ATR activation by Cr-DNA damage is a major survival response establishing late S and G2 checkpoints after Cr(VI) exposure

Nuclear SUMOylation and Proteotoxic Stress Responses to Metals with Different Ligand Preferences

Proteins are vulnerable to damage by a broad range of electrophiles, and cells contain several proteotoxic stress-monitoring systems. Main transcriptional responses to protein damage are driven by cytosolic HSF1 and NRF2 using soft nucleophile Cys-SH as sensors of electrophiles. It is unclear what stress responses are activated by poorly SH-reactive hard electrophiles. We examined protein damage responses in normal human lung cells with equitoxic doses of three carcinogenic metals with different electrophilic softness: soft, cadmium(II), intermediate, cobalt(II), and hard, chromium(III) delivered into cells using chromium(VI)/chromate. Cd(II) strongly activated cytosolic NRF2 and HSF1, produced soluble and insoluble polyubiquitinated proteins in the cytosol, and moderately elevated ER and mitochondrial unfolded protein responses and nuclear polySUMOylation. Cr(III) primarily induced nuclear protein damage and polySUMOylation and was negative for the activation of all cytoplasmic stress responses. Co(II) triggered HSF1, NRF2, and other responses seen with both Cr(III) and Cd(II) except for cytosolic polyubiquitin aggregates. Physiological levels of the antioxidant ascorbate inhibited but did not eliminate NRF2 activation by Co(II) and enhanced polySUMOylation by Cr(VI/III). For all three metals, SUMOylated proteins accumulated in nuclear PML bodies, and their formation was suppressed by PML knockdown. Inhibition of SUMOylation decreased transcription and, even more severely, protein expression of NRF2 and HSF1 targets by Cd(II) and Co(II), revealing the importance of this nuclear response in the functionality of cytosolic stress-activated pathways. Our findings demonstrate that soft and hard metal electrophiles elicit distinct proteotoxic stress responses, with the notable inability of the hard electrophile Cr(III) to trigger cytosolic damage-monitoring systems.

Prolonged Particulate Hexavalent Chromium Exposure Induces DNA Double-Strand Breaks and Inhibits Homologous Recombination Repair in Primary Rodent Lung Cells

Hexavalent chromium [Cr(VI)] is a known lung carcinogen and a driving mechanism in human lung cells for Cr(VI)-induced lung cancer is chromosome instability, caused by prolonged Cr(VI) exposure inducing DNA double-strand breaks, while simultaneously inhibiting the repair of these breaks. In North Atlantic right whales, Cr(VI) induces breaks but does not inhibit repair. It is unclear if this repair inhibition is specific to human lung cells or occurs in other species, as it has only been considered in humans and North Atlantic right whales. We evaluated these outcomes in rodent cells, as rodents are an experimental model for metal-induced lung carcinogenesis. We used a guinea pig lung fibroblast cell line, JH4 Clone 1, and rat lung fibroblasts. Cells were exposed to two different particulate Cr(VI) compounds, ranging from 0 to 0.5 ug/cm2, for 24 or 120 h and assessed for cytotoxicity, DNA double-strand breaks, and DNA double-strand break repair. Both particulate Cr(VI) compounds induced a concentration-dependent increase in cytotoxicity and DNA double-strand breaks after acute and prolonged exposures. Notably, while the repair of Cr(VI)-induced DNA double-strand breaks increased after acute exposure, the repair of these breaks was inhibited after prolonged exposure. These results are consistent with outcomes in human lung cells indicating rodent cells respond like human cells, while whale cells have a markedly different response.

The effect of the nucleolar protein ZNF385A on the ribosomal DNA copy number variation in response to Cr(VI)-induced DNA damage

Hexavalent chromium [Cr(VI)] is a widely distributed carcinogen in industrial contexts and general environmental contexts. Emerging research highlights the central role of ribosomal DNA (rDNA) in DNA Damage Responses (DDRs). However, there remains a lack of investigation into the potential dose-dependent relationship between exposure to Cr(VI) and alterations in rDNA copy number (CN), as well as the related mechanisms underlying these effects. A molecular epidemiological investigation involving 67 workers exposed to Cr(VI) and 75 unexposed controls was conducted. There was a notable increase in ZNF385A expression, variations in rDNA CN, and elevated γH2AX levels in the peripheral blood of Cr(VI)-exposed workers. Restricted cubic spline (RCS) models showed that blood Cr levels in the exposed population exhibited non-linear dose-dependent relationships with γH2AX, rDNA CN, and ZNF385A. Of considerable interest, there were robust and positive associations between ZNF385A and both γH2AX and rDNA CN. Further in vitro experiments provided concrete evidence that Cr(VI) simultaneously caused an increase in ZNF385A expression and variations in rDNA CN. ZNF385A-depleted cells showed increased sensitivity to Cr(VI)-mediated DDRs and alterations in rDNA CN. This study indicated that ZNF385A played a highly significant role in the rDNA CN variation in response to Cr(VI)-induced DNA damage.