Biological activity and toxicity of the Chinese herb Magnolia officinalis Rehder & E. Wilson (Houpo) and its constituents

2.1. Lignans

Major significant bioactive components isolated from the bark of M. officinalis or M. obovata appear to be the polyphenolic neolignans, magnolol and honokiol (Kong et al., 2005; Amblard et al., 2007; Lin et al., 2011). Magnolol was named after its source, genus Magnolia plants, and honokiol was named after “Honoki”, a Japanese name of M. obovata Thunb. (Maruyama and Kuribara, 2000). Magnolol and honokiol are two hydroxylated biphenolic isomers (neolignans, C₁₈H₁₈O₂) (Lin et al., 2007), the ortho-ortho-C-C and ortho-para-C-C dimers of 4-allyl-phenol, respectively (Kong et al., 2005) (Table 3).

Commercially available Magnolia bark extracts appear to be marketed based on their high phenolic content, with magnolol and honokiol ranging from 40% to 90% of total polyphenols; the evaluation of the quality of commercial Houpo samples, according to region, harvesting, and processing, is effectively based on a quantitative determination of the levels of magnolol and honokiol in the bark (EPCNF, 2009). The Chinese (CPC, 2010) and European pharmacopoeia (Council of Europe, 2013) require a minimum of 2.0% of the total amount of honokiol and magnolol, with reference to the dried drug. Other neolignans include 4-O-methylhonokiol (0.012% in M. obovata and 0.0003% in M. officinalis bark) and obovatol (only found in M. obovata at 0.33%) (Lee et al., 2011).

2.2. Alkaloids

Other bioactive polar compounds from Magnolia bark are alkaloids (about 1% of the bark), mainly benzyltetrahydroisoquinoline and aporphine types (Yu et al., 2012), based on the precursor amino acids tyrosine and dopa. Alkaloids that possess an isoquinoline skeleton are among the most common of all alkaloids. From the bark of M. officinalis, different tertiary and quaternary alkaloids have been isolated and structurally elucidated (Yan et al., 2013), including (1) the aporphine alkaloids N-methylcoxylonine, (S)-magnoflorine, magnofficine, (R)-asimilobine, corytubérine, anonaine, liriodenine and (2) the benzyltetrahydroisoquinoline alkaloids (R)-magnocurarine, (S)-tembetarine, lotusine, (R)-oblongine, reticuline (Guo et al., 2011; Yan et al., 2013). Magnoflorine and magnocurarine are considered as the major and most potent of Magnolia bark alkaloids (Dharmananda, 2002) (Tables 4 and 5).

Alkaloids are found in leaves (at full maturation or beginning of fall), branches, and bark of Magnolia species (Ziyaev et al., 1999). These alkaloids include tertiary and the highly polar quaternary ammoniums.

3.1. Cytotoxic activity: eventual therapeutic application in cancer

Cancer represents a major health problem worldwide. In 2012, the World Health Organization (WHO) recorded 14 million new cases of cancer and 8.2 million cancer-related deaths, with 4.3 million under the age of 70 years (WHO, 2016). Significant progress in the fight against cancer still calls for both advances in cancer diagnosis and development of preventive and therapeutic strategies (Arora et al., 2012). Herbal therapies from alternative and complementary medicines may hold a key to such advances; for example, TCM proposes preparations to treat cancers, but the effective component(s) or their mode of action at cellular and molecular levels is largely unknown (Yang et al., 2003).

Studies in in vitro and animal models have shown that Magnolia components, especially the neolignans honokiol and magnolol, are able to target many pathologically relevant pathways (Arora et al., 2012). These findings have increased interest in using neolignans as possible novel chemotherapeutic agents; notably, mechanistic studies have been carried out on honokiol to assist the development of novel synthetic analogues and to provide clues for rational combinations with conventional chemo-or radiotherapy (Fried and Arbiser, 2009).

3.1.1 Mechanisms of action

As Arora et al. (2012) recapped, cancer progresses through a series of genetic and epigenetic aberrations leading to dysregulation of key cell signaling pathways involved in growth, malignant behavior, and therapy-resistance. Honokiol and its analogs target multiple signaling pathways, including nuclear factor kappa B (NF-κB), signal transducer and activator of transcription 3 (STAT3), epidermal growth factor (EGFR, also called ErbB1), mammalian target of rapamycin (mTOR, a protein kinase centrally involved in the control of cell metabolism, growth, and proliferation), and the caspase-mediated common pathway, which modulate cancer initiation and progression (Kumar et al., 2013). Also, to fight cancer, lignans-induced apoptosis seems to be a key feature (Lee et al., 2011).

