The majority of research performed on cellular stress and apoptosis focuses on mitochondrial dysfunction; however, the importance of the endoplasmic reticulum dysfunction and the link to metabolic diseases has gained a substantial interest. This review focuses on the potential of terpenoids to influence endoplasmic reticulum stress and the possible role terpenoids play as the treatment of metabolic diseases.
Metabolic diseases develop as a result of a cascade of cellular pathways. In most cases, cells are able to compensate for the disruption of the cellular homeostasis although the initiation of response pathways; however, chronic stress initiates apoptotic pathways. This reviewed (1) showed the importance of phytoterpenoids to influence endoplasmic reticulum (ER) stress and homeostasis, (2) showed how regulating ER stress affect the cell survival and death, and (3) highlighted some examples of how the progression of metabolic diseases can be influenced by ER.
Due to the substantial number of terpenoids that have been identified in literature, this review gave examples of 21 terpenoids that have been documented to have an effect on the different proteins associated with ER stress, how these plant terpenoids influence ER dysfunction and metabolic diseases such as diabetes, cancer, liver, and neurological diseases and parasitic infections.
Current findings of the role of medicinal plants in the traditional treatment of metabolic diseases
An extensive percentage of the world's developing countries depend on the use of traditional medicines as an alternative to Western medicines. The use of plants and plant-derived remedies to treat a variety of different ailments have been used for centuries within different cultures globally. Plant-derived traditional remedies are used to treat a wide variety of ailments, ranging from mild dermatological problems to serious metabolic conditions, such as diabetes, cardiovascular disease and cancer.[1–6]
Research in the fields of ethnopharmacology is growing exponentially with the discovery of numerous novel treatments derived from natural plant products. Over the last few decades, a substantial amount of research has focused on identifying the different medicinal plants and the biologically active compounds (e.g. terpenoids, flavonoids, alkaloids, glycosides, saponins) that have the potential to be an alternative therapeutic treatment for a variety of different ailments. This review will focus on terpenoids and their potential use in alleviating endoplasmic reticulum (ER) stress found in different metabolic disorders.
The endoplasmic reticulum and its importance
The ER is an essential cellular organelle responsible for the production, folding, post-transcriptional modifications of nascent proteins, maintaining calcium homeostasis, and the degradation of secretory and transmembrane proteins via the ER-associated degradation (ERAD) pathway.[7,8] For example, the ER degradation enhancing α-mannosidase-like protein 1 (EDEM-1) family, expression of genes involved in DNA repair, genes involved in the maintenance and expansion of the ER, proteins involved in lipid biosynthesis and maintaining the redox potential of the cell.[9–11]
By definition, ER homeostasis is the balance between the ER's protein loading and its protein folding capacity. Any disturbance in the ER's homeostasis would lead to the production and accumulation of unfolded and incorrectly folded proteins. It is the accumulation of these proteins within the ER that is responsible for the ER stress and the initiation of the unfolded protein response (UPR).
Terpenoids linked to endoplasmic reticulum stress
Terpenoids can be modified to produce thousands of different intermediates and metabolites, to date approximately 40 000 plant and animal terpenoids have been identified; however, the majority have not been extensively studied.[13,14] All terpenoids are derived from one of two isomers, isopentyl diphosphate and dimethylallyl diphosphate via the traditional mevolonate or methylerythritol phosphate pathway. However, there have been documented cases where plants use both pathways in the synthesis of terpenoids.[15–17]
Because of the vast number of terpenoids that have been identified, a small fraction has been studied as possible treatments for a variety of ailments. This review has selected a few terpenoids that have shown promise as possible treatment options in metabolic diseases. These include: abscisic acid,[18,19] lycopene,[20,21] carotene, cannabidiol (CBD),[23–25] cannabinol (CBN), tetrahydrocannabinol (THC), geraniol, limonene, genipin, linalool,[28,29] menthol, perillyl alcohol, kujigamberol, marrubiin,[34,35] sarcodonin,[36,37] ganoderiol F, artemisinin,[39–41] dehydrocostus lactone,[42–44] farnesol,[28,45] parthenolide and thapsigargin (Table 1).
Table 1. A summary of the chemical structures of the few terpenoids mentioned in this review that have shown promise in alleviating metabolic diseases
Structure and classification of different terpenoids
Rosa arvensis, Bibes nigrum, Fagus sylvatica, Malus domestica
ER stress was first described by Kozutsumi and associates when they observed an increased production of glucose-regulated protein 78 (GRP78; also known as immunoglobulin heavy-chain binding protein (BiP)/heat-shock 70 kDa protein) resulting from the accumulation of unfolded proteins in the ER. To prevent ER stress from developing, cells need to tightly regulate ER homeostasis. ER stress can be induced through the treatment with compounds that inhibit glycosylation (e.g. tunicamycin); disruptions in the cell's calcium homeostasis; the overexpression of proteins, accumulation of incorrectly folded/unfolded proteins, starvation due to nutrient depletion, oxidative conditions, hypoxia, accumulation of free fatty acids and, in some cases, viral infections.[7,8,49–52]
Once ER homeostasis has been disrupted, ER stress triggers the activation of three transmembrane proteins: inositol requiring enzyme 1 (IRE1), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6). These transmembrane proteins become activated with the release of GRP78, which preferentially binds to accumulated unbound or incorrectly folded proteins, allowing for the dimerization of IRE1 and PERK complexes into ‘active’ complexes and allows the translocation of ATF6 to the golgi apparatus. The activation of these proteins and protein complexes, subsequently regulates the signalling cascade of the UPR.
