Cytotoxic Natural Products from Marine Sponge-Derived Microorganisms

A growing body of evidence indicates that marine sponge-derived microbes possess the potential ability to make prolific natural products with therapeutic effects. This review for the first time provides a comprehensive overview of new cytotoxic agents from these marine microbes over the last 62 years from 1955 to 2016, which are assorted into seven types: terpenes, alkaloids, peptides, aromatics, lactones, steroids, and miscellaneous compounds.


Introduction
The search for cytotoxic agents from marine resources has always attracted the attention of natural products chemists [1,2]. More than 10% of the screened marine sponges display cytotoxic activities [3][4][5]. Marine sponges are well known to be hosts for a large community of microorganisms, which comprise a significant percentage (up to 50%-60%) of the biomass of the sponge host [6,7]. A growing body of evidence has indicated that marine sponges undergo symbiotic relationships with microbes such as bacteria and fungi, which are likely to be the prolific producers of bioactive secondary metabolites [8,9]. This review provides a comprehensive overview of 107 new cytotoxic agents metabolized by marine sponge-derived microbes, which are assorted into seven types, including terpenes, alkaloids, peptides, aromatics, lactones, steroids, and miscellaneous compounds discovered from 1955 to 2016.

Diterpenes
Four novel decalin derivatives, tandyukisins (23)(24)(25)(26), were produced by a strain of Trichoderma harzianum OUPS-111D-4 originally derived from the marine sponge Halichondria okadai collected in Osaka Bay, Japan (Chart 5). Cytotoxic assays suggest that compound 23 exhibited moderate cytotoxicity against murine leukemia cell lines P388 and L1210 and human leukemia cell line HL-60. Compounds 24-26 had moderate cytotoxicity against a disease-oriented panel of 39 human cancer cell lines (HCC panel). However, these diterpenes showed slightly selective growth inhibition against the central nervous system cancer SNB-75 cell line in the HCC panel [17,18].

Sesterterpenoids
Chemical examination of the marine fungus Aspergillus ustus isolated from a Mediterranean sponge Suberites domuncula yielded five new ophiobolin-type sesterterpenoids 18-22 [16] (Chart 4). These compounds were assayed for their cytotoxic activity against the murine lymphoma cell line L5178Y at 10 mg/mL.

Diterpenes
Four novel decalin derivatives, tandyukisins (23)(24)(25)(26), were produced by a strain of Trichoderma harzianum OUPS-111D-4 originally derived from the marine sponge Halichondria okadai collected in Osaka Bay, Japan (Chart 5). Cytotoxic assays suggest that compound 23 exhibited moderate cytotoxicity against murine leukemia cell lines P388 and L1210 and human leukemia cell line HL-60. Compounds 24-26 had moderate cytotoxicity against a disease-oriented panel of 39 human cancer cell lines (HCC panel). However, these diterpenes showed slightly selective growth inhibition against the central nervous system cancer SNB-75 cell line in the HCC panel [17,18].

Meroterpenoids
Chemical investigation of the EtOAc extract of the culture medium of the marine-derived fungus Aspergillus insuetus OY-207 led to the isolation of a novel meroterpenoid, insuetolide C (27) (Chart 6). The strain OY-207 was isolated from a Mediterranean sponge Psammocinia sp.

Sesterterpenoids
Chemical examination of the marine fungus Aspergillus ustus isolated from a Mediterranean sponge Suberites domuncula yielded five new ophiobolin-type sesterterpenoids 18-22 [16] (Chart 4). These compounds were assayed for their cytotoxic activity against the murine lymphoma cell line L5178Y at 10 mg/mL.

Diterpenes
Four novel decalin derivatives, tandyukisins (23)(24)(25)(26), were produced by a strain of Trichoderma harzianum OUPS-111D-4 originally derived from the marine sponge Halichondria okadai collected in Osaka Bay, Japan (Chart 5). Cytotoxic assays suggest that compound 23 exhibited moderate cytotoxicity against murine leukemia cell lines P388 and L1210 and human leukemia cell line HL-60. Compounds 24-26 had moderate cytotoxicity against a disease-oriented panel of 39 human cancer cell lines (HCC panel). However, these diterpenes showed slightly selective growth inhibition against the central nervous system cancer SNB-75 cell line in the HCC panel [17,18].

