**3. Anticancer activity of capsaicin and its analogs**

There is a strong epidemiological and experimental evidence that the phytochemical diet found in fruits, vegetables, whole grains, spices, and teas provides various inhibitory effects against the initiation, development, progression, and metastasis of cancer [14]. Capsaicin, a bioactive phytochemical abundant in chili peppers, is in between them. Capsaicin is a derivative of homovanillic acid, which has been shown to modify the function of many genes associated with cancer cell life span, growth arrest, angiogenesis, and metastasis [15, 16].

Tumorigenesis is a multistage process, which usually begins over an extended period. Cancer cells develop special properties not acquired by most healthy cells. Multiple genetic alterations and aberrant signaling pathways initiate and advance cancer. Determining the molecular targets involved in the tumor development process will provide opportunities to develop a successful cancer-fighting strategy. Studies assessing the capsaicin effect to inhibit cell proliferation by mechanisms are not fully understood in many types of cancer cells [17]. The capsaicin's suggested anticancer pathways include increased cell cycle arrest and apoptosis.

#### **3.1 Capsaicin and apoptosis**

Apoptosis is a vital mechanism against the growth of cancer, and it is strongly correlated with the loss of apoptotic signals in malignancy. It has been shown that capsaicin induces apoptosis in many different types of cancer cell lines, including pancreas, colon, prostate, liver, esophagus, bladder, skin, leukemia, lung, and endothelial cells, keeping the normal cells unharmed. A recent review noted capsaicin appears to induce apoptosis in more than 40 distinct lines of cancer cells [13, 18].

Two major signaling systems are the intrinsic mitochondrial death pathway and the extrinsic death receptor pathway, which activate executioner/effector caspases and lead to apoptosis. In specific, the mitochondrial pathway is involved in the complete execution of apoptosis; thus, the mitochondrion has been named as apoptotic mechanism's gatekeeper, and the mitochondrial death pathway proteins and pathways had become important targets for new treatments [19]. Many proteins involved in the mitochondrial death pathway have been targeted by capsaicin to induce apoptosis in different cancer cell lines. For instance, capsaicin treatment activated the cluster of differentiation 95 (CD95)-mediated apoptotic intrinsic and extrinsic pathways [20] and suppressed antiapoptotic protein expression, B-cell lymphoma 2, which causes caspase-9 and -3 activation, loss of mitochondrial membrane potential, and subsequent rises in cytochrome c release [21].

Capsaicin's proapoptotic activity has been found to be mediated via transient vanilloid potential receptor (TRPV1) in many types of cancers [21–23]. It is a nonselective cation channel pertaining to the transient receptor potential channel (TRP) family [24]. It does prefer Ca2+ to Na+ . Thus it contributes to changes in the concentration of cytosolic free Ca2+ and it is capsaicin's primary cell target.

#### **3.2 Reactive oxygen species**

Earlier research in pancreatic cells showed that the apoptosis effects of capsaicin were correlated with reactive oxygen species (ROS) production, c-Jun N-terminal Kinase (JNK) activation, mitochondrial depolarization, cytochrome c release in the cytosol, and caspase-3 cascade activation [25]. There is also quite a complex relationship that exists between capsaicin exposure and ROS production. ROS is conventionally considered cytotoxic and mutagenic in normal cells and can induce cell death, apoptosis, and senescence at high levels [26].

Capsaicin has been suggested to induce apoptosis in cancerous cells through the production of higher rates of intracellular ROS. This observation reveals capsaicin as the primary signaling molecule [27, 28]. Capsaicin is capable of activating apoptosis through nonreceptor mechanisms [25].

A number of studies in recent years have shown that oxidative stress causes cellular apoptosis through both mitochondria-dependent and mitochondriaindependent pathways [29].

Mitochondria account for 50% of the total cytoplasmic volume in most cells and they participate more than any other organelle in metabolic functions, particularly those involved in cellular energy production. It also consumes nearly 90% of cellular oxygen and is the main source of ROS produced during breathing, and it is engaged in maintaining the intracellular redox state. Along with their longstanding role in energetics, mitochondria depict its prime focus in mammalian cells for many cell death signals. Interactions at the mitochondrion eventually determine whether a cell survives or dies in reaction to many physiological or therapeutic stimuli of cell death. A number of proteins involved in the mitochondrial death pathway were shown to be targeted by capsaicin in order to initiate apoptosis in various cancer cell lines.

