The aim of this study was to determine the total protein amounts, protein profiles, in vitro cytotoxicities, antimicrobial activities and hemolytic effects of skin secretions of the Lissotriton vulgaris and Triturus ivanbureschi.
Skin secretions were obtained, clarified, supernatants snap-frozen then lyophilized. Total protein amounts were determined by BCA assay kit. Protein profiles were revealed by the SDS-PAGE. The cytotoxicity and antimicrobial activity were determined by using MTT assay and minimum inhibitory concentration (MIC) method. Hemolytic effects were measured on rabbit red blood cells.
Lissotriton vulgaris and T. ivanbureschi skin secretions have totally 18 and 20 protein fractions. IC50 values were detected between 1.40 and 40.28 μg/mL. The MIC results were found between 7.8 and 250 μg/mL. Lissotriton vulgaris skin secretion showed low hemolytic effect while T. ivanbureschi skin secretion showed high hemolytic effect.
This study is the first report showing the potential of L. vulgaris and T. ivanbureschi skin secretions for cytotoxicity, antimicrobial and hemolytic activity as an alternative therapeutic approach for traditional uses. Further studies need to focus on purification of the active components from these skin secretions and mode of action on cancer cell lines and microorganisms as anti-agents.
Bu çalışmanın amacı, Lissotriton vulgaris ve Triturus ivanbureschi deri salgılarının total protein miktarlarını, protein profillerini, in vitro sitotoksisitelerini, antimikrobiyal aktivitelerini ve hemolitik etkilerini belirlemektir.
Ham deri salgıları, sağıldı, arındırıldı, supernatant kısımlar donduruldu ardından liyofilize edildi. Deri salgılarının total protein miktarları BCA kiti ile belirlendi. Protein profilleri SDS-PAGE ile ortaya koyuldu. Sitotoksik ve antimikrobiyal etkiler MTT testi ve MIC metodu kullanılarak belirlendi. Hemolitik etkiler, tavşan kırmızı kan hücreleri üzerinde hesaplandı.
Lissotriton vulgaris deri salgısında 18, T. ivanbureschi deri salgısında 20 protein fraksiyonu bulunmaktadır. IC50 değerleri 1.40–40.28 μg/mL arasında tespit edildi. MIC sonuçları 7.8–250 μg/mL arasında bulundu. Lissotriton vulgaris deri salgısı tavşan kırmızı kan hücreleri üzerinde düşük hemolitik etki gösterirken, T. ivanbureschi deri salgısı yüksek hemolitik etki gösterdi.
Bu çalışma, L. vulgaris ve T. ivanbureschi deri salgılarının geleneksel kullanımdaki alternatif terapötik potansiyellerini sitotoksisite, antimikrobiyal ve hemolitik aktivite bakımından ortaya koyan ilk rapor niteliğindedir. Gelecek çalışmalarda deri salgılarının anti-ajanlar olarak kullanılabilmesi için içeriğindeki aktif bileşenlerin saptanması, hücre hatları ve mikroorganizmalar üzerindeki etki mekanizmasının belirlenmesine odaklanılmalıdır.
Skin secretions from many species of amphibians contain a wide range of compounds with biological activity in very high concentration that has excited interest because of their potential for novel drug development . Also researches have shown that skin secretions from many species of Anura are containing high concentrations of peptides with cytotoxic activities against prokaryotic and eukaryotic cells . Amphibian skin/parotoid gland secretions are rich sources of antimicrobial activity against strains of antibiotic-resistant microorganisms such as bacteria and fungi, also highly cytotoxic effects on cancer cell lines . Besides, it is known that dermal glands indicate physiological functions on skin and play several roles of protection from predators [4, 5].
The secreted materials of amphibian skin have been used extensively by several cultures. Chansu and Cinobufacini (Huachansu) are traditional Chinese medicines that have been used for centuries as therapy for inflammation, anesthesia and infections in China and other Asian countries . Studies on bufadienolidic steroids led to isolation of marinobufagin, bufalin, telocinobufagin, and cinobufagin. These bufadienolidic steroids have shown efficiency against cancer cells in animal models and in migration of colon cancer cells (Colon 26-L5) [7, 8], human leukemia cells (K562, U937, ML1 and HL-60)  and breast tumor cells (MCF-7) . In Telocinobufagin studies, bufadienolid was found active against Staphylococcus aureus and Escherichia coli . Three components of Chansu (Bufalin, Telocinobufagin and Cinobufagin) demonstrate a collection of biological activities, including cardiotonic, anesthetic, antineoplastic activity and blood pressure stimulation [10, 12].