Alkaloids
Chemical examination of the cultured mycelium of a bacterium Alteromonas sp. from the sponge Halichondria okadai led to the isolation of one novel tetracyclic alkaloid: 31 (Chart 7). It exhibited  [21].
Two new congener alkaloids, communesins 34 and 35 (Chart 7), were detected in the ethyl acetate extract of a Penicillium sp. which was isolated from the Mediterranean sponge Axinella verrucosa. Communesin 34 was observed to be most active on the human acute T lymphoblastic leukemia cell line MOLT-3 with an ED 50 value of 8.6 µg/mL. Conversely, 35 possessed a strong inhibitory effect on the human acute B lymphoblastic leukemia cell line SUP-B15 with an ED 50 value of 9.0 µg/mL [22]. A new sorbicillin-derived compound, 36, metabolized by Penicillium chrysogenum associated with the Mediterranean sponge Ircinia fasciculata was found to exhibit a strong cytotoxic activity against L5178y cells and low toxicity to cervical carcinoma HeLa S3 cells and pheochromocytoma PC12 cells [23]. Another Penicillium strain, P. aurantiogriseum SP0-19, was isolated from the marine sponge Mycale plumose and shown to produce two novel quinazoline alkaloids: aurantiomides 37 and 38.

Peptides
Two highly N-methylated linear octapeptides, RHM1 (58) and RHM2 (59) (Chart 10), were produced by an atypical strain of Acremonium sp. cultured from a Teichaxinella sp. marine sponge (collected in Papua New Guinea) and were shown to have mild cytotoxicity against murine L1210

Polyketides
Chemical investigation of Penicillium brocae, obtained from a tissue sample of a Fijian sponge Zyzyya sp., led to the isolation of three novel polyketides: brocaenols A-C (65-67) (Chart 11).

Anthraquinones
From a strain of the fungus Emericella variecolor derived from the marine sponge Haliclona valliculata (collected at Secca di Capo di Fonza, Elba, Italy), a new natural product called evariquinone 71 (Chart 13) was isolated and found to display antiproliferative activity towards tumor cell lines KB (60% inhibition) and NCI-H460 (69% inhibition) at 3.16 mg/mL [41].

Bicoumarin
Fractionation of the EtOAc extract of a static culture of Aspergillus niger from a Mediterranean sponge Axinella damicornis, yielded one new secondary metabolite: 3,3′-bicoumarin bicoumanigrin (78) (Chart 14). MTT assay indicated that this compound exhibited moderate inhibitory effects on the growth of leukemia and carcinoma cell lines using incorporation of 3 H-thymidine as a marker [44].

Ethers
Two new prenylated diphenyl ethers (79 and 80) (Chart 14) were purified from the fungus strain of Aspergillus versicolor Hmp-F48 associated with marine sponge Hymeniacidon perleve. Compounds 79 and 80 showed moderate inhibitory activities against the human promyelocytic leukemia cell line

Anthraquinones
From a strain of the fungus Emericella variecolor derived from the marine sponge Haliclona valliculata (collected at Secca di Capo di Fonza, Elba, Italy), a new natural product called evariquinone 71 (Chart 13) was isolated and found to display antiproliferative activity towards tumor cell lines KB (60% inhibition) and NCI-H460 (69% inhibition) at 3.16 mg/mL [41].

Anthraquinones
From a strain of the fungus Emericella variecolor derived from the marine sponge Haliclona valliculata (collected at Secca di Capo di Fonza, Elba, Italy), a new natural product called evariquinone 71 (Chart 13) was isolated and found to display antiproliferative activity towards tumor cell lines KB (60% inhibition) and NCI-H460 (69% inhibition) at 3.16 mg/mL [41].

Bicoumarin
Fractionation of the EtOAc extract of a static culture of Aspergillus niger from a Mediterranean sponge Axinella damicornis, yielded one new secondary metabolite: 3,3′-bicoumarin bicoumanigrin (78) (Chart 14). MTT assay indicated that this compound exhibited moderate inhibitory effects on the growth of leukemia and carcinoma cell lines using incorporation of 3 H-thymidine as a marker [44].

Ethers
Two new prenylated diphenyl ethers (79 and 80) (Chart 14) were purified from the fungus strain of Aspergillus versicolor Hmp-F48 associated with marine sponge Hymeniacidon perleve. Compounds  (78) (Chart 14). MTT assay indicated that this compound exhibited moderate inhibitory effects on the growth of leukemia and carcinoma cell lines using incorporation of 3 H-thymidine as a marker [44].

Ethers
Two new prenylated diphenyl ethers (79 and 80) (Chart 14) were purified from the fungus strain of Aspergillus versicolor Hmp-F48 associated with marine sponge Hymeniacidon perleve. Compounds 79 and 80 showed moderate inhibitory activities against the human promyelocytic leukemia cell line HL-60 with IC 50 values of 6.35 and 19.97 µM, respectively [45].