It was reported that a 12-h exposure to high concentrations of capsaicin was necessary to induce higher apoptosis rates in Cellosaurus Colo 16 (COLO 16) cells [26]. More than half experienced apoptosis, which was correlated with progressive irreversible dispersion of mitochondrial transmembrane potential and elevated superoxide levels, illustrates the destruction of mitochondria and subsequent breakdown of mitochondrial electron transfer.

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is a part of the complex I of the mitochondrial electron transport chain. Capsaicin has been shown to specifically inhibit mitochondrial NADPH oxidase activity by competitively binding this enzyme to the ubiquinone/coenzyme Q site. Therefore, if capsaicin blocks the transportation of electrons in mitochondria, irreversible dispersion of mitochondrial transmembrane potential will occur. This will initiate apoptosis and change the mitochondrial permeability to release cytochrome c and eventual activation of proapoptotic pathways.

Capsaicin can block NAPDH oxidase in the plasma membrane by acting as a Q antagonist coenzyme. Capsaicin's vanillyl moiety is structurally similar to coenzyme Q's cyclic part, which could address the fact that vanilloids serve as antagonists to the coenzyme Q. Suppression of plasma membrane NAPDH oxidase was correlated with capsaicin's prooxidant and proapoptotic properties in some transformed cells, and activated T cells [30].

**111**

*Anticancer Effect of Capsaicin and Its Analogues DOI: http://dx.doi.org/10.5772/intechopen.91897*

mammalian cell cycle [31, 32].

**3.3 Capsaicin and cell cycle**

arrest [34].

**3.4 Capsaicin and p53**

of apoptosis [35].

agent in human gastric cancer [37].

Another mechanism proposed by capsaicin's anticancer activity is interaction with AMP-dependent protein kinase (AMPK), which is the cell's primary metabolic gatekeeper, belonging to the family of protein kinase stimulated during enzyme-depleting metabolic states like hypoxemia, thermal shock, oxidative stress, and physical activity. It acts as a significant metabolic transition for maintaining energy homeostasis and shown to be an intrinsic controller of the

There is an increase in intracellular Adenosine diphosphate (ADP)/Adenosine triphosphate (ATP) and/or Adenosine monophosphate (AMP)/ATP ratios [33] during energetic imbalance and it promotes activation of AMPK. Activation of AMPK increases oxidative stress in many human cancer cells and induces apoptosis. This stimulates catabolic pathways and, at the same time, inhibits the rate of anabolic

Capsaicin treatment of human colorectal adenocarcinoma (HT29) cell line is shown to cause AMPK activation and inhibition of acetyl-CoA carboxylase (ACC), a well-known AMPK substrate, which indicates capsaicin inhibits lipid biosynthe-

The cycle of cells is a series of stages that cells undergo to allow them to divide and produce new cells. The cell cycle is divided into phases G0/G1, S, and G2/M stages. The cyclins, cyclin-dependent kinases (CDKs), and the CDK inhibitors are essential parts of the cell cycle. There are DNA checkpoints to assure DNA replication integrity. Such checkpoints and repair pathways render cellular responses to damage to DNA easier [14]. When stimulated, the CDKs provide the cells with a driving force to pass from one stage to the next, but if cyclin and/or CDKs are impaired, cell cycle arrest [17, 33, 34] happens. Thus, any alteration in these pathways raises the cancer risk. It was reported that capsaicin inhibits CDK2, CDK4, and CDK6 inhibiting the proliferation of 5637 bladder carcinoma cells via cycle