The PERK pathway is largely involved in the attenuation of protein translation. The IRE1 and ATF6 signalling pathways are primarily responsible for the transcriptional up-regulation of ER chaperones and initiate the ERAD pathway. Thus, the activation of UPR pathways may lead to the attenuation of protein translation, except ER resident chaperones and the proteins associated with ERAD (e.g. EDEM). The expression of these proteins are up-regulated to decrease the demands of ER function and to degrade any incorrectly folded and unfolded proteins that have accumulated in the ER lumen. In cases where the ER stress remains irresolvable, the cells initiate the process of apoptosis. Apoptosis unlike necrosis is a cell-programmed death response to a variety of stimuli and is characterized by a variety of cellular changes including chromatin and cytoplasm condensation (resulting in the organelles becoming densely packed), cell shrinkage and pyknosis.
The UPR pathways are complex, a ‘cross-talk’ between the UPR signalling cascades and mitochondrial dysfunction is well-documented, and the functional relationship between the ER and mitochondria is essential to maintain cellular homeostasis. If persistent ER stress and the activation of the IRE1 signalling cascade occurs, the initiation of apoptosis may occur either via the phosphorylation of β-cell lymphoma 2 (Bcl-2) and Bcl-2 interacting mediator of cell death (Bim), or the release of cytochrome C, which would initiate the caspase signalling pathway (Figure 1). There have also been documented cases indicating that mitochondrial dysfunction increases the level of ER stress as seen by the up-regulation of phosphorylated eukaryote translation initiation factor 2 alpha (eIF2α), GRP78, C/EBP homologous protein (CHOP) and the activation of c-Jun N-terminal kinase (JNK) signalling.
The UPR (Figure 1) can be regulated by a number of proteins, the most common include: BiP/GRP78, growth arrest and DNA damage-inducible protein 34 (GADD34), 58 kDa inhibitor of protein kinase (P58IPK), and wolfram syndrome 1 (WFS1/wolframin). GRP78 is an ER chaperone and is considered a master UPR regulator. This protein is responsible for the prevention of premature UPR activation. GADD34 is responsible for the dephosphorylation of eIF2α, through the recruitment of phosphatsase 1, restoring the eIF2α to its unphosphorylated state. P58IPK is considered a negative feedback regulator of PERK signalling. Similarly to GRP78, this protein's main function is to maintain ER homeostasis. The phosphorylation of eIF2α, results in the attenuation of general protein translation. The expression of P58IPK is mediated by ATF6 in the pancreas and interacts with the kinase domain of PERK, inhibiting the kinase activity. WFS1 is an important and key negative regulator of UPR. If a mutation in this protein's gene occurs, people can develop Wolfram syndrome. WFS1 is located in the ER and appears to have a protective function against ER stress. Very little is known about the protein's function; thus, the protein's functional mechanism is unclear. However, Oslowski and Urano have recently reported that WFS1 may play a role in regulating ATF6α activation.
Relationship between the unfolded protein response and terpenoids
Inositol requiring enzyme 1 or endoplasmic reticulum to nucleus signalling 1 pathway, and the associated terpenoids
The activation of the type I transmembrane protein, IRE1 and the subsequently signalling pathway is considered to be one of the main UPR pathways. IRE1 is triggered as early response to the accumulation of incorrectly folded proteins in the ER under both pathological and physiological conditions. When the ER is under a tolerable amount of stress, IRE1 undergoes dimerization, and autophosphorylation after GRP78 has been released, resulting in the activation of the IRE1 RNAse domain. However, it has been suggested that the IRE1 signalling pathway may remain partially active in cells at a basal level. The IRE1 signalling pathways have a dual cellular function, as this signalling pathway has both pro-apoptotic branch via the activation of tumour necrosis factor receptor-associated factor 2 (TRAF2), apoptosis signal regulating kinase 1 (ASK1), JNK signalling pathway, and a pro-survival branch via the activation of X-box binding protein (XBP1).[69,70] Activation of IRE1 occurs when the cell is experiencing severe hypoxia or if there is an increase in the expression of interleukins (IL-1β, IL-6 and IL-8), vascular endothelial growth factor (VEGF) A, connective tissue growth factor and matrix proteins, such as thrombospondin 1 and metallopeptidase 9.[71–73] Both branches of the IRE1 pathway has been documented to be modulated by several interacting proteins including β-cell lymphoma 2 (Bcl-2), Bcl-2-associated X protein (BAX) and Bcl-2 antagonist/killer protein (Figure 1). Protein tyrosine phosphatase 1 B (PTP1B) has also been documented to affect the IRE1 pathway. PTP1B has been observed to disrupt the splicing of XBP1, subsequently attenuating the up-regulation of XBP1's target genes (e.g. EDEM) and induce the phosphorylation of JNK.