Miscellaneous Compounds
Novel metabolites trichodenones A-C (101-103) (Chart 18) were detected in the culture broth of Trichoderma harzianum OUPS-N 115 originally separated from marine sponge Halichondria okadai (collected in the Tanabe Bay, Japan) and shown to possess strong cytotoxicity against P388 cells [55]. Chemical investigation of the fungal strain Penicillium citrinum SpI080624G1f01, derived from the Demospongiae sponge (collected from Ishigaki Island, Japan), afforded a new compound, JBIR-59 (104) (Chart 18), which had L-glutamate toxicity against tumor cell line N18-RE-105 with an EC50 value of 71 μM [56]. One novel sterol bendigole 105 (Chart 18) produced by Actinomadura sp. SBMs009 from the marine sponge Suberites japonicus displayed a moderate cytotoxic effect on the L929 cells with an IC50 value of 30 μM [57]. Two new structurally unique compounds bearing a nitrogen and sulfurcontaining tricyclic ring system, ulbactin F (106) and its diastereomeric isomer ulbactin G (107) (Chart 18), were isolated from the culture extract of Brevibacillus sp. associated with an unidentified marine sponge (Iwate, Japan). Bioassay testing indicated that 106 and 107 had a strong inhibitory effect on epidermoid carcinoma cell line A431 at non-cytotoxic concentrations with IC50 values of 6.4 and 6.1

Miscellaneous Compounds
Novel metabolites trichodenones A-C (101-103) (Chart 18) were detected in the culture broth of Trichoderma harzianum OUPS-N 115 originally separated from marine sponge Halichondria okadai (collected in the Tanabe Bay, Japan) and shown to possess strong cytotoxicity against P388 cells [55]. Chemical investigation of the fungal strain Penicillium citrinum SpI080624G1f01, derived from the Demospongiae sponge (collected from Ishigaki Island, Japan), afforded a new compound, JBIR-59 (104) (Chart 18), which had L-glutamate toxicity against tumor cell line N18-RE-105 with an EC50 value of 71 μM [56]. One novel sterol bendigole 105 (Chart 18) produced by Actinomadura sp. SBMs009 from the marine sponge Suberites japonicus displayed a moderate cytotoxic effect on the L929 cells with an IC50 value of 30 μM [57]. Two new structurally unique compounds bearing a nitrogen and sulfurcontaining tricyclic ring system, ulbactin F (106) and its diastereomeric isomer ulbactin G (107) (Chart 18), were isolated from the culture extract of Brevibacillus sp. associated with an unidentified marine sponge (Iwate, Japan). Bioassay testing indicated that 106 and 107 had a strong inhibitory effect on epidermoid carcinoma cell line A431 at non-cytotoxic concentrations with IC50 values of 6.4 and 6.1

Miscellaneous Compounds
Novel metabolites trichodenones A-C (101-103) (Chart 18) were detected in the culture broth of Trichoderma harzianum OUPS-N 115 originally separated from marine sponge Halichondria okadai (collected in the Tanabe Bay, Japan) and shown to possess strong cytotoxicity against P388 cells [55]. Chemical investigation of the fungal strain Penicillium citrinum SpI080624G1f01, derived from the Demospongiae sponge (collected from Ishigaki Island, Japan), afforded a new compound, JBIR-59 (104) (Chart 18), which had L-glutamate toxicity against tumor cell line N18-RE-105 with an EC 50 value of 71 µM [56]. One novel sterol bendigole 105 (Chart 18) produced by Actinomadura sp. SBMs009 from the marine sponge Suberites japonicus displayed a moderate cytotoxic effect on the L929 cells with an IC 50 value of 30 µM [57]. Two new structurally unique compounds bearing a nitrogen and sulfur-containing tricyclic ring system, ulbactin F (106) and its diastereomeric isomer ulbactin G (107) (Chart 18), were isolated from the culture extract of Brevibacillus sp. associated with an unidentified marine sponge (Iwate, Japan). Bioassay testing indicated that 106 and 107 had a strong inhibitory effect on epidermoid carcinoma cell line A431 at non-cytotoxic concentrations with IC 50 values of 6.4 and 6.1 µM, respectively [58]. In summary, microorganisms associated with marine sponges are a prolific source of novel cytotoxic natural products with rich chemical structures. The utilization of natural products as sources of new drugs is still alive and well, especially in the area of cancer [59]. Generally, any cytotoxic chemical with an IC50 or ED50 value <1 μM has great potential for application in the discovery of new anti-tumor drugs/leads, for example, tetracyclic alkaloid 31 and dankastatin C (57). These candidates may play an important role in defeating human cancer.