The ubiquitous transcription factors, NF-κB and STAT3, control the expression of a wide array of genes notably involved in development, cell growth, and differentiation, immunity, metabolism, inflammation, and cancer (Grivennikov and Karin, 2010). Aberrant activation of these transcription factors is not only common in many malignancies, in both the early and late developmental steps (Arora et al., 2012), but also in tumorigenesis, metastasis, and angiogenesis through the induction of genes participating in malignant conversion and tumor promotion (Lee et al., 2011). In many tumors, a constitutive activation of NF-κB modulates the expression of anti-apoptotic (upregulation) and pro-apoptotic (downregulation) genes and activates cell cycle and proliferation (Garg and Aggarwal, 2002; Karin, 2006; Lee et al., 2011; Arora et al., 2012). STATs notably participate in oncogenesis, through upregulation of genes encoding apoptosis inhibitors and cell cycle regulators (Yu et al., 1995; Buettner et al., 2002). Lignans, especially honokiol, have been shown to affect NF-κB signaling, not through a direct effect on NF-κB-DNA binding, but upstream of NF-κB activation, at the level of IKK, the inhibitor of κB (IκB) kinase (Fried and Arbiser, 2009). They inhibit NF-κB activation through the suppression of protein kinase B (AKT), which prevents IKK activation, IκBα phosphorylation, and NF-κB nuclear translocation. STAT3 can be inhibited by lignans through interleukin-6 (IL-6), one of the many growth factors that modulate its expression, and by the repression of upstream protein tyrosine kinases, cellular sarcoma (c-Src), Janus kinase 1 (JAK1), and JAK2 (Fried and Arbiser, 2009; Arora et al., 2012).

EGFR is a membrane receptor, a glycoprotein with tyrosine kinase activity, which is frequently overexpressed and/or amplified in various cancer types. Aberrant EGFR signaling has been shown to promote cell survival and proliferation (Arora et al., 2012). Failure to attenuate receptor signaling by receptor downregulation can also lead to cellular transformation; the mechanisms by which EGFR becomes oncogenic are numerous, often specific for each cancer type (Zandi et al., 2007). Studies in several human cells, obtained from breast cancer, melanoma, and head and neck squamous cell carcinoma, indicate that a growth inhibitory effect of lignans is associated with down-modulation of EGFR signaling and phosphorylation (Fried and Arbiser, 2009; Arora et al., 2012; Kaushik et al., 2012).

In most cancer cells, but also in the carcinogenic process, an aberrant activation of mTOR is primarily caused by the activation of the phosphatidyl-inositol 3-kinase (PI3K)-AKT pathway (Guertin and Sabatini, 2007). It is suggested that lignans, and especially honokiol, may suppress the activation of mTOR and its downstream signaling, resulting in apoptosis induction, by inhibiting the extracellular signal-regulated kinases (ERK) and AKT pathways or by upregulating phosphatase and tensin homolog (PTEN) (Liu et al., 2008; Fried and Arbiser, 2009; Lee et al., 2011; Arora et al., 2012; Kaushik et al., 2012).

3.1.2 Cytotoxic activity of Magnolia lignans

Several constituents of M. officinalis, mainly lignans, have been widely reported to be cytotoxic, which is a possible clue to new anti-cancer compounds (Lee et al., 2011). At low concentrations (up to 3 µmol/L), they can induce apoptosis in human cancer cell lines, whereas they do not inhibit the growth of human untransformed cells (Kong et al., 2005). Honokiol was reported to possess higher activity than magnolol in the induction of apoptosis (Kong et al., 2005). Obovatol increased the susceptibility to chemotherapeutic agents at low concentrations, around 5 µmol/L (Lee S.Y. et al., 2009) (Table 6).

3.2. Anti-inflammatory activity

Inflammation, a protective response involving immune cells, blood vessels, and molecular mediators, is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants (Ferrero-Miliani et al., 2007). Generally, the production of inflammatory molecules is triggered by mitogenic stimulation with various bacterial products, such as lipopolysaccharides (LPS).

