Cardiotonic glycoside toxins, originally isolated from plants or amphibians, have been proposed as mammalian cardiotonic hormones. This paper is a review and update of the discovery of [i] a new class of steroid hormones, [ii] the path for their biosynthesis and [iii] some preliminary data on their function. The compounds are phosphoester conjugates and share a characteristic structural feature, a lactone ring, with [a] one class of synthetic potassium sparing diuretics and with [b] plant and amphibian cardiotonic steroids. Purification was initially monitored by cross reaction with steroid sulfate assays and later with digoxin-specific assays. Six compounds were purified by HPLC to near homogeneity and characterized by Tandem mass spectroscopy (MS-MS) and 31P-NMR. Three were digoxin-like materials (DLM) with 23 carbon atoms. The two extra carbon atoms form a spiral lactone E-ring. Several additional spiral lactones have been identified by MS-MS. In a pilot study, based on MS analysis, we evaluated phosphocholine steroid levels in individual serum samples in patients with pre-eclampsia (n=20). The control group of normotensive pregnant women (n=20) was used to estimate the mean and standard deviation. Twelve of the women with pre-eclampsia had a z-score over 2 for at least one of the four phosphocholine steroids. In contrast, only 1 sample from the normotensive women had a z-score over 2. The observation that there are two patterns, one with elevated phosphocholine steroid levels and one without the elevation, suggests that there may be two different underlying causes of pre-eclampsia. We now need to extend the study to determine which pattern predicts progression to life-threatening hypertension in pregnant women. This pilot study illustrates that it is possible to evaluate individual endogenous cardiotonic hormones without relying on antibodies developed to plant or amphibian toxins.

The Na+/K+ ATPase is a protein involved in the active transport of ions across the cellular membrane. Ouabain is a cardiotonic glycoside that, by inhibiting the Na+/K+ pump, interferes with cell processes mediated directly by the pump, but also indirectly influences other cellular processes, such as cellular cycle and proliferation, growth, cell differentiation, angiogenesis, migration, adhesion and invasion. We used the SK-BR-3 breast cancer cell line, mesenchymal stem cells (MSCs), and tumor-associated fibroblasts (TAFs) in vitro for determining the changes induced by Ouabain in concentration of 10-5 M, 10-6 M, 10-8 M, 10-9 M on these cellular types, employing specific methods: SEM for morphological evaluation; immunocytochemistry, flowcytometry to assess immunophenotypical markers expression, cell cycle and Annexin V/PI assay for cellular proliferation and viability testing; flowchamber assay to evaluate functional adhesion behavior; RT-PCR for Na+/K+ ATPase subunits molecular expression. Being indicative of the pro-apoptotic and inhibitory effect of Ouabain on tumor invasion and metastasis, the results support the addition of Ouabain to the oncological therapeutic arsenal, trailing the “repurposing drugs” approach.

Cardiotonic steroids (CTS) are a group of steroid compounds derived from certain plants and animals. CTS are selective inhibitors of Na,K-ATPase, so for a long time they were widely used for medical purposes to treat heart failure and some other diseases. However, over time, doctors gradually began to refuse this group of drugs due to their narrow therapeutic range and a number of serious side effects. However, in parallel with the rise and fall of interest in CTS as a drug, interest in them as a single class of high-affinity Na,K-ATPase inhibitors has only increased. Numerous data on the effect of CTS on signaling cascades and cell viability made impelling a search for a rationale for the existence of such a subtle biological regulator. A large number of studies devoted to the potential use of CTS in the treatment of neurodegenerative, oncological, immune and other diseases "revitalized" the already "written off" group of the drugs. If initially the role of exogenous (mainly plant origin) CTS was studied, then by the 90s of the XX century the study of the role of endogenous CTS and their regulatory effects became a popular trend in biology and physiology. The role of endogenous CTS was studied in details in the pathogenesis of cardiovascular diseases [1]. A large number of researchers have unanimously stated that endogenous CTS are very complex regulators of the development of pathological processes; however, the exact cause-and-effect relationship that would unambiguously explain the processes of synthesis, utilization, and the role of endogenous CTS has not yet been established. The effects of CTS in the frame of various regulatory schemes have been described, but the predominant number of studies was devoted to arterial hypertension, heart and renal failure, cancer [2–4], aging [5,6], and neuroinflammation. The effects of both endo- and exogenous CTS on blood cells and have been studied to a much lesser extent.