The Bioactive Secondary Metabolites from Talaromyces species

Abstract The focus of this review is placed on the chemical structures from the species of the genus Talaromyces reported with reference to their biological activities. 221 secondary metabolites, including 43 alkaloids and peptides, 88 esters, 31 polyketides, 19 quinones, 15 steroid and terpenoids, and 25 other structure type compounds, have been included, and 66 references are cited. Graphical Abstract

papers which reported the secondary metabolites from the species named Talaromyces were covered.
The genus Talaromyces (Trichocomaceae) is an important fungal genus because of its ubiquity which were isolated from soil, plants, sponges, and foods. Some of the species are heat resistant. Some of the species are famous because of their enzymes applicable in the synthesis of saccharides, preparation of chiral building blocks or biotransformations, and for its application in pest biocontrol. Many of its species are used in food and agricultural production. Interestingly, the T. pinophilus strain EMOO 13-3 is able to degrade agricultural waste [2]. However, although endemic in maize, T. funiculosus also occurs in a wide range of other foods and sometimes causes spoilage [1]. Considering their importance, members of this genus have attracted the attention of chemists. Many studies have focused on the secondary metabolites.

The Secondary Metabolites
The secondary metabolites of Talaromyces mainly include alkaloids, peptides, lactones, polyketides, and miscellaneous structure type compounds. T. flavus, a microorganism remarkable for its secondary metabolites with unique biological activities, is the commonest species of the genus Talaromyces [3]. All of the natural products from the species of this genus are classified. The reported bioactivities are also represented below.

