Fungal Depsides Naturally Inspiring Molecules: Comparison
Please note this is a comparison between versions V2 by Lindsay Dong and V3 by Lindsay Dong.

Depsides are a group of polyketides consisting of two or more ester-linked hydroxybenzoic acid moieties. They possess valuable bioactive properties, such as anticancer, antidiabetic, antibacterial, antiviral, anti-inflammatory, antifungal, antifouling, and antioxidant qualities, as well as various human enzyme-inhibitory activities.

  • fungi
  • depsides
  • biosynthesis
  • spectral data
  • biological activities

1. Introduction

Depsides are simple polyketides that are formed by the condensation of two or more hydroxybenzoic acid moieties via ester linkage; the COOH group of one molecule is esterified with a phenolic OH group of the second molecule. They could be β-orcinol (β-orsellinic acid) or orcinol (orsellinic acid) derivatives, relying on the existence of the C3 methyl group on both rings (Figure 1). The ring with an ester-carbonyl is referred to as ring A and the other as ring B. Their major structural variations are the attached alkyl chains’ length, the degree of chain oxidation, and the degree of methylation of OH and COOH groups [1][18]. The OH groups usually exist at the aromatic carbons, C-3′/C-4′/C-2 or C-4, and other oxygenated substituents are usually connected to the skeleton, such as carboxyl and methoxyl substituents [2][19].

Figure 1. Basic structures of orcinol paradepsides, orcinol metadepsides, β-orcinol paradepsides, β-orcinol metadepsides, and mixed orcinol β-orcinol depsides.
Depsides are common lichen metabolites [3][4][5][20,21,22]. However, they have also been reported in some higher plants and fungi [2][6][7][8][9][10][19,23,24,25,26,27]. Contrary to the lichen depsides, fungal depsides are not widely distributed and are isolated only from a restricted number of fungi. It was reported that depsides possess remarkable bioactivities: anticancer, anti-diabetic, antibacterial, antiviral, anti-inflammatory, antifungal, antifouling, antioxidant, and various enzyme inhibitory activities.

2. Biosynthesis of Depsides

Depsides are acetyl-poly-malonyl-derived polyketides that are biosynthesized by polyketide synthase (PKS) [11][12][13][14][15][16][17][28,29,30,31,32,33,34]. PKS is composed of a minimal set of KS (ketosynthase), AT (acyltransferase), and ACP (acyl carrier protein) domains [13][30]. The non-reduced framework of the depside rings reveals that its corresponding PKS belongs to non-reducing PKSs (NR-PKSs). Depsides consist of two orsellinic acid molecules, connected by an ester linkage. Therefore, orsellinic acid can be considered the constructing unit of all depsides [11][28]. Biosynthetically, orsellinic acid is produced from a linear tetraketide chain. This chain is formed through an acetate-malonate pathway that is catalyzed by PKSs [12][13][29,30]. The tetraketide chain forming β-orsellinic acid (methyl-3-orsellinate) is produced by introducing a CH3 group obtained from SAM (S-adenosylmethionine) by the methyltransferase (CMeT) domain of the corresponding PKS [14][31]. Then, the non-enzymatic 2,7-aldol condensation of these chains produces orsellinic and β-orsellinic acids. Furthermore, the molecular skeleton is probably designed by post-biosynthetic tailoring enzymes, such as cyclases and hydrolases [1][18]. p-Depsides are produced by the condensation of either orsellinic acid and orcinol derivatives or by two methyl-3-orsellinate or orsellinate moieties, through the formation of an ester [18][12]. The consequent condensation of an additional unit produces a tri-depside, and two moieties yield a tetra-depside [15][32]. Moreover, depsides containing alkyl side-chains can be produced by the reduction of the terminal ketone groups, resulting in the required saturated alkyl moieties. m-Depsides are formed through the hydroxylation of the para-depside B-ring, subsequently followed by rearrangement [16][33] (Figure 2).
Figure 2. Biosynthesis of depsides [1][11][12][13][14][15][16][18,28,29,30,31,32,33].

