Lichenochemicals: extraction, purification, characterization, and application as potential anticancer agents

Tridepsides as potential bioactives: a review on their chemistry and the global distribution of their lichenic and non-lichenic natural sources

Tridepsides, as fully oxidized polyketides, have been known to exist in lichens for more than a century. Recent studies have showed that these possible defensive lichenochemicals possess various biological activities. Also, a candidate biosynthetic gene cluster was recently reported for gyrophoric acid (GA), an important tridepside. The present study focused on biosynthesis, natural sources, biological activities, and bioanalytical methods of tridepside molecules. Our survey shows that, so far, lichenic tridepsides have been reported from 37 families, 111 genera, and 526 species of lichen. Because many of their species contain tridepsides, the families Parmeliaceae, Lobariaceae, and Peltigeraceae can be considered critical lichenic sources of tridepsides. Furthermore, several species of Hypotrachyna in Parmeliaceae family showed lichenic tridepsides, suggesting that this genus is a viable source of tridepsides. This research also explored tridepsides from non-lichenic sources, such as non-lichenized fungi, lichenicolous fungi, endophytes, parasites, and liverworts, which offer substantial potential as biotechnological sources to produce tridepsides, which are produced in small amounts in lichen thalli. Two lichenic tridepsides have also been detected in non-lichenic sources: GA and tenuiorin (TE). Additionally, no significant correlation was found between tridepside biosynthesis and geographical distribution patterns for several potentially tridepside-producing lichens. We further showed that GA is the most studied tridepside with various reported biological activities, including anticancer, wound healing, photoprotection, anti-aging, antioxidant, cardiovascular effect, DNA interaction, anti-diabetes, anti-Alzheimer's, anti-bacterial, and antifungal. Last but not least, this study provides an overview of some bioanalytical methods used to analyze tridepsides over the past few years.

ROS-Induced DNA-Damage and Autophagy in Oral Squamous Cell Carcinoma by Usnea barbata Oil Extract-An In Vitro Study

Oxidative stress is associated with aging, cancers, and numerous metabolic and chronic disorders, and phenolic compounds are well known for their health-promoting role due to their free-radical scavenging activity. These phytochemicals could also exhibit pro-oxidant effects. Due to its bioactive phenolic secondary metabolites, Usnea barbata (L.) Weber ex. F.H. Wigg (U. barbata) displays anticancer and antioxidant activities and has been used as a phytomedicine for thousands of years. The present work aims to analyze the properties of U. barbata extract in canola oil (UBO). The UBO cytotoxicity on oral squamous cell carcinoma (OSCC) CLS-354 cell line and blood cell cultures was explored through complex flow cytometry analyses regarding apoptosis, reactive oxygen species (ROS) levels, the enzymatic activity of caspase 3/7, cell cycle, nuclear shrinkage (NS), autophagy (A), and synthesis of deoxyribonucleic acid (DNA). All these studies were concomitantly performed on canola oil (CNO) to evidence the interaction of lichen metabolites with the constituents of this green solvent used for extraction. The obtained data evidenced that UBO inhibited CLS-354 oral cancer cell proliferation through ROS generation (316.67 × 10⁴), determining higher levels of nuclear shrinkage (40.12%), cell cycle arrest in G0/G1 (92.51%; G0 is the differentiation phase, while during G1 phase occurs preparation for cell division), DNA fragmentation (2.97%), and autophagy (62.98%) than in blood cells. At a substantially higher ROS level in blood cells (5250.00 × 10⁴), the processes that lead to cell death-NS (30.05%), cell cycle arrest in G0/G1 (86.30%), DNA fragmentation (0.72%), and autophagy (39.37%)-are considerably lower than in CLS-354 oral cancer cells. Our work reveals the ROS-mediated anticancer potential of UBO through DNA damage and autophagy. Moreover, the present study suggests that UBO pharmacological potential could result from the synergism between lichen secondary metabolites and canola oil phytoconstituents.

Isolation, Characterization, and Breast Cancer Cytotoxic Activity of Gyrophoric Acid from the Lichen Umbilicaria muhlenbergii

Lichens produce a large variety of secondary metabolites with diverse bioactivities, chemical structures, and physicochemical properties. For this reason, there is a growing interest in the use of lichen-derived bioactive molecules for drug discovery and development. Here, we report on the isolation, identification, and cytotoxic evaluation of gyrophoric acid (GA) from the lichen Umbilicaria muhlenbergii, a largely unexplored and scantly described lichen species. A simple purification protocol was developed for the fractionation of lichen crude extracts with silica gel column chromatography using solvents with changing polarity. GA was identified in one of the fractions with Fourier transform infrared spectroscopy (FTIR), ion trap mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (1H-NMR and 13C-NMR). The FTIR spectra demonstrated the presence of aromatic and ester functional groups C=C, C-H, and C=O bonds, with the most remarkable signals recorded at 1400 cm−1 for the aromatic region, at 1400 cm−1 for the CH3 groups, and at 1650 cm−1 for the carbonyl groups in GA. The MS spectra showed a molecular ion [M-1]− at (m/z) 467 with a molecular weight of 468.4 and the molecular formula C24H20O10. that correspond to GA. The 1H-NMR and 13C-NMR spectra verified the chemical shifts that are typical for GA. GA reduced the cell viability of breast cancer cells from the MCF-7 cell line by 98%, which is indicative of the strong cytotoxic properties of GA and its significant potential to serve as a potent anticancer drug.

