2-Hydroxycarbazole induces Ca2+ release from sarcoplasmic reticulum by activating the ryanodine receptor

Neferine induces autophagy-dependent cell death in apoptosis-resistant cancers via ryanodine receptor and Ca2+-dependent mechanism

Resistance of cancer cells to chemotherapy is a significant clinical concern and mechanisms regulating cell death in cancer therapy, including apoptosis, autophagy or necrosis, have been extensively investigated over the last decade. Accordingly, the identification of medicinal compounds against chemoresistant cancer cells via new mechanism of action is highly desired. Autophagy is important in inducing cell death or survival in cancer therapy. Recently, novel autophagy activators isolated from natural products were shown to induce autophagic cell death in apoptosis-resistant cancer cells in a calcium-dependent manner. Therefore, enhancement of autophagy may serve as additional therapeutic strategy against these resistant cancers. By computational docking analysis, biochemical assays, and advanced live-cell imaging, we identified that neferine, a natural alkaloid from Nelumbo nucifera, induces autophagy by activating the ryanodine receptor and calcium release. With well-known apoptotic agents, such as staurosporine, taxol, doxorubicin, cisplatin and etoposide, utilized as controls, neferine was shown to induce autophagic cell death in a panel of cancer cells, including apoptosis-defective and -resistant cancer cells or isogenic cancer cells, via calcium mobilization through the activation of ryanodine receptor and Ulk-1-PERK and AMPK-mTOR signaling cascades. Taken together, this study provides insights into the cytotoxic mechanism of neferine-induced autophagy through ryanodine receptor activation in resistant cancers.

Ca2+ signals generated by CatSper and Ca2+ stores regulate different behaviors in human sperm

[Ca²⁺]_i signaling regulates sperm motility, enabling switching between functionally different behaviors that the sperm must employ as it ascends the female tract and fertilizes the oocyte. We report that different behaviors in human sperm are recruited according to the Ca²⁺ signaling pathway used. Activation of CatSper (by raising pH_i or stimulating with progesterone) caused sustained [Ca²⁺]_i elevation but did not induce hyperactivation, the whiplash-like behavior required for progression along the oviduct and penetration of the zona pellucida. In contrast, penetration into methylcellulose (mimicking penetration into cervical mucus or cumulus matrix) was enhanced by activation of CatSper. NNC55-0396, which abolishes CatSper currents in human sperm, inhibited this effect. Treatment with 5 μm thimerosal to mobilize stored Ca²⁺ caused sustained [Ca²⁺]_i elevation and induced strong, sustained hyperactivation that was completely insensitive to NNC55-0396. Thimerosal had no effect on penetration into methylcellulose. 4-Aminopyridine, a powerful modulator of sperm motility, both raised pH_i and mobilized Ca²⁺ stored in sperm (and from microsomal membrane preparations). 4-Aminopyridine-induced hyperactivation even in cells suspended in Ca²⁺-depleted medium and also potentiated penetration into methylcellulose. The latter effect was sensitive to NNC55-039, but induction of hyperactivation was not. We conclude that these two components of the [Ca²⁺]_i signaling apparatus have strikingly different effects on sperm motility. Furthermore, since stored Ca²⁺ at the sperm neck can be mobilized by Ca²⁺-induced Ca²⁺ release, we propose that CatSper activation can elicit functionally different behaviors according to the sensitivity of the Ca²⁺ store, which may be regulated by capacitation and NO from the cumulus.

1-(1-Hy-droxy-8-methyl-9H-carbazol-2-yl)ethanone

The title compound, C(15)H(13)NO(2), crystallizes with four independent mol-ecules (A, B, C and D) in the asymmetric unit. The carbazole units are almost planar [maximum deviations = 0.015 (3) for A, 0.024 (3) for B, 0.026 (3) for C and 0.046 (3) Å for D]. In all four mol-ecules, there is an O-H⋯O hydrogen bond involving the hy-droxy substituent and the carbonyl O atom of the adjacent acetyl group, which forms a six-membered ring. In the crystal, the four independent mol-ecules are linked via N-H⋯O and C-H⋯O inter-actions.

