Regulation of cadmium-induced apoptosis by PKCδ in U937 human promonocytic cells

The Effects of Agaricus blazei Murill Polysaccharides on Cadmium-Induced Apoptosis and the TLR4 Signaling Pathway of Peripheral Blood Lymphocytes in Chicken

In this study, we investigated the effects of Agaricus blazei Murill polysaccharides (ABP) on cadmium (Cd)-induced apoptosis and the TLR4 signaling pathway of chicken peripheral blood lymphocytes (PBLs). Seven-day-old healthy chickens were randomly divided into four groups, and each group contained 20 males. The cadmium-supplemented diet group (Cd group) was fed daily with full feed that contained 140 mg cadmium chloride (CdCl₂)/kg and 0.2 mL saline. The A. blazei Murill polysaccharide diet group (ABP group) was fed daily with full feed with 0.2 mL ABP solution (30 mg/mL) by oral gavage. The cadmium-supplemented plus A. blazei Murill polysaccharide diet group (Cd + ABP group) was fed daily with full feed containing 140 mg CdCl₂/kg and 0.2 mL ABP solution (30 mg/mL) by gavage. The control group was fed daily with full feed with 0.2 mL saline per day. We measured the apoptosis rate and messenger RNA (mRNA) levels of apoptosis genes (caspase-3, Bax, and Bcl-2), the mRNA levels of TLR4 and TLR4 signaling pathway-related factors (MyD88, TRIF, NF-κB, and IRF3), the TLR4 protein expression, and the concentrations of inflammatory cytokines (IL-1β, IL-6, and TNF-α) in chicken PBLs. The results showed that the PBL apoptosis rate was significantly increased, the mRNA levels of caspase-3 and Bax were significantly increased, while that of Bcl-2 was significantly reduced. The Bax/Bcl-2 ratio was significantly increased in the Cd group at 20, 40, and 60 days after treatment compared with that in the control group. After treatment with ABP, the above changes were clearly suppressed. At the same time, ABP reduced the concentrations of IL-1β, IL-6, and TNF-α induced by Cd. We also found that ABP inhibited the TLR4 mRNA level and protein expression and inhibited the mRNA levels of MyD88, TRIF, NF-κB, and IRF3. The results demonstrated that Cd could induce apoptosis, activate the TLR4 signaling pathway, and induce the expression of inflammatory cytokines in chicken PBLs, and that the administration of ABP clearly inhibited Cd-induced effects on chicken PBLs.

Cadmium-induced heme-oxygenase-1 expression plays dual roles in autophagy and apoptosis and is regulated by both PKC-δ and PKB/Akt activation in NRK52E kidney cells

Heme oxygenase-1 (HO-1) protects cells against cadmium (Cd)-induced oxidative stress. However, the mechanism underlying this protection is not well understood. In this study, we elucidated the role of HO-1 in Cd-induced cytotoxicity. Exposure of NRK52E cells to Cd induced protein kinase B (PKB)/Akt, protein kinase C (PKC)-δ, and glycogen synthase kinase (GSK) 3αb phosphorylation, and eukaryotic initiation factor (eIF) 2α dephosphorylation. Pharmacological inhibition of Akt resulted in HO-1 suppression and eIF2α activation, which partially suppressed CHOP and PARP-1 cleavage, but promoted autophagy and decreased cell viability. Pharmacological inactivation of PKC-δ markedly suppressed Cd-induced phospho-serine (p-Ser) GSK3αβ, and HO-1, and partially inhibited PARP-1 cleavage, but massively induced autophagy and decreased cell viability. Pharmacological upregulation of p-Ser GSK3αβ enhanced Cd-induced HO-1, CHOP, and PARP-1 cleavage, but decreased autophagy. Genetic deficiency of GSK3β suppressed HO-1 and PARP-1 cleavage and increased autophagy. Genetic suppression of HO-1 reduced Cd-induced PARP-1 cleavage, but increased LC3-II. Cd exposure led to accumulation of p-PKC-δ, p-Ser GSK3αβ, and HO-1 in the nucleus and particulate fractions, suggesting that they have dual functions in response to Cd. N-acetylcysteine treatment suppressed Cd-induced activation of PKC-δ and Akt. These results indicate that HO-1 induced by Cd exposure is regulated by PKC-δ, p-Ser GSK3αβ, and PKB/Akt, which restrain autophagic cell death, but mildly induce apoptosis in NRK52E cells. Together, the results suggest that HO-1 expression in response to Cd maintains cellular homeostasis during oxidative stress.