Various signaling cascades composed of pattern recognition receptors (e.g. Toll-like receptor 4 (TLR4)), non-receptor type tyrosine kinases (eg. Src and Syk), serine/threonine kinases (protein kinase C (PKC), PKA, PI3K, and AKT), MAPK, and various redox-sensitive transcription factors, such as NF-κB, are regarded as critical components participating in the process (Kim and Cho, 2008; Simmonds and Foxwell, 2008).

Nitric oxide (NO·), a physiological mediator of endothelial cell relaxation, also plays a key role in the inflammatory pathway. Synthesised by many cell types involved in immunity and inflammation, through the inducible NO· synthase (iNOS), in response to pro-inflammatory cytokines and bacterial LPS, NO· is a major defense molecule, toxic against infectious organisms, and is a key player in the pathogenesis of a variety of inflammatory diseases (Lee et al., 2011). NO· notably regulates the functional activity, growth and death of many immune and inflammatory cell types including macrophages, T lymphocytes, antigen-presenting cells, mast cells, neutrophils, and natural killer cells (Coleman, 2001; Ricciardolo et al., 2004; Garcia and Stein, 2006; Tripathi et al., 2007).

The factor NF-κB (Section 3.1) promotes the transcription of genes involved in pro-inflammatory responses. In macrophages, NF-κB is activated by inflammatory extracellular signals such as LPS, IL-1 and tumor necrosis factor α (TNF-α), and regulates a number of inflammatory genes, inducing further inflammatory mediators, including NO· and cytokines (Zhang et al., 2013). A second major transcription factor, the activator protein 1 (AP-1), an ubiquitous dimeric protein complex composed of Jun and Fos subfamilies (Curran and Franza, 1988), is also activated by many pathophysiological stimuli, including LPS, reactive oxygen species (ROS), mitogenic growth factors, inflammatory cytokines, growth factors of the transforming growth factor-β (TGF-β family, ultraviolet (UV) and ionizing irradiation, cellular stress, antigen binding, and neoplastic transformation. In response to different stimuli, AP-1 is able to activate different sets of genes for differentiation, proliferation, apoptosis (Wisdom, 1999), and inflammation (Newton and Dixit, 2012) responses.

Within the repertoire of protein kinase cascade networks, the MAPK family contains at least 3 protein kinases in series that culminate in the activation of a multifunctional MAP kinase (Pearson et al., 2001). MAP kinases are major components of pathways controlling embryogenesis, cell differentiation, cell proliferation and cell death (Pearson et al., 2001), and play a significant role in carcinogenesis and in leukocytes’ recruitment to sites of inflammation (Herlaar and Brown, 1999). As for AP-1, the MAPKs differentially activate inflammatory pathways, depending on the stimuli and cell types (Lee et al., 2011).

The cyclooxygenase 2 (COX2), induced in cells by proinflammatory cytokines and growth factors, is also increased in certain inflammatory states. COX2 is a key enzyme in the synthesis of pro-inflammatory eicosanoids (prostaglandins (PGs), thromboxanes (TBXs), leukotrienes, etc.).

As discussed in the previous section, the NO production, the expressions of iNOS, IL-1β, TNF-α and COX, and the generation of eicosanoids, in addition to the activation levels of MAPKs, AP-1, and NF-κB pathways, may reflect the degree of inflammation and have become important indicators with which to assess inflammatory and anti-inflammatory processes (Lee et al., 2011). An impressive series of studies has shown that all of these major indicators are positively impacted by Magnolia lignans, even if the full anti-inflammatory mechanisms have not yet been elucidated (Choi et al., 2007; Lin et al., 2007; Munroe et al., 2007; Kang et al., 2008; Lee et al., 2011). Upregulating signaling cascades composed of Ras, Raf, and MAPK, downregulating the activation of NF-κB, and blocking NF-κB activation mediated by CD40 and latent membrane protein 1 are speculated to be the action mechanism of Magnolia lignans (Lin et al., 2007; Kim and Cho, 2008; Lee et al., 2011).

Among the lignans (Table 7), honokiol and magnolol were notably shown to inhibit the formation of eicosanoids (prostaglandin D2 (PGD2), PGE2, leukotriene C4 (LTC4), LTB4, and thromboxane B2 (TXB2)), probably through inhibition of phospholipase A2, COX, 5-lipoxygenase, LTC4 synthase, and LTA4 hydrolase activities (Lin et al., 2007).