3. Biological Activities

3.1. Antitumor Activity

Lünne et al. [19][37] evaluated the antitumor effect of lecanoric acid (1) and ethyl lecanorate (2) purified from Claviceps purpurea on HepG2 (human liver cancer cells) and CCF-STTG1 (human astrocytoma cells) using the CTC (5-cyano-2,3-bis(4-methylphenyl)-2H-tetrazolium chloride) assay. Both metabolites produced a dose-dependent antitumor effect on the tested cell lines. They reduced the CCF-STTG1 cell viability down to ~60%, at a concentration of 40 μM, and HepG2 cell viability by ~30% and 40%, respectively. Similar to HepG2 cells, 2 had the strongest antitumor effect on CCF cells (IC50 value of 54 μM) [19][37]. In the MTT ((3-(4,5-dimethylthiazol-2-yl))-2,5-diphenyl-2H-tetrazolium bromide) assay, aspergisides A (3), B (4), and C (5) showed weak antitumor activity, with IC50 values in the range of 45–114 µM toward Vero, MCF-7, and KB cell lines, compared with doxorubicin [20][38]. MS-3 (22) was inactive against Ehrlich ascites for leukemia and carcinoma in vivo; however, it was active toward Yoshida sarcoma cells (ID5o value of 85 µg/mL) in vitro. Its activity was suggested to be due to a glyoxalase inhibition, as it possessed a glyoxalase inhibitory effect with an ID50 value of 12 µg/mL in the spectrophotometric assay [21][39]. In addition, 23 possessed significant antitumor activity toward A549 and HepG2, with IC50 values of 13.14 and 49.02 µM, respectively, compared to cisplatin (IC50 14.33 and 18.74, respectively) in the MTT assay [22][40]. Compounds 2729 were assayed against NCI-H187, Vero, BC, and KB cells, employing an MTT assay. Compounds 27 and 28 exhibited a significant antitumor effect against BC, with IC50 values of 8.8 and 4.4 µM, respectively, compared to ellipticine (IC50 0.49 µM), while they showed weak to moderate effectiveness toward other cell lines, with IC50 values ranging from 13.0 to 34.3 µM [23][41]. Arenicolins A (30) and B (31), two new depsides having C-glycosyl moiety and dual heptyl side-chains, were isolated from Penicillium arenicola and assessed for antitumor activity at a concentration of 30.0 μM toward IMR-32, HCT-116, and BT-474 cell lines using an ICC (immunocytochemistry) assay. Compound 30 reduced cell viability with IC50 values of 6.0, 7.3, and 9.7 μM, respectively, compared to 5-FU (5-fluorouracil, IC50 6.5 μM for HCT-116 and 5.7 μM for IMR-32). However, 31 did not have a significant antitumor effect toward the tested cell lines at a concentration of > 30 μM [24][42].

3.2. Antimicrobial Activity

Phainuphong et al. purified three new depsides, aspergisides A–C (35) from Aspergillus unguis, and assessed their antimicrobial potential against MRSA (methicillin-resistant S. aureus), S. aureus, C. albicans, flucytosine-resistant C. neoformans, and M. gypseum [20][38]. Compound 3 had a weak antibacterial activity toward S. aureus and MRSA, with an MIC (minimal inhibitory concentrations) value of 8 µg/mL, while 4 and 5 were inactive, with MIC values of 32–200 µg/mL in the agar diffusion method [20][38]. Setophoma sp. associated with guava fruits produced compounds 68 and 6669 [2][19]. They did not have growth inhibition activity toward E. coli. However, 6669 demonstrated inhibition of S. aureus with MIC values of 100, 6.25, 50, and 25 µg/mL, respectively, in comparison to tetracycline (MIC 3.12 µg/mL) [2][19]. Compounds 6 and 7 were inactive. Moreover, all compounds did not exhibit quorum-sensing inhibitory activity. Studying the structural activity relationship revealed that the activity increased with the full methylation of the B-ring; however, the additional CH3 group at ring A, especially at C-2, resulted in a decrease in activity [2][19]. The agonodepsides A (12) and B (13) were isolated from the filamentous fungus, F7524 [25][50]. In the fluorometric InhA assay, 12 inhibited M. tuberculosis InhA with an IC50 value of 75 µM, while 13 was inactive at 100 µM, compared with triclosan (IC50 3.0 µM) [25][50].