The Effects of Cadmium at Low Environmental Concentrations on THP-1 Macrophage Apoptosis

Cadmium at environmental concentrations is a risk factor for many diseases, including cardiovascular and neurodegenerative diseases, in which macrophages play an important role. The aim of this study was to evaluate the effects of cadmium at low environmental (nanomolar) concentrations on apoptotic processes in THP-1(acute monocytic leukemia cells line)-derived macrophages, with special focus on mitochondrial events involved. Macrophages were incubated with various cadmium chloride (CdCl₂) solutions for 48 h at final concentrations of 5 nM, 20 nM, 200 nM and 2 µM CdCl₂. Cell viability was measured using flow cytometry. Flow cytometric measurement (annexin V/FITC (annexin V/fluorescein isothiocyanate) and PI (propidium iodide) double staining) was used to quantify the extent of apoptosis. Fluorescence and confocal microscopy were used for imaging of apoptosis process. Changes in mitochondrial membrane potential were monitored using cytofluorimetry after cell staining with JC-1(5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazol-carbocyane iodide) probe. Mitochondrial ROS (reactive oxygen species) levels were measured cytofluorimetrically after incubation of cells with mitochondrial superoxide indicator (MitoSOX) red fluorescent marker. The mRNA expression of Bcl-2 and Bax was analysed with qRT-PCR. Our study demonstrates that cadmium, even at low environmental concentrations, exerts mitochondrial toxicity in THP-1 macrophages. Forty-eight-hour exposure to very low concentrations reduces cell viability and results in cell death by apoptosis and necrosis. The decrease in mitochondrial membrane potential, increased ROS production, increased Bax and decreased Bcl-2 mRNA expression are mitochondrial events involved in cadmium-induced apoptosis.

Shift in monocyte apoptosis with increasing viral load and change in apoptosis-related ISG/Bcl2 family gene expression in chronically HIV-1-infected subjects

Although monocytes and macrophages are targets of HIV-1-mediated immunopathology, the impact of high viremia on activation-induced monocyte apoptosis relative to monocyte and macrophage activation changes remains undetermined. In this study, we determined constitutive and oxidative stress-induced monocyte apoptosis in uninfected and HIV(+) individuals across a spectrum of viral loads (n = 35; range, 2,243 to 1,355,998 HIV-1 RNA copies/ml) and CD4 counts (range, 26 to 801 cells/mm(3)). Both constitutive apoptosis and oxidative stress-induced apoptosis were positively associated with viral load and negatively associated with CD4, with an elevation in apoptosis occurring in patients with more than 40,000 (4.6 log) copies/ml. As expected, expression of Rb1 and interferon-stimulated genes (ISGs), plasma soluble CD163 (sCD163) concentration, and the proportion of CD14(++) CD16(+) intermediate monocytes were elevated in viremic patients compared to those in uninfected controls. Although CD14(++) CD16(+) frequencies, sCD14, sCD163, and most ISG expression were not directly associated with a change in apoptosis, sCD14 and ISG expression showed an association with increasing viral load. Multivariable analysis of clinical values and monocyte gene expression identified changes in IFI27, IFITM2, Rb1, and Bcl2 expression as determinants of constitutive apoptosis (P = 3.77 × 10(-5); adjusted R(2) = 0.5983), while changes in viral load, IFITM2, Rb1, and Bax expression were determinants of oxidative stress-induced apoptosis (P = 5.59 × 10(-5); adjusted R(2) = 0.5996). Our data demonstrate differential activation states in monocytes between levels of viremia in association with differences in apoptosis that may contribute to greater monocyte turnover with high viremia.

IMPORTANCE

This study characterized differential monocyte activation, apoptosis, and apoptosis-related gene expression in low- versus high-level viremic HIV-1 patients, suggesting a shift in apoptosis regulation that may be associated with disease state. Using single and multivariable analysis of monocyte activation parameters and gene expression, we supported the hypothesis that monocyte apoptosis in HIV disease is a reflection of viremia and activation state with contributions from gene expression changes within the ISG and Bcl2 gene families. Understanding monocyte apoptosis response may inform HIV immunopathogenesis, retention of infected macrophages, and monocyte turnover in low- or high-viral-load states.