Natural products of amphibian skin and their proficiency are not limited with bufodienolidic steroids. Amphibian skin secretion studies have showed that Anura families such as Ranidae, Hylidae, Bombinatoridae, Pipidae, Alytidae, Ascaphidae and Leptodactylidae have various therapeutic potentials as anti-diabetic agents, immunomodulatory agents, anti-viral and anti-cancer agents . Approximately 30 years have passed since the discovery of the Magainins in the skin of the African clawed frog, Xenopus laevis in the family Pipidae . Magainin from X. laevis show potential as anti-cancer agents displaying tumoricidal activity against human small cell lung cancer cell lines , the RT4, 647V, and 486P bladder cancer cell lines , and against suspension cultures of a wide range of hematopoietic cell lines . Another biologically active compound of amphibian skin is steroidal samandarine from salamanders. Studies show that samandarine have high toxicity is likely due to its potent local anesthetic activity .
To sum up, there are a lot of researches related to amphibian skin secretions that have been carried out on frogs and toads. In salamanders and newts, the studies have been carried out only on Andrias davidianus (Chinese giant salamander) , Taricha granulosa (rough-skinned newt) , Tylototriton verrucosus (crocodile newt, Himalayan newt) , Plethodon cinereus (red-backed salamander) , and Salamandra salamandra terrestis (European fire salamander) .
The smooth newt, Lissotriton vulgaris and Balkan-Anatolian crested newt, Triturus ivanbureschi that are studied in this survey have similar distribution range in western and European part of Turkey. The main purpose of this study was to investigate cytotoxic, antimicrobial and hemolytic effects of L. vulgaris and T. ivanbureschi skin secretions on various cancer and non-cancerous cells, microorganisms and rabbit red blood cells to form an estimate of their potential use in medicine as therapeutic agents for the first time.
Materials and methods
Smooth newt – Lissotriton vulgaris specimens (five males, five females) were collected during the field excursion in Urla/Izmir province, western Turkey in March 2016. Balkan-Anatolian crested newt – Triturus ivanbureschi specimens (five males, five females) were collected during the field excursion in Uzunköprü/Edirne province, Turkish Thrace in April 2013. These two species can be easily distinguished from each other. Lissotriton vulgaris is an obviously smaller newt species than T. ivanbureschi. Besides, male specimens of L. vulgaris have continuous dorsal fin in reproduction period, while dorsal fins of the male specimens of T. ivanbureschi have an interruption at the base of the tail.
Collection of skin secretions
Skin secretions obtained by mild electrical stimulation (5–10 V) by stimulator (C.F. Palmer, London) according to Tyler et al. . Each individual was rinsed with ultra-pure water into the tubes. Skin secretions clarified by centrifugation (5635×g for 10 min.), supernatants were snap-frozen by liquid nitrogen then lyophilized and stored at +4°C until the bioactivity assays are set up. Secretion harvesting was performed in the field, and then newts were released to their natural habitats, unharmed.
Determination of the protein profiles by the SDS-PAGE and protein concentration
Protein content was assayed triplicate for each diluted skin secretion (2 mg/mL) samples in ultra-pure water, using bovine serum albumin as a standard by BCA assay kit (Thermo Scientific, MA, USA). The protein content was calculated with using a UV/Vis spectrophotometer (Thermo Multiskan Spectrum, Bremen, Germany) at 562 nm.
Electrophoretic processes were performed with sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) using a discontinuous buffer system according to Laemmli . The running process was executed on Bio-Rad Mini Protein Tetra Cell electrophoresis device. TGX Stain Free (Bio-Rad, CA, USA) gel was used which does not need any staining processes. The Thermo PageRuler Plus Protein Standard (10, 15, 25, 35, 55, 70, 100, 130 and 250 kD) was used as a marker. In protein profile studies, samples were repeated three times on gel. Approximate molecular weights of the protein fractions were calculated and photographed with using a software program (Image Lab, Bio-Rad, CA, USA) of the gel imaging system ChemiDoc (Bio-Rad, CA, USA).
