Tetraphenylphosphonium ion is a true indicator of negative plasma-membrane potential in the yeast Rhodotorula glutinis. Experiments under osmotic stress and at low external pH values

Salinomycin as a potent anticancer stem cell agent: State of the art and future directions

Cancer stem cells (CSCs) are a small subpopulation of cells within a tumor that can both self‐renew and differentiate into other cell types forming the heterogeneous tumor bulk. Since CSCs are involved in all aspects of cancer development, including tumor initiation, cell proliferation, metastatic dissemination, therapy resistance, and recurrence, they have emerged as attractive targets for cancer treatment and management. Salinomycin, a widely used antibiotic in poultry farming, was identified by the Weinberg group as a potent anti‐CSC agent in 2009. As a polyether ionophore, salinomycin exerts broad‐spectrum activities, including the important anti‐CSC function. Studies on the mechanism of action of salinomycin against cancer have been continuously and rapidly published since then. Thus, it is imperative for us to update its literature of recent research findings in this area. We here summarize the notable work reported on salinomycin's anticancer activities, intracellular binding target(s), effects on tumor microenvironment, safety, derivatives, and tumor‐specific drug delivery; after that we also discuss the translational potential of salinomycin toward clinical application based on current multifaceted understandings.

Mechanisms underlying beta2-adrenoceptor-mediated nitric oxide generation by human umbilical vein endothelial cells

Endothelial beta(2)-adrenoceptor (beta(2)AR) stimulation increases nitric oxide (NO) generation, but the underlying cellular mechanisms are unclear. We examined the role of l-arginine transport and of phosphorylation of NO synthase 3 (NOS-3) in beta(2)AR-mediated NO biosynthesis by human umbilical vein endothelial cells (HUVEC). To this end, we assessed l-arginine uptake, NOS activity (from l-arginine to l-citrulline conversion), membrane potential (using [(3)H]tetraphenylphosphonium), as well as serine phosphorylation of NOS-3 (by Western blotting and mass spectrometry), in HUVEC treated with betaAR agonists or cyclic AMP-elevating agents. beta(2)AR stimulation increased l-arginine transport, as did cyclic AMP elevation with either forskolin or dibutyryl cyclic AMP, and this increase was inhibitable by N-ethylmaleimide. Blockade of l-arginine uptake by l-lysine inhibited NOS activity and, conversely, blockade of NOS using N(omega)-nitro-l-arginine methyl ester (l-NAME) inhibited l-arginine transport. beta(2)AR stimulation also caused a membrane hyperpolarization inhibitable by l-NAME, suggesting that the increase in l-arginine uptake occurred in response to NO-mediated hyperpolarization. beta(2)AR activation also increased NOS activity and phosphorylation of NOS-3 on serine-1177, and these increases were attenuated by inhibition of protein kinase A (PKA), phosphatidylinositol 3-kinase (PI3K) or Akt, and abolished by coinhibition of PKA and Akt. These findings suggest that beta(2)AR-mediated NOS-3 activation in HUVEC is mediated through phosphorylation of NOS-3 on serine-1177 through both the PKA and the PI3K/Akt systems, and is sustained by an increase in l-arginine uptake resulting from NO-mediated membrane hyperpolarization.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.

Chemoattractant-induced membrane hyperpolarization in Dictyostelium discoideum. A possible role for cyclic GMP

Membrane potentials of the cellular slime mold Dictyostelium discoideum were monitored after chemotactic stimulation by measuring the distribution of the lipophilic cation tetraphenylphosphonium. Stimulation with the chemoattractant cAMP induces a transient membrane hyperpolarization which reaches its most negative value between 1-3 min after stimulation. This hyperpolarization is consistent with the opening of potassium channels. Measurements in streamer F mutant cells reveal that cGMP likely plays a role in the regulation of the cAMP-induced hyperpolarization.

Analysis of the H+/sugar symport in yeast under conditions of depolarized plasma membrane

H+/sugar symport in the obligatory aerobic yeast Rhodotorula glutinis was analyzed under conditions where the plasma membrane was selectively depolarized by the lipophilic cation tetraphenylphosphonium (TPP+). Control experiments showed that this treatment did not impair the transmembrane delta pH, the cell energy charge, and the function of plasma membrane H+-ATPase. The kinetic data were fitted to elementary functions derived from a model constructed on the basis of some simplifying premises for ordered (either C + H+ + S or C + S + H+) and random reaction mechanisms. In addition, the comparison of the kinetic parameters in fully energized and depolarized cells provided information about the free carrier charge. It was concluded that the binding sequence of formation of the ternary carrier/H+/substrate complex follows a random mechanism and that the carrier bears a negative charge.

Tetraphenylphosphonium is an indicator of negative membrane potential in Candida albicans

The characteristics of the uptake of lipophilic cations tetraphenylphosphonium (TPP+) into Candida albicans have been investigated to establish whether TPP+ can be used as a membrane potential probe for this yeast. A membrane potential (delta psi, negative inside) across the plasma membrane of C. albicans was indicated by the intracellular accumulation of TPP+. The steady-state distribution of TPP+ was reached within 60 min and varied according to the expected changes of delta psi. Agents known to depolarize membrane potential caused a rapid and complete efflux of accumulated TPP+. The initial influx of TPP+ was linear over a wide range of TPP+ concentrations (2.5-600 microM), indicating a non mediated uptake. Thus, TPP+ is a suitable delta psi probe for this yeast.