The UPR signalling pathway the expression and regulation of different genes and proteins that are associated with cell survival and apoptosis. However, with the apoptotic signalling pathway, there is ‘cross-talk between the ER and the mitochondia protein signalling pathway (Figure 1).[8,54–64]
IRE1 has been documented to have two catalytic domains. The first has a serine/threonine transautophosphorylation activity and the second, an endoribonuclease activity, which is responsible for initiating the splicing of XBP1.[75,76] The cleavage of XBP1 messenger RNA (mRNA) in an unconventional splicing event, forcing a translational frame producing the active transcription factor XBP1. The active form of XBP1 is responsible for regulation and expression of a variety of cellular chaperones and ERAD enzymes. The active transcription factor (XBP1s) is integral to the anti-apoptotic signalling branch of the IRE1 signalling pathway. The active XBP1 is responsible for regulation the expression of proteins implemented in ensuring correct protein folding, trafficking and secretion; active XBP1 has also been linked to the restoration of ER homeostasis and general cell survival.[69,77]
IRE1 has been documented to be responsible for a number of cellular functions, including the reduction of the ER workload, although the cleavage of ER associated mRNA and enhancing the synthesis of pro-insulin; thus, IRE1 has an important role in the production of insulin. However, under intolerable ER stress, IRE1 can activate an apoptotic pathway (Figure 1). This occurs when IRE1 recruits the protein TRAF2 and phosphorylates JNK, which is responsible for the regulation of the Bcl-2 family of proteins that induce apoptosis. TRAF2 is an adaptor protein that binds to JIK : IRE1 complex and promotes the activation of JNK indirectly through the ASK1. Although the IRE1 signalling pathway and the link between mitochondrial dysfunction and apoptosis via the activation of caspases is well-established; a limited number of studies on the effect terpenoids have on the IRE1 signalling cascade have been documented (Table 2).[25,38,44,78]
Table 2. Examples of terpenoids that have an effect on various parts of the IRE1 signalling pathway
Induces the accumulation of calcium in the cytosol, possibly via the PPARγ and the calcium/calmodulin dependent kinase II signalling pathway and induces the activation and accumulation of p38 in the non-small lung cancer cells A549 and NCI-460.
Double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase pathway and the associated terpenoids
The second most characterized ER type I transmembrane protein is PERK (also known as pancreatic eukaryotic initiation factor 2α kinase; EIF2AK3).[54,67] This protein complex plays an important role in the functioning of β-cells under normal physiological conditions. PERK aids in the regulation of β-cell development and proliferation, and the production of pro-insulin in response to hyperglycaemic conditions. Under conditions that induce ER stress, PERK, similarly to IRE1, dimerizes and autophosphorylates once GRP78 has been released, triggering the phosphorylation of eIF2α, which is a subunit of the guanosine triphosphate-binding complex eIF2, attenuating general mRNA translation under tolerable ER stress conditions.
Under normal conditions, protein translation is initiated once eIF2 recruits the methionyl-tRNA to the ribosome. Once PERK is activated and eIF2α is phosphorylated, the activity of eIF2 complex is disrupted, and general protein translation ceases, reducing the protein load in the ER. The phosphorylation of the alpha subunit of eIF2 favours the translation of ATF4 mRNA, which subsequently produces the active transcription factor and nephrin. However, it must be noted that the phosphorylation of eIF2α is not always synonymous with ER stress, and as such, ER dysfunction should be confirmed with additional experiments other than the detection of phosphorylated eIF2α. Under irresolvable ER stress conditions, the continuous phosphorylation of eIF2α, induction of ATF4 and ATF3 would induce cell death through the up-regulation of the pro-apoptotic factor CHOP (Figure 1). Recently, Oslowski and Urano discover a novel transcription factor that appears to antagonize apoptosis and through the transcriptional regulation of protein kinase B (AKT1) mediates β-cell survival.
ATF4 regulates the expression of the genes responsible for the restoration of ER homeostasis (e.g. GADD34), the production and metabolism of amino acids, regulating the antioxidant stress response, the transcription of other prosurvival genes (GRP78, GRP94), Tribbles homologue 3 (TRB3), the receptor activator nuclear factor-kappa B (NF-κB) ligand VEGF as well as the expression of CHOP (also known as DNA-damage inducible transcript/DNA-damage inducible gene 153).[7,50,81–83] Increased expression of GADD34 increases the rate eIF2α dephosphorylation in a negative feedback loop involving protein phosphatase 1, allowing for protein translation to be resumed.[84,85] Increased expression of CHOP can be induced through the initiation of all three UPR transmembrane sensors, e.g. IRE1, PERK and ATF6; therefore, it has been traditionally considered an apoptotic inducer. CHOP has been documented to activate the ER oxidase, GADD34, TRB3, and the proapoptotic factors Bim and death receptor 5 while decreasing the expression of the prosurvival protein Bcl-2.[83,86–91] This is not always the case in all cell types and some neurological diseases, e.g. murine models with Pelizaeus–Merzbacher disease.
