A number of studies have demonstrated the antioxidant properties of extracts prepared from different parts of MI. Dhital (11) demonstrated that the leaf methanol extract of MI exerts moderate free radical scavenging effects (12). In another study, among the aqueous bark extracts prepared from the Keitt, Kent, Honey (originally Ataulfo) and Tommy Atkins MI varieties, the aqueous bark extract of Kent was reported to possess the most pronounced antioxidant properties. Moreover, the Ataulfo extract was found to exert the most prominent cellular anti-oxidant effects (12). The leaf methanol extract of Mahajanaka, a mango variety found in Thailand, has been shown to exert potent free radical scavenging effects. The same leaf extract has also been shown to inhibit lipopolysaccharide-induced nitric oxide production in RAW264.7 cells (13). In another study by John et al 2012, exposure to an aqueous ethanol extract of MI stem bark increased the red blood cell count in Wistar strain albino rats, indicating its ability to enhance erythropoiesis and exert protective effects against oxidative damage. Furthermore, the increment of monocytes and neutrophils following exposure to the aqueous ethanol extract indicated its ability to enhance and modulate immunological activities (14). The leaf ethanolic extract of MI has been found to have antioxidant properties in vivo. In a previous study, the attenuation of cerebral oxidative status in rats was observed when the rats were fed various different doses of MI leaf ethanol extract. Moreover, increased levels of malondialdehyde, and reduced superoxide dismutase and glutathione peroxidase activities were observed in the hippocampus of the tested animals (15). The ethanolic extract of MI fruits has also been found to exert protective effects against cognitive impairment and oxidative stress (16). These observations indicate the ability of the MI plant extract to function as an antioxidant in vitro and in vivo.

The anti-inflammatory properties of mango peel, seed and pulp of Sri Lankan mango varieties have been investigated (17). Experiments conducted using the human red blood cell membrane stabilization assay demonstrated that the peel, pulp and seed ethyl acetate extracts of three different mango varieties (Willard, Vellaicolomban and Karthacolomban) exerted anti-inflammatory activities. Among these, the ethyl acetate extract of Karthacolomban seeds exhibited the highest anti-inflammatory activity (17). The ethanol leaf extract of MI was found to possess analgesic and anti-inflammatory effects in-vivo (18). Wistar Hannover rats treated with MI leaf ethanol extract have also been shown to exhibit diminished inflammatory activities induced by 4% formalin (19). In the study by Kim et al (20), polyphenolic derivatives of MI were shown to modulate dextran sulfate sodium (DSS)-induced colitis in rats, suggesting that MI polyphenolic derivatives can attenuate inflammatory responses. In another study, the methanol extract of MI stem bark was found to exert anti-inflammatory effects against DSS-induced colitis (21). Moreover, the administration of aqueous stem bark extracts of MI resulted in a reduction in the levels of thiobarbituric acid reactive substances, the expression of tumour necrosis factor-α (TNF-α), cyclooxigenase-2, inducible nitric oxide synthase and TNF receptor-2 in colonic tissue and, and in serum TNF-α and interleukin (IL)-6 levels (21). Collectively, these findings suggest that mango peel, seed and leaf extracts possess anti-inflammatory properties.

The study by Perpétuo and Salgado (22) demonstrated the effects of diets containing mango flour on blood glucose levels in diabetic rats. Diabetic rats fed mango flour exhibited reduced blood glucose levels from the 10th day of feeding. Moreover, a 64% reduction in glycogen levels was observed compared to the control group (22). MI flavonoids have been reported to reduce blood glucose levels in Swiss albino mice with induced diabetes (23). Furthermore, MI leaf aqueous extract was found to increase high-density lipoprotein levels (23). The ethanol (95%) extract of MI leaves and mangiferin modulated the endocannabinoid (CB) system and peroxisome proliferator-activated receptor-γ (PPARγ) mRNA expression in cafeteria diet-fed rats, suggesting a possible role for MI in controlling obesity and metabolic syndrome (24). The endocannabinoid system has been reported to contribute to weight gain and glucose intolerance, while PPARγ is one the key regulators of adipose cell development and differentiation (24).

