The SigmaR1 chaperone drives breast and colorectal cancer cell migration by tuning SK3-dependent Ca2+ homeostasis.

The remodeling of calcium homeostasis contributes to the cancer hallmarks and the molecular mechanisms involved in calcium channel regulation in tumors remain to be characterized. Here, we report that SigmaR1, a stress-activated chaperone, is required to increase calcium influx by triggering the coupling between SK3, a Ca2+-activated K+ channel (KCNN3) and the voltage-independent calcium channel Orai1. We show that SigmaR1 physically binds SK3 in BC cells. Inhibition of SigmaR1 activity, either by molecular silencing or by the use of sigma ligand (igmesine), decreased SK3 current and Ca2+ entry in breast cancer (BC) and colorectal cancer (CRC) cells. Interestingly, SigmaR1 inhibition diminished SK3 and/or Orai1 levels in lipid nanodomains isolated from BC cells. Analyses of tissue microarray from CRC patients showed higher SigmaR1 expression levels in cancer samples and a correlation with tumor grade. Moreover, the exploration of a cohort of 4937 BC patients indicated that high expression of SigmaR1 and Orai1 channels was significantly correlated to a lower overall survival. As the SK3/Orai1 tandem drives invasive process in CRC and bone metastasis progression in BC, our results may inaugurate innovative therapeutic approaches targeting SigmaR1 to control the remodeling of Ca2+ homeostasis in epithelial cancers.

Modulation of calcium-dependent chloride secretion by basolateral SK4-like channels in a human bronchial cell line.

The human bronchial cell line16HBE14o- was used as a model of airway epithelial cells to study the Ca(2+)-dependent Cl(-) secretion and the identity of K(Ca) channels involved in the generation of a favorable driving force for Cl(-) exit. After ionomycin application, a calcium-activated short-circuit current ( I(sc)) developed, presenting a transient peak followed by a plateau phase. Both phases were inhibited to different degrees by NFA, glybenclamide and NPPB but DIDS was only effective on the peak phase. (86)Rb effluxes through both apical and basolateral membranes were stimulated by calcium, blocked by charybdotoxin, clotrimazole and TPA. 1-EBIO, a SK-channel opener, stimulated (86)Rb effluxes. Block of basolateral K(Ca) channels resulted in I(sc) inhibition but, while reduced, I(sc) was still observed if mucosal Cl(-) was lowered. Among SK family members, only SK4 and SK1 mRNAs were detected by RT-PCR. KCNQ1 mRNAs were also identified, but involvement of K(cAMP) channels in Cl(-) secretion was unlikely, since cAMP application had no effect on (86)Rb effluxes. Moreover, chromanol 293B or clofilium, specific inhibitors of KCNQ1 channels, had no effect on cAMP-dependent I(sc). In conclusion, two distinct components of Cl(-) secretion were identified by a pharmacological approach after a Cai2+ rise. K(Ca) channels presenting the pharmacology of SK4 channels are present on both apical and basolateral membranes, but it is the basolateral SK4-like channels that play a major role in calcium-dependent chloride secretion in 16HBE14o- cells.

Characterization of the taurine transport pathway in A6 kidney cells.

We investigated the role of taurine in cell homeostasis and characterized the taurine transport pathway in cultured kidney cells (A6). The taurine concentration in A6 cells varies with the osmolarity of the culture medium, suggesting that taurine participates in cell osmolarity. Under isosmotic conditions, 14C-taurine efflux through the apical membranes (aJtaur) was 6-7 times lower than that through the basolateral membranes (bJtaur). Under hyposmotic conditions, aJtaur remained almost unchanged. On the contrary, bJtaur increased 8 times in comparison with isosmotic conditions. In hyposmotic conditions, bJtaur was inhibited by 500 microM DIDS, 50 microM NPPB, 10 microM of the two oxonol derivatives DISBAC(2)3 and WW-791, and 100 microM ketoconazole. Conversely, 100 microM 1,9-dideoxyforskolin, 10 microM tamoxifen, 100 microM niflumic acid and 50 microM verapamil had no inhibitory effects. Cell volume regulation upon hyposmotic stress was also found to be inhibited by DISBAC(2)3 (K0.5 of 5+/-1 microM) and by ketoconazole. Nystatin was used to permeabilize the apical membranes with the aim to further characterize bJtaur. 14C-taurine transepithelial fluxes in nystatin-treated cells were found to be linear over taurine concentrations ranging from 3.5 microM to 35 mM. Clamping the transepithelial voltage at positive values (serosal side) slightly stimulated the 14C-taurine transport. Similar time courses of 14C-taurine, 36Cl and 86Rb transepithelial fluxes were found under osmotic stimulation followed by DIDS inhibition in nystatin-treated cells. In whole cell patch-clamp experiments, DISBAC(2)3 application resulted in a strong and reversible decrease of the global Cl- current which was stimulated by hyposmotic stress. Our study indicates that taurine participates in the control of A6 cell osmolarity and that the transporting taurine pathway (efflux) is on the basolateral membranes. In addition to usual chloride channel blockers, oxonol was found to be a potent blocker of the taurine transport and of the swelling-activated chloride current. Using a pharmacological approach, we could not distinguish between a common or different pathway for Cl- and taurine.

