Structure Elucidation and Cytotoxic Evaluation of New Polyacetylenes from a Marine Sponge Petrosia sp.

The sponge Petrosia sp. yielded five polyacetylenic compounds (1–5), including two new polyacetylenes, petrosianynes A (1) and B (2). The structures of these compounds were elucidated by detailed spectroscopic analysis and by comparison with the physical and spectral data of related known analogues. Compounds 1–5 exhibited significant cytotoxic activity against a limited panel of cancer cell lines.


Introduction
Since the early days of marine natural products chemistry, sponges (Porifera) have occupied a superior place in the focus of researchers aiming to reveal the therapeutic potentials of these benthic organisms [1]. Thousands of secondary metabolites have been identified from these organisms and their contributions to the chemical library continue with fascinating results. Such a sponge-centric theme comes as no surprise, because such organisms have been spotted since antiquity and their unique secondary metabolites suggested a myriad of potential applications [2][3][4]. Among the well-studied Porifera genera is Petrosia sp. They have been subjected to intensive scrutiny following the separation of different acyclic polyacetylenes with potent biological activities from sponges belonging to this genus [5][6][7]. Despite the fact that certain acyclic polyacetylenes can be found in terrestrial plants, especially the family Asteraceae, Petrosia sp. have been known to be a source of polyacetylenes with unique structural features as it has been demonstrated over the last two decades of research [8]. It is estimated that 33% of all known polyacetylenes have been isolated from Petrosia sp. [9]. In addition to the unique structural features of the isolated polyacetylenes, a plethora of interesting biological activities has been reported for this class of secondary metabolites, including antimicrobial [10], antifungal [11,12], reverse transcriptase inhibitory [13] and antitumor activities [9,14,15]. In a search for bioactive metabolites from marine organisms, the sponge Petrosia sp. (Figure 1) was selected for a detailed investigation, as its EtOAc crude extract showed significant cytotoxicity in six human tumor cell lines. Bioassay-guided fractionation resulted in the isolation of two new polyacetylenic compounds (1 and 2) and three known polyacetylenes (3)(4)(5) (Figure 2). The cytotoxic activity of the metabolites (1-5) against human T cell lymphoblast-like cell line (CCRF-CEM), human T lymphoblast, acute lymphoblastic leukemia (MOLT-4), human chronic myelogenous leukemia (K-562), human colon adenocarcinoma (DLD-1), human prostate carcinoma (LNCaP) and human hormone-dependent breast cancer (T-47D) cell lines was evaluated.

Results and Discussion
Following the chromatographic separation of Petrosia sp., the EtOAc soluble fraction yielded five polyacetylenic compounds, two of which were new natural products (1 and 2). All compounds were obtained as colorless oils. The new compounds were given the trivial names petrosianynes A (1) and B (2). Three compounds 3-5 were found to be identical to the known polyacetylenes 15,16-dihydropetrosianyne (3) [16], petrosynol (4) [17] and petrosynone (5) [10], respectively, through the comparison of their physical and spectroscopic data with those reported in the literature.
Compound 1, which was isolated as a colorless oil, was found to have the molecular formula C30H44O4 deduced by HRESIMS at m/z 491.3135 [M + Na] + . The IR spectrum of 1 showed absorption bands due to a hydroxyl (3425 cm −1 ) and alkyne (3280 and 2225 cm −1 ) moieties. The MS, 1 H and 13 C NMR spectra ( Table 1) of 1 showed characteristic signal patterns that were reminiscent of the C2-symmetrical structure. The planar structure and all of the 1 H and 13 C chemical shifts of 1 were elucidated by 2D NMR spectroscopic analysis, especially the 1 H-1 H COSY and HMBC correlations ( Figure 3). Thus, 1 was found to possess two terminal acetylenes at C-1/C-2 and C-29/C-30, two disubstituted acetylenes at C-12/C-13 and C-18/C-19, one double bond at C-15/C-16 and four hydroxy groups at C-3, C-14, C-17 and C-28. In addition, the spectroscopic data of 1 (IR, 1 H and 13 C NMR) were similar to those of 4, which was isolated from the same sponge. Comparison of the 1 H and 13 C NMR data of 1 with 4 [17] showed that the signals corresponding to a 1,2-disubstituted double bond in 4 were replaced by signals of a single bond in 1. As the new metabolite 1 was isolated together with 3 [16] and 4 from the same species and possesses a similar molecular skeleton, it was proposed that the three compounds are synthesized through a common biosynthetic pathway and thus have the same absolute configurations at C-3, C-14, C-17 and C-28. On the basis of the above analyses, the structure of 1 was established and the compound was named petrosianyne A.    Metabolite 2 was obtained as a colorless oil. Its HRESIMS indicated the molecular formula C30H40O3, with eleven degrees of unsaturation. The IR spectrum suggested the presence of a hydroxyl (3414 cm −1 ) and alkyne (3284 and 2209 cm −1 ) moieties. The 13 C NMR data of 2 (Table 1) Table 2). Compounds 1 and 3-5 exhibited significant cytotoxic activity against three cancer cells, CCRF-CEM, MOLT-4 and K-562 (IC50 < 4 μg/mL), and compound 1 also exhibited significant cytotoxicity against LNCaP and T-47D. In addition, compound 3 exhibited significant cytotoxicity toward DLD-1 and LNCaP cell lines. Compound 2 was not cytotoxic (IC50 > 4 μg/mL) toward the above six cancer cell lines. According to the structures of 1-4, compound 2, the 14-deoxy derivative of 4, has IC50 values of >4 μg/mL against the above carcinoma cell lines; therefore, it was suggested that the hydroxy group of C-14 is important for the cytotoxic activity of compounds 1-4.