In addition to the activation of the eIF2 signalling cascade, PERK has also been documented to phosphorylate the nuclear respiratory factor 2, which functions by binding to the antioxidant response element, resulting in the transcriptional activation of the gene-encoding detoxification enzymes. Unlike the IRE1 signalling pathway, the PERK pathway is more simplistic because this pathway is not considered the initial UPR pathway, and a limited amount research has been performed on the effects of terpenoids. In some cases, terpenoids may have an effect on more than one protein in the signal pathway, for example CBD (Table 3).[25,94]
Table 3. Examples of some terpenoids that affect the cellular proteins specific to the PERK pathway
Activating transcription factor 6 pathway and the associated terpenoids
The third UPR signalling pathway primarily involves the activation and cleavage of ATF6. Unlike IRE1 and PERK, ATF6 is classified as a type II transmembrane protein. Under ER stress conditions, GRP78 dissociates from the luminal domain of ATF6, releasing the protein into the cytoplasm and facilitating the translocation of ATF6 to the Golgi apparatus via the coat protein II vesicles. At the Golgi apparatus, the transcription factor is cleaved into its active form by site-1 and site-2 proteases.[95,96] The active transcription factor, which is the cytoplasmic bZIP domain, is subsequently translocated to the nucleus where it up-regulates the expression of homeostatic factors such as chaperones (e.g. GRP78 and calnexin – allowing for incorrectly folded proteins to be refolded), as well as proteins associated with ERAD and lipid biosynthesis[7,12,95,97–101] (Figure 1). In general the detection of the cleaved ATF6 is considered a good biomarker of ER stress; however, it may be difficult to detect. As far as the authors of this review are aware, the only terpenoid that has been documented to affect ATF6α is CBD, which induces the cleavage of the transcription factor in the nucleus.
Aside from the proteins already mentioned, which are specific to the different UPR pathways, there are several proteins that are associated with more than one of the UPR pathways or other cellular pathways that may be associated with ER stress (Table 4).[25,30,33,44,102–109]
Table 4. Terpenoids that have been shown to affect some of the proteins associated with more than one unfolded protein response pathway
DHE increased the expression of the chaperone in lung cancer cell lines A549 and NCI-H460.
Genipin suppresses the up-regulation of Bip in Neuro2a cells after the cells were treated with the calcium ionophore A23187. Genipin has also been documented to have neuroprotective properties in the rat primary hippocampal neurons when treated with β-amyloid peptide. In Neuro2a cells, genipin has shown to have neuroprotective effects against the cytotoxicity reaction induced by serum deprivation and oxidative stress.
Kujigamberol has been noted to inhibit GSK3-β, that is believed to restore growth in the mutant yeast strain YNS17 by affecting the calcium signal transduction. Kujigamberol is also cytotoxic to the promyelocytic leukemia cell line HL60.
Both terpenoids up-regulates the expression of the pro-apoptotic protein CHOP via the phosphorylation of PERK and the activation of the PERK signalling cascade in the hepatic stellate cells LX-2 and A549 cell lines and lung cancer cell line, NCI-H460, and the multiple myeloma cells OPM2. MM.1S.
Is there a role for terpenoids from medicinal plants in alleviating endoplasmic reticulum stress and metabolic disorders?
Literature has indicated that the core programs that contribute to the proper executive phase of apoptosis appear to be expressed constitutively in virtually every cell. The intracellular signals (e.g. UPR, mitochondrial dysfunction, mammalian target of rapamycin, NF-κB) that are involved in the induction of apoptosis are often regulators of other cellular responses. However, there is accumulating evidence that multiple signal pathways can interact during a process called ‘cross-talk’. A cell's response to a given stimulus may alter significantly not only between cellular compartments but also within a cell populations. Because of the ‘cross-talk’ that occurs between signalling pathways both intrinsic and extrinsic, the effect different terpenoids have on the specific signalling pathways may also have an effect-related pathway that may not necessarily be considered in the original experiments. The progression of metabolic diseases and, in some cases, viral infections is complex and involves the activation of different signalling pathways. This review focused on the role of the ER and the UPR pathways; however, the ‘cross-talk’ that occurs between the UPR signalling pathways and other cellular pathways need to be kept in mind.
As in the case of most of the different active components found in plants, there are several different types of terpenoids/isoprenols that can be classified as polyisoprenoid alcohols, phytol, carotenoids and abietic acid, all of which can be derivatized to produce additional terpenoids. Terpenoids have been documented to have several different functions such as the regulation of cation channels, induce apoptosis and suppress tumour proliferation,[13,111–114] which raises the question: Can terpenoids be used to prevent or aid in the alleviation of metabolic diseases? If so, what are the biological target, and do these phytochemicals distinguish between healthy and ‘diseased/infected’ cells? For the treatment of metabolic disorders, a number of possible therapeutic targets have been identified including nuclear receptors and several different proteins involved in the URP signalling cascades.