Cell culture, in vitro cytotoxicity assay and determination of half maximal inhibitory concentration (IC50)
HeLa (human cervix adenocarcinoma), A549 (human alveolar adenocarcinoma), Caco-2 (human colon colorectal adenocarcinoma), MPanc-96 (human pancreas adenocarcinoma), PC-3 (human prostate adenocarcinoma), U-87 MG (human glioblastoma-astrocytoma), MDA-MB-231 (human mammary gland adenocarcinoma) cancer cells and as a non-cancerous cell lines, HEK-293 (human embryonic kidney) were used for determination of the cytotoxicity. Cell lines were purchased from ATCC (Manassas, VA, USA). All cells were cultivated in Dulbecco’s modified Eagle’s medium F12 (DMEM/F12), supplemented with 10% fetal bovine serum (FBS), 2 mM/L glutamine, 100 U/mL of penicillin and 100 μg/mL of streptomycin (Lonza, Visp, Switzerland). The cells were incubated at 37°C in a humidified atmosphere of 5% CO2. The cells that actively proliferate in the logarithmic phase were used in the tests.
Cytotoxicity of crude skin secretions were determined by colorimetric MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide] assay . The optical density (OD) was measured at 570 nm (with a reference wavelength 630 nm) by UV/Vis spectrophotometry (Thermo Multiskan Spectrum, Bremen, Germany). Cytotoxicity was assayed triplicate. All cell lines were cultivated in 96-well microplates for 24 h with an initial concentration of 1×105 cells/mL. Subsequently, the cultured cells were treated with different doses of skin-secretions and incubated at 37°C for 48 h. The plant-derived sesquiterpene-lactone (parthenolide) was used as positive cytotoxic control agent. The viability (%) was determined by the following formula:
In cell culture studies for untreated cell lines (negative controls) cytotoxicity was set to 0%. The IC50 values were calculated by fitting the data to a sigmoidal curve and using a four parameter logistic model and presented as an average of three independent measurements. The IC50 values were reported at 95% confidence interval and calculations were performed using Prism 5 software (GraphPad5, San Diego, CA, USA). The values of the blank wells were subtracted from each well of treated and control cells and half maximal inhibition of growth (IC50) were calculated in comparison to untreated controls.
Microorganisms and antimicrobial assay
Following microorganisms were used for antimicrobial assays: The Gram-negative bacteria strains: enteropathogenic Escherichia coli 0157:H7 (RSKK 234), non-pathogenic E. coli ATCC 25922, Klebsiella pneumoniae ATCC 10031 and Salmonella thyphimurium CCM 5445. The Gram-positive bacteria strains: Bacillus cereus ATCC 7064, Enterococcus fecalis ATCC 29212, vancomycin-resistant E. faecium DSM 13590, methicillin-resistant Staphylococcus aureus ATCC 43300, and S. epidermidis ATCC 12228. Candida albicans ATCC 10239 and C. tropicalis RSKK 2412 were used as yeasts. The lyophilized bacteria and yeast strains were obtained from Ege University, Faculty of Science, Department of Basic and Industrial Microbiology.
Antimicrobial effects of skin secretions were determined by broth micro-dilution method. Test microorganisms were grown in MH broth for 5 h (exponential phase) and adjusted to 0.5 McFarland turbidity standard (A600=1.0), corresponding to 1.5×106 colony forming unit (CFU)/mL. MICs were determined according to the Clinical and Laboratory Standard Institute . Serial dilutions of skin secretions (0.9–500 μg/mL) were prepared in 96-well microtiter trays, at a final volume of 80 μL. Then, 20 μL of the adjusted bacterial and fungal inocula (1.5×105 CFU/mL) were added to each well and incubated at 37°C for 24 h. Inhibition of microorganisms’ growth was determined by visual observation. The minimum inhibitory concentration (MIC) was defined as the lowest concentration of skin secretions required to inhibit microbial growth. Each dilution series included control wells, which consisted of 80 μL of it and 80 μL of Mueller Hinton broth. Ampicillin and flucytosine were used as standard drugs for comparison. All assays were studied three replicates.