Ischemia induces regulator of G protein signaling 2 (RGS2) protein upregulation and enhances apoptosis in astrocytes

Regulator of G protein signaling (RGS) family members, such as RGS2, interact with Galpha subunits of heterotrimeric G proteins, accelerating the rate of GTP hydrolysis and attenuating the intracellular signaling triggered by the G protein-coupled receptor-ligand interaction. They are also reported to regulate G protein-effector interactions and form multiprotein signaling complexes. Ischemic stress-induced changes in RGS2 expression have been described in astrocytes, and these changes are associated with intracellular signaling cascades, suggesting that RGS2 upregulation may be an important mechanism by which astrocytes may regulate RGS2 function in response to physiological stress. However, information on the functional roles of stress-induced modulation of RGS2 protein expression in astrocyte function is limited. We report the role of ischemic stress in RGS2 protein expression in rat C6 astrocytoma cells and primary mouse astrocytes. A marked increase in RGS2 occurred after ischemic stress induced by chemicals (sodium azide and 2-deoxyglucose) or oxygen-glucose deprivation (OGD, real ischemia). RGS2 mRNA expression was markedly enhanced by 1 h of exposure to chemical ischemia or 6 h of OGD followed by 2 or 6 h of recovery, respectively. This enhanced expression in primary astrocytes and C6 cells was restored to baseline levels after 12 h of recovery from chemically induced ischemic stress or 4-6 h of recovery from OGD. RGS2 protein was also significantly expressed at 12-24 h of recovery from ischemic insult. Ischemia-induced RGS2 upregulation was associated with enhanced apoptosis. It significantly increased annexin V-positive cells, cleaved caspase-3, and enhanced DNA ladder formation and cell cycle arrest. However, a small interfering RNA (siRNA)-mediated RGS2 knockdown reversed the apoptotic cell death associated with ischemia-induced RGS2 upregulation. Upregulated RGS2 was significantly inhibited by SB-203580, a p38 MAPK inhibitor. Rottlerin, a potent inhibitor of PKCdelta, completely abrogated the increased RGS2 expression. We also examine whether ischemia-induced RGS2-mediated apoptosis is affected by siRNA-targeted endogenous PKCdelta downregulation or its phosphorylation. Although RGS2 upregulation was not affected, siRNA transfection significantly suppressed endogenous PKCdelta mRNA and protein expressions. Ischemia-induced PKCdelta phosphorylation and caspase-3 cleavage were dose dependently inhibited by PKCdelta knockdown, and this endogenous PKCdelta suppression reversed ischemia-induced annexin V-positive cells. This study suggests that ischemic stress increases RGS2 expression and that this condition contributes to enhanced apoptosis in C6 cells and primary astrocytes. The signaling it follows may involve PKCdelta and p38 MAPK pathways.

Circulating monocytes in HIV-1-infected viremic subjects exhibit an antiapoptosis gene signature and virus- and host-mediated apoptosis resistance

Mechanisms that may allow circulating monocytes to persist as CD4 T cells diminish in HIV-1 infection have not been investigated. We have characterized steady-state gene expression signatures in circulating monocytes from HIV-infected subjects and have identified a stable antiapoptosis gene signature comprised of 38 genes associated with p53, CD40L, TNF, and MAPK signaling networks. The significance of this gene signature is indicated by our demonstration of cadmium chloride- or Fas ligand-induced apoptosis resistance in circulating monocytes in contrast to increasing apoptosis in CD4 T cells from the same infected subjects. As potential mechanisms in vivo, we show that monocyte CCR5 binding by HIV-1 virus or agonist chemokines serves as independent viral and host modulators resulting in increased monocyte apoptosis resistance in vitro. We also show evidence for concordance between circulating monocyte apoptosis-related gene expression in HIV-1 infection in vivo and available datasets following viral infection or envelope exposure in monocyte-derived macrophages in vitro. The identification of in vivo gene expression associated with monocyte resistance to apoptosis is of relevance to AIDS pathogenesis since it would contribute to: 1) maintaining viability of infection targets and long-term reservoirs of HIV-1 infection in the monocyte/macrophage populations, and 2) protecting a cell subset critical to host survival despite sustained high viral replication.