3.3. Therapeutic activity in asthma

Asthma is a chronic disease characterized by recurrent attacks of breathlessness and wheezing, which vary in severity and frequency from person to person (WHO, 2016). According to WHO estimates, 235 million people suffer from asthma, with prevalences ranging from 1% to 18% across countries. Moreover, it represents one of the most prevalent chronic conditions among children. In addition to large financial burdens, asthma also leads to serious health consequences and compromises life quality (Lin et al., 2015).

The causes of asthma are not completely understood and the pathology cannot be cured, but appropriate management can control the disorder (WHO, 2016). In China as well as throughout Asia, myths and misconceptions on Western medicine and corticosteroids are prevalent, resulting in non-adherence to conventional treatments and in the recourse to traditional herbal medicines (Hon et al., 2015). For example, Saiboku-To, a Japanese mixture of ten different herbal extracts (Table 1) that include M. officinalis, has been investigated for corticosteroid-dependent severe asthma to reduce the maintenance doses of corticosteroids. This herbal complex medicine is also known for its use in general bronchial asthma (Lee et al., 2011) and the TCM herb “Houpo” (bark of M. obovata and/or M. officinalis), is widely used for the treatment of chest tightness and asthma (Ko et al., 2003). Homma et al. (1993) suggested that magnolol, as an inhibitor of 11β-hydroxysteroid dehydrogenase and T-lymphocyte proliferation, might be responsible for the therapeutic effect of Saiboku-To resulting in corticosteroid-sparing.

The effects of Magnolia on asthma are from two types: (1) M. officinalis (as well as Saiboku-To) inhibits human lymphocyte blastogenesis, in vitro, in a dose-dependent way. It seems to have anti-asthmatic effect through suppression of type IV (lymphocyte-mediated) allergic reaction (Lee et al., 2011); (2) Magnolia extracts induce bronchodilatation by a muscle relaxation that is associated with a Ca²⁺ antagonist effect, probably due to magnolol and honokiol. Moreover, alkaloids such as (R)-coclaurine and (S)-reticuline inhibit acetylcholine (ACH)-induced contraction of muscles (Ko et al., 2003; Lee et al., 2011). The effects of honokiol and magnolol on muscular contractile responses and intracellular Ca²⁺ mobilization were investigated in the non-pregnant rat uterus; both lignans (at concentrations 1–100 μ mol/L) inhibited spontaneous and uterotonic agonists (carbachol,

PGF-2α, and oxytocin), and an experimental Ca²⁺ channel activator in a concentration-dependent manner. The inhibition rate of honokiol on spontaneous contractions appeared to be slower than that of magnolol-induced responses (Lu et al., 2003) (Table 8).

3.4. Treatment of gastrointestinal disorders

Diseases of the GI tract are highly common and varied, including irritable bowel syndrome, functional dyspepsia, abdominal pain, abdominal bloating, nausea, vomiting, diarrhoea, constipation, etc., which have a substantial effect on quality of life and health-care costs (Hu et al., 2009). M. officinalis is commonly used in TCM for treating such GI disorders, probably through an antispasmodic effect that results in relaxation of GI tract smooth muscles (Chan et al., 2008; Lee et al., 2011).

Among various regulatory factors that modulate smooth muscle motility in GI tracts, ACH and serotonin (5-hydroxytryptamine, 5-HT) are considered as major neurotransmitters that regulate the GI motility. Magnolol and honokiol significantly inhibit the contractility of isolated gastric fundus strips of rats treated with ACH or serotonin, and of isolated ileum in guinea pigs treated with ACH or CaCl₂; both behave as non-competitive muscarinic antagonists. Magnolol and honokiol inhibited contractions induced by ACH (in Ca²⁺-free medium) and extracellular Ca²⁺-dependent contractions induced by ACH; this Ca²⁺ blockade effect is also discussed for anti-asthmatic effects in Section 3.3 (Lu et al., 2003; Chan et al., 2008; Jeong et al., 2009; Lee et al., 2011; Herrmann et al., 2014).

Alkaloids may also confer some of the antispasmodic effect of Magnolia bark when used in high-dose decoctions to alleviate bronchiole spasms and intestinal spasms. Magnocurarine was investigated as a potential muscle relaxant drug in Japan (Dharmananda, 2002) (Table 9).