Hemolytic activity assay
The hemolytic effect of crude L. vulgaris and T. ivanbureschi skin secretions were measured according to the modified method of . Red blood cells were obtained from healthy New Zealand rabbit (Bornova Veterinary Control and Research Institute, Izmir, Turkey). Blood was collected with BD Vacutainer TM (NH 143 I.U., Belliver Industrial Estate, Plymouth, UK). Aliquots of 7 mL of blood were washed three times with sterile saline solution (0.89%, w/v NaCl, pyrogen free) by centrifugation at 2000×g for 5 min. The cell suspension was prepared by finally diluting the pellet to 0.5% in saline solution. A volume of 0.05 mL of the cell suspension was mixed in U bottom 96-well microplate with 0.05 mL diluents containing 50, 5 and 0.5 μg/mL concentration of crude L. vulgaris and T. ivanbureschi skin secretions in saline solutions. The mixtures were incubated for 30 min at 37°C and centrifuged at 800×g for 10 min. The free hemoglobin in the supernatants was measured spectrophotometrically at 412 nm. Saline and distilled water were included as minimal and maximal hemolytic controls. The hemolytic percent developed by the saline control was subtracted from all groups. Each experiment included triplicates at each concentration.
Protein contents and electrophoretic protein profiles
The total protein and peptide concentrations were determined by BCA assay for L. vulgaris and T. ivanbureschi as 1775 and 1470 μg/mL, respectively.
The electrophoretic profiles of the skin secretions revealed numerous proteins with a wide spectrum of molecular masses (Figure 1). In L. vulgaris skin secretion, totally 18 protein fractions were determined. The approximate molecular weights of these protein fractions were calculated as 265, 252, 115, 83, 69, 60, 53, 47, 40, 34, 28, 23, 21, 18, 16, 13, 11 and 9 kDa. In T. ivanbureschi skin secretion, totally 20 protein fractions were found. These are calculated as 272, 263, 200, 109, 89, 78, 68, 64, 54, 50, 47, 43, 38, 30, 27, 22, 20, 16, 11 and 9 kDa, respectively.
The cytotoxicity of crude skin secretions is measured against the following cell lines: HeLa, A549, Caco-2, MPanc-96, PC-3, U-87 MG, MDA-MB-231 cancer cells and as a non-cancerous cell line, HEK-293. Crude skin secretions of the L. vulgaris and T. ivanbureschi inhibit cell viability in a dose-dependent manner (Figure 2). The IC50 values of skin secretions for treated cell lines are shown in Table 1.
|Cell lines IC50 (μg/mL)||HEK-293 (non-cancerous kidney)||Caco-2 (colon)||HeLa (cervical)||A549 (lung)||U-87 MG (glioblastoma)||MPanc-96 (pancreas)||PC-3 (prostate)||MDA-MB-231 (breast)|
Parthenolide was used as positive control.
Crude L. vulgaris and T. ivanbureschi skin secretions were showed high cytotoxic effects on all cancer and non-cancerous cell lines with IC50 values varying between 1.58–26.15 and 1.40–40.28 μg/mL, respectively. Additionally, we found the highest anticancer activity for both secretion samples on MDA-MB-231 cell line (IC50: 1.58 and 1.40 μg/mL). These results are higher than plant-derived sesquiterpene lactone parthenolide, remarkably. The lowest cytotoxic effect observed on human cervix adenocarcimoma (HeLa) by T. ivanbureschi crude skin secretion (IC50: 40.28 μg/mL). Morphological changes were observed after 48 h exposure to different doses of skin secretions. Increasing concentrations resulted in increased number of rounded cells, growth inhibition and the rate of various morphological abnormalities with larger areas devoid of cells when compared with the untreated control cells.
