Spirocyclic Lignans from Guaiacum (Zygophyllaceae) Induce Apoptosis in Human Breast Cancer Cell Lines

Abstract

We investigated the composition of extracts derived from Guaiacum spp. (Zygophyllaceae), a group of neotropical tree species with varied uses in Central- and South American traditional medicine. Activity-guided fractionation of Guaiacum heartwood extracts led to the identification of four new spirocyclic lignans, named ramonanins A-D (1-4). The ramonanins exhibit cytotoxic activity against human breast cancer cell lines with an IC50 value of 18 μM and induce cell death via apoptotic mechanisms. The ramonanins are derived from four units of coniferyl alcohol and feature an unusual spirocyclic ring system.

Several plant-derived cancer drugs were discovered by examining the use of plants in traditional medicine,^1,2 for example etoposide and teniposide, which are derived from the lignan podophyllotoxin, isolated from Podophyllum spp. (Berberidaceae). Podophyllum was used by several American and Asian cultures for the treatment of skin cancers and warts.^3,4 Here we report results from our chemical analysis of two neotropical tree species of the genus Guaiacum, which have a long history of use by indigenous societies of Central and South America to treat inflammatory diseases and cancer.^5,6 In addition, Guaiacum extracts and “gum” have been employed extensively as antioxidant food additives.⁷ Correspondingly, the chemistry of Guaiacum species has been subject to numerous studies.^8,9

Results and Discussion

We tested methanol and chloroform extracts from G. officinale and G. sanctum heartwood for cytotoxicity in a series of human cancer cell lines including lung, colon, cervical, and breast cancer cell lines. Alamar blue assays indicated that G. sanctum chloroform extracts affected cell viability of breast cell cancer lines at concentrations as low as 16 μg/mL, whereas lung, colon, and cervical cell lines were affected only at much higher concentrations of 125-250 μg/mL. Methanol extracts were generally less active. Cytotoxicity of the G. sanctum chloroform extracts was further characterized using sulforhodamine B (SRB) assays, which indicated strong antiproliferative activity with an IC50 of 20 μg/mL for human breast cancer cell line MB-MDA 231. Analysis of MB-MDA 231 cells treated with G. sanctum extract at 20 μg/mL by fluorescence microscopy after staining with Hoechst 33342 and propidium iodide revealed 70% apoptotic cell death after 12 h of exposure.

Chromatographic fractionation of G. officinale and G. sanctum heartwood extracts produced several active subfractions, suggesting that not a single compound, but several different compounds were responsible for the observed activity. G. officinale and G. sanctum heartwood extracts showed very similar HPLC-MS chromatograms and activity profiles. As G. officinale wood was much more readily available, G. officinale extracts were used for further study. The most active G. officinale fractions were selected for additional fractionation via HPLC, which led to isolation of two active compounds, named ramonanin A and ramonanin B. Ramonanins exhibit modest cytotoxic activity against human breast cancer cell line MD-MBA 231 with an IC50 value of 18 μM. Because crude Guaiacum extracts appeared to induce apoptosis, we examined MD-MBA 231 cells treated with the more abundant ramonanin A for morphological features consistent with apoptotic cell death.^10,11 Ramonanin A-treated MD-MBA 231 cells showed an increase in segregated and condensed chromatin, condensation of the cytoplasm and nucleus, and loss of membrane symmetry; characteristic features of apoptotic cell death,^12,13 which appeared in a time- and dose- dependent manner. Control cells retained an intact plasma membrane with normal nuclear morphology (Figure S1).

We further investigated whether the ramonanins affect cell cycle progression by using fractional DNA content analysis. MD-MBA-231 cells were treated with ramonanin A or camptothecin, and cell cycle distribution was monitored via flow cytometry using fluorescence-activated cell sorting. We found that ramonanin A caused concentration-dependent decreases in S-phase events from 13% for untreated control cells to less than 4% for cells treated with 30 μg/mL ramonanin A or B (Figure 1). Furthermore, the decrease of S-phase populations was accompanied by an increase in the percentage of cells in sub-G0 phase. Therefore, it appears that the ramonanins strongly disrupt cell cycle progression at the G1/S phase transition. Furthermore, the results indicate that the ramonanins activate apoptotic cell death at a rate similar to that induced by camptothecin.

