Blockade of ALK4/5 signaling suppresses cadmium- and erastin-induced cell death in renal proximal tubular epithelial cells via distinct signaling mechanisms.

Various types of cell death, including apoptosis, necrosis, necroptosis, and ferroptosis, are induced in renal tubular epithelial cells following exposure to environmental stresses and toxicants such as osmotic stress, ischemia/reperfusion injury, cisplatin, and cadmium. This is known to cause renal dysfunction, but the cellular events preceding stress-induced cell death in renal tubules are not fully elucidated. The activin receptor-like kinase (ALK) 4/5, also known as activin-transforming growth factor (TGF) ß receptor, is involved in stress-induced renal injury. We, therefore, studied the role of ALK4/5 signaling in HK-2 human proximal tubular epithelial cell death induced by cisplatin, cadmium, hyperosmotic stress inducer, sorbitol, and the ferroptosis activator, erastin. We found that ALK4/5 signaling is involved in cadmium- and erastin-induced cell death, but not sorbitol- or cisplatin-induced apoptotic cell death. Cadmium exposure elevated the level of phosphorylated Smad3, and treatment with the ALK4/5 kinase inhibitors, SB431542 or SB505124, suppressed cadmium-induced HK-2 cell death. Cadmium-induced cell death was attenuated by siRNA-mediated ALK4 or Smad3 silencing, or by treatment with SIS3, a selective inhibitor of TGFß1-dependent Smad3 phosphorylation. Furthermore, ALK4/5 signaling activated Akt signaling to promote cadmium-induced HK-2 cell death. In contrast, siRNA-mediated Inhibin-bA silencing or treatment with TGFß1 or activin A had little effect on cadmium-induced HK-2 cell death. On the other hand, treatment with SB431542 or SB505124 attenuated erastin-induced ferroptosis by hyperactivating Nrf2 signaling in HK-2 cells. These results suggest that blockade of ALK4/5 signaling protects against cadmium- and erastin-induced HK-2 cell death via Akt and Nrf2 signaling pathways, respectively.

Involvement of Notch1 signaling in malignant progression of A549 cells subjected to prolonged cadmium exposure.

Cadmium exposure is known to increase lung cancer risk, but the underlying molecular mechanisms in cadmium-stimulated progression of malignancy are unclear. Here, we examined the effects of prolonged cadmium exposure on the malignant progression of A549 human lung adenocarcinoma cells and the roles of Notch1, hypoxia-inducible factor 1α (HIF-1α), and insulin-like growth factor 1 receptor (IGF-1R)/Akt/extracellular signal-regulated kinase (ERK)/p70 S6 kinase 1 (S6K1) signaling pathways. Exposing A549 cells to 10 or 20 µm cadmium chloride (CdCl2) for 9-15 weeks induced a high proliferative potential, the epithelial-mesenchymal transition (EMT), stress fiber formation, high cell motility, and resistance to antitumor drugs. Of note, the CdCl2 exposure increased the levels of the Notch1 intracellular domain and of the downstream Notch1 target genes Snail and Slug. Strikingly, siRNA-mediated Notch1 silencing partially suppressed the CdCl2-induced EMT, stress fiber formation, high cell motility, and antitumor drug resistance. In addition, we found that prolonged CdCl2 exposure induced reduction of E-cadherin in BEAS-2B human bronchial epithelial cells and antitumor drug resistance in H1975 human tumor-derived non-small-cell lung cancer cells depending on Notch1 signaling. Moreover, Notch1, HIF-1α, and IGF-1R/Akt/ERK/S6K1 activated each other to induce EMT in the CdCl2-exposed A549 cells. These results suggest that Notch1, along with HIF-1α and IGF-1R/Akt/ERK/S6K1 signaling pathways, promotes malignant progression stimulated by prolonged cadmium exposure in this lung adenocarcinoma model.

PI3K signaling mediates diverse regulation of ATF4 expression for the survival of HK-2 cells exposed to cadmium.