3.3. Antifouling Activity

The anti-larval settlement activities of 15, 48, 56, 58, 59, and 7079 were assessed towards cyprid larvae of B. amphitrite [26][52]. Compounds 15, 48, 56, 7173, and 7678 deterred larval settlement, with EC50s ranging from 2.95 to 69.19 µM in comparison to butenolide (EC50 4.62 µM). At a concentration of 10 µM, 15, 7173, and 77 exhibited narcotic potential toward B. amphitrite cyprids. They caused the loss of the phototactic response of cyprids, in addition to decreasing the appendage activity and cyprids becoming completely immobilized. The recovery rates of cyprids treated with 15, 7173, and 77 (concentration of 10 µM) revealed that larvae possessed the highest recovery rate after treatment with 71, while no larvae recovered after treatment with 15 for 24 h. From all tested compounds, 71 had an excellent antifouling potential and cyprids treated with it had the highest recovery rate. Thus, 71, 72, and 77 were reversible inhibitors. Conversely, 5859, 74, and 75 had no effect [26][52].

3.4. Anti-Diabetic Activity

Rivera-Chávez et al. reported that the tridepside, 59 (dose 3.1–31.6 mg/kg), reduced glucose blood levels after 30 min of oral administration of the sucrose load in mice (3.0 g/kg); however, only the highest dose (31.6 mg/kg) caused a marked reduction in blood glucose levels in NA-STZ (nicotinamide-streptozotocin) diabetic mice, indicating that 59 (doses of 3.1 and 10 mg/kg) reduced the blood glucose levels in both diabetic and normal mice [27][67].

3.5. D-Glucose-6-Phosphate Phosphohydrolase Inhibitory Activity

G6Pase (D-glucose-6-phosphate phosphohydrolase) is a hepatic metabolism-regulating enzyme, that catalyzes the last steps of glycogenolysis and gluconeogenesis pathways [28][93]. Its inhibition decreases the output of hepatic glucose from both pathways, leading to lowering the blood glucose levels in diabetes. The tetra-depside, 97 isolated from Chloridium sp. CL48903 prohibited G6Pase in rat liver microsomes (IC50 1.6 µM) at a concentration of 133 µM, using a colorimetric assay and hepatocyte glucose output (81% inhibition), indicating the role of 97 as a G6Pase inhibitor [29][77].

3.6. α-Glucosidase Inhibitory (αGI) Activity

The α-glucosidase enzyme is an important therapeutic target for treating carbohydrate-mediated diseases. It catalyzes the breakdown of oligo- and disaccharides into monosaccharides in the final stage of carbohydrate digestion, leading to a rise in glucose levels [30][31][32][33][94,95,96,97]. Several studies revealed that α-glucosidase inhibitors (αGIs) slow down the digestion and absorption of carbohydrates, and thus reduce the postprandial blood glucose level [30][31][32][33][94,95,96,97]. The serious side effects of the current αGIs, such as liver injuries and gastrointestinal damage, have directed research efforts toward discovering and developing new and safer anti-diabetic agents.

3.7. Protein Tyrosine Phosphatase Inhibitory (PTP1BI) Activity

PTP1B (protein-tyrosine phosphatase 1B) is a negative regulator of the insulin signaling pathway. The inhibition of PTP1B activity has great promise for alleviating insulin and leptin resistance; hence, PTP-1BIs (PTP1B inhibitors) show potential for treating T2DM and other metabolic disorders [34][98].
Cosmospora sp. produced aquastatin A (32) (IC50 0.19 µM) that showed modest but selective PTP1BI activity over other PTPs (protein tyrosine phosphatases) such as TCPTP (T-cell protein tyrosine phosphatase) (IC50 0.51 µM), SHP-2 (IC50 > 44 µM), CD45 (IC50 > 44 µM), and LAR (IC50 > 44 µM), compared with ursolic acid (IC50 2.5 µM). It was suggested that the 2,4-dihydroxy-6-pentadecylbenzoic acid moiety is critical for PTP1BI activity [35][60].