MIC was determined using broth dilution method (0.9–500 μg/mL). The MIC values of skin secretions against various Gram-negative and Gram-positive bacterial strains and yeasts are presented in Table 2.
|Microorganisms||MIC (μg/mL) values for skin secretion and antimicrobial agents|
|L. vulgaris||T. ivanbureschi||Ampicillin||Flucytosine|
|E. coli O157:H7 RSKK234||7.8||125||4.6||–|
|E. coli ATCC 25922||125||–||4.6||–|
|S. aureus ATCC 43300||31.25||250||4.6||–|
|S.epidermidis ATCC 12228||15.6||125||2.3||–|
|E. faecalis ATCC 29212||15.6||15.6||9.3||–|
|E. faecium DSM 13590||31.25||31.25||4.6||–|
|K. pneumonia ATCC 10031||125||–||9.3||–|
|S. thyphimurium CCM 5445||125||125||4.6||–|
|B. cereus ATCC 7064||125||125||9.3||–|
|C. albicans ATCC 10239||7.8||15.6||–||9.3|
|C. tropicalis RSKK 2412||31.25||–||–||9.3|
Ampicillin and flucytosine were used as positive controls. –, Not detected.
The values indicate that skin secretion have MIC values varying from 3.9 to 250 μg/mL. Skin secretion of L. vulgaris showed the most potent activity against enteropathogenic E. coli O157:H7 RSKK234 and C. albicans ATCC 10239 with a MIC value of 7.8 μg/mL. Especially, antifungal activity against C. albicans ATCC 10239 has a higher MIC value than positive control drug flucytosine, remarkably. Also, skin secretion of L. vulgaris showed moderate antibacterial activity against E. faecalis ATCC 29212, S. epidermidis ATCC 12228, S. aureus ATCC 6538P, vancomycin-resistant E. faecium DSM 13590 and C. tropicalis RSKK 2412 (MIC=15.6 and 31.25 μg/mL). Skin secretion of T. ivanbureschi exhibited the most potent activity against E. faecalis ATCC 29212 and C. albicans ATCC 10239 with a MIC value of 15.6 μg/mL.
The hemolytic effects of crude L. vulgaris and T. ivanbureschi skin secretions on rabbit red blood cells were shown in Table 3. Hemolytic activities were observed at concentrations between 0.5 and 50 μg/mL. Hemolytic effect of L. vulgaris skin secretion was found only in the highest concentration (50 μg/mL) with 13.6% hemolytic percent. No hemolytic activity was observed in other concentrations of the L. vulgaris skin secretion. Hemolytic activities were seen at all concentrations of the T. ivanbureschi skin secretions in dose dependent manner. It showed very high hemolytic activity at 50 and 5 μg/mL concentrations which were more than the positive control distilled water (hemolytic percents: 115.1% and 103.7%).
|Controls and samples||Concentration (μg/mL)||Absorbance value OD (412 nm)||Hemolytic percent (%)|
Hemolytic percent of saline and distilled water were included as minimal and maximal hemolytic control, respectively. All values represent the mean±SD (n=3 test). –, Not detected.
The skin secretions of amphibians show differentiation across different species with biologically active compounds such as peptides/proteins, steroids, alkaloids, biogenic amines and enzymes . They might be useful as pharmacological implements in drug research and potential drug design models. In this study, cytotoxic, antimicrobial and hemolytic properties of crude L. vulgaris and T. ivanbureschi skin secretions were determined. Also, protein profiles of these crude skin secretions were revealed.
The antimicrobial and cytotoxic activities are results of the active peptide inducing alterations in the hydrophobic–hydrophilic seal of the cell membrane, effecting lysis of the bacterial or cancer cells .
Salamanders and newts are known to secrete toxic and noxious compounds, such as neurotoxic tetrodotoxin . Nevertheless, the exact mode of action of the salamander alkaloids is almost unknown. They have local anesthetic effects by nerve-blocking activity. Respiratory paralysis was found as the reason of death according to in vivo studies . Moreover, salamander alkaloids, especially samandarone, exhibit distinct antimicrobial activities, although being less potent than most antibiotics [22, 33]. Also, several peptides such as corticotropin-releasing peptides and vasoactive intestinal peptides were isolated from the newt, Cynops pyrrhogaster by Teranishi et al. .
