Sodium arsenite and cadmium chloride induction of proteasomal inhibition and HSP accumulation in Xenopus laevis A6 kidney epithelial cells

Lysine-63-linked polyubiquitination: a principal target of cadmium carcinogenesis

Cadmium is an environmental pollutant that constitutes a major danger to human health. It is considered a definite human carcinogen. The lung and kidney are the most sensitive organs for cancer development, and we recently provided the first evidence of direct upregulation of lysine-63-linked polyubiquitination by cadmium, particularly in response to environmentally relevant concentrations. Investigations of K63 polyubiquitination have greatly progressed, and various strategies have been reported for studying this molecular process in different biological systems under both physiological and stress conditions. Furthermore, the mechanisms underlying cadmium-induced accumulation of K63-polyubiquitinated proteins in lung and renal cells continue to be of interest given the unknown mechanism involved in the carcinogenesis of this metal. Cadmium is persistent within the cytosol and induces oxidative stress, which continuously damages proteins and causes K63 polyubiquitination, leading to the regulation/activation of different cellular signaling pathways. The aim of this review was to perform a critical analysis of the knowledge about K63 polyubiquitination induced by cadmium and its effect on selective autophagy, CYLD, the NF-KB pathway and Hif-1α. We also report data obtained in different experimental studies using cadmium, highlighting similarities in the induction of the ubiquitination system. A more detailed discussion will concern the role of K63 polyubiquitination in cadmium-exposed renal proximal convoluted tubules and lung cells since they are suitable model systems that are extremely sensitive to environmental stress, and cadmium is one of the most carcinogenic metals to which humans are exposed. We ultimately concluded that K63 polyubiquitination may be the origin of cadmium carcinogenesis in the lung and kidney.

Graphical Abstract

Pathways of cadmium carcinogenesis: Cadmium mimics zinc and induces Lysine-63-linked polyubiquitination, which promotes three intracellular processes: (1) accumulation of ubiquitinated proteins, (2) stabilization of hypoxic inducible factor-1α and (3) activation of the nuclear factor-kappaB pathway, which results in the blockade of selective autophagy, angiogenesis, inflammation and cell proliferation.

Environmental factors modulating protein conformations and their role in protein aggregation diseases

The adverse physiological conditions have been long known to impact protein synthesis, folding and functionality. Major physiological factors such as the effect of pH, temperature, salt and pressure are extensively studied for their impact on protein structure and homeostasis. However, in the current scenario, the environmental risk factors (pollutants) have gained impetus in research because of their increasing concentrations in the environment and strong epidemiologic link with protein aggregation disorders. Here, we review the physiological and environmental risk factors for their impact on protein conformational changes, misfolding, aggregation, and associated pathological conditions, especially environmental risk factors associated pathologies.

Stress granules counteract senescence by sequestration of PAI-1

Cellular senescence is a physiological response by which an organism halts the proliferation of potentially harmful and damaged cells. However, the accumulation of senescent cells over time can become deleterious leading to diseases and physiological decline. Our data reveal a novel interplay between senescence and the stress response that affects both the progression of senescence and the behavior of senescent cells. We show that constitutive exposure to stress induces the formation of stress granules (SGs) in proliferative and presenescent cells, but not in fully senescent cells. Stress granule assembly alone is sufficient to decrease the number of senescent cells without affecting the expression of bona fide senescence markers. SG‐mediated inhibition of senescence is associated with the recruitment of the plasminogen activator inhibitor‐1 (PAI‐1), a known promoter of senescence, to these entities. PAI‐1 localization to SGs increases the translocation of cyclin D1 to the nucleus, promotes RB phosphorylation, and maintains a proliferative, non‐senescent state. Together, our data indicate that SGs may be targets of intervention to modulate senescence in order to impair or prevent its deleterious effects.

Alleviation Mechanisms of Selenium on Cadmium-Spiked Neutrophil Injury to Chicken

To determine the negative effects of cadmium (Cd) exposure and the protective role of selenium (Se) on Cd-spiked neutrophils of chicken, forty-eight 28-day-old Isa Brown male chickens were divided randomly into four groups. Group I (control group) was fed with the basic diet containing 0.2 mg/kg Se. Group II (Se-treated group) was fed with the basic diet supplemented with Na₂SeO₃, and the total Se content was 2 mg/kg. Group III (Se/Cd-treated group) was fed with the basic diet supplemented with Na₂SeO₃; the total Se content was 2 mg/kg and supplemented with 150 mg/kg CdCl₂. Group IV (Cd-treated group) was fed with the basic diet supplemented with 150 mg/kg CdCl₂. Analyses of inflammatory factors, cytokines, and heat shock protein (Hsp) messenger RNA (mRNA) expression were detected by real-time PCR (RT-PCR). Additionally, we evaluated the phagocytic rate of neutrophils in peripheral blood. First, we observed that Cd significantly induced the mRNA expression levels of inflammatory factors NF-κB, iNOS, COX-2, and TNF-α, while Se/Cd treatment reduced their mRNA expression, although these expression levels remained higher than that of the control group. In addition, the mRNA expression levels of cytokines (IL-2, IL-4, and IL-10) for the Se-treated group exhibited significant differences between the Se/Cd-treated group and the Cd-treated group. Furthermore, the mRNA expression levels of Hsps demonstrated that the Se/Cd-treated group and the Cd-treated group were significantly higher (P < 0.05) than the control group and the Se-treated group. These results demonstrated that Se presented partial protection on Cd-spiked neutrophils of chicken with Hsps being involved in the process of the Cd-spiked toxic effects in chicken peripheral blood neutrophils.

