Cytotoxic Nitrogenous Terpenoids from Two South China Sea Nudibranchs Phyllidiella pustulosa, Phyllidia coelestis, and Their Sponge-Prey Acanthella cavernosa

A detailed chemical investigation of two South China Sea nudibranchs Phyllidiella pustulosa and Phyllidia coelestis, as well as their possible sponge-prey Acanthella cavernosa, led to the isolation of one new nitrogenous cadinane-type sesquiterpenoid xidaoisocyanate A (1), one new naturally occurring nitrogen-containing kalihinane-type diterpenoid bisformamidokalihinol A (16), along with 17 known nitrogenous terpenoids (2–15, 17–19). The structures of all the isolates were elucidated by detailed spectroscopic analysis and by the comparison of their spectroscopic data with those reported in the literature. In addition, the absolute stereochemistry of the previously reported axiriabiline A (5) was determined by X-ray diffraction (XRD) analysis. In a bioassay, the bisabolane-type sesquiterpenoids 8, 10, and 11 exhibited cytotoxicity against several human cancer cell lines.


Introduction
Sea slugs of the genus Phyllidiella and Phyllidia are prolific in the South China Sea. They are well known for their ability to ingest toxic nitrogenous sesquiterpenoids from their diets, and use either these metabolites themselves or their biosynthetically transformed derivatives as a weapon for chemical defense [1][2][3][4][5][6][7]. An intriguing ecological study showed that when sea slugs are under attack, they release a lot of mucus containing these nitrogenous metabolites to poison their enemies [8]. The dietary origin of nitrogenous sesquiterpenoids has been supported by chemical investigations involving the isolation of such metabolites from both nudibranchs and their sponge-preys [9][10][11][12][13].
In our previous chemical investigation on South China Sea (Hainan) nudibranchs and sponges, nitrogenous terpenoids were isolated and structurally characterized [1,17,18,[32][33][34]. In the course of our continuing project on searching for chemically fascinating and biologically active secondary metabolites from Hainan marine molluscs, as well as the chemical ecology study between nudibranchs and their sponge-preys, we made different collections of two nudibranchs, Phyllidiella pustulosa and Phyllidia coelestis, as well as their sponge-prey Acanthella cavernosa, from the same location (Xidao Island, Hainan Province, China), with the aim of accumulating their nitrogenous metabolites for further study of their bioactivities, as well as studying the dietary relationship between P. pustulos, P. coelestis, and their sponge-prey A. cavernosa.

Results
Chemical investigation of the collected two nudibranchs, P. pustulosa and P. coelestis, as well as one sponge, A. cavernosa, led to the isolation of one new cadinane-type sesquiterpenoid (1), one new naturally occurring kalihinane-type diterpenoid (16), along with 14 known sesquiterpenoids (2−15) and three known diterpenoids (17−19) (Figure 1). All the compounds contain nitrogen atoms in different functional groups, such as isocyanate, isothiocyanate, and formamide. Herein, we describe the isolation, structure elucidation, and cytotoxic activity of these compounds, as well as their possible biosynthetic origin influenced by the prey-predator relationship.