Figure 1.

Effect of ramonanin A (1) and B (2) on cell cycle distribution compared to camptothecin. Human breast cancer cell line MD-MBA 231 was treated at the indicated concentrations for 24 h and analyzed via fluorescence-activated cell sorting (FACS). (A) Percentage of cells in S phase. (B) Percentage of cells in sub-G0 phase.

For structure elucidation, samples of ramonanin A (1) and B (2) were analyzed using NMR spectroscopy and mass spectrometry. Analysis of dqfCOSY, HSQC, and HMBC spectra revealed the presence of four 4-hydroxy-3-methoxyphenyl moieties in both ramonanin A and B, which along with a molecular formula determined as C₄₀H₄₀O₁₀ suggested tetrameric structures derived from four phenylpropanoid units (Table 1 and Table S1). Analysis of the HMBC spectra confirmed this hypothesis and revealed a furano-2-oxaspiro[4,5]decane core bearing the 4-hydroxy-3-methoxyphenyl substituents in positions 1 and 3 of the spirocycle and 1′ and 3′ of the annelated furan. Important HMBC-correlations are shown in Figure 2A. Placement of the four methines in positions 1, 3, 8, and 10 follows from HMBC crosspeaks of the methine protons with carbons of the four aromatic rings. HMBC correlations of the methylene protons and of H-8 and H-10 determined placement of the two double bond carbons forming the 7-11 double bond. Lastly, HMBC correlations of the methine protons H-1 and H-3 in conjunction with HMBC signals of the 4-CH2, C-6, and C-13 methylenes enabled assembly of the spirocyclic portion of the molecule.

Table 1.

NMR Spectroscopic Data (600 MHz, CDCl₃) for Ramonanin A (1).