The cellular response to ER stress does not simply involve the activation of UPR pathways to determine whether the cell survives or triggers apoptosis. Under tolerable ER stress conditions where the IRE1 pathway is triggered, the signalling cascade may initiate the ER overload response as an alternative pathway to activate NF-κB or GSK3-β to trigger anti-apoptotic responses.[115,116]
Nuclear receptors are of great interest, as they appear to be the main target for the different active compounds present in plants.[117,118] Nuclear receptors can generally be classified into: (1) steroid hormones such as mineral corticoid, glucocorticoid, androgen, estrogen and progesterone receptors; (2) receptors that have the conserved characteristics of nuclear receptors but are not associated with endogenous ligands; and (3) receptors that to associate with endogenous ligands, such as the peroxisome proliferation-activated receptors (PPARs), thyroid hormone receptor and retinoic acid receptors. In normal cell differentiation, very low levels of NF-κB is required for controlled cell proliferation; however, under conditions where uncontrolled cell proliferation occurs (cancerous/tumour cells), the expression and activity of NF-κB is increased. Thus, the potential to use terpenoids (e.g. pathenolide, artemisinin, resveratol) to specifically target proteins crucial to cell proliferation as well as having the ability to target proteins involved in apoptotic pathways, for example p53 pathway demonstrates the potential terpenoids to influence multiple cellular processes. As a result of some terpenoids, especially the lactone-derived terpenoids are in use in clinical trials for the treatment of cancers. The use of the endocannabinoid system to mediate the activation of HSCs and subsequently fibrosis has been documented, and the use of cannabinoids in the treatment of cancer or the alleviation of associated conditions (e.g. pain and inflammation) is well-established.[121–123]
Metabolic diseases and infections that have been linked to endoplasmic reticulum stress and unfolded protein response pathways
The importance of the ER and its dysfunctional prevalence is increasingly being observed in different metabolic diseases and infections that may trigger a metabolic response. A variety of diseases are associated with ER stress and dysfunction, and as such, this review looks to provide a few examples of diseases, indicating the different links to ER stress, the respective proteins associated with the UPR signalling pathways and some terpenoids that have been documented to have an effect on the UPR pathways and diseases.
As previously mentioned, the use of plants and plant-derived compounds are well-established as medicinal treatments for a wide variety of ailments and diseases (as an alternative to synthetic medicines). Various terpenoids have been identified for their potential and promising role in treating diseases[25,30,33,44,102–109] (Table 4).
ER dysfunction has been documented to be linked to a variety of different diseases including lung diseases, cancers, diabetes and diabetic-related diseases/complications, malaria, neurological diseases, inflammatory diseases, viral infections etc. This review provides a few examples of diseases that have been linked to ER stress and dysfunction, and identifies terpenoids that have been associated with the different treatments[13,20,21,32,34,35,40,41,55,56,114,124–155] (Table 5).
Table 5. Examples of the terpenoids that have shown promise to alleviate metabolic disorders
Hypothesized to kill the Plasmodium spp. gametocytes by inhibiting the activity of the sarcoendoplasmic reticulum Ca2+ ATPase pump.
Leishmania spp. have been documented to reduce the expression of inducible nitric oxide synthase (iNOS) and subsequently nitric oxide in macrophages from BALB/c mice. By treating Leishmania-infected macrophages with artemisinin, the expression of iNOS and NO increased, and the levels of INF-γ decreased to a level comparable with uninfected macrophages.
When bound to Fe(II), artemisinin is cleaved, producing free radicals toxic to tumour cells and may contribute to the anti-leishmanial activity.
Artemisinin inhibits the activity of NF-κB that is elevated in tumour cells, rendering the cells sensitive to chemotherapy.
This lactone has been well-documented to diffuse through the cell membranes and block the calcium pump via lipophilic interactions, inhibiting the Ca2+ transporting ATPases in Plasmodium falciparum. The same mechanism is proposed in cancer cells.
Blocking the SERCA pump, prevents the Ca2+ flux between the ER and the cytosol, resulting in the accumulation of Ca2+ and subsequent ER stress and apoptosis.
Modulates the expression of the pro-apoptotic factors (BAX, Bcl-2) that would subsequently result in mitochondrial dysfunction and apoptosis via the induction of the caspase signalling cascade in Hela cells.
The main mechanism of killing leishmanial parasites is by controlling the expression of NO. Linalool has been shown to increase the production of NO in L. amaxonensis-infected macrophages and not in the mammalian Vero cells (kidney epithelial cells from African green monkeys). The chronic increase in NO would induce the activation of the UPR process.
Linalool also affects mitochondrial dysfunction, resulting in mitochondrial swelling.
Linalool has also been documented to change the chromatin organization.
In adipose tissue, PPARγ activation has been seen to improve insulin resistance.
Suppresses the hepatic 3-hydroxy-3 methylglutaryl coenzyme A (HMG-CoA) reductase activity and reduces serum cholesterolin rats and humans.
Has a concentration dependent impact on tumour cell proliferation in the murine cells: P388, B16 melanoma, Morris hepatoma 7777 cells and human MIA PaCa2 pancreatic ductal adenocarcinoma cells. By suppressing the mevalonate pathway in tumour cells and not normal cells.
May serve as an intermediate in the synthesis of different terpenoids in the mevalonate pathway.
Has an in vivo antitumour activity against murine hepatoma, melanoma and leukaemia cells.
Mannose-6-phosphate/insulin-like growth factor II receptor
Has a chemopreventive/therapeutic properties against, liver, mammary, cancer in Fisher 344 rats and human MIA PaCa-2 cells.
Perillyl alcohol inhibited tumour growth in Fisher 344 rats by increasing the expression of mRNA encoding mannose-6-phosphate/insulin-like growth factor II receptor and transforming growth factor B types I, II and III receptors.
Decreases fasting glucose, improves glucose intolerance and increases the expression of PPARγ and its responsive genes. Abscisic acid increases the PPARγ reporter activity in bone marrow-derived macrophages, which is dependent on the expression if LANCL2 as demonstrated by LANCL2 knock-down studies.
Abscisic acid shown to suppress the LPS-mediated expression of prostaglandin E2, MCP-1 production and inflammation through the activation of PPARγ and suppressing NF-κB in T cells.