Contribution of changes in the chloride driving force to the fading of I(GABA) in frog melanotrophs.

Chloride redistribution during type A gamma-aminobutyric acid (GABA(A)) currents (I(GABA)) has been investigated in cultured frog pituitary melanotrophs with imposed intracellular chloride concentration ([Cl(-)](i)) in the whole cell configuration or with unaltered [Cl(-)](i) using the gramicidin-perforated patch approach. Prolonged GABA exposures elicited reproducible decaying currents. The decay of I(GABA) was associated with both a transient fall of conductance (g(GABA)) and shift of current reversal potential (E(GABA)). The shift of E(GABA) appeared to be time and driving force dependent. In the gramicidin-perforated patch configuration, repeated GABA exposures induced currents that gradually vanished. The fading of I(GABA) was due to persistent shifts of E(GABA) as a result of g(GABA) recovering from one GABA application to another. In cells alternatively clamped at potentials closely flanking resting potential and submitted to a train of brief GABA pulses, a reversal of I(GABA) was observed after 150 s recording. It is demonstrated that, in intact frog melanotrophs, shifts of E(GABA) combine with genuine receptor desensitization to depress I(GABA). These findings strongly suggest that shifts of E(GABA) may act as a negative feedback, reducing the bioelectrical and secretory responses induced by an intense release of GABA in vivo.

The sigma-ligand (+)-pentazocine depresses M current and enhances calcium conductances in frog melanotrophs.

Gramicidin-perforated patch clamp experiments and microfluorimetric measurements were performed to study the ionic mechanisms involved in the sigma-receptor-mediated stimulation of frog (Rana ridibunda) pituitary melanotrophs. The sigma-ligand (+)-pentazocine (50 microM) depressed a sustained outward K(+) current. The kinetic properties of this K(+) component, investigated by analyzing tail currents, were reminiscent of those of the M current (I(M)), with an activation threshold close to -60 mV, a -21-mV half-maximal activation potential, and two-component exponential deactivation kinetics at -90 mV. (+)-Pentazocine (20 microM) produced a 12-mV rightward shift of the activation curve and accelerated the deactivation rate of the tail current. It is also demonstrated that (+)-pentazocine (20 microM) reversibly increased both voltage-dependent calcium conductances and internal calcium level. Altogether, these results suggest that the sigma-receptor-induced modulation of I(M) and calcium currents likely underlies the increase of intracellular [Ca(2+)].

A-Current down-modulated by sigma receptor in frog pituitary melanotrope cells through a G protein-dependent pathway.

Gramicidin perforated patch-clamp recordings were used to study the effects of two sigma 1 receptor ligands, (+)-N-cyclopropylmethyl-N-methyl-1, 4-diphenyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO 1784) and (+)-pentazocine, on the transient outward potassium current (IA) in cultured frog melanotrope cells. (+)-Pentazocine reversibly decreased the current amplitude in a dose-dependent manner. The effects of (+)-pentazocine were mimicked by JO 1784 and were markedly reduced by the sigma 1 receptor antagonist, N, N-dipropyl-2-[4-methoxy-3-2(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE 100). Inactivation rate of IA was best fitted with a double exponential function, yielding time constants of 23.7 and 112.5 ms. (+)-Pentazocine (20 microM) accelerated the current decay, decreasing the time constants to 10.7 and 59 ms, respectively. Current-voltage experiments revealed that (+)-pentazocine (20 microM) did neither modify the open-state I/V curves nor the voltage dependence of IA. However, (+)-pentazocine (20 microM) shifted the steady-state inactivation curve toward more negative potentials and increased the time constant of the time-dependent removal of inactivation. In whole-cell experiments, internal dialysis of guanosine-5'-O-(3-thiophosphate) (100 microM) irreversibly prolonged the response to (+)-pentazocine. In addition, cholera toxin pretreatment (1 microgram. ml-1; 12 h) suppressed the inhibition of IA by (+)-pentazocine (20 microM). It is concluded that in frog melanotrope cells, a cholera toxin-sensitive, G protein-dependent inhibition of IA through a sigma 1 receptor activation, at least partially, underlies the excitatory effect of sigma ligands.

Sigma ligands stimulate the electrical activity of frog pituitary melanotrope cells through a G-protein-dependent inhibition of potassium conductances.