3.8. Diacylglycerol Acyltransferase Inhibitory (DGATI) Activity

Postprandial hypertriglyceridemia is considered the main risk factor for cardiovascular functions. Thus, triglyceride synthesis inhibition has remarkable therapeutic potential in metabolic disorder treatment. The enzymes known as diacylglycerol acyltransferases (DGATs) catalyze the final and only committed step in the biosynthesis of triglycerides [36][99]. Therefore, these enzymes could be a potential therapeutic target to combat cardio-metabolic disorders [37][38][36][71,82,99]. Compound 9 also inhibited TG synthesis (IC50 91 µM), as well as PC and PE syntheses, indicating that it had a non-specific DGATI effect [39][76]. The compounds 86 and 8890 were purified from Humicola sp. by Tomoda et al. [40][74]. Compound 88 was the most potent DGATI, with an IC50 of 10.2 µM, followed by 86 (IC50 17.5 µM), 89 (IC50 19.2 µM), and 90 (IC50 51.6 µM). They also inhibited the formation of triacylglycerol using Raji cells on the intact cell assay, with IC50 values ranging from 2.82 to 17.2 µM. At high concentrations, 86 moderately inhibited the formation of phosphatidylethanolamine (PE) and phosphatidylcholine (PC), whereas 89 possessed a weak effect, indicating that 89 specifically suppressed the formation of triacylglycerol (TG) [40][74].

3.9. Activity of 11β-Hydroxysteroid Dehydrogenase Inhibitory (11β-HSDI) Enzyme

High levels of glucocorticoid produce insulin resistance and glucose intolerance, leading to metabolic syndrome (MS) [41][100]. The enzyme 11β-HSD (11β-hydroxysteroid dehydrogenase) is accountable for the production of glucocorticoids in tissues, thus it plays a remarkable role in T2DM and MS. The 11β-HSD1 inhibitors (11β-HSD1Is) could be considered promising therapeutics in treating MS. Compounds 3841 exhibited powerful and selective inhibitory activities against 11β-HSD1 in the HTRF immunoassay. They inhibited human 11β-HSD1 activity in a dose-dependent manner with IC50 values ranging from 240 to 6600 nM. Compounds 38 and 40 were the most active with IC50s 240 and 230 nM, respectively, while they did not prohibit 11β-HSD2 (IC50 > 10,000 nM) [42][64].

3.10. Anti-Inflammatory Activities

Compound 23, biosynthesized by Stereum hirsutum, exhibited noticeable NO inhibitory potential (IC50 19.17 µM) in the LPS-induced macrophages, compared with hydrocortisone (IC50 48.15 µM) [22][40]. Moreover, 48 (ID50 12 µM) and 49 (ID50 9 µM) possessed considerable anti-inflammatory potential for the conversion of 14C-arachidonic acid into PGF2α plus PGE2 by the microsomes of ram seminal vesicles [43][44][69,81]. ID50s of the conversion of arachidonic acid (AA) into PGH2 (prostaglandin H2), PGH2 into (prostaglandin E2), and thromboxane A2 (TXA2) synthetase are 10, 40, 150 µM, respectively, for 48 in comparison to indomethacin (ID50 30 for PGH2 and 130 µM for PGE2) and imidazole (ID50 200 µM for TXA2 synthetase); meanwhile, 49 had ID50 values of 40, 9, and 350 µM, respectively. Compound 48 had a strong inhibitory effect on the conversion of AA into PGH2, while 49 specifically inhibited the step involving PGE2 synthesis from PGH2. Moreover, they inhibited TXA2 synthesis in bovine platelet microsomes (ID50 values of 150 and 350 µM, respectively), which was comparable to imidazole (200 µM) [43][44][69,81]. Both compounds (dose 50 mg/kg, orally) showed no significant anti-inflammatory effects on carrageenan-induced edema in rats. However, 49 caused a 70% inhibition of this edema system at an intravenous (IV) dose of 5 mg/kg, while 48 displayed no activity, even with IV administration [44][81].