Commonly used ryanodine receptor activator, 4-chloro-m-cresol (4CmC), is also an inhibitor of SERCA Ca2+ pumps

4-Chloro-m-cresol (4CmC) is an extensively used activator of ryanodine receptors (RyRs). Studies have shown that 4CmC, at a concentration of 1 mM, is sufficient to cause Ca(2+) release through RyRs. Here, we show that mM concentrations of 4CmC also inhibit the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA), (IC(50) 2-3mM) and cause Ca(2+) release. 4CmC also causes increased intracellular [Ca(2+)] levels in COS-7 cells, which lack functional RyRs. Thus, any increase in [Ca(2+)] levels associated with use of 4CmC (>or= 1mM) could lead to non-specific Ca(2+) changes due to SERCA inhibition rather than RyR activation.

Activation of the sheep cardiac Ca2+ release channel by simple heteroaromatics

The broad range of ligands known to modulate ryanodine receptor activity includes a class of heteroaromatic compounds displaying relatively poor efficacy. Greater understanding of the physicochemical properties that predispose these molecules to interaction with the channel should facilitate the rational design of more potent analogues. To this end we are examining the structure-activity relationship for simple heteroaromatic compounds. Efficacy is assessed by the ability to stimulate [3H]ryanodine binding to heavy sarcoplasmic reticulum vesicles. The propensity to activate the channel requires notably little chemical functionality and is associated with the capacity for charge-transfer complex formation in conjunction with steric bulk.

Inhibition of the type 1 inositol 1,4,5-trisphosphate receptor by 2-aminoethoxydiphenylborate

2-Aminoethoxydiphenylborate (2-APB) inhibits the extent of inositol 1,4,5-trisphosphate (InsP(3))-induced Ca(2+) release from cerebellar microsomes with a potency that is dependent upon the InsP(3) concentration used. At high InsP(3) concentrations (10 microM), the concentration of 2-APB required to cause half-maximal InsP(3)-induced Ca(2+) release (IC(50)) was greater than 1 mM, while at 0.25 microM InsP(3) this reduced to 220 microM. The fact that the inhibition of the extent of InsP(3)-induced Ca(2+) release (IICR) by 2-APB was not restored to control levels by high concentrations of InsP(3), in addition to the fact 2-APB did not substantially inhibit [3H]InsP(3) binding to its receptor, indicates that the inhibition is not competitive in nature. Since the cooperativity of IICR as a function of InsP(3) was reduced in the presence of 2-APB (Hill coefficient changing from 1.9 in the absence of 2-APB to 1.4 in the presence of 1 mM 2-APB), this suggests that it is acting as an allosteric inhibitor. 2-APB also reduces the rate constants for IICR. In cerebellar microsomes this release process is biphasic in nature, with a fast and slow phase. 2-APB appears particularly to affect the fast-phase component. Although 2-APB does not inhibit the ryanodine receptor, it does inhibit the Ca(2+) ATPase activity as well store-operated Ca(2+) entry channels, which may limit its use as a specific membrane permeant InsP(3) receptor inhibitor.

Response of neurons to an irreversible inhibition of endoplasmic reticulum Ca(2+)-ATPase: relationship between global protein synthesis and expression and translation of individual genes

