Capsicodendrin from <i>Cinnamosma fragrans</i> Exhibits Antiproliferative and Cytotoxic Activity in Human Leukemia Cells: Modulation by Glutathione

Capsicodendrin (CPCD, <b>1</b>), an epimeric mixture of a dimeric drimane-type sesquiterpene, is one of the major compounds present in the three endemic species of Madagascan traditional chemopreventive plants: <i>Cinnamosma</i> species (<i>C. fragrans, C. macrocarpa</i>, and <i>C. madagascariensis</i>). Despite the popular use of <i>Cinnamosma</i> in Madagascan traditional medicine and the reported antiproliferative properties of CPCD, elucidation of its mechanism(s) of action is still to be accomplished. In the present study, CPCD at low micromolar concentrations was cytotoxic and induced apoptosis in human myeloid leukemia cells in a time- and concentration-dependent manner. The activity of CPCD in HL-60 and K562 cells was modulated by glutathione (GSH), since depletion of this intracellular thiol-based antioxidant with buthionine sulfoximine resulted in significantly (<i>p</i> < 0.05) greater potency in antiproliferation assays. GSH depletion also significantly potentiated the cytotoxic activity in CPCD-treated human HL-60 cells. Single-cell gel electrophoresis (Comet) assays revealed that GSH depletion in HL-60 cells enhanced the formation of DNA strand breaks in the presence of CPCD. Although CPCD does not contain an obvious Michael acceptor in its structure, ¹H NMR analyses indicated that cinnamodial (<b>2</b>), a monomer of CPCD, was formed within a few hours when dissolved in DMSO-<i>d</i>₆ and interacts with GSH to form a covalent bond via Michael addition at the C-7 carbon. Together the results strongly suggest that <b>2</b> is responsible for the DNA-damaging, pro-apoptotic, and cytotoxic effects of CPCD and that depletion of GSH enhances overall activity by diminishing covalent interaction between GSH and this 2-alkenal decomposition product of CPCD

Dose-toxicity parameters of sesquiterpenes isolated from CINEX on adult females of <i>Ae</i>. <i>aegypti</i> (LVP strain).

<p>^{<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#t001fn001" target="_blank">*</a>} = significantly different from CDIAL as determined by a F-test (P < 0.05).</p

A natural agonist of mosquito TRPA1 from the medicinal plant <i>Cinnamosma fragrans</i> that is toxic, antifeedant, and repellent to the yellow fever mosquito <i>Aedes aegypti</i> - Fig 3

<p><b>Comparative toxic resistance of a pyrethroid-resistant strain of <i>Ae</i>. <i>aegypti</i> (Puerto Rican strain, PR) to CDIAL (A, B) vs. cypermethrin (C,D).</b> The concentration/dose-toxicity relationships of CDIAL against the LVP strain of <i>Ae</i>. <i>aegypti</i> are from <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.g002" target="_blank">Fig 2</a> (data points are omitted for clarity). A,C) Concentration-toxicity relationships in 1st instar larvae 24 h after adding indicated compound to the rearing water. Efficacy was defined as the percentage of larvae that were dead within 24 h. Values are means ± SEM based on 3–12 independent replicates of 5 larvae per concentration. The EC₅₀ of CDIAL in PR (97.1 μM; 95% CI = 82.4–114.5 μM) was slightly (1.4-fold), but significantly (P < 0.05; F-test), greater than that in LVP (70 nM; 95% CI = 53.8–91.0 μM). The EC₅₀ of cypermethrin in PR (88.0 nM; 95% CI = 46.2–167.7 nM) was dramatically (131-fold) and significantly (P < 0.05; F-test) greater than that in LVP (0.68 nM; 95% CI = 0.34–1.34 nM). B,D) Dose-toxicity relationships in adult females 24 h after applying indicated compound to the thoracic cuticle. Efficacy was defined as the percentage of adults that were incapacitated (dead or flightless) within 24 h. Values are means ± SEM based on 3–8 independent replicates of 10 females per dose. The ED₅₀ of CDIAL in PR (0.45 nmol/mg; 95% CI = 0.34–0.58 nmol/mg) was slightly (1.5-fold), but significantly, (P < 0.05; F-test) greater than that in LVP (0.29 nmol/mg; 95% CI = 0.25–0.34 nmol/mg). The ED₅₀ of cypermethrin in PR (2,685.0 fmol/mg; 95% CI = 1315.0–5484.0 fmol/mg) was dramatically (84-fold) and significantly (P < 0.05; F-test) greater than that in LVP (27.3 fmol/mg; 95% CI = 12.1–61.4 fmol/mg).</p

Repellency of DEET, CINEX, and isolated sesquiterpenes in adult female <i>Ae</i>. <i>aegypti</i> (LVP strain) as determined via a non-choice membrane blood-feeding bioassay.

<p>Cages of 20 mosquitoes were allowed to feed on a blood source (defibrinated rabbit blood) for 1 h; the feeding membrane was treated with DEET (20.8 μg/cm² = 108.75 nmol/cm²), CINEX (20.8 μg/cm²), or a sesquiterpene (67.45 nmol/cm²) dissolved in 100% acetone. Control membranes were treated with 100% acetone. The number of mosquitoes that fed from each cage was determined. The percent reduction in the number of mosquitoes that fed from the treatment cage (relative to the mean feeding percentage of the control cages) was calculated to determine ‘percent mosquitoes repelled’. Values are means ± SEM; N = number of independent replicates of 20 mosquitoes each. Lower-case letters indicate statistical categorization of the means as determined by a one-way ANOVA and Holm-Sidak’s posttest (P < 0.05).</p

Antifeedant activity of CINEX and isolated sesquiterpenes as determined via choice CAFE assays in adult female <i>Ae</i>. <i>aegypti</i> (LVP strain).