3.11. Antimalarial Activity

Malaria is among many prevalent health concerns and is caused by the Plasmodium parasite in several of the world’s tropical regions [45][105]. The emergence of malaria strains that are drug-resistant to the available therapeutics makes the discovery of new antimalarial agents a great scientific challenge [46][47][14,106].
The two new depside galactopyranosides, 28 and 29, and their aglycone 27, isolated from Acremonium sp., were tested against Plasmodium falciparum K1 using a microculture radioisotope technique. Only compound 27 was active towards P. falciparum K1, with an IC50 value of 9.9 µM compared with dihydroartemisinin (IC50 0.0039 µM). However, 28 and 29 had weak effects (IC50 > 10 µM) [23][41].

3.12. Antioxidant Activity

The depsides, 23 and 24 showed weak radicals scavenging capacity with EC50 > 200 µM [22][40]. In the DPPH (2-diphenyl-1-picrylhydrazyl) assay, 11, 45, and 46 also had weak antioxidant activity, compared with ascorbic acid [48][46].

3.13. Ca2+/CaM Dependent Phosphodiesterase Inhibitory (CaM-PDEI) Activity

Nakanishi et al. reported that 34 and 35, purified from Sporothrix sp., inhibited heart and bovine brain PDEs (IC50 4.3 and 1.8 µM and 5.9 and 15.0 µM, respectively) [49][63]. Moreover, they prohibited the CaM-dependent activities of CaM-PDEs but had a low effect against their CaM-independent effects, suggesting that these compounds interacted with CaM to inhibit Ca2+/CaM-dependent enzymes. On the other hand, they had no inhibitory activities on protein kinase C [49][63]. Moreover, PS-990 (47), isolated from Acremonium sp., inhibited brain CaM-PDE with an IC50 value of 3 µg/mL and did not elevate the intracellular cyclic AMP level. It markedly induced the neurite extension of Neuro2A (mouse neuroblastoma) at concentrations ranging from 10 to 30 µg/mL, suggesting its neuritogenic effect. It inhibited both cell growth and thymidine incorporation into the cells at the same concentration range. Interestingly, 47 reversibly induced neurite formation, with cell growth arrest through a mechanism other than increasing the intracellular cyclic AMP concentration [50][51][66,110].

3.14. Antiviral Activity

HCMV (human cytomegalovirus) is the most familiar viral cause of congenital infections, which can lead to severe birth defects. Its current treatments include viral DNA polymerase inhibitors, which block the late stages of HCMV replication; however, they do not prohibit the viral induction of multiple cell activation events [52][111]. Thus, it may be beneficial to discover new treatments for HCMV infections.
Compounds 2729 were assessed against HSV-1 (Herpes simplex virus type 1), using the SBR technique. Only 28 showed potent activity, with an IC50 value of 7.2 µM, compared with acyclovir (IC50 10.2 µM), while 27 and 29 displayed weak activity, with IC50 values of > 1000 and > 50 µM, respectively [23][41]. Cytonaema sp. yielded novel p-tridepsides; 84 and 85 showed in vitro inhibitory activities to hCMV protease, with IC50 values of 43 and 11 µM, respectively, in the scintillation proximity assay 73].

3.15. Human Leukocyte Elastase (HLE) Inhibitory Activity

HLE is one of the most destructive enzymes that can degrade tissue matrix proteins, such as collagen, elastin, fibronectin, proteoglycan, and laminin, by activating progelatinase, procollagenase, and prostromelysin [8][25]. It is released from PMNLs (polymorphonuclear leukocytes) as a result of inflammatory mediators and stimuli. HLE is considered an important therapeutic target for treating many inflammation-linked disorders [53][112].
The depsides, 25 (IC50 45.1 µM) and 26 (IC50 92.6 µM), weakly inhibited HLE in the spectro-photometric immunoassay, while the tridepside, 83 (IC50 1.8 µM), exhibited high HLE inhibitory activity compared to ulinastatin (IC50 1.1 µg/mL), which was 25–50-fold greater than that of depsides [8][25].