position	δ C	proton	δ H	JHH,, Hza	ROESYb,c	HMBCc,d
1	87.8	1-H	4.58		3-H, 6-Ha, 6-Hb, 2‴-H, 6‴-H	4, 5, 6, 13, 1‴, 6‴, 2‴
3	82.7	3-H	5.38	J3-H,4-CHaHb = J3-H,4- CHaHb = 2.3	1-H, 2′‴-H, 6′‴-H, 4-CHaHb	4, 4-CH2, 5 (wk), 1′‴, 2′‴, 6′‴
4	157.6
4-CH2	106.9	4-CHaHb	5.13		6-Hb, 8-H, 10-H, 12-Ha, 4-CHaHb	3, 4, 5, 13 (wk), 1′‴ (wk)
		4-CHaHb	4.92		3-H, 2′‴-H, 6′‴-H, 4-CHaHb	3, 4, 5, 13 (wk), 1′‴ (wk)
5	47.6
6	30.1	6-Ha	2.12	J6-Ha,6-Hb = 16.8, J6-Ha,12-Ha ≈ 3, J6-Ha,12-Hb ≈ 1.5	1-H, 13-Hb, 2‴-H, 6‴-H	1, 4, 5, 7, 11
		6-Hb	2.00	J6-Hb,12-Ha ≈ 2.5, J6-Hb,12-Hb < 1 J6-Hb,13-Ha ≈1	1-H, 8-H, 12-Hb, 4-CHaHb	4, 5, 7, 8, 11, 13
7	135.9
8	88.9	8-H	5.58	J8-H,10-H = 3, J8-H,6-Ha ≈ 1.5, J8-H,6-Hb ≈ 1	6-Hb, 4-CHaHb, 2″-H, 6″-H	7, 11, 1″, 2″, 6″
10	88.5	10-H	5.53	J10-H,6-Ha ≈ 2, J10-H,6-Hb ≈ 3	12-Ha, 12-Hb, 4-CHaHb, 2′-H, 6′-Hb	7, 11, 1′, 2′, 6′
11	132.8
12	18.5	12-Ha	1.96	J12-Ha,12-Hb = 17.3, J12-Ha,13-Ha = 5.6, J12-Ha,13-Hb = 10.9	10-H, 4-CHaHb, 12-Hb, 13-Ha, 2′‴-H, 6′‴-H
		12-Hb	1.78	J12-Hb,13-Ha ≈ 2.5, J12-Hb,13-Hb = 5.5	10-H (wk), 12-Ha, 13-Ha (wk), 13-Hb, 2′-H, 6′-H	5, 7, 10 (wk), 11, 13
13	26.1	13-Ha	1.62	J13-Ha,13-Hb = 13.7	4-CHaHb, 12-Ha, 12-Hb, 13-Hb, 2‴-H, 6‴-H, 2′‴-H, 6′‴-H	1, 4, 5, 6, 11, 12
		13-Hb	0.91		1-H (wk), 6-Ha, 12-Hb, 13-Ha, 2′-H, 6′-H, 2‴-H, 6‴-H	1, 4, 5, 6, 11, 12
1′	132.07
2′	109.92	2′-H	6.73	J2′-H,6′-H = 2.0	10-H, OCH3′, 12-Hb, 13-Hb	1′, 3′, 4′, 6′, 10
3′ 146.49
4′ 145.42
5′	113.91	5′-H	6.81	J5′-H,6′-H = 8.0		2′, 3′, 4′, 6′
6′	119.77	6′-H	6.67		10-H, 12-Hb, 13-Hb	2′, 3′(wk), 4′, 5′, 10
1″	132.07
2″	110.28	2″-H	6.79	J2″-H,6″-H = 2.0	6-Ha, 6-Hb, 8-H, 13-Hb (wk), OCH3″	1″, 3″, 4″, 6″, 8
3″	146.49
4″	145.43
5″	114.43	5″-H	6.88	J5″-H,6″-H = 8.0		1″, 3″, 6″(wk), 8 (wk)
6″	120.35	6″-H	6.81		6-Ha, 6-Hb, 8-H	1″, 3″, 4″, 6″, 8
1‴	128.32
2‴	109.69	2‴-H	6.77	J2‴-H,6‴-H = 2.0	1-H, 6-Ha, 13-Ha, 13-Hb, OCH3‴	1‴, 3‴, 4‴, 6‴, 1
3‴	146.00
4‴	145.12
5‴	113.59	5‴-H	6.82	J5‴-H,6‴-H = 8.0		1‴, 2‴, 3‴, 1 (wk)
6‴	120.12	6‴-H	6.75		1-H, 6-Ha, 13-Ha, 13-Hb	2‴, 4‴, 5‴(wk), 1
1′‴	133.00
2′‴	110.04	2‴-H	6.96 4	J2′‴-H,6′‴-H = 2.0	3-H, 4-CHaHb, OCH3′‴, 12-Ha, 13-Ha	1′‴, 3′‴, 4′‴, 6′‴, 5′‴(wk), 3
3′‴	146.29
4′‴	145.20
5′‴	114.14	5‴-H	6.90	J5′‴-H,6′‴-H = 8.0		1′‴, 2′‴, 3′‴, 4′‴, 3 (wk)
6′‴	120.12	6‴-H	6.94		3-H, 4-CHaHb, 12-Ha, 13-Ha	2′‴, 4′‴, 3′‴(wk), 3
OCH3′	55.66		3.73		2′	3′
OCH3″	55.69		3.75		2″	3″
OCH3‴	55.66		3.76		2‴	3‴
OCH3′‴	55.72		3.88		2′‴	3′‴

^a(¹H,¹H)-Coupling constants were estimated from the 1D ¹H NMR or the dqfCOSY spectrum.

^bROESY correlations were observed using a mixing time of 300 ms.

^cAbbreviation: wk = weak, but clearly discernable HMBC or ROESY correlation.

^dHMBC correlations (optimized for 6 Hz) are from the proton(s) stated to the indicated carbon.

Figure 2.

(A) Important HMBC correlations used for structural assignments. Blue arrows signify correlations used for placing the aromatic substituents, red arrows represent correlations determining placement of C-7 and C-11, and black arrows indicated correlations establishing connectivity around the C-5 spirocenter. (B) Relative configuration in ramonanin A (1) and B (2).