Animal Material
The sponge Petrosia (Vosmaer, 1887) sp. belongs to the class Demospongiae, order Haplosclerida (Topsent 1928), family Petrosiidae (Van Soest, 1980) [18]. It was collected by scuba divers at a depth of 15-20 m from coral reefs off the coast of Pingtung, Taiwan, in February 2013. This sponge is irregular in shape and dark purple in color (Figure 1). The ectosomal skeleton is dense, granular, with a tangential network of single strongyles forming a subrectangular mesh, with free strongyles grouped at the nodes of the network (Figure 4). Abundant abruptly curved microxeas were observed. Choanosomal spicule tracts were slightly differentiated with stout longitudinal tracts and connecting short, irregular tracts of strongyles and free strongyles. Megascleres measured from 188-322 µm and microscleres from 72-100 µm ( Figure 5). The sponge was identified as Petrosia sp. and a voucher specimen was deposited at the National Museum of Marine Biology and Aquarium, Taiwan (specimen no. SP2013-1). Taxonomic identification was performed by Prof. Li-Lian Liu, National Sun Yat-sen University, Kaohsiung, Taiwan.

Supplementary Files
HR-ESI-MS, 1 H NMR, and 13 C NMR spectra for the two new compounds (1 and 2) as well as the 1 H NMR, and 13 C NMR spectra for compounds 3-5 are available as Supplementary Information.

Cytotoxicity Testing
The MTT assay was performed as described previously with some modification [19]. Cytotoxicity assays of compounds 1-5 were conducted against human T cell lymphoblast-like cell line (CCRF-CEM), human T lymphoblast, acute lymphoblastic leukemia (MOLT-4), human chronic myelogenous leukemia (K-562), human colon adenocarcinoma (DLD-1), human prostate carcinoma (LNCaP) and human hormone-dependent breast cancer (T-47D) using a MTT colorimetric method. The tested human cancer cell lines were seeded at 4 × 10 4 per well in 96-well culture plates before treatment with different concentrations of the tested compounds. Compounds 1-5 were dissolved in DMSO (less than 0.02%) and made immediately to 1.25, 2.5, 5, 10 and 20 μg/μL prior to the experiments. After treatment for 72 h, the cytotoxicities of the tested compounds were determined using a MTT cell proliferation assay (thiazolyl blue tetrazolium bromide, Sigma-M2128, Sigma-Aldrich, St. Louis, MO, USA). The MTT is reduced by the mitochondrial dehydrogenases of viable cells to a purple formazan product. The MTT-formazan product was dissolved in DMSO. Light absorbance values (OD = OD570 − OD620) were recorded at wavelengths of 570 and 620 nm using an ELISA reader (Anthos labtec Instrument, Salzburg, Austria) to calculate the concentration that caused 50% inhibition (IC50), i.e., the cell concentration at which the light absorbance value of the experimental group was half that of the control group. These results were expressed as a percentage of the control ± SD established from n = 4 wells per one experiment from three separate experiments.