in red to low-affinity targets and parameters (effects with >1 µM to ≤100 µM) presented in dark blue. If a compound did not affect a specific factor/parameter or showed low effectiveness (>100 µM), the factor/parameter is marked in light gray. Factors and parameters lacking data are marked in white. The heat map is considered as simplified guide for orientation and does not provide a detailed summary of the topic. All concentrations are given micromole (µM). Abbreviations used are: Actv, activation; A, activity; BA, binding affinity; C, cleavage; E, expression; PF, product formation; P, production; T, translocation. mediated by 12-and 15-LO remained unchanged (Jang et al., 2016). The discrepancy in IC 50 values in the inhibition of cellfree 5-LO likely depends on the different assay conditions. While Pein et al. analyzed specific 5-LO products by reverse-phase high-performance liquid chromatography with ultraviolet detection, Jang et al. used an indirect colorimetric assay, which determines the formation of hydroperoxides. For SCMs, namely 5′-T-COOH and 3′-T-COOH, no inhibitory effect was observed at the tested concentrations up to 3 µM, except for a-5′-T-COOH (IC 50 750 nM) (Pein et al., 2018).

Chromenols
Like SCAs, d-SCE has been shown to interfere with the NF-kB and the MAPK pathways. In TNF-a-stimulated endothelial cells (Gwon et al., 2017) and LPS-stimulated microglia cells (Kim et al., 2018), p65 translocation and the phosphorylation of IkB-a were inhibited by 40 µM and 60 µM d-SCE, respectively. In the same cell models inflammation-induced phosphorylation of JNK and ERK was diminished by d-SCE, whereas p38 remained unchanged (Kim et al., 2018) (Table 5).

LOW AND HIGH-AFFINITY MOLECULAR TARGETS
The heat map in Figure 8 provides a simplified overview about high-and low-dose bioactivities of the different chromanols and chromenols for a rapid assessment. The selection of compounds and parameters is based on a comprehensive review of the current literature about chromanols and chromenols and focusses on the important biological functions described for these compounds in the context of inflammation and cancer. For reasons of simplification, we did not take into account compound-specific uptake kinetics or cell type-or animal model-specific differences. For more detailed information, the reader is referred to Tables 1-5 which summarize our current knowledge on the chromanols and chromenols described in the respective sections. For comparison, presented concentrations are IC 50 values or the lowest reported concentrations affecting the respective parameters.
In the studies considered here, T3s often showed higher effectiveness on the induction or suppression of biological activities linked to inflammation and cancer than TOHs. Furthermore, oxidative modification of the terminal side-chain often substantially increases the anti-inflammatory capacity of respective compounds compared to parental compounds, such as TOHs and T3s. Amplexichromanols, sargachromanols and sargachromenols are also characterized by oxidative modifications of the side-chain, which might rationalize potent interactions with inflammatory targets, which needs further investigation. Notably, regulation of different target genes, proteins, and nuclear receptors can hardly be generalized. For instance, within the group of investigated targets, 5-LO is mostly inhibited by a few compounds, with d-T3-13′-COOH showing strongest inhibitory effects (IC 50 35 nM) and a-TOH showing the least (IC 50 1 µM). In contrast, the COX-2-regulated formation of signaling molecules is most efficiently inhibited by g-T3. In summary, especially 5-LO seems to represent a high affinity (affected at concentrations <1 µM) and therefore specific target for the LCMs of vitamin E. Most of the other observed effects, like mediation of caspase activity, anti-proliferative effects, inhibition of NO formation, are probably the result of a stimulation involving low-affinity targets (affected at concentrations ≥1 µM). However, as implied by the heat map in Figure 8, further studies are required for a comprehensive evaluation of the potential of chromanol and chromenol structures to serve as lead structures for the development of future anti-inflammatory therapeutic approaches.