Alkaloids and Peptides
Alkaloid is a kind of important natural products. Many alkaloids have various kinds of biological activities, such as antibacterial, antifungal, cytotoxic, and nematicidal. The structures of alkaloids isolated from Talaromyces species are mainly nitrogen heterocyclic derivatives.
Two prenylated indole alkaloids, talathermophilins A and B (1 and 2), were isolated from a thermophilic fungus T. thermophilus strain YM1-3. And the ratio of 1 and 2 in the culture broths was unexpectedly rather constant (about 2:3), which even remained unchanged despite the addition of exogenous 1 or 2 suggesting that talathermophilins might be of special function for the extremophilic fungus. Those both compounds showed nematicidal toxicity (ca. 38 and 44 % inhibition, respectively) toward the worms of the free-living nematode Panagrellus redivivus at a concentration of 400 lg/mL for 72 h. The family of prenylated indole alkaloids is a well-known group of secondary metabolites mainly produced by Aspergillus and Penicillium sp. This is a first report about pyranoindol alkaloids from Talaromyces [4]. Other fourindole alkaloids with various levels of prenylation, talathermophilins C-E (3)(4)(5) and cyclo (glycyltryptophyl) (6), from the thermophilic fungus T. thermophilus strain YM3-4 which was collected in hot springs, were also elucidated by the same research group in 2011 [5]. Interestingly, authors found that only a very small group of amino acids (glycine, alanine, proline, and its derivatives) could be naturally chosen as a starting building block to form the 2,5-diketopiperazine with tryptophan [4,5].
An unprecedented class of PKS-NRPS hybrid metabolites possessing a 13-membered lactam-bearing macrolactone, thermolides A-F (16)(17)(18)(19)(20)(21), were also obtained from T. thermophilus YM3-4. They showed that compounds 16 and 17 displayed potent inhibitory activity against three notorious nematodes with LC 50 values of 0.5-1 lg/mL, as active as commercial avermectins. This is the first report on the discovery of hybrid macrolides from a fungus origin [8]. Afterwards, a combination of chemical screening, genome analyses, and genetic manipulation led to the identification of the thermolide biosynthetic genes from sister thermophilic fungi T. thermophilus and Thermomyces lanuginosus C5. And a novel macrolactone, thermolide G (22), was obtained from the cultural broth of Thermomyces lanuginosus C5. Their results revealed the first fungal hybrid iterative polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) genes involved in the biosynthesis of bacterial-like hybrid macrolactones instead of typical fungal tetramic acids-containing metabolites [9].
A peptide analogue N-benzoylphenylalanyl-N-benzoylphenylalaninate (35) was isolated from the fungus T. thailandiasis, which was firstly found from a higher plant, Croton hieronymi [12]. Two new cyclic peptides, talaromins A and B (36 and 37) were yielded from the endophytic fungus T. wortmannii, isolated from Aloe vera by the group of Peter Proksch and Abdessamad Debbab. Both cyclopeptides contain ring systems comprised of six a-amino acid residues connected to b-amino acid. The absolute configurations of the a-amino acids were determined by Marfey's method. Both compounds showed no activity when evaluated for their cytotoxicity against L5178Y mouse lymphoma cells and no antibacterial activity against a broad spectrum of bacterial strains up to a concentration of 64 lg/mL [13]. 9-(3-L-alanylamino-3-carboxypropyl)adenine (NK374200, 38) with a peptidyl adenine nucleus was isolated from the culture broth of the fungus Talaromyces sp., which had been isolated from a soil sample. 38 was screened in various biological assay systems, and found to have anti-mosquito larval activity [14].
The Bioactive Secondary Metabolites from Talaromyces species 5
The Bioactive Secondary Metabolites from Talaromyces species 7 Three new oxaphenalenone dimers, bacillosporins A-C (66-68), were isolated from T. bacillosporus NHL 2660. 66 had the antibacterial activity against Bacillus subtilis and Sarcina lutea [24]. Other oligophenalenone dimers, bacillisporins D and E (69 and 70) and duclauxin (71), were isolated from the fungus T. bacillisporus from a soil sample. They were screened for in vitro cytotoxicity again three human tumor cell lines MCF-7, NCI-H-460 and SF-268, and 71 exhibited moderate inhibitory effects against all three cell lines but 70 showed little activity [25]. In 2015, two new oxaphenalenone dimers, talaromycesone A (72) and talaromycesone B (73), were isolated from the culture broth and mycelia of a marine fungus Talaromyces sp. strain LF458. 72 exhibited potent antibacterial activities with IC 50 3.70 lM against human pathogenic Staphylococcus strains, and 72 also displayed potent acetylcholinesterase inhibitory activities with IC 50 7.49 lM [26].