3.5. Treatment of diabetes

Magnolia plants have been used as Korean and Brazilian complementary and alternative medicines for the treatment of diabetes and diabetic complications; it has been shown that long-term supplementation of honokiol and magnolol ameliorates body fat accumulation, insulin resistance, and adipose inflammation in high-fat fed mice (Kim et al., 2013). Honokiol and magnolol reduce fasting blood glucose and plasma insulin levels in type 2 diabetic rats without altering body weight, and induce glucose uptake in adipocytes. Honokiol also enhances insulin signaling pathways such as the Ras/ERK1/2 and phosphoinositide-3-kinase/AKT signaling pathways, and ameliorates alcoholic steatosis by blocking fatty acid synthesis regulated by the sterol regulatory element-binding protein 1c (SREBP1c) (Kim et al., 2013). An ethanolic extract of the Magnolia cortex showed an in vitro inhibitory effect on the formation of advanced glycation endproducts (AGEs), which play an important role in the development of diabetic complications. Furthermore, magnolol inhibits AGE formation and sorbitol accumulation in streptozotocine-induced diabetic rats (Sohn et al., 2007). Honokiol and magnolol also improve both glucose and lipid metabolisms and inhibit acyl-CoA cholesterol acyltransferase (ACAT), which catalyzes the formation of cholesteryl esters from cholesterol and long-chain fatty acyl-CoA. Magnolol can activate the peroxisome proliferator-activated receptor γ (PPARγ) as a ligand, induce adipocyte differentiation, and enhance insulin-stimulated glucose uptake in animal models (Lee et al., 2015).

3.6. Effects on neuronal disease

The bark of M. officinalis is one of the most important traditional herbal medicines in China and Japan used to treat clinical depression and anxiety-related disorders (Nakazawa et al., 2003). Even the oldest known TCM book, Shennong Bencao Jing, mentions this tranquilizing action (Tachikawa et al., 2000). Kampo preparations, such as Hange-Koboku-To, Yoku-Kan-san, Saiboku-To, and Kami-Kihi-To, have been historically prescribed for conditions akin to clinical depression, anxiety-related disorders, such as anxiety neurosis, insomnia, and anxiety hysteria, as well as for thrombotic stroke and GI complaints (Maruyama et al., 1998). The ingredients that have been reported to possess pharmacological effects on the nervous systems are β-eudesmol, α-and β-pinenes, and bornyl acetate, as essential oils; magnolol and honokiol, as diphenyl compounds; and magnocurarine and magnoflorine, as alkaloids (Tachikawa et al., 2000). Metabolites formed by the gut flora have been proposed as the actual active agents (Nakazawa et al., 2003).

3.6.1 Treatment of anxiety

Anxiety disorders are considered the most common psychiatric diagnoses affecting between 10% and 30% of the general population. Excess anxiety can be debilitating and lower the quality of life (Wittchen and Hoyer, 2001; Lee et al., 2011). Despite well-known side effects such as sedation, muscle relaxation, amnesia, and dependence, benzodiazepines are extensively used for the treatment of several forms of anxiety (Wittchen and Hoyer, 2001). Various herbal medicines, traditionally used for tranquillo-sedative effects, are now being investigated as anxiolytic drugs, including the bark of M. officinalis (Lee et al., 2011).

The central depressant effects of lignans may contribute not only to an anticonvulsant effect but also, at low doses, to an anxiolytic activity. This can be partially attributed to their interaction with the γ-aminobutyric acid receptor A (GABA_A), a known target for benzodiazepines and other anxiolytics. The activity of hippocampal glutamic acid decarboxylase (GAD), an enzyme involved in GABA synthesis, is significantly increased in honokiol-treated mice, suggesting that honokiol may alter the brain’s synthesis of GABA. GABA_A receptors have subunit heterogeneity that influences their function; magnolol and honokiol were found to modulate all receptors, regardless of their sub-units, but receptors with the δ-subunit were 2 to 3 times more sensitive (Alexeev et al., 2012; Chen et al., 2012; Woodbury et al., 2013).

3.6.2 Treatment of depression

Depression is a mood-altering disease affecting energy, sleep, appetite, libido, and the ability to function. The symptoms of depression include an intense feeling of sadness, hopelessness, despair, and the inability to experience pleasure in usual activities (Lee et al., 2011). Most common anti-depressant drugs (tricyclic/polycyclic antidepressants, selective serotonin reuptake inhibitor, monoamine oxidase inhibitors, serotonin-norepinephrine reuptake inhibitors, lithium) potentiate the action of monoamines (norepinephrine, dopamine, and serotonin) in the brain, an observation that resulted in the so-called “theory of monoamines” that postulates an association between depression and monoamine depletion. Efforts in the field are still sorely needed, as recent studies indicate that about 30% of depressive patients fail to respond satisfactorily to antidepressant treatment.

