The monoterpenoid, citral, when delivered through PEG-b-PCL nanoparticles inhibits in vivo growth of 4T1 breast tumors. Here, we show that citral inhibits proliferation of multiple human cancer cell lines. In p53 expressing ECC-1 and OVCAR-3 but not in p53-deficient SKOV-3 cells, citral induces G1/S cell cycle arrest and apoptosis as determined by Annexin V staining and increased cleaved caspase3 and Bax and decreased Bcl-2. In SKOV-3 cells, citral induces the ER stress markers CHOP, GADD45, EDEM, ATF4, Hsp90, ATG5, and phospho-eIF2α. The molecular chaperone 4-phenylbutyric acid attenuates citral activity in SKOV-3 but not in ECC-1 and OVCAR-3 cells. In p53-expressing cells, citral increases phosphorylation of serine-15 of p53. Activation of p53 increases Bax, PUMA, and NOXA expression. Inhibition of p53 by pifithrin-α, attenuates citral-mediated apoptosis. Citral increases intracellular oxygen radicals and this leads to activation of p53. Inhibition of glutathione synthesis by L-buthionine sulfoxamine increases potency of citral. Pretreatment with N-acetylcysteine decreases phosphorylation of p53 in citral-treated ECC-1 and OVCAR-3. These results define a p53-dependent, and in the absence of p53, ER stress-dependent mode of action of citral. This study indicates that citral in PEG-b-PCL nanoparticle formulation should be considered for treatment of breast and other tumors.
Plumbagin, an anti-cancer agent, is toxic to cells of multiple species. We investigated if plumbagin targets conserved biochemical processes. Plumbagin induced DNA damage and apoptosis in cells of diverse mutational background with comparable potency. A 3–5 fold increase in intracellular oxygen radicals occurred in response to plumbagin. Neutralization of the reactive oxygen species by N-acetylcysteine blocked apoptosis, indicating a central role for oxidative stress in plumbagin-mediated cell death. Plumbagin docks in the ubiquinone binding sites (Q0 and Qi) of mitochondrial complexes I–III, the major sites for oxygen radicals. Plumbagin decreased oxygen consumption rate, ATP production and optical redox ratio (NAD(P)H/FAD) indicating interference with electron transport downstream of mitochondrial Complex II. Oxidative stress induced by plumbagin triggered an anti-oxidative response via activation of Nrf2. Plumbagin and the Nrf2 inhibitor, brusatol, synergized to inhibit cell proliferation. These data indicate that while inhibition of electron transport is the conserved mechanism responsible for plumbagin’s chemotoxicity, activation of Nrf2 is the resulting anti-oxidative response that allows plumbagin to serve as a chemopreventive agent. This study provides the basis for designing potent and selective plumbagin analogs that can be coupled with suitable Nrf2 inhibitors for chemotherapy or administered as single agents to induce Nrf2-mediated chemoprevention.
The terpene-enriched steam distilled extract of ginger rhizomes has potent anti-tumor activity. Screening of the components of ginger extract demonstrated citral, a commonly used food flavoring agent, as the major monoterpene that inhibits proliferation of human and murine cancer cell lines A2780, ECC-1, OVCAR-3, SKOV-3, ID8 and MOVCAR. Citral initiates a rapid decrease in intracellular levels of glutathione and a corresponding increase in oxygen radicals. In wild-type p53-expressing ECC-1 cells, citral treatment increases phosphorylation of the Ser-15 residue of p53 and causes apoptosis. Inhibition of intracellular oxygen radicals by N-acetylcysteine and p53 activity by pifithrin-α inhibits citral-induced apoptosis in ECC-1 cells. Apoptosis in mutant p53 (Arg248Glu) expressing OVCAR-3 cells was also inhibited by N-acetylcysteine and was associated with a decrease in phosphorylation of Ser-15 of p53. Finally, in cells not expressing p53, citral inhibited proliferation, but not through apoptosis. Instead in the p53-negative SKOV-3 cells, citral treatment caused endoplasmic reticulum stress indicated by increase in phosphorylation of eIF2α and the expression of CHOP, GADD45, EDEM, ATF4 and Hsp90 and increased ATG5 and LC3-II suggesting the cells were undergoing autophagy. The study identifies the mechanism that allows citral to produce its cytotoxic effects. Combined with our recent results that nanoparticle-encapsulated citral decreases in vivo growth of 4T1 murine breast tumors, the results from the current study suggest that citral and its analogs be considered as chemotherapeutic agents. Since citral mediates its cytotoxic effects primarily by inducing oxidative stress, it is likely that this monoterpenes and its derivatives can be used for the treatment of a variety of solid tumors including ovarian cancer. Citation Format: Arvinder Kapur, Mildred Felder, Lucas Fass, Justanjyot kaur, Austin Czarnecki, Kavya Rathi, Manish Patankar. The monoterpene, citral, increases intracellular oxygen radicals and inhibits cancer cell proliferation by inducing apoptosis and endoplasmic reticulum stress . [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr B73.