The peel acetone extract of MI possesses hypoglycaemic effects. The study by Gondi and Rao illustrated that the leaf extract of MI can be used against streptozotocin-induced diabetes in rats (25). Rats fed various doses of MI peel extracts were found to have lower levels of glycated haemoglobin (25). Narasimhan et al (26) also demonstrated that albino rats of the Wistar strain fed various doses of ferulic acid, a major compound found in mango, exhibited an enhanced glycogen synthesis, improved blood glucose tolerance and reduced gluconeogenic enzyme activities. Moreno et al (27) demonstrated that MI extracts inhibited the action of lipoprotein lipase and hormone sensitive lipase in male Wistar rats. Animals receiving stem bark and leaf extracts exhibited an inhibition of isoproterenol-stimulated glycerol release and an increment in faecal fat. Moreover, animals receiving MI leaf extract also exhibited an inhibition of pancreatic lipase (27). A study on lean and obese individuals identified that a mango supplementation decreased plasminogen activator inhibitor-1, glycated haemoglobin and inflammatory cytokine (IL-8 and monocyte chemoattractant protein-1) levels in obese individuals and controlled blood pressure, indicating a positive role of mango consumption against metabolic and obesity-related chronic diseases (28).

Acetone extracts of MI leaves have been shown to exhibit antibiotic activity against antibiotic-sensitive and multidrug-resistant bacteria, such as Salmonella typhi, Staphylococcus aureus and Salmonella typhimurium (29). Furthermore, a methanol extract of the seed kernal of MI has been found to exert inhibitory effects against Staphylococcus aureus, Escherichia coli and Bacillus subtilis (29). In the study by Manzur et al (30), aqueous and ethanolic extracts of MI leaves were found to exert inhibitory effects against Stapylococcus species isolated from cows. In another study, Mushore and Matuvhunye (31) demonstrated that an aqueous extract of stem bark of MI can reduce the growth of S. aureus. A recent study demonstrated that the hexane peel extract prepared from the Sindhura and Banisha mango varieties found in India exerted growth inhibitory effects against Escherichia coli (32). In addition, methanolic and ethyl acetate peel extracts was found to exert inhibitory effects against Bacillus subtilis and Pseudomonas aeruginosa, respectively (32). These findings suggest the potential use of MI extracts and by-products as anti-microbial agents. Further studies are required however, to elucidate anti-microbial mechanisms and to isolate active compounds.

Various extracts and compounds isolated from MI have been reported to exert anticancer effects in vitro and in vivo. Noratto et al (33) evaluated the anticancer effects of polyphenolic extracts prepared from five mango varieties (Haden, Francis, Honey, Kent and Tommy Atkins) in breast (MDA-MB-231), lung (A549), prostate (LNCaP), colon (SW-480) and leukaemia (Molt-4) cancer cell lines. The Ataulfo and Haden polyphenolic extracts exhibited cell growth inhibitory potential in all tested cancer cell lines in a dose-dependent manner. The Ataulfo extract exhibited the most potent inhibitory effects in the MDA-MB-231 cells, while the Haden extract displayed the most prominent inhibitory effects in the Molt-4 cells. Fitriasih et al (34) also demonstrated that the treatment of MCF-7 breast cancer cells with a methanol extract of MI leaves induced apoptosis by decreasing Bcl-2 expression and increasing Bax expression. In another study, when MDA-MB-231 cells were exposed to polyphenols extracted from MI pulp, cell growth was inhibited and the expression of PI3K signalling-associated miRNAs was modulated (35). Furthermore, Abdullah et al (36) reported that MI seed ethanolic extract induced the apoptosis of MCF-7 breast cancer cells.

