Early effects of the antineoplastic agent salinomycin on mitochondrial function

Abstract

Salinomycin, isolated from Streptomyces albus, displays antimicrobial activity. Recently, a large-scale screening approach identified salinomycin and nigericin as selective apoptosis inducers of cancer stem cells. Growing evidence suggests that salinomycin is able to kill different types of non-stem tumor cells that usually display resistance to common therapeutic approaches, but the mechanism of action of this molecule is still poorly understood. Since salinomycin has been suggested to act as a K+ ionophore, we explored its impact on mitochondrial bioenergetic performance at an early time point following drug application. In contrast to the K+ ionophore valinomycin, salinomycin induced a rapid hyperpolarization. In addition, mitochondrial matrix acidification and a significant decrease of respiration were observed in intact mouse embryonic fibroblasts (MEFs) and in cancer stem cell-like HMLE cells within tens of minutes, while increased production of reactive oxygen species was not detected. By comparing the chemical structures and cellular effects of this drug with those of valinomycin (K+ ionophore) and nigericin (K+/H+ exchanger), we conclude that salinomycin mediates K+/H+ exchange across the inner mitochondrial membrane. Compatible with its direct modulation of mitochondrial function, salinomycin was able to induce cell death also in Bax/Bak-less double-knockout MEF cells. Since at the concentration range used in most studies (around 10 μM) salinomycin exerts its effect at the level of mitochondria and alters bioenergetic performance, the specificity of its action on pathologic B cells isolated from patients with chronic lymphocytic leukemia (CLL) versus B cells from healthy subjects was investigated. Mesenchymal stromal cells (MSCs), proposed to mimic the tumor environment, attenuated the apoptotic effect of salinomycin on B-CLL cells. Apoptosis occurred to a significant extent in healthy B cells as well as in MSCs and human primary fibroblasts. The results indicate that salinomycin, when used above μM concentrations, exerts direct, mitochondrial effects, thus compromising cell survival.

Repurposing atovaquone: Targeting mitochondrial complex III and OXPHOS to eradicate cancer stem cells

“…Similarly, metformin, which has complex I inhibitory effects, induced rapid apoptosis of pancreatic CSCs [21]. Salinomycin, an antibiotic that was recently identified as a selective inhibitor of CSCs [22] has been shown to reduce cell survival, at least partially, by impairing mitochondrial bioenergetic performance [23]. Finally, pyrvinium pamoate, an FDA-approved anti-parasitic agent, behaves as an OXPHOS inhibitor targeting mitochondrial complex II and efficiently prevents mammosphere formation in the nano-molar range, with an IC-50 of 50 nM [18].…”

The pharmacological regulation of cellular mitophagy

“…30 However, its mode of action is distinct to that of valinomycin and instead of depolarizing the IMM, salinomycin induces its transient hyperpolarization. 30,31 In contrast to valinomycin, salinomycin increases the efflux of K + from the mitochondrion by catalyzing their electroneutral exchange against protons and subsequently modifies the pH gradient across the IMM. 31 The acidification of the mitochondrial matrix impairs mitochondrial respiration and thus, limits the generation of ATP, eventually leading to mitochondrial failure and activation of the mitochondrial removal process.…”

“…30,31 In contrast to valinomycin, salinomycin increases the efflux of K + from the mitochondrion by catalyzing their electroneutral exchange against protons and subsequently modifies the pH gradient across the IMM. 31 The acidification of the mitochondrial matrix impairs mitochondrial respiration and thus, limits the generation of ATP, eventually leading to mitochondrial failure and activation of the mitochondrial removal process. 30,31 Another example is the antibiotic antimycin A that elicits a mitochondria-targeted autophagic response by inhibiting the respiratory complex III and leading to increased levels of reactive oxygen species (ROS) ( Table 1).…”

“…31 The acidification of the mitochondrial matrix impairs mitochondrial respiration and thus, limits the generation of ATP, eventually leading to mitochondrial failure and activation of the mitochondrial removal process. 30,31 Another example is the antibiotic antimycin A that elicits a mitochondria-targeted autophagic response by inhibiting the respiratory complex III and leading to increased levels of reactive oxygen species (ROS) ( Table 1). 32 Compared to protonophores and other mitochondrial toxins, antimycin A causes a relatively limited decrease in ΔΨm that is rapidly balanced by the reverse hydrolysis activity of the F1Fo-ATPsynthase.…”

Nigericin decreases the viability of multidrug-resistant cancer cells and lung tumorspheres and potentiates the effects of cardiac glycosides

“…NIG is a K + /H + ionophore derived from S. hygroscopicus that has been recently shown to overcome multidrug resistance of cancer cells 27 and to target CSCs. 15,[28][29][30] However, a recent study found that stem-like ovarian cancer cells expressing high levels of ABC drug transporters were largely resistant to ionophore antibiotics (IAs) such as salinomycin and NIG suggesting that IAs alone cannot eliminate certain CSLCs. 31,32 NIG also inhibits cellular processes associated with increased stemness such as metastasis and epithelial mesenchymal transition (EMT) in colorectal 16 and breast cancer cells.…”