Protein profiles of crude skin secretions of salamanders and newts are almost unknown. Up to now, only crude skin secretion of A. davidianus is determined. The molecular masses of the proteins in the skin secretions of the smooth newt and Balkan-Anatolian crested newt are distributed over a wide range between 9 and 272 kDa, while those of the A. davidianus skin secretion are above 66 kDa and between 14 and 66 kDa . There is no further study on bioactivity of skin secretions of newts.
However, some comparable bioactivity studies on skin secretion of frogs and toads have been conducted. Cunha Filho et al.  indicated that B. rubescens skin-parotoid gland secretions have antimicrobial activity on S. aureus (MIC: 128 μg/mL) and E. coli (MIC: 16–64 μg/mL). We found higher effect on E. coli (MIC: 7.8 μg/mL) and S. aureus (MIC: 31.25 μg/mL) for L. vulgaris skin secretion when compared to the study of Cunha Filho et al. . Zare-Zardini et al.  determined the antimicrobial and hemolytic effect of maximin peptide from B. kavirensis skin secretion with MIC values for B. cereus (18.5 μg/mL), S. aureus (16.3 μg/mL), K. pneumonia (8.9 μg/mL), E. coli (8.1 μg/mL), C. albicans (32.1 μg/mL). Also, they found 2.5% hemolytic effect with 60 μg/mL maximin on human erythrocytes. Our results on same microbial strains for L. vulgaris and T. ivanbureschi skin secretions with MIC values for B. cereus (both 125 μg/mL), S. aureus (31.25 and 250 μg/mL), K. pneumonia (125 μg/mL and ineffective), E. coli (7.8 and 125 μg/mL), C. albicans (7.8 and 15.6 μg/mL), respectively. Also, we found higher hemolytic effects on rabbit erythrocytes with 50 μg/mL skin secretion, 13.6% for L. vulgaris and 115.1% for T. ivanbureschi. van Zoggel et al.  studied the cytotoxicity of dermaseptin peptides from P. bicolor skin secretion on human prostate adenocarcinoma (PC-3) and found 75% inhibition with 10 μg/mL concentration. We found 60% inhibition on the same cell line with 5 μg/mL skin secretion of L. vulgaris and 50% viability 5 μg/mL skin secretion treatment of T. ivanbureschi. Cunha Filho et al.  performed the cytotoxicity of R. schneideri macrogland secretion derivatives on HCT-8 cell line and they found the IC50 values up to 1.20 μM with no hemolytic activity. We found the IC50 values on Caco-2 as 24.40 μg/mL for L. vulgaris and 5.60 μg/mL for T. ivanbureschi skin secretion with high hemolytic activity. These results are demonstrating potential for cancer and antimicrobial treatment.
In conclusion, biological activities and protein profiles of skin secretions of the smooth newt, L. vulgaris and Balkan-Anatolian crested newt, T. ivanbureschi were revealed for the first time. Besides, the results indicated that these crude skin secretions have a high inhibition on cancer cell lines and microorganisms with relatively high hemolytic activity and wide range molecular masses of proteins. Further studies need to focus on purification of the active components from these skin secretions and mode of action on cancer cell lines and microorganisms as anti-agents.
This work was partly supported by the Scientific and Technical Research Council of Turkey (TÜBİTAK) under Grant 114Z599 (Funder Id: 10.13039/501100004410) and Ege University Scientific Research Project Commission (Project no: 2015-FEN-004). We would like to express our gratitude to AREL (Research and Education Laboratory, Ege University School of Medicine) for their support laboratory facilities during the study.
Conflict of interest statement: The authors have no conflict of interest.
Ethical considerations: MK, HTY, AN and BG have ethical permission from Ege University Animal Experiments Ethics Committee (2014-002) for skin secretion milking procedures. Also, they have special permission for the fieldworks from the Republic of Turkey, Ministry of Forestry and Water Affairs, Directorate of Nature Conservation and National Parks (2014-51946) to collect newts for secretion sampling.