Decreases LPS-mediated inflammation and regulates the innate immune responses through the bifurcating pathway using LANCL2.
Suggested to be an endogenous pro-inflammatory cytokine in human granulocytes that has the ability to stimulate the secretion of insulin.
Carotenoids (e.g. α-carotene, β-carotene and lycopene)
Inflammation associated with obesity and atherosclerosis
α-Carotene acts as the precursor for the synthesis of vitamin A
β-carotene inhibits the expression of inflammatory genes in LPS-stimulated macrophages.
Lycopene is a strong antioxidant that has been documented to have several mechanisms of activity, including anti-angiogenesis and induction of apoptosis through the inhibition of insulin-like growth factor 1-stimulated cell proliferation, inhibition of LDL oxidation and lipid peroxidation. Lycopene also reduces the expression of inflammatory markers IL-1β, IL-6 and MCP-1.
Treating murine peritoneal macrophages infected with L. donovani promastigotes, increased iNOS expression would induce UPR, resulting in the release in cytokines. The expression of IL10 and IL4 were down-regulated and the expression of IL-12, INF-γ and TNF-α were up-regulated in the macrophages.
18β-glycyrrhetinic acid induced the migration of NF-κB into the nucleus of the parasite-infected macrophages, decreasing the levels of IκB in the cytoplasm. The decrease in IκB in the cytoplasm increased the phosphorylation of IκBα, activating the kinase activity. However, 18β-glycyrrhetinic acid did not directly affect the activity of IκB kinase.
Using a murine model using Swiss mice, where the mice were exposed to an iNOS inhibitor, the gastroprotective properties of marrubiin was strongly related to the production of nitric oxide, the increased synthesis of mucus and the decrease in gastric acid secretion.
Using INS-1 cells, marrubiin increased insulin expression in a dose-dependent manner, up-regulated the expression of glucose transport 2 genes, and increased the respiratory rate and mitochondrial membrane potential under hyperglycaemic conditions.
Using male Wistar rats, marrubiin, increased insulin secretion under hyperglycaemic conditions, which was linked to mitochondrial metabolism. As ATP production increased, stimulated by an increase in the mitochondrial membrane potential, increasing oxygen consumption via the respiratory chain.
In the obese Wistar rat models, marrubiin treatment was seen to increase HDL-cholesterol levels while normalizing the total cholesterol levels by decreasing the LDL-cholesterol, the atherogenic index.
Marrubiin normalized IL-1β and IL-6 levels, and decreased TNF-α levels in obese rats. The anti-inflammatory activity of marrubiin is proposed to be linked to the suppression of the NF-κB signalling pathway.
Marrubiin has also been shown normalize fibrin levels in the obese rat model.
This terpenoid inhibits platelet aggregation and subsequently suppressing coagulation.
Marrubiin inhibits collagen and thrombin-induced Ca2+ mobilization.
Diabetes is a common metabolic disorder that is characterized with chronically high glucose levels in the blood because of the inability of the body to utilize glucose as a result of an absence or deficiency in insulin production by β-cells. Diabetes is generally classified as either type 1 diabetes mellitus (T1DM) or type 2 diabetes mellitus (T2DM); however, a rare medical condition, Wolcott–Rallison syndrome, which is an autosomal recessive form of juvenile onset diabetes, has also been documented.
Experimental, clinical and genetic evidence indicating that ER stress plays a role in β-cell dysfunction and death during the progression of T1DM, T2DM and genetic forms such as Wolcott–Rallison syndrome is well-documented and as such is briefly mentioned in this review. In T1DM, β-cells are the direct target of the autoimmune invasion that results in insulitis and β-cell death, and as such, T1DM is classified as an autoimmune form of diabetes. In T1DM, ER stress has been documented to occur resulting from exposure to pro-inflammatory cytokines, increased nitric oxide production, depletion of the ER calcium stores and the activation of the transcription factor ATF3, resulting in the up-regulation of CHOP and subsequently inducing the apoptotic pathways.[157,158]
In T2DM, metabolic stress is thought to activate the innate immune system, resulting in an increase in cytokines, islet-associated macrophages, insulin resistance, and the reduction of β-cell mass and ultimately apoptosis. A number of factors contribute to cell apoptosis in T2DM, including glucotoxicity, lipotoxicity, ER stress, oxidative stress and amyloid deposition. The apoptosis of β cells in T2DM may be contributed to a variety of different components, including the activation of the IRE : TRAF2 : JNK signalling pathway, the up-regulation of CHOP ATF4, spliced XBP1 and GSK3-β.[161,162]
The first documented case, linking ER stress to diabetes was reported in 1972 by Wolcott and Rallison. They documented the presence of a genetic mutation in the gene EIF2AK3, encoding the catalytic domain of the UPR transmembrane sensor PERK, thus resulting in a defective PERK signalling pathway, that leads to β-cell dysfunction and apoptosis.[163,164]
Diabetes has also been linked to a variety of different ailments, for example blindness, cardiovascular diseases, kidney failure and obesity. The occurrence of insulin resistance in T2DM is well-established. Insulin resistance has been linked to the prothrombic state and coagulation. The prothrombic state that is characterized by increased fibrinogen levels, increased plasminogen activator inhibitor and abnormalities in platelet function that would result in thrombosis. Insulin resistance affects the prothrombic state and coagulation by cosegregating with abnormalities that may occur in coagulation. Well-documented terpnoids that have been linked to diabetes and diabetes-related ailments include cannabinoids and abscisic acid (Table 4). The antithrombic effect of Cannabis sativa and the cannabinoids THC and CBN were observed in vivo and in vitro using obese rats and human plasma respectively. Indicating the potential use of cannabinoids in patients where traditional anticoagulants (e.g. heparin and warfarin) cannot be used. In 3T3-L1 cells, THC has been documented to decrease the rate of adipogenesis and increase insulin-induced glucose uptake, relative to increasing concentrations of the cannabinoid. The cannabis extract has also been shown to reduce the effects caused by a cafeteria-induced obesity, especially with regards to fat depots, insulin level regulation and cytokine production. Abscisic acid has been documented to up-regulate the expression of PPARγ, and increase glucose intolerance and decreases the level of fasting glucose.
