Apple fruit

Apple  (Malus sp., Rosaceae) is cultivated worldwide as a fruit tree, and is the most widely grown species in the genus Malus. The apple tree originated in Central Asia, where its wild ancestor, Malus sieversii, is still found today. Apples have been grown for thousands of years in Asia and Europe, and were brought to North America by European colonists.

Apples are popular because of the many ways that they can be consumed and because of their convenience and durability. Apples can be processed into sauce, slices, or juice and are favored for pastries, cakes, tarts, and pies.

The apple pulp has been processed into candies (fruit leathers) and used as a source of pectin. The apple juice can be consumed fresh, either natural or filtered, fermented into alcoholic beverages such as cider or wine, distilled into brandy, or transformed into vinegar.

Is apple a fruit ?

By definition, a fruit is the sweet and fleshy product of a tree or other plant that contains seed and can be eaten as food.

In fact apples are common fruits consumed worldwide!

Apple fruit facts

The apple fruit matures in late summer or autumn and cultivars exist with a wide range of sizes. Commercial growers aim to produce an apple that is 7.0 to 8.3 cm (2.75 to 3.25 in) in diameter, due to market preference. Some consumers, especially those in Japan, prefer a larger apple, while apples below 5.7 cm (2.25 in) are generally used for making juice and have little fresh market value. The skin of ripe apples is generally red, yellow, green, pink, or russetted although many bi- or tri-colored cultivars may be found 1). The skin may also be wholly or partly russeted i.e. rough and brown. The skin is covered in a protective layer of epicuticular wax 2). The exocarp (flesh) is generally pale yellowish-white, though pink or yellow exocarps also occur.

Commercially, apples can be stored for some months in controlled atmosphere chambers to delay ethylene-induced ripening. Apples are commonly stored in chambers with higher concentrations of carbon dioxide and high air filtration. This prevents ethylene concentrations from rising to higher amounts and preventing ripening from occurring too quickly. Ripening continues when the fruit is removed from storage. For home storage, most cultivars of apple can be held for approximately two weeks when kept at the coolest part of the refrigerator (i.e. below 5 °C). Some, including ‘Granny Smith’ and ‘Fuji’, can be stored up to a year without significant degradation.

Non-organic apples may be sprayed with 1-methylcyclopropene blocking the apples’ ethylene receptors, temporarily preventing them from ripening.

Apple nutrition facts

A typical apple serving weighs 242 grams and provides 126 calories with a moderate content of dietary fiber (Table 1). Otherwise, there is generally low content of essential nutrients.

Apples are a rich source of various phytochemicals including flavonoids (e.g., catechins, flavanols, and quercetin) and other phenolic compounds (e.g., epicatechin and procyanidins) found in the skin, core, and pulp of the apple 3).

Phenolic compounds, such as polyphenol oxidase, are the main driving force behind browning in apples. Polyphenol oxidase catalyzes the reaction of phenolic compounds to o-quinones causing the pigment to turn darker and therefore brown.

Ideain (cyanidin 3-O-galactoside) is an anthocyanin, a type of pigment, which is found in some red apple cultivars 4).

Phlorizin is a flavonoid that is found in apple trees, particularly in the leaves, and in only small amounts if at all in other plants, even other species of the Malus genus or related plants such as pear trees 5).

Table 1. Apple nutrition facts

[Source: United States Department of Agriculture Agricultural Research Service 6)]

apple fruit

Apple fruit benefits

Apples are a rich source of dietary phytochemicals such as flavonoids. They also contain high levels of polyphenols and other phytochemicals 7). Polyphenols in apples and their extracts (juices) have been studied in several human studies that have shown promising results related to their beneficial effects 8). For example, consumption of at least one apple a day was reported to reduce the risk of colorectal cancer 9). The study also predicted that the risk of colorectal cancer reduced by approximately 50% upon consumption of more than one apple a day. In many laboratory test tubes and animals studies on the anticancer effects of apple extracts have been evaluated, including those of phytochemical compounds in these extracts 10), 11) and apple juice fractions 12).

In previous studies, it was demonstrated that phloretin isolated from apple peels exhibits significant antihepatic tumor proliferation capacity through in animal study via the inhibition of type 2 glucose transporter (GLUT2) 13). A study further demonstrated that phloretin significantly potentiates paclitaxel-induced DNA laddering effects in a human liver cancer cell model 14). This observation indicated that phytochemical components in apples exhibit a beneficial effect on human health.

For cancer chemoprevention, dietary nutrients should be more readily available. Many studies have demonstrated the chemopreventive effects of dietary polyphenols, especially the most abundant subclasses, including flavonoids (60% of all polyphenols) and phenolic acids (30% of total polyphenols) 15). Flavonoids are divided into various groups based on their molecular structure, several of which are present in significant quantities in apple, including flavanols, flavonols, and anthocyanidins as well as dihydrochalcones and hydroxycinnamic acids 16), 17). The chemical structures of several representative polyphenols present in apple are shown in Figure 1 18).