Physodic acid sensitizes LNCaP prostate cancer cells to TRAIL-induced apoptosis

In spite of the extensive research for developing new therapies, prostate cancer is still one of the major human diseases with poor prognosis and high mortality. Therefore, with the aim of identifying novel agents with antigrowth and pro-apoptotic activity on prostate cancer cells, in the present study, we evaluated the effect of lichen secondary metabolite physodic acid on cell growth in human prostate cancer cells. In addition, we tested the apoptotic activity of physodic acid on TRAIL-resistant LNCaP cells in combination with TRAIL. The cell viability was measured using MTT assay. LDH release, a marker of membrane breakdown, was also measured. For the detection of apoptosis, the evaluation of DNA fragmentation and caspase-3 activity assay were employed. The expression of proteins was detected by Western blot analysis. It was observed that physodic acid showed a dose-response relationship in the range of 12.5-50 μM concentrations in LNCaP and DU-145 cells, activating an apoptotic process. In addition, physodic acid sensitizes LNCaP cells to TRAIL-induced apoptosis. The combination of physodic acid with other anti-prostate cancer therapies could be considered a promising strategy that warrants further investigations.

Pro-Apoptotic Potential of Pseudevernia furfuracea (L.) Zopf Extract and Isolated Physodic Acid in Acute Lymphoblastic Leukemia Model In Vitro

Acute lymphoblastic leukemia (ALL) is the most frequently diagnosed type of leukemia among children. Although chemotherapy is a common treatment for cancer, it has a wide range of serious side effects, including myelo- and immunosuppression, hepatotoxicity and neurotoxicity. Combination therapies using natural substances are widely recommended to attenuate the adverse effects of chemotherapy. The aim of the present study was to investigate the anti-leukemic potential of extract from the lichen Pseudevernia furfuracea (L.) Zopf (PSE) and isolated physodic acid (Phy) in an in vitro ALL model. A screening assay, flow cytometry and Western blotting were used to analyze apoptosis occurrence, oxidative stress, DNA damage and stress/survival/apoptotic pathway modulation induced by the tested substances in Jurkat cells. We demonstrate for the first time that PSE and Phy treatment-induced intrinsic caspase-dependent cell death was associated with increased oxidative stress, DNA damage and cell cycle arrest with the activation of cell cycle checkpoint proteins p53, p21 and p27 and stress/survival kinases p38 MAPK, JNK and PI3K/Akt. Moreover, using peripheral T lymphocytes, we confirmed that PSE and Phy treatment caused minimal cytotoxicity in normal cells, and therefore, these naturally occurring lichen secondary metabolites could be promising substances for ALL therapy.

Lichens as a repository of bioactive compounds: an open window for green therapy against diverse cancers

Lichens, algae and fungi-based symbiotic associations, are sources of many important secondary metabolites, such as antibiotics, anti-inflammatory, antioxidants, and anticancer agents. Wide range of experiments based on in vivo and in vitro studies revealed that lichens are a rich treasure of anti-cancer compounds. Lichen extracts and isolated lichen compounds can interact with all biological entities currently identified to be responsible for tumor development. The critical ways to control the cancer development include induction of cell cycle arrests, blocking communication of growth factors, activation of anti-tumor immunity, inhibition of tumor-friendly inflammation, inhibition of tumor metastasis, and suppressing chromosome dysfunction. Also, lichen-based compounds induce the killing of cells by the process of apoptosis, autophagy, and necrosis, that inturn positively modulates metabolic networks of cells against uncontrolled cell division. Many lichen-based compounds have proven to possess potential anti-cancer activity against a wide range of cancer cells, either alone or in conjunction with other anti-cancer compounds. This review primarily emphasizes on an updated account of the repository of secondary metabolites reported in lichens. Besides, we discuss the anti-cancer potential and possible mechanism of the most frequently reported secondary metabolites derived from lichens.

A Review of Anti-Cancer and Related Properties of Lichen-Extracts and Metabolites

BACKGROUND

Lichens are a composite consortium of fungus and alga. The symbiotic organisms are naturally equipped with distinct characteristics as compared to constituting organisms separately. Lichens due to their peculiar anatomy and physiology, are the reservoir of more than 600 unique secondary metabolites, also known as 'lichen substances'. Since ancient times, many ethnic groups from various parts of the world had knowledge about the applications of lichens as major provenance of food/fodder, medicine, dyes, spices, perfumes, etc. Lichen substances have shown impressive antioxidant, antimicrobial, antiviral, antitumor, and anti-inflammatory activities under experimental conditions. Usnic acid, a well-known metabolite, found in several species of lichens, possesses potent antioxidant and anti-inflammatory activities. It also has significant anti-proliferative potential as revealed through testing in different cancer cell lines. Atranorin, Lecanoric acid, Norstictic acid, Lobaric acid, Stictic acid, Ramalin, Gyrophoric acid, Salazinic acid, Protolichesterinic, and Fumarprotocetraric acid are some of the other purified lichen metabolites with potent anti-cancer activities.

OBJECTIVE

This study presents an overview of lichen derived extracts/compounds augmenting the anti-cancer (related) properties.

METHOD

The review comprehends different studies (in vivo and in vitro) backing up the possibility of lichen extracts and metabolites towards their use as antioxidant, anti-proliferative, anti-inflammatory and EMT-inhibiting agents.

RESULTS

The review focuses on anti-cancer and related properties of lichen extracts and metabolites that include their anti-oxidative, anti-inflammatory, anti-proliferative and pro-apoptotic, cancer stemness reduction, activities and, the potential of inhibition of cancer-associated Epithelial-mesenchymal transition (EMT) that is responsible for multiple drug-resistance and metastasis of cancer cells in a large proportion of cases.

CONCLUSION

Lichens can be the repertoire of a plethora of lichen metabolites with putative bioactive potential, which is needed to be explored in order to find out novel anti-cancer drugs.