Cadmium exposure causes endoplasmic reticulum (ER) stress and accumulation of activating transcription factor 4 (ATF4), an ER stress marker. To elucidate the role of phosphatidylinositol-3-kinase (PI3K) signaling in this process, we examined the effects of PI3K signaling on cadmium chloride (CdCl2) exposure-induced ATF4 expression in HK-2 human renal proximal tubular cells. ATF4 knockdown by siRNA enhanced CdCl2-induced cellular damage, indicating a cytoprotective function of ATF4. Treatment with LY294002, a PI3K inhibitor, suppressed CdCl2-induced ATF4 expression and Akt phosphorylation at Thr308 with little effect on phosphorylation of eukaryotic translation initiation factor 2 subunit α at Ser51. Activation of PI3K signaling with epidermal growth factor treatment enhanced CdCl2-induced Akt phosphorylation and ATF4 expression. Suppression of CdCl2-induced ATF4 expression by LY294002 treatment was markedly blocked by cycloheximide, a translation inhibitor, but not by MG-132, a proteasome inhibitor, or actinomycin D, a transcription inhibitor. CdCl2 exposure also induced phosphorylation of mammalian target of rapamycin (mTOR) at Ser2448, glycogen synthase kinase-3α (GSK-3α) at Ser21, GSK-3ß at Ser9, and 90 kDa ribosomal S6 kinase 2 (RSK2) at Ser227 in HK-2 cells. Treatment with rapamycin, an mTOR inhibitor, MK2206, an Akt inhibitor, and BI-D1870, a RSK inhibitor, partially suppressed CdCl2-induced ATF4 expression. Conversely, SB216763, a GSK-3 inhibitor, markedly inhibited the potency of LY294002 to suppress CdCl2-induced ATF4 expression. These results suggest that PI3K signaling diversely regulates the expression of ATF4 in a translation-dependent manner via downstream molecules, including mTOR, GSK-3α/ß, and RSK2, and plays a role in protecting HK-2 cells from cadmium-induced damage.

Phosphorylation of FOXO3a by PI3K/Akt pathway in HK-2 renal proximal tubular epithelial cells exposed to cadmium.

We examined the effects of cadmium chloride (CdCl2) exposure on the phosphorylation and function of the forkhead box class O (FOXO) transcription factor FOXO3a in HK-2 human renal proximal tubular cells. Phosphorylation of FOXO3a (at Thr32 and Ser253) and its upstream kinase, Akt (at Thr308 and Ser473) were markedly increased following exposure to 10 or 20 µM CdCl2. Treatment with wortmannin (500 nM), an inhibitor of phosphoinositide-3-kinase (PI3K), suppressed CdCl2-induced phosphorylation of Akt and FOXO3a at their Akt phosphorylation sites. CdCl2-induced phosphorylation of FOXO3a was markedly suppressed by the epidermal growth factor receptor inhibitor, AG1478 (1 µM), the Ca(2+)/calmodulin-dependent kinase II inhibitor, KN-93 (10 µM), and the Src inhibitor, PP2 (10 µM), but only partially suppressed by the insulin-like growth factor-1 receptor inhibitor, PPP (2.5 µM). Furthermore, the p38 inhibitor, SB203580 (20 µM), suppressed CdCl2-induced phosphorylation of Akt and FOXO3a, suggesting possible cross-talk between p38 mitogen-activated protein kinase and Akt. Although phosphorylation of FOXO3a was associated with reduced levels of nuclear FOXO3a, this change in cellular localization was transient. Silencing of FOXO3a expression using short interfering RNA suppressed CdCl2-induced cellular damage and accumulation of cytoplasmic nucleosomes in HK-2 cells. These results show that cadmium exposure induces phosphorylation of FOXO3a through the PI3K/Akt signaling pathway and suggest that FOXO3a phosphorylation (inactivation) transiently promotes survival of HK-2 cells. Phosphorylation of FOXO3a by the PI3K/Akt pathway may regulate cell fate in proximal tubules exposed to cadmium.

Zinc chloride exposure increases heme oxygenase-1 expression in MDPC-23 odontoblast-like cells.