Antibacterial binaphtho-a-pyrones, talaroderxines A and B (74 and 75) were isolated from a new heterothallic ascomycetous fungus, T. derxii, cultivated on rice. The antibacterial activities of the metabolites from T. derxii and their derivations against Bacillus subtilis indicated that only talaroderxine, the mixture of 74 and 75, showed antibacterial activity, which was almost as strong as that of viriditoxin. And talaroderxine had inhibitory activity toward 5-lipoxygenase, its IC 50 value was determined as 3.8 9 10 -6 M [27].
Eight new dinapinones, AB1, AB2, AC1, AC2, AD1, AD2, AE1 and AE2 (76-83) were obtained from the culture broth of T. pinophilus FKI-3864. All these dinapinones possessed the same biaryl dihydronaphthopyranone skeleton consisting of a heterodimer with one monapinone A and one different monapinone. The effect of dinapinones was evaluated on the synthesis of [ 14 C] triacylglycerol (TG) and [ 14 C] cholesterol ester from [ 14 C] oleic acid in CHO-K1 cells and the results indicated that dinapinone (77) showed potent inhibition of TG synthesis in intact mammalian cells with an IC 50 value of 1.17 lM, whereas the other dinapinones showed weak inhibition of TG synthesis [28].
A coumarin 90 was obtained from the organic extracts of the soil fungus T. flavus [30]. Two new coumarins, talacoumarins A (91) and B (92), were isolated from the ethyl acetate extract of the wetland soil-derived fungus T. flavus BYD07-13. They were evaluated for anti-Ab42 aggregation, cytotoxic, and antimicrobial activities and the results showed that 91 and 92 had moderate anti-Ab42 aggregation activity, and this was the first report on the Ab42 inhibitory aggregation activity of coumarins [31].
Merodrimanes, thailandolides A (99) and B (100), a drimane linked through a tertiary oxygen to the dihydroisocoumarin, were isolated from a previously undescribed fungus T. thailandiasis [12]. A new meroterpenoid, chrodrimanin C (101) together with chrodrimanins A and B (102 and 103) from the strain YO-2 of Talaromyces sp. Chrodrimanin B exhibited insecticidal activity with an LD 50 value of 10 lg/g of diet, while chrodrimanins A and C were inactive [32]. Four new meroterpenoids, named chrodrimanin D-G (104-107), and a known compound chrodrimanin H (108) were also isolated from the strain YO-2 of Talaromyces sp. Chrodrimanins D, E and F (104-106) showed insecticidal activity against silkworms with respective LD 50 values of 20, 10 and 50 lg/g of diet [33].
A phthalide derivative 109 and a spiro-phthalide derivative 110 were obtained from the organic extracts and from the water extracts of the soil fungus T. flavus [30,34]. Another phthalide compound FKI-0076 B, vermistatin 111, was obtained from Talaromyces sp. during the screening programme for synergist of azoles antifungal antibiotics [20]. 111 was also isolated from the extract of T. flavus IFM52668 [16], and from the culture broth T. flavus FKI-0076 which isolated from a soil sample [21]. Other two analogues penisimplicissin (112) and hydroxydihydrovermistatin (113) were isolated from the fungus T. thailandiasis [12].
Three new phthalide derivatives, talaromycolides A-C (114-116), and a known compound rubralide C (117), were isolated from the methanolic extracts of the green Chinese onion-derived fungus T. pinophilus AF-02. Talaromycolides A-C are rare phthalide derivatives with a novel linkage position between the phenyl and phthalide moieties, and exhibited significant antibacterial activity in response to some of the tested strains, Bacillus subtilis, B. megaterium, Escherichia coli, Clostridium perfringens, Micrococcus tetragenus, and no activity against the strain of MRSA (methicillin-resistant Staphylococcus aureus) [7].
A six-membered ring lactone (118) was isolated from the water extracts of the soil fungus T. flavus [34]. Two lactones (119 and 120) were isolated from an endophytic fungus, a close relative of Talaromyces sp., found in association with Cedrus deodara. They displayed a range of cytotoxicities against human cancer cell lines (HCT-116, A-549, HEP-1, THP-1, and PC-3), and induced apoptosis in HL-60 cells, as evidenced by fluorescence and scanning electron microscopy studies [35]. In the course of screening for apoptosis inducers in ras dependent Ba/F3-V12 cells, a new active compound, rasfonin (121) was isolated from the fermented mycelium of Talaromyces sp. 3656-A1. The cytotoxic activity indicated that rasfonin induced cell death in Ba/F3-V12 cells in an IL-3-free medium containing Dex (2 9 10 -7 M) with an IC 50 of 0.16 lg/mL and no cell death was observed in the presence of IL-3 at concentrations less than 1.25 lg/mL of rasfonin (IC 50 1.8 lg/mL) [36].