3.16. Indoleamine 2,3-Dioxygenase Inhibitory (IDOI) Activity

IDO (indoleamine 2,3-dioxygenase) catalyzes the tryptophan catabolism initial step via the KP (kynurenine pathway) [54][68]. Dysregulation of the KP is accompanied by the IDO activity elevation and production of quinolinic acid (an excitotoxin), which has been engaged in the pathogenesis of neurodegenerative disorders, neuroinflammatory, HIV encephalitis, age-related cataract, and depression [54][68]. Therefore, IDO is a promising target of new therapeutics for treating neurological disorders and cancer, as well as other disorders characterized by a defect in tryptophan metabolism.
Compounds 49, 54, and 65 isolated from Coniochaeta sp., inhibited the activity of IDO with IC50 values of 21.2, 14.5, and, 26 µM, respectively in comparison to menadione (IC50 3.7 µM) [54][68].

3.17. Adenosine Triphosphatase Inhibitory Activity

Na+/K+-ATPase (sodium/potassium adenosine triphosphatase) is an integral membrane protein that is accountable for maintaining Na+ and K+ gradients across the plasma membrane, an important process for mammalian cell survival. Currently, it is extensively studied as a potential target for cancer treatment, especially in glioblastoma and lung cancer [55][113]. The proton pump, H+/K+ ATPase, plays an important role in the stomach acidification process. Its inhibition in gastric parietal cells decreases gastric acid overproduction [56][114]. H+/K+ ATPase inhibitors can be utilized as a target for developing drugs against gastric acid production disturbances.
Aquastatin A (32) was biosynthesized by Fusarium aquaeductuum. It inhibited Na+/K+-ATPase (adenosine triphosphatase) (IC50 7.1 µM) and H+/K+-ATPase (IC50 6.2 µM) [57][59].

3.18. Proteasome Inhibitory Activity

Proteasome comprises one or two 19S RPs (regulatory particles) and 20S CPs (core particles). In humans, the 20S CP formation is assisted by proteasome-specific chaperones: PAC1–PAC4 and POMP (proteasome maturation protein) [58][59][115,116]. Proteasome accounts for misfolded, unneeded, or damaged cellular protein degradation. Therefore, it is a crucial target for the future treatment of various diseases, such as neurodegenerative and autoimmune diseases, cystic fibrosis, cancer, diabetes, and atherosclerosis [58][115]. The compound 105 (IC50 0.020 µM) had a potent PAC3 (proteasome-assembling chaperone 3 homodimer) inhibitory effect, while 103 and 107 (IC50 > 250 µM) did not inhibit the PAC3 homodimer [59][116].

3.19. Phospholipase Inhibitory Activities

Phospholipase A2 (PLA2) catalyzes the hydrolysis of membrane phospholipids into arachidonic acid; therefore, its inhibitors have the potential for treating various inflammatory disorders [60][117]. Compound 107 showed a strong reversible and noncompetitive inhibition of human PLA2-II (Ki value of 0.098 μM, IC50 value of 0.076 μM); however, it showed weak inhibition of human PLA2-I (IC50 of 18 μM). Its inhibitory effect toward PLA2-II human and PLA2 Naja mocambique was noticeably reduced by methylation of the two COOH groups. Furthermore, 107, upon co-injection with carrageenan, remarkably reduced PLA2 activity and exudate volume in the carrageenan-induced pleurisy rat model [61][80].

4. Conclusions

Most of the reported depsides have been evaluated for their α-GI (α-glucosidase inhibitory), antimicrobial, antitumor, antifouling, PLA2 (phospholipase A2), and DGATI (diacylglycerol acyltransferase inhibitory) abilities (Figure 3). Thus, these studies revealed that fungal depsides are a rich source for the discovery of effective and novel pharmaceutical leads and should be further exploited.
They also demonstrate inhibitory activities against various enzymes that can be utilized as targets for the treatment of various diseases. These metabolites could have potential as lead compounds for treating metabolic syndrome, obesity, and diabetes via the inhibition of various enzymes, such as HSD, PTP1BI, α-GI, G6Pase, and DGAT. However, extensive explorations of their mechanism of action, as well as structure modification, chemical synthesis, and structure/activity relationship analysis are needed. Despite the extensive structural diversity of depsides, none of them has been approved by the FDA, and none of them has as yet progressed to clinical trials. Therefore, the impact of fungal depsides on human health concerns has to be considered in several ways.
Figure 3. Biological activities of isolated depsides and the number of articles.