A putative lectin-binding receptor mediates cadmium-evoked calcium release

Reduction in cadmium-induced toxicity by c-Jun modulation in mouse embryo limb bud cells

BACKGROUND

While it is known that cadmium-exposed embryonic cells have increased activation of c-Jun N-terminal kinase (JNK), the role of this stress signaling pathway in the embryotoxic response is not clear. Thus, the effects of modification of the transcription factor c-Jun, one of the downstream targets of JNK, on cadmium-induced embryotoxicity were investigated in primary cultures of mouse embryo limb bud cells.

METHODS

Cultures of limb bud cells harvested on day 11 of gestation were pretreated with antisense oligonucleotides (ASO) to c-Jun to reduce its expression, and then incubated with cadmium in the form of cadmium chloride. Toxicity was measured through assessments of cell proliferation and differentiation, while the effectiveness of the ASO in reducing c-Jun was assessed through Western blotting using phosphorylation-specific antibodies.

RESULTS

When cells were treated with ASO c-Jun, the total amounts of c-Jun and also cadmium-induced c-Jun activation were diminished. Cadmium-induced cytotoxicity, indicated by reduced cell numbers and differentiation, was found to decrease when cells were exposed to the antisense oligonucleotides to c-Jun. In addition, limb cell numbers and differentiation were also enhanced by exposure to ASO in the absence of cadmium.

CONCLUSION

The JNK pathway, and particularly the downstream effector c-Jun, appears to play an important role in regulating cell survival and differentiation in mouse embryo limb bud cells both in the presence and absence of the toxic metal cadmium.

Interplay of calcium and cadmium in mediating cadmium toxicity

The environmentally important toxic metal, cadmium, exists as the Cd(2+) ion in biological systems, and in this state structurally resembles Ca(2+). Thus, although cadmium exerts a broad range of adverse actions on cells by virtue of its propensity to bind to protein thiol groups, it is now well appreciated that Cd(2+) participates in a number of Ca(2+)-dependent pathways, attributable to its actions as a Ca(2+) mimetic, with a central role for calmodulin, and the Ca(2+)/calmodlin-dependent protein kinase II (CaMK-II) that mediates effects on cytoskeletal dynamics and apoptotic cell death. Cadmium interacts with receptors and ion channels on the cell surface, and with the intracellular estrogen receptor where it binds competitively to residues shared by Ca(2+). It increases cytosolic [Ca(2+)] through several mechanisms, but also decreases transcript levels of some Ca(2+)-transporter genes. It initiates mitochondrial apoptotic pathways, and activates calpains, contributing to mitochondria-independent apoptosis. However, the recent discovery of the role CaMK-II plays in Cd(2+)-induced cell death, and subsequent implication of CaMK-II in Cd(2+)-dependent alterations of cytoskeletal dynamics, has opened a new area of mechanistic cadmium toxicology that is a focus of this review. Calmodulin is necessary for induction of apoptosis by several agents, yet induction of apoptosis by Cd(2+) is prevented by CaMK-II block, and Ca(2+)-dependent phosphorylation of CaMK-II has been linked to increased Cd(2+)-dependent apoptosis. Calmodulin antagonism suppresses Cd(2+)-induced phosphorylation of Erk1/2 and the Akt survival pathway. The involvement of CaMK-II in the effects of Cd(2+) on cell morphology, and particularly the actin cytoskeleton, is profound, favouring actin depolymerization, disrupting focal adhesions, and directing phosphorylated FAK into a cellular membrane. CaMK-II is also implicated in effects of Cd(2+) on microtubules and cadherin junctions. A key question for future cadmium research is whether cytoskeletal disruption leads to apoptosis, or rather if apoptosis initiates cytoskeletal disruption in the context of Cd(2+).

Involvement of cholinoceptors in cadmium-induced endothelial dysfunction

Cadmium (Cd) toxicity was produced in male rats to study the role of cholinoceptors in Cd-induced endothelial dysfunction. The changes in the tension of the aortic rings to constrictor and dilator agonists were compared with those of controls. A Cd-induced significant increase in phenylephrine response was associated with a decrease in basal dilator prostanoid release. In Cd-exposed rings, despite an obvious depression in the acetylcholine (ACh) response, the receptor-independent dilation to the calcium ionophore A23187, which elicits a receptor-independent endothelial relaxation, was slightly elevated (p<0.01), but the smooth muscle cell response to the NO donor, sodium nitroprusside (SNP) remained unaltered. Cadmium decreased both the maximal response to ACh (10(-5) M) and its pirenzepine (Prz) sensitive component. The M1 type cholinoceptor-mediated response to ACh decreased in Cd-exposed rings to 10.30 +/- 5.00% from 38.40 +/- 6.90% (p<0.001). Cadmium also reduced the share of indomethacin 1.64% to 13.92 +/- 2.89% (p<0.01), which correlated well with the changes in the M1-mediated response (r=0.991, p<0.0001). Most of the deleterious effect of Cd appears to be restricted to the M1-dependent ACh response. These findings suggest that Cd produces an endothelial dysfunction by impairing the M1 type cholinoceptor mediated response, which seems to be involved in prostanoid release.