CONCLUSION
For our review, we selected chromanols and chromenols for which data on anti-inflammatory and anti-carcinogenic effects were available in public databases of the scientific literature. The structures of our interests were tocopherols, tocotrienols, and their respective metabolites (which are produced in the liver under physiological and pathophysiological conditions) as well as structurally related compounds including sargachromanols, sargachromenols, and amplexichromanols. Criteria for the evaluation of compounds as possible lead structures for future therapeutic targets were their effects on key inflammatory and apoptotic pathways, proliferation, and interaction with (nuclear) receptor and enzymes that connect inflammation with carcinogenesis. Within this group of selected structures, tocopherols, more precisely a-TOH, are by far the most extensively studied compounds. However, the effects of TOHs are mostly only marginal compared to other compounds described in this review.
It should be noted that the methylation pattern of the chromanol ring system significantly affects inflammation and carcinogenesis. For instance, non-a-TOH and non-a-T3 forms affect eicosanoid-and cytokine-mediated inflammation as well as the cleavage of caspases that mediate apoptosis. Further, T3s are more potent in inhibiting caspase cleavage compared to the respective TOH forms. Tocopherol-and T3-derived metabolites and carboxychromanols more than hydroxychromanols inhibit LO, and in particular 5-LO, effectively and reduce the viability of multiple cancer cell lines. Furthermore, sargachromanols interact with MAPK and NF-kB pathways, assuming their crosstalk with both, carcinogenesis and inflammation, while sargachromenols mediate anti-carcinogenic effects. Although our knowledge about biological activities of amplexichromanols is sparse, first results indicate their potential for pharmacological applications.
The development of clinically relevant nitric oxide-, eicosanoid-, or cytokine-inhibiting agents or agents that interact with signaling pathways of inflammation is challenging with respect to selectivity and toxicity. Next, although blocking inflammation is meant to be protective, its permanent or long-term inhibition may cause damage to the body (Brasky et al., 2017). Although detrimental effects of naturally occurring chromanols and chromenols cannot be excluded yet, they are less likely for this group of lead compounds in light of the good tolerability of TOHs and T3s at low to moderate doses. Further studies are required to evaluate whether the observed effects of chromanols and chromenols on inflammation and carcinogenesis are indeed beneficial in humans. Until today, no human clinical trials have been published that provide valid information on the biological activity, bioavailability, kinetics, systemic distribution, or local accumulation of these compounds. However, this groups of molecules appears to be promising as lead structures for future anti-inflammatory and/or anti-cancerogenic therapeutic approaches.

LIMITATIONS
Our review is based on a recent systematic review of Birringer et al. (2018), which presented the first comprehensive overview on the diversity of chromanol and chromenol structures and their biological functions. The aim of our review was to more selectively describe the effects on signaling pathways involved in inflammation, apoptosis, cell proliferation, and carcinogenesis and the underlying modes of action for selected chromanols and chromenols. We are aware of the lack of data for a variety of chromenol structures in our overview. We therefore focused on chromanols and chromenols only where adequate data was available that reported anti-inflammatory and anti-carcinogenic properties. For a more detailed description of the structural and chemical properties of all 230 chromanol and chromenol structures, the reader is referred to (Birringer et al., 2018).

AUTHOR CONTRIBUTIONS
MW and SK wrote the manuscript. MW, SK, MS, MB, and SL designed and structured the manuscript, MS, MB, SL, AK, and OW supervised the project and carefully read, evaluated, and discussed the content of the manuscript.

FUNDING
SL and OW were supported by the Free State of Thuringia and the European Social Fund (2016 FGR 0045), and The Deutsche Forschungsgemeinschaft (CRC 1278 "Polymer-based nanoparticle libraries for targeted anti-inflammatory strategies (PolyTarget). Work of SL and AK was supported by the Deutsche Forschungsgemeinschaft (DFG, RTG 1715), and AK was funded by the DFG (KO 4589/7-1). In addition, work of OW and SL is also funded within the Collaborative Research Centre (SFB) 1278 (PolyTarget) by the DFG. Other sources of funding include the Forschungskreis der Ernährungsindustrie (FEI) as part of an AiF (Arbeitsgemeinschaft industrieller Forschungsvereinigungen "Otto von Guericke") project of the Industrielle Gemeinschaftsforschung (IGF), and the German Federal Ministry of Education and Research (nutriCARD, grant agreement number 01EA1411A).