Obesity is a common occurrence in diabetic individuals. Obesity linked to diabetes has been extensively documented to induce ER stress in a number of organs, including the liver, heart, pancreas, kidneys, adipose tissue and the central nervous system.[169–178] For example, it has been observed that obesity can activate ER stress in the liver by suppressing the insulin receptor signalling through the activation of JNK and the phosphorylation of the insulin receptor substrate[171,179] In a study by Zhang and associates, the effect of genipin on mitochondrial respiration and pancreatic islet function was tested. They observed that genipin not only increase mitochondrial membrane potential by closing KATP plasma membrane channels and increased ATP levels but also showed an acute reversal of high glucose and obesity-associated β-cell dysfunction. These results were observed when genipin was used to inhibit uncoupling protein 2 (UCP2) expression in a dose-dependent manner; thus, genipin has the ability to affect an ER-associated protein, Grp78, and a mitochondrial protein, UCP2.
Inflammatory bowel diseases
The two main inflammatory bowel diseases that affect the colon or the intestines are Crohn's disease and ulcerative colitis, both have been linked to ER stress, especially the IRE1 signalling pathway and its involvement in inflammation.[181–183] However, the activation of the PERK and ATF6 pathways has also been noted. Thus, targeting the UPR pathways associated with the expression of proteins (e.g. pro-inflammatory cytokines) that exaggerate inflammation could possibly serve as a valuable treatment plan to alleviate the discomfort and slowdown the progression of the diseases. Several terpenoids have been documented to alleviate inflammation, examples include abscisic acid and carotenoids (Table 4).
The field of cancer research is vast due to the wide variety of cancers and carcinomas afflicting the global population. The prevalence of cancer has increased substantially over the last few decades, and as such a substantial amount of research has been and continues to be performed searching for natural compounds that show potential in the treatment of cancers in general.
Auf and associates showed that there is a correlation between IRE1 signalling pathway and the reduction of functional tumour blood vessels in glioma, cell invasion and vessel co-option. They showed that by blocking the IRE1 pathway, they modified the glioma expansion by reducing angiogenesis and promoting cell invasion, disrupting the normal tissue architecture and induced reactive gliosis. They also found that interfering with IRE1 activity resulted in the down-regulation of VEGF-A, IL-1β, IL-6 and IL-8, thus indicating that the IRE1 signalling pathway may be essential for angiogenesis and tumour progression and anti-inflammatory action.
The use of C. sativa as part of cancer treatments is well-established and has been used to relieve a variety of ailments, ranging from pain (Sativex®, GW Pharmaceuticals, Salisbury, UK), inflammation and nausea (Cesamet®, Valeant Pharmaceuticals, Toronto, Canada) for decades, thus is possibly the best example of the use of terpenoids in the treatment of cancers.Cannabis sativa contains a variety of phytochemicals; however, the terpenoids that have shown promise in cancer treatments are commonly known as cannabinoids (THC, CBD, CBN). In relation to terpenoids, cannabinoids are highly regarded for their general antitumour properties against a wide variety of cancers. Antitumour properties have been documented in gliomers, melanomas, lymphomas and carcinomas.
In general, cannabinoids have been documented to affect the endocannabinoid system by binding to the cannabinoid receptors 1 and 2 (CB1 and CB2). THC impairs cell proliferation via the CB2 receptor, inhibition of the protumourigenic protein kinase B, impairing angiogenesis and subsequently induce the apoptotic signalling cascade. Treatment with THC has resulted in increased cell proliferation; however, in these cases, it was noted that the increased cell proliferation was due to the use of low dosages and compromised immune systems of the animal models.[187–190] For THC treatment to be effective in inhibiting cell proliferation, the dosage and general immunity need to be taken into account. CBD induces ER stress in a variety of cancers, including gliomas, liver carcinoma and pancreatic cancer.[94,122,191] CBD, unlike THC, has a low affinity for the endocannabinoid receptors CB1 and CB2. It has been proposed to bind to the vanilloid receptors, induce ER stress and inhibit the AKT/mammalian target of rapamycin 1 signalling cascade, initiating the ‘cross-talk’ between the ER stress pathways and mitochondrial dysfunction, and ultimately resulting in autophagy and apoptosis.