1. Xu X, Lai R. The chemistry and biological activities of peptides from amphibian skin secretions. Chem Rev 2015;115:1760–846.Search in Google Scholar
2. Conlon JM. Structural diversity and species distribution of host-defense peptides in frog skin secretions. Cell Mol Life Sci 2011;68:2303–15.Search in Google Scholar
3. Nalbantsoy A, Karış M, Yalçın HT, Göçmen B. Biological activities of skin and parotoid gland secretions of bufonid toads (Bufo bufo, Bufo verrucosissimus and Bufotes variabilis) from Turkey. Biomed Pharmacother 2016;80:298–303.Search in Google Scholar
4. Clarke BT. The natural history of amphibian skin secretions, their normal functioning and potential medical applications. Biol Rev 1997;72:365–379.Search in Google Scholar
5. Daly JW. The chemistry of poisons in amphibian skin. Proc Natl Acad Sci 1995;92:9–13.Search in Google Scholar
6. Chen KK, Kovarikova A. Pharmacology and toxicology of toad venom. J Pharm Sci 1967;56:1535–41.Search in Google Scholar
7. Ogasawara M, Matsubara T, Suzuki H. Screening of natural compounds for inhibitory activity on colon cancer cell migration. Biol Pharm Bull 2001;24:720–3.Search in Google Scholar
8. Li C, Hashimi SM, Cao S, Qi J, Good D, Duan W, Wei MQ. Chansu inhibits the expression of cortactin in colon cancer cell lines in vitro and in vivo. BMC Complement Altern Med 2015;15:207.Search in Google Scholar
9. Zhang LS, Nakaya K, Yoshida T, Kuroiwa Y. Induction by bufalin of differentiation of human leukemia cells HL60, U937, and ML1 toward macrophage/monocyte-like cells and its potent synergistic effect on the differentiation of human leukemia cells in combination with other inducers. Cancer Res 1992;52:4634–41.Search in Google Scholar
10. Nogawa T, Kamano Y, Yamashita A, Pettit GR. Isolation and structure of five new cancer cell growth inhibitory bufadienolides from the Chinese traditional drug Ch’an Su. J Nat Prod 2001;64:1148–52.Search in Google Scholar
11. Cunha Filho GA, Schwartz CA, Resck IS, Murta MM, Lemos SS, Castro MS, et al. Antimicrobial activity of the bufadienolides marinobufagin and telocinobufagin isolated as major components from skin secretion of the toad Bufo rubescens. Toxicon 2005;45:777–82.Search in Google Scholar
12. Gomes A, Giri B, Saha A, Mishra R, Dasgupta SC, Debnath A, Gomes A. Bioactive molecules from amphibian skin: their biological activities with reference to therapeutic potentials for possible drug development. Indian J Exp Biol 2007;45:579–93.Search in Google Scholar
13. Conlon JM, Mechkarska M, Lukic ML, Flatt PR. Potential therapeutic applications of multifunctional host-defense peptides from frog skin as anti-cancer, anti-viral, immunomodulatory, and anti-diabetic agents. Peptides 2014;57:67–77.Search in Google Scholar
14. Zasloff M. Magainins, a class of antimicrobial peptides from Xenopus skin: Isolation, characterization of two active forms, and partial cDNA sequence of a precursor. Proc Natl Acad Sci 1987;84:5449–53.Search in Google Scholar
15. Ohsaki Y, Gazdar AF, Chen HC, Johnson BE. Antitumor activity of magainin analogues against human lung cancer cell lines. Cancer Res 1992;52:3534–8.Search in Google Scholar
16. Lehmann J, Retz M, Sidhu SS, Suttmann H, Sell M, Paulsen F, et al. Antitumor activity of the antimicrobial peptide magainin II against bladder cancer cell lines. Eur Urol 2006;50:141–7.Search in Google Scholar
17. Cruciani RA, Barker JL, Zasloff M, Chen HC, Colamonici O. Antibiotic magainins exert cytolytic activity against transformed cell lines through channel formation. Proc Natl Acad Sci 1991;88:3792–6.Search in Google Scholar
18. Daly JW, Spande TF, Garraffo HM. Alkaloids from amphibian skin: a tabulation of over eight-hundred compounds. J Nat Prod 2005;68:1556–75.Search in Google Scholar