In the physiological state, there appears to be a regulatory link between endoplasmic reticulum (ER) Ca(2+) homoeostasis and the initiation of neuronal protein synthesis. Exposing neuronal cell cultures to thapsigargin (Tg), an irreversible inhibitor of sarcoplasmic/ER Ca(2+)-ATPase (SERCA), induced an almost complete suppression of protein synthesis, which recovered to approx. 60% of control 24 h after Tg exposure. This is an experimental model where the regulatory link between the initiation of protein synthesis and ER Ca(2+) homoeostasis recovers, despite an irreversible suppression of SERCA activity [Doutheil, Treiman, Oschlies and Paschen (1999) Cell Calcium 25, 419--428]. The model was used to investigate the relationship between transcription and translation of various stress genes that respond to conditions causing graded suppression of protein synthesis. Expression patterns revealed three groups of genes. The mRNA levels of genes responding to conditions of ER stress (grp78, grp94, gadd34 and gadd153) were increased up to 200-fold after Tg exposure, whereas those coding for ER-resident proteins (SERCA 2b and Bcl-2) were increased up to 6-fold in treated cultures, and those coding for cytoplasmic proteins (heat-shock protein 70 and p67) were increased only 2--4-fold. Analysis of translation of these mRNAs suggests an imbalance in the synthesis of apoptosis-inducing (GADD153) and tolerance-activating (GRP78 and Bcl-2) proteins after blocking of the ER Ca(2+) pump. The observation that the relationship between Tg-induced changes in mRNA and protein levels varied considerably for the various genes studied implies that translation of the respective transcripts is differently regulated under conditions causing graded suppression of global protein synthesis. Detailed analysis of the response of neuronal cells to transient disturbance of ER Ca(2+) homoeostasis may help to elucidate the mechanisms underlying neuronal cell injury in those neurological disorders in which an impairment of ER function is thought to contribute to the pathological process of deterioration.

Bupivacaine suppresses [Ca(2+)](i) oscillations in neonatal rat cardiomyocytes with increased extracellular K+ and is reversed with increased extracellular Mg(2+)

Lidocaine is used to treat cardiac arrhythmias, whereas bupivacaine is noted for its cardiotoxicity. A precise mechanism for these differences is unclear, and there is no well defined antidote for local anesthetic cardiotoxicity. Our study compares the effect of lidocaine and bupivacaine on oscillations of intracellular Ca(2+) coupled with contractions in neonatal rat cardiomyocytes by using digital imaging. In medium containing 5.6 mM K(+), both 42 microM lidocaine and 5.5 microM bupivacaine significantly reduced the oscillation rate. The oscillatory patterns were highly irregular, and the rates were increased in the presence of bupivacaine in 7.6 mM K(+) medium, eventually degenerating into a loss of oscillations after several minutes of bupivacaine exposure. Irregular oscillations did not occur with lidocaine until the K(+) concentration was increased to 10 mM. Increasing the Mg(2+) and Ca(2+) concentrations by 2 mM each recovered oscillation that had been suppressed by bupivacaine in high K(+) buffer. Evaluation of intracellular Ca(2+) oscillations in neonatal rat suggests that increased extracellular K(+) may be an important component of bupivacaine cardiotoxicity.

IMPLICATIONS

Evaluation of intracellular Ca(2+) oscillations in neonatal rat myocytes suggests that increased extracellular K(+) may be an important component of bupivacaine cardiotoxicity.

Three different vasoactive responses of rat tail artery to nicotine

The vasoactive effects of nicotine on isolated rat tail artery tissues were studied. Nicotine transiently contracted rat tail artery tissues (EC50, 55.6 ± 2 µM) in an extracellular Ca2+ dependent and endothelium-independent fashion. The blockade of alpha1-adrenoceptors, but not alpha2-adrenoceptors or P2X purinoceptors, inhibited the nicotine-induced contraction by 38 ± 7% (p < 0.05). Nicotine (1 mM) depolarized membrane by 13 ± 3 mV, but did not affect L-type Ca2+ channel currents, of the isolated rat tail artery smooth muscle cells. The phenylephrine-precontracted tail artery tissues were relaxed by nicotine (EC50, 0.90 ± 0.31 mM), which was significantly inhibited after the blockade of nicotinic receptors. Simultaneous removal of phenylephrine and nicotine, after a complete relaxation of the phenylephrine-precontracted tail artery strips was achieved by nicotine at accumulated concentrations (>=10 mM), triggered a Ca2+-dependent rebound long-lasting vasoconstriction (n = 20). This rebound contraction was abolished in the absence of calcium or in the presence of tetracaine in the bath solution. Pretreatment of vascular tissues with a nicotinic receptor antagonist did not affect the nicotine-induced vasoconstriction or nicotine withdrawal induced rebound contraction. The elucidation of the triphasic vascular effects of nicotine and the underlying mechanisms is important for a better understanding of the complex vascular actions of nicotine.Key words: nicotine, smokeless tobacco, vascular smooth muscles, contraction, relaxation.