Accumulation of heme oxygenase-1 (HSP32) in Xenopus laevis A6 kidney epithelial cells treated with sodium arsenite, cadmium chloride or proteasomal inhibitors

The present study examined the effect of sodium arsenite, cadmium chloride, heat shock and the proteasomal inhibitors MG132, withaferin A and celastrol on heme oxygenase-1 (HO-1; also known as HSP32) accumulation in Xenopus laevis A6 kidney epithelial cells. Immunoblot analysis revealed that HO-1 accumulation was not induced by heat shock but was enhanced by sodium arsenite and cadmium chloride in a dose- and time-dependent fashion. Immunocytochemistry revealed that these metals induced HO-1 accumulation in a granular pattern primarily in the cytoplasm. Additionally, in 20% of the cells arsenite induced the formation of large HO-1-containing perinuclear structures. In cells recovering from sodium arsenite or cadmium chloride treatment, HO-1 accumulation initially increased to a maximum at 12h followed by a 50% reduction at 48 h. This initial increase in HO-1 levels was likely the result of new synthesis as it was inhibited by cycloheximide. Interestingly, treatment of cells with a mild heat shock enhanced HO-1 accumulation induced by low concentrations of sodium arsenite and cadmium chloride. Finally, we determined that HO-1 accumulation was induced in A6 cells by the proteasomal inhibitors, MG132, withaferin A and celastrol. An examination of heavy metal and proteasomal inhibitor-induced HO-1 accumulation in amphibians is of importance given the presence of toxic heavy metals in aquatic habitats.

The effect of manganese-induced cytotoxicity on mRNA expressions of HSP27, HSP40, HSP60, HSP70 and HSP90 in chicken spleen lymphocytes in vitro

The purpose of this study was to investigate the effect of manganese (Mn)-induced cytotoxicity on heat shock proteins in chicken spleen lymphocytes. Lymphocytes were cultured in medium in the absence and presence of MnCl2 (2 × 10(-4), 4 × 10(-4), 6 × 10(-4), 8 × 10(-4), 10 × 10(-4), and 12 × 10(-4) mmol/L) for 12, 24, 36, and 48 h in vitro. Then, the mRNA levels of HSP27, HSP40, HSP60, HSP70, and HSP90 were examined by real-time quantitative PCR. The results showed that the mRNA levels of HSP27, HSP40, HSP60, HSP70, and HSP90 in all treatment groups at all time points, except mRNA levels of HSP27 at 48 h, had the same tendency. As manganese concentration increased, the mRNA expression of the heat shock proteins first increased and then decreased. In other words, we demonstrated that the mRNA expression of the heat shock proteins was induced at lower concentrations of manganese and was inhibited at higher concentrations. Mn had a dosage-dependent effect on HSP27, HSP40, HSP60, HSP70, and HSP90 mRNA expression in chicken spleen lymphocytes in vitro.

Enhanced HSP30 and HSP70 accumulation in Xenopus cells subjected to concurrent sodium arsenite and cadmium chloride stress

Heat shock proteins (HSPs) are molecular chaperones that aid in protein folding, translocation and in preventing stress-induced protein aggregation. The present study examined the effect of simultaneous sodium arsenite and cadmium chloride treatment on the pattern of HSP30 and HSP70 accumulation in A6 kidney epithelial cells of the frog, Xenopus laevis. Immunoblot analysis revealed that HSP30 and HSP70 accumulation in concurrent stressor treatments were significantly higher than the sum of HSP30 or HSP70 accumulation in individual treatments. This finding suggested a synergistic action between sodium arsenite and cadmium chloride. KNK437 inhibitor studies indicated that the combined stressor-induced accumulation of HSPs may be regulated, at least in part, at the level of transcription. Immunocytochemistry revealed that simultaneous treatment of cells with the two stressors induced HSP30 accumulation primarily in the cytoplasm in a punctate pattern with some dysregulation of F-actin structure. Increased ubiquitinated protein accumulation was observed with combined sodium arsenite and cadmium chloride treatment compared to individual stressors suggesting an impairment of the ubiquitin proteasome degradation system. The addition of a mild heat shock further enhanced the accumulation of HSP30 and HSP70 in response to relatively low concentrations of sodium arsenite plus cadmium chloride.

Heat shock protein 70 (Hsp70) inhibits oxidative phosphorylation and compensates ATP balance through enhanced glycolytic activity

To address possible effects of heat shock protein 70 (Hsp70) on energy metabolism, we established a cell line expressing different levels of Hsp70 and evaluated changes in glucose and lactate metabolites, as well as ATP levels accordingly. In addition, activities of enzymes involved in glycolysis [phosphofructokinase (PFK) and lactate dehydrogenase (LDH)], Krebs cycle [citric synthase (CS)], and oxidative phosphorylation {NADH dehydrogenase [complex I (CI)] and ubiquinol:cytochrome-c reductase [complex III (CIII)]} were analyzed. The results show that both glucose consumption and lactate excretion were elevated significantly in cells expressing increased levels of Hsp70. Simultaneously, the activities of glycolytic enzymes PFK and LDH were increased markedly in cells overexpressing Hsp70. Activities of enzymes CI and CIII, both involved in oxidative phosphorylation, decreased upon increased expression of Hsp70. These findings were supported by nonsignificant reductions of CS activities in cells that overexpressed Hsp70, whereas intracellular ATP levels remained constant over a wide range of Hsp70 expression. In conclusion, overexpression of Hsp70 in HeLa cells results in downregulation of oxidative phosphorylation, in particular, multiprotein CIII, the main source of reactive oxygen species. In exchange, upregulation of the glycolytic pathway compensates for the homeostasis of cellular ATP supply.