The p53 tumor suppressor prevents cell proliferation by inducing cell cycle arrest and apoptosis in response to cell stress such as damage to DNA, hypoxia, and activation of oncogenes. Phosphorylation is critical for p53-dependent transactivation at the Ser-15 residue [17]. P53 promotes apoptosis by a linear pathway involving Bax transactivation, cytosol-to-membrane Bax translocation, mitochondrial cytochrome c release, and caspase-9 activation, followed by caspase-3, -6, and -7 activation. Research studies indicate p53 is a target of capsaicin's anticancer action. Capsaicin was found to induce p53 phosphorylation of residue Ser-15 and enhanced p53 acetylation by sirtuin 1 downregulation, which is responsible for the initiation

Incubated adenocarcinoma gastric cell line (AGS) of human gastric cancer with different capsaicin concentrations in the presence and absence of p53 siRNA showed that capsaicin induces apoptosis via p53 upregulation in AGS cells and that the apoptotic effect of capsaicin is p53-dependent [36]. It also observed that the tendency of capsaicin to induce the expression of proapoptotic proteins, such as Bax, caspase-3, and caspase-8, was almost entirely diminished by hitting down p53. Effects of capsaicin on the same cell type reported that caspase-3 activity increased with capsaicin exposure, indicating that capsaicin may serve as an anti-tumorigenic

sis. The above concepts were implicated in apoptosis caused by capsaicin.

reactions to regain the correct energy charge for adenylates.

*Anticancer Effect of Capsaicin and Its Analogues DOI: http://dx.doi.org/10.5772/intechopen.91897*

Capsicum

(TRP) family [24]. It does prefer Ca2+ to Na+

cell death, apoptosis, and senescence at high levels [26].

tosis through nonreceptor mechanisms [25].

breakdown of mitochondrial electron transfer.

**3.2 Reactive oxygen species**

independent pathways [29].

various cancer cell lines.

tion of proapoptotic pathways.

and activated T cells [30].

Capsaicin's proapoptotic activity has been found to be mediated via transient vanilloid potential receptor (TRPV1) in many types of cancers [21–23]. It is a nonselective cation channel pertaining to the transient receptor potential channel

Earlier research in pancreatic cells showed that the apoptosis effects of capsaicin were correlated with reactive oxygen species (ROS) production, c-Jun N-terminal Kinase (JNK) activation, mitochondrial depolarization, cytochrome c release in the cytosol, and caspase-3 cascade activation [25]. There is also quite a complex relationship that exists between capsaicin exposure and ROS production. ROS is conventionally considered cytotoxic and mutagenic in normal cells and can induce

Capsaicin has been suggested to induce apoptosis in cancerous cells through the production of higher rates of intracellular ROS. This observation reveals capsaicin as the primary signaling molecule [27, 28]. Capsaicin is capable of activating apop-

A number of studies in recent years have shown that oxidative stress causes cellular apoptosis through both mitochondria-dependent and mitochondria-

Mitochondria account for 50% of the total cytoplasmic volume in most cells and they participate more than any other organelle in metabolic functions, particularly those involved in cellular energy production. It also consumes nearly 90% of cellular oxygen and is the main source of ROS produced during breathing, and it is engaged in maintaining the intracellular redox state. Along with their longstanding role in energetics, mitochondria depict its prime focus in mammalian cells for many cell death signals. Interactions at the mitochondrion eventually determine whether a cell survives or dies in reaction to many physiological or therapeutic stimuli of cell death. A number of proteins involved in the mitochondrial death pathway were shown to be targeted by capsaicin in order to initiate apoptosis in

It was reported that a 12-h exposure to high concentrations of capsaicin was necessary to induce higher apoptosis rates in Cellosaurus Colo 16 (COLO 16) cells [26]. More than half experienced apoptosis, which was correlated with progressive irreversible dispersion of mitochondrial transmembrane potential and elevated superoxide levels, illustrates the destruction of mitochondria and subsequent

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is a part of the complex I of the mitochondrial electron transport chain. Capsaicin has been shown to specifically inhibit mitochondrial NADPH oxidase activity by competitively binding this enzyme to the ubiquinone/coenzyme Q site. Therefore, if capsaicin blocks the transportation of electrons in mitochondria, irreversible dispersion of mitochondrial transmembrane potential will occur. This will initiate apoptosis and change the mitochondrial permeability to release cytochrome c and eventual activa-

Capsaicin can block NAPDH oxidase in the plasma membrane by acting as a Q antagonist coenzyme. Capsaicin's vanillyl moiety is structurally similar to coenzyme Q's cyclic part, which could address the fact that vanilloids serve as antagonists to the coenzyme Q. Suppression of plasma membrane NAPDH oxidase was correlated with capsaicin's prooxidant and proapoptotic properties in some transformed cells,

concentration of cytosolic free Ca2+ and it is capsaicin's primary cell target.