Notably, major polyphenols, such as mangiferin, norathyriol and quercetin have been reported to exert inhibitory effects against P-glycoprotein, a member of the adenosine triphosphate binding cassette transporter superfamily responsible for multidrug resistance in human cancers (37). The PI3K/AKT/mTOR signalling pathway is a frequently activated signalling pathway in a range of human cancers (38). Banerjee et al (39) demonstrated that when BT474 breast cancer cells were exposed to MI pulp extracts, the expression of PI3K signalling-associated proteins, such as p-PI3K, p-AKT and AKT was reduced. Furthermore, female athymic BALB/c nude mice bearing BT474 tumours, exhibited a reduction in tumour size and PI3K signalling-associated proteins when fed the pulp extract (39). These findings indicate that MI extracts and phytochemicals may have potential for use in the development of drugs for cancer treatment. However, the possible toxic effects of mango extracts and compounds need to be determined and managed before using these as therapeutics for cancer in clinical practice.

Two new halogenated compounds (chloromangiferamide and bromomangiferic acid), quercetin and catechin (41), as well as a new resorcinolic lipid (42) were isolated from the bark of MZ. Of these compounds, chloromangiferamide have been shown to exert selective anticancer effects in MDA-MB-231 triple-negative breast cancer cells with less cytotoxicity to MCF-10A normal mammary epithelial cells. Furthermore, experiments performed using qPCR assays revealed that chloromangiferamide regulated the expression of genes associated with the cell cycle, apoptosis, topoisomerases, drug metabolism, receptor tyrosine kinase signalling, histone deacetylases (HDAC1-4, 6-8 and 11), protein kinases, phosphatases, growth factors and PI3K signalling in MDA-MB-231 cells (41). The isolated new resorcinolic lipid has been shown to exert potent cytotoxic effects through a mechanism related to oxidative stress in MCF-7 oestrogen receptor positive breast cancer cells (42).

Studies conducted with MZ bark extracts have also demonstrated notable scientific findings, which support its traditional use in cancer treatment. The hexane extract of MZ bark was previously reported to exert cytotoxic effects in breast (MCF-7 and MDA-MB-231) and ovarian (SKOV-3) cancer cells through the induction of apoptosis. The gas chromatography-mass spectrometry (GC-MS) analysis of the hexane extract of MZ bark identified certain unknown compounds, which indicated the presence of new phytochemicals in the bark of MZ (40). Furthermore, the chloroform extract of MZ fruit peel induced the apoptosis of MCF-7 breast cancer cells through a mechanism related to oxidative stress, suggesting the potential use of MZ fruit peel as a cost-effective source for anticancer compounds (43). However, in order to elucidate the complete anticancer mechanisms exerted by MZ extracts and isolated compounds, in vivo investigations are necessary to determine the in vivo anticancer efficacy and toxic effects of the MZ extracts.

Preliminary investigations conducted in the authors' laboratories identified that extracts prepared from MZ leaves exerted anticancer effects in breast and lung cancer cells in vitro (unpublished data). In vitro investigations performed using MI and MZ extracts are summarized in Table I, while in vivo investigations performed using various MI extracts are listed in Table II. The chemical structures of the compounds present in MI and MZ are illustrated in Fig. 2. In addition, the pharmacological activities of various compounds isolated from MI and MZ are summarised in Table III. A comparison of the anticancer activities of various MI and MZ extracts is presented in Table IV. Furthermore, the potential efficacy of chloromangiferamide to target HDAC genes in MDA-MB-231 triple-negative breast cancer cells is illustrated by the schematic diagram in Fig. 3.

Nucleic acid material of eukaryotes consists of DNA and histone proteins (83). Histone proteins are found as octamers and are wrapped by 1.65 turns of DNA. These octamers consist of two copies of histone subunits known as H2A, H2B, H3 and H4. The N-terminal tails of these subunits contain higher amounts of lysine resulting in an overall positive charge. The chromatin structure functions as a protective mechanism for genetic information and regulatory mechanism to control the access to enzymes and proteins necessary for DNA replication and gene expression (84).