In vitro radioligand binding and cellular functional assays revealed that Magnolia bark supercritical carbon dioxide extracts interact with the adenosine A1 receptor, dopamine transporter, and dopamine D5 receptor (antagonist activity), serotonin receptors (5-HT1B and 5-HT6, antagonist activity) and the GABA benzodiazepine receptor at a concentration of 100 μg/ml or lower (Koetter et al., 2009). Honokiol and magnolol exhibit neurotrophic function by enhancing hippocampal ACH release, and magnolol modulates the central serotonergic activity (Nakazawa et al., 2003). Major receptors of biogenic amine neurotransmitters (dopamine, noradrenaline, serotonin, and histamine) are coupled with G-proteins, activating or inhibiting adenylate cyclase. Honokiol and magnolol could normalize the biochemical abnormalities in brain serotonin and its metabolites, serum corticosterone levels, and platelet adenylate cyclase activity via up-regulating the cyclic adenosine monophosphate pathway (Waymire, 1997; Lee et al., 2011).

3.6.3 Effects on Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia. There are no available treatments that stop or reverse the progression of the disease, which worsens as it progresses, and eventually leads to death (WHO, 2016). Studies of patients with AD have revealed ACH-depleted cerebral regions; centrally acting cholinergic drugs have been reported to increase the ACH levels in these brain regions affected by AD and to improve cognition and memory. Maintaining ACH levels in brain, by antagonizing the activity of acetylcholinesterase (ACHE), the enzyme that degrades ACH, is one of the few therapeutic options in AD patients, which could be important to delay the aggravation of the disease (Lee et al., 2011). However, ACHE inhibitors have limitations, including short half-lives, peripheral cholinergic side effects, and notable hepatotoxicity (Lee et al., 2011). Beside ACH depletion, abnormal phosphorylation of the protein tau, oxidative stress, and altered protein processing resulting in abnormal β-amyloid peptide (Aβ) accumulation are associated with the progressive development of AD, leading to neuronal cell death (Zaki et al., 2014).

Ethanol extracts of M. officinalis and 4-O-methylhonokiol were found to dose-dependently attenuate the scopolamine-induced increase in ACHE activity in the cortex and hippocampus of mice and inhibited ACHE activity in vitro (Lee et al., 2010). Beside ACH depletion, oxidative damages are also associated in the progressive development of AD. Actually, oxidative stress-induced neuronal cell death by accumulation of Aβ is a critical pathological mechanism in AD. The antioxidants honokiol, magnolol, and 4-O-methylhonokiol significantly decrease Aβ-induced cell death; these neuroprotective effects are possibly mediated through reduced ROS production as well as suppression of intracellular calcium elevation and inhibition of caspase-3 activity (Hoi et al., 2010; Woodbury et al., 2013). Lignans are also able to decrease the activity of β-secretase, preventing the release of Aβ from amyloid precursor protein (APP) (Lee et al., 2010).

3.7. Therapeutic activity in cardiovascular diseases

Cardiovascular diseases (CVDs), a group of disorders of the heart and blood vessels, are the prime cause of death worldwide (WHO, 2016). The cardiovascular protection potentiality of M. officinalis due to its antioxidant components was described around the 1990s (Lo et al., 1994). A role for oxidative stress has been postulated in many CVDs as ROS contribute to cardiovascular lesions by promoting inflammation, altering vasomotion, inducing cell death, causing platelet aggregation, and stimulating vascular smooth muscle cell (VSMC) proliferation; induced vascular stenosis, cell death, and inflammation are major progressive factors in cardiovascular dysfunctions (Lee et al., 2011). The potential of M. officinalis to afford cardiovascular protection was attributed to its antioxidant components, including lignans and, notably, honokiol and magnolol. Those effects are dose-related, and are the consequence of different molecular mechanisms’ regulation (Ho and Hong, 2012).