MAPK and mTOR pathways are involved in cadmium-induced neuronal apoptosis

Cadmium (Cd) may be accumulated in human body through long-term exposure to Cd-polluted environment, resulting in neurodegeneration and other diseases. To study the mechanism of Cd-induced neurodegeneration, PC12 and SH-SY5Y cells were exposed to Cd. We observed that Cd-induced apoptosis in the cells in a time- and concentration-dependent manner. Cd rapidly activated the mitogen-activated protein kinases (MAPK) including extracellular signal-regulated kinase 1/2 (Erk1/2), c-Jun N-terminal kinase (JNK) and p38. Inhibition of Erk1/2 and JNK, but not p38, partially protected the cells from Cd-induced apoptosis. Consistently, over-expression of dominant negative c-Jun or down-regulation of Erk1/2, but not p38 MAPK, partially prevented Cd-induced apoptosis. To our surprise, Cd also activated mammalian target of rapamycin (mTOR)-mediated signaling pathways. Treatment with rapamycin, an mTOR inhibitor, blocked Cd-induced phosphorylation of S6K1 and eukaryotic initiation factor 4E binding protein 1, and markedly inhibited Cd-induced apoptosis. Down-regulation of mTOR by RNA interference also in part, rescued cells from Cd-induced death. These findings indicate that activation of the signaling network of MAPK and mTOR is associated with Cd-induced neuronal apoptosis. Our results strongly suggest that inhibitors of MAPK and mTOR may have a potential for prevention of Cd-induced neurodegeneration.

Interactive effects of cadmium and all-trans-retinoic acid on the induction of forelimb ectrodactyly in C57BL/6 mice

BACKGROUND

Most toxicological studies have tested single chemical agents at relatively high doses, and fewer studies have addressed the toxic effects of chemical interactions. It is important to understand the toxicity of chemical mixtures in order to assess the more realistic risks of environmental and occupational exposures. A number of chemicals are known to induce a predominantly postaxial forelimb ectrodactyly in C57BL/6 mice, including acetazolamide, ethanol, cadmium, valproic acid, carbon dioxide, dimethadione, phenytoin, and 13-cis-retinoic acid and all-trans-retinoic acid (RA). In the present study, the interactive effects of coadministration of cadmium and RA on developing limbs were investigated.

METHODS

Pregnant C57BL/6 mice were treated with different intraperitoneal (IP) doses of cadmium chloride (CdCl2) and/or RA on gestational day (GD) 9.5, and fetuses were collected on GD 18 and double stained for examination of skeletal defects.

RESULTS

When RA was given simultaneously with cadmium, a significant increase in the incidence and severity of forelimb ectrodactyly (predominantly postaxial) was observed compared to the results with corresponding doses of cadmium or RA alone. When mice were exposed to subthreshold doses of both cadmium (0.5 mg/kg) and RA (1 mg/kg), the combined treatment exceeded the threshold, resulting in forelimb ectrodactyly in 19% of the fetuses. Moreover, coadministration of cadmium and RA at doses exceeding the respective thresholds showed a synergistic effect, that is, 92% of fetuses were found with the forelimb defect as opposed to 10% if the response were additive.

CONCLUSIONS

The findings demonstrate that concurrent exposure to these teratogens can have a synergistic effect and that subteratogenic doses may combine to exceed a threshold.

6-hydroxydopamine-induced apoptosis is mediated via extracellular auto-oxidation and caspase 3-dependent activation of protein kinase Cdelta

6-Hydroxydopamine is a neurotoxin commonly used to lesion dopaminergic pathways and generate experimental models for Parkinson disease, however, the cellular mechanism of 6-hydroxydopamine-induced neurodegeneration is not well defined. In this study we have explored how 6-hydroxydopamine neurotoxicity is initiated. We have also investigated downstream signaling pathways activated in response to 6-hydroxydopamine, using a neuronal-like, catecholaminergic cell line (PC12 cells) as an in vitro model system. We have shown that 6-hydroxydopamine neurotoxicity is initiated via extracellular auto-oxidation and the induction of oxidative stress from the oxidative products generated. Neurotoxicity is completely attenuated by preincubation with catalase, suggesting that hydrogen peroxide, at least in part, evokes neuronal cell death in this model. 6-Hydroxydopamine does not initiate toxicity by dopamine transporter-mediated uptake into PC12 cells, because both GBR-12909 and nisoxetine (inhibitors of dopamine and noradrenaline transporters, respectively) failed to reduce toxicity. 6-Hydroxydopamine has previously been shown to induce both apoptotic and necrotic cell-death mechanisms. In this study oxidative stress initiated by 6-hydroxydopamine caused mitochondrial dysfunction, activation of caspases 3/7, nuclear fragmentation, and apoptosis. We have shown that, in this model, proteolytic activation of the proapoptotic protein kinase Cdelta (PKCdelta) is a key mediator of 6-hydroxydopamine-induced cell death. 6-Hydroxydopamine induces caspase 3-dependent cleavage of full-length PKCdelta (79 kDa) to yield a catalytic fragment (41 kDa). Inhibition of PKCdelta (with rottlerin or via RNA interference-mediated gene suppression) ameliorates the neurotoxicity evoked by 6-hydroxydopamine, implicating this kinase in 6-hydroxydopamine-induced neurotoxicity and Parkinsonian neurodegeneration.