Recently, we demonstrated that the monoterpene, citral, is potent inhibitor of cell proliferation. Here, we are demonstrating that the α,β-unsaturated carbonyl functionality of citral is responsible for its biological activity causing substantial increase in intracellular oxygen radicals. The resulting oxidative stress activates p53. Neutralization of oxygen radicals by N-acetylcysteine attenuates the cytotoxic effect of citral and inhibits activation of p53. Inhibition of p53 by pifithrin-α inhibits citral-induced apoptosis. Oxidative stress caused by citral initiates downstream events causing apoptosis of cancer cells. With the identification of α,β-unsaturated carbonyl group as the obligatory functionality for the anti-cancer effects of citral, we surveyed other small molecule drugs that shared this chemical entity. Cinnamaldehyde, nonenal, perillaldehyde, curcumin, and plumbagin have the α,β-unsaturated carbonyl functional groups and have demonstrated anti-cancer effects. Using curcumin and plumbagin as prototypes, we were able to demonstrate similarities in the cytotoxic mechanism with citral. The oxidative stress caused by curcumin and plumbagin is the initiating step required for cancer cell apoptosis. Fluorescent Lifetime Imaging Microscopy (FLIM) showed that exposure to plumbagin results in an increase in free NADH levels and a decrease in the NADH/FAD ratio. Additionally, in silico modeling showed that plumbagin and curcumin docked to the active site of complex I and III of the mitochondrial electron transport chain. Using the Seahorse analyzer, our experiments showed that plumbagin and curcumin inhibited the oxygen consumption rate in treated cells. These results indicate that the α,β-unsaturated carbonyl containing compounds produce their cytotoxic effects by interfering with the electron transport chain. The cancer cells respond to the surge in oxygen radicals by increasing the expression of Nrf-2, the transcription factor that regulates major anti-oxidant responses. This compensatory mechanism is a form of chemoresistance that allows cancer cells to evade the cytotoxic effects of agents such as citral. We will present data from strategies to inhibit the Nrf-2-mediated compensation and thereby enhance the cytotoxic activity of citral, curcumin and plumbagin. The data obtained through these experiments is providing important information that will allow designing of novel agents that can interfere with the electron transport chain and inhibit tumor growth by inducing oxidative stress. Note: This abstract was not presented at the meeting. Citation Format: Arvinder Kapur, Amruta P. Nayak, Thomas Beres, Kavya Rathi, Mildred Felder, Amani Gillette, Spencer Ericksen, Melissa Skala, Lisa Barroilhet, Manish S. Patankar. Targeting tumors by inhibiting mitochondrial electron transport [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5460. doi:10.1158/1538-7445.AM2017-5460
The monoterpenoid, citral, when delivered through PEG-b-PCL nanoparticles inhibits in vivo growth of 4T1 breast tumors. Here, we show that citral inhibits proliferation of multiple human cancer cell lines. In p53 expressing ECC-1 and OVCAR-3 but not in p53-deficient SKOV-3 cells, citral induces G1/S cell cycle arrest and apoptosis as determined by Annexin V staining and increased cleaved caspase3 and Bax and decreased Bcl-2. In SKOV-3 cells, citral induces the ER stress markers CHOP, GADD45, EDEM, ATF4, Hsp90, ATG5, and phospho-eIF2α. The molecular chaperone 4-phenylbutyric acid attenuates citral activity in SKOV-3 but not in ECC-1 and OVCAR-3 cells. In p53-expressing cells, citral increases phosphorylation of serine-15 of p53. Activation of p53 increases Bax, PUMA, and NOXA expression. Inhibition of p53 by pifithrin-α, attenuates citral-mediated apoptosis. Citral increases intracellular oxygen radicals and this leads to activation of p53. Inhibition of gluthatione synthesis by L-buthionine sulfoxamine increased potency of citral. Pretreatment with N-acetylcysteine decreases phosphorylation of p53 in citral-treated ECC-1 and OVCAR-3. These results define a p53-dependent, and in the absence of p53, ER stress-dependent mode of action of citral. This study indicates that citral in PEG-b-PCL nanoparticle formulation should be considered for treatment of breast and other tumors. Citation Format: Arvinder Kapur, Mildred Felder, Lucas Fass, Justanjot Kaur, Austin Czarnecki, Kavya Rathi, San Zeng, Kathryn Kalady Osowski, Colin Howell, May P. Xiong, Rebecca J. Whelan, Manish S. Patankar. MODULATION OF OXIDATIVE STRESS AND SUBSEQUENT INDUCTION OF APOPTOSIS AND ENDOPLASMIC RETICULUM STRESS ALLOWS CITRAL TO DECREASE CANCER CELL PROLIFERATION [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr AP15.
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