. Thus it contributes to changes in the

**110**

Another mechanism proposed by capsaicin's anticancer activity is interaction with AMP-dependent protein kinase (AMPK), which is the cell's primary metabolic gatekeeper, belonging to the family of protein kinase stimulated during enzyme-depleting metabolic states like hypoxemia, thermal shock, oxidative stress, and physical activity. It acts as a significant metabolic transition for maintaining energy homeostasis and shown to be an intrinsic controller of the mammalian cell cycle [31, 32].

There is an increase in intracellular Adenosine diphosphate (ADP)/Adenosine triphosphate (ATP) and/or Adenosine monophosphate (AMP)/ATP ratios [33] during energetic imbalance and it promotes activation of AMPK. Activation of AMPK increases oxidative stress in many human cancer cells and induces apoptosis. This stimulates catabolic pathways and, at the same time, inhibits the rate of anabolic reactions to regain the correct energy charge for adenylates.

Capsaicin treatment of human colorectal adenocarcinoma (HT29) cell line is shown to cause AMPK activation and inhibition of acetyl-CoA carboxylase (ACC), a well-known AMPK substrate, which indicates capsaicin inhibits lipid biosynthesis. The above concepts were implicated in apoptosis caused by capsaicin.

#### **3.3 Capsaicin and cell cycle**

The cycle of cells is a series of stages that cells undergo to allow them to divide and produce new cells. The cell cycle is divided into phases G0/G1, S, and G2/M stages. The cyclins, cyclin-dependent kinases (CDKs), and the CDK inhibitors are essential parts of the cell cycle. There are DNA checkpoints to assure DNA replication integrity. Such checkpoints and repair pathways render cellular responses to damage to DNA easier [14]. When stimulated, the CDKs provide the cells with a driving force to pass from one stage to the next, but if cyclin and/or CDKs are impaired, cell cycle arrest [17, 33, 34] happens. Thus, any alteration in these pathways raises the cancer risk. It was reported that capsaicin inhibits CDK2, CDK4, and CDK6 inhibiting the proliferation of 5637 bladder carcinoma cells via cycle arrest [34].

#### **3.4 Capsaicin and p53**

The p53 tumor suppressor prevents cell proliferation by inducing cell cycle arrest and apoptosis in response to cell stress such as damage to DNA, hypoxia, and activation of oncogenes. Phosphorylation is critical for p53-dependent transactivation at the Ser-15 residue [17]. P53 promotes apoptosis by a linear pathway involving Bax transactivation, cytosol-to-membrane Bax translocation, mitochondrial cytochrome c release, and caspase-9 activation, followed by caspase-3, -6, and -7 activation. Research studies indicate p53 is a target of capsaicin's anticancer action. Capsaicin was found to induce p53 phosphorylation of residue Ser-15 and enhanced p53 acetylation by sirtuin 1 downregulation, which is responsible for the initiation of apoptosis [35].

Incubated adenocarcinoma gastric cell line (AGS) of human gastric cancer with different capsaicin concentrations in the presence and absence of p53 siRNA showed that capsaicin induces apoptosis via p53 upregulation in AGS cells and that the apoptotic effect of capsaicin is p53-dependent [36]. It also observed that the tendency of capsaicin to induce the expression of proapoptotic proteins, such as Bax, caspase-3, and caspase-8, was almost entirely diminished by hitting down p53. Effects of capsaicin on the same cell type reported that caspase-3 activity increased with capsaicin exposure, indicating that capsaicin may serve as an anti-tumorigenic agent in human gastric cancer [37].