Polyketides
Polyketides, pyrones, xanthones, are both a major focus of many research efforts and a rich source of novel metabolites of Talaromyces.
Hydroxymethylmaltol (132) was isolated from the water extracts of the soil fungus T. flavus [34]. Funicone (133) and a new funicone derivative, 9,14-epoxy-11-deoxyfunicone (134), were isolated from the strain T. flaus IFM52668. As the results of the antifungal assay showed that 133 had the characteristic inhibition against a human pathogenic filamentous fungus, A. fumigates (11-mm inhibition zone at 100 lg/disc), whereas 134 showed the weak antifungal activity against A. niger (10-mm inhibition zone at 200 lg/disc) [16]. Deoxyfunicone (135) and actofunicone (136) were obtained from the culture broth T. flavus FKI-0076 which isolated from a soil sample. 135 and 136 showed no effect on the growth of Candida albicans up to 300 lM, and a slight inhibition (35 %) was observed at that concentration for NG-012. But in the absence of the funicones, the IC 50 value of miconazole against C. albicans was calculated to be 19 lM, however, in combination with the funicones (50 lM), the IC 50 values were decreased to 1.6-3.7 lM, demonstrating that they reinforced the inhibition C. albicans activity of miconazole [20,21].
Abenzopyrone derivate 137 was isolated from the organic extracts of the soil fungus T. flavus [30]. Benzopyrone derivatives 138 and 139 were isolated from a culture broth of a fungus, Talaromyces sp. 138 exhibited the weak anti-HBV activity with an IC 50 value of 72.4 lM [42].
Two xanthones, norlichexanthone (140) and secalonic acid A (141), were obtained from the extract of the mangrove endophytic fungus Talaromyces sp. ZH-154 which was isolated from the stem bark of Kandelia candel (L.) Druce, Rhizophoraceae. 141 exhibited high activities against six selected strains.
Moreover, in vitro cytotoxic activities indicated that 141 displayed very strong cytotoxicity against KB and KBv200 cell lines with IC 50 values of 0.63 and 1.05 lg/mL, closed to those of the positive control (0.56 and 0.78 lg/mL). Whereas, the xanthone dimer 141 showed higher bioactivity than the xanthone monomer 140 [43].
Kasanosins A (151) and B (152), novel azaphilones, were isolated from cultures of Talaromyces sp. derived from the seaweed. 151 and 152 selectively inhibited the activities of eukaryotic DNA polymerases b and k (pols b and k) in family X of pols, and 151 was a stronger inhibitor than 152, and the IC 50 values of 151 on rat pol b and human polk were 27.3 and 35.0 lM, respectively. And the results also suggested that 151 and 152 could identify the inhibition between pols b, k, and terminal deoxynucleotidyl transferase (TdT) in family X [47]. Kasanosin C (153) and entonaemin A (154) were isolated from the solid fermentation of Talaromyces sp. T1BF derived from the old bast tissue of Taxus yunnanensis [48]. A known polyketide (155) was isolated from the strain T. wortmanii [49]. Deacetylisowortmin (156) was isolated from the endophytic fungus T. wortmannii LGT-4 [50].
162 inhibited DNA amplification by polymerase chain reaction (PCR) with Thermus thermophilus DNA polymerase and the IC 50 value was 2.6 lg/mL. 162 dosedependently inhibited the telomerase activity of MT1 (human leukemia) and almost completely inhibited the activity at 50 lM. But 162 showed no antimicrobial activity [53].
Two bisdihydroanthracenone atropodiastereomeric pairs, homodimeric flavomannin A (173) and flavomannin B (174), two new unsymmetrical dimers 175 and 176, and two new mixed dihydroanthracenone/anthraquinone dimers 177 and 178, were isolated from T. wortmannii, an endophyte of Aloe vera. The compounds exhibited antibacterial activity, including (multi) drugresistant clinical isolates and compounds 173-178 were predominantly active against Staphylococci, with MIC values from 4 to 8 lg/mL. Reporter gene analyses indicated induction of the SOS response for some of the derivatives, suggesting interference with DNA structure or metabolism. But the compounds showed no cytotoxic activity, encouraging their further evaluation as potential starting points for antibacterial drug development [56].
Two new tricyclic polyketides, vanitaracin A (179) and B (180), were isolated from a culture broth of a fungus, Talaromyces sp. 179 and 180 were evaluated for anti-HBV activity using HBV-susceptible HepG2-hNTCP-C4 cells and 179 exhibited the strong anti-HBV activity with an IC 50 value of 10.5 lM [42]. Stemphyperylenol (181) was isolated from the extract of the mangrove endophytic fungus Talaromyces sp. ZH-154, and showed inhibitory activity against Sarcina ventriculi with a MIC value of3.12 lg/mL, lower than that of ampicillin (12.5 lg/mL) [43].