3.7.1 Effects in atherosclerosis

Atherosclerosis is a condition, in which plaques made of cholesterol, fatty substances, cellular waste products, calcium, and fibrin build up inside the arteries (AHA, 2016). Oxidized low-density lipoproteins (LDL) induce arterial wall cells to produce chemotactic factors, adhesion molecules, cytokines, and growth factors that play a role in the development of these plaques (Young and Woodside, 2001; Lee et al., 2011). Honokiol was shown to alleviate the detrimental effects of oxidized LDL, reducing the expression of endothelial iNOS and adhesion molecules, and attenuating the induced cytotoxicity, apoptosis, ROS generation, intracellular calcium accumulation, the subsequent mitochondrial membrane potential collapse, cytochrome c release, and activation of caspase-3 in human umbilical vein endothelial cells (HUVECs) (Ou et al., 2006). Moreover, magnolol effects include protection of the cardiovascular system by interfering with ROS generation (Lee et al., 2011). Magnolia compounds with an anti-atherosclerosis effect were listed in Table 10 (Lee et al., 2011).

3.7.2 Anti-platelet activity

Magnolia compounds have been known for more than 20 years to interfere in some major pathways of platelet activation/inhibition: (1) TBXs, some of the most powerful agonists for platelet activation and major contributors to thrombus formation; TXB2, whether induced by collagen, arachidonic acid, or thrombin, is inhibited by magnolol and honokiol; also five aporphine alkaloids isolated from leaves of M. obovata, N-acetylanonaine, N-acetylxylopine, N-formylanonaine, liriodenine, and lanuginosine, show potent antiplatelet activities, probably by interfering with TXA2 production and/or binding (Teng et al., 1988; Pyo et al., 2003; Ho and Hong, 2012); (2) the activation of PPAR isoforms (α, β/δ, and γ), a pathway known to inhibit platelet aggregation; magnolol (20–60 μmol/L) dose-dependently enhances the activity and intracellular level of PPAR-β/γ in platelets (Shih and Chou, 2012); and (3) the cytosolic Ca²⁺ influx and/or mobilization from intracellular stores that plays a crucial role in the platelet responses to various agonists (Lee et al., 2011); the rise of intracellular calcium caused by arachidonic acid or collagen is suppressed by both magnolol and honokiol (Teng et al., 1988; Pyo et al., 2003; Lee et al., 2011).

3.8. Antimicrobial activities

Antibiotic treatments have become a cornerstone for human health; however, due to long, wide, and irrational applications of antibacterial agents in treatments in various fields other than in the clinic, resistant strains have evolved as problematic pathogens and now pose a serious challenge. In the fight against these resistant pathogens, plant extracts heavily exploited in different civilizations (Amblard et al., 2007; Zuo et al., 2015) could yield clues to new therapeutic options (Okusa et al., 2014; Rasamiravaka et al., 2014; Ngezahayo et al., 2015; Ngezahayo et al., 2016).

The antimicrobial activities of honokiol and magnolol, the main constituents of M. officinalis, were effective against Gram-positive bacteria, such as Streptococcus mutans (Bang et al., 2000), Staphylococcus aureus, Listeria monocytogenes, Streptococcus faecalis, Escherichia coli, Salmonella typhimurium, Bacillus anthracis, Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, Micrococcus luteus, Bacillus subtilis, Capnocytophaga gingivalis, Veillonella disper, but also against fungal strains such as Candida albicans as a representative fungus for candidiasis, Trichophyton mentagrophytes for superficial dermatomycosis and Cryptococcus neoformans for deep dermatomycosis. The lignans showed a significant antimicrobial activity against these microorganisms with minimum inhibitory concentrations (MICs) ranging 10–240 μ mol/L (Chang et al., 1998; Bang et al., 2000; Ho et al., 2001; Hu et al., 2011).

Lignans, mainly honokiol and magnolol, have marked dose-dependent antimicrobial effects (Table 11) (Kim et al., 2015). Previous studies have shown that phenolic compounds can affect microbial growth by altering microbial cell permeability and permitting the loss of macromolecules. Once the phenolic compounds have crossed the cell membrane, interactions with membrane enzymes and proteins will cause an opposite flow of protons, affecting cellular activity. It has also been suggested that phenols react primarily with the phospholipid components of the cell membrane, causing an increase in permeability (Hu et al., 2011; Zuo et al., 2015). For antiviral activity of aporphine alkaloids, they were shown to interfere with the viral replicative cycle. Although DNA synthesis was reduced, their exact target remains to be elucidated (Boustie et al., 1998).