cells Review Cardiac Glycosides as Autophagy Modulators Jan Škubník , Vladimíra Svobodová Pavlí ˇ cková , Jana Psotová and Silvie Rimpelová *   Citation: Škubník, J.; Svobodová Pavlíˇ cková, V.; Psotová, J.; Rimpelová, S. Cardiac Glycosides as Autophagy Modulators. Cells 2021, 10, 3341. https://doi.org/10.3390/cells10123341 Academic Editors: Mojgan Djavaheri-Mergny and Mohammad Amin Moosavi Received: 5 November 2021 Accepted: 24 November 2021 Published: 28 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Biochemistry and Microbiology, University of Chemistry and Technology, Technická 3, 166 28 Prague, Czech Republic; jan.skubnik@vscht.cz (J.Š.); vladimira.pavlickova@vscht.cz (V.S.P.); jana.psotova@vscht.cz (J.P.) * Correspondence: silvie.rimpelova@vscht.cz Abstract: Drug repositioning is one of the leading strategies in modern therapeutic research. Instead of searching for completely novel substances and demanding studies of their biological effects, much attention has been paid to the evaluation of commonly used drugs, which could be utilized for more distinct indications than they have been approved for. Since treatment approaches for cancer, one of the most extensively studied diseases, have still been very limited, great effort has been made to find or repurpose novel anticancer therapeutics. One of these are cardiac glycosides, substances commonly used to treat congestive heart failure or various arrhythmias. Recently, the antitumor properties of cardiac glycosides have been discovered and, therefore, these compounds are being considered for anticancer therapy. Their mechanism of antitumor action seems to be rather complex and not fully uncovered yet, however, autophagy has been confirmed to play a key role in this process. In this review article, we report on the up-to-date knowledge of the anticancer activity of cardiac glycosides with special attention paid to autophagy induction, the molecular mechanisms of this process, and the potential employment of this phenomenon in clinical practice. Keywords: bufalin; digoxin; ouabain; peruvoside; Na + /K + -ATPase; autosis; LC3-II; Beclin 1; mTOR 1. Introduction According to the estimated projection of US cancer incidence and death in 2040 published by Rahib et al. [1] in 2021, the incidence of cancer will generally increase. In comparison with 2020, the number of diagnosed cancer cases in 2040 will increase by nearly 150,000 to 1,881,000. Although the incidence will increase, the number of deaths will most probably be lower than in 2020. The estimate of 410,000 deaths is, however, worrying. The ratio of deaths makes cancer still one of the most threatening diseases. Currently, not a few cancer-related deaths in 2021 will be indirectly connected with reduced access to health care due to the coronavirus disease 2019 (COVID-19) pandemic [2]. The COVID-19 pandemic has also shown that besides developing new drugs and treatment strategies, it is also possible to search for useful active compounds among already approved drugs, which are used to treat other indications [3]. This approach, called “drug repositioning”, has also long been implemented in cancer treatment. It enables a partial speeding up of the process of a drug’s clinical evaluation since basic information on the therapeutic parameters of a repurposed drug is already known [4]. As an example, cardiac glycosides (CGs) have emerged as potentially very useful anticancer therapeutics. CGs are widely distributed substances of both plant and animal origin that have found an application in the treatment of cardiovascular diseases, especially heart failure. Their effects have been known since ancient times [5]. Not only were they used for healing, but also as a part of arrow poisons [6,7]. First, CGs have been incorporated into modern medicine by an English doctor and botanist William Withering, who summarized his findings in his book “An account of the foxglove, and some of its medical uses” published in 1785 [8]. Almost 200 years later, in 1967, Shiratori [9] reported on the inhibition of neoplastic cells by CGs for the first time. Since then, the interest in CGs as potential anticancer chemotherapeutics has increased. Cells 2021, 10, 3341. https://doi.org/10.3390/cells10123341 https://www.mdpi.com/journal/cells