<p>Groups of 5 mosquitoes were allowed to feed equally on two capillaries of 10% sucrose with 0.01% trypan blue; the control capillary included 1% acetone (the solvent), and the treatment capillary included 1% acetone and CINEX (0.48 μg/μl) or a sesquiterpene (0.75 or 1.5 mM). In ‘Mock’ experiments, both capillaries included 1% acetone alone. The difference in volume consumed between the capillaries was used to calculate the antifeedant activity (see <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#sec002" target="_blank">Methods</a> for details). Values are means ± SEM; N = number of independent replicates of 5 mosquitoes each. Lower-case letters indicate statistical categorization of the means as determined by a one-way ANOVA and Holm-Sidak’s posttest (P < 0.05).</p

Effects of CDIAL on <i>TRPA1-/- Ae</i>. <i>aegypti</i>.

<p>A) Antifeedant activity of CDIAL (1.5 mM) in adult females of the parental ORL and <i>TRPA1-/-</i> strains of <i>A</i>. <i>aegypti</i>, as determined via the CAFE choice bioassay. Antifeedant activity was calculated as described in <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.g004" target="_blank">Fig 4</a>. Values are means ± SEM; N = number of independent replicates of 5 mosquitoes each. P value indicates significant difference from ORL strain as determined via an unpaired t-test. B) Dose-toxicity relationship of CDIAL in adult females of ORL and <i>TRPA1-/- Ae</i>. <i>aegypti</i>. Efficacy was defined as the percentage of adults that were incapacitated (dead or flightless) within 24 h. Values are means ± SEM, based on 3–6 independent replicates of 10 mosquitoes per dose. The ED₅₀ values and Hill slopes for the two strains were not significantly different from each other (P > 0.05; F-test). ORL: ED₅₀ = 0.33 nmol/mg (95% C.I. = 0.28–0.38 nmol/mg); Hill slope = 4.77 (95% C.I. = 2.70–6.83). <i>TRPA1-/-</i>: ED₅₀ = 0.30 nmol/mg (95% C.I. = 0.24–0.37 nmol/mg); Hill slope = 2.87 (95% C.I. = 1.16–4.58).</p

Structural comparison of representative drimane sesquiterpenes.

<p>CDIAL, CFRAG, and CMOS were the focus of the present investigation. The conjugation of Ald1 with the double-bond at C-7 and C-8 makes CDIAL a strong electrophile (<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.s001" target="_blank">S1 Fig</a>). The presence of Ald2 attached to a quaternary carbon (C-9) bearing a hydroxyl group also contributes to the electrophilic nature of the molecule (<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.s001" target="_blank">S1 Fig</a>).</p

Effects of isolated sesquiterpenes on the electrophysiological activity of <i>Xenopus</i> oocytes expressing <i>Ag</i>TRPA1 as determined by two-electrode voltage clamping.

<p>A) Representative traces of membrane current (I_m) in <i>Ag</i>TRPA1-expressing (TRPA1) or H₂O-injected oocytes. Horizontal bars indicate the addition of 10 μM CDIAL, CFRAG, CMOS, or ruthenium red (RR) to the extracellular bath. The bidirectional arrows show the peak changes in membrane current (ΔI_m). The dashed lines indicate the activation (<i>1</i>) and deactivation (<i>2</i>) rates (ΔI_m/Δt). For H₂O-injected oocytes, the addition of CDIAL (shown), CFRAG (<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.s003" target="_blank">S3 Fig</a>), or CMOS (<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0006265#pntd.0006265.s003" target="_blank">S3 Fig</a>) did not noticeably elicit a change in I_m. B) Summary of ΔI_m elicited by each sesquiterpene (10 μM) in TRPA1-expressing oocytes. Values are means ± SEM; N = number of oocytes measured. Lower-case letters indicate statistical categorization of the means as determined by a one-way ANOVA and Holm-Sidak’s posttest (P < 0.05). C) Summary of ΔI_m/Δt during periods ‘<i>1</i>’ and ‘<i>2</i>’ shown in panel A for CDIAL and CFRAG. Values are means ± SEM; N = number of oocytes measured. The mean ΔI_m/Δt during period ‘<i>1</i>’ for CDIAL was significantly greater than that for CFRAG as determined by an unpaired t-test (P < 0.01). ‘***’ and ‘**’ indicate significant difference (P < 0.001 and P < 0.01, respectively) between activation (‘<i>1</i>’) and deactivation (‘<i>2</i>’) rates for each molecule as determined by a paired t-test.</p

Antiproliferative Trihydroxyalkylcyclohexenones from <i>Pleiogynium timoriense</i>

Investigation of a DCM extract of the bark of <i>Pleiogynium timoriense</i> from the former Merck collection of natural product extracts for antiproliferative activity indicated that it was active with an IC₅₀ value of 1.3 μg/mL against the A2780 ovarian cancer cell line. Bioassay-directed fractionation of this extract yielded the three new bioactive trihydroxyalkylcyclohexenones <b>1</b>–<b>3</b>. Their structures were determined by a combination of spectroscopic and chemical methods. Compounds <b>1</b>–<b>3</b> exhibited submicromolar antiproliferative activity against the A2780 human ovarian cancer cell line, with IC₅₀ values of 0.8, 0.7, and 0.8 μM, respectively