Acanthella cavernosa
The frozen A. cavernosa animals were cut into pieces and exhaustively extracted by acetone. The Et 2 O-soluble portion of the acetone extract was repeatedly chromatographed to yield pure compounds 4, 5, 15, 16, 18, and 19 ( Figure 1). The known compounds were readily identified as one cadinane-type sesquiterpenoid: 10-formamido-4-cadinene (4) [24], one eudesmane-type sesquiterpenoid: axiriabiline A (5) [32], one spiroaxane-type sesquiterpenoid: axamide-3 (15) [27], along with two kalihinane-type diterpenoids: 10β-formamido-5β-isothiocyanatokalihinol-A (18) [14] and 10β-formamido-5-isocyanatokalihinol-A (19) [14] by comparing their NMR spectroscopic data and specific optical rotation with those reported in the literature.  Figure S9). The IR spectrum ( Figure S10) of 16 showed absorptions at ν max 1665 cm −1 and 3440 cm −1 , indicating the presence of the amide carbonyl and hydroxy groups, respectively. The 13 C NMR and DEPT spectra of 16 displayed 22 carbon signals, including five sp 3 methyls, six sp 3 methylenes, five sp 3 methines, four sp 3 quaternary carbons, and two sp 2 methines. The spectroscopic data (Table 1, Figures S11 and S12) showed highly similarity to those of co-occurring related known compounds 18 and 19, indicating that 16 is also a kalihinane-type diterpenoid. In fact, they differed from each other only by the substitution at C-5 position of the kalihinane ring. Bearing in mind the two additional protons present in its molecular formula in comparison to 19, a −NHCHO group (δ H 8.10 s, δ C 167.6, CH) should be attached to the C-5 of compound 16. Intriguingly, resonances for both formamides were observed as a plethora of signals between δ H 8.0 and 8.3. These included eight signals arising from the four isomeric arrangements possible for the two formamides at C-5 and C-10 [47]. Detailed analysis of the 1D and 2D NMR spectra, including 1 H-1 H COSY, HSQC, and HMBC ( Figures S13-S15), allowed the establishment of the planar structure of 16 (Figure 2), the same as a known compound named bisformamidokalihinol A, which was obtained from the hydrolysis of kalihinol A with acetic acid [48].
The relative configuration of 16 was also determined to be the same as co-occurring compounds 17-19 by careful interpretation of its NOESY spectrum with the clear NOE correlations of H-1/H-7, H-5/H-6/H 3 -20, and H 3 -19/NHCHO at C-5 ( Figure 2 and Figure S16). Since the absolute configuration of 17 has been previously determined by total synthesis [29], from a biogenetic point of view, the absolute configuration of compound 16 was tentatively assigned as 1S,4R,5R,6S,7S,10S,11R,14S.
It is worth noting that compound 5 was previously isolated from the Hainan sponge Axinyssa variabilis, and its absolute configuration was determined by a combination of ROESY experiment and time dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculation [32]. In this work, we obtained a single crystal of 5, and X-ray diffraction (XRD) analysis on a suitable crystal of 5 by employing Ga Kα radiation (λ = 1.34139 Å) with small Flack parameter 0.02 (16) allowed not only the unambiguous definition of the planar structure as illustrated in Figure 3, but also the revision of its absolute configuration from 4S,5R,10S to 4R,5S,10S. and hydroxy groups, respectively. The 13 C NMR and DEPT spectra of 16 displayed 22 carbon signals, including five sp 3 methyls, six sp 3 methylenes, five sp 3 methines, four sp 3 quaternary carbons, and two sp 2 methines. The spectroscopic data (Table 1, Figures S11 and S12) showed highly similarity to those of co-occurring related known compounds 18 and 19, indicating that 16 is also a kalihinane-type diterpenoid. In fact, they differed from each other only by the substitution at C-5 position of the kalihinane ring. Bearing in mind the two additional protons present in its molecular formula in comparison to 19, a −NHCHO group (δH 8.10 s, δC 167.6, CH) should be attached to the C-5 of compound 16. Intriguingly, resonances for both formamides were observed as a plethora of signals between δH 8.0 and 8.3. These included eight signals arising from the four isomeric arrangements possible for the two formamides at C-5 and C-10 [47]. Detailed analysis of the 1D and 2D NMR spectra, including 1 H-1 H COSY, HSQC, and HMBC (Figures S13−S15), allowed the establishment of the planar structure of 16 (Figures 2), the same as a known compound named bisformamidokalihinol A, which was obtained from the hydrolysis of kalihinol A with acetic acid [48]. The relative configuration of 16 was also determined to be the same as co-occurring compounds 17-19 by careful interpretation of its NOESY spectrum with the clear NOE correlations of H-1/H-7, H-5/H-6/H3-20, and H3-19/NHCHO at C-5 ( Figure 2 and Figure S16). Since the absolute configuration of 17 has been previously determined by total synthesis [29], from a biogenetic point of view, the absolute configuration of compound 16 was tentatively assigned as 1S,4R,5R,6S,7S,10S,11R,14S.
It is worth noting that compound 5 was previously isolated from the Hainan sponge Axinyssa variabilis, and its absolute configuration was determined by a combination of ROESY experiment and time dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculation [32]. In this work, we obtained a single crystal of 5, and X-ray diffraction (XRD) analysis on a suitable crystal of 5 by employing Ga Kα radiation (λ = 1.34139 Å) with small Flack parameter 0.02 (16) allowed not only the unambiguous definition of the planar structure as illustrated in Figure  3, but also the revision of its absolute configuration from 4S,5R,10S to 4R,5S,10S. Aware of the potent cytotoxicity exhibited by marine nitrogenous terpenoids, we performed in vitro biological evaluation of all the isolated metabolites on several tumor cell lines. The results ( Table 2) showed that compounds 8, 10, and 11 exhibited strong cytotoxicity against human cancer cell line SNU-398 with IC50 values of 0.50, 2.15, and 0.50 μM, respectively. In addition, compound 8 also displayed broad cytotoxicity against the other three cancer cell lines, including A549, HT-29, and Capan-1, with IC50 values of 8.60, 3.35, and 1.98 μM, respectively. It is interesting to note that, although only three compounds showed cytotoxicity, they are all of the same bisabolane type. Therefore, a preliminary structure-activity relationship could be addressed, that is, the bisbolane skeleton might be good for activity, while regarding the inactive compounds 7 and 9, the terminal olefin or the formamide group might be harmful for activity. More diverse bisabolanes should be discovered and tested for cytotoxicity to support our proposal. Aware of the potent cytotoxicity exhibited by marine nitrogenous terpenoids, we performed in vitro biological evaluation of all the isolated metabolites on several tumor cell lines. The results ( Table 2) showed that compounds 8, 10, and 11 exhibited strong cytotoxicity against human cancer cell line SNU-398 with IC 50 values of 0.50, 2.15, and 0.50 µM, respectively. In addition, compound 8 also displayed broad cytotoxicity against the other three cancer cell lines, including A549, HT-29, and Capan-1, with IC 50 values of 8.60, 3.35, and 1.98 µM, respectively. It is interesting to note that, although only three compounds showed cytotoxicity, they are all of the same bisabolane type. Therefore, a preliminary structure-activity relationship could be addressed, that is, the bisbolane skeleton might be good for activity, while regarding the inactive compounds 7 and 9, the terminal olefin or the formamide group might be harmful for activity. More diverse bisabolanes should be discovered and tested for cytotoxicity to support our proposal.