Analysis of ROESY spectra established the relative configuration of ramonanin A (1) and B (2), which differ with regard to the relative configurations at C-1 and C-3 (Figure 2B). ROESY crosspeaks between 4-CH2 and the H-8 and H-10 methines established the configuration at the spirocyclic center relative to that at C-8 and C-10 and cis-substitution of the dihydrofuran ring for both ramonanins A and B. Further, cis-substitution of the tetrahydrofuran ring follows from ROESY crosspeaks between H-1 and H-3 observed for both compounds. The two compounds are distinguished in that ramonanin A shows ROESY crosspeaks between H-1 and the C-6 protons, whereas ramonanin B shows crosspeaks between H-1 and the C-13 protons, establishing the relative configurations as shown (Figure 2B).

Subsequent analysis of G. officinale HPLC fractions adjacent to those containing ramonanin A and B revealed smaller amounts of two additional ramonanin stereoisomers, ramonanin C (3) and D (4). Analysis of the ROESY spectra of ramonanin C and D indicated that they feature the same relative configuration as ramonanin A at C-1, C-3, and C-5, but differ with regard to the orientation of the substituents at C-8 and C-10. As opposed to ramonanin A, ramonanin C does not show ROESY crosspeaks between 4-CH₂ and H-8 or H-10, and instead shows crosspeaks between 4-CH₂ and protons of the two phenyl substituents at C-8 and C-10, indicating that 4-CH₂ and these two aromatic substituents are oriented cis with respect to the furanobenzene bicycle (see Tables S2 and S3). Similarly, the presence of ROESY correlations between H-8 and 4-CH₂ as well as between the aromatic substituent at C-10 and 4-CH₂ established the relative configuration of ramonanin D (Scheme 1).

Scheme 1.

Structures of ramonanin C (3), ramonanin D (4), coniferyl alcohol (5), and nectandrin B (6), and possible biogenesis of the ramonanins (1-4) from putative diene precursor 7.

The furano-2-oxaspiro[4,5]decane core of the ramonanins is unprecedented among known natural products; nonetheless, these compounds appear to be derived from fairly straight-forward oligomerization of four units of coniferyl alcohol-like precursors. Intriguingly, the ramonanins could be considered as dimers of two identical 1,3-di-(4-hydroxy-3-methoxyphenyl) substituted furanes such as 7 (Scheme 1). Dimerization of 7, perhaps via a Diels-Alder-like mechanism, would directly lead to ramonanins A-D. Although compound 7 was not found in the current study and has not been identified previously from any other source, partially reduced derivatives such as nectandrin B (6) have been identified from many plant species,⁸ including Guaiacum spp,⁹ and were also found to be present in our G. officinale heartwood extracts. In conclusion, we have identified a new group of lignans that induce cell death via apoptotic mechanisms. The established use of Guaiacum extracts in traditional medicine and as a food additive may encourage more detailed study of these compounds’ mode of action.

Experimental Section

General Experimental Procedures

NMR spectra were recorded on Varian INOVA 500 MHz (500 MHz for ¹H and 126 MHz for ¹³C) and Varian INOVA 600 NMR (600 MHz for ¹H, 151 MHz for ¹³C) using DBG (¹³C-direct detection) and HCN indirect detection probes. Non-gradient phase-cycled dqfCOSY spectra were acquired using the following parameters: 0.6 s acquisition time, 500-600 complex increments, 8-32 scans per increment. Gradient and non-gradient HSQC, HMQC, and HMBC spectra were acquired with these parameters: 0.25 s acquisition time, 300-600 increments, 4-32 scans per increment. HMBC spectra were optimized for J_H,C = 6 Hz. Susceptibility-matched NMR tubes (Shigemi) were used for sample amounts smaller than 2 mg. NMR spectra were processed using Varian VNMR and MestreLabs MestReC and MNOVA software packages. HRMS was performed on a LTQ Orbitrap Velos (Thermo Scientific). HPLC–MS was performed using an Agilent 1100 Series HPLC system equipped with a diode array detector and connected to a Quattro II spectrometer (Micromass/Waters) operated in positive-ion electrospray ionization mode. Data acquisition and processing for HPLC-MS was controlled Waters MassLynx software. Optical rotations were measured using a Perkin-Elmer 241 polarimeter and a 1-mL cuvette.

