Arenobufagin

Arenobufagin is a cardiotoxic bufanolide steroid secreted by the Argentine toad Bufo arenarum. It has effects similar to digitalis, blocking the Na+/K+ pump in heart tissue.Structure of ArenobufaginStructure of a Cardiac Glycoside Arenobufagin is a cardiotoxic bufanolide steroid secreted by the Argentine toad Bufo arenarum. It has effects similar to digitalis, blocking the Na+/K+ pump in heart tissue. The compound Arenobufagin is one of the major components of toad venom( (Venenum bufonis) . It is derived from the dried skin of giant toads, such as the Bufo gargarizans or the Bufo melanostictus Suhneider. Arenobufagin is specifically secreted by the Rhinella arenarum, which is found in South American nature. The venom of these toad species contains about 1,75% of arenobufagin. The other, major, part of the venom consists mostly of similar looking bufagins, which are all toxic steroides. Toads produce their venom when they are scared, injured or provoked, as a defence mechanism against their predators Arenobufagin is a component of a certain toad venom which goes by the name of Chan’su. For centuries, this venom has been used in Chinese traditional medicine in the treatment of liver cancer. Also, it is reported to have a simulative effect upon the heart. It is derived from the dried skin secretions of giant toads. In a 2013 study, it was shown that arenobufagin inhibits the growth of certain liver tumor cells. In fact, this compound showed the most potent antitumor activity of fifteen bufadionolides isolated from Chan’su. As already mentioned, arenobufagin has been widely used in traditional Chinese medicine (TCM) to tackle down carcinogenesis , since it inhibits cell growth in several cancer cells. It is one of the central active ingredients of toad venom for treatment. This happens both in purified form or in combination with other ingredients, which are mostly herbal components. Toad venom is in fact still used in clinical practice of in TCM to treat hepatocellular carcinoma (HCC). So far there is still little known about the anti-angiogenic properties of Arenobufagin. One studie shows that Arenobufagin inhibits VEGF-induced endothial cell tube formation. Under normal circumstances, arenobufagin is a solid. In Chinese traditional medicine, it in ingested either orally or topically, for example to the skin. Little is known about its toxicokinetics. Arenobufagin is believed to play a role in the regulation of the transport of water and electrolytes across cell membranes under physiological conditions. Experimental results show that Arenobufagin inhibits the sodium-potassium adenosine triphosphatase (Na+/K+-ATPase) . It is one of the most potent blockers that are known to science, along with ouabain. It has been suggested that uncharged and non-polar amino acids may participate in the binding of arenobufagin to the extracellular surface of the ATPase. If this is the case, the steroid nucleus of arenobufagin probably also contributes to the binding. Furthermore, the presence of a sugar group in ouabain and the absence of one in arenobufagin suggests that it may me more lipophilic that ouabain and as such form a more stable complex with the Na+-K+ pump. Besides its effects on the Na+-K+ pump, arenobufagin also has some other effects on cells, which were discovered in experiment with cancer cell lines.It causes apoptosis by tempering with mitochondria. In cells treated with it, a decreasing mitochondrial potential was detected (in a dose-dependent manner), as well as a high Bax/Bcl-2 ratio, which is associated with apoptosis, or programmed cell death. Bax translocation from cytosol to mitochondria was also found to be increased.In addition to these effects, arenobufagin also induces morphological changes in organelles, blebbing of plasma membrane, shrinkage of nuclear membrane and chromatin condensation. These observation also indicate the occurrence of apoptosis. Last but not least, specific cleavage of poly (ADP-ribose) polymerase (PARP) and a decrease in pro-caspase9 and 3 were also induced by arenobufagin treatment. PARP is mainly involved in cell repair and programmed cell death.After treatment with arenobufagin, some cells make more autophagosomes and lysosomes, whereas other cells undergo apoptosis. Arenobufagin also leads to increased expression of LC3-II, Biclin1(initial vesicle formation), Atg5 (elongation and completion), Atg9, Atg16L1 and p62/SQSTM1, all proteins which induce autophagy. Blockers of autophagy increase the cytotoxic effect of arenobufagin. Addition of 3-MA increases the proportion of cells in which there in enlarged PARP cleavage and limited caspase-9 and caspase-3 cleavage. This indicates that autophagy pathways protect the cell against apoptosis by arenobufagin.It was hypothesized that arenobufagin may inhibit the PI3K/Akt pathway in controlling cell death and differentiation in response to external stimuli. It was found that arenobufagin inhibited the proteins Akt, PDK1 and PI3K, whereas it stimulated the PHEN-protein. It does this by altering the degree of phosphorylation. All these proteins are involved in the PI3/Akt pathway. Further down the pathway, arenobufagin indirectly inhibits the mTOR-protein, which is involved in apoptosis as well as in autophagy. The biotransformation of arenobufagin by Alternaria alternata leads to the following three metabolites: 3-oxo-arenobufagin (1a), ψ-bufarenogin (1b), and 3- oxo- ψ-bufarenogin (1c). The biotransformation processes consists of a main reaction whereas the dehydrogenation of the 3-hydroxyl group takes place. This process is followed by isomerization.