DNA methylation exclusively takes place in the cytosine bases of CpG dinucleotides in the human genome (85). Short CpG contents (0.5-4 kb), referred to as CpG islands, are commonly found in the human gene promoter regions (86). The epigenetic silencing of tumour suppressor genes (for example CDKN1C, CDKN2A, RUNX3, WT1, FOXA2, DAPK, TMS1, BCL2, HOXD11, GPC3, LAMA3 and LKB1) due to the hypermethylation of CpG islands is frequently observed in human cancers (86). Several oncogenes are upregulated through promoter hypermethylation (6). For example, the promoter of the gene SOSTDC1, which encodes for a bone morphogenetic protein, is frequently upregulated through hypermethylation (85,86). The FLT4 gene, that encodes a tyrosine kinase receptor for vascular endothelial growth factors C and D, is frequently upregulated through promoter hypermethylation. CYBA, a gene that encodes for cytochrome B light chain is also upregulated through DNA hypermethylation (86). APC is a tumour suppressor gene known to regulate the Wnt signalling pathway. In colorectal cancer the hypermethylation of the APC gene promoter induces the downregulation of APC (87). The RASAL2 gene that encodes for Ras-GTPase-activating protein 2 is also upregulated through promoter hypomethylation in distinct human cancers (88). DNA methyl transferase inhibitors are useful drugs in cancer treatments. Deoxycytidine is one of the very first drugs which undergoes a series of phosphorylation steps and incorporates into CpG sites of DNA, leading to the formation of covalent bonds between DNA methyl transferase 1 (DNMT1) catalytic sites (89,90). 5-Aza-2'-deoxycytidine is a Food and Drug Administration (FDA)-approved DNMT inhibitor used in the treatment of myelodysplastic syndromes (91).

Histone acetylation is another major epigenetic modification event, which involves the acetylation of lysine residues at the ε-amino groups of histones (92). This epigenetic modification is controlled by two different groups of enzymes known as histone acetyl transferases (HATs) and histone deacetylases (HDACs). HATs transfer the acetyl group from Acetyl-Co A to amino acid group of the targeted lysine residues found in the histone tails, while HDACs counteract the actions of HATs (92).

Based on the structural homology, HATs are classified into three different families known as Gcn5-related N-acetyltransferases (GNATs), MOZ, Ybf2, Sas2, TIP60 (MYST) and orphan [p300/CREB binding protein (CBP) and nuclear receptors] (93). These enzymes neutralize positive charges of the histone proteins, resulting in weakened interactions between histones and DNA, rendering chromatin less condensed and DNA more accessible to transcription factors (94). Histone acetylation is however targeted to specific regions of DNA through sequences specific co-factors of HATs, including CBP, p300, MYST, and GNAT. The deacetylation of histones leads to condensed chromatin, resulting in the transcriptional silencing of genes (95).

Apart from the acetylation of histones, several other modifications, such as the phosphorylation, methylation and ubiquitination of histone subunits have also been identified. Research performed using normal tissues, tumours and mouse models has reported that the loss of acetylated Lys16 (K16-H4) of histone H4 is a frequent event in human cancers (95). Moreover, a reduction in histone acetylation has been shown to be associated with invasion and metastasis in gastrointestinal tumours (95). Mutations, such as missense mutations, truncations, translocations and frame shift mutations of different families of HATs have been identified in several human cancers, including colorectal, head, neck, breast and lung cancers (96).

Investigations carried out with histone deacetylase inhibitors (HDACis) have proven the role of HDACs in the regulation of p21 protein (97). p21 Protein is a cyclin-dependant kinase inhibitor which inhibits cyclin D1, A and E. The inhibition of p21 leads to cell cycle arrest at the G1 or G2/M phases (98). In prostate cancer cells, exposure to trichostatin A, an HDAC inhibitor, has been found to induce the activation of p-glycoprotein expression through the modulation of the histone markers, H3K4me2, H3K4me3, H3K9Ac, H4Ac and H3Ac, indicating an involvement of an epigenetic mechanism in the regulation of p-glycoprotein expression (99).