Solvents and Sephadex LH-20 were obtained from Sigma-Aldrich and Acros. Silica gel (KG60, 40-63 μm) and pre-coated TLC plates (silica gel KG60-F254) were obtained from Merck (Darmstadt, Germany) and Teledyne ISCO. For thin layer chromatography, compounds were visualized using UV light and/or vanillin/H2SO4 reagent. Flash chromatography was performed using a Teledyne ISCO CombiFlash system. For semi-preparative HPLC, a Dionex LC system with a P580 pump, ASI-100 autosampler, UVD 170U detector and Gibson 206 fraction collector was equipped with a preparative reverse phase Phenomenex Aqua column 5 μ C18 125 (250 × 10.00 mm).

Plant Materials

G. sanctum and G. officinale L. wood samples were collected in Punta Cana, Dominican Republic. Collected branches approximately 10 mm to 20 mm in diameter were stripped of their bark and stored. The wood was cut into small pieces, air dried, and ground. Voucher specimens were deposited at the L. H. Bailey Hortorium Herbarium (BH) at Cornell University, Ithaca, NY, herbarium@cornell.edu (accession number BH000035546, authentication by Francisco Guanchez, 2000).

Extraction and Isolation

Dried and powdered G. officinale heartwood (500 g, origin Dominican Republic) was extracted exhaustively with CHCl₃ for three days in a Soxhlet apparatus followed by extraction with MeOH. The CHCl₃extract yielded 28.6 g of a yellow oil, which was fractionated further over Sephadex LH-20, using a mixture of CH₂Cl₂ and acetone (85:15 v/v) as eluent. Nineteen fractions were collected and tested for cytotoxicity and induction of apoptosis (FACS, see below). Activity (Alamar blue assay) was found to be highest in fraction F13, showing 75% apoptosis. Fraction F13 pooled with fraction F14 yielded 0.476 g of material, which was fractionated again over Sephadex LH-20, now using 100% acetone. Most active fractions (Alamar blue assay) were combined and purified further via semi-preparative HPLC yielding 0.27 g of a yellow oil (HPLC conditions: isocratic 70% MeOH in H₂O, run time of 35 min). Ramonanin A and B eluted at 22.5 and 23.6 min, respectively, and were obtained in amounts of 6.3 mg and 2.3 mg. Ramonanin C and D eluted at 24.2 min and 24.8 min, respectively, and were obtained in amounts of 1.5 mg and 0.8 mg. Additional samples of ramonanin A, C, and D were obtained from processing of additional G. officinale wood samples. HPLC-MS analyses of CHCl₃ wood extracts indicated the presence of ramonanins A and C in all G. sanctum and G. officinale wood samples analyzed. Because of their lower abundance, ramonanins B and D could not always be detected in unfractionated wood extracts.

• Ramonanin A (1): colorless wax; [α]d −2.2 (c 0.6 MeOH, 22 °C); UV (MeOH) λ_max (log ε) 230 (4.1), 279 nm (3.6); ¹H NMR, ¹³C NMR, ROESY, and HMBC data, see Table 1; HR-ESI⁺MS obsd m/z 703.2533 [M + Na]⁺, calcd for C₄₀H₄₀NaO₁₀, 703.2519.

• Ramonanin B (2): colorless oil; [α]D −4.0 (c 0.2, MeOH, 22 °C); UV (MeOH) λ_max (log ε) 230 (4.1), 279 nm (3.6); ¹H NMR, ¹³C NMR, ROESY, and HMBC data, see Supporting Table 1; HR-ESI⁺MS obsd m/z 703.2541 [M + Na]⁺, calcd for C₄₀H₄₀NaO₁₀, 703.2519.

• Ramonanin C (3): colorless oil; [α]D −5.2 (c 0.2, MeOH, 22 °C); UV (MeOH) λ_max (log ε) 230 (4.1), 279 nm (3.6); ¹H NMR, ¹³C NMR, ROESY, and HMBC data, see Supporting Table 2; HR-ESI⁺MS obsd m/z 703.2544 [M + Na]⁺, calcd for C₄₀H₄₀NaO₁₀, 703.2519.

• Ramonanin D (4): colorless wax; [α]d −5.1 (c 0.3, MeOH, 22 °C); UV (MeOH) λ_max (log ε) 230 (4.1), 279 nm (3.6); ¹H NMR, ¹³C NMR, ROESY, and HMBC data, see Supporting Table 3; HR-ESI⁺MS obsd m/z 703.2529 [M + Na]⁺, calcd for C₄₀H₄₀NaO₁₀, 703.2519.