Histidine--rich glycoprotein in the blood of the bivalve Mytilus edulis: role in cadmium speciation and cadmium transfer to the kidney

Cadmium transfer from M. edulis L. blood plasma to tissues was investigated in relation to its chemical speciation in the blood. 109Cd was injected into the posterior adductor muscle along with synthetic chelators oxine, ethylenediaminetetraacetic acid (EDTA) or cyclohexanediaminetetraacetic acid (CDTA). Chelator concentrations were chosen to bind up from 0 to 98.7% of the total blood Cd, based on speciation calculations using the geochemical speciation model MINTEQA2. Increases in synthetically chelated Cd were accompanied by linear decreases in the calculated percentages of Histidine-rich Glycoprotein (HRG)-bound Cd, and nonlinear decreases in Cd(2+) and Cd chloro-complexes. Cadmium uptake by the kidneys decreased with increasing percentage synthetic chelation, while uptake by other tissues was not affected by chelation. Results indicate that there is at least one mechanism of Cd uptake common to all M. edulis tissues, and an additional, more rapid uptake mechanism in the kidneys that is mediated by CdHRG.

Evidence that E-cadherin may be a target for cadmium toxicity in epithelial cells

E-cadherin is a Ca(2+)-dependent cell adhesion molecule that plays an important role in the development and maintenance of epithelial polarity and barrier function. This commentary describes the results of recent studies showing that the environmental pollutant Cd(2+) can damage the E-cadherin-dependent junctions between many types of epithelial cells and reviews the evidence indicating that this effect results from the direct interaction of Cd(2+) with the E-cadherin molecule. In addition, the implications of these findings with respect to the mechanisms of Cd(2+) toxicity in specific target organs such as lung, kidney, bone, and the vascular endothelium are discussed.

Role of Ca(2+) in the regulation of nickel-inducible Cap43 gene expression

We have recently cloned a gene, Cap43, that was significantly induced by exposure to nontoxic levels of both water-soluble and -insoluble Ni(2+) compounds. In this paper, we utilized the expression levels of this gene as a tool to identify second messengers involved in nickel-inducible transcription. We report here that the Ca(2+) ionophore A23187 substantially stimulated Cap43 gene expression. In addition, we found that BAPTA-AM, a specific chelator of free intracellular Ca(2+), consistently attenuated the induction of Cap43, indicating that elevation of intracellular Ca(2+) was essential for this response. TPEN, a chelator of heavy metals, such as Ni(2+) with a very low affinity for Ca(2+), did not attenuate Cap43 induced by Ni or calcium ionophore, suggesting that elevations of Ca(2+) but probably not elevations of other metal ions were involved in the induction of Cap43. A direct measurement of Ca(2+) levels using the fluorescent probe Fluo-3 AM showed elevations of free intracellular Ca(2+) in Ni-treated cells. A strong induction of Cap43 by okadaic acid suggested the involvement of a serine/threonine phosphorylation in a signaling pathway that was presumably activated by Ni and that led to enhanced Cap43 gene expression. However, calcium-dependent protein kinase(s) involved in the nickel-activated signaling pathway remains to be identified.

Excitatory effect of Cd2+ on cat adrenal chromaffin cells

To understand the mechanism(s) underlying the Cd2+- and Co2+-induced increases in the cytosolic free Ca2+ concentration ([Ca]i) in cat adrenal chromaffin cells, we used nystatin-perforated patch recording method and fura-2 microfluorometry. Under the current-clamp conditions, the external application of 5x10(-7) M Cd2+ slowly depolarized the cells resulting in the bursting of action potentials. Under the voltage-clamp conditions, Cd2+ evoked a slow inward current accompanied by a decrease of K+ conductance at a holding potential of -40 mV, and Co2+ mimicked Cd2+ action. In some cells (16%), Cd2+ evoked an additional rapid transient outward current associated with an increased K+ conductance and a successive slow inward current. The Cd2+-induced inward current was activated in a concentration-dependent manner with a half-maximum concentration of 9.3x10(-8) M. The Cd2+- and Co2+-induced [Ca]i increases measured with fura-2 microfluorometry were maximal at 10(-6) and 10(-5) M, respectively, and the higher concentrations of both cations caused the smaller responses. Additional transient increase in [Ca]i was often evoked upon the removal of relatively higher concentrations of these metals. It was concluded that the Cd2+-induced membrane depolarization due to the decrease in K+ conductances evoked the bursting firings resulting in the increase in [Ca]i, and consequently might stimulate the catecholamine secretion.

Effect of calmodulin-inhibitors and verapamil on the nephrotoxicity of cadmium in rat

Recent reports indicate that calmodulin inhibitors (CIs) can modify cadmium (Cd) toxicity in rodents. Pretreatment with CIs prevents Cd-induced testicular damage in mice and reduces the severity of such damage in rats. On the other hand it has been suggested that the cellular transport of Cd can be partly inhibited by the calcium-channel inhibitor, verapamil. The aim of this study was to determine whether these inhibitors can prevent the toxic effects of Cd on the kidney which is the critical organ. For that purpose, we have examined the effects of two CIs (trifluoperazine and chlorpromazine) and of verapamil on the development of tubular damage in female Sprague-Dawley rats. The animals were injected subcutaneously 5 days a week for 8 weeks with cadmium chloride (1 mg Cd/kg), alone or in association with trifluoperazine (20 mg/kg), chlorpromazine (15 mg/kg) or verapamil (2 x 5 mg/kg). The development of renal dysfunction was followed by measuring the urinary excretion of the low molecular weight protein Clara cell protein (CC16). In Cd-treated rats, the urinary excretion of CC16 started to increase from week 6 to reach at the end of experiment values more than 100-times above normal. CIs or verapamil did not influence the rise of urinary CC16 induced by Cd. The three inhibitors, by contrast, enhanced the accumulation of Cd in the liver and, at the exception of chlorpromazine, in the kidneys of Cd-treated rats. Although interfering with the metabolism of Cd, CIs and verapamil do not prevent renal damage in rats chronically exposed to this heavy metal.