19. Guo W, Ao M, Li W, Wang J, Yu L. Major biological activities of the skin secretion of the Chinese giant salamander, Andrias davidianus. Z Naturforsch C 2012;67:86–92.Search in Google Scholar
20. Cardall BL, Brodie Jr ED, Brodie ED, Hanifin CT. Secretion and regeneration of tetrodotoxin in the rough-skin newt (Taricha granulosa). Toxicon 2004;44:933–8.Search in Google Scholar
21. Jiang WB, Hakim M, Lei LU, Bo-Wen LI, Shi-Long YA, Yu-Zhu SO, et al. Purification and characterization of cholecystokinin from the skin of salamander Tylototriton verrucosus. Zool Res 2015;36:174–7.Search in Google Scholar
22. Fredericks LP, Dankert JR. Antibacterial and hemolytic activity of the skin of the terrestrial salamander, Plethodon cinereus. J Exp Zool Part A 2000;287:340–5.Search in Google Scholar
23. Mebs D, Pogoda W. Variability of alkaloids in the skin secretion of the European fire salamander (Salamandra salamadra terrestris). Toxicon 2005;45:603–6.Search in Google Scholar
24. Tyler MJ, Stone DJ, Bowie JH. A novel method for the release and collection of dermal, glandular secretions from the skin of frogs. J Pharmacol Toxicol Methods 1992;28:199–200.Search in Google Scholar
25. Laemmli UK. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 1970;227:680–5.Search in Google Scholar
26. Mosmann T. Rapid colorimetric assay for cellular growth and survival: application to proliferation and cytotoxicity assays. J Immunol Methods 1983;65:55–63.Search in Google Scholar
27. CLSI (Clinical and Laboratory Standard Institute). Performance standards; for antimicrobial susceptibility testing, nineteenth informational supplement, 2009; (M100–S19).Search in Google Scholar
28. Yang ZG, Suna HX, Fang WH. Haemolytic activities and adjuvant effect of Astragalus membranaceus saponins (AMS) on the immune responses to ovalbumin in mice. Vaccine 2005;23:5196–203.Search in Google Scholar
29. Sciani JM, de-Sá-Júnior PL, Ferreira AK, Pereira A, Antoniazzi MM, Jared C, et al. Cytotoxic and antiproliferative effects of crude amphibian skin secretions on breast tumor cells. Biomed Prev Nutr 2013;3:10–8.Search in Google Scholar
30. Pukala TL, Bowie JH, Maselli VM, Musgrave IF, Tyler MJ. Host-defence peptides from the glandular secretions of amphibians: structure and activity. Nat Prod Rep 2006;23:368–93.Search in Google Scholar
31. Yotsu-Yamashita M, Mebs D. The levels of tetrodotoxin and its analogue 6-epitetrodotoxin in the red-spotted newt, Notophthalmus viridescens. Toxicon 2001;39:1261–3.Search in Google Scholar
32. Habermehl G. Venoms of amphibia. In: Florkin M, Scheer BT, editors. Chemical zoology. Vol. 9. New York, NY, USA: Academic Press, 1974:161–83.Search in Google Scholar
33. Preusser HJ, Habermehl G, Sablofski M, Schmall-Haury D. Antimicrobial activity of alkaloids from amphibian venoms and effects on the ultrastructure of yeast cells. Toxicon 1975;13:285–8.Search in Google Scholar
34. Teranishi H, Muneoka Y, Takao T, Shimonishi Y, Kojima M. Isolation and characterization of four VIP-related peptides from red-bellied newt, Cynops pyrrhogaster. Regul Peptides 2004;123:173–9.Search in Google Scholar
35. Zare-Zardini H, Ebrahimi L, Ejtehadi MM, Hashemi A, Azam AG, Atefi A, et al. Purification and characterization of one novel cationic antimicrobial peptide from skin secretion of Bufo kavirensis. Turk J Biochem 2013;38:416–24.Search in Google Scholar
36. van Zoggel H, Hamma-Kourbali Y, Galanth C, Ladram A, Nicolas P, Courty J, et al. Antitumor and angiostatic peptides from frog skin secretions. Amino Acids 2012;42:385–95.Search in Google Scholar
37. Cunha Filho GA, Resck IS, Cavalcanti BC, Pessoa CÓ, Moraes MO, Ferreira JR, et al. Cytotoxic profile of natural and some modified bufadienolides from toad Rhinella schneideri parotoid gland secretion. Toxicon 2010;56:339–48.Search in Google Scholar
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