Cell Line Culture Conditions and Reagents

The following human cancer cell lines were obtained from ATCC (Manassas, VA): CaLu-6 (lung), HCT-116 and HT-29 (colon), HeLa (cervical), SKBR-3 (breast), MD-MBA-468 (breast) and MD-MBA 231 (breast). Cells were maintained in RPMI 1640 medium (PAA Laboratories GmbH, Teddington Middlesex, UK) supplemented with 10% heat-inactivated tetracycline free fetal calf serum (PAA Laboratories) and 2 mM L-glutamine, 100 units/mL penicillin, and 100 μg/mL streptomycin (Gibco, Grand Island, NY). Cells were propagated and cultured at 37°C in a 100% relative-humidity atmosphere consisting of 5% CO₂ and 95% air. Cells were trypsinized with 0.05% Trypsin in aqueous 0.2% EDTA (w/v) solution.

Cytotoxicity Assays

For Alamar blue cell viability assays, cells were seeded onto a standard 96-well microtiter plate at a density of 2×10⁴ cells/well in 100-μL tissue culture medium. After 24 h at densities of 5,000 cells per well, cells were treated with Guaiacum extracts and fractions. All samples were added as solutions in DMSO (10 μg extract/1 μL DMSO). DMSO (0.1%) was used a negative control, and camptothecin (CPT) and 5-fluorouracil (5-FU) were used as positive controls. Following a 48 h incubation period, cells were evaluated immediately by visualizing color densities and spectrophotometrically at 540 nm.

Sulforhodamine (SRB) blue assay.¹⁴ MB-MDA 231 cells were seeded as described above. After a 24 h incubation period, Guaiacum extracts and fractions were added as solutions in DMSO to achieve final concentrations of 200, 20, 2, 0.2 μg extract/1 mL of media. Cultures were treated with trichloracetic acid (10%), washed with cold tap water, stained with SRB (100 μL, 0.4% (w/v) in 1 % HOAc), and incubated at room temperature for 10 min. Unbound dye was removed by five washes with 1% HOAc. After drying, dye-stained protein was extracted with 10 mM TRIS base. Absorbance of stained protein was measured using an automated 96-well microtiter ELISA plate reader (SAFIRE) at 540 nm.

Apoptosis and Cell Cycle Analysis

The morphology of cell cultures were analyzed for apoptosis using a double-fluorescence staining technique with a Hoechst 33342/propidium iodide (H/PI) assay.¹⁵ MD-MBA 231 was seeded at low density (1.0 × 10 4 cells/mL) in 6-well plates. After 24 h growth, camptothecin (CPT, as solution in DMSO), the ramonanins (as solution in DMSO), or DMSO (as control) were added. Samples were collected after 6, 12, and 24 h, at which both the supernatant and adhered cells were stained in 20 μg/mL propidium iodide (emitting red fluorescence) and 100 μg/mL Hoechst 33342 (emitting blue fluorescence) for 15 min at 37 °C in the dark. Double fluorescence was detected with a Zeiss fluorescence microscope (Carl Zeiss, Inc., Thornwood, NY) using an epifluorescence system with a long pass filter cube A.

For detecting apoptosis, cells were seeded in 24-well microtiter plates at a density of 1.5×10⁴ cells per well in 3 mL tissue culture medium. After 24 h, cells were treated with Guaiacum fractions, isolated ramonanins, or camptothecin. Test compounds were dissolved in DMSO (10-100 μg sample/1 mL DMSO). Following a second 24 h incubation period, cell cycle distribution was analyzed using a fluorescence-activated cell sorter (FACS Caliber Becton Dickinson, San Jose, CA), and double staining with Annexin V/propidium iodide.¹⁰ For DNA content analysis, 24 h after exposure to ramonanin A, 1.5 mL cells were re-suspended. Cells were centrifuged at 1000 g at room temperature, washed in 0.5 mL of hypotonic staining solution, and analyzed by flow cytometry on the same day. Resulting DNA distributions were analyzed for the fraction of cells in apoptosis in G0/G1, and S, phases of the cell cycle. Singlet events were gated and 10,000–20,000 events were acquired within the gated region. All experiments were repeated at least three times, each in duplicate.