Steroids and Terpenoids
A steroid 182 was isolated from the genus of Talaromyces sp. T1BF for the first time which isolated from an endophyte from Taxus yunnanensis by chromatography techniques [57]. A new natural product 3-acetyl ergosterol 5,8endoperoxide (183) was isolated from the culture of the marine-sponge associated fungus T. trachyspermus (KUFA 0021) [40]. Secovironolide (184) was purified from the culture broth of T. wortmanni and is the first example of a furanosteroid scaffold bearing a five-membered B ring.
A new nardosinane-type sesquiterpene, talaflavuterpenoid A (191), was isolated from the wetland soil-derived fungus T.

Others
(-)-Epoformin(197) and (1S*,3R*,5R*)-3-methyl-2-oxabicyclo[3.3.1]nonan-7-one (198) were isolated from an endophytic fungus Talaromyces sp., found in association with Cedrus deodara. The sulforhodamine B cytotoxicity assay indicated that 197 was found to be the most active followed by compound 198 [35]. Four new spiroketaltalaromycins (199-202) had been isolated from the strain T. stipitatus [60]. A new metabolite, trachyspic acid (203) that inhibited heparanase, was isolated from the culture broth of T. trachyspermus SANK 12191. Its structure was determined from NMR spectral analyses and chemical reactions as a tricarboxylic acid derivative containing a spiroketal. The IC 50 value of trachyspic acid against heparanase was 36 lM [61]. A novel benzene derivative (204) was isolated from a culture broth of a fungus, Talaromyces sp., and it was evaluated for anti-HBV activity using HBV-susceptible HepG2-hNTCP-C4 cells, but 204 exhibited the weak anti-HBV activity [42]. 5-Hydroxymethylfurfural (205) and two benzene derivatives 206 and 207 were isolated from the organic extracts of the soil fungus T. flavus [30]. 207 was also evaluated for its ability to inhibit HIV-1 integrase in coupled and strand-transfer assays and the data indicated that 207 with IC 50 values of 19 lM in the coupled assay and 25 lM in the strand-transfer assay [62]. Two benzene derivatives 208 and 209 from the genus of Talaromyces sp. T1BF which isolated from an endophyte from Taxus yunnanensis by chromatography techniques [57].
Three diphenyl ether derivatives including two new natural products, tenelates A (210) and B (211), together with the known compound, tenellic acid C (212), were isolated from the mangrove endophytic fungus Talaromyces sp. , from the South China Sea [63]. Three new derivatives of p-hydroxybenzoic acid (213-215) had been isolated from the culture filtrate of T. derxii [64].
A new antibiotic, fosfonochlorin (221), was found in the culture filtrate of four strains of fungi freshly isolated from soil samples including T. flavus. The biological activity indicated that it was active against Proteus mirabilis and P. vulgaris and weakly active against Salmonella enteritidis, Klebsiella pneumoniae and Providencia rettgeri, and its synergistic effect with glucose-6-phosphate was observed on Staphylococcus aureus and Escherichia coli [65].
A new antifungal antibiotic, named talaron, had been isolated from the culture of T. vermiculatus (M-3224). Talaron is water-soluble acidic polysaccharide containing nitrogen and phosphorus, and its molecular weight was estimated to be 7000-8000. Talaron had strong fungicidal activity against filamentous dermatophytes and exhibited inhibitory activity against the spore germination of Trichophyto asteroides and showed cytotoxic effect at 1 mcg/ mL on HeLa cells, and at 0.2 mcg/mL on mouse embryo fibroblast cells, but no antibacterial activity [66].

Conclusions
The Talaromyces genus includes many species with a variety of uses, some of which are important in the food products and agriculture. Since, several anthraquinone metabolites from T. avellaneus were isolated in 1965 [54], lots of secondary metabolites described in this report were obtained from this genus fungi which from a soil sample, from the plant, or from a marine sponge. The 221 compounds, including 43 alkaloids and peptides, 88 esters, 31 polyketides, 19 quinones, 15 steroid and terpenoids, and 25 other structure compounds, described in this review were isolated from 28 species, which 19 species have been determined and 9 species were not given the specific names ( Table 1). The secondary metabolite studies were mainly performed on the commonest species of the genus, T. flavus [3]. The stereochemistry of many compounds was determined via circular dichroism spectrum [7], Mosher's analysis method [8], Marfey's method [13], a single-crystal X-ray diffraction experiment using Cu Ka radiation [59], or quantum chemical calculation [6]. Those fungi were cultivated with varying media: potato dextrose, barley grains [10], rice [25], WSP30 [26], ISP 2 broth [44], or other modified medium.
In the early years of secondary metabolite of those genus species research was less emphasis on biological testing, but increasingly there has been a focus on the biological properties of these compounds. Inhibitory activity to tumour cells [17], bacteria [7], fungi [10], HBV [42], nematode [8], HIV-1-integrase [62], caspase-3 [11], mosquito larval [14], 5-lipoxygenase [27], and other activities were performed. Some of the isolated compounds have been used as pigments.
Studies on total synthesis and biotransformation of some of those compounds have been described. Structure-activity relationships have also been undertaken. Recently, The Bioactive Secondary Metabolites from Talaromyces species  21 there has been great interest in the study of biosynthesis genes based on secondary metabolites from the genus. However, systematic secondary metabolites-biosynthesis genes relationship might give insight into the molecular level, seem to be absent. This might be a promising direction in which work in the field of the secondary constituents from this genus fungi may proceed.

Compliance with ethical standards
Conflict of interest The authors declare no conflict of interest.