Discussion
In recent years, several marine molluscs were found by our group to contain the same or similar secondary metabolites as those in marine corals or sponges, which was further proved to be due to the predator-prey relationship between these animals. For example, isoquinolinequinones were discovered from both the nudibranch Jorunna funebris and its sponge-prey Xestospongia sp. [49,50], while cladiellane-type diterpenoids were isolated from both the nudibranch Tritoniopsis elegans and its soft coral prey Cladiella krempfi [51]. In this study, similar results were observed by the chemical investigation of the three title animals. As shown in Figure 4, by comparison of the typical nitrogenous terpenoids in the two nudibranchs P. pustulosa and P. coelestis with those in the sponge A. cavernosa, four common structural skeletons were observed in both P. pustulosa and A. cavernosa, including cadinane, eudesmane, aromadendrane, and kalihinane, whereas one common eudesmane skeleton was found in all three animals. In addition, our previous chemical investigation of the marine sponge A. variabilis from the same water area in the South China Sea revealed the main secondary metabolites as bisabolene sesquiterpenoids [52], which was the common skeleton found in both P. pustulosa and P. coelestis (Figure 4). Therefore, on the basis of these research observations, we hold the belief that the two nudibranchs P. pustulosa and P. coelestis feed on the sponges A. cavernosa and A. variabilis and accumulate the useful dietary metabolites from the sponges, especially those toxic isocyanide derivatives, to be employed as their own chemical defensive agents for surviving in the harsh marine living environment. More intriguingly, it is obvious that one nudibranch can feed on various sponges to obtain diverse isocyanide metabolites, so as to use them as specially appointed chemical weapons on particular occasions.
In summary, the chemical investigation of the two nudibranchs P. pustulosa and P. coelestis, as well as the sponge A. cavernosa, led to the isolation and determination of 19 nitrogenous terpenoids with high chemical diversity. In fact, a total of seven different chemical skeletons were observed: four cadinane-type sesquiterpenoids (1-4), two eudesmane-type sesquiterpenoids (5-6), five bisabolene-type sesquiterpenoids (7)(8)(9)(10)(11), two aromadendrane-type sesquiterpenoids (12 and 13), one pupukeanane-type sesquiterpenoid (14), one spiroaxane-type sesquiterpenoid (15), and four kalihinane-type diterpenoids (16)(17)(18)(19). Their structures including relative stereochemistry were elucidated by comprehensive NMR analyses. The absolute configuration of two new metabolites (1 and 16) were tentatively assigned based on the biogenetic consideration, whereas that of the known compound 5 was revised by the XRD analysis. In bioassay, the bisabolane-type sesquiterpenoids 8, 10, and 11 displayed considerable cytotoxicity against several cancer cell lines, which is worth further pharmacological study. Further chemical ecological research on the basis of the predator-prey relationship to prove our hypothesis would be interesting to be conducted in the future.