HDACis, also known as chromatin-modifying agents, have been identified as a promising class of anticancer agents. HDACis have the ability to re-establish dysregulated acetylation profiles of cancer cells and re-activate the expression of epigenetically silenced tumour suppressor genes, allowing cancer cells to undergo programmed cell death (apoptosis) (6). To date, several pan or isoform specific HDACis (natural and synthetic) have been developed and, these HDACis allow for the identification of the role of various HDACs in tumorigenesis (100). Vorinostat (SAHA) for the treatment of cutaneous T-cell lymphomas (CTCL), panobinostat (LBH-589) for the treatment of multiple myeloma, belinostat (PXD101) for the treatment of peripheral T-cell lymphomas (PTCL) and romidepsin for the treatment of CTCL and PTCL are the only four FDA-approved HDACis in clinical use to date (100). Natural compounds, such as curcumin, epigallocatechin gallate, sulforaphanes, kaempferol, resveratrol and butein are some examples for natural HDACis (7).

A group of short RNA molecules (18-22 nucleotides), known as miRNAs, also contribute to the epigenetic regulation of gene expression (101). miRNAs are transcribed by RNA polymerase II to yield primary-miRNAs (pri-miRNAs), which are then cleaved into 65-70 nucleotide-long hairpin RNA duplexes by DROSHA and PASHA (microprocessors) (101,102). These are then exported to the cytoplasm and further cleaved by a protein complex known as Dicer to generate functional miRNAs. A complex comprised of miRNAs and a protein known as RISC (RNA induced silencing complex) bind to the target mRNA and degrade it, causing gene silencing (103). miRNAs play crucial roles in the development, apoptosis, cell differentiation and proliferation under normal conditions (104).

There is experimental evidence to indicate that a considerable number of miRNAs can play oncogenic or tumour-suppressive roles (105). The upregulation of oncogenic miRNAs, also referred as oncomirs, is frequently observed in a range of human cancers (105). It has been reported that any functional irregularity in the expression and function of tumour suppressor miRNAs can cause tumorigenesis (105). The Let-7 family miRNAs (let-7a, let-7b and let-7g), miR-205, members of the miR-200 family (miR-200a, b and c, miR141 and miR-429), miR-200c, miR-145 and miR-142-3p have been reported to function as tumour suppressor miRNAs, while oncogenic roles of miRNAs, such as miR-21, miR-10a, miR-10b, miR-155, miR-17-92, miR-17-5p, miR-27a, miR-96 and miR-182 have also been identified (98,106,107). It has been found that epigenetic modifications, such as histone deacetylation in promoter sites and DNA methylation in CpG islands cause alternations in mRNA expression. Therefore, epigenetic mechanisms provide promising drug targets since DNA-demethylating agents and HDACis can be used to re-establish mRNA expression implicated due to epigenetic alternations (99). Several miRNAs have also been identified to play a regulatory role in cancer stem cells (CSCs). For example, the Let-7 family miRNAs have been reported to play a role in CSC differentiation (99).

Several mango compounds have been reported to function as epigenetic modulators. Epicatechin gallate has been found to upregulate HDAC5 and HDAC7, while downregulating HDAC 1 and HDAC3. Moreover, the DNMT inhibitory activities of catechin have also been reported (108).

Experiments performed using A/J mice have revealed that epicatechin gallate leads to a significant elevation in the levels of miRNAs related to lung tumours induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. These miRNAs include mmu-miR-2137, mmu-miR-449a, mmu-miR-144, mmu-miR-193, mmu-miR-5030 and mmu-miR-2861. Moreover, the downregulation of mmu-miR-969, mmu-miR-449c, mmu-miR-7a, mmu-miR-205 and mmu-miR-218 has also been observed due to the exposure to epicatechin gallate (109).

A previous study performed using ATDC5 cells revealed that mangiferin upregulated miR-181a expression. The upregulation of miR-181a resulted in protection from lipopolysaccharide-induced cell damage, by suppressing the levels of PTEN and the NF-κB pathway (110). Another study performed using glioma cells demonstrated that mangiferin upregulated miR-15b expression, resulting in the suppression of MMP-9 in U87 cells, indicating the ability of mangiferin to reduce metastasis (111).