Over the last few decades, additional research has been performed in search of terpenoids other than cannabinoids that show potential in the treatment of cancers. Additional terpenoid examples and derived therapeutic drugs used in the potential treatment of cancer are available in other review articles. For example, artemisinin (commonly used to treat malaria), thapsigargin and partenolide have been used in the cancer clinical trials because of their ability to induce ER stress by blocking the sarco/ER calcium ATPase pump, and subsequently affecting a variety of downstream metabolic signalling pathways integral to cell survival and proliferation.[126,193]
As previously mentioned, the ER is essential for the correct folding of proteins and as such ER dysfunction would be expected to play an important role in the development and progression of diseases that may arise from the resultant effects of accumulated incorrectly folded proteins or even protein overloading. The idea that ER dysfunction and the subsequent accumulation of incorrectly folded neuronal proteins may play an important role in the progression of most neurological diseases is gaining popularity; however, it is important to note that not all neurological diseases arise from ER dysfunction. Factors such as the duration and severity of ER stress, the amount of cellular energy expended to alleviate ER dysfunction, and ER overload may also play a role in the different neurological diseases. In some cases, neurological diseases such as Huntington's disease may induce ER stress as a consequence of the disease, instead of ER stress being associated with the progression of the neurodegenerative disease. Neurological diseases that have been linked to ER dysfunction include: epilepsy, cerebral malaria, Alzheimer's, Parkinson's, multiple sclerosis, sleep apnoea and prion diseases.
In the past, the intrinsic and extrinsic apoptotic pathways studied in neuronal cells after seizures focused on the mitochondrial and death receptor analysis, respectively. However, in recent years, the importance and role of ER stress and dysfunction has emerged as an alternative intrinsic pathway.[55,194,195] A number of research articles and reviews are available, linking neurological diseases to ER dysfunction and stress, and as such, this review will briefly provide two examples, epilepsy and cerebral malaria, that have not been studied to the same extent as the more common neurological diseases.[54,80,196–200]
In general, the role of the ER in neurodegenerative diseases has increasingly been recognized; however, very little is known about the role of ER stress in epilepsy. A recent study by Liu and associates have indicated the expression of ER stress related chaperones (GRP78 and GRP94) were up-regulated after seizures in the temporal neocortex of epileptic patients, confirming what had previously been seen in literature. They also documented the presence of ER stress through the activation of the IRE1α : TRAF2 : JNK signalling pathway, thus indicating that in cases where brain injury has occurred as a result of epileptic seizures, ER stress and specifically the IRE1α signalling pathway may play an important role in the apoptosis of neuronal cells. Other evidence linking ER stress to epilepsy include disturbances in the calcium homeostasis, increased expression of pro-inflammatory cytokines (IL-1β, IL-6 and TNFα), and the up-regulation of XBP1.[202–204]
The contraction and progression of cerebral malaria develops as a result of complications after the infection of Plasmodium falciparum. Even though a substantial amount of people die from the disease annually, the pathogenesis in humans remains largely unstudied because of experimental analysis only occurring post-mortem. In recent years, a murine model has been used to study the pathogenesis of cerebral malaria after infected with P. berghei. The murine model is used to study cerebral malaria because it displays symptoms typical of humans cerebral malaria. Observations from these studies have indicated that any complications arising from infections, which may involve the activation of platelets, increase expression of cytokines, disruption of the blood-brain barrier and the blockage of microvessels by infected red blood cells. At the stage, where the infection results in cell death, several signalling pathways have been implicated, including mitochondrial dysfunction, activation of calcium-dependent and independent kinases (e.g. JNK), caspases and recently ER stress.[205–207] Anand and Babu illustrated the role of ER stress in neuronal cell death in mice infected with P. berghei. They observed the activation of all the signalling pathways involved in UPR and that neuron apoptosis occurred via the activation of the IRE1-TRAF2-ASK-JNK signalling pathway; the increased levels and accumulation of CHOP, phosphorylated eIF2α, ATF4, GADD34, BAX, caspase 3 cleavage and the down-regulation of the prosurvival proteins (Bcl-2, GRP78, calreticulin, calnexin). This indicates the involvement of both the IRE1 and PERK signalling pathways in the progression of the disease.
Like most disease states, viral infections have been documented to affect ER homeostasis. Once the cells are infected, the viral replication proteins increase pressure on the cellular functions and induce the ER's UPR pathways. However, because irresolvable ER stress generally results in the induction of apoptosis, which would not be beneficial to the survival of the virus. To overcome this obstacle, several viruses have been documented with the ability to regulate the UPR pathways, ensuring that favourable conditions are maintained. Hepatitis C virus affects the ATF6 and IRE1 pathways by cleaving ATF6 into its active conformation, subsequently up-regulating the expression of chaperones, and prevents the activation of XBP1. The dengue virus induces the expression of XBP1, EDEM1, ER DNA J homologue 4, p58IPK and GRP78.[209,210] West Nile virus strains affect all the UPR pathways by up-regulating pro-apoptotic factors (CHOP, caspase 3) and GADD34, XBP1 and INF signalling pathways.
In general, the majority of research that is performed on signal cascades involve the prominent protein that initiate the signalling pathways; the effect terpenoids have on the expression or function of the UPR proteins TRAF2, JIK, JUN, SAPK and ATF4 have not been well-established. The exuberant number of terpenoinds that have been identified, increase the possibility that more plant-derived compounds can be used to identify novel treatments for more diseases and viral infections, thus producing a sustainable alternative to the treatment of diseases.