General Experimental Procedures
Optical rotations were measured in CHCl3 on a Perkin-Elmer 241MC polarimeter (PerkinElmer Inc., Waltham, MA, USA). IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) with KBr pellets; peaks are reported in cm −1 . 1D and 2D NMR spectra were measured on a Bruker DRX-400 or Bruker DRX-500 spectrometer (Bruker Biospin AG, Fällanden, Germany), using the residual CHCl3 signal (δH 7.26 ppm) as an internal standard for 1 H NMR and CDCl3 (δC 77.00 ppm) for 13 C NMR. Chemical shifts are expressed in δ (ppm) and coupling constants (J) in Hz. 1 H and 13 C NMR assignments were supported by 1 H-1 H COSY, HSQC, HMBC, and NOESY experiments. EIMS and HREIMS spectra were recorded on a Finnigan-MAT-95 mass spectrometer (FinniganMAT, San Jose, CA, USA). HRESIMS spectra were recorded on an Agilent G6250 Q-TOF (Agilent, Santa Clara, CA, USA). Reversed-phase (RP) HPLC purification was carried out on an Agilent 1260 series liquid chromatography equipped with a DAD G1315D detector at 210 and 254 nm and with a semi-preparative ODS-HG-5 column (5 μm, 250 × 9.4 mm). Commercial silica gel (Qingdao Haiyang Chemical Group Co., Ltd., Qingdao, China, 200-300 and 300-400 mesh) was used for column chromatography, and precoated silica gel plates (Yan Tai Zi Fu Chemical Group Co., Yantai, China, G60 F-254) were used for analytical Thin-layer chromatography (TLC). Spots were detected on TLC under UV light or by heating after spraying with anisaldehyde H2SO4 reagent. All the chemicals were obtained from commercial sources. All solvents used for column chromatography (CC) were of analytical grade, and solvents used for HPLC were of HPLC grade.