OBJECTIVE: The aim of this study is to clarify the effects of zinc chloride (ZnCl2) exposure on the induction of heme oxygenase-1 (HO-1) expression and its regulatory mechanisms in MDPC-23 mouse odontoblast-like cells. METHODS: MDPC-23 cells were incubated with ZnCl2, and the levels of HO-1 protein, phosphorylated forms of mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated protein kinase (ERK), c-Jun NH2-terminal kinase (JNK), and p38, and phosphorylated forms of amino kinase terminal (Akt) and nuclear factor-κB (NF-κB) p65 were determined with western immunoblotting. The level of HO-1 mRNA was determined with RT-PCR analysis. After pretreatment with inhibitors of ERK, JNK, p38, phosphoinositide-3 kinase (PI3K), and NF-κB, HO-1 protein level was determined in MDPC-23 cells exposed to ZnCl2. RESULTS: Following exposure to 500µM ZnCl2, the levels of both HO-1 mRNA and protein were markedly increased. The phosphorylated forms of ERK, JNK, and p38 increased after ZnCl2 exposure. Furthermore, the expression of HO-1 was markedly suppressed by treatment with the p38 inhibitor, SB203580, and mildly suppressed by the MAPK/ERK kinase inhibitor, U0126. However, treatment with the JNK inhibitor, SP600125, did not suppress ZnCl2-induced HO-1 expression. In addition, the phosphorylated forms of Akt, a downstream kinase of PI3K, and NF-κB p65 increased after ZnCl2 exposure. Treatment with the PI3K inhibitor, LY294002, and the NF-κB inhibitor, Bay11-7082, suppressed ZnCl2-induced HO-1 expression. CONCLUSION: These results suggest that ZnCl2 exposure induces HO-1 expression via multiple intracellular signalling pathways, including p38, ERK, PI3K/Akt, and NF-κB, in this odontoblast-like cell line.

Cadmium activates extracellular signal-regulated kinase 5 in HK-2 human renal proximal tubular cells.

We examined the effects of cadmium chloride (CdCl(2)) exposure on the phosphorylation and functionality of extracellular signal-regulated kinase 5 (ERK5), a recently identified member of the mitogen-activated protein kinase (MAPK) family, in HK-2 human renal proximal tubular cells. Following exposure to CdCl(2), ERK5 phosphorylation increased markedly, but the level of total ERK5 was unchanged. ERK5 phosphorylation following CdCl(2) exposure was rapid and transient, similar to the time course of ERK1/2 phosphorylation. Treatment of HK-2 cells with the MAPK/ERK kinase 5 inhibitor, BIX02189, suppressed CdCl(2)-induced ERK5 but not ERK1/2 phosphorylation. The CdCl(2)-induced increase of phosphorylated cAMP response element-binding protein (CREB) and activating transcription factor-1 (ATF-1), as well as the accumulation of mobility-shifted c-Fos protein, were suppressed by BIX02189 treatment. Furthermore, BIX02189 treatment enhanced cleavage of poly(ADP-ribose) polymerase and increased the level of cytoplasmic nucleosomes in HK-2 cells exposed to CdCl(2). These findings suggest that ERK5 pathway activation by CdCl(2) exposure might induce the phosphorylation of cell survival-transcription factors, such as CREB, ATF-1, and c-Fos, and may exert a partial anti-apoptotic role in HK-2 cells.

Effects of salubrinal on cadmium-induced apoptosis in HK-2 human renal proximal tubular cells.

Cadmium exposure is known to cause endoplasmic reticulum (ER) stress. In our current study, we examined the effects of salubrinal, a selective inhibitor of eukaryotic translation initiation factor 2 subunit α (eIF2α) dephosphorylation, on apoptotic cell death and ER stress-signaling events in HK-2 human renal proximal tubular cells exposed to cadmium chloride (CdCl(2)). Using phase-contrast microscopy and a cell viability assay, we observed that salubrinal suppressed CdCl(2)-induced cellular damage and cell death. Treatment with salubrinal reduced the number of TUNEL-positive cells and the cleavages of caspase-3 and poly(ADP-ribose) polymerase, but not the cleavage of light chain 3B, indicating protection from CdCl(2)-induced apoptosis but not autophagy. Although eIF2α remained phosphorylated after CdCl(2) exposure to salubrinal-treated HK-2 cells, the expression of activating transcription factor 4 (ATF4) and the 78 kDa glucose-regulated protein (GRP78) was not increased. On the other hand, CdCl(2)-induced expression of C/EBP homologous protein (CHOP) was reduced by salubrinal treatment. Expression of ATF4, an upstream regulator of GRP78 and CHOP, appeared to be a prerequisite for full protection by salubrinal against cadmium cytotoxicity, because CdCl(2)-induced cellular damage was not fully suppressed in ATF4-deficient cells. Phosphorylated forms of mitogen-activated protein kinases (MAPKs), including c-Jun NH(2)-terminal kinase (JNK), p38, and extracellular signal-regulated protein kinase (ERK), increased after CdCl(2) exposure, whereas salubrinal suppressed the phosphorylation of JNK and p38 but not ERK. These results suggest that salubrinal protects CdCl(2)-exposed HK-2 cells from apoptosis by suppressing cell death signal transduction pathways.