We have investigated the effects of sigma ligands [1,3-di(2-tolyl)guanidine (DTG) and (+)-pentazocine] on the electrical activity of cultured frog pituitary melanotrope cells by using the patch-clamp technique. DTG and (+)-pentazocine (10 microM each) induced a reversible depolarization associated with an increase in membrane resistance and action potential firing. In voltage-clamp experiments, DTG and (+)-pentazocine elicited inward currents whose intensity augmented with membrane depolarization. The currents vanished or reversed between -90 and -100 mV, at values close to the K+ equilibrium potential (E(K)+ = -102 mV). DTG (2-500 microM) and (+)-pentazocine (0.2-200 microM) reduced the outward delayed rectifier K+ current [IK (V)] in a dose-dependent manner with EC50 of 64 and 37 microM, respectively. In contrast, naloxone (50 microM) and pirenzepine (10 microM) did not affect the sigma ligand-induced inhibition of IK (V). Addition of guanosine-5'-O-(3-thiophosphate) in the pipette solution irreversibly sustained the DTG-induced current whereas guanosine-5'-O-(2-thiodiphosphate) virtually suppressed the response. Cholera toxin-pretreatment (1 microgram/ml; 18 hr) abolished the inward current and the inhibition of IK (V) induced by sigma ligands. In contrast, pretreatment with pertussis toxin (1 microgram/ml; 18 hr) had no effect. Taken together, these data indicate that DTG and (+)-pentazocine activate the electrical activity of cultured frog melanotrope cells by reducing both a tonic K+ current and a voltage-dependent [IK (V)] K+ conductance through the activation of a cholera toxin-sensitive G-protein.

Adenosine potentiates the delayed-rectifier potassium conductance but has no effect on the hyperpolarization-activated Ih current in frog melanotrophs.

The effects of adenosine on the voltage-sensitive delayed-rectifier K+ (IK) currents and hyperpolarization-activated cationic inward current (Ih) were studied in cultured frog melanotrophs using the whole-cell configuration of the patch-clamp technique. The A1 receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA; 50 microM) reversibly increased IK. Perfusion of dibutyryl-cAMP (1 mM) in the external solution did not modify the R-PIA-induced enhancement of IK. Pretreatment of melanotrophs with pertussis toxin (1 microg/ml; 12 h) totally abolished the R-PIA-evoked response. Application of hyperpolarizing voltage pulses from -60 to -120 mV to melanotrophs induced a two-component inward current corresponding to an Ih-like conductance. This conductance was characterized by a high K+ selectivity and a low Na+ permeability and was resistant to tetrodotoxin (1 microM). R-PIA had no effect on Ih. The present study demonstrates that in frog melanotrophs adenosine inhibits the electrical activity by activating IK through an A1 receptor subtype coupled to a pertussis toxin-sensitive pathway independent of the cAMP/PKA system. This study also demonstrates the existence of a Ih conductance in frog melanotrophs which is not modulated by A1 receptors.

Gramicidin-perforated patch revealed depolarizing effect of GABA in cultured frog melanotrophs.

1. In frog pituitary melanotrophs, GABA induces a transient stimulation followed by prolonged inhibition of hormone secretion. This biphasic effect is inconsistent with the elevation of cytosolic calcium and the inhibition of electrical activity also provoked by GABA in single melanotrophs. In the present study, standard patch-clamp configurations and gramicidin-perforated patches were used to investigate the physiological GABAA receptor-mediated response and intracellular chloride concentration ([Cl-]i) in cultured frog melanotrophs. 2. In the gramicidin-perforated patch configuration, 1 microM GABA caused a depolarization associated with an action potential discharge and a slight fall of membrane resistance. In contrast, at a higher concentration (10 microM) GABA elicited a depolarization accompanied by a transient volley of action potentials, followed by a sustained inhibitory plateau and a marked fall of membrane resistance. Isoguvacine mimicked the GABA-evoked responses, indicating a mediation by GABAA receptors. 3. In gramicidin-perforated cells, the depolarizing excitatory effect of 1 microM GABA was converted into a depolarizing inhibitory action when 0.4 microM allopregnanolone was added to the bath solution. 4. After gaining the whole-cell configuration, the amplitude and/or direction of the GABA-evoked current (IGABA) rapidly changed before stabilizing. After stabilization, the reversal potential of IGABA followed the values predicted by the Nernst equation for chloride ions when [Cl-]i was varied. 5. In gramicidin-perforated cells, the steady-state I-V relationships of 10 microM GABA- or isoguvacine-evoked currents yielded reversal potentials of -37.5 +/- 1.6 (n = 17) and -38.6 +/- 2.0 mV (n = 8), respectively. These values were close to those obtained by using a voltage-ramp protocol in the presence of Na+, K+ and Ca2+ channel blockers. The current evoked by 1 microM GABA also reversed at these potentials. 6. We conclude that, in frog pituitary melanotrophs, chloride is the exclusive charge carrier of IGABA. In intact cells, the reversal potential of IGABA is positive to the resting potential because of a relatively high [Cl-]i (26.5 mM). Under these conditions, GABA induces a chloride efflux responsible for a depolarization triggering action potentials. However, GABA at a high concentration or in the presence of the potentiating steroid allopregnanolone exerts a concomitant shunting effect leading to a rapid inhibition of the spontaneous firing.