General Experimental Procedures
Optical rotations were measured in CHCl 3 on a Perkin-Elmer 241MC polarimeter (PerkinElmer Inc., Waltham, MA, USA). IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) with KBr pellets; peaks are reported in cm −1 . 1D and 2D NMR spectra were measured on a Bruker DRX-400 or Bruker DRX-500 spectrometer (Bruker Biospin AG, Fällanden, Germany), using the residual CHCl 3 signal (δ H 7.26 ppm) as an internal standard for 1 H NMR and CDCl 3 (δ C 77.00 ppm) for 13 C NMR. Chemical shifts are expressed in δ (ppm) and coupling constants (J) in Hz. 1 H and 13 C NMR assignments were supported by 1 H-1 H COSY, HSQC, HMBC, and NOESY experiments. EIMS and HREIMS spectra were recorded on a Finnigan-MAT-95 mass spectrometer (FinniganMAT, San Jose, CA, USA). HRESIMS spectra were recorded on an Agilent G6250 Q-TOF (Agilent, Santa Clara, CA, USA). Reversed-phase (RP) HPLC purification was carried out on an Agilent 1260 series liquid chromatography equipped with a DAD G1315D detector at 210 and 254 nm and with a semi-preparative ODS-HG-5 column (5 µm, 250 × 9.4 mm). Commercial silica gel (Qingdao Haiyang Chemical Group Co., Ltd., Qingdao, China, 200-300 and 300-400 mesh) was used for column chromatography, and precoated silica gel plates (Yan Tai Zi Fu Chemical Group Co., Yantai, China, G60 F-254) were used for analytical Thin-layer chromatography (TLC). Spots were detected on TLC under UV light or by heating after spraying with anisaldehyde H 2 SO 4 reagent. All the chemicals were obtained from commercial sources. All solvents used for column chromatography (CC) were of analytical grade, and solvents used for HPLC were of HPLC grade.

Biological Material
The molluscs and sponges were collected using scuba at Xidao Island, Hainan Province, China, in March 2014, at a depth of −15 to −20 m, and identified by Professor Xiu-Bao Li from Hainan University. The voucher sample is deposited at the Shanghai Institute of Materia Medica, CAS.

Extraction and Isolation of 1-19
The lyophilized bodies of P. pustulosa (24 specimens, 11.1 g, dry weight) were carefully dissected into internal organs and mantle that were separately extracted by acetone using ultrasound. Filtration of the two homogenates gave an aqueous-Me 2 CO filtrate that was concentrated in vacuo to give a gummy residue. The residue was suspended in H 2 O and extracted sequentially with diethyl ether and n-BuOH. The lyophilized bodies of P. coelestis (seven specimens, 25.5 g, dry weight) were extracted by acetone using ultrasound. The extracts of both internal organs and mantle were combined due to the similar TLC results, to give 700 mg extract. An approach similar to the abovementioned fractional method was applied to give a total of seven fractions (A-G). Compounds 8 (5.2 mg) and 9 (3.4 mg) were obtained directly from fractions B and G after purification by HPLC, respectively. Fraction B was chromatographed over Sephadex LH-20 eluting with PE/CHCl 3 /MeOH (2:1:1), followed by HPLC purification to give compounds 10 (1.5 mg) and 11 (1.2 mg). Fraction F was treated by the same procedure as above to give compound 6 (1.7 mg).
The frozen A. cavernosa sponges (55 g, dry weight) were cut into pieces and extracted exhaustively with acetone at room temperature (6 × 2.0 L). The organic extract was evaporated to give a brown residue, which was then partitioned between H 2 O and Et 2 O. The upper layer was concentrated under reduced pressure to give a red residue (1.0 g). The resultant residue was separated into six fractions (A-F) by gradient silica gel column chromatography. The resulting fractions were then fractionated into sub-fractions by Sephadex LH-20. The sub-fraction F6 was purified by semi-preparative HPLC (70% MeOH to 100% MeOH in 20 min), yielding compounds 16 (4.0 mg), 18 (2.0 mg), and 19 (1.9 mg). The sub-fraction E4 of fraction E gave compounds 4 (3.1 mg), 6 (4.1 mg), and 15 (2.7 mg).  and refined using full-matrix least-squares difference Fourier techniques. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were placed in idealized positions and refined as riding atoms with their related isotropic parameters. Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC 1880256. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).

Bioassay Procedures
Cytotoxic Activity Compounds 1-19 were evaluated for their cytotoxic activity against four human cancer cell lines (A549, HT-29, SNU-398, and Capan-1) using the sulforhodamine B (SRB, Sigma, St. Louis, MO, USA) method. Four cell lines were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). The cytotoxic activity in vitro was indicated in terms of IC 50 (µM), that is, the concentration of a compound that inhibited the proliferation rate of tumor cells by 50% as compared to the untreated control cells. Vincristine was used as a reference drug.