Figure 1. Chemical structures of some selected typical biocompounds in apple juice

biocompounds in apple juice

[Source 19)]

Antioxidant activity of apple polyphenols

Generation of oxygen radicals causes chronic diseases such as diabetes mellitus 20), retinal degeneration, neurodegenerative disorders, aging, and cancer. Several studies have demonstrated that apple polyphenols, including phloretin, exhibit promising antioxidants effects by playing a role in significant mechanisms responsible for the prevention of illnesses triggered by oxidative stress 21). For example, in a previous study on Wistar rats, diabetes was induced by a single dose of streptozotocin. Rats in the diabetic group received either apple juice (15 mL/kg) or apple peel extract (1 g/kg) for 21 days. At the end of the study, lipid profile parameters were measured in serum samples and lipid peroxidation level, antioxidant enzyme activities, and level of inflammatory markers were evaluated in pancreatic tissue samples. The study concluded that supplementation with apple juice/extract may have protective effects against deleterious complications of diabetes mellitus due to its antioxidant effects 22). In a different study on human participants, after 2 weeks of dietary intervention in 25 healthy individuals, the influence of apple and grape juices consumption on body antioxidant status was investigated. The results indicated that such a dietary consumption increased their plasma total antioxidant capacity and decreased their serum and plasma concentration of malondialdehyde 23).

Anticancer activity of apple polyphenols

In addition to its antioxidant activity, studies 24), 25), 26) have demonstrated that apple polyphenols have significant effects in affecting signaling pathways that control cell survival, growth, and proliferation both in vitro and in vivo. The results have shown that phloretin inhibited proliferation and induced apoptosis in nonsmall cell lung cancer cells (A549, Calu-1, H838, and H520) in a dose-dependent manner; phloretin also suppressed the invasion and migration of these cells. In one study, it was found that phloretin (50–150μM) significantly potentiates paclitaxel (10nM)-induced DNA laddering formation in human hepatoma (Hep G2) cells. The antitumor therapeutic efficacy of phloretin (10 mg/kg body weight) was determined by combined treatment of cells with antitumor drug (paclitaxel, 1 mg/kg body weight) in an SCID mouse model 27). Recently it was also further demonstrated that apple phloretin inhibits human colorectal cancer and liver cancer cells through inhibition of GLUT2 (Figure 2). These results further provide evidence to the hypothesis that glucose deprivation therapy has some important beneficial effects on human cancer therapy 28). To test this hypothesis, another group evaluated the antiproliferative activity of apple juices in vitro in MCF-7 and MDA-MB-231 human breast cancer cells. The study results showed that Pelingo apple juice has promising effects to inhibit breast cancer cell proliferation 29). It was demonstrated that 3-beta-trans-cinnamoyloxy-2alpha-hydroxy-urs-12-en-28-oic acid, which is one of the main components of apple peels, showed potent in vitro and in vivo antitumor activity against mammary tumor in a nude mouse xenograft model at a dose of 50 mg/kg/d without body weight loss and mortality 30).

Figure 2. Apple polyphenol phloretin inhibits growth of cancer cells through inhibition of type 2 glucose transporter

Apple polyphenol phloretin
[Source 31)]

Apple polyphenols inhibit cell migration and invasion

Antimetastasis effects of biocompounds in apple have been studied by our group. Our results have indicated that phloretin is an inhibitor of GLUT2 32) and that targeting GLUT2 significantly inhibited COLO 205 colon cancer cell proliferation, migration, and invasion in vitro and in vivo 33). In this study 34), p53-mediated signals were important. Inhibition of the wild-type p53 by dominant negative p53 will attenuate the phloretin-induced colon cancer migration and its related signals. In colorectal cancers, studies have demonstrated that the activation of nuclear factor-κB (NF-κB) occurs via lipopolysaccharide (LPS) binding to the Toll-like receptor 4 (TLR4). Modification of polysaccharide components in apple altered the LPS/TLR4/NF-κB pathway; consequently, supplementation of apple polysaccharide significantly inhibited the migratory ability in vitro on the LPS/TLR4/NF-κB pathway in colorectal cancer cells (HT-29 and SW620 cells) 35). In a study on liver cancer cells, the effect of apple polyphenol extract on the proliferation and invasion of rat ascites hepatoma cell line (AH109A) was examined in vitro. The apple polyphenol extract suppressed both proliferation and invasion of the hepatoma cell line in a dose-dependent manner up to 200 μg/mL. In an in vivo study, apple polyphenol also reduced the growth and metastasis of solid hepatomas and significantly suppressed the serum lipid peroxide level in rats transplanted with AH109A 36).

Apple polyphenols induced apoptotic cancer cell death

Previous results demonstrated that apple polyphenol phloretin (50–150μM) significantly potentiates paclitaxel (10nM)-induced DNA laddering formation in Hep G2 cells. It was also demonstrated that the caspases 3, 8, and 9 were involved in apoptosis, as evidenced by activity assays 37). Previous studies in this area have also demonstrated that phloretin inhibited leukemia cell growth 38) and induced apoptosis of melanoma cells through deprivation of glucose uptake by inhibition of glucose transmembrane transport 39). Using 18F-fluorodeoxyglucose micropositron emission tomography the effects of phloretin-induced suppression of liver tumor growth were demonstrated to involve regulation of glucose transportation. The 18F-fluorodeoxyglucose uptake in the phloretin-treated Hep G2 tumor-bearing mice was significantly suppressed as compared with the control mice. Effects of phloretin on glioblastoma cancer cells have been investigated via induction of apoptosis and cells’ growth cycle arrest. The identified mechanisms demonstrated increased expression of p27 and decreased expression of cdk2, cdk4, cdk6, cyclin D, and cyclin E. Moreover, the phosphatidylinositol-3-kinase/Akt and the mammalian target of rapamycin (PI3K/Akt/mTOR) signaling cascades were suppressed by phloretin in a dose-dependent manner 40). Phloretin-based combination treatment enhanced the anticancer effects of cisplatin on nonsmall cell lung cancer cell lines by suppressing the expression of Bcl-2, increasing the protein expression of cleaved caspases 3 and 9, and deregulating the expression of matrix metalloproteinase-2 and metalloproteinase-9 on gene and protein levels 41). The results suggest that inhibition of intracellular glucose uptake was the most important mechanism responsible for the cancer cell killing effects. Because many cancer cells rely on aerobic glycolysis for energy production, Xintaropoulou et al 42) targeted this pathway as a potential strategy to inhibit cancer cell growth. In that study, inhibition of five glycolysis pathway molecules (GLUT1, HKII (hexokinase II), PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3), PDHK1 (pyruvate dehydrogenase kinase I), and LDH (lactate dehydrogenase)) using nine inhibitors (phloretin, quercetin, STF31 (Glut1 inhibitor), WZB117 (Glut1 inhibitor), 3PO (3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one, glycolytic inhibitor), 3-bromopyruvate, dichloroacetate, oxamic acid, NHI-1 (lactate dehydrogenase A inhibitor)) was investigated in panels of breast and ovarian cancer cell line models. Their results indicated that growth of breast and ovarian cancer cell lines was more sensitive to the glycolytic pathway, with increased sensitivity to the inhibitors under normoxic conditions 43).

Signaling molecules and disease protection effects of phloretin

As described earlier, apple polyphenols induced anticancer activity mainly through their antioxidant activity. Such results have been confirmed by basic in vitro studies. Moreover, phloretin-induced cell cycle arrest was associated with increased expression of p27 and decreased expression of cdk2, cdk4, cdk6, cyclin D, and cyclin E 44). Inhibition of intracellular signaling pathways as well as the PI3K/Akt/mTOR and ERK/Nrf2 signaling cascades was suppressed by phloretin in a dose-dependent manner 45). In addition, many previous studies have also proposed that phloretin triggered the mitochondrial apoptosis pathway 46), 47) and generated reactive oxygen species (ROS) 48). Most of these studies were accompanied by induction of cell growth arrest and apoptosis through upregulation of proapoptotic molecules such as Bax, Bak, and poly (ADP-ribose (adenosine diphosphate-ribose)) polymerase (cleaved) and downregulation of Bcl-2. The antioxidant agents N-acetyl-l-cysteine and glutathione weakened the effect of phloretin on glioblastoma cells. In conclusion, these results demonstrate that phloretin exerts a potent chemopreventive activity in human glioblastoma cells through the generation of ROS. Such effects may have some potential applications for clinical patients. For example, in acute hepatitis patients, liver damage is induced by several damaging factors, among which viral exposure, alcohol consumption, and drug and immune system issues are most popular 49). In addition to antioxidant effects, phloretin is also able to modulate inflammatory responses. A previous study demonstrated that phloretin suppressed the activation and function of mouse dendritic cells 50). The study results showed that phloretin disturbed the multiple intracellular signaling pathways in dendritic cells induced by the TLR4 agonist LPS, including ROS, mitogen-activated protein kinases (extracellular signal-regulated kinase, c-Jun N-terminal kinase, p38 mitogen-activated protein kinase), and NF-κB, thus reducing the production of inflammatory cytokines and chemokine 51).

References   [ + ]

Sign up to get VIP access to eBooks